P-39 vs P-40

P-39 Expert said:

P-39 manual shows 9.4gal oil with 15.8gal overload, P-63 manual shows 9.6gal/13.7gal overload. Pretty much the same.

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Weight of the lubrication oil was not listed under weight of lubrication system, even the trapped oil was listed separately. Here we can see that 'Lubrication system' weights 56.2 lbs empty, same figure is repeated in the 'America's hundred thousand'. Same book states the 'Lubricating system' weight on the P-63 at 135-137 lbs.
Lets also recall that racing P-39s, like the Cobra II, featured an extra oil cooler, supposedly taken from a P-40. Despite still using a 1-stage engine, ie. without the hydraulic coupling adding to the cooling load. FWIW: link

Yes, that postwar racing P-39 had a G model Allison which was too late for combat.

P-39 would overheat on the ground if standing still, but taxiing was easier than in a tail dragger as visibility was excellent over the nose so takeoff was pretty quick. That problem was cured in the P-63.

Shortround6 said:

Please try to compare oil needed/used for normal flying (combat) for a 120-126 gallon fuel capacity and ignore max capacity oil used for ferrying with max drop tanks.

With the "normal" fuel supply the two stage engine only needed a few extra gallons but they were in the fluid coupling and in the larger oil cooler and not just sitting in the oil tank.

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P-63 used the size inlets they did because Bell engineers were stupid?
Or because the engine needed more cooling air?
P-39 inlets
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Just asking, are you saying that the fluid in the hydraulic clutch was circulated with the oil? I had always assumed that the fluid in the hydraulic coupling was sealed inside like on the DB 600 series.

It wasn't sealed in the case of the DB 600 series of engines either. The oil in the fluid coupling needed to be cooled and the easiest way to do it was to share the oil supply and and oil cooler of the main engine oil.
Otherwise you need a seperate oil supply and a separate oil cooler. Most automatic transmission cars run a line to and from the automatic transmission (torque converter/fluid coupling) to either a small coil in the radiator or a small separate radiator cooler in front of it.

GregP said:

... unless you're in it at the time.

Nylon letdown over the guys you just bombed or straffed isn't a recipe for continued good health.

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It can also be quite hard to get out of an aircraft in a flat spin.

I don't know if anyone was able to work out a recovery from that. iirc it became an issue on the early planes when the cannon ammo was expended. that moved the CG further back...in a way similar to the 51 fuse tank. there were certain things you couldn't do when it was full...on the 39 there were certain things you couldn't do after you shot all your cannon rounds.

bobbysocks said:

I don't know if anyone was able to work out a recovery from that. iirc it became an issue on the early planes when the cannon ammo was expended. that moved the CG further back...in a way similar to the 51 fuse tank. there were certain things you couldn't do when it was full...on the 39 there were certain things you couldn't do after you shot all your cannon rounds.

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I'm inclined to agree with FlyboyJ in post #17. You weren't just flying along fat dumb and happy and all of a sudden you fell into a spin (or tumbled). Nose ammo had to be expended, you had to be in a near vertical climb near the stall and then you had to pull back on the stick further to even come near a tumble if it really existed. Expending the nose ammo didn't affect any other flight regime including approach and landing. Virtually every single seat trainee flew a P-39 in training, if the plane had been that dangerous it would have showed up pretty quick.

Hello Gentlemen,

I was meaning to post this in one of the other P-39 threads but got tired of reading to see what was already there to be sure the issue had not already been covered.

While I do like how the P-39 appears, my opinion is that it has a severe design flaw in the way that disposable loads were located in the aeroplane. As has been mentioned before, this resulted in a marginal stability problem, flat spin characteristics and of course the infamous "Tumbling".
Many years ago, I put together a spreadsheet detailing the various weights and their moment arms to calculate changes in CoG as different items were added or removed.
The aircraft I was attempting to model was the P-39D and I actually rounded up quite a lot of data at the time.
Unfortunately that was several computers ago I have no idea where that spreadsheet or data ended up.

Attached are some images showing equipment locations in the P-39Q if anyone else wants to attempt the same thing. I actually had collected much more information on the P-39D.

Everyone discusses the ammunition load of the P-39 and how the CoG shifts back past the safe range when the ammunition is expended. There happens to be a considerable difference in the CoG change depending on if the Motor cannon happens to be a 20 mm versus a 37 mm though I can't remember the specifics.
That is not the only thing worthy of note.
The Fuel Tanks also slightly ahead of the typical CoG which means that when Fuel is expended, the CoG shifts back even further.
The last "Major" item is one we normally don't think about.
The Pilot also happens to be ahead of the CoG and although the Pilot is not a disposable load, they don't all weigh the same. If the Pilot is very light and carrying very minimal equipment, stability gets even worse.

It was brought up that an aeroplane should be safe to fly as long as CoG was within established limits.
Although I do not have the calculations in front of me at the moment, I was convinced that the under some rare occasions, the CoG would be Aft of those limits and handling would be dangerous.

It was also brought up that the P-39 was often used in training later in the war without any great problems.
Consider what happens with a fighter trainer:
It is not likely to be loaded with cannon ammunition in which case there would be some amount of ballast as a substitute. Since the ballast would not change, the Aft migration of CoG would be limited. A student pilot would also most likely be keeping a reasonable fuel reserve especially with an aeroplane with such short legs.

- Ivan.

Shortround6 said:

P-63 used the size inlets they did because Bell engineers were stupid?
Or because the engine needed more cooling air?

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The single inlet does not appear significantly larger in frontal area than the two single inlets combined. So
P-63 used the size inlets they did because Bell engineers were stupid?
Or because the engine needed more cooling air?
Or because the wider centre section allowed a single inlet with better cooling air aerodynamics and efficiency than the two inlets and the serpentine ducting of the P-39?
Or a mixture of more cooling air volume with better cooling air aerodynamics?

P-39 Expert said:

Virtually every single seat trainee flew a P-39 in training, if the plane had been that dangerous it would have showed up pretty quick.

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It did show up. I quote myself from an old thread:

In accident statistics the P-39 stands out like the proverbial sore thumb, in that it was the most dangerous plane to fly. From Army Air Forces Statistical Digest (tables 174 and 214), continental US only :

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MiTasol said:

The single inlet does not appear significantly larger in frontal area than the two single inlets combined. So
P-63 used the size inlets they did because Bell engineers were stupid?
Or because the engine needed more cooling air?
Or because the wider centre section allowed a single inlet with better cooling air aerodynamics and efficiency than the two inlets and the serpentine ducting of the P-39?
Or a mixture of more cooling air volume with better cooling air aerodynamics?

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I would note that the P-63 used a single oil cooler, (oval?) in the center of the wing under the engine and used two coolant radiators in separate ducts but adjacent to the oil cooler, one on each side. Sort of the reverse of the P-39 which used a single coolant radiator in the center and two oil coolers, each in a separate duct.

Now I am left to wonder why they spent all that time and effort redesigning the entire cooling system (both prestone/water and oil) if the cooling set up on the P-39 was fine the way it was.
Now just because just about every test says that the P-39 could not meet USAAF cooling standards in one way or another (anything from both oil and coolant not meeting standards in level flight and climb or perhaps only failing in one area on on flight regime) that shouldn't mean you can't stuff an engine making hundreds more hp into the same airframe with the same cooling system/s and not have it work, right?

Bell engineers were just doing "make work projects" to keep their draft deferments from running out and being drafted into the infantry?

P-39 Expert said:

I'm inclined to agree with FlyboyJ in post #17. You weren't just flying along fat dumb and happy and all of a sudden you fell into a spin (or tumbled). Nose ammo had to be expended, you had to be in a near vertical climb near the stall and then you had to pull back on the stick further to even come near a tumble if it really existed. Expending the nose ammo didn't affect any other flight regime including approach and landing. Virtually every single seat trainee flew a P-39 in training, if the plane had been that dangerous it would have showed up pretty quick.

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That's mostly true. However I don't agree that you had to be in a near vertical climb to initiate a stall that could lead to a flat spin. Stalling happens when exceeding maximum angle of attack (of the wing). It can happen when airspeed is too low in straight and level flight, or at high airspeed during high G turns. The higher the airspeed in a turn, the more angle of attack is needed to maintain lift and thus it's "easy" to stall a wing if you pull back too hard on the stick. Any stall that isn't recovered from promptly can possibly lead to a flat spin.

If the CG is past the aft limit as a consequence of spending all ammunition, and the aircraft is placed in a stall (intentionally or not), and it isn't recovered immediately, I can imagine it going into a flat spin relatively easily if one isn't careful and/or trained to handle the situation.

I am however surprised that a combat aircraft like the P-39 came with a "no spinning" instruction, just as I am surprised that a combat aircraft can exceed C of G limitations by expending ammunition. But on the other hand, with the engine in the back and such a short nose, I guess it's not that far fetched.

I'm also left wondering if the fuel level could exacerbate the situation (disclaimer, I don't know the locations of the fuel tanks on the P-39). Presumably in the wings and possibly somewhere forward of the engine?

Hello JimmyZ,

There are actually plenty of aeroplanes for which intentional spins are prohibited.
There are also some for which a spin is not recoverable. (The F7F Tigercat is an example.)

I believe I addressed some of the points you brought up in this thread at Post Number 49.
P-39 vs P-40

It gets really confusing when two threads have exactly the same Title and very similar discussions.

- Ivan.

P-39 Expert said:

I'm inclined to agree with FlyboyJ in post #17. You weren't just flying along fat dumb and happy and all of a sudden you fell into a spin (or tumbled). Nose ammo had to be expended, you had to be in a near vertical climb near the stall and then you had to pull back on the stick further to even come near a tumble if it really existed. Expending the nose ammo didn't affect any other flight regime including approach and landing. Virtually every single seat trainee flew a P-39 in training, if the plane had been that dangerous it would have showed up pretty quick.

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it did show up. the 357th started off in 39s at Tonopah. they suffered more than a few training accidents and because of it the 39 earned a line in an old pilots song "she'll stall and she'll spin and soon auger in. Don't give me a p39"

and the P40 would also overheat if standing still. my father trained in 40s in Florida. he told me when they were in line waiting to take off, if they were going to be sitting still in the Florida heat for more than several minutes they shut the engine down. they had to keep a Putt-putt ( battery jumper) there to help start them back up if needed.

bobbysocks said:

it did show up. the 357th started off in 39s at Tonopah. they suffered more than a few training accidents and because of it the 39 earned a line in an old pilots song "she'll stall and she'll spin and soon auger in. Don't give me a p39"

and the P40 would also overheat if standing still. my father trained in 40s in Florida. he told me when they were in line waiting to take off, if they were going to be sitting still in the Florida heat for more than several minutes they shut the engine down. they had to keep a Put-Put ( battery jumper) there to help start them back up if needed.

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That song was sung about a lot of planes.
P-39 would overheat on the ground. Not normally a problem since you had full forward visibility while taxiing so you just drove it to the end of the runway like a car, ran your mag and prop checks and took off. Unlike tail draggers that had to serpentine their way down the taxiway so they could see what was in front of them.

Regarding post #53, even some civil aircraft can have the CG move aft with fuel burn. The Beech Bonanza comes to mind immediately, but it is nowhere NEAR as far aft as the P-39 with no ammunition, or the design would have generated alarming accident statistics. It can move out of envelope as you burn fuel in the Beech, but not by much.

I'm thinking that the P-39 was probably fine unless you were near stall and also were not paying attention. I have not investigated the stall characteristics of the P-39 myself, but if you had stall warning and didn't release back pressure, you'd best at least not pull any harder. In combat, noticing those indications can go bye-bye and you react when looking out at the enemy instead of at the needle and ball.

From the looks of the training accident statistics, it caught many by surprise. In peacetime today, we don't use up ammunition, and they are ballasted for good CG in all cases.

There is a clip on the net showing a P-63 accident. He tried a loop from too low a speed with something like low cruise power on it, and didn't make past vertical. A shame since he probably had more than 50% power left unused. I cannot understand why people would perform a maneuver in an airshow that they have not practiced recently at some length up at a safe altitude.

sometimes the difference down low is the heat coming off of the runway isn't accounted for. that totally affects the air density. so a maneuver you practiced where you only lost 1000 feet now may take 1200. if you are at 1195 feet at the top of your maneuver it might make for a very bad day. I witnessed this at a local airshow during the "drunk pilot" skit. scratch one perfectly good citabria and almost a pilot. lucky man survived...somewhat banged up but breathing

michaelmaltby said:

... make of this what you will .

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

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I am not sure what plans are in the future for any of us, but it should be pointed out that it
was a Yak-9 at 0:22 that shot that Bf 109 (F-4?) out of the sky and not a P-39. I'm just
saying.

, Jeff

GregP said:

There is a clip on the net showing a P-63 accident. He tried a loop from too low a speed with something like low cruise power on it, and didn't make past vertical. A shame since he probably had more than 50% power left unused. I cannot understand why people would perform a maneuver in an airshow that they have not practiced recently at some length up at a safe altitude.

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Hello GregP,
I have watched that video. It seemed like the pilot really had no chance completing the maneuver.
Do you suppose it is possible that there might have been some kind of engine / throttle fault that prevented him from using full power?
It reminds me a bit of the Mosquito crash in which one engine failed when the aircraft was near vertical and the asymmetric thrust pulled the plane around so fast that there was never a hope of recovery at that altitude.
The announcer can be heard saying, "Oh no.... Oh Dear.... Oh My...." or something along those lines as the Mosquito went in.

Hello Corsning,
I believe I have seen this film also.
An older Pokryshkin can be heard saying "Airacobra was a very good aeroplane" during an interview.
I can;t recall what else he said during the rather short excerpt from the interview.

I believe that part of the difference in conclusions might have been due to expectations and part may have been due to differences in equipment.
Note that some Yak fighters were modified to carry an internal bomb load behind the cockpit! THAT would cause a horrendous change in CoG when the bombs were dropped.
Other Yak fighters carried a 45 mm (?) cannon firing through the propeller shaft. The recoil was really more than the aeroplane could take but apparently longevity and solid construction was not as high in consideration.
Some of their wooden aircraft structures were rather questionable in strength and durability because of manufacturing faults.
In that context, the P-39 would look pretty good!

Note also that the Russians had a tendency to remove some of the wing armament from some of the Lend Lease fighters.
Would removing the wing guns from a P-39 improve the margin for stability?

- Ivan.