Why a Rear Engine For the P-39?

[P-39 Expert]

All this P-39 CG problem is largely overblown.
Only came into effect when the nose ammunition was expended.
Still it didn't affect normal flight operation or approach/landing. Only when almost vertical near the stall and further back stick pressure is applied. The plane may tumble or may not, various pilots say yes and various say no (Chuck Yeager).
This didn't have anything to do with engine location, just the location of expendable ammunition forward of the CG.
Total weight of the nose ammunition was 60# of 37mm ammo and 125# of .50cal ammo (185#) less the weight of the shell casings which remained in the armament bay after firing.
Don't do any wild acrobatics after expending the nose ammunition.

Regarding the 37mm cannon, the Russians considered it more reliable than the 20mm cannon in the P-400. Cannon reliability was improved in the L model and subsequent models (M, N and Q) by installation of the little exhaust ports just forward of the gun bay access doors. This allowed heated cockpit air to be ducted up through the rudder pedal boxes to the cannon and nose MGs and then exhausted through the exhaust ports allowing a constant flow of heated air through the cockpit and the gun bay. The cannon being an open bolt design would freeze at altitude unless it was heated.
P-39 would easily mount the 37mm or 20mm cannons. The 20mm cannon only held 60 rounds (5-6 seconds firing time vs 13 seconds for the 37mm). Would have been nice to have a P-38 style 20mm with a 150 round magazine and jam clearing mechanism. But the P-39 soldiered on with the 37mm which was reliable and devastating.

 

[swampyankee]

Excessively aft C/G affects stability at all times, not just "when almost vertical* near the stall," and not just when the stick is pulled back. Since the landing flare tends to be very near stall, and may even rely on the fact that stall may not be instantaneous, aft c/g would, most definitely, affect landing behavior.



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* Stalls tend to occur at about 15 to 20 degrees angle of attack.

 

[OldParts]

As I think I mentioned in an earlier post, Eric Brown thought his "hack" P-39 was a lovely plane to fly and very aerobatic. Of course, he was not flying it in combat. I think most of us who are "old" (in either age or afflicted with making arguments over technology now almost 80 years old) have read that the "tumble" tendency of the P-39 was either mythical or at least over-stated.

Example of a center of gravity issue (plus some other mistakes) resulted in an accident: Air Midwest Flight 5481 - Wikipedia

 

[P-39 Expert]

Excessively aft C/G affects stability at all times, not just "when almost vertical* near the stall," and not just when the stick is pulled back. Since the landing flare tends to be very near stall, and may even rely on the fact that stall may not be instantaneous, aft c/g would, most definitely, affect landing behavior.



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* Stalls tend to occur at about 15 to 20 degrees angle of attack.

Extensive testing of the "tumbling" effect stated specifically that the P-39 CG with nose ammo expended did not affect normal operation or approach/landing.

 

[Ivan1GFP]

Excessively aft C/G affects stability at all times, not just "when almost vertical* near the stall," and not just when the stick is pulled back. Since the landing flare tends to be very near stall, and may even rely on the fact that stall may not be instantaneous, aft c/g would, most definitely, affect landing behavior.

Hello Swampyankee,
One thing worth noting is that the CoG of Airacobra would migrate forward slightly as the landing gear was lowered.
The Mains only go sideways, but the Nose gear moves forward quite a bit, so the CoG issues would be less in the landing configuration. This kind of thing is even more noticeable in the P-38 and I believe there is even a manual entry that suggests lowering the landing gear in case of a CoG problem.
Agreed on the other points.

- Ivan.



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* Stalls tend to occur at about 15 to 20 degrees angle of attack.[/QUOTE]

 

[XBe02Drvr]

Swampyankee is right, aft CG affects stability at all times. It's major effect is to reduce stick force gradient, meaning it produces a greater response for the same inputs, leading to a tendency for inept or inexperienced pilots to overcontrol and get themselves in trouble by pulling more AOA than intended while inducing unintended yaw by clumsy footwork. We all know what that leads to. The reduced nose down pitch moment when the aircraft "goes ballistic" in the resulting assymetric stall tends to lead to a flat rather than nose-down spin, or a Lomcevak-like tumble. Just a momentary thrill to Yeager or Brown, but likely fatal to a bewildered nugget aviator.
This same stick gradient behavior inspires talented pilots like Brown and Yeager to wax poetic about its "light and intuitive responsiveness".
As mentioned above, it probably will have less affect on landing behavior because of the forward CG shift with gear extension.
Cheers,
Wes

 

[Greyman]

As I think I mentioned in an earlier post, Eric Brown thought his "hack" P-39 was a lovely plane to fly and very aerobatic.

Eric Brown, 1946:
In March we had a visit from one of the test pilots of the Bell Aircraft Corporation of the USA. Just for a laugh I asked him to test my old Bell Airacobra, which I had been using for so many hops around the country. He took off, did one very quick circuit, and came back ashen-faced.

'I have never,' he said, 'flown in an aeroplane in such an advanced state of decay. This machine should be scrapped forthwith.' So, on 28th March, I went up for one last aerobatic session in her, then bade her a sentimental farewell. The last laugh was on me.

 

[XBe02Drvr]

Example of a center of gravity issue (plus some other mistakes) resulted in an accident: Air Midwest Flight 5481 - Wikipedia

I'm a little sensitive to this, as I experienced a similar situation in a 1900C (without the disastrous finale, of course). 400 pounds of company materials got loaded in aft baggage without being added to the manifest we were given. So we departed Boston for Burlington four inches out of limits aft and at MGTOW, while our load sheet showed within limits and 400 pounds under gross.
The plane handled squirrelly as hell and was real twitchy in all axis, but especially pitch. The 1900 tended to load aft, and we often flew with CG at the aft limit, where it had a tendency to squirrelliness, so we weren't particularly alarmed by the behavior at first, but as fuel burned off , and it didn't get any better we began to wonder. Unfortunately, we had an FAA inspector aboard plugged into the intercom, so we couldn't talk about it, just make hand signals back and forth. We made it to Burlington and even managed a greaser landing, but when two husky rampies hopped up in aft baggage to throw down the bags, the old girl sat right down on her tail, launching the Fed from the airstair door where he was standing to an undignified heap in a puddle on the ramp. The excrement impacted the rotary impeller with a vengeance right about then.
Cheers,
Wes

 

[pbehn]

Eric Brown, 1946:
In March we had a visit from one of the test pilots of the Bell Aircraft Corporation of the USA. Just for a laugh I asked him to test my old Bell Airacobra, which I had been using for so many hops around the country. He took off, did one very quick circuit, and came back ashen-faced.

'I have never,' he said, 'flown in an aeroplane in such an advanced state of decay. This machine should be scrapped forthwith.' So, on 28th March, I went up for one last aerobatic session in her, then bade her a sentimental farewell. The last laugh was on me.

Was that the plane he had been practicing landing on carriers in?

 

[OldParts]

I have a question for those of you who have flying experience and or more aeronautical knowledge than me. I understand the tricycle gear was preferred for aircraft with a higher landing speed (and consequently a higher stall speed for a given angle of attack -- I am including the angle of incidence in angle of attack which is I think called alpha) and have read that was one reason it was preferred for the P-39. My questions is, when landing a tricycle gear plane compared to a tailwheel aircraft and if all else is equal -- I guess primarily wing loading, is the flare as great or is it a faster approach at a more shallow angle. Since I have never landed either type of plane then I have no idea. In Rotary Wing, I guess the main idea is to stay out of the dead man zone (too low and too slow to autorotate and not having sufficient power stop your decent?). I do know that you want to land on the main gear with a tricycle wheeled a/c -- so you have to flare but how much?

 

[Greyman]

Was that the plane he had been practicing landing on carriers in?

Yes, not sure he kept the arrester hook all that time, but it sounds like something he'd do.

 

[OldParts]

Was that the plane he had been practicing landing on carriers in?

I don't know. I am searching for Volume 1 of Wings of the Weird and Wonderful but I have not been able to locate it. I will keep searching. I am almost certain that he gives the registration or other number for the a/c. I just have too many books and am unorganized.

 

[Shortround6]

I believe he only landed once. I believe an actual landing was not part of the test program. The test program was in connection with a possible aircraft with NO LANDING GEAR to land on a flexible flight deck. They were interested in view from the cockpit and other things that required a close approach but not actual touch down/hooking the arrester gear.

 

[OldParts]

In Wings of the Weird and Wonderful (Vol. 1 and hence forth written by me as WoW&W1, Brown reports that the A/C he landed was AH574 which was also the hack. Brown reports he declared an emergency since he was not scheduled to actually land the P-39 The carrier was the Pretoria Castle and capable of only 17 knots. Brown and the Captain had previously served together and he was not asked to specify the emergency. It was the Airacobra's only deck landing according to Brown. He describes the landing as easy but the take off with little wind and only 17 knots from the carrier as falling off instead of flying off. The carrier was already swinging to port to miss him (pages 26-27). Only the following page he talks about flying AH574 (after receiving the Bell test pilot's opinion) and doing aerobatics over Farnborough for 50 minutes. He closes with: "When I landed I can truly say it was with a heavy heart that I finally parted company with my old friend." I have read a most of Brown's books and since he really loved overpowered airplanes (especially when evaluating twins), I wonder if flying a stripped down Airacobra. Shortround6 is correct that this was preliminary to the flexible deck experiments. I finally found WoW&W1 in a box with Hugh Dundas's Flying Start (which could lead to some other interesting conversations).

 

[BiffF15]

Eric Brown, 1946:
In March we had a visit from one of the test pilots of the Bell Aircraft Corporation of the USA. Just for a laugh I asked him to test my old Bell Airacobra, which I had been using for so many hops around the country. He took off, did one very quick circuit, and came back ashen-faced.

'I have never,' he said, 'flown in an aeroplane in such an advanced state of decay. This machine should be scrapped forthwith.' So, on 28th March, I went up for one last aerobatic session in her, then bade her a sentimental farewell. The last laugh was on me.

Greyman,

The thing in my opine to remember, when reading what Eric Brown commented on regarding the aircraft he flew, is that he didn't fly the vast majority of them in combat, nor did he have a considerable amount of time in any one plane. You could put a highly experienced test pilot in an Eagle, and I would bet most any operational IP would do much better in the employment arena. There is a big difference between a highly experienced test pilot and a very experienced IP in the same type of plane. One knows much more about using it as a weapon than the other.

Also most WW2 fighters were probably a joy to fly, however that doesn't translate directly into how they well they could be used as a weapon. The P-39 was probably a joy to fly with the CG in the correct range. That doesn't mean it was with an aft CG, at the edge of it's range, with a wiley bandit giving you hell.

Cheers,
Biff

 

[OldParts]

I in no way meant to imply that the P-39 was a good fighter in 1940 or 1941 (or even 1939). I am more interested in Brown's opinion of its handling characteristics. He was not fond of many of the planes he tested. I don't know how many hours he had on type by type; but he shot down two FW-200s flying from the Audacity (the first of all escort carriers) and had a probably with an FW-190 flying a Spitfire. But the P-39 was an inadequate plane in many ways.

 

[BiffF15]

I in no way meant to imply that the P-39 was a good fighter in 1940 or 1941 (or even 1939). I am more interested in Brown's opinion of its handling characteristics. He was not fond of many of the planes he tested. I don't know how many hours he had on type by type; but he shot down two FW-200s flying from the Audacity (the first of all escort carriers) and had a probably with an FW-190 flying a Spitfire. But the P-39 was an inadequate plane in many ways.

OldParts,

This is one of two threads on the P-39. My comment was meant for both threads, and should be read as a "reply all" via email more than directed at Greyman.

Cheers,
Biff

 

[Zipper730]

I'd say that P-39 (but definitely not the XP-39) have had one of best radiator set-ups.

Really?

If you move the engine back in the fuselage, you relocate the CofG for the fuselage assembly aft and your 25-30% MAC CofLift has to migrate rearward to compensate. Result: longer nose, shorter tail, unless you're willing to accept the increased wetted surface and heavier tail moment that lengthening it entails. Eventually you wind up with a BF-109 tail, but that was a front engine plane. Remember, the P-39 was supposed to be a " big engine, small airfame" machine.

I was under the impression that almost all the weight was concentrated in the middle, the CL/CG were closer than typical making for a plane that could potentially be rather twitchy.

I once inadvertantly flew a Beech 1900 from Boston to Burlington VT with the CofG 4 inches out of limits aft. Can you spell SQUIRRELLY? The plane was borderline divergent stable, and reacted abruptly to any pitch inputs.

Pretty scary... I could imagine such a situation could result in a loss of control.

Turns out the rampies in Boston had loaded 400 pounds of company shipments in aft baggage without listing it on the manifest. We got in hot water with the Feds over that one.

Yikes...

Stabilizer with no down load?

I thought there were certain cases where it was okay as long as the tail could keep lifting after the wing stopped. Of course with the tail behind the wing...

 

[XBe02Drvr]

n

when landing a tricycle gear plane compared to a tailwheel aircraft and if all else is equal -- I guess primarily wing loading, is the flare as great or is it a faster approach at a more shallow angle.

All else being equal (two different versions of the same aircraft, but one converted to the opposite type gear) will fly their approaches at the same speed and angle and their flares will be similar with a couple exceptions. As long as they're flying machines, they're pretty much the same, it's the transition to a wheeled vehicle where the difference shows up. A taildragger has two landing options: a three-pointer or a wheel landing. In a three point, the pilot keeps holding it off as it slows until it settles on super slow and all done flying. This is best done in slower lighter planes with larger control surfaces for better low speed control. Also better with minimum crosswind component. But oh so pretty. Faster heavier planes tend to use wheel landing technique, where the plane is "flown in formation with the runway" and eased down until the wheels roll smoothly (we hope) onto the runway surface and it's still a flying machine which is rolling its wheels on the ground and gradually transferring its weight onto the wheels as it slows. This gives better control in a crosswind or with a plane that has smaller high speed control surfaces. Once the main wheels are on and the aircraft is slowing, a little gentle forward stick will keep it pinned to the ground and keep the tail from settling until the speed as slowed enough that it won't try to fly again when the AOA increases as the tail settles. This is ground loop country where the slightest deviation from a straight path will encourage the tail to try to get in front of the nose. A taildragger has to be flown alertly from tiedown to tiedown with no relaxation of vigilance.
A trigear is more of a "winged rocking chair", very forgiving in ground handling. The only caveat is that some of them (like the P-39) have a long nosegear, such as Mooneys, Comanches, and many early jets, in order to give the wing a positive AOA on the ground. When landing one of these, it is necessary to hold the nose up longer until a deep flare is established before touchdown (like a 3 point taildragger) in order to avoid a "wheelbarrow" arrival. If you come in fast and level and try to "plant it" by chopping the power, the nosewheel will hit first, followed by the mains, which cocks your nose in the air, and because you're still moving fast, you're back in the air with no power and lots of drag (dead man zone!), whereupon you drop in on the nosegear again and repeat the process until you run out of runway or bash the nosegear into the engine compartment. This is called "crow-hopping" and has been the demise of many a fine airplane. The solution to this dilemma is to put the power to it at the top of the bounce, take it around the pattern, give yourself time to settle down, then try again.
Hope this helps.
Cheers,
Wes

 

[daveT]

P-39 Tumble and Spin Test WW2 Film
The U.S. Army Air Forces produced a short film to discuss the spin characteristics and procedures for the Bell P-39 Airacobra fighter. The film disputes a tumbling phenomenon described by some pilots.
BUT, at the end of the film a P-39 crashes due to a tumbling spin.

View: https://www.youtube.com/watch?v=BWwI6gZw67g