XP-39 Airplane in wind tunnel

[Shortround6]

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.

 

[chris mcmillin]

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...

 

[johnbr]

Bell XP-39 Airacobra Archives - This Day in Aviation

 

[johnbr]

 

[Elmas]

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...

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...

 

[chuter]

...
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.

 

[chris mcmillin]

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.

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...

 

[chris mcmillin]

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...

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...
View attachment 540022
And for me that's enough in this thread...

 

[Elmas]

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?

 

[chris mcmillin]

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...

 

[Elmas]

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...

 

[Shortround6]

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.

 

[chris mcmillin]

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...

 

[chris mcmillin]

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...

 

[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.

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.

 

[P-39 Expert]

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.

 

[Shortround6]

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.

 

[P-39 Expert]

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.

 

[wuzak]

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.

 

[Shortround6]

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