Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: lo-muf on November 10, 2002, 02:12:06 PM
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Hi, I have a question that follows me since some time, among other questions like: who I am, where are we going, what's the meaning of life. The question is: why the English, who had completely ruined a nice project of an average ground attack plane by fitting a Rolls Royce Merlin into a P-51B, did not the very same thing with a P-39 Airacobra (which they had under the name of P-400)? Yes, it's the well known "what if" territory but this plane was already fast with the Allison, the brit engine could only add more speed to a 37mm cannon making of it a nice performer in the ETO as a pure fighter rather than mere ground attack stuff...
Was the Merlin too heavy to fit it in the Airacobra fuselage, so the plane simply would have been seated on his tail? Was too big the delusion so the P-39 didn't had another chance to improve his design?
Or simply was too risky to divert a badly needed engine for the Spitfire/Lancaster to a big flying question mark?
All written above cames when you don't have a woman, so I eventually apologize for the raving :)
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the P-39 had dangerous flying characteristics - tendency to tumble & get into unrecoverable (inverted, flat) spins.
low rate of fire on 37mm made it somewhat difficult to use in air-to-air combat, but not too hard against ground targets
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Originally posted by whgates3
the P-39 had dangerous flying characteristics - tendency to tumble & get into unrecoverable (inverted, flat) spins.
low rate of fire on 37mm made it somewhat difficult to use in air-to-air combat, but not too hard against ground targets
The "tumbling" issue reported about the P39 is apocryphal.
"Earl" who flew P39s, P400s (export variant of P39) and P47s in WWII, and Skyraiders in Vietnam, and played AW denied that the handling characteristics of the the P39 were any more lethal than any other high-powered prop aircraft and put these comments down to pilot conservatism. If it looks right it will fly right is a maxim that Pilots and Designers tend to stick to and to some people the idea that the engine was behind them, and the propshaft ran between their legs made it "look" wrong. As Earl said if you hit the ground at 300mph it doesn't matter if the engine is behind or in front you're going to die.
The Russians made good use of the P39 and P63 in both Ground Attack and Med Alt fighter roles. Perception and urban legend did the P39 in in Western eyes anyway, more so than any unpleasant handling characteristics.
palef.
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Using soviet success with the p39 to refute the stall issue is misgueded. They identified exactly the same behavior in thier tests.
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The 37mm cannon sounds good but isnt of much use in a dogfight. (Try furballling in a Yak9t sometime)
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Originally posted by Pongo
Using soviet success with the p39 to refute the stall issue is misgueded. They identified exactly the same behavior in thier tests.
Pongo, Earl was quick to point out that all aircraft have handling issues that require you to either stay in the envelope or ride the edge of it. If you have a documented sequence of events that happen if you exceed that envelope, why do it?
I repeat that the P39 was regarded as 2nd rate because of tumbling and flat spins is misleading at best.
palef
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Hi Palef,
>The "tumbling" issue reported about the P39 is apocryphal.
I used to think the same until recently I found a translation of a paragraph from the "Tsagi" book on WW2 aircraft, in this case on the P-63:
"In order to become acquainted with the new aircraft the VVS sent test pilot A, G Of kochetkova and testing engineer F. P. Suprun to the USA. Spin characteristics was the problem, which interested Soviet specialists the most. G. Kochstkov was interested in flight testing the aircraft under these conditions. And it turned out that the assurances Bell that the P-63 would not enter into flat spin, were unfounded. In a test flight A. G. Kochetkov proved that like its predecessor the P-63 had a tendency towards flat spins. After numerous, but futile attempts to pull the fighter out of the spin he was forced to bail out of the aircraft and parachute to the ground. This test forced the aircraft to go through a number of design in order to move the center of gravity forward. Additional flight limitations were also introduced. "
>"Earl" who flew P39s, P400s (export variant of P39) and P47s in WWII, and Skyraiders in Vietnam, and played AW denied that the handling characteristics of the the P39 were any more lethal than any other high-powered prop aircraft and put these comments down to pilot conservatism.
P-400 (and P-39) veteran M. F. Kirby mentioned that he never heard of a single instance of the P-39 doing the feared flat spin, but he also pointed out that the pilots did their best to avoid getting into any kind of spin anyway.
Regards,
Henning (HoHun)
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Thanks HoHun!
Always nice to update the ol' database.
It would be very nice to get hold of an English translation of Tsagi.
Regards
palef
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The Merlin engine was no better than the Allison. It was a little smaller but was actually less durable to damage and abuse.
The reason the P-51B performed so much better than the P-51A was it had a better propeller and supercharger. The Merlin's slightly smaller size and different layout made it easier to fit the 2-stage/2 speed supercharger into the limited space.
The P-40E was modified to carry a Merlin engine and a number of planes of this type were built (known as P-40F), but since it had the same propeller and lousy supercharger it didn't perform much better at all. The P-39 would be no different.
Eventually an improved version of the P-39 known as the P-63 was built. It featured an improved Allison engine, 4-blade propeller and a better supercharger and it did indeed have pretty good performance (and it still hads that 37mm cannon). Top speed was over 400 MPH at altitude.
Also of note is a late-model P-40 (XP-40Q) was built with a 4-blade propeller and a good supercharger (once again it still had an Allison). Speed increased from around 370 MPH for the P-40N up to about 420 MPH for the XP-40Q.
J_A_B
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Bell tested the P-39 extensivly and found the only thing close to a 'tumble', was if the stick was held back through the stall, the plane would do a series of whip-stalls, which when seen from the ground, could look like a tumble.
Centering the stick would set the plane in to a normal spin and normal recovery procedures could be used.
What the P-39 did have, was the lightest controls of any aircraft in WW2 (number escapes me...3.5lbs/G??) and also the most sensitive controls, making it easy to overfly it, giving the P-39 it's bad reputation.
Daff
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Without going back to reference this, I believe that the reason the P-39/P-400 program was not pursued like the P-51 and the P-47 program was purely political. Fitted with the Merlin, this Airframe would undoubtedly have been a fine performer both in the attack role and with some armement alterations, inn the air to air role.
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^^ Yes, the P-39 was the cleanest airframe of any of the US WW2 fighters and had some kind of charging option for higher alts been seriously pursued it could have been a serious contender in the ETO.
Daff
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Originally posted by -ammo-
Without going back to reference this, I believe that the reason the P-39/P-400 program was not pursued like the P-51 and the P-47 program was purely political.
Wasn't the P-39's range a bit of an issue? If I recall my numbers correctly, the P-39 only carried 120 USG internally, versus 159 USG for the P-40 and 180 USG for the P-51 (without fuselage tank)...
While we're on the subject of the P-39, does anyone recall the comparative tests done against the captured A6M2? One of my references quotes the test report as stating that the P-39D began the test using 70" of manifold pressure! (The engine then - surprise, surprise - began to detonate and they reduced power to 52" MP.)
Does anyone have any idea why the evaluation group would try such an extreme level of overboost?
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It was not uncommon in the field for "local mods" to take place..the engine manufacturers tended to make restrictions on performance on the safe side....the field engine mechanics knowing better "eh..eh..eh" would soup-up the engine a bit...like when we overclock our cpu for faster performance..
If you remove the boost protection from the engine you could get 70" of manifold pressure..but look out...
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Another thing to consider is that the design of the reduction gear and driveshaft for the P-39 was not a trivial task and it was customized for the V-1710. If the V-1650 were substituted it would have required design of a new drivetrain suitable for that engine.
On the other hand, versions of the V-1650 already existed which were suitable for installation in the Mustang, so replacing the V-1710 with the V-1650 was a somewhat easier task in that aircraft.
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Simple, while the Brits generally liked their allison mustangs they vehemently hated their P400s and sent them back to the USA. :D
That said I would love to get an early P39D/P400 in US Pacific markings, please!!!
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this is from an interview w/ Dr. Robert Gilruth who worked at Langley Memorial Aeronautical Laboratory '36 - '45 (later he was director of the Manned Spacecraft Center '61 - '72):
...WOLKO: This is again getting back now to Bell Aircraft, on the P-39, did you ever get the P-39 down at Langley to test it?
GILRUTH: Yes.
WOLKO: Was there a tumble problem with that aircraft?
GILRUTH: People thought it did.
WOLKO: That was one of the rumors that went around with the P-39.
GILRUTH: Yes, people tended to think it did. I don't think we were ever that adventurous with it. But it would dig in, and the force per G was pretty nominal until you got to high Gs and then the force per G went to about zero, and it would pull itself in. I know that. I remember that. And that was bad.
WOLKO: Was that because of the engine in the rear?
GILRUTH: I have no idea why. I don't think so. I think it had something to do with the fabric on the elevators. There are all kinds of possibilities. For so many years we just took ailerons and other control surfaces and put canvas over them and dope and when you got to high speed, you had no idea what the shape of that aileron was. It depended so much on the internal pressure, the tension on the fabric and the dynamic pressure.
WOLKO: It could balloon out on you.
GILRUTH: It could balloon out or it could suck in, depending on how it was vented. And similarly with the elevators. So in those days, you could get all kinds of things that you wouldn't understand. It was the British that finally said, "Isn't it time we quit fooling around with the fabric on something as sensitive as that?" My goodness, sure.
WOLKO: So the British were first to put metal skin on their...
here is a pic of Dr. Gilruth showing JFK a small model of the Apollo Command Module
(http://spaceflight.nasa.gov/gallery/images/gilruth/gilruth_institutional/lores/s62-03989.jpg)
the following is from a section of an autobiography of Robert E. Riddle, at the time flying w/ the 28th squadron in Panama (Later Lt. Riddle became an 11 victory ace w/ the 31st Figther Group, 307th Squadron in the MTO)
".... Whatever its faults, I actually enjoyed flying the machine. Besides having a center of gravity far back, little servo tabs on ailerons and elevators made the controls extremely sensitive. An uncoordinated maneuver could cause the ship to snap roll and in some cases tumble end-over-end until dropping into a flat spin. The snap roll was much more likely at slow speeds and took it's toll on pilots landing out of a tight circular approach. But I found that at speed, the P-39 was as easy to hold in a three-G shuddering high speed stall as any other plane I ever flew. After many complaints and some fatalities attributed to the stalled "tumbling" action, an experienced Army test pilot was assigned to investigate. His report stated that he was unable to induce such a reaction from the ship in any attitude. But I have seen it happen and a friend of mine twice experienced it while attempting to perfect a vertical circular loop..."
and of course theres this song sung by USAAF pilots
Don't give me a p-39,
With the engine mounted behind,
It'll tumble and roll,
And dig a deep hole
Don't give me a P-39.
It'll stall out and spin,
And soon auger in,
Don't give me a P-39
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Hi Whgates,
Thanks for the quotes!
>GILRUTH: Yes, people tended to think it did. I don't think we were ever that adventurous with it. But it would dig in, and the force per G was pretty nominal until you got to high Gs and then the force per G went to about zero, and it would pull itself in. I know that. I remember that. And that was bad.
"Bad" sounds like a mild understatement here. What Gilruth describes is the same as the feared "control reversibility" of the P-51 when it had a full fuselage tank.
When the pilot is pulling at the stick to make a hard high-speed turn and the stick force suddenly disappears, he is able to pull more Gs than the aircraft (or the pilot himself) can endure. If the force diminishes before he can react, he actually will pull excessive Gs since he's still applying a force on the stick.
Among the criteria the NACA had defined to ensure military aircraft could be operated safely was a minimum stick force per G to prevent just this kind of self-destructive handling characteristic. (Of course, if it only happened in a certain part of the envelope, it might have escaped the test pilots.)
Regards,
Henning (HoHun)
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ja, the gilruth stuff was kinda off topic, but the dude's résumé is so impressive i had to include it. Lt. Riddle's accout is much more convincing - sounds like the engineers at Langley didn't wring the bird out nearly as thoroughly as the fighter pilot trainees did. it dose make sense to me that a plabe with it's engine mounted like the P-39s was, would, when outside the flight envelope, tumble or do just about anything and be very tough to recover
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Hi Whgates,
>ja, the gilruth stuff was kinda off topic, but the dude's résumé is so impressive i had to include it.
I'd say there's a close connection to the flat spin topic, though: If you encounter sudden reversibility in an asymmetric flight condition, you might not overstress your aircraft but actually whip it violently into a spin instead.
Regards,
Henning (HoHun)
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My impressions is that "tumble" phenomena was connected with center of gravity and I don't know if NACA, AF or manufacturer tested all possible combinations. Maybe it happened just with given CG.
gripen
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the P-51 had handling trouble with a full fuselage tank (~80 gallons or about 700 lbs) and didn't really manuver well (this is according to book i read by a 31st FG pilot) until fuselage tank was down to ~30 gallons. i would assume that a heavy mass near the CG would cause handling problems due to inertial effects. if this is the case then the allison engine in the P-39 (nearly 1600 lbs) could be a serious pain in the oscar....
"plabe"...LOL