# XP-39 Wind Tunnel Tuft Tests



## kool kitty89 (Apr 20, 2016)

_View: https://www.youtube.com/watch?v=vKxe8jc9_rI_


I'm surprised I've never stumbled on this video before, it's amazingly informative. The few photos of the XP-39 floating around and limited information (often misinformation) regarding its layout and turbocharger (and radiator) installation don't do it justice at all. (in fact, none seem to show the starboard side of the aircraft at all)

It might just be me, but the entire cooling system installation (oil cooler, intercooler, and to at least some extent coolant radiator) look shockingly sloppy and patched together ... almost taped-on. (except the coolant radiator which just seems awkwardly placed, or at least its exit duct mounted mid-wing towards the trailing edge ... below, and possible ahead of the intercooler scoop and turbo air intake) No boundary layer splitter/sleeves/cowls either, but that's not really surprising compared to the rest.


I had an inkling that the overall installation on the XP-39 was so bad as to be totally non-indicative of a practical turbocharger installation (even a reasonably acceptable draggy one with external intercooler -like the P-38J), but I didn't think it was this bad.


Additionally, note the comment about the wing stalling all at once (or almost simultaneously), though I'm not sure if this is relevant to the production P-39. (stall warning is supposed to be good, it's just the spin characteristics that are poor, at least when loaded tail-heavy or out of ammunition -the NACA-recommended 13 inch rear-fuselage extension probably didn't help, let alone the load of equipment crammed behind the engine)


Also interesting that the XP-39 has a nicer clear-view canopy, both taller and lacking the thick 'anti-roll bulkhead' behind the pilot. (an odd addition, I don't think any other WWII aircraft implemented roll-bars, and if they did they were omited in the bubble canopy redesign. (granted, this is a feature I noticed long ago with the more common pictures of the XP-39, though I only realized the full function of that bulkhead recently)

See:
http://img525.imageshack.us/img525/2236/bellairacobrai1939airen.jpg

(also note the small 60-round 20 mm ammunition drum, and the holes in the forward nose bulkhead for the omitted pair of 30 cal nose guns present on the YP-39 and P-39C, though there looks like enough space for another pair of .50 cal guns, but perhaps not ammunition boxes)




Edit:
on this issue (cut from earlier in my post)
This actually makes the quoted speed of the XP-39 with turbocharger rather more impressive with all that drag holding it back, admittedly with a lot less weight as well. (also makes me slightly more dubious over the 'excessive drag' created by attempted saddle-mounted turbo installations and other experimental fits tried ... unless those were all only on paper, so less attempted and more anticipated)

I see Shortround addressed this bit of misinformation as well a few years back:


Shortround6 said:


> Tomo, this has been gone over in other threads, but basically, The XP-39 did not meet the performance numbers promised.
> 
> There is no real evidence that it ever came close to the numbers given for in many books/web sites. And there is evidence that it couldn't have done what is claimed during the time period it is claimed it flew those numbers.
> 1. There was a potential problem with drive-shaft vibration that called for a redesigned heavier drive shaft to be fitted, This was not done until after the NACA wind tunnel tests and until fitted the engine was restricted to 2600rpm. No where near full power.
> ...



340 mph at 20,000 ft I would buy with that 'streamlining' on the XP-39.


Though in that same thread:
XP-39: pros cons


> And on the XP-39 the oil cooler and radiator were in the wing root area ahead of the wheel, but to have good airflow you also need an exhaust duct behind the the radiators/coolers.


Shortround got this bit wrong, given the oil cooler is clearly in a bulky external box on the XP-39 (an understandable mistake given the confusing information floating around, and that video being the only source showing the oil cooler at all)

The XP-39 used the wing center-section for the turbocharger, with the radiator in the port wing center section (outlet in trailing edge). The XP-39A (and all later models) appear to have moved the radiator to the center section (port side) and the oil cooler(s) alongside it (starboard side) with the intakes for each in the corresponding wing leading edge.

On a side note, scanning the remainder of that thread's discussion: to increase fuel capacity, the best option seems to be deleting the wing armament and adding fuel cells to the outer wing compartments (that would otherwise hold the guns and magazines) and possibly in the wing leading edge. (the portion ahead of the gun bay is already clear to accommodate the blast tubes, but inboard of that may not be, and much further outboard gets too tight for useful fuel compartments very fast, so a pair of small leading-edge reserve tanks seems mostly likely) Beyond that you'd have to resort to tip-tanks, which would've been neat (and likely helped roll rate and possibly even stall/spin -due to wing-fence/winglet-like behavior) but the tip-tank arrangement seems to be too unusual and counter-intuitive for the time to be an obvious solution. (or sheer coincidence that no one stumbled on that prior to the XP-80A)

Oh, and retaining the .30 cal guns in the nose should have improved CoG a bit too. (.50s would've been better on all accounts if they could fit them ... even if some cheek bulges were needed ... maybe they could add ballast to the spinner, but useful 'ballast' would obviously be preferred)

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## kool kitty89 (Apr 20, 2016)

Thinking on the 'wing stalls almost simultaneously' aspect, I realized wind tunnel tests of this sort simulate glide conditions, thus omitting the impact of prop-wash. The accelerated air over the wing center section would thus tend to make the tips stall much earlier than the inboard portion of the wing (especially at low speeds where prop wash has the most impact on overall airflow -transonic prop-tip shockwave situations aside).

Again, this might not have bearing on the stall/spin characteristics of the production P-39, but it's at least interesting to consider. (I still think partial-span leading edge slots are a good compromise for complexity/weight/drag in improving aileron/tip stall characteristics -like the Hudson, N9M, or Me 163 used- without the difficulties of slats -same goes for compromising on non-functional slats like the Whirlwind had)

Wiki actually notes that particular type of partial-span slot implementation:
Leading edge slot - Wikipedia, the free encyclopedia (though the list of examples is a bit off, I don't think the Beaufort -or any Blenheim derived airframes- used leading edge slots operationally).

The hudson seems to avoid complexity further by cutting slots between wing ribs rather than making a continuous slot.
http://www.cybermodeler.com/hobby/builds/ca/images/ca_0448_26.jpg
https://s-media-cache-ak0.pinimg.com/736x/2f/9d/be/2f9dbe761332155a40167e493e00b822.jpg


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## Shortround6 (Apr 20, 2016)

Quite a few aircraft used "letter box" slots but they caused more drag than the retracting slats when the slats were retracted. It was a balancing act to get the aileron response at low speed desired without causing too much drag. Slats/slots were almost de rigueur for a few years just before WW II. It depended a bit on who had the patents. The first 50 Halifaxes had slats. The XB-24 had slots. I am not sure how long the B-24 series kept them. The XB-35 flying wing had them. Grumman Avengers had them. Curtiss Helldivers had slats.


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## Shortround6 (Apr 21, 2016)

I have looked at a P-39 book by Birch Matthews which has several photos of the XP-39 showing the right side. the oil cooler is in a duct on the fuselage about the same place as that "box" in the video but is much smaller and more of a half tube (round instead of square). 
Given the XP-39s rather alarming tendency to overheat even in ground running and taxiing they were doing some rather mad scrambling to solve the cooling problem before the plane was shipped to Langley. This cooling duct could very well be a hasty improvisation. there is mention in the text of several different ducts being tried in order to solve the oil cooling problem.

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## GregP (May 2, 2016)

The Allison uses the engine oil for a significant amount of cooling. The Merlin, on the other hand, doesn't. As a result, most Allison-powered aircraft will have two oil coolers for an Allison and one radiator ... and a Merlin-powered aircraft will sometimes have two radiators, but only usually has one oil cooler.

The front intake of the P-40, take an N model for instance has two oil coolers up high in the intake and one radiator in the lower center. The P-38 is the same way today but, back when they actually used the turbochargers, the late-model intake had two oil coolers and the center was the intake for the intercooler. Today there are two P-38s that occasionally fly WITH the turbos, but the rest plug up the outboard turbo intakes and use the lower center opening as the carburetor air intake.

*Edit*: You are right, it's 2 coolant radiators and an oil cooler in the P-40N. Very similar shape, but coolant. And thanks for making me look closer! It also happens to share some parts with the P-38 and P-39 lie starters and cuno filters. Many Allisons shared the same carburetors. If I am not mistaken, many shared magnetos, too. It was probably GFE - Government Furnished Equipment. *End of Edit*.

Many Merlin planes, such as the Spitfire, split the coolant radiator into two small under-wing units.

Coolant is the Merlin's lifeblood. It doesn't run long without it. If you lose oil cooling, you can throttle back and get home if it isn't too far away. With the Allison, you need the radiator AND the oil coolers. if you loose oil cooling, you'll get a less damaged range than the Merlin and will have to shut it down or burn. The upside is if you loose the coolant radiator, it will fly a LOT longer than Merlin with the same fault at reduced power because much cooling in the Allison comes from the oil.

I don't believe either one is inherently superior to the other ... just different solutions to cooling a big V-12.

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## tomo pauk (May 3, 2016)

Greg - a good overwiev, thanks. 
BTW, Spitfire, at least service machines, never had two coolant radiators.


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## wuzak (May 5, 2016)

tomo pauk said:


> Greg - a good overwiev, thanks.
> BTW, Spitfire, at least service machines, never had two coolant radiators.



The two stage Merlins had two radiators:
[qoute]Because the intercooler required a radiator, the radiator under the starboard wing was halved in size and the intercooler radiator housed alongside. Under the port wing a new radiator fairing housed a square oil cooler alongside the other half-radiator unit. When the engine was running at low speed, one radiator section provided enough coolant; a thermostatic switch turned off the starboard radiator section until more power was called for and extra engine cooling was required.[/quote

Supermarine Spitfire (late Merlin-powered variants) - Wikipedia, the free encyclopedia


[qoute]The Mk. IX radiators had enlarged frontal area as compared to the earlier Spitfire marks. Both radiators were identical rather than being mirror images of each other, divided into two sections - the starboard being an oil cooler, port side being occupied by the intercooler.[/quote]

Supermarine Spitfire Mk. IX in Detail -The Powerplant

Certainly single stage Merlin Spitfires and the Mk XII had only one coolant radiator.

In any case, P-38s had two radiators per engine.

http://i.imgur.com/W9W2rs4.jpg


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## tomo pauk (May 5, 2016)

Indeed, both you and Greg are right re. Spitfire radiators. Aplologies.


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## kool kitty89 (May 5, 2016)

Shortround6 said:


> Quite a few aircraft used "letter box" slots but they caused more drag than the retracting slats when the slats were retracted. It was a balancing act to get the aileron response at low speed desired without causing too much drag. Slats/slots were almost de rigueur for a few years just before WW II. It depended a bit on who had the patents. The first 50 Halifaxes had slats. The XB-24 had slots. I am not sure how long the B-24 series kept them. The XB-35 flying wing had them. Grumman Avengers had them. Curtiss Helldivers had slats.


Retracting slats seemed to be tricky to engineer in a reliable, lightweight, compact, structurally sound manner, so (patents aside) had a lot more difficulties in implementing than fixed slots. Messerschmitt got it right pre-war and continued to do so during the war on multiple aircraft, but that was rare.

Fixed slots seem like a reasonable solution for aircraft where wing-tip stalling behavior is dangerously unacceptable either at low speed (take-off and landing) or general maneuvering (high-speed stalls and spins). I believe this was the primary reason the Me 163 employed them given the dangerous of stall characteristics on tailless aircraft. (I'm not positive, but I believe some of the YP-49's dangerous, structurally compromising high-speed stall behavior was related to similar issues; the YP-49 had letterbox style slots but they appear to have had flaps/fairings that closed during normal flight and were not aerodynamically operated like slats)

Given the high-speed nature of the Me 163, I imagine fixed slots were used as the added drag they presented was acceptable and worth the weight and simplicity gains, especially for a wooden wing. (plus only partial-span in spite of lack of prop wash, so likely more useful for improving stall characteristics than actually improving stall speeds)

Wing-tip slots do allow for other drag-cutting measures like using lower drag (lower lift) airfoil profiles, eliminating span-wise wash-out (twist) normally used for improving stall characteristics and possibly even eliminating dihedral (though this has some other stability trade-offs). In the P-39's case, and its use of NACA 0015 airfoil at the root (relatively low-lift, low drag symmetrical airfoil), using that 00xx profile for the entire span might have been a reasonable option for reducing drag in conjunction with adding the slots.

In the case of something like the F4U, it seems more like the critical need for carrier landing characteristics would have merited any added drag wing-tip letterbox slots would add. (particularly as it would still have been lower drag than the F6F, and available sooner)




GregP said:


> The Allison uses the engine oil for a significant amount of cooling. The Merlin, on the other hand, doesn't. As a result, most Allison-powered aircraft will have two oil coolers for an Allison and one radiator ... and a Merlin-powered aircraft will sometimes have two radiators, but only usually has one oil cooler.
> 
> The front intake of the P-40, take an N model for instance has two oil coolers up high in the intake and one radiator in the lower center. The P-38 is the same way today but, back when they actually used the turbochargers, the late-model intake had two oil coolers and the center was the intake for the intercooler. Today there are two P-38s that occasionally fly WITH the turbos, but the rest plug up the outboard turbo intakes and use the lower center opening as the carburetor air intake.


You've mentioned this before, but as I recall in the last time it came up, someone pointed to the outboard radiators on the P-40 being glycol coolers and the central one being the oil cooler. (I forget who provided the reference, but it confirmed the labels on various cut-away examples of the P-40)

There might be some misinformation going on elsewhere, though, and the merlin powered P-40s may have swapped the radiator and oil cooler positions, among other things, leading to some confusion. That or the P-40N uses a different arrangement than earlier models.

http://img43.imageshack.us/img43/2537/curtissp40ekittyhawk1we.jpg


That aside, the large oil cooling capacity does at least mesh with the similarly sized oil cooler (port) and prestone/glycol radiator (starboard) intakes in the wing roots


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## tomo pauk (May 5, 2016)

Serial-produced P-40, whether with V1710 or with V-1650, were with 2 coolant radiators:
Allison V-1710 Installation in P-40


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## kool kitty89 (May 5, 2016)

Do note the size of the oil tank vs coolant tank depicted there, though. The large oil capacity might be significant.


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## Shortround6 (May 5, 2016)

Oil was a consumable. Coolant was not. Engines were rated not only by fuel used per HP/hr but by oil used per HP/hr. Long flights required more oil than short ones, see larger oil tanks fitted to Spitfires for ferry flights for example.
Coolant was supposed to stay in the cooling system and not be used up at the rate of a gallon an hour or more.


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## wuzak (May 5, 2016)

tomo pauk said:


> Serial-produced P-40, whether with V1710 or with V-1650, were with 2 coolant radiators:
> Allison V-1710 Installation in P-40



Thanks for that picture Tomo.

I have sometimes wondered if, given their size, the radiators could have been mounted on the leading edge of the wing - perhaps near the root or at the front of the landing gear fairing. Then they would only need a small duct for the oil radiator and the nose could have been sleeker.


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## wuzak (May 5, 2016)

The P-40F appears to have had only a single radiator.







Looks a little bigger than the paired Allison radiators

http://fsfiles.org/flightsimshotsv2/images/2015/04/18/aFlyingHeritageCollection113.jpg

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## tomo pauk (May 6, 2016)

Indeed, the P-40 have had only one radiator, rather big (from Service manual for the P-40F):











wuzak said:


> ...
> I have sometimes wondered if, given their size, the radiators could have been mounted on the leading edge of the wing - perhaps near the root or at the front of the landing gear fairing. Then they would only need a small duct for the oil radiator and the nose could have been sleeker.



I like the proposal. Or, similar like it was done on the P-40Q, though it seems like the oil coolers went at the wings, between the wheel struts and guns. Now where is my pic of such modified 'plain' P-40, all together with clipped wings...

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## kool kitty89 (May 8, 2016)

tomo pauk said:


> I like the proposal. Or, similar like it was done on the P-40Q, though it seems like the oil coolers went at the wings, between the wheel struts and guns. Now where is my pic of such modified 'plain' P-40, all together with clipped wings...


Take a look at the XP-40K

http://img.wp.scn.ru/camms/ar/375/pics/3_103_b1.jpg

Military Aviation Archives - P-40 Warhawk Historical Development Photo Set
http://www.milavnarc.com/photos/wor...cal_development_photo_set/large/22 XP-40K.jpg

Radiators and oil coolers installed in a flat conformal arrangement between the landing gear strut bulges and look like they might be placed to avoid reducing the center section fuel tank capacity.

I haven't seen any information on performance, but it looks rather nice at least. (extending the intake ducts to a unified scoop below the nose might have made for better ram effect and reduced drag relative to mass flow through the cooling matrix -and better cooling on the ground and at low speeds due to prop ram effect)

Come to think of it, the P-39 could probably have employed fuel tankage in the center section if (somewhat draggier) semi-embedded lower-fuselage radiators had been used instead, perhaps somewhat spitfire or 109F/G-like but set below the wing roots, inboard of the gear and outboard of the belly rack (rather than in the wings). That would also free up the leading edge wing root space for fuel tanks (a rather prime location given the thick ring roots and existing open structure to accommodate the oil and radiator intake ducts). Large, center-section tanks should also hold more fuel relative to their weight than slim, flat wing fuel cells (much better volume to surface area ratio -so much less of that thick, heavy composite rubber self-sealing tank material for a given fuel capacity).

Delete the wing guns and outer most fuel cells (and maybe clip the wing tips) and you'll increase roll-rate quite a bit too. (and again, 2 .30. 2 .50, and 1 20mm cannon in the nose seems like a reasonable armament, provided a P-38 style cocking mechanism is included to compensate for reliability issues of the American Hispano's firing pin ... or just use the Navy's solution of slathering all the 20 mm rounds with so much wax that they seat into the chamber properly) Fitting a .50 cal within each wing would be nice, and maybe possible if fuel cells were deleted from further inboard of the wing than the .30 cal location, but that's just a guess. (it was really strange the P-63 had external guns AND stupidly limited fuel capacity, with the wings mostly empty in spite of being fairly large and thick -they SHOULD have been able to carry more fuel than the Mustang AND at least 1 .50 cal inside the outer wing portions ... as it was they made poorer long-range fighters than the P-39Q)

Actually, making the radiators semi-external could also allow shifting them forward enough to partially address the CoG issues in the P-39. (using fuel tanks to do this would be a bad idea as they shift weight when empty vs full ... though I suppose having reserve leading edge tanks shifting CoG forward would be OK given they'd always be full in combat)

The P-39 really seems like it should have been able to accept a 2-stage V-1710 without lengthening the rear fuselage as well. It had the physical space there already, but it was being occupied by tons of equipment including some relatively heavy things (like the entire radio array rather near the tail -probably not helping CoG at all). I wonder if the radio equipment could have been relocated behind the cockpit bulkhead ahead of the engine. (delete the rear canopy glazing for a more turtle/razorback configuration and rely on an external rather than internal rear view mirror -the super thick bulckhead framing behind the pilot made for a huge blindspot anyway, and an external rear view mirror wouldn't really be worse -particularly if the modeling in Il-2 is anything to go by ... that rear bulkhead really kills the visibility -switching from the cab style cockpit to a sliding malcolm hood would IMPROVE visibility substantially even if the entire rear glazing was replaced with solid metal)


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## wuzak (May 8, 2016)

No, not enough space for a two stage Allison in the P-39.


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## Shortround6 (May 8, 2016)

I am always amazed at the idea that the P-39 could fit a two stage supercharger when the Bell engineers lengthened the fuselage by about two feet to make the XP-39E and the P-63. The XP-39E was actually ordered as the P-76 to the tune of 400 aircraft but contract was later cancelled or transfered to the P-63.
If the Bell engineers figured they needed bigger fuselage ( and relocated wing) to make it work I don't know why people think it would have been so easy to stuff the needed components in the existing P-39.


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## Zipper730 (Jan 16, 2017)

kool kitty89 said:


> I'm surprised I've never stumbled on this video before, it's amazingly informative. The few photos of the XP-39 floating around and limited information (often misinformation) regarding its layout and turbocharger (and radiator) installation don't do it justice at all. (in fact, none seem to show the starboard side of the aircraft at all)


Yeah, all I've seen was the left


> It might just be me, but the entire cooling system installation (oil cooler, intercooler, and to at least some extent coolant radiator) look shockingly sloppy and patched together ... almost taped-on.


Looks remarkably boxy, though. It's interesting that their earlier configuration was rounder...

Photo by David Lednicer

BTW: The radiator for the engine was on the left side, the inter-cooler was the boxy structure, and the intercooler was the smaller installation below right?



> No boundary layer splitter/sleeves/cowls either, but that's not really surprising compared to the rest.


The P-38 and P-40 didn't have any form of boundary-layer splitter from what it appears. They did incorporate doors/gills to regulate the airflow (The P-39 did not).



> NACA-recommended 13 inch rear-fuselage extension probably didn't help


What was the XP-39's length compared to later models?



> Also interesting that the XP-39 has a nicer clear-view canopy, both taller and lacking the thick 'anti-roll bulkhead' behind the pilot.


The lowering of the canopy reduced drag if I recall...



> Though in that same thread:
> XP-39: pros cons


I'll look through this thread next...




wuzak said:


> No, not enough space for a two stage Allison in the P-39.


If NACA wanted to add 15 inches to the design, and the P-63 was two feet longer, why didn't they just add the two feet? That would have fixed most of the problems (the P-63 did okay even without a turbo)


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## wuzak (Jan 16, 2017)

Zipper730 said:


> The P-38 and P-40 didn't have any form of boundary-layer splitter from what it appears. They did incorporate doors/gills to regulate the airflow (The P-39 did not).



The P-40 did not need a boundary layer splitter - there was little or no boundary layer at that point, as the radiator intake was right behind the spinner.





http://www.aviation-history.com/curtiss/p40-23.jpg

The P-38 did, albeit a duct rather than a splitter.





http://usautoindustryworldwartwo.com/images/Harrison Radiator/p-38-107w-3.jpg





http://usautoindustryworldwartwo.com/images/Harrison Radiator/p-38-107w-2.jpg

And the P-39 had flaps to control the mass flow through the radiator





http://hyperscale.com/features/2002/images/p39kws_2.jpg

(I know it is a model, but you get the picture)


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## Zipper730 (Jan 16, 2017)

wuzak said:


> The P-40 did not need a boundary layer splitter - there was little or no boundary layer at that point, as the radiator intake was right behind the spinner.


That's why the belly-radiator didn't work on all aircraft?


> The P-38 did, albeit a duct rather than a splitter.


I never saw the radiator up close like this before, but I get it.


> And the P-39 had flaps to control the mass flow through the radiator


I'm curious if it would have been possible to extend the radiator a bit forward like the XP-40Q?[/QUOTE]


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## wuzak (Jan 17, 2017)

Zipper730 said:


> That's why the belly radiator didn't work on all aircraft?



Possibly.




Zipper730 said:


> I'm curious if it would have been possible to extend the radiator a bit forward like the XP-40Q?



Like on the P-63?

Or would you rather some underwing radiators like on the Aeobonita?





http://www.fiddlersgreen.net/aircraft/Bell-XFL-Airabonita/IMAGES/Bell-XFL1-Airabonita-Inflight.jpg


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## wuzak (Jan 17, 2017)

I think one or two of the XP-40Q prototypes had leading edge radiators.

The P-39 had its radiator buried in the fuselage, fed by leading edge intakes (P-63 too, I think).


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## Zipper730 (Jan 17, 2017)

> Like on the P-63?


The P-63 or F4U arrangement could work. Something like the De Havilland Mosquito or Hornet could work as well.



> Or would you rather some underwing radiators like on the Aeobonita?


Those didn't seem to work out too good, though I do remember seeing three movable doors on the underside of your model, and a small radiator could cover that.


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## Shortround6 (Jan 17, 2017)

I am not sure what you are trying to accomplish?

The cooling problems were fixed on the XB-39B and YP-39As. Air went in the wing root intakes, was routed to the center section between the wheel wells and out through the doors/flaps underneath. This worked just fine for all the production P-39s until 1944. There were two radiators and one oil cooler. 
P-63 needed bigger ducts, more cooling for several reasons. One is that while the P-39 set up not only worked for the initial 1090hp engine it was able to work for the later 1200-1325hp engines, of course they only made big power down low where the air is dense so mass airflow through the system wasn't too big a problem. Also the single stage engines only used up about 100hp in friction and about 200hp to drive the supercharger so total cooling load was about 1500-1600hp in the cylinders. 
With the P-63 flight at higher altitudes was planned with less dense air so you need more cubic feet even for the same power. The mechanical drive 2nd stage (or Aux stage) required up to 300 hp to drive it (going by memory could be wrong) if operating at it's highest gear ratio (variable drive) so the cooling system has to be big enough for the 1800-1900hp cooling load to get 1325hp to the prop at 25,000ft. 
An inter-cooler was planned for the P-63 (at least for some engines) but the sub contractor couldn't deliver (or more than one sub contractor?). At least that is what one book claims. Where it was supposed to go I don't know.
Please note that for inter-coolers to be effective they need 2-3 (or more) times the amount of cooling air as intake for the engine. In other words, even a a well designed system is going to need a scoop 2-3 times the size of the scoop behind the canopy of the of the P-39 or P-63. 
Turbos had their own cooling problems which is why every production installation except the P-47 had them hanging part way out of the airplane. You have to keep the turbine blades from getting so hot that they fail. And a lot of the service installations were not far from that point. There was a reason that they were placed a number of feet from the engine (exhaust could cool a little) and still had pieces of steel plate to catch/deflect thrown turbine blades from hitting aircrew.

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## GregP (Jan 17, 2017)

This may or may not be a good place to bring this up, but the current Mercedes Formaul 1 engine has a rather revolutionary turbocharger. The exhaust turbine is on one end of the engine and the shaft runs all the way through the engine block to the other side, so the compressor wheel is nowhere NEAR as hot as for a conventional turbocharger, being far removed from exhaust heat of any kind.

Nobody thought of that back in WWII, but I wonder if it could have significantly affected turbo developement all those years ago? Interesting, but sort of an over-the-top "what-if" that really requires no further development. I mentiom it only for completeness as, had someone thought of it, it is possible turbos would have been much more developed by the time the big pistons were going extinct, and higher altitude pistons might have been the norm.

End of side track ... back to reality, please continue.


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## Shortround6 (Jan 17, 2017)

I am not sure the problem was heat transfer from the turbine section to the compressor section. 

P-47 turbo:




There seems to be a heat shield / cooling duct between the Turbine casing and the compressor casing. Granted some heat could still get through and some be conducted along the shaft. 

Only gone through a couple of websites in the Mercedes F1 so I may be reading something wrong but it appears the Turbine drives a generator that feeds a battery pack. The supercharger is electric powered? Battery pack also feeds power to electric motors in rear wheels upon demand (under braking wheel motors act as generators to feed electricity back into the battery) 

Haven't found the boost level yet but would bet dollars to donuts that the F1 engine is running higher pressure than the P-47 turbo alone.


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## wuzak (Jan 17, 2017)

Shortround6 said:


> I am not sure the problem was heat transfer from the turbine section to the compressor section.
> 
> P-47 turbo:
> 
> ...



The main advantage of the Mercedes turbo is for packaging in the car.

The current F1 V6 Hybrids consist of a 1.6l V6, turbocharger with attached motor/generator unit (MGUH, H for heat), a battery pack and a motor/generator unit connected to the crankshaft of the engine (MGUK, K for kinetic).

The MGUH can be used to drive the turbo, when rpm is low or when in qualifying mode allowing the wastegates to open, reducing back pressure and increasing power.

And it can also be used to generate power, recovered from the exhaust energy. This can be sent to the battery pack, or directly to the MGUK connected to the engine. The regulations specify that only 4MJ of energy can be sent from the battery pack to the MGUK, but i=unlimited energy can be transferred between the battery and the MGUH and between the MGUK and MGUH. The effect is that for most of the time the engine works as a turbo-compound, feeding recovered power back to the output shaft.

The compressors run a pressure ratio in the region of 3 - 4:1. Exactly how much is secret.

Mercedes' layout allows the MGUH to be placed between the turbine and compressor and in the engine's vee.
Ferrari's original turbo also placed the MGUH between the turbine and compressor, but the whole assembly was behind the engine. This has since been changed to a system that Renault has run, with turbo and compressor together, with the MGUH in front of the compressor in the vee.
Honda have the MGUH between turbine and compressor, but in their case the compressor was in the engine's vee. For 2017 they are changing to the Mercedes layout. Renault and Ferrari may do also - restrictions which prevented them from changing their layouts before have been lifted.

The net result of this is an engine which produces nearly 800hp from the ICE alone, around 950hp with all the energy recovery deployed. All with a restricted fuel flow rate of 100kg/h. With thermal efficiency is approaching 50%.

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## GregP (Jan 17, 2017)

Hi Shortround,

Nice pic. GE C-23, I think. Did you notice it is labeled incorrectly in today's terminology? The unit WAS called both a supercharger a turbo-supercharger in WWII, but is a turbocharger today. I suppose period terminology is only fitting, though.

About the pic Shortround ... all the parts were metal, right? If metal was any good as an insulator, nobody would use metal pots and pans to cook with.

In the case of the Mercedes F1 engines of today, the compressor is far removed from exhaust heat and the cooling is much superior to regular turbochargers. They rather obviously haven't made public how much less intercooling is required, but that engine is making an easy 50 - 80 Hp more than others. Most of the non-Mercedes F1 engines are making some 820 Hp while the Mercedes, with the same displacement, is making 870+ HP, and quite reliably. That's 6 to 7.9% more power with no other obvious technology changes. In a 2,000 HP engine, that might give 120 - 158 more HP for free. While not exactly earth-shaking, more it is better.

Again, though, I am NOT intending to open a Formula 1 discussion, just wondering how much it would help a 1940's-technology turbocharger to have separated the turbine wheel and the compresor wheel by a distance of some 2 meters. I don't really expect anyone can answer that and do NOT want to take us off topic. If we DO discuss it, maybe in a separte thread?

Seems like a potentially-intertesting subject, but also one for which we would all have zero data and no possibility of data. That just makes for a long technical thread that is of limited value without a conclusion based in fact.

After I posted I looked above, and I see we're wandering off-topic badly and will desist starting here, unless in a separate thread. Good post, Wayne. Thing is, everybody has the same rules and everybody is running the MGUH and all the rest, not just Mercedes. Their ONLY advantage is the better turbocharger. Everyone else has all the rest of very equal technology. That was my point. The Mercedes advantage almost has to be mostly turbocharger only, or else you are implying everyone else isn't as good as Mercedes with technology. And I think all the teams have very good technical expertise, not just Mercedes.

So, maybe a separate thread? In any case, good post, as usual.

Back to XP-39!

So far, Shortround's assessment taht the P-39 was troo small to accept everything required for a turbochargerd piston fighter early in the U.S. P-39 development seems to be the best summary of the type. Perhaps we might consider all development, including maybe a 2-stage Merlin, in a P-63?

At least that is more or less on topic. The P-63 had potential and was a very good fighter, but it also wasn't a long-range fighter. I am not sure if the airframe could handle both a Merlin setup AND more fuel for increased range, and still remain a good fighter, but we CAN make a supposition that it didn't have to fight wih a large full-fuel load.

The P-51 was abysmal when the fuselage tank behind the pilot was full and it also had drop tanks, but it was employed that way nonetheless on thousands of occasions because lighter P-51s were covering for the heavier ones until the heavier ones got lighter and could fight. That is, the burdened fighters were at the rear, burning fuel and the lighter ones fought up front and then left for home when the rear echelon guys got lighter and could handle things.

Perhas a similar thing could be worked out for a 2-stage Merlin-powered P-63. Again, though, it would have to gain something over the P-51 before it would make sense to acquire it for USAAF use. Since we obviously have zero data on that, it is another argument possibly without a good answer, other than our opinions.

I lean toward the 2-stage Merlin making the P-63 a better plane than it was, and I already think it was pretty good relative to a P-51. Making it even better might make it a good candidate for acquisition, but I'd have to see data ... and there isn't any. So, my opinion is that it would probably gain at least parity with the P-51 with just the Merlin swap, and I dont know if extra fuel could be accommodated or not. I'm SURE you ccould get the fuel in there, but the question in my mind is whether or not you could get it in and still remain with a good CG limit. The P-39 had issues with that. Changing it might introduce the same in the P-63, and the only way to know would be to have someone familiar with the P-63 design analysis calculate estimated performance, including extra fual and CG envelope.

Personally, I lack a wind tunnel, the requisite equipment, an accurate scale model, the funds, and the desire to do it. That pretty much rules me out, even from the interest side, since there is no possible use for the data 70 years after the fact.

I think we are stuck with the P-39 not being able to be developed into a useful fighter at it's real-world size, and maybe the P-63 could have been. In the event, we all know what happened. They built a lot of P-63s, but we didn't use them.


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## Shortround6 (Jan 17, 2017)

The insulation doesn't come from the metal it comes from the airspaces on either side of the heat shield. 

Much like the heat-shields on catalytic converters keep your floor mats from melting 




doesn't mean they don't get warm.

Old text book says an auxiliary supercharger that is raising the ambient air pressure to sea level pressure and is 65% efficient will raise the air temperature by 210-220 degrees F at 25,000ft. Granted the ambient air at 25,000ft is pretty cold but the compressor section is going to be pretty warm all on it's own. 

Heat gets transferred 3 ways. Conduction (heat passed through common parts like the shaft of the turbo assembly). Convection ( hot air/gases leaves the hot surface/combustion and travels to the new surface/object. And radiant heat, Those infrared waves that travel in straight lines (and can go through glass). Metal heat shield blocks the radiant heat (at least until the heat shield gets really hot.) and with air flowing on both sides of the heat shield the convection heat path is removed. With air flowing over the heat shield the heat shields ability to operate as a radiator is diminished. Conducted heat has to get into the shaft, flow down the shaft and then heat up the compressor disk. However the heat has had to flow past two oil lubricated bearings which may take some of the heat away. 

Cut away Turbo





Heat shield and airflow provided by turbine blades?


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## wuzak (Jan 17, 2017)

GregP said:


> In the case of the Mercedes F1 engines of today, the compressor is far removed from exhaust heat and the cooling is much superior to regular turbochargers. They rather obviously haven't made public how much less intercooling is required, but that engine is making an easy 50 - 80 Hp more than others. Most of the non-Mercedes F1 engines are making some 820 Hp while the Mercedes, with the same displacement, is making 870+ HP, and quite reliably. That's 6 to 7.9% more power with no other obvious technology changes. In a 2,000 HP engine, that might give 120 - 158 more HP for free. While not exactly earth-shaking, more it is better.



The distance between the compressor and the turbine on the Mercedes F1 engine is about a foot.

Ferrari are said to be line ball with Mercedes in terms of power, except in qualifying mode.

Renault and Honda are not far behind.

Over-cooling of the intake air destroys the efficiency of these engines, and hence reduces power. Both Mercedes and Ferrari use an air to air interooler and a water to air intercooler. Presumably this is to give finer control of cooling.


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## GregP (Jan 17, 2017)

I'm not going to continue to hijack this thread, Wayne.

If you want to talk about, fine, Please, in a new thread.

Cheers.


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## Zipper730 (Jan 18, 2017)

shortround said:


> I am not sure what you are trying to accomplish?


Somewhere along the way, it was mentioned that the intakes had issues flowing around the tight kinks produced by the landing-gear door positions. So I figured...


> The cooling problems were fixed on the XB-39B and YP-39As.


Of course, there was no secondary stage of supercharging and no cooler needed as a result.


> There were two radiators and one oil cooler.


Okay, so the center was the oil-cooler, and the left and right were the radiators?


> P-63 needed bigger ducts, more cooling for several reasons. One is that while the P-39 set up not only worked for the initial 1090hp engine it was able to work for the later 1200-1325hp engines, of course they only made big power down low where the air is dense so mass airflow through the system wasn't too big a problem. Also the single stage engines only used up about 100hp in friction and about 200hp to drive the supercharger so total cooling load was about 1500-1600hp in the cylinders.


I never realized friction would have been factored seperately, I figure anything needed to drive the supercharger would be weighted against the extra horsepower produced...


> Please note that for inter-coolers to be effective they need 2-3 (or more) times the amount of cooling air as intake for the engine. In other words, even a a well designed system is going to need a scoop 2-3 times the size of the scoop behind the canopy of the of the P-39 or P-63.


What reduces the cooling-air requirements and space requirements?


> Turbos had their own cooling problems which is why every production installation except the P-47 had them hanging part way out of the airplane.


How did they get around it?


> You have to keep the turbine blades from getting so hot that they fail.


I understand that, admittedly it's mostly from a knowledge of jet-engines, but...


> There was a reason that they were placed a number of feet from the engine (exhaust could cool a little) and still had pieces of steel plate to catch/deflect thrown turbine blades from hitting aircrew.


I thought that was simply so the exhaust would be reasonably smooth through the turbine, but I had no idea of the use of armor plates to catch the turbine so the pilot wouldn't get skewered.


BTW: I'm not capable of quoting and pasting at this time for some reason, so I'm doing it the old fashioned way...


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## tomo pauk (Jan 19, 2017)

The P-39 have had two oil coolers/radiators, and one coolant radiator. P-63 was with two oil coolers/radiators and two coolant radiators.

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## Shortround6 (Jan 19, 2017)

Zipper730 said:


> Somewhere along the way, it was mentioned that the intakes had issues flowing around the tight kinks produced by the landing-gear door positions. So I figured...
> Of course, there was no secondary stage of supercharging and no cooler needed as a result.



Well you can't have everything 

You want the turbo and intercooler you accept the weight and drag and the poorer performance at 15,000 and below. Turbo only shows real advantage at 20,000ft and up with the 15-20,000ft area being a toss-up/crossover. There is no free lunch.



> Okay, so the center was the oil-cooler, and the left and right were the radiators?


I was in error, Tomo is correct. One square or rectangular radiator in the center and a round oil cooler to either side in their own ducts.



> I never realized friction would have been factored seperately, I figure anything needed to drive the supercharger would be weighted against the extra horsepower produced...



When figuring out the size of the radiators/oil coolers you need to find out what the power being produced in cylinders was, not the power going to the propshaft. Friction changed from about 100hp in the early C series engines (long nose) to around 200hp in the late engines that made 1600hp and up in WER mode. Different piston rings, different valve springs and a few other details. Power to drive pumps (oil, water and fuel) was usually included in friction) They actually hooked the engine up to an electric motor and subtracted-added parts while measuring how much power it took to turn the engine over at the desired RPM. Like running it without connecting rods/pistons or cylinder heads to figure friction in the main bearings.



> What reduces the cooling-air requirements and space requirements?


Nothing unless you can repeal the laws of physics. You can sometimes do a better job of packaging but please remember that every bend in the ducts and every change in the cross section of the ducts can bring their own losses so you can't just jam everything in tighter. B-17s actually had slightly different full throttle heights on inner and outer engines due to differences in the duct work. Same engines, same turbo, same intercooler.



> How did they get around it?



P-47 used a big enough air intake to be able to split off some air to cool the turbo.


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## Zipper730 (Jan 19, 2017)

tomo pauk said:


> The P-39 have had two oil coolers/radiators, and one coolant radiator. P-63 was with two oil coolers/radiators and two coolant radiators.


Which was due to thee more powerful engine?



Shortround6 said:


> Well you can't have everything


What about surface evaporative cooling for the engine and a normal radiator for the intercooler ?


> You want the turbo and intercooler you accept the weight and drag and the poorer performance at 15,000 and below.


How much drag would you say would present?


> Turbo only shows real advantage at 20,000ft and up with the 15-20,000ft area being a toss-up/crossover.


15,000 to 20,000 feet being about the same seems acceptable, and much of the performance advantages the USAAF had demonstrated over the Luftwaffe were over 22,000 feet...


> When figuring out the size of the radiators/oil coolers you need to find out what the power being produced in cylinders was, not the power going to the propshaft. Friction changed from about 100hp in the early C series engines (long nose) to around 200hp in the late engines that made 1600hp and up in WER mode.


How does the radiator size scale in proportion to these figures (i.e. hp to square or cubic inch).


> B-17s actually had slightly different full throttle heights on inner and outer engines due to differences in the duct work. Same engines, same turbo, same intercooler.


I never knew that...


> P-47 used a big enough air intake to be able to split off some air to cool the turbo.


I was thinking about the inter-cooler, and I was wondering: Did the US ever use liquid cooling for this purpose other than the Merlin 60's?


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## wuzak (Jan 20, 2017)

Zipper730 said:


> I was thinking about the inter-cooler, and I was wondering: Did the US ever use liquid cooling for this purpose other than the Merlin 60's?



Yes, some versions of the V-1710 two stage engine did.

But definitely not for the air cooled engines.


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## Shortround6 (Jan 20, 2017)

Zipper730 said:


> What about surface evaporative cooling for the engine and a normal radiator for the intercooler ?


Surface cooling only worked for racing planes. It generally required a lot of maintenance to deal with leaks let alone enemy fire.



> How much drag would you say would present?


 the two mock ups Bell tried with add on turbos were 30-40mph slower than standard P-39s at low altitude. Even cutting that to 20mph with a lot of attention to detail leaves the P-39 with little speed advantage at low altitude over the Zero.


> How does the radiator size scale in proportion to these figures (i.e. hp to square or cubic inch).


It doesn't because not all radiators were the same depth front to back, and not all designers used the same pressure drop through the radiator.


> I was thinking about the inter-cooler, and I was wondering: Did the US ever use liquid cooling for this purpose other than the Merlin 60's?


Not on service aircraft, not sure about prototypes. Liquid cooling helps with packaging, it is a lot easier to run a few fluid filled lines than large air ducts. It doesn't help so much with actual heat dissipation. Instead of cooling air flowing through inter-cooler with hot air in the alternate passages you replace it with water/antifreeze. Yes the intecooler may be able to be made smaller but now you need the intercooler radiator and you need XXX number of pounds per minute of cooling air to cool the inter-cooler fluid.


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## GregP (Jan 20, 2017)

Shortround6 said:


> Surface cooling only worked for racing planes. It generally required a lot of maintenance to deal with leaks let alone enemy fire.
> 
> the two mock ups Bell tried with add on turbos were 30-40mph slower than standard P-39s at low altitude. Even cutting that to 20mph with a lot of attention to detail leaves the P-39 with little speed advantage at low altitude over the Zero.
> 
> ...


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## tomo pauk (Jan 20, 2017)

P-39 with turbo & intercooler attached:

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## Zipper730 (Jan 20, 2017)

wuzak said:


> Yes, some versions of the V-1710 two stage engine did.


What aircraft used these?


> But definitely not for the air cooled engines.


That makes sense


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## wuzak (Jan 20, 2017)

XP-51J?
F-82 maybe.

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## Shortround6 (Jan 20, 2017)

I would note that trying to use the cooling system of the engine is pretty much useless to the intercooler. The US was trying to reduce the temperature of the air entering the Carburetor on turbo-ed engines to a max of 100 degrees F. They didn't always meet this goal but trying to use engine coolant wasn't going to work as engine coolant was usually near boiling even when cruising. A liquid intercooler needs it's own pump, radiator and lines. 
On the P-51 normal carb inlet temperature was 15C to 40C normal with 50C max. Coolant temp was 60 C minimum for take-off, 100-110C normal and 121 C max or 125C max for -9 engines.


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## Zipper730 (Jan 20, 2017)

Shortround6 said:


> Surface cooling only worked for racing planes.


It was sort of a joke, though (from a purely intellectual standpoint) I was curious how much drag it would reduce.

Poor damage resistance and high maintenance would definitely qualify: Still, from an intellectual standpoint, how much drag does it reduce?


> the two mock ups Bell tried with add on turbos were 30-40mph slower than standard P-39s at low altitude.


The P-39 could do how much at sea-level?


> Even cutting that to 20mph with a lot of attention to detail leaves the P-39 with little speed advantage at low altitude over the Zero.


Yeah, it's almost the same


> Liquid cooling helps with packaging, it is a lot easier to run a few fluid filled lines than large air ducts.


The idea was based on smaller size: A pound and a half of bricks versus a pound of cotton.


> Yes the intecooler may be able to be made smaller but now you need the intercooler radiator and you need XXX number of pounds per minute of cooling air to cool the inter-cooler fluid.


Worked decently on the P-51 and Spitfire…


> I would note that trying to use the cooling system of the engine is pretty much useless to the intercooler. The US was trying to reduce the temperature of the air entering the Carburetor on turbo-ed engines to a max of 100 degrees F. They didn't always meet this goal but trying to use engine coolant wasn't going to work as engine coolant was usually near boiling even when cruising. A liquid intercooler needs it's own pump, radiator and lines.


The Merlin 60's did this right?


> On the P-51 normal carb inlet temperature was 15C to 40C normal with 50C max. Coolant temp was 60 C minimum for take-off, 100-110C normal and 121 C max or 125C max for -9 engines.


Quite a difference in temperature...


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## wuzak (Jan 20, 2017)

Zipper730 said:


> I assume this principle is based on reducing pressure and increasing speed correct?



The idea was to reduce pressure _drop_ and this was achieved by using a divergent/convergent duct.

The duct leading to the radiator would be divergent, that is its cross sectional area would increase. Thus the air flow velocity would be reduced and the pressure drop across the cooler reduced. Then the outlet duct is convergent, that is the area reduces. That way the heated air from the radiator is accelerated and, hopefully, will provide some thrust. 

This also has the benefit of keeping the air in contact with the radiator for longer, thus increasing heat transfer.

The radiator was a compromise between heat transfer and minimal pressure drop. You could reduce the latter by having a thinner radiator with wider spaced cooling passages/fins, but then you don't get the required heat transfer.

Duct design was important. if the divergence is too sever you will get flow separation and turbulence, reducing the effectiveness of the installation.

The RAE tested the Spiteful radiator and found issues with the boundary layer (the installation had no boundary layer bleed off) and separation, and that the top 20% of the radiator was not cooling at all.

They tried several different methods to improve the system. One was to lengthen the intake section, which had one of the best reductions in internal drag - but that was completely offset by an external drag increase.

Some systems were tried with boundary layer bleeds. One went across the top of the duct from front to back. Another released the boundary layer air through teh upper surface of the wing - drag was reduced, but so was the lift of the wing.

The best solution they found was guide vanes before and after the radiator, at about 20-25% of radiator height (from the top).




Zipper730 said:


> The Merlin 60's did this right?



No. The 2 stage Merlins and Griffons used a water/glycol mix for the intercooler fluid, but they did not use the engine coolant. The engine cooling system was separate from the intercooler system. 

If the intercooler system was hit the engine could still continue, but boost had to be lowered.

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## Zipper730 (Jan 21, 2017)

wuzak said:


> The idea was to reduce pressure _drop_ and this was achieved by using a divergent/convergent duct.
> 
> The duct leading to the radiator would be divergent, that is its cross sectional area would increase. Thus the air flow velocity would be reduced and the pressure drop across the cooler reduced.


Wait, I thought a divergent shape would cause a gain in pressure similar to a bell-mouth, not reducing a drop...


> Then the outlet duct is convergent, that is the area reduces. That way the heated air from the radiator is accelerated and, hopefully, will provide some thrust.


Like a ramjet


> This also has the benefit of keeping the air in contact with the radiator for longer, thus increasing heat transfer.


So slowing it allows more interaction with the radiator and that produces better transfer of heat, because the slower airflow also has higher pressure it's compressed more so when it's heated, it expands out with more force. If it's slowed too much, it won't be able to provide sufficient coolant (air) through the duct rapidly enough to carry away the heat, and if the compression makes excessive heat, it won't be cool enough to absorb the radiator heat?


> The radiator was a compromise between heat transfer and minimal pressure drop. You could reduce the latter by having a thinner radiator with wider spaced cooling passages/fins, but then you don't get the required heat transfer.


I figure you'd want nice thin sheets spaced closely together for maximum heat-transferring surface area...


> Duct design was important. if the divergence is too sever you will get flow separation and turbulence, reducing the effectiveness of the installation.


Okay


> The RAE tested the Spiteful radiator and found issues with the boundary layer (the installation had no boundary layer bleed off) and separation, and that the top 20% of the radiator was not cooling at all.


When you say no boundary layer bleed off, do you mean like a splitter or diverter?


> They tried several different methods to improve the system. One was to lengthen the intake section, which had one of the best reductions in internal drag - but that was completely offset by an external drag increase.
> 
> Some systems were tried with boundary layer bleeds. One went across the top of the duct from front to back. Another released the boundary layer air through teh upper surface of the wing - drag was reduced, but so was the lift of the wing.
> 
> The best solution they found was guide vanes before and after the radiator, at about 20-25% of radiator height (from the top).


Was this known to be an effective solution before the 1944?


> No. The 2 stage Merlins and Griffons used a water/glycol mix for the intercooler fluid, but they did not use the engine coolant. The engine cooling system was separate from the intercooler system.
> 
> If the intercooler system was hit the engine could still continue, but boost had to be lowered.


Redundancy?


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## tomo pauk (Jan 21, 2017)

wuzak said:


> XP-51J?
> F-82 maybe.



V-1710 as installed in the P-82s were not intercooled, that was one of reasons they were so unreliable when trying to make promissed 2200 HP. Also a major reason for not being capable to beat Merlin 61 from 1942 with those V-1710s even in 1945 in altitude power.
Only the XP-51J was to feature intercooled 2-stage V-1710s.


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## P-39 Expert (Apr 9, 2018)

Shortround6 said:


> I am always amazed at the idea that the P-39 could fit a two stage supercharger when the Bell engineers lengthened the fuselage by about two feet to make the XP-39E and the P-63. The XP-39E was actually ordered as the P-76 to the tune of 400 aircraft but contract was later cancelled or transfered to the P-63.
> If the Bell engineers figured they needed bigger fuselage ( and relocated wing) to make it work I don't know why people think it would have been so easy to stuff the needed components in the existing P-39.


Not trying to argue, but the space for the engine/second stage mechanical supercharger in the P-63 is exactly the same size as the P-39. The P-63 moved the coolant tank from behind the engine to up behind the pilot's seat above the engine and put the second stage blower where the coolant tank had been. Same thing for the XP-39E. The fuselage was lengthened but it was the tail cone aft of the coolant tank, not the engine compartment.


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## Shortround6 (Apr 9, 2018)

P-39 fuselage drawing.




P-39 engine




P-63 engine. Please note the collection of machinery spaced off the rear of the engine.
Also please note the relocated position of the cockpit and engine of the P-63 compared to the P-39.

Bell had two chances at redoing the P-39, the P-39E (also with a longer fuselage and many other changes) and the P-63 (different wing too). and yet we modern day engineers are so much smarter that we can fit stuff in where large numbers of full time engineers could not??


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## Shortround6 (Apr 9, 2018)

P-39, note where the upward angle starts at the rear of the engine.




P-63










looks like the coolant tank was in front of the engine and not on top of it.


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## Zipper730 (Apr 9, 2018)

P-39 Expert said:


> Not trying to argue, but the space for the engine/second stage mechanical supercharger in the P-63 is exactly the same size as the P-39. The P-63 moved the coolant tank from behind the engine to up behind the pilot's seat above the engine and put the second stage blower where the coolant tank had been.


Just out of curiosity, why this change in position?


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## P-39 Expert (Apr 10, 2018)

Shortround6 said:


> View attachment 489169
> 
> P-39 fuselage drawing.
> View attachment 489170
> ...


Here are two drawings, one of the P-39 and one of the P-63. The distance from the front of the engine compartment to the back are both exactly 90.25 inches. They did lengthen the rear fuselage but it was aft of the engine compartment, not within the engine compartment. 

They also did move the wing back a bit (or the fuselage up) on the P-39E and P-63 necessitating the lengthened tail cone for balance and stability.


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## P-39 Expert (Apr 10, 2018)

Zipper730 said:


> Just out of curiosity, why this change in position?


They had to move the coolant tank in order to put the second stage supercharger there.


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## Zipper730 (Apr 10, 2018)

P-39 Expert said:


> They had to move the coolant tank in order to put the second stage supercharger there.


No, I _get_ that, what I'm wondering is if there was physical room off the bat to position the coolant tank forward, and arrange a turbocharger aft of the engine: Why did they place the turbo under the engine from the get-go?


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## Shortround6 (Apr 10, 2018)

There were more than just space/volume considerations. 
weight/center of gravity comes into it. As does ducting, both exhaust and inlet plus inter-cooler


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## P-39 Expert (Apr 11, 2018)

Zipper730 said:


> No, I _get_ that, what I'm wondering is if there was physical room off the bat to position the coolant tank forward, and arrange a turbocharger aft of the engine: Why did they place the turbo under the engine from the get-go?


I believe so since the size of the engine compartment in both the P-39 and P-63 were almost exactly the same, and the P-63 moved the coolant tank up just ahead of the engine while the P-39 still had it behind the engine.


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## Zipper730 (Apr 11, 2018)

Shortround6 said:


> There were more than just space/volume considerations.
> weight/center of gravity comes into it.


Do you have any idea how much the turbo would weigh?


> As does ducting, both exhaust and inlet plus inter-cooler


I'm curious if there was enough space to put the turbocharger behind the engine and still have enough room for ducting and intercooler spacing?


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## Zipper730 (Apr 30, 2018)

I should have found this already: Where was the engine radiator on the XP-39


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## pbehn (May 1, 2018)

When discussing the Meredith effect, as with everything there is no free lunch, on full power the water pumps on a Merlin consumed 40BHP.


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## P-39 Expert (May 1, 2018)

Zipper730 said:


> I should have found this already: Where was the engine radiator on the XP-39


Coolant radiator was in left (facing forward) wing leading edge. Did not have an adjustable outlet.


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## Zipper730 (May 1, 2018)

P-39 Expert said:


> Coolant radiator was in left (facing forward) wing leading edge. Did not have an adjustable outlet.


And on the P-39 the coolant radiator was put in the leading edges of both wings?


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## Shortround6 (May 1, 2018)

no.

XP-39 with radiator inlet.





in left wing root.
XP-39 with radiator outlet about midwing 




intersects the wing walkway.

P-39s had an inlet in each wing but the radiator was in the wing center section under the engine. Outlet was at the rear of the wing with an oil cooler flap on each side.

see bottom picture in Post #20 of this thread


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## Shortround6 (May 1, 2018)

duplicate


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## P-39 Expert (May 2, 2018)

Like I have said before, that turbo installation in the P-39 would have made Rube Goldberg proud. I don't think it would ever have been reliable and Bell/AAF made a wise move in deleting it in order to get the P-39 into series production in time for WWII.

All they really needed to make the early P-39s(D,F,K and L) high altitude planes (in 1942) was less weight.


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## Zipper730 (May 2, 2018)

P-39 Expert said:


> Like I have said before, that turbo installation in the P-39 would have made Rube Goldberg proud.


No, I get that in the way it was installed below the engine: However, if like on the P-63, the oil tank was moved forward and the turbo was placed behind the engine instead of under it, would such a design have been workable?

Also, would there have been enough room to mount the intercooler as placed?


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## wuzak (May 2, 2018)

P-39 Expert said:


> All they really needed to make the early P-39s(D,F,K and L) high altitude planes (in 1942) was less weight.



And a better high altitude supercharger for the engine.

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## P-39 Expert (May 3, 2018)

Zipper730 said:


> No, I get that in the way it was installed below the engine: However, if like on the P-63, the oil tank was moved forward and the turbo was placed behind the engine instead of under it, would such a design have been workable?
> 
> Also, would there have been enough room to mount the intercooler as placed?


The oil tank wasn't moved on the P-63, it was the coolant tank that was moved up between the pilot and engine that made room for the mechanical second stage supercharger (not a turbo). There was enough room there for the mechanical second stage but not a turbo which required more space. Plus all the internal ducting (exhaust to turbo to intercooler to carburetor) and the intercooler itself, none of which was used/needed on the P-63. Adding the turbo where the oil tank was would have caused a CG problem also. Workable is the key word, and that would have been difficult without substantially enlarging the P-39/63. The P-47 used this arrangement (turbo in the rear fuselage) and was a much larger plane than the P-39/63. Much easier/quicker to use the mechanical second stage rather than the turbo. Just my opinion.


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## P-39 Expert (May 3, 2018)

wuzak said:


> And a better high altitude supercharger for the engine.


Obviously the higher supercharger gear ratio (9.6 vs 8.8) in the P-39M/N/Q provided about 100 extra HP and better performance but the earlier D,F,K and L with the 8.8 engines would have benefited greatly from an easy weight reduction program. 
The P-39C (8.8 gears) had the same engine and aerodynamics as the P-39D/F but was about 10mph faster (379mph vs 369mph) and climbed a whopping 1000fpm faster (3720fpm vs 2720fpm), good performance for 1942. The difference was weight since the P-39C weighed only 7075# and the D/F/K/L weighed about 7650#. 
Simply remove the (useless) four .30 caliber light machine guns from the wings, the nose armor plate (100#) and the small armor panels in the cockpit turnover bulkhead around the rear armor glass but leave all the rest of the armor plate/glass protecting the pilot and oil tank. A P-39F being produced around the time of Pearl Harbor weighed 5409# empty, add pilot 200#, fuel 720# (std 120gal load), oil 71#, guns/ammo 580# (37mm plus 2x.50calMG and gunsight) and remaining armor plate/glass 129# and this plane weighs 7109#. Not quite 7075# but close enough for government work. That would have been a hot ship in 1942.


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## Zipper730 (May 4, 2018)

P-39 Expert said:


> The oil tank wasn't moved on the P-63, it was the coolant tank that was moved up between the pilot and engine that made room for the mechanical second stage supercharger (not a turbo).


Sorry about that...


> There was enough room there for the mechanical second stage but not a turbo which required more space. Plus all the internal ducting (exhaust to turbo to intercooler to carburetor) and the intercooler itself, none of which was used/needed on the P-63.


The P-63 was originally planned around having an intercooler (not sure where), but ultimately they went with anti-detonant...


> Adding the turbo where the oil tank was would have caused a CG problem also.


So the turbocharger weighed more than the supercharger, as well as all it's ducting and the intercooler?


> Workable is the key word, and that would have been difficult without substantially enlarging the P-39/63.


Would lengthening the P-39 to the P-63's length have provided any fix? If not, would an arrangement similar to the YP-37 and YB-38 work (also below the engine but better flow)


> Much easier/quicker to use the mechanical second stage rather than the turbo.


I agree, it's just kind of an intellectual exercise -- see what would/would not work. That, and I don't think Allison had a 2nd stage developed at the time (that said, it could be bolted on without difficulty due to the design).


> Simply remove the (useless) four .30 caliber light machine guns from the wings, the nose armor plate (100#) and the small armor panels in the cockpit turnover bulkhead around the rear armor glass but leave all the rest of the armor plate/glass protecting the pilot and oil tank.


The removal of the 4 x 7.62's seems like a good idea to be honest, I never really understood why they put them in the design: I figure that 2 x 0.50 in the wing, 2 x 0.50 in the nose, and 1 x 37mm would be quite good. As for the nose armor plate, and small panels around the rear-armor glass, do you mean the bulletproof glass or firewall & that metal thing protecting the pilot's head?


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## P-39 Expert (May 4, 2018)

Zipper730 said:


> Sorry about that...
> The P-63 was originally planned around having an intercooler (not sure where), but ultimately they went with anti-detonant...
> So the turbocharger weighed more than the supercharger, as well as all it's ducting and the intercooler?
> Would lengthening the P-39 to the P-63's length have provided any fix? If not, would an arrangement similar to the YP-37 and YB-38 work (also below the engine but better flow)
> ...


The turbo and the mechanical second stage (MSS) probably weighed about the same but if the turbo went where the oil tank was it would be further aft of the CG than the MSS causing (more) balance issues. The ducting issue was mainly internal space not weight.
The P-63 was lengthened but it was all in the tail cone, not in the engine compartment. Engine compartment was the same size on both P-39 and P-63. Lengthening was for stability, not internal space for the MSS.
Allison MSS was in production in March 1943, just for time reference.
Those 4x.30cal wing guns ruined the P-39. Without them the plane was lighter and the space vacated by the guns could have been used for more fuel if desired. Main problem was they weren't powerful enough to be effective as the British proved in the Battle of Britain. No other AAF production fighter used them. We can blame our friends the British for the .30 caliber wing guns, they specified them on the export P-400. 
Regarding the armor plate, keep the rear armor glass and delete the small pieces of armor on the turnover bulkhead (that metal thing protecting the pilot's head). 
P-39 was just too small for a turbo, intercooler and all the internal ducting IMO. Fun to talk about though.


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## wuzak (May 4, 2018)

Zipper730 said:


> The P-63 was originally planned around having an intercooler (not sure where), but ultimately they went with anti-detonant...



It was a liquid to air intercooler, very much like the one on the Merlin, positioned above the engine stage supercharger. This require relocating the carburetor to the intake of the auxiliary supercharger.

The radiator for the intercooler was, presumably, located with or near the engine radiator.


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## wuzak (May 4, 2018)

P-39 Expert said:


> Those 4x.30cal wing guns ruined the P-39. Without them the plane was lighter and the space vacated by the guns could have been used for more fuel if desired. Main problem was they weren't powerful enough to be effective as the British proved in the Battle of Britain. No other AAF production fighter used them. We can blame our friends the British for the .30 caliber wing guns, they specified them on the export P-400.



The 0.30" mg was a common fitment to American aircraft at the time. The P-40B, in production in 1941, had 2 x 0.50"mgs and 4 x 0.30" mgs.

The early P-39s carried 2 x 0.30" mgs as well as 2 x 0.50" mgs and 1 x 37mm boat anchor. Well before the British ordered the Airacobra I.

If the British specified the extra wing guns it was probably because they felt it was under-armed. Not sure if the 0.50" mgs could have fitted in the P-39's wings, but if they couldn't it would explain why the 2 x 0.30" mgs in each wing were chosen.

Oh, and the BoB was won using, almost exclusively, 0.303" mgs.

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## Zipper730 (May 4, 2018)

P-39 Expert said:


> The turbo and the mechanical second stage (MSS) probably weighed about the same but if the turbo went where the oil tank was it would be further aft of the CG than the MSS causing (more) balance issues. The ducting issue was mainly internal space not weight.


Okay


> The P-63 was lengthened but it was all in the tail cone, not in the engine compartment. . . . Lengthening was for stability, not internal space for the MSS.


Stability usually implies a relationship in control surface effectiveness as well as mass...


> Those 4x.30cal wing guns ruined the P-39. Without them the plane was lighter and the space vacated by the guns could have been used for more fuel if desired.



How much weight could be trimmed off by removing the 4 x 0.30" and all their ammo?
How much fuel could be put in that volume
Would 2 x 0.50 with normal rpg levels add any significant weight?



> P-39 was just too small for a turbo, intercooler and all the internal ducting IMO. Fun to talk about though.


It is, isn't it?

I'm not sure how this would have worked, but I know this configuration was pretty similar and worked on the XB-38...


... the problem with the P-37 was the configuration of the radiators producing an excessively long nose.


> Allison MSS was in production in March 1943, just for time reference.


And the reason for this delay had to do with the fact that

There was little demand for inline engines in the United States
The airlines had little interest as air-cooled engines were more reliable; the NACA cowl greatly improved cooling, and reduced drag
Raceplane designers seemed to vary: The land-based guys used radials almost exclusively because the engines were lighter for the same power, and used very well designed cowlings, and built their planes as light as possible; the seaplane designers usually used liquid cooled or even surface-evaporative cooling and used the struts and braces as radiator surfaces.
The USN had little interest in inline engines after the late 1920's or early 1930's: Radials were more compact and more reliable, though they were willing, at times, to evaluate inlines if they provided a marked improvement in performance, and as a result funded the Lycoming H-2470, Pratt & Whitney X-1800/H-2240/2600, H-3130/3730
The USAAC really was the only organization that wanted inline engines, and to this effect, they funded the Continental O-1230/I-1430 Hyper-engine, the V-1710, X-3420/V-3420, and the Lycoming O-1230

The massive funding of a large number of inline-engines: Including the Continental O-1230/I-1430, Lycoming O-1230, H-2470, Pratt & Whitney X-1800/H-2240/2600, H-3130/3730, and the Allison V-1710, X-3420/V-3420 meant that essentially 7 designs were being developed (some of them were reconfigurations and resizing) with little commercial demand in a depression; the result was a massive breadth but little depth to it all. 
The USAAC mostly favored turbochargers over superchargers
They were sometimes willing to look into two-stage superchargers: The problem was not that they looked into turbochargers (the USN did that too at times), it's that they put turbochargers so far ahead in the running, that secondary stage superchargers were basically in the background not a close or moderate second
The USN did look into the idea of twin-stage supercharging and made it work with the F4F-3, F4U-1 making their first flights in 1940.




> Regarding the armor plate, keep the rear armor glass and delete the small pieces of armor on the turnover bulkhead (that metal thing protecting the pilot's head).


I'm not sure the pilots would've liked that!


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## P-39 Expert (May 4, 2018)

wuzak said:


> It was a liquid to air intercooler, very much like the one on the Merlin, positioned above the engine stage supercharger. This require relocating the carburetor to the intake of the auxiliary supercharger.
> 
> The radiator for the intercooler was, presumably, located with or near the engine radiator.


Makes a lot of sense, there was additional room in the P-63 radiator area for an intercooler if necessary. I like the way Bell got around the intercooler too.


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## wuzak (May 4, 2018)

Zipper730 said:


> I'm not sure how this would have worked, but I know this configuration was pretty similar and worked on the XB-38...
> View attachment 492104
> 
> ... the problem with the P-37 was the configuration of the radiators producing an excessively long nose.
> And the reason for this delay had to do with the fact that



The XB-38 used the standard B-17 turbo location, had a chin mounted intercooler and leading edge radiators (between the nacelles).

Not sure why the standard B-17 intercoolers couldn't be used with a chin mounted radiator - would have made for a simpler conversion - take off the R-1820s and bolt up the V-170 power egg.


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## Zipper730 (May 4, 2018)

wuzak said:


> It was a liquid to air intercooler, very much like the one on the Merlin, positioned above the engine stage supercharger. This require relocating the carburetor to the intake of the auxiliary supercharger.


Oh, okay


> Not sure if the 0.50" mgs could have fitted in the P-39's wings, but if they couldn't it would explain why the 2 x 0.30" mgs in each wing were chosen.


They couldn't fit 1 x 0.50 in each wing?


> The XB-38 used the standard B-17 turbo location, had a chin mounted intercooler and leading edge radiators (between the nacelles).


While a little beyond the scope of the P-39 discussion, if the YP-37 had the following repositioned in the following ways

Radiators and intercooler to the wing leading-edges
Auxiliary fuel tank behind the wing
Possibly the repositioning of the radio and/or battery behind the cockpit
Cockpit moved further up
Would the design have fit fairly closed to the P-40?


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## Shortround6 (May 4, 2018)

One .50 weighs about 3 times what a .30 weighs.
.50 cal ammo weighs 30 lbs per hundred.
.30 cal ammo weighs 6-6.5 lbs per hundred.
If four .30s with 1200 rounds total ruined the P-39 then adding two .50s was hopeless.

Blaming 180 pounds of guns and ammo for ruining plane seems a bit extreme.

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## P-39 Expert (May 4, 2018)

Zipper730 said:


> Okay
> Stability usually implies a relationship in control surface effectiveness as well as mass...
> 
> 
> ...


The 4 x .30caliber wing guns weighed around 377# with 1000rpg (just guns & ammo, not including mounts, chargers, heaters and ammo boxes). Pull all that out and you are looking at 400# weight saved. Later models loaded the .30s with 300rpg to save weight. This reduced the weight of the four guns and ammo to approximately 200# total. So the amount you save depends on the model, either 400# or 200# depending on load.
Fuel where the wing guns were is estimated at 50gal total, 25gal in each wing. Post war racers put 100gal in the same space but were using thin walled tanks instead of the leakproof rubber tanks used in WWII. Estimate is based on measurements. Weight of two .50cal MGs and 300rpg weighed 330# on the P-39Q, but those were hung underneath the wing.
Regarding the P-37, it did result in a long nose. I think reliability of the turbo was the big problem at that stage of the game.
Agree with you completely regarding radial vs inline. For commercial applications radials were superior. Only purpose for an inline engine is for fighter planes IMO. 
The small pieces of armor plate in the turnover bulkhead were to either side of the armor glass protecting the pilot's head. That piece of armor glass was large enough to protect the pilot without the small side pieces, so remove them and save the weight. Later models dispensed with the rear armor glass and substituted conventional armor plate to protect the pilot's head like on other bubble canopy planes. Saved a little weight too.


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## wuzak (May 4, 2018)

P-39 Expert said:


> Weight of two .50cal MGs and 300rpg weighed 330# on the P-39Q, but those were hung underneath the wing.



With the wing guns suspended beneath, and not inside, the wing, was any extra fuel carried in the wing of the P-39Q?


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## Shortround6 (May 4, 2018)

wuzak said:


> With the wing guns suspended beneath, and not inside, the wing, was any extra fuel carried in the wing of the P-39Q?


No.
The guns were underneath, but the ammo was pretty much in the wing in the old .30 cal ammo storage area.


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## P-39 Expert (May 4, 2018)

Shortround6 said:


> One .50 weighs about 3 times what a .30 weighs.
> .50 cal ammo weighs 30 lbs per hundred.
> .30 cal ammo weighs 6-6.5 lbs per hundred.
> If four .30s with 1200 rounds total ruined the P-39 then adding two .50s was hopeless.
> ...


The 200# in the N with the uprated engines was not a big deal, but 400# in the earlier models with the 8.8 supercharger was a killer. Especially since they weren't very effective against other aircraft.


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## Zipper730 (May 4, 2018)

P-39 Expert said:


> The 4 x .30caliber wing guns weighed around 377# with 1000rpg (just guns & ammo, not including mounts, chargers, heaters and ammo boxes). Pull all that out and you are looking at 400# weight saved.


So that lowers the OEW to 5062, and 7100 for takeoff?


> Fuel where the wing guns were is estimated at 50gal total, 25gal in each wing. Post war racers put 100gal in the same space but were using thin walled tanks instead of the leakproof rubber tanks used in WWII.


So that adds 301 pounds back into the plane in fuel, no idea how much the self sealing tank rubber weighs.


> Agree with you completely regarding radial vs inline. For commercial applications radials were superior. Only purpose for an inline engine is for fighter planes IMO.


They were okay on some bombers (Mosquito in particular) too...


> The small pieces of armor plate in the turnover bulkhead were to either side of the armor glass protecting the pilot's head. That piece of armor glass was large enough to protect the pilot without the small side pieces, so remove them and save the weight.


How much weight was saved?



Shortround6 said:


> The guns were underneath, but the ammo was pretty much in the wing in the old .30 cal ammo storage area.


Could the guns and/or ammo fit inside the wing in their entirety?


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## P-39 Expert (May 4, 2018)

Zipper730 said:


> So that lowers the OEW to 5062, and 7100 for takeoff?
> So that adds 301 pounds back into the plane in fuel, no idea how much the self sealing tank rubber weighs.
> Initially with the YP-37, yes -- I'm not sure if it was as bad as the P-39 though.
> They were okay on some bombers (Mosquito in particular) too...
> ...


Guns and ammo were actually included in useful load, not empty weight. 5462# empty, useful load 1700#, loaded 7162# or so. That would have been a hot rod even with the 8.8 engine. 
Self sealing tanks weighed about 20# each, but those were for 10gal (avg) tanks, so maybe 50# per 25gal tank? 
Removing the turnover armor saved 16#. Add that to the nose armor of $100# and save 116#, still have 115# of armor plate and glass protecting the pilot & oil tank.


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## Zipper730 (May 4, 2018)

Shortround6 said:


> If four .30s with 1200 rounds total ruined the P-39 then adding two .50s was hopeless.


Too much weight eh?



P-39 Expert said:


> Guns and ammo were actually included in useful load, not empty weight.


The guns are factored into the OEW, the ammo isn't. Manufacturer empty omits some extra things as well, and I think the British called it Tare weight... still I see your point.


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## P-39 Expert (May 4, 2018)

Zipper730 said:


> Too much weight eh?
> 
> The guns are factored into the OEW, the ammo isn't. Manufacturer empty omits some extra things as well, and I think the British called it Tare weight... still I see your point.


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## P-39 Expert (May 4, 2018)

See attached. Listed Empty Weight at the top, added guns, armor plate, pilot and oil for a subtotal Tactical Weight Empty, than added fuel, extra oil and ammo for total Gross Weight. Have heard Tactical Weight Empty referred to as Empty Equipped also.

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## Zipper730 (May 19, 2018)

P-39 Expert said:


> Regarding the P-37, it did result in a long nose. I think reliability of the turbo was the big problem at that stage of the game.


Yes, but if I recall, they were able to make it work, and it neatly fits under the engine. 

The intercooler position isn't much different than on the P-39...


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