Why a Rear Engine For the P-39?

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MIflyer

1st Lieutenant
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May 30, 2011
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Here's an article on why the P-39 had a rear engine. I guess it was ahead of its time. I can't think of but maybe two fighters built after 1945 that had the engine in the front. front.
 
I know there was a lot of justification, but Mr Woods is definitely over-selling the rather spurious advantages of the engine location: the P-39 and P-63 were simply too slow for nose shape to be significant. Indeed, the net drag of the P-39 was probably increased by the engine location, as moving the engine back required proportionately larger tail surfaces, to get the required tail volume (area times distance from center of gravity) and placed more keel area ahead of the center of gravity, further increasing the size of the tail surfaces needed.

As for front-engined fighters after 1945? For practical reasons, it's not common to put jet engines in the nose of an aircraft. There were a few Soviet aircraft to do so, and a Spanish trainer. More telling, no other fighters using the P-39 configuration went beyond prototypes.
 
The C series Allison engines used in the early P-40's and P-38's were specifically designed with a long tapering gearcase reduce drag. But that gear design limited the engine power and was soon discarded. Like curved wingtips, the value of those kind of features were vastly overrated.

You look at an Allison engined Mustang and wonder why everyone did not design airplanes that way, if they were so concerned about drag. The P-35, P-36, P-40, P-43, and even the P-39 look "fat" compared to a Mustang
 

For subsonic aircraft -- something all piston-engine fighters were -- a major source of drag is skin friction, which is directly related to surface area, as long as there is no separation. Another major source of drag is cooling, which can easily be 25% of the zero-lift drag of an airplane. This latter is probably -- I think definitely -- the biggest factor in in the Mustang's low drag, which is generally reported to have a zero-lift drag coefficient of about 0.017, vs most fighters (radial and V-12) which run about 0.022 to 0.025. Test data, reported among other places, in von Mises, shows that the optimum fine-ness ratio of a fuselage with tail fins is about 7.
 
MIflyer said:
You look at an Allison engined Mustang and wonder why everyone did not design airplanes that way, if they were so concerned about drag. The P-35, P-36, P-40, P-43, and even the P-39 look "fat" compared to a Mustang
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The P-35, P-36 and P-43 were radial engined fighters, so they are always going to look fat.

The P-40 was developed from a radial engined fighter (the P-36), so it retains some of the flab.

But I don't think the P-39 could be considered fat compared to the Mustang.
 
wuzak said:
The P-40 was developed from a radial engined fighter (the P-36), so it retains some of the flab.

.
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Not only that but in putting an in line engine in, the radiator was mounted there with it, which is not the best place, compared to the P51 and others which was designed from the start with an inline engine with radiators mounted elsewhere.
 
There was an article I posted in which the author, Lee Atwood, said he thought use of the Meredith Effect on the Mustang countered for more than the laminar flow wing. That was his opinion

But compare the profile of the Mustang to the other US fighters. "Fattest point" is where the pilot's head is. And it looks so much slimmer than the others.

And the FW-190 does not look fat.
 

The fattest point on the Mustang is behind the pilot - right where its big fat belly is.
 
MIflyer said:
There was an article I posted in which the author, Lee Atwood, said he thought use of the Meredith Effect on the Mustang countered for more than the laminar flow wing. That was his opinion

.
Click to expand...
I may have read it, as I understand it it was the whole "pie". NA took the aerodynamics to a new level. The wings were a part of it, the Meredith effect and radiator design especially the inlet was another while the whole approach to fit of panels, fastenings etc was at a higher level, such that a standard production P-51 was like PR Spitfires which had been specially worked on with gaps filled sanded etc.
 

I once heard the P-39 described as the first 'weapons system' aircraft. The engine was in the back so the 37mm cannon could go in front. This same cannon was in the original P-38 too, since both planes were designed to intercept and shoot down large bombers. The Army Air Corps let the P-38 keep its turbochargers, but took the turbo out of the P-39.

The P-38 tried to hit the target performance by power, using two engines. The P-39 tried to keep the aircraft as small as possible. Different approaches to the same problem.

An advantage of having the engine in the center of the aircraft was that the engine mount was part of the aircraft structure, saving a little weight. It also meant the aircraft would be more manueverable.One disadvantage of the engine/cannon combo for the P-39 was the change in balance when the cannon and the two .50 cals had fired a lot of their ammo; the nose became too light. I walways wondered why they never put a water tank in the back that would drop some water for each firing burst to keep the balance.
 
The mid-engine configuration was used in quite a few types, it's just that Bell was able to get the combination to work well enough that it became a largely produced type.

Other mid-engined types would be the Douglas XB-42 "Mixmaster", Göppingen Gö9 (which was a testbed for the Do335), Kyushu J7W1 "Shiden", Yokosuka R2Y "Keiun" which looked a great deal like the proposed Me509 project and the Piaggio P.119 (which actually used a radial engine).
 
pbehn said:
Not only that but in putting an in line engine in, the radiator was mounted there with it, which is not the best place, compared to the P51 and others which was designed from the start with an inline engine with radiators mounted elsewhere.
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I'd say that P-39 (but definitely not the XP-39) have had one of best radiator set-ups. Definitely better than some classics that were desinged around V12 engines, like the Bf 109E and subsequent, Hurricane or Spitfire.
 
jmcalli2 said:
I walways wondered why they never put a water tank in the back that would drop some water for each firing burst to keep the balance.
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1) Where would you find room to put it?
2) What about the weight and its decaying effect on performance? If you add this weight to the tail you have to add more weight in the nose to compensate. These added weights will negatively affect the polar moment of inertia which your mid engine mount was supposed to gain.
3) What do you do if the dump valve freezes up and you get into a fight?
4) How do you vent the water/meth mixture in such a way that it doesn't get all over the tail surfaces and:
a) freeze on ("blue ice") or
b) attack the skin material or
c) penetrate the rudder and elevator structures and freeze in place. (FLUTTERBUG!!)
Cheers,
Wes
 
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Yes, the P-39 had a real overheating problem, so bad that in hot climates it even affected the sequence in which the aircraft were launched. You did not want P-39s to have to wait, idling, on the ground while other aircraft took off and you did not want them to have to taxi long distances. Put them close to the runway and let them take off first.

This probably was not as much of a problem for the Soviets. But at the same time air leaks into the cockpit made it uncomfortable to fly in cold climates.
 
jmcalli2 said:
The P-38 tried to hit the target performance by power, using two engines. The P-39 tried to keep the aircraft as small as possible. Different approaches to the same problem.
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Not quite the same problem. The requirement that lead to the P-38 called for twice the endurance with the same armament and performance (speed and climb) as the requirement that lead to the XP-39.

A cautionary tale for those who propose long range fighters using mid to late 1930s technology.

Kelly Johnson thought he could meet the long endurance requirement with a 1500hp engine but since no such engine existed in year/s they started design work he was forced to go to a twin engine aircraft and because the twin was a larger/highdrag aircraft than a single he was forced to go to 2000hp.
 

Hello Swampyankee,
I don't see how the engine located behind the cockpit made any difference in the CoG location of the P-39.
The CoG in the ideal case would still have been located around 25% MAC.
The location of the engine just meant that other components which unfortunately included a lot of disposable loads were moved around to compensate.
When the aircraft was loaded, the CoG was pretty normal. It was just when many of the disposable loads were expended that the balance was not so good.
If larger tail surfaces were required, it would have been because the moment arm of the tail were shorter than average and not because the engine is in the back. The SB2C Helldiver is a good example of what happens when the tail gets too short.
As for changes in the keel area, what pieces do you see that would have affected that? The volume of the nose was actually relatively small and the cockpit was only very slightly ahead of the CoG.

As I see it, the biggest problems with this design were:
The disposable loads were too far from the CoG and all ahead of it.
The cooling system was very marginal even for the original installed power and was not in a place where it could be easily improved.
The rear engine location put the carburetor intake in a low pressure area which limited ram effect.

I have also heard it mentioned that the elevators were too sensitive and ailerons were too heavy, so control harmony was poor but that wasn't a characteristic inherent in the rear engine design.

- Ivan.
 
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