# XP-39 Airplane in wind tunnel



## johnbr (Feb 4, 2019)



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## Wurger (Feb 4, 2019)




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## Gnomey (Feb 6, 2019)

Good one!


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## spicmart (Mar 6, 2019)

Apart from the non-laminar profile wing the P-39 looks like the most streamlined fighter ever.

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## tomo pauk (Mar 6, 2019)

spicmart said:


> Apart from the non-laminar profile wing the P-39 looks like the most streamlined fighter ever.



It have had many things that improved streamlining - fully covered U/C, well burried radiators, a ram air intake tucked behind cockpit, reasonably thin wing (nothing special, though) of modest size (smallest wing on ww2 US fighters). On same or even lower power it was faster than G.55 or Spitfire, let alone P-40.
We can recall that P-39 in racing trim (including extra radiator under belly) was very succesful in post-war air races.

Unfortunately, the XP-39 was a mess from aerodynamic point of wiev.

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## Zipper730 (May 26, 2019)

spicmart said:


> Apart from the non-laminar profile wing the P-39 looks like the most streamlined fighter ever.


It does have a streamlined quality that makes it look more like a jet than a propeller driven plane. From the streamlined nose, the raised canopy, to the overall elegant looks.



tomo pauk said:


> It have had many things that improved streamlining - fully covered U/C, well burried radiators, a ram air intake tucked behind cockpit, reasonably thin wing (nothing special, though) of modest size (smallest wing on ww2 US fighters).


I thought the intake produced fairly poor ram compression compared to other aircraft like the P-40? As for a buried-radiator: I would assume the idea is to avoid having anything protrude into the path of the airflow?

As for the wings: They don't seem unusually thin, though the wing-area probably is somewhat small (that said, I'm not sure how good a turning fighter it was).


> On same or even lower power it was faster than G.55 or Spitfire, let alone P-40.


That I'm surprised about, especially with the Spitfire seeming even skinnier than the P-39 (from what I grasp, the thin fuselage was supposed to avoid changing the direction of the airflow excessively, which produces minimal turbulence, and that keeps the airflow over the horizontal and vertical stabs good, and makes the surfaces able to be smaller).


> Unfortunately, the XP-39 was a mess from aerodynamic point of wiev.


And it all owed to the turbocharger arrangement. I do remember a proposal for a higher altitude V-1710, with different gearing. I'm not sure if it had two speeds, and what the gearing was.


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## fubar57 (May 27, 2019)

Zipper730 said:


> That I'm surprised about, especially with the Spitfire seeming even skinnier than the Spitfire



LOL....Wut?

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## Zipper730 (May 27, 2019)

Fubar57,

Good catch: I fixed it.


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## tomo pauk (May 28, 2019)

Zipper730 said:


> I thought the intake produced fairly poor ram compression compared to other aircraft like the P-40?



Yes, ram air intake was not exactly a strong point on the P-39 with regard to engine-related ram effect, though it improved streamlining - the 'no free lunch' rule applies as ever.



> That I'm surprised about, especially with the Spitfire seeming even skinnier than the P-39 (from what I grasp, the thin fuselage was supposed to avoid changing the direction of the airflow excessively, which produces minimal turbulence, and that keeps the airflow over the horizontal and vertical stabs good, and makes the surfaces able to be smaller).



No worries, P-39 was one skinny fighter.



> And it all owed to the turbocharger arrangement. I do remember a proposal for a higher altitude V-1710, with different gearing. I'm not sure if it had two speeds, and what the gearing was.



I'm not sure where the statement ends, and the question starts here.


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## Elmas (May 28, 2019)

By my personl point of view, P-39 was a mess not for aerodynamics, but for the distribution of the masses, expecially consumables.
That affected quite a lot Static balance (Moment of first order, expecially after ammunition expended) but even more the Moment of second order, namely the "Moment of inertia" of the airplane, making handling tricky, to say the least.

Moment of inertia - Wikipedia


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## Zipper730 (May 28, 2019)

tomo pauk said:


> No worries, P-39 was one skinny fighter.


True, I actually thought of a drawing project that involved "Spitfiring" a P-39. The idea was to put elliptical wings on it.


> I'm not sure where the statement ends, and the question starts here.


What gear-ratios were proposed for the higher altitude V-1710? Was it single or twin-speed?



Elmas said:


> By my personl point of view, P-39 was a mess not for aerodynamics, but for the distribution of the masses, expecially consumables.


What changes could have been made? I had actually thought of creating a thread about theoretical ways WWII airplanes could have been made better with the knowledge of the time.


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## tomo pauk (May 28, 2019)

Zipper730 said:


> True, I actually thought of a drawing project that involved "Spitfiring" a P-39. The idea was to put elliptical wings on it.



I've never heard about such a proposal. Any sources?



> What gear-ratios were proposed for the higher altitude V-1710? Was it single or twin-speed?



Early altitude-rated V-1710s have had supercharger gear ratios of 8.77:1, that went to 8.80:1. Such engines were a bit better, for example, than DB 601A or M-105 in altitude power.
Gearing was changed to 9.60:1 to improve altitude performance, basically making it comparable with Merlin III on a test stand (the V-1710 having better carb and exhausts than early Merlins, so in everyday use such V-1710s might give a bit better altitude power, that was sorely needed for the heavy P-39/40/51).
Problem with such V-1710s was that initial prototypes have had one of the shafts involved being to weak for the horsepower needed to turn the S/C at greater rpm. Meaning that such V-1710s became available by some time of Autumn of 1942, or about 4 years after Merlin III entered service.
1-stage supercharged V-1710s were always with 1-speed S/C gearing in-service, there was a few prototypes with 2-speed S/C gearing.
2-stage supercharged V-1710s were with 1-speed gearing for engine-stage S/C, while the auxiliary S/C was driven via hydraulic coupling, thus it was variable speed within a specified range.


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## Zipper730 (May 28, 2019)

tomo pauk said:


> I've never heard about such a proposal. Any sources?


I said I thought of a drawing project. I was the person who was proposing to draw it up...


> Early altitude-rated V-1710s have had supercharger gear ratios of 8.77:1, that went to 8.80:1. Such engines were a bit better, for example, than DB 601A or M-105 in altitude power.
> Gearing was changed to 9.60:1 to improve altitude performance, basically making it comparable with Merlin III on a test stand (the V-1710 having better carb and exhausts than early Merlins


So that would have produced a critical altitude of around 16,250 to 18500 feet with ram compression?


> Problem with such V-1710s was that initial prototypes have had one of the shafts involved being to weak for the horsepower needed to turn the S/C at greater rpm.


So that's why it was cancelled? The shaft that the supercharger was connected to was too weak?


> Meaning that such V-1710s became available by some time of Autumn of 1942, or about 4 years after Merlin III entered service.


I thought the P-63 had a two-stage supercharger with a hydraulic clutch for the aux-blower?


> 1-stage supercharged V-1710s were always with 1-speed S/C gearing in-service, there was a few prototypes with 2-speed S/C gearing.


I guess they didn't pursue this due to shaft strength?


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## pbehn (May 28, 2019)

The P-39 was years after the Spitfire, it should be better in every respect.

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## Zipper730 (May 28, 2019)

pbehn said:


> The P-39 was years after the Spitfire, it should be better in every respect.


True but there were ways the P-51 was beaten by the Spitfire and it flew later.


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## pbehn (May 28, 2019)

Zipper730 said:


> True but there were ways the P-51 was beaten by the Spitfire and it flew later.


True but mainly because of the focus of the design. In terms of aerodynamics there were huge advances especially in USA between the first flight of the Spitfire and the other two.


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## Shortround6 (May 28, 2019)

Zipper730 said:


> I guess they didn't pursue this due to shaft strength?



I believe it was the strength of the gears themselves. The first engines with 9.60 gears were supposed to be delivered in Nov-Dec of 1941? However the solution to the problem was to make the area of the block holding the gears (or the gear case cover?) longer so wider gears which could spread the load/force over more tooth area could be fitted. Note that you cannot reffit and old engine with the new gears, although it might be possible to fit the old gears in a new engine (perhaps using spacers?) Anyhow, the changes to block castings took a while to implement and that is what caused the delay in service use. 


Zipper730 said:


> I thought the P-63 had a two-stage supercharger with a hydraulic clutch for the aux-blower?


it was a hydraulic torque converter which is not quite the same thing. It was variable speed much like the torque converter used on the DB engines.


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## Zipper730 (May 29, 2019)

Shortround6 said:


> I believe it was the strength of the gears themselves.


What gear ratio did the early Merlins operate at?


> the solution to the problem was to make the area of the block holding the gears (or the gear case cover?) longer so wider gears which could spread the load/force over more tooth area could be fitted.


Was that a normal practice?


> it was a hydraulic torque converter which is not quite the same thing.


What is the difference?


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## wuzak (May 29, 2019)

Zipper730 said:


> What gear ratio did the early Merlins operate at?



Which early Merlin?

Merlin 

The ratio varied a bit. But it is not directly comparable to the V-1710 as the supercharger was larger (10.25" for most IIRC vs 9.5").


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## wuzak (May 29, 2019)

Zipper730 said:


> What is the difference?



Pratt & Whitney 2 stage engines used a hydraulic clutch to change supercharger drive gears.

The torque converter/fluid coupling drove the supercharger directly.


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## Elmas (May 29, 2019)

Zipper730 said:


> _omissis_
> What changes could have been made? I had actually thought of creating a thread about theoretical ways WWII airplanes could have been made better with the knowledge of the time.



No changes, unfortunately. No matter how pretty P-39 could have been, the mass distribution was flawed, and there were no modifications that could have improved the design.
USAAF was more than happy to get rid of it at the first useful opportunity, giving them as a present to a low-ranking Allies...


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## pbehn (May 29, 2019)

Elmas said:


> No changes, unfortunately. No matter how pretty P-39 could have been, the mass distribution was flawed, and there were no modifications that could have improved the design.
> USAAF was more than happy to get rid of it at the first useful opportunity, giving them as a present to a low-ranking Allies...


Even if the mass distribution was OK, if you change the engine substantially it isn't and you need a new airframe.


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## Elmas (May 29, 2019)

Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".


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## pbehn (May 29, 2019)

Elmas said:


> Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
> I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
> But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".


I was thinking more along the lines of fitting the Merlin, most allied aircraft had some engine change or other, with the P-39 it required a complete re design to do it.


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## Shortround6 (May 29, 2019)

I don't know why people want to reinvent the wheel. 

The P-63 was a redesigned P-39. It solved the handling problems.

it also used no parts in common with the P-39 except screws and rivets.

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## pbehn (May 29, 2019)

Shortround6 said:


> I don't know why people want to reinvent the wheel.
> 
> The P-63 was a redesigned P-39. It solved the handling problems.
> 
> it also used no parts in common with the P-39 except screws and rivets.


Could you just slot a Griffon or Sabre in there Mr Bell.


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## wuzak (May 29, 2019)

Elmas said:


> Even without the turbocharger P-39 had a C.G dangerously aft, so I can imagine how it could be with turbocharger in the rear of the fuselage.
> I'm inclined to think that stripping P-39 of the turbo was a technique to get an aeroplane "more or less" flyable at the expense of high flying performance.
> But it is not only , as I stated before, just a matter of "static balance" but also of "dynamic balance".



The turbo was below the engine, not in the aft fuselage.


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## Elmas (May 29, 2019)

wuzak said:


> The turbo was below the engine, not in the aft fuselage.



Of course. But it was always _behind_ the C.G.


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## Zipper730 (May 29, 2019)

Shortround6 said:


> The P-63 was a redesigned P-39. It solved the handling problems.


Why didn't the USAAF buy the P-63?


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## Zipper730 (May 29, 2019)

wuzak said:


> Pratt & Whitney 2 stage engines used a hydraulic clutch to change supercharger drive gears.


So that was about friction reduction?


> The torque converter/fluid coupling drove the supercharger directly.


Okay, I got you


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## wuzak (May 29, 2019)

Zipper730 said:


> So that was about friction reduction?



No, it operated like a friction clutch.


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## Zipper730 (May 29, 2019)

wuzak said:


> No, it operated like a friction clutch.


I'm confused...


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## pbehn (May 29, 2019)

Zipper730 said:


> Why didn't the USAAF buy the P-63?


To replace what? Months before the start of the jet age?


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## Shortround6 (May 29, 2019)

Zipper730 said:


> Why didn't the USAAF buy the P-63?


Who do you think paid for over 3300 of them? 

Design work started in early 1941, The US army ordered two Prototypes in June of 1941. 

Out of 3300 the Russians got just over 2400.

and NO, not all of the rest were the famous pinball target aircraft.

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## wuzak (May 29, 2019)

Zipper730 said:


> I'm confused...



Rolls-Royce used friction clutches (like what is used in a manual/stick shift car) to change supercharger gears. One for each gear. To change the gear the clutch in the current gear is released, while the one for the other gear is engaged, slipping as the supercharger is brought to the speed of the new gear.

Pratt & Whitney used fluid couplings to do the same thing. Once the supercharger is up to speed the fluid coupling does not vary its ratio.

Rolls-Royce engineered a small amount of slippage in the supercharger drive clutches to reduce the effects of torsional vibration from the crankshaft affecting the supercharger gears (that is, reduced shock loadings). The fluid coupling in Pratt & Whitney's superchargers would naturally have some slippage. 

A variable speed fluid coupling has a variable speed ratio. This is done by controlling the amount of fluid on the casing. This is different to a torque converter, which varies its speed ratio depending on load.

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## chris mcmillin (May 30, 2019)

What I always thought was the turbo in a ventral scoop with intakes for the inter/after coolers and an oil to coolant heat exchanger for the oil cooling and the deeper profile allowing a better velocity drop for the radiators and a controlled exit like the NAA design. 
This way the original fuse shape would be unchanged but the power would be maintained and cooling drag optimized if not reduced.

Fuel capacity and wing armament would still be a problem. With the wing as thin as it is, the armament could be removed and fuel capacity increased by another cell in the ammo and breech areas. Two .50's and a 37 mm seemed to be capable enough for those high scoring Russian guys! (I'm pretty sure the Airacobra is the highest scoring allied fighter, at 50 kills.)

Oh, maybe wing pylons for drops too. In the high 20's fuel burn is really low, like 45 gph at 300 miles true. Two 50 gallon drops would be more than 2 hours endurance increase. An escort fighter before the Mustang!

Chris...


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## Shortround6 (May 30, 2019)

Might have been faster at high altitude, was 30mph slower at low altitudes. This was a mock up. 
P-39s were running on the margin for cooling both the glycol mixture and and the oil as it was. Trying to cool an 1150-1425hp engine at 25,000ft needs bigger radiators or a lot of attention to the radiators and ducts. 

The space in the outer wings doesn't work well for fuel if you are using self sealing tanks. the tanks wind up rather thin and spread out for the amount of fuel they hold meaning even the empty tanks are rather heavy. 

You are also forced into using under wing tanks/bombs, perhaps not a bad thing but that much more stuff to sort out. 

The P-39 was already a pint and a half (750ml) in a pint (500ml) bottle, trying to stuff the turbo in means you are going for the full quart (1000ml) in the pint (500ml) bottle.


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## chris mcmillin (May 30, 2019)

Elmas said:


> By my personl point of view, P-39 was a mess not for aerodynamics, but for the distribution of the masses, expecially consumables.
> That affected quite a lot Static balance (Moment of first order, expecially after ammunition expended) but even more the Moment of second order, namely the "Moment of inertia" of the airplane, making handling tricky, to say the least.
> 
> Moment of inertia - Wikipedia




I dont see any real evidence for balance problems, more like hearsay and myth written by uninformed writers.

The cannon brass is retained after expending the lead, the cannon and guns are always retained, the balance is figured for full and empty on all consumables, ammo, fuel, drops, gear up and down. Just to add, this is a front line fighter that near 10,000 were built. All of these fantasy aft CG problems were nothing but, real engineers built this fighter, real test pilots did the engineering flight test, it wasnt pulled out of someone's rear end... like much that is written about them certainly. The idea it was as you describe is ridiculous.

When I read these handling reports from keyboard jocks online, I always wonder how much time they have in the Airacobra. You? How much time in any airplane? 
If not just your opinion, how about a paper written by a combat pilot or commander? Like Kit Carson did, better yet the paper by Buzz Wagner. Real pilots, in era, flying combat airplanes...

The problems with the Airacobra were spares, gasoline quality and lack of support in the PNG. Any problems it had were doubled by these things. By comparison, the P-51 had many problems that had engineering resources thrown onto it plenty fast because of it's time period and political position because of high ranking AAF people pushing it. Shedding tails, shedding wings, and actual aft CG instability were addressed quick and plenty. Not so with the Bell contingent in 1942.

Chris...


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## pbehn (May 30, 2019)

Fubar, put your overalls on, we are swapping tanks and guns again.


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## chris mcmillin (May 30, 2019)

Shortround6 said:


> Might have been faster at high altitude, was 30mph slower at low altitudes. This was a mock up.
> P-39s were running on the margin for cooling both the glycol mixture and and the oil as it was. Trying to cool an 1150-1425hp engine at 25,000ft needs bigger radiators or a lot of attention to the radiators and ducts.
> 
> The space in the outer wings doesn't work well for fuel if you are using self sealing tanks. the tanks wind up rather thin and spread out for the amount of fuel they hold meaning even the empty tanks are rather heavy.
> ...



No, no, I meant a real ventral scoop designed with the engineering elegance of the NAA west coast guys. You obviously missed my point. One, okay... P-51D shaped, ventral scoop architecture in which turbo, inter and after coolers and glycol radiator was internally ducted, with proper, low drag intake and adjustable outlet. Oh, and sorry, the 2,200 hp P-51H's -9 had the oil cooled by the coolant/oil heat exchanger. Eliminated weight and drag and area of another radiator. 

See, I've actually installed thin bags. These fuel tanks are actually real. It could've been done, that is why I wrote what I wrote. What, belts and rails aren't heavy, along with four rifle caliber useless guns, give me the gas. 

Under wing stores are not something odd.

Yes, the airplane was little and that is why it turned out the way it did. I'm just giving my 2 cents, and they are shiny.

Chris...


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## Shortround6 (May 30, 2019)

There may be difference between modern fuel bladders and WW II self sealing tanks/bags. I am not sure that putting in non-self sealing bags was a good idea. 
Self sealing means it will take more than one hit from a RCMG and after leaking for a short period of time it will seal up the hole/s. 
Most AiraCobras were loaded with about 300 rounds per gun for those useless guns, (which were more powerful, faster firing and had a shorter time of flight than the cowl guns in a Zero or the RCMG in the Ki-43) not the full capacity 1000 rounds (which would take 50 seconds to fire at best rate) and that means about 75-78lbs of ammo. That is a whopping 13 US gallons of fuel. the guns were under 95lbs, another 16 gallons (rounded up).

As far as the ammo weight in the nose goes, for 200rpg for two guns, if I have done the math right, the plane has fired 43lb of bullets (worst case/heaviest bullets) and used up around 13.5-14lbs of powder. since it kept the cases in a collector box it kept around 54lbs of fired brass. I have no information on the 37mm ammo. 

There may have been a prohibition about flying the plane with *no* ammo but since the Brass and links account for about 1/2 the ammo weight any test that does not include that amount of ballast may be suspect. 

A WW II book on engines and engine installations used 10 cubic ft as an estimate for the amount of room needed for a turbo installation for a 1000hp engine. This does not include the radiators or oil coolers, just the turbo, the intercoolers and the ducting. You are trying to stuff an awful lot of things into that P-51 style duct.

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## chris mcmillin (May 30, 2019)

As for the wings: They don't seem unusually thin, though the wing-area probably is somewhat small (that said, I'm not sure how good a turning fighter it was).

The Airacobra wing was too thin to mount .50 cal M2's in, so I figure it was pretty thin.

I have watched them perform at airshows in the 70's when they were using some big G's, they rotate at an impressive rate. Same with the King Cobra, very small turning circle in a 250 kt overhead approach demonstration. 

Chris...


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## johnbr (May 31, 2019)

Bell XP-39 Airacobra Archives - This Day in Aviation

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## johnbr (May 31, 2019)



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## Elmas (May 31, 2019)

chris mcmillin said:


> I dont see any real evidence for balance problems, more like hearsay and myth written by uninformed writers.
> 
> The cannon brass is retained after expending the lead, the cannon and guns are always retained, the balance is figured for full and empty on all consumables, ammo, fuel, drops, gear up and down. Just to add, this is a front line fighter that near 10,000 were built. All of these fantasy aft CG problems were nothing but, real engineers built this fighter, real test pilots did the engineering flight test, it wasnt pulled out of someone's rear end... like much that is written about them certainly. The idea it was as you describe is ridiculous.
> 
> ...



And you, have you got an idea of what a "Moment of inertia" is?
As a M. Sc in Structural engineering I can understand very well problems about C.G. and I worked in the the design of small airplanes...
This one is the latest, for example

Ultralight biplane

of wich I did all calculations to find a suitable C.G.
This plane flies beautifully, but unfortunately I have not the permission of the Owner to publish photos or videos in flight.

In 1944 P-39s were issued to Italian Cobelligerent Air Force, all formed by well seasoned Pilots, survivors of three years of war against overwhelming Air forces.
One of these Pilots said in an interview, that is on the Youtube, in italian, of course:
_"When we were assigned to P-39, we were very upset, we could not believe Allied used an airplane that was so dangerous. I owe my life to the fact that, to the contrary of what we Italian Pilots were used to do, I never attempted to perform aerobatics with P-39.
And to add insult to injury, other Italian Pilots, still on Macchi 205 or in very old Spitfire V, broke our balls by calling us "i camionisti" (the truck drivers) for the car style door of the airplane..."_
In this book you will see the (very poor..) esteem Italian Pilots had of P-39...




And for me that's enough in this thread...


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## chuter (May 31, 2019)

Shortround6 said:


> ...
> As far as the ammo weight in the nose goes, for 200rpg for two guns, if I have done the math right, the plane has fired 43lb of bullets (worst case/heaviest bullets) and used up around 13.5-14lbs of powder. since it kept the cases in a collector box it kept around 54lbs of fired brass. I have no information on the 37mm ammo.
> 
> There may have been a prohibition about flying the plane with *no* ammo but since the Brass and links account for about 1/2 the ammo weight any test that does not include that amount of ballast may be suspect.
> ...




Yes, there was a prohibition. With full ammo load CG was at max fwd, with spent ammo (ammo cases and links) CG was at max aft. Russia's famous issues with spins was due to them tossing the included flight manuals (English) and test flying the newly assembled aircraft straight away without ammo or ballast. It was some time before a Bell rep showed up and, much to his alarm, discovered what they were doing.


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## chris mcmillin (May 31, 2019)

Shortround6 said:


> There may be difference between modern fuel bladders and WW II self sealing tanks/bags. I am not sure that putting in non-self sealing bags was a good idea.
> Self sealing means it will take more than one hit from a RCMG and after leaking for a short period of time it will seal up the hole/s.
> Most AiraCobras were loaded with about 300 rounds per gun for those useless guns, (which were more powerful, faster firing and had a shorter time of flight than the cowl guns in a Zero or the RCMG in the Ki-43) not the full capacity 1000 rounds (which would take 50 seconds to fire at best rate) and that means about 75-78lbs of ammo. That is a whopping 13 US gallons of fuel. the guns were under 95lbs, another 16 gallons (rounded up).
> 
> ...



The fuel bag outer dimension would be the same as the internal dimension of the space opened by removing the guns and ammo stuff, modern or self sealing. The goo layer is on the inside of a self sealing tank and the thing would be filling a cubic space. At 6.5 lbs per gallon, I'm not sure how much cubic space a gallon is, but 25 gallons is a half an hour at high altitude cruise power and if both wings had an extra 25 gallons, that is a lot of performance extension.

Interesting about the .30's. I didn't know that about their ballistics. I do know a lot of Russians removed them and just used the nose armament, kind of like Mölders and Galland did on 109F's

I'm not sure the weight of the ammo and equipment in the wings is a direct 1 to 1 weight transfer to fuel. Would love to know fuel it would hold. It was 25 gallons in each wing bag on Howie Keefe's Mustang and it did not take advantage of the whole interior of the wing ammo bay.

The nose weight stuff is interesting and very close to what I thought in terms of change after depletion of ammo. Is the .50 brass bagged or ejected on the Airacobra? I dont know.

True, but the existing radiator stuff in the wing center section would be removed and redesigned in my fantasy. It would all fit, and be cleaner and have better efficiency in coolant and oil cooling. The turbo was already there in the XP so it fits in a non-optimized way anyway. That short path, constant diameter intake and crappy little exits for the little round oil cooler and radiators was so bad, it is like Curtiss engineers came over to design it. (The P-40 is the same, such a quick overheater on the ground Evergreen had a spray bar system on theirs for taxiing on modern airports.) It could've been done and much better than the crap you see pictures of. That Jules Vern experimental crap Bell did is so weird one wonders how they hit the perfect set-up on the Airacobra, King Cobra and X-1!?
Chris...


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## chris mcmillin (May 31, 2019)

Yes, as a matter of fact I do. I express it in an actual, practical manner when I go to work and fly the big jets. With an ATP certificate and 30,000 hours over a life spanning flying since my first logbook entry at 9 signed off by my father, my experience indicates a typical CG range and handling similar to all types of WWII airplanes for the Airacobra. Also when I fly warbirds, or my own aerobatic airplane or antique. Having flown many types, many times, while in the present and growing up while actually speaking to those that did fly the fighters so spoken about here first hand, I have a very practical idea about the CG change in flight and how a mid engined fighter would fly after it depleted its ammo. Which would be quick and sporty from start to finish with not enough change to matter to the pilot flying a fighter in combat. The BS was left behind when speaking to these men. The metal is the same today.
Mr. Martin sold his P-39 to Mr. Randall who was owner of Fender guitar on our airport, for a sporty aerobatic plane. He delighted in flying it.

Meanwhile to the combat point the Americans had already proven the type in PNG and the Russians had super star aces killing Bf and FW aces with them. Still the highest kill count of any allied fighter.
Strange anecdote with zero context in the actual reality of it. Especially with the experience and fuel available the Italians that late in the war. You would think they would be getting 1500 hp with 150 octane and know the proper loading to get the most out of it as a bomber, and a fighter after bomb release. No reasons are offered, just like your original premise offered nothing but opinion.
Chris...



Elmas said:


> And you, have you got an idea of what a "Moment of inertia" is?
> As a M. Sc in Structural engineering I can understand very well problems about C.G. and I worked in the the design of small airplanes...
> This one is the latest, for example
> 
> ...


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## Elmas (May 31, 2019)

chris mcmillin said:


> _omissis_
> Mr. Martin sold his P-39 to Mr. Randall who was owner of Fender guitar on our airport, for a sporty aerobatic plane. He delighted in flying it.
> _omissis_



With 37 mm cannon, HMGs, complete armour, full ammo, protected fuel tanks and a full load of gas onboard?


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## chris mcmillin (May 31, 2019)

Elmas said:


> With 37 mm cannon, HMGs, complete armour, full ammo, protected fuel tanks and a full load of gas onboard?


Obviously not. But the controls the same size and ratios, moments the same dimension, cg range the same limits. So you are thinking properly balanced the airplane handled well? 
Chris...


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## Elmas (May 31, 2019)

chris mcmillin said:


> Obviously not. But the controls the same size and ratios, moments the same dimension, cg range the same limits. So you are thinking properly balanced the airplane handled well?
> Chris...



It is in that "moments the same dimension" that I see that you have, please forgive me, I should say, a rather vague idea of what a Moment of Inertia is, however full can be your aeronautical career, for wich I have the deepest esteem.

No problem, between Pilots and Engineers relationships have not always been idyllic.

Moment of Inertia is a _Mass multplied for a distance squared_, so even very small variations in the distance from C.G. of a mass give marked differences in a Moment of second order.
In this example all masses are concentrated in the discs at the end of the arms, let's assume arms have a mass = 0.






All the three sistems are equilibrated and have the same mass, but the torque necessary to put the three in motion and rotate is quite different, depending, as I already said, by the square of the distance. So, if I double the distance, I must have four times the torque and so on.

Why C-5 has wings bent this way?



Because it is so big, and the longitudinal Moment of inertia so high, that a certain degree of "instability" is necessary to allow manouvers without using ailerons as big as two thirds of the wing.

In P-39 masses were already distributed, from the start, in the worst possible way, as all airplanes of WWII era with unconventional mass distribution, like Saab 21, born as an interceptor and soon relegated to ground attack. Bell engineers bet, and loose: had not been P-39 such a pretty airplane, certainly we would not have been here arguing about his qualities and defects.

I have no doubt that a P-39, stripped of one or two tons, with the necessary ballast strategically put where it was most needed, could have been a very good "sport" airplane: it is as a practical weapon for a _modern Western Air Force,_ where the Pilot's life was the most precious thing (Russians were able to sustain losses much more heavy than those USAAF and RAF could, and a couple of dozens of Pilots dead for a flat spin meant nothing for them), that P-39 was at fault.
I completely agree with USAAF Top Brass (for which prettiness af an airplane counted nothing... and not to speak of RAF) that wanted to get rid of P-39s ASAP...


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## Shortround6 (May 31, 2019)

chris mcmillin said:


> Especially with the experience and fuel available the Italians that late in the war. You would think they would be getting 1500 hp with 150 octane and know the proper loading to get the most out of it as a bomber, and a fighter after bomb release.



I am not sure anybody in the Mediterranean theater was getting 150 octane fuel unless it was in the last few months of the war. Let alone the minor allies. 
The P-39 didn't need it in any case. It could pull over 1400hp on 100/130 and the supercharger wasn't going to give you any more air over 9000ft. 150 octane might have allowed a bit more power at low altitudes (under 5000ft?) the 150 fuel prevented detonation, it didn't make the engine parts any stronger.


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## chris mcmillin (May 31, 2019)

Elmas,
Quite right on all of your engineering of which we have had little difference.

Engineers and pilots, yes the MD-80 cockpit engineers I hope spend eternity in one!

One thing in which I would counter is the assumption that the airplane was to be made easy to fly by a new, low time pilot without adequate training. I would say none of the WWII airplanes were necessarily hard to fly with adequate training at full weight, bar bell effects of the the moments of inertia at those high weights that you describe and all. But, at the same time once the airplane was close to or slightly outside of those parameters of controlled flight when at full gross weight that exacting and quick control was necessary to correct the airplane back to controlled flight with the airplane's rather large area control surfaces. An analogy could be made to comparing Mr. Randall's converted TP-39Q sport ship to a regular interested sports car driver in an Indy car at 195 mph well trimmed for downforce and proper mechanical trim. Easy to go this fast within the speed parameters for the more or less regular guy. The fully loaded Airacobra for battle is like running an Indy car full tilt at say 230 mph laps trying to exact every ounce of speed out of it sacrificing handling a bit to get it and the top driver using his fine abilities to keep it underneath him and out of the wall. Not all men have the same abilities, but top men could keep the tool useful.
Maybe the Macchi flies like a Cub! My friend says the Spitfire is a dream.
Chris...


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## chris mcmillin (May 31, 2019)

Shortround6,
Is the Allison -93 that low in critical altitude? My friend has one from a P-39 in his Jurca Spitfire replica and I thought it was around 16,000.
Probably right about 150, and 100/130 was plenty to avoid the heat soak problems leading to detonation with which the poor PNG guys had to deal. 

Still why the Italians crying over the airplane and exaggeration of its handling and performance limitations? Politically posturing for Thunderbolts or Mustangs? Carrying 1000 pounders on each wing? Or one on the centerline? Improper loading accidents causing an overabundance of caution by commanders? Elmas didn't elaborate.

Chris...


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## Shortround6 (May 31, 2019)

chris mcmillin said:


> Is the Allison -93 that low in critical altitude? My friend has one from a P-39 in his Jurca Spitfire replica and I thought it was around 16,000.


Which critical ALtitude?

The engine will give 1125hp at 15,500ft _military power. _at 3000rpm and 44.5in MAP, the supercharger is maxed out, there is no more manifold pressure to be had. This is with the 9.60 supercharger gears. At 9,000ft the supercharger could supply 57in of boost and that was the critical altitude for _WEP. _Once you are over 9000ft the pressure starts dropping even at full throttle. This is what the books/charts say but some of them don't agree with each other and/or confuse altitudes with and without RAM. 
Obviously the pilot could open the throttle more (if the supercharger was adjusted to permit it) at even lower altitudes and get more pressure/power but 57in was Allison's and the Army's instructions/recommendations were not to do so as the 9.60 gears heated the air up more than the 8.80 gears and the pressures that were "safe" with the 8.80 gear engines might not be save with the 9.60 gear engines. 

BTW most US self sealing tanks had at least 3 layers, the outer layer which was on the hard side to resist abrasion, the sealing layer of "goo" and an inner layer to keep the gasoline and "goo" separate. It was a reaction between the gasoline and the "goo" that caused the swelling and the goo to stiffen up and plug the whole, A reaction you don't what going on in normal flying. This a is a very general statement as actual construction and materials used in the tanks varied from aircraft type to aircraft type and in some cases form year to year or changes were were made after a certain production batch. P-40s for instance changing from a lined metal tank to a non metallic tank. But since the tanks were in spaces in the wing and behind the seat the non metallic tank had to hold it's shape with little or no support from the structure (aside from brackets). This was after the P-36 and very early P-40s used plain metal tanks. British P-40s got tanks wrapped in a treated leather coating (?) 
PBYs had integral tanks in the wing and when fitted with self sealing fuel cells lost hundreds of gallons of capacity. The fuel tanks could be opened up and the self sealing cells dropped into the space. One manual even gives fuel capacity for one side of the plane using the original fuel tank and the other fitted with the self sealing cells. It also gives capacity for both tanks "bare" and both tanks self sealing. 
There were some major advances in "rubber" technology during and after the war that allowed for new, thinner, flexible"rubber" fuel cells to be fitted to a number of private and commercial aircraft. Most warbirds got rid of the original self sealing tanks as they deteriorate with age and "debris" clogs up the fuel filters and lines.


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## P-39 Expert (Jun 2, 2019)

The -93 was the two stage model used in the P-63, critical altitude over 20000', right?

Chris, be careful talking about the P-39 on here, most still believe that if one climbed over 12000' that the engine blew up and the plane flipped into a tumble. Or that the Russians used it as a tank buster. When the cannon didn't misfire.

The P-39 didn't need a turbo and the installation would never have worked properly anyway. 

All the early P-39 (D through L) needed was less weight easily accomplished by removing the peashooter wing guns (200# including ammo, ammo boxes, mounts, chargers and heaters) and the 100# nose armor plate (move the radios from the tail cone to above the engine right behind the pilot for balance). That alone gives you another 350-400 feet per minute climb at all altitudes. Only about 2000 of these (D-L) were manufactured before switching to the -85 engine with the 9.6 gears which gave another 100HP over about 15000'.

The P-39N (late '42) didn't need anything. It would outclimb anything in combat in '43 except a two stage Merlin Spitfire which it never faced. Dump the wing guns and the nose armor and it would have been even better.

Don't get me started on the Q with those gondola wing guns. Waste of a perfectly good airplane. Russians had the right idea, dump the wing guns and performance improves dramatically.


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## Shortround6 (Jun 2, 2019)

P-39 Expert said:


> Chris, be careful talking about the P-39 on here, most still believe that if one climbed over 12000' that the engine blew up and the plane flipped into a tumble. Or that the Russians used it as a tank buster. When the cannon didn't misfire.




Jeez, get it right.
over 10,000ft and engine just stopped running, could usually be restarted at under 7,000ft though.
The flipping tumble was totally independent of altitude and most prevalent at balked landing.
The tumble was just part of the trouble, flipping into an inverted flat spin was the real killer. 
The Russians knew better than to use it a tank buster as the 37mm shells bounced of anything thicker than sheet metal like ping pong balls bounce off a concrete wall. 

We all know that the P-39 was the greatest aircraft of the war, and had Bell (known for honesty and integrity and 100% accurate performance estimates) only bribed the government officials like the the airplane makers the P-39 would have won the war on it's own.


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## P-39 Expert (Jun 3, 2019)

Hilarious. Regarding Bell, I have a close friend in the steel business and he has told me that the purchasing department won't even let him quote any of their contracts. He's sure that they are fishing for a little kickback from him or his company. I'm sure that goes on pretty much everywhere there are government contracts. BUT I AM SURE THAT BELL'S DEALINGS WITH THOSE BRITISH CRIMINALS WAS ENTIRELY ABOVE BOARD. THEIR INSISTENCE ON PUTTING IN THOSE DAMN .30 CALIBER GUNS RUINED THE REST OF THE ENTIRE P-39 PRODUCTION RUN. ROLL BRITTANIA MY ASS. LIMEY BASTARDS. Rant over.


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## wuzak (Jun 3, 2019)

P-39 Expert said:


> Hilarious. Regarding Bell, I have a close friend in the steel business and he has told me that the purchasing department won't even let him quote any of their contracts. He's sure that they are fishing for a little kickback from him or his company. I'm sure that goes on pretty much everywhere there are government contracts. BUT I AM SURE THAT BELL'S DEALINGS WITH THOSE BRITISH CRIMINALS WAS ENTIRELY ABOVE BOARD. THEIR INSISTENCE ON PUTTING IN THOSE DAMN .30 CALIBER GUNS RUINED THE REST OF THE ENTIRE P-39 PRODUCTION RUN. ROLL BRITTANIA MY ASS. LIMEY BASTARDS. Rant over.



Get it right....

It's RULE BRITANNIA.


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## Shortround6 (Jun 3, 2019)

P-39 Expert said:


> BELL'S DEALINGS WITH THOSE BRITISH CRIMINALS WAS ENTIRELY ABOVE BOARD. THEIR INSISTENCE ON PUTTING IN THOSE DAMN .30 CALIBER GUNS RUINED THE REST OF THE ENTIRE P-39 PRODUCTION RUN




You may want to rethink that and get off the anti-british track.
"USAAC was generally pleased with the Airacobra, and an initial order for 80 production examples (Bell Model 13) was issued on August 10, 1939 under Contract AC13383. "
"Armament was one 37-mm cannon, two 0.50-inch and two 0.30-inch machine guns, all in the nose."

So the British just moved two of the .30 cal guns from the nose to the wings, and since they were using a cannon over 100lbs lighter than the American cannon, added another 24lb machine gun in each wing. Somehow this lighter weight armament than the US was using ruined the rest of the Production run? 







P-39C


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## P-39 Expert (Jun 3, 2019)

wuzak said:


> Get it right....
> 
> It's RULE BRITANNIA.


I thought it was "Roll" because of their sea heritage. Good to know.


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## P-39 Expert (Jun 3, 2019)

Shortround6 said:


> You may want to rethink that and get off the anti-british track.
> "USAAC was generally pleased with the Airacobra, and an initial order for 80 production examples (Bell Model 13) was issued on August 10, 1939 under Contract AC13383. "
> "Armament was one 37-mm cannon, two 0.50-inch and two 0.30-inch machine guns, all in the nose."
> 
> ...


I think the Army would have wised up and deleted those nose .30s to restore the full 30 rounds for the 37mm cannon. Having the two nose .30's reduced the cannon armament to 15 rounds.


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## pbehn (Jun 3, 2019)

P-39 Expert said:


> I thought it was "Roll" because of their sea heritage. Good to know.


Its a song.


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## P-39 Expert (Jun 3, 2019)

Potato/poTAHto.


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## wuzak (Jun 3, 2019)

P-39 Expert said:


> I thought it was "Roll" because of their sea heritage. Good to know.



It is related to their sea heritage. The chorus is:

Rule, Britannia! Britannia, rule the waves!
Britons never, never, never shall be slaves.


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## Shortround6 (Jun 3, 2019)

P-39 Expert said:


> I think the Army would have wised up and deleted those nose .30s to* restore* the full 30 rounds for the 37mm cannon. Having the two nose .30's reduced the cannon armament to 15 rounds.



There was no "*restore*" the full 30 rounds. The YP-39s (the model right before the C, there were no As or Bs) that had guns (most were completed without guns and some had the gins added later), had the 37mm with 15 rounds (the 30 round capacity required a different feedway). the two .50 cal guns with 200rpg and two .30 cal guns with 500rpg. 
There was no early version of the P-39 with *just* the 37mm and two .50s unless it was one of the YP-39s without the full armament layout. 

lets also remember that the USAAC told Bell (and other companies) what they wanted for guns, the companies might push back and say it won't fit or it will reduce performance by X amount, but the companies did NOT tell the USAAC what the armament _should_ be ( they could offer a certain set up in the sales pitch like Republic and the eight .50s in the P-47B).
Larry Bell pushed for the .30s to be taken out but then his company had lied about the basic airplane could do performance wise and he may have been looking for the last few percent of performance to keep up the numbers.


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## P-39 Expert (Jun 3, 2019)

wuzak said:


> It is related to their sea heritage. The chorus is:
> 
> Rule, Britannia! Britannia, rule the waves!
> Britons never, never, never shall be slaves.


I stand corrected. Good to know. Should I go back and correct my original post?


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## wuzak (Jun 3, 2019)

Shortround6 said:


> and some had the *gins* added later



Did they fit a cup holder, or was it fed through a tube?

And did it come with tonic?

Reactions: Like Like:
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## P-39 Expert (Jun 3, 2019)

Shortround6 said:


> There was no "*restore*" the full 30 rounds. The YP-39s (the model right before the C, there were no As or Bs) that had guns (most were completed without guns and some had the gins added later), had the 37mm with 15 rounds (the 30 round capacity required a different feedway). the two .50 cal guns with 200rpg and two .30 cal guns with 500rpg.
> There was no early version of the P-39 with *just* the 37mm and two .50s unless it was one of the YP-39s without the full armament layout.
> 
> lets also remember that the USAAC told Bell (and other companies) what they wanted for guns, the companies might push back and say it won't fit or it will reduce performance by X amount, but the companies did NOT tell the USAAC what the armament _should_ be ( they could offer a certain set up in the sales pitch like Republic and the eight .50s in the P-47B).
> Larry Bell pushed for the .30s to be taken out but then his company had lied about the basic airplane could do performance wise and he may have been looking for the last few percent of performance to keep up the numbers.


Absolutely the Army told the contractors what they wanted and the contractor built it. In any event, the 37mm cannon needed 30 rounds. And Bell was no more crooked than any other defense contractor, just smaller and less influential.


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## Shortround6 (Jun 3, 2019)

wuzak said:


> Did they fit a cup holder, or was it fed through a tube?
> 
> And did it come with tonic?




Ah, now we know why the British airacobras performed so poorly. it was the several hundred pound gin dispenser


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## wuzak (Jun 3, 2019)

P-39 Expert said:


> Absolutely the Army told the contractors what they wanted and the contractor built it. In any event, the 37mm cannon needed 30 rounds. And Bell was no more crooked than any other defense contractor, just smaller and less influential.



Because it couldn't hit the side of a barn from 20 paces?


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## Shortround6 (Jun 3, 2019)

P-39 Expert said:


> Absolutely the Army told the contractors what they wanted and the contractor built it. In any event, the 37mm cannon needed 30 rounds. And Bell was no more crooked than any other defense contractor, just smaller and less influential.




You said they restored the 30 round capacity, they didn't.
Doesn't matter if they needed 30 rounds or not (and 15 rounds would last the 37 M4 about 6 seconds, which is just a bit less than 60 rounds lasts a Hispano or 55-60 rounds lasts an MGFF/M or japanese type 99 mark I)

As to Bell's honesty, I can't think of another company that lied about the performance of a prototype aircraft to the extent that Bell did. Plenty of companies gave optimistic estimates of performance before the prototype flew (some were very optimistic). But Bell at least had few hours of flight testing before the XP-39 was shipped to Langley. There Is strong evidence that the prototype never flew at full power let alone achieved the performance numbers claimed for it. Langley said it wouldn't come close to the numbers from wind tunnel results and yet nearly a year later Bell was still advertising it as a 400mph airplane. Based on the performance estimates and not test results? 

BTW, Curtiss had to either refund or accept over 14,000 dollars less in payment when the 2nd XP-46 failed to meet the promised performance figures.


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## P-39 Expert (Jun 10, 2019)

So, those liars at Bell somehow got a second plane into production (P-63) before the end of the war. More lying?


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## Shortround6 (Jun 10, 2019)

P-39 Expert said:


> So, those liars at Bell somehow got a second plane into production (P-63) before the end of the war. More lying?




Imaterial.

The XP-39 never flew 400mph or even 390mph before being sent to Langley, It never climbed to 20,000ft in 5 minutes. There was a potential vibration problem with the driveshaft and they never ran the engine faster than 2600rpm before going to Langley. They used a different diameter drive shaft with different thickness walls after the XP-39 was rebuilt and on every P-39 after that. 
Yet they claimed the plane did 390mph and had that fantastic climb rate. If that is telling the truth then I'm a little green man from Mars. 
ANd then we have the rather modified plane the British tested at the Bell Factory to get within a few mph of the promised performance. Modifications that were never done to another military P-39 out of thousands built. More TRUTH????

I will let the Performance of the XP-76 off the hook as mere exuberance in the design phase since they didn't keep claiming the same performance after they flew the thing.


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## wuzak (Jun 10, 2019)

Shortround6 said:


> I will let the Performance of the XP-76 off the hook as mere exuberance in the design phase since they didn't keep claiming the same performance after they flew the thing.



XP-76/XP-39E or P-63?


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## Shortround6 (Jun 10, 2019)

Sorry. XP-77





Planned engine used a supercharger that never materialized (it had a supercharger, just not one that would give 400-500hp at 27,000ft) 
It wound up seriously overweight (sound familiar?) even after deleting the 20mm cannon through the prop hub,


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## fubar57 (Jun 10, 2019)

XP-76 was a waste any way


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## P-39 Expert (Jun 11, 2019)

fubar57 said:


> XP-76 was a waste any way


Way too heavy at 8900# and never got the 4 blade propeller. Six .50MGs AND a 37mm cannon. AAF should have just put the two stage engine in a P-39 already in production.


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## Peter Gunn (Jun 11, 2019)

P-39 Expert said:


> Way too heavy at 8900# and never got the 4 blade propeller. Six .50MGs AND a 37mm cannon. AAF should have just put the two stage engine in a P-39 already in production.


I didn't think a two stage supercharger would fit in the existing P-39 airframe.


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## P-39 Expert (Jun 11, 2019)

The P-39E used a P-39D fuselage. The area behind the engine where the second stage went was exactly the same size as the P-63 which had the second stage. The coolant expansion tank was reconfigured and moved forward between the pilot and engine to make room for the second stage. The increase in fuselage length was aft of the bulkhead forming the back of the engine compartment.


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