P-39- Did the engine behind the pilot cause any problems?

[B-25 Pilot]

Hey everyone,

I am doing a school project and
I chose the P-39 to be on of the planes in my book. But I have a question, did the engine behind the pilot cause any problems?

Thanks ahead of time.

 

[fubar57]

Good luck with your book

 

[soulezoo]

Short answer is yes.

There are a couple of long P-39 threads here in this sub-forum. There's one on the XP-39 just on page two here. There's a lot of good reading and info on them. Some banter as well.

Good luck with your book.

 

[pbehn]

Putting the engine where the fuel normally is means you need to find a place for the fuel and weapons and put in a shaft to reach the propeller.

 

[GregP]

To be specific, the placement of the engine does NOT cause inherent problems, assuming the design is properly thought out. There are design considerations, to be sure, but not necessarily problems that can't be overcome.

In the case of the P-39, the design was not quite well thought-out. Yes, it had a driveshaft running under the pilot's seat and through his legs, but this was not a real problem. There were three basic problems with the P-39:

1) The engine was an Allison V-1710. There was nothing inherently wrong with the Allison (there were a few issues that got fixed over time), but the original specification was as an engine for late 1920's airships (dirigibles). As a result, it was specified with a single-stage supercharger. This meant that the engine performed quite well up to about 15,000 - 18,000 feet. The power dropped off pretty fast after that altitude. As a result, the P-39 was not a high-altitude fighter, making it unsuited to higher-altitude conflicts, like in Europe. It didn't have that problem in the Pacific, Mediterranean, or CBI, although it wasn't exactly on anyone's list as a favorite fighter for combat missions.

2) The arrangement of the armament was not well-designed, and the center-of-gravity moved aft when the ammunition was used up. This made for tricky handling anywhere near a stall in hard maneuvers, as fighters are prone to using. As a result, the airplane could be made to tumble if it got a bit slow and the pilot pulled hard on the control stick in combat. This was reported during the war, but was never confirmed by wind-tunnel testing until after the war. Generally, the P-39 handled well. But when the ammo was mostly gone, you had to be careful when yanking it around the sky.

3) The P-39 was never envisioned as a high-altitude, long-range fighter. So, it didn't have enough fuel to be used as a bomber escort unless the mission were relatively short.

I summary, the faults above are not unique to the engine-behind-the-pilot, but WERE present in the P-39.

* They COULD have used a 2-stage, supercharged engine or, at least, an engine rated for higher altitudes ... but didn't. The P-40 had the same engine and had the same low-altitude restrictions. The P-38 used the same engine (actually power section), but it had turbochargers and could get to high altitudes and operate effectively there. The P-39 airframe was small, and there was not really room for an effective turbocharger installation. The P-63 was similar to the P-39, but was larger and HAD room for another engine boost stage. That's another story altogether.

* The movement of the CG aft as ammunition was used up could have been fixed or at least addressed, but was never anticipated for some reason. I blame that squarely on design. The placement of the center-of-gravity at all extremes SHOULD have been anticipated before the first flight. The P-40 didn't have this issue. Neither did the P-38.

* Last, the short range was due to lack of vision when the specifications were drawn up. They COULD have made room for more fuel, had it been asked for. When I say "made room," I mean in the design stage, not added on after the aircraft was built.

While the P-39 was not a BAD fighter, neither was it a top-notch fighter as we used it. The Soviet Union was sent P-39s and used them to great advantage in low-altitude, ground-support war operations ... it was our design, but the Soviets liked it better than we did. Go figure.

 

[tomo pauk]

...
... it was our design, but the Soviets liked it better than we did. Go figure.

Soviets liked it just for one reason - it was a better fighter than what they produced.

I'll refrain from commenting other stuff listed in the post.

 

[DarrenW]

We have a resident "P-39 Expert" on the forum so hopefully he'll chime in soon and help set the record straight concerning this aircraft.

 

[B-25 Pilot]

Thanks everyone for the help!

 

[Koopernic]

Hey everyone,

I am doing a school project and
I chose the P-39 to be on of the planes in my book. But I have a question, did the engine behind the pilot cause any problems?

Thanks ahead of time.

From an aircraft stability point of view it creates problems for the designer to solve. You want the centre of pressure of the aircraft/missile to be behind the centre of mass like a dart. Placing the engine in the middle tends to put the centre of mass and centre of pressure closer together reducing stability margins.

The remedy might be increasing area of the empennage (tail fin and stabiliser) or extending the tail to increase tail moment arm by increasing length. Both these cause increases in weight and drag. Slats or anything that helps stall/spin recovery can help and devices such as cuffs to even up stall angle against the circulating slip stream from prop rotation that makes one wing stall ahead of the other. You might shift mass from equipment such as guns and undercarriage into the nose (as the P39 did).

Anecdotally I believe the P39 did have problems with spin recovery, the solution was to make sure spin recovery was well trained and drilled into pilots. There is a report here:
http://www.wwiiaircraftperformance.org/P-39/P-39Q_Spin.pdf
P-39 Airacobra
"Its unique engine location behind the cockpit caused some pilot concern, but this proved to be no more of a hazard in a crash landing than with an engine located forward of the cockpit. The P-39's spin characteristics, however, could be quite a problem if recovery techniques were ignored. "
By contrast the Me 109 was aid to be the easiest aircraft in the world to recover from.

Jet aircraft have similar problems since the engines tend to be placed in the middle. The gyroscopic effect of the propeller and rotating slip stream and propeller P-factor create a lot of nasty handling problems but they also solve some hence jet aircraft tended to snake or have slightly dynamic stability problems (a slight tendency to snake)

I don't want to exaggerate this as the P-39 was a good aircraft but, yes it did have its challenges but they could be dealt with. Bell and the NACA were careful with the P-39 successor the P63.

There is quite a reasonable explanation in this wiki article of static stabillity margin:
Static margin - Wikipedia

 

[GregP]

Not too sure the P-39 was better than an La-5 or Yak-3, but would agree it was better than earlier LaGGs and Yaks as well as the I-15 / I-16 generation for sure. I could be wrong since the P-39 would be flying in its best envelope in Russian use ... it MIGHT be better than the La-5 / Yak-3. The Yaks were VERY good, but lightly-armed. I've always considered the La-5 / Yak-3 to be the start of the really good Soviet fighters.

I was thinking they probably liked the P-39 mostly because:
1: It came to them free.
2: It had a radio that worked.
3: The Soviets didn't fight at high altitude very much. They stayed low, flew ground support and hit the German troops forcing the Luftwaffe to come down and fight or stay high and watch their troops get killed by low-flying ground-pounders. The P-39 was no slouch below 12,000 feet and could hold its own there and possibly better than just hold its own, even against Fw 190s and Bf 109s. It just never got the chance in the ETO because the ETO was fought mostly at high altitudes relative to the Russian Front fighting.

Again, could be wrong ... but free fighters give any politician or accountant a warm, fuzzy feeling inside.

Nice discussion about static margin above.

 

[Koopernic]

Not too sure the P-39 was better than an La-5 or Yak-3, but would agree it was better than earlier LaGGs and Yaks as well as the I-15 / I-16 generation for sure. I could be wrong since the P-39 would be flying in its best envelope in Russian use ... it MIGHT be better than the La-5 / Yak-3. The Yaks were VERY good, but lightly-armed. I've always considered the La-5 / Yak-3 to be the start of the really good Soviet fighters.

I was thinking they probably liked the P-39 mostly because:
1: It came to them free.
2: It had a radio that worked.
3: The Soviets didn't fight at high altitude very much. They stayed low, flew ground support and hit the German troops forcing the Luftwaffe to come down and fight or stay high and watch their troops get killed by low-flying ground-pounders. The P-39 was no slouch below 12,000 feet and could hold its own there and possibly better than just hold its own, even against Fw 190s and Bf 109s. It just never got the chance in the ETO because the ETO was fought mostly at high altitudes relative to the Russian Front fighting.

Again, could be wrong ... but free fighters give any politician or accountant a warm, fuzzy feeling inside.

Nice discussion about static margin above.

The MiG 3 apparently had terrible spin characteristics so the P-39 would have seemed relatively viceless compared to the MiG 3 when the Soviets got them. It just goes to show that from a stability point of view a well designed mid engine aircraft can be better than a poorly developed nose powered aircraft. I think the MiG 3 vices could have been fixed but Joseph Stalin wanted the engines for the IL-2 Sturmovik so the MiG 3 was not improved or developed further.

I don't think Lend Lease to the Russians was entirely free. Unlike Lend Lease to the UK it had to be paid for. It was more in the nature of a guaranteed loan but the goods were highly discounted by 90%. The debt wasn't settled till 2006 though the Russians did supply a lot of chromium to the US during the war and other minerals. A Lend Lease item to the British was free if lost in combat and usually 10% if it was kept rather than destroyed after the conflict. Technically of course the British paid for it by leasing a few Caribbean naval bases to the US.

 

[GregP]

The UK made the last WWII debt payment to the U.S.A. and Canada in 2006.

The USSR never did repay the WWII lend-lease debt. They paid part of it and, since we never really expected any payment, maybe it was a win.

I'm sorry there was ever a reason for WWII debt in the first place. It would be interesting to go back to the 1880s and find a way to keep the Triple Alliance and other agreements alive. Maybe WWI would never have happened, thus maybe not generating a way or a need for Hitler to come into the limelight. von Metternich set up alliances that were allowed to expire and set up WWI. Funny how agreements penned in the mid-1800s managed to lead to WWI and WWII so many years later after they expired. I suppose that's off-topic, so I'll just let it die here.

 

[DarrenW]

By contrast the Me 109 was aid to be the easiest aircraft in the world to recover from.

Not to sound confrontational, but why the plug for the BF 109? Though probably better than the P-39, I'm sure there are many aircraft of the day with superior stall characteristics to that of the German fighter (most naval types for instance).

 

[tomo pauk]

Not too sure the P-39 was better than an La-5 or Yak-3, but would agree it was better than earlier LaGGs and Yaks as well as the I-15 / I-16 generation for sure. I could be wrong since the P-39 would be flying in its best envelope in Russian use ... it MIGHT be better than the La-5 / Yak-3. The Yaks were VERY good, but lightly-armed. I've always considered the La-5 / Yak-3 to be the start of the really good Soviet fighters.

There was no Yak-3 in service before 1944. La-5 was good, but it's full potential was not realized due to many things, 1st and foremost lax fit & finish.
Vast majority of Soviet fighters in 1942-43 were Yak series, that also suffered from bad fit & finish that hampered performance. The P-39, while far from an ideal fighter, didn't have had problems with fit & finish.
Similar conclusion happened when M4 tank was compared with T-34 (Yugoslav tankies noted in late 1940s that switch from T-34 to Sherman was akin to the switch from tractor to taxi).

I was thinking they probably liked the P-39 mostly because:
1: It came to them free.
2: It had a radio that worked.
3: The Soviets didn't fight at high altitude very much. They stayed low, flew ground support and hit the German troops forcing the Luftwaffe to come down and fight or stay high and watch their troops get killed by low-flying ground-pounders. The P-39 was no slouch below 12,000 feet and could hold its own there and possibly better than just hold its own, even against Fw 190s and Bf 109s. It just never got the chance in the ETO because the ETO was fought mostly at high altitudes relative to the Russian Front fighting.
Again, could be wrong ... but free fighters give any politician or accountant a warm, fuzzy feeling inside.
Nice discussion about static margin above.

1. - It mattered to the Soviet leadership, not so much to the pilots.
2. - Yes. Radio suite was elaborate on P-39s, so much that rumor has it that Soviets flew the P-39s with just one radio set (out of 3 total possible).
3. - Soviets didn't complained much, if at all, about the 37mm cannon, while stripping the P-39s by removing the outer LMGs & ammo, some armor and radio(s) helped with performance to be better than what was expected 'per book', unlike with Soviet fighters were series-made aircraft were of lower performance than 'per book'. The V-1710 will handle over-boosting much better than M-105 series of engines. So all in all, there is no wonder Soviet pilots rated P-39s very high.

 

[pbehn]

Development of the P-39 was difficult, putting a bigger engine in or a different type creates more issues.

 

[GregP]

The Allison and the Merlin are almost interchangeable, in their single-stage varieties anyway, as far as mounting goes. The Allison V-1710 has an SEA 50-spline prop shaft and the Merlin had either an SAE 50-spline or a Rotol #51 prop shaft. Easy to change between them if you have the parts. Both are 60° V-12 engine and both have a 6-inch stroke. The Merlin has a 5.4-inch bore and the Allison has a 5.5-inch bore for a difference of 61 cubic inches. Both weigh in and around 1,500 pounds and have quite similar outlines.

The main difference, for mounting purposes anyway, is the fact that the Merlin has an updraft carburetor and the Allison has a downdraft carburetor. Easy to see ... a P-51B-K has the carb intake on the bottom of the prop spinner and the duct runs under the engine to turn upward into the carb. In an Allison bird, the carb intake is on top of the spinner, as on an Allison P-40 or a P-51A. The duct runs across the top of the engine and turns downward into the carb.

It would be hard to change an Allison for a 2-stage Merlin, but going between single-stage units is no big deal. They did exactly that with the P-40. All the P-40s are Allison-powered except for the P-40F and L, which have single-stage Merlin in them.

Here is a P-40E. Note the carb intake on top of the spinner.
Redirect Notice

Here is a P-40F. Note the lack of carb intake on top.
Redirect Notice

I like the looks of the F myself, but the performance gain was very little, and wasn't worth the effort.

 

[tomo pauk]

The Allison and the Merlin are almost interchangeable, in their single-stage varieties anyway, as far as mounting goes. The Allison V-1710 has an SEA 50-spline prop shaft and the Merlin had either an SAE 50-spline or a Rotol #51 prop shaft. Easy to change between them if you have the parts. Both are 60° V-12 engine and both have a 6-inch stroke. The Merlin has a 5.4-inch bore and the Allison has a 5.5-inch bore for a difference of 61 cubic inches. Both weigh in and around 1,500 pounds and have quite similar outlines.

The main difference, for mounting purposes anyway, is the fact that the Merlin has an updraft carburetor and the Allison has a downdraft carburetor. Easy to see ... a P-51B-K has the carb intake on the bottom of the prop spinner and the duct runs under the engine to turn upward into the carb. In an Allison bird, the carb intake is on top of the spinner, as on an Allison P-40 or a P-51A. The duct runs across the top of the engine and turns downward into the carb.

... [/QUOTE]

The P-39 used the V-1710 with remote reduction gear. There was no such Merlin in production, meaning that Merlin-powered P-39 was impossible unless someone devotes time & money into making such one. The prototype Merlin with remote reduction gear was the Packard Merlin V-1650-5, intended for P-63, but I don't think that engine/aircraft combination ever flew.

I like the looks of the F myself, but the performance gain was very little, and wasn't worth the effort.

Too far away from the topic.

 

[GregP]

It does really not seem like that much trouble to so, Tomo. The gearbox and driveshaft were already in production. All they had to do was make it fit the Merlin nose case or come up with a remote nose case mount like on an E-series Allison as opposed to an F-series. The Merlin was already being produced with the correct SAE 50-spline prop shaft.

You are technically correct (no surprise there), but the problems involved in mating the existing remote gearbox and driveshaft to a Merlin are quite small when compared with other changes that were made to many aircraft during the war, while being bombed. The Japanese went from a V-12 to a radial in going from the Ki-61 to the Ki-100, and they didn't seem to have much trouble. The P-51 went from an Allison to a Merlin in the B-model without a lot of fanfare. The Tempest went through several engines without mush trouble other than the engines themselves. The P-40 even went from an Allison to a Merlin when it seemed a thing to try.

I see changing the engine mounts so a Merlin fits where the Allison did, finding a bottom air intake and duct, and designing one driveshaft component that mates to said Merlin. I would not think it all that difficult when compared with changing the ammunition location to eliminate the aft CG issues. What problems do you see that seem difficult?

 

[tomo pauk]

...
What problems do you see that seem difficult?

No problems at all.
I also have Brooklyn bridge on sale.

 

[GregP]

Don't buy it. Coronavirus in the bricks.