XP-39 II - The Groundhog Day Thread

[P-39 Expert]

Going into a fatal spin when nose ammo was expended wasn't a real CG problem?

Actually you are referring to the "Tumble" where the P-39 supposedly could tumble end over end when the nose ammo was expended.

You guys make it sound like immediately after the nose ammo was expended the plane crashed. I know most of you are just pulling my chain, but honestly? Even with all the nose ammo expended the P-39 would execute any normal fighter maneuver easily with no trouble. Rolls, turns, climbs, dives, loops, approach and landing, whatever. Only when purposely attempting to stall the plane at the top of a loop could the tumble be induced, and then not on a consistent basis. Tumbling a P-39 was easy to avoid. Chuck Yeager couldn't make it tumble and stated that it wouldn't tumble.

How many P-39s were lost to this supposed tumble? Anyone know?

How many P-38s were lost diving from altitude? Or lost an engine on takeoff? Either engine, since both turned the wrong way. How many P-40s crashed on takeoff or landing, especially in a crosswind? How many P-47s crashed on takeoff with the ubiquitous drop tanks from their long takeoff run? Or had to use WEP just to take off? How many P-51s were lost in a tight turn with a full fuselage tank? I know that was alleviated by only partially filling the fuselage tank, but why have a fuel tank if it can't be filled to capacity? How many Wildcats crashed on takeoff or landing at land bases with that narrow, soft landing gear? How many Corsairs crashed on carrier landings due to a severe wing drop during a stall? Corsair couldn't even get carrier qualified until late in the war.

No AAF or USN fighter was without it's faults.

 

[GrauGeist]

From Bill Overstreet's biography:

Bill was in combat training in June 28th, 1943 when he had his first crash, at the controls of an Bell P-39 Airacobra, which went into a dreaded flat spin, a condition uniquely devastating for the model and which claimed many a pilot's life. Bill and his squadron-mates were practicing aerobatic maneuvers when his plane strated tumbling and he couldn't control it. Bill went to release the Airacobra's doors but the air pressure prevented them from opening. He finally managed to get a knee against one door with his shoulder against the other, trying to overcome the pressure, and the moment he got out, he pulled the ripcord on his parachute. The moment the chute snapped open Bill found himself standing amidst the wreckage of his plane right by the propeller. He was so close to the ground when he escaped his doomed plane that none of his flight-mates even saw his chute deploy, Bill belives he was perhaps the first pilot to survive the crash of a tumbling P-39, and he made a point on tracking down the man who packed his chute to personally thank him for a job well done.

Overstreet was best known for his piloting skills and flying his P-51C (that had custom white-wall tires) with the 357th FS in the ETO.

 

[DerAdlerIstGelandet]

" Only when purposely attempting to stall the plane at the top of a loop could the tumble be induced"

So all those pilots who entered a stall and discovered the problem were trying to stall the P-39?

I get loving an aircraft, but damn...

 

[XBe02Drvr]

Actually you are referring to the "Tumble" where the P-39 supposedly could tumble end over end when the nose ammo was expended.

As has been established here upthread, the "tumble" is essentially a Lomcevak, which the acrobatic world has accumulated plenty of experience with since Airacobra days. Given a CG in the "approved for acrobatics" range, which is usually midrange and slightly forward of midrange of the "normal" CG range, a Lomcevak normally devolves into a typical upright, nose low, garden variety, incipient spin. Now take that same aircraft, load it to its "normal" aft CG limit, and your Lomcevak will occur with a lot more "snap" to it and can easily terminate in a high rate, upright or inverted* flat spin. The P39, with its symmetrical airfoil, can be expected to stall negative or positive with equal ease during the wild fluctuations of the tumble, and lacks the polar moment of forward mounted engine mass to "steer" it in the right direction and unblank the horizontal tail. The ultimate version of "going ballistic"!

*"upright" or "inverted" in this context doesn't refer to the aircraft's attitude relative to earth, but to whether the wing is stalled positive or negative.

 

[DerAdlerIstGelandet]

Besides, Bell was not able to recreate the spin, tumble, whatever you want to call it, because they were flying it with full simulated ammo load for weight. Once they removed that weight and the aircrafts CG shifted aft they were able to recreate it.

 

[GregP]

Hey P-39 Expert, the "tumble" was NOT at the top of a loop when purposely stalled. It was when past rearward CG and inadvertently-stalled with power-on.

Below is a P-63 crash at Biggin Hill. The pilot is flying a perfectly good P-63, but is flying it at low cruise power. He then tries to do a loop which, in any WWII fighter, has a basic speed range that should be used. The pilot here is at lower-than-recommended speed, MUCH lower-than-recommended power, and simply runs out of airspeed and power when he gets vertical. The rest is poor recovery technique - though he DID avoid a spin, at least initially - and then he stalls it a second time during the pull-out with insufficient altitude to recover.



This was a very preventable accident. Don't fly aerobatics if you are afraid to use the engine as required for the maneuver. The P-63 has plenty of power for aerobatics, but trying a loop at a low-cruise hp setting is just not going to work very well. There is a reason the POH has the entry speeds that are published in it.

 

[Ivan1GFP]

Attached is a description of how to initiate a "safe" tumble in the Airacobra.
Imagine what happens if you do not take the precautions this fellow did BEFORE initiating a tumble.
I believe the same pilot was pretty accomplished with aerobatic flying in the Airacobra and had a few observations.
First of all, before flying aerobatics, fly straight and level and give it a sudden pull on the stick.
Observe the reaction.
If the aircraft immediately pitches down and oscillates slightly and returns to level flight it is safe to fly any maneuver.
If the aircraft pauses before pitching down slowly and does not return immediately to level flight, it is NOT safe to fly aerobatics.

- Ivan.

 

[XBe02Drvr]

First of all, before flying aerobatics, fly straight and level and give it a sudden pull on the stick.
Observe the reaction.
If the aircraft immediately pitches down and oscillates slightly and returns to level flight it is safe to fly any maneuver.
If the aircraft pauses before pitching down slowly and does not return immediately to level flight, it is NOT safe to fly aerobatics.

Best lifesaver I've seen all day!

 

[FLYBOYJ]

That was the P-39Q with the underwing .50calMG pods, made the plane a little more unstable.

As you know, spins were prohibited in the pilot's manual on all AAF and USN fighters.

They were - the point here is you either don't understand or continue to ignore the fact that because the P-39 operated on the most aft portion of it's CG envelope, it had handling misgivings despite being in "balance" (as you say).

 

[fubar57]

From Crowood Aviation, "Bell P-39 Airacobra" -Dorr & Scutts: "In Russia: Russian pilots often had opportunities to engage the Messerschmidt Bf 109 at low altitude, where the American built fighter performed best. The 'cobrastochka' ('dear little Cobra') clearly performed better in Russian hands than American. Lt. Col. Alexander I. Pokryshkin became the Allie's second-ranking ace of the war with fifty-nine aerial victories, forty-eight of them accomplished with his Airacobra. Eight other Soviet P-39 pilots are reported to have claimed twenty or more aerial victories. Pokryshkin assessed the Bell fighter much like many others: "It was a shapely aircraft. One thing I particularly liked about the Airacobra was the armament. That was really something to shoot the enemy down with - a hard-hitting 37mm cannon, two fast-firing heavy machine guns, and four normal caliber machine guns. I wasn't put off when other pilots warned me that the Cobra was dangerously prone to spinning because of a C of G located well aft" (my bold from now on) .....American ambassador W. Averell Harriman was quoted by Soviet foreign minister Vladimir Molotov on the significance of the P-39: "There is one type of aircraft, specifically the Airacobra, which is used very well by the Soviet Air Force. Harriman says the Russians use this aircraft even better than the Americans. Therefore it would be a benefit for Vandenberg (Maj. General Hoyt S. Vandenburg, Twelve Air Force commander and future US chief of staff) to become acquainted with the experience of Soviet pilots...Vandenberg would like to be informed about this and visit Soviet squadrons composed of Airacobra pilots." Molotov blessed the proposed meeting as 'useful' Bell Aircraft expressed a similar request. Gen. Vandenberg and company representatives visited the 6th ZAB (Zapasnaya Aviabrigada, or Reserve Air Brigade) in Ivanovo and two other units. The Soviet pilots had some requests - for example, they wanted improvement in the ballistics and rate of fire of the 37mm cannon. The Russians asserted that the late-model P-39Qs were much less stable than earlier Airacobras, and that the armoured headrest introduced on this model impaired rearward visibility. Bell representatives said they would take these complaints into account, although it is not clear what they might have done about them. The visit of the American delegation to the 6th ZAB was marred by the crash of a P-39Q-5 when the pilot became disoriented after recovering from a spin and had to bail out. Sure enough, the spin had been induced by too far aft center of gravity. If anything, the Russians became more obsessed with the downside of Airacobra's flying characteristics than the Americans. A number of evaluation programmes were undertaken including these: spinning and aerobatic tests of P-39Q-10-BE (43-2467), which showed that the Q model spun more powerfully and irregulaly than earlier P-39 variants....spinning and aerobatic tests of a P-39Q-15-BE (44-29115) with altered centre of gravity and reinforced tail unit and rear fuselage. The tests apparently raised no problem which justified this much effort and expense; however, there are indications that some Russians continued to have problems with the C. G. situation"

 

[FLYBOYJ]

You guys make it sound like immediately after the nose ammo was expended the plane crashed.

Not at all! Just pointing out the potential for an inexperienced or negligent pilot!

How many P-38s were lost diving from altitude? Or lost an engine on takeoff? Either engine, since both turned the wrong way.

Many from dives, agree! Now your comment "Either engine, since both turned the wrong way" makes totally no sense! Do you know what a critical engine is? Do you know why there were many engine out facilities on the P-38? It definitely wasn't from "Either engine, since both turned the wrong way."

No AAF or USN fighter was without it's faults.

Agree - to include the P-39

 

[P-39 Expert]

They were - the point here is you either don't understand or continue to ignore the fact that because the P-39 operated on the most aft portion of it's CG envelope, it had handling misgivings despite being in "balance" (as you say).

I completely understand the CG calculations.

I just believe that your CG concerns for the P-39 are a little overboard.

How many P-39s were lost to spinning/tumbling?

 

[FLYBOYJ]

I just believe that your CG concerns for the P-39 are a little overboard.

Based on the data presented, show me where I'm "a little overboard."

The P-39 got a reputation for tumbling a spinning, was all that a myth?

If you really understood "Weight and Balance" calculations, you would not have talked about moving equipment with out "doing the math." Have you ever weighed a real airplane? Have you ever flown a tail heavy airplane? There are some of us on here who have to include warbirds so if some of us are emphasizing something in our posts it's for a reason!

 

[Ivan1GFP]

Hello fubar57,

That particular book is a great reference and is also where I got my tumble description from.
The authors are actually Dorr, not Dover and Scutts. Robert Dorr actually lives in our neighborhood and donates some of his books to the local public library. I have also met him at the local IPMS annual events a couple times.

I came across a report on Soviet Spin Tests of the P-39Q and was wondering why it was done when the P-39 type had been in service for so long. Now it makes sense if the P-39Q had particularly bad spin characteristics.

- Ivan.

 

[fubar57]

Thanks Ivan, corrected the name

 

[P-39 Expert]

Based on the data presented, show me where I'm "a little overboard."

The P-39 got a reputation for tumbling a spinning, was all that a myth?

Depends on the actual facts. I don't know how many P-39s were lost to spinning or tumbling. Do you? I'm just asking.

P-40s got a reputation for being extremely hard to take off and land, especially in a crosswind. The P-40 had to be taken off and landed. The P-39 didn't have to be put in a situation where it would spin.

 

[fubar57]

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[FLYBOYJ]

Depends on the actual facts. I don't know how many P-39s were lost to spinning or tumbling. Do you? I'm just asking.

I don't but there were enough lost for it to be given a reputation!

P-40s got a reputation for being extremely hard to take off and land, especially in a crosswind. The P-40 had to be taken off and landed. The P-39 didn't have to be put in a situation where it would spin.

We're not talking about P-40s and you are correct. My earlier post was edited;

Have you ever weighed a real airplane? Have you ever flown a tail heavy airplane? There are some of us on here who have to include warbirds so if some of us are emphasizing something in our posts it's for a reason!

 

[Ivan1GFP]

I completely understand the CG calculations.

I just believe that your CG concerns for the P-39 are a little overboard.

How many P-39s were lost to spinning/tumbling?

Hello P-39 Expert,

Losses from spinning in combat, we probably don't know.
Losses from inadvertent spins in stateside training: a few.
Notes from experienced P-39 pilots to be wary of the flat spin, many.

My own belief is that some of the "tumbles" really were just very odd gyrations that happened during accelerated stalls in a P-39 with its CoG too far aft. When controls are released, the nose does not drop immediately as one might expect from a stable aircraft and the aircraft makes an unpredictable rotation or two before the nose ends up pointed into the airstream. Consider it a very unpredictable departure.

- Ivan.

 

[P-39 Expert]

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