XP-39: pros cons

[tomo pauk]

...
You need intercoolers to make high power at altitude. Otherwise you are just adding weight, bulk and cost to make a slightly better low altitude engine. The supercharger adds considerable heat to the intake charge. For instance the supercharger on a Merlin XX added 148 degrees celsius (over 260 degrees F) to the temperature of the incoming air when in high gear. using more compression in the supercharger/s just adds more heat and at some point you reach the detonation point.
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Excellent post, however I'd say that intercooler was not mandatory for 2-stage engines to work well at high altitude. The gain vs. 1-stage versions of same engine was about 60% on the DB 605L vs. 605A at 35000 ft, similar for the best 1-stage V-1710 vs. the 2-stage versions. Both DB-603L (or was it LA?) and Jumo 213F were 2-stage engines without intercoolers.
Cooling drag of the Jumo 213E was about 35% greater than of the 213E, so we might take that factor in a consideration. One of things that made the otherwise unremarkable XP-40Q2 and P-63C as fast, if not faster than Spitfire IX/VIII.

 

[wuzak]

The auxiliary supercharger weighed about 125lbs and added about 21 in to the length of the engine. Two weights are given. 175lbs for the one ones with the carb on the auxiliary supercharger and 125lbs for the ones with the carb still on the engine. It could be that the difference is the weight of the carb, and that it is counted as part of the auxiliary supercharger when attached?

It could be that the one with teh carb on the auxiliary supercharger also had a liquid to air intercooler.

 

[Zipper730]

That's the big question, why did it take so long to develop the mechanical two stage supercharger?

If I was to make a guess, the USAAF seemed to prefer the turbocharger, whereas the Navy preferred twin-stage supercharging.

A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.

From what you said (regarding the P-63), the supercharger was added behind the engine, with the cooler moved forward...

Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).

Why was the pusher so desirable for the YFM-1

 

[Shortround6]

Why was the pusher so desirable for the YFM-1

Because if you made it with tractor propellers the gunners would be shooting backwards

The 37mm guns were flexible

and could be aimed over a cone independent of the aircrafts direction and in fact cold be aimed at different targets.

 

[Zipper730]

Because if you made it with tractor propellers the gunners would be shooting backwards

What was the motive for such a weird design? As I understand it, I twas supposed to be a patrol-interceptor that could be an escort-fighter.

 

[wuzak]

What was the motive for such a weird design? As I understand it, I twas supposed to be a patrol-interceptor that could be an escort-fighter.

To blow enemy bombers out of the sky.

 

[Dimlee]

To blow enemy bombers out of the sky.

And it still blows many minds. What a cool "steampunk" machine.

 

[J.A.W.]

And it still blows many minds. What a cool "steampunk" machine.

Yeah that thing is a real-life 'Astounding SF'/'Sky Captain & the World of Tomorrow' - type of machine..

 

[Zipper730]

In their report they (the NACA) claim the XP-39, as they received it was good for 340mph at 20,000ft and just under 280mph at sea level.

Was this with or without the turbocharger?

There were severe problems with the airflow for the radiator, oil cooler and intercooler.

The production P-39 would redesign the radiator & oil-cooler to cover the left & right inboard wing for this reason?

For the last, US practice of the time was that the intercooler should remove 1/2 of the heat added by the turbo-supercharger to the intake air before it entered the engine carburetor. In the XP-39 the NACA estimated the intercooler (based on airflows) was removing only 25% in high speed flight and about 12% during climb. This significantly affected power output. The inter cooler was small in size to fit the airframe and had a high pressure drop across it. this meant high drag. The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.

Since this is kind of a 'what-if' exercise: Would it be fair to say that the intercooler was affected by the following issues

1. The intercooler lacked a splitter and, as a result, more turbulent flow entered, reducing the effectiveness of the intercooler for a given area?
2. The front of the intercooler duct was not shaped to correctly reduce the velocity and achieved desired pressure drop?
3. The rear of the intercooler duct lacked a flap to allow sufficiently high area at low speeds and sufficiently low area at high speeds?
4. The area of the intercooler was inadequate even if 1-3 were optimal?

Bell did at least two mock-ups of turbo/intercooler units in 1941 to be 'added' to the P-39. the extra drag of these units caused 40-45mph speed loss at the lower altitudes.

So if 340 was achieved at higher altitudes, you'd be down to 295-300?

Regarding the YFM-1

Because if you made it with tractor propellers the gunners would be shooting backwards

Early on, I thought they simply had the guns able to move because of the fact that the ballistic arc was substantial. Still, I'm surprised they didn't just put the guns in the fuselage below the optical sight, or in the wing-roots

The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.

This came up in a parallel thread on the P-39/P-63 -- it had a liquid to air intercooler.

Allison had a 9.5" supercharger for the V-1710 and a 10.0" supercharger for the V-3420. The auxiliary supercharger was 12.1875"

I thought GE was making most all superchargers at first?

 

[Zipper730]

@P-39 Expert

It's kind of a little off topic but I have three questions that you might have answers for

1. How much weight would be saved/added by removing the car-doors and going with a sliding canopy?
2. Other than the intercoolers and things of that sort: Why was the turbocharger harder to mount on the P-39 than the YP-37 (despite the nose being absurd -- the turbocharger was mounted decently under the engine without any real trouble
3. The redesign of the tail-cone on the P-63: Was this information learned from experience on the P-63, or did it come from some other source?

While I am in agreement that from a practical standpoint that a twin-stage supercharger would be better, Allison didn't have a workable system at the time, but did have a turbocharger. All of this is clearly an intellectual exercise

 

[P-39 Expert]

@P-39 Expert

It's kind of a little off topic but I have three questions that you might have answers for

1. How much weight would be saved/added by removing the car-doors and going with a sliding canopy?
2. Other than the intercoolers and things of that sort: Why was the turbocharger harder to mount on the P-39 than the YP-37 (despite the nose being absurd -- the turbocharger was mounted decently under the engine without any real trouble
3. The redesign of the tail-cone on the P-63: Was this information learned from experience on the P-63, or did it come from some other source?

While I am in agreement that from a practical standpoint that a twin-stage supercharger would be better, Allison didn't have a workable system at the time, but did have a turbocharger. All of this is clearly an intellectual exercise

Thanks for the questions.
1. Probably a little weight would have been saved by replacing the car doors with a sliding canopy, but I bet more weight would have been saved by the accompanying deletion of the turnover bulkhead that wouldn't be needed with a sliding canopy. Save a little weight and have much better visibility. Replace the armored glass and small armor plates in the turnover bulkhead with normal armor plate behind the pilot. To me the biggest quick opportunities for weight reduction would be the deletion of the 4x.30 caliber peashooter guns in the wings along with their mounts, chargers, heaters and ammunition boxes (about 200# with 300rpg, closer to 400# with 1000rpg) and deleting the piece of armor plate in the nose (about 100#) and moving the radios from the tail cone to above the engine right behind the pilot to retain balance. These items would have helped the earlier P-39D/F/K/L with the 8.8 geared engines climb a lot faster, about 1.2fpm per every pound of weight saved. Reduce weight by 300# and increase climb rate by 360fpm, a worthwhile endeavor IMO.
2. Regarding the turbo, if the YP-37 turbo was without any real trouble then it would have been produced in volume. The turbo at that time was still unreliable, it took lots of internal space, and the P-39 radiators (coolant, oil and intercooler) were a real Rube Goldberg arrangement with poor airflow and no adjustable exit flaps. In other words maybe 25% functional. The only other place to put the turbo was the rear fuselage, but then you've grown into a P-47.
3. The tail cone was lengthened to compensate for moving the wing back about a foot. Moving the wing back reduces the distance from the CG to the tail, so the tail needed to be lengthened to compensate for stability. The tail wasn't lengthened to provide space for the mechanical second stage, there was already room there, just move the coolant tank up right behind the pilot and put the second stage where the coolant tank was.

In summary, the turbo wasn't ready and the promised mechanical second stage offered almost as much horsepower with a much simpler and cleaner installation. Now, the two stage mechanical V-1710-93 didn't start production until May 1943 but it's installation into the P-39 and P-40 would have transformed them into superb high altitude planes. Hope this helps.

 

[Shortround6]

Was this with or without the turbocharger?

With, the turbo charger.

The production P-39 would redesign the radiator & oil-cooler to cover the left & right inboard wing for this reason?

The air inlets were in the wing roots, the actual radiator and oil coolers were in the belly of the plane where the turbo had been.

It is a model but gives the best idea as to what they changed. Radiator in the center, oil cooler on each side.

Since this is kind of a 'what-if' exercise: Would it be fair to say that the intercooler was affected by the following issues

1. The intercooler lacked a splitter and, as a result, more turbulent flow entered, reducing the effectiveness of the intercooler for a given area?
2. The front of the intercooler duct was not shaped to correctly reduce the velocity and achieved desired pressure drop?
3. The rear of the intercooler duct lacked a flap to allow sufficiently high area at low speeds and sufficiently low area at high speeds?

1. The area of the intercooler was inadequate even if 1-3 were optimal?[/QUOTE]

No 4 is the primary, they didn't have enough airflow by a factor of 4 when climbing, a splitter and.or modified air intake is not going to solve the problem of too small aaaaan intercooler to begin with.

So if 340 was achieved at higher altitudes, you'd be down to 295-300?

No, the turbo installation cost 40-45 mph off the speed of of an unmodified P-39. A "D" I believe?

In other words if the standard D could do 336mph at 5,000ft it was down to 300mph or under with the add on turbo installations

Much like trying to fly/fight with a large drop tank under the plane as far as drag goes. This was a mock up which is why the exhaust doesn't connect.

 

[Zipper730]

Thanks for the questions.

No problem

1. Probably a little weight would have been saved by replacing the car doors with a sliding canopy, but I bet more weight would have been saved by the accompanying deletion of the turnover bulkhead that wouldn't be needed with a sliding canopy.

I thought most aircraft had roll-bars up top?

To me the biggest quick opportunities for weight reduction would be the deletion of the 4x.30 caliber peashooter guns in the wings along with their mounts, chargers, heaters and ammunition boxes (about 200# with 300rpg, closer to 400# with 1000rpg) and deleting the piece of armor plate in the nose (about 100#) and moving the radios from the tail cone to above the engine right behind the pilot to retain balance.

Was it a standard practice among any aviation service to mount the radio above the engine?

2. Regarding the turbo, if the YP-37 turbo was without any real trouble then it would have been produced in volume. The turbo at that time was still unreliable, it took lots of internal space, and the P-39 radiators (coolant, oil and intercooler) were a real Rube Goldberg arrangement with poor airflow and no adjustable exit flaps. In other words maybe 25% functional.

Firstly: I was under the impression the problems the P-37 were the fact that the design's long nose and cockpit position would basically provide nearly nonexistent visibility over the nose, particularly when taxiing or pulling-g for deflection shots (if you thought the F4U was bad, you ain't seen nothin' yet!).

3. The tail cone was lengthened to compensate for moving the wing back about a foot. Moving the wing back reduces the distance from the CG to the tail, so the tail needed to be lengthened to compensate for stability. The tail wasn't lengthened to provide space for the mechanical second stage, there was already room there, just move the coolant tank up right behind the pilot and put the second stage where the coolant tank was.

Ok

Now, the two stage mechanical V-1710-93 didn't start production until May 1943 but it's installation into the P-39 and P-40 would have transformed them into superb high altitude planes.

Yeah, it definitely would have -- I'm curious why we didn't procure the P-63?

With, the turbo charger.

Okay, I understand

The air inlets were in the wing roots, the actual radiator and oil coolers were in the belly of the plane where the turbo had been.
View attachment 522424
It is a model but gives the best idea as to what they changed. Radiator in the center, oil cooler on each side.

Ok, so the modified arrangement actually made it impossible to stuff the turbocharger in?

No 4 is the primary, they didn't have enough airflow by a factor of 4 when climbing, a splitter and.or modified air intake is not going to solve the problem of too small an intercooler to begin with.

Would having two scoops on either side fix it?

 

[P-39 Expert]

No problem
I thought most aircraft had roll-bars up top?
Was it a standard practice among any aviation service to mount the radio above the engine?
Firstly: I was under the impression the problems the P-37 were the fact that the design's long nose and cockpit position would basically provide nearly nonexistent visibility over the nose, particularly when taxiing or pulling-g for deflection shots (if you thought the F4U was bad, you ain't seen nothin' yet!).

Secondly: Regarding the engine and oil-cooler, that was ultimately addressed in real life and seems that much of the same system could be used without incident.

Thirdly: The turbocharger and intercooler presented three problems. The first was that the airflow to the turbocharger wasn't optimal and was exposed to daylight (and the drag coming along with this), the second was the arrangement of the intercooler.

We'd discussed the idea of moving the coolant tank forward and positioning the turbocharger rearward in another thread, but the differences designs of the mechanical secondary stage (P-63) and the turbocharger (XP-39) in terms of weight and airflow pathway differences would effectively make the aircraft considerably too tail-heavy. Suffice it to say, I don't know if there was inherently any problem with having the turbo underneath the engine (it was used on the XB-38), or if it was the devil in the details so to speak. Would it be feasible to angle the turbo in a position that avoided having it pointed rearwards or straight down but pointed backwards to some extent at an angle? It would avoid the extreme CG of having it pointed dead aft like the P-63's supercharger layout, and it would avoid pointing it down?

Lastly: The intercooler design seemed to have problems in the fact that, at the most basic level, the duct was designed completely wrong. For starters, it had no splitter to remove turbulent airflow -- this can be rectified with a splitter; the outlet had no variable position flap so the area was probably too low for low-speed flight, not low enough for high-speed flight, so the simple solution would be to put a movable flap there to correct for it.

In terms of greater details the area of the duct may not have been adequate: That could be fixed by increasing the size of the existing duct or putting two smaller ducts in the aircraft, one on each side much like the P-38.
Ok
Yeah, it definitely would have -- I'm curious why we didn't procure the P-63?

Okay, I understand
Ok, so the modified arrangement actually made it impossible to stuff the turbocharger in?
Would having two scoops on either side fix it?
That's not a mockup of a jet-engine or ramje?

Usually not a rollbar with a bubble canopy. I think the normal back/head plate armor may have acted as a rollbar.

P-39 radios were often mounted above the engine right behind the pilot. Look for it in P-39 photos.

I'm really no advocate of the turbo in either the P-39 or P-40. Allison mechanical two stage would have been almost as powerful and much simpler/more compact. And an intercooler isn't needed except for WEP, so don't have WEP. Only helps under the critical altitude anyway.

We did procure the P-63, they began trickling out in October '43 with only 28 completed by the end of that year and another 1800 by the end of '44. Interesting how the engine had been in production since May but the first production P-63 rolled out in October. My plan (hindsight) would have been to just put the two stage in the P-39 and skip the P-63, but...

By the way, the V-1710-93 two stage engine wasn't really a new engine, it was the same constantly updated engine that was going into contemporary P-38s, P-40s and P-51s. Only thing new was the separate mechanical second stage.

 

[Zipper730]

Usually not a rollbar with a bubble canopy. I think the normal back/head plate armor may have acted as a rollbar.

How much weight would come out of removing the roll-bar and doors, and replacing with sliding canopy and adding normal armor?

P-39 radios were often mounted above the engine right behind the pilot. Look for it in P-39 photos.

Okay, so this would be simply taking it from the XP-39 to it's natural evolution?

I'm really no advocate of the turbo in either the P-39 or P-40.

Of course, as I said it's an intellectual exercise only based on what existed at the time.

We did procure the P-63

Why didn't we approve them for combat?

 

[wuzak]

We'd discussed the idea of moving the coolant tank forward and positioning the turbocharger rearward in another thread, but the differences designs of the mechanical secondary stage (P-63) and the turbocharger (XP-39) in terms of weight and airflow pathway differences would effectively make the aircraft considerably too tail-heavy. Suffice it to say, I don't know if there was inherently any problem with having the turbo underneath the engine (it was used on the XB-38), or if it was the devil in the details so to speak. Would it be feasible to angle the turbo in a position that avoided having it pointed rearwards or straight down but pointed backwards to some extent at an angle? It would avoid the extreme CG of having it pointed dead aft like the P-63's supercharger layout, and it would avoid pointing it down?

The turbo was not under the engine one the XB-38. In fact they were in the same place as on the B-17 - under the nacelle, behind the firewall.

 

[Shortround6]

I'm really no advocate of the turbo in either the P-39 or P-40. Allison mechanical two stage would have been almost as powerful and much simpler/more compact. And an intercooler isn't needed except for WEP, so don't have WEP. Only helps under the critical altitude anyway.

Oh boy, that is a major misconception. Please see the P-38 and it's history of a too small intercooler affecting not only WEP but the altitude at which military power could be used as the military power was increased (big hint, the critical altitude/FTH dropped). You might want to tell P & W that the R-1830 in the F4F didn't need intercoolers or the R-2800 used the F4U, F6F and P-61 didn't need intercoolers if they didn't use WEP.

And in actual fact, an intercooler can INCREASE the Critical Altitude of a single stage engine if you have the volume/space to install one ( One or two versions of the JUmo 211 used intercoolers on bombers)

If you are not interested in increasing the critical altitude of the engine/plane just drop the whole 2nd stage, mechanical or turbo.

We did procure the P-63, they began trickling out in October '43 with only 28 completed by the end of that year and another 1800 by the end of '44. Interesting how the engine had been in production since May but the first production P-63 rolled out in October. My plan (hindsight) would have been to just put the two stage in the P-39 and skip the P-63, but...

The minor fact that the V-1710-93 two stage engine didn't complete it's official type test until Nov 27th 1943 didn't have anything to do with this delay did it?
P-63s completed before that date with V-1710-93 engines were flying with restrictions. Not really suitable for combat until the restrictions were lifted with completion of the test.
f you aren't going to use WEP or use the second stage to improve altitude performance what is the point?

By the way, the V-1710-93 two stage engine wasn't really a new engine, it was the same constantly updated engine that was going into contemporary P-38s, P-40s and P-51s. Only thing new was the separate mechanical second stage.

Part of the delay in testing was the time needed to develop the water injection and the improved pistons and piston rings needed to support the higher power. Late model P-38 engines benefited from these parts. Allison powered P-51s had stopped being built months before these developments were put in production. Likewise late model P-40s had no real need of these parts.

BTW the WER test wasn't done until Dec 1943

 

[P-39 Expert]

How much weight would come out of removing the roll-bar and doors, and replacing with sliding canopy and adding normal armor?
Okay, so this would be simply taking it from the XP-39 to it's natural evolution?
Of course, as I said it's an intellectual exercise only based on what existed at the time.
But we didn't approve them for combat...

No idea on the weight savings on removing the turnover bulkhead and replacing the doors with a sliding canopy. We'd lose the doors, turnover bulkhead, it's rear armor glass and couple of small pieces of integral armor, then add the sliding canopy and head/body armor plate. Regarding weight I think it's largely a wash but the better visibility would be worth it.

I think the XP-39s natural evolution should have been lighter P-39D/F/K/L, then the M/N/Q with the more powerful V-1710-85 with 9.6 gears, then the -93 with the mechanical second stage which instead became the P-63. Seems like Allison was thwarted at every turn. The 9.6 supercharger step up gears were planned for introduction around new years '42, but tests showed they couldn't stand the increased MP so the gears had to be redesigned wider and that took until about August/October of '43 (8-10 months) to get the engine into the P-39M in November. Then the second stage took seemingly forever, originally scheduled around new year '43 but not actually in production until May. But so goes wartime production of new engines. Ideally the AAF would have had the N model around the beginning of '42 and the two stage around the beginning of '43. But actually they had the N at the end of '42 and the two stage P-63 just starting production the end of '43.

 

[P-39 Expert]

Oh boy, that is a major misconception. Please see the P-38 and it's history of a too small intercooler affecting not only WEP but the altitude at which military power could be used as the military power was increased (big hint, the critical altitude/FTH dropped). You might want to tell P & W that the R-1830 in the F4F didn't need intercoolers or the R-2800 used the F4U, F6F and P-61 didn't need intercoolers if they didn't use WEP.

And in actual fact, an intercooler can INCREASE the Critical Altitude of a single stage engine if you have the volume/space to install one ( One or two versions of the JUmo 211 used intercoolers on bombers)

If you are not interested in increasing the critical altitude of the engine/plane just drop the whole 2nd stage, mechanical or turbo.



The minor fact that the V-1710-93 two stage engine didn't complete it's official type test until Nov 27th 1943 didn't have anything to do with this delay did it?
P-63s completed before that date with V-1710-93 engines were flying with restrictions. Not really suitable for combat until the restrictions were lifted with completion of the test.
f you aren't going to use WEP or use the second stage to improve altitude performance what is the point?


Part of the delay in testing was the time needed to develop the water injection and the improved pistons and piston rings needed to support the higher power. Late model P-38 engines benefited from these parts. Allison powered P-51s had stopped being built months before these developments were put in production. Likewise late model P-40s had no real need of these parts.

BTW the WER test wasn't done until Dec 1943

Yes, an intercooler would be slightly better, but the P-63 didn't have one and used water injection instead. The purpose of the second stage is to provide high altitude performance. Intercoolers or water injection were used for WEP which was used below the critical altitude where the P-39 already had excellent performance. The P-39 could have had WEP with water injection like the P-63 if considered necessary.

The official test was completed in November '43 but less than 28 P-63s had been completed by then. No big deal.

If you don't use WEP for the two stage engine then you don't need the extra time to develop water injection or the improved pistons/rings, but these items benefited the other Allison engines also.

 

[Zipper730]

Much like trying to fly/fight with a large drop tank under the plane as far as drag goes. This was a mock up which is why the exhaust doesn't connect.

And that was a proposed intercooler? I'm curious if a belly radiator of some sort or two ventral radiators like the Spitfire would be workable

The turbo was not under the engine one the XB-38. In fact they were in the same place as on the B-17 - under the nacelle, behind the firewall.

So it was further back? On a fighter it'd be under the pilot more or less?

No idea on the weight savings on removing the turnover bulkhead and replacing the doors with a sliding canopy. We'd lose the doors, turnover bulkhead, it's rear armor glass and couple of small pieces of integral armor, then add the sliding canopy and head/body armor plate. Regarding weight I think it's largely a wash but the better visibility would be worth it.

What about the fact that the control linkages would be easier to lay out because of the fact that you don't have any interruptions in the fuselage because of the doors?

There was supposedly a proposal for some kind of higher altitude V-1710 that got cancelled, I'd almost swear I remember hearing that with the P-39. Was this a single stage supercharger with a different gear ratio or a twin-speed set-up? Or was it twin-speed?


BTW: While I'm beating a dead horse, I remembered something you stated about the P-63 -- it's wing was moved back by a foot. The changes needed to mount the twin-stage supercharger added some weight and that's why some tankage was moved around and, while a turbocharger would be bulkier and heavier -- if the wings stayed forward by their normal amount, that would mean the space between the wing and the elevator would be much larger, and the wing seems to bear a significant amount of the aircraft's weight, and while I could definitely be wrong, would you be able to have enough leverage to make everything work?