Improvements to the P-39

[wuzak]

In the thread about a turbocharged P-36 there was some discussion about improvements to the P-39 - mainly armament and some suggestions about the engine.

For armament it was thought that the M4 cannon could be replaced with a 20mm cannon or 0.50" HMG firing through the hub with 2 x 0.50" synchronised cowl guns.

What if we remove the cowl guns and hub cannon completely, moved the cockpit forward and installed a fuel tank between the cockpit and engine.

Have 4 0.5" MGs in the fuselage sides, the ammunition boxes under the fuel compartment behind the pilot, synchronised to fire through the propeller arc.

Some suggested 2 stage engines for the P-39, but a 2 speed drive for the supercharger would help some, as would improvements to the supercharger itself. Have the 7.48 and 9.60 gear ratios. Should maintain the performance down low, and add a little up higher.

 

[tomo pauk]

The layout of P-39C internals, from the lovely illustrated V-1710 manual. The fuel tanks are still non self sealing, thus having 170 gals of capacity, there are still 2 synchronised .30s in the nose, just one radio, no drop tank facility. The armor protection will be installed with the P39D, along with other changes.

Armor layout of the P-39D and subsequent (before the Soviets started ripping of parts of it ). Interestingly enough, the P-63 dispensed with the aft-most armor plate.

 

[tomo pauk]

...
What if we remove the cowl guns and hub cannon completely, moved the cockpit forward and installed a fuel tank between the cockpit and engine.
ave 4 0.5" MGs in the fuselage sides, the ammunition boxes under the fuel compartment behind the pilot, synchronised to fire through the propeller arc.

Reduction of the armament is a must. While not so excessively heavy as the P-40E and later, the P-39D and subsequent were 1000 lbs heavier than the Spitfire V, that have had more power.
This one has 5 HMGs, 4 in/under wings and 5th firing through the prop, cab is forward, fuel tank between pilot and engine:

Some suggested 2 stage engines for the P-39, but a 2 speed drive for the supercharger would help some, as would improvements to the supercharger itself. Have the 7.48 and 9.60 gear ratios. Should maintain the performance down low, and add a little up higher.

The 7.84.1 ratio gives 1325 HP military (15 min) power at ~2500 ft, no ram, as in A-36. The take off power could also be upped from 1325 HP, since the last engines with 8.80:1 S/C gearing were already at that T.O. power. However, the 2-speed drive would require 'shift' in the time line by some 2 years? The sooner the Allison comes out with 9.60:1 S/S gearing, the better for P-39.
I'd propose relocating the ram air intake to the roughly 10 or 2 o'clock position vs. 12 o'clock, so it is not shadowed by the cockpit, that would hopefully improve the altitude capability a bit. Introduction of water injection would've helped from SL to 15-18000 ft.

 

[wuzak]

Not sure that you can have the 0.50" firing through the hub with the cab forward design.

Agree about the air intake - perhaps one on either side?

 

[tomo pauk]

Perhaps it might fit, looking at the cutaway posted above the HMGs use blast tubes to 'clear' the cowling. The P-63 was with HMGs moved in the front, used very short blast tubes. It the proposal above, only half of the HMG length is behind the reduction gear.
But then, even if it does not fit, loss is not that great.
I'm okay with either one or two intakes.

The earlier introduction of better propeller should've helped, too. Props installed in the different P-39s:

 

[drgondog]

The key issue with large fuel cell in front of cockpit is the same as having a larges supply of ammunition - namely a large CG change as the fuel is consumed... suspect stability is seriously affected and the P-39 already had those issues.

 

[wuzak]

The key issue with large fuel cell in front of cockpit is the same as having a larges supply of ammunition - namely a large CG change as the fuel is consumed... suspect stability is seriously affected and the P-39 already had those issues.

Would it need an extension at the front or moving the engine back to get the fuel on CoG?

 

[wuzak]

The key issue with large fuel cell in front of cockpit is the same as having a larges supply of ammunition - namely a large CG change as the fuel is consumed... suspect stability is seriously affected and the P-39 already had those issues.

The fuel cell would be behind the cockpit, in front of the engine.

Just over the leading edge of the wing in Tomo's picture.

 

[tomo pauk]

The internal fuel tankage was one of major shortcomings of the P-39, not helped by usually small amount of fuel in the external tank, usualy 75 gals or smaller. The elusive 175 gal tank sometimes mentioned is so elusive it is not featured in two P-39 manuals I took a look today
The fuel carried should also not be over-done, we'd be back on the overweight P-39. So, before the uprated engine is available, perhaps best bet would be to design a drop tank of interim dimensions, bigger than the 75 gal, but not that long to interfere with front U/C leg. A 120-130 gals should give some boost to the endurance/radius. Further, having two wing racks should add flexibility, since it would not require the 'P-39 custom' drop tanks.
Once the engine power is up, talk mid 1942, in the place once occupied by wing guns and ammo maybe some fuel tanks could be installed, so the internal fuel is at 150 gals or thereabout.

Interestingly enough, the P-63 featured the external self sealing 'combat' tank, slipper form, carried under the belly, 65 gals.

 

[GregP]

There is some room in the wings and some room in the fuselage for fuel Not sure how much but most wing fuel tanks don't seriously affect the CG as fuel is used. If you were to move the pilot forward maybe 2 - 3 feet, you could probably get another 100 - 120 gallons behind him, but how many guys want to sit in front of a gas tank. A forced landing might be interesting. Of course, the Bf 109 guys used their fuel tank as a seat, so maybe the consideration is moot. If you DO move the pilot forward 2 - 3 feet, is it still possible to have fuselage armament other than the cannon? I don't know but, if so, the barrels will be shorter, for sure.

The P-39 basically has a "slow" airfoil and wasn't ever going to be serious competition for the P-51, but it could certainly be a tough nut to crack below 15,000 feet when properly flown at moderate airspeeds.

The P-63 was bigger all around for a reason, and space was a prime example of it.

I have heard that fitting a P-63 prop to a P-39 was suicide, but have little in the way of hard evidence. Mike Carroll found out the hard way when his P-39 proved uncontrollable after takeoff using a P-63 prop. I believe the surface area added forward just overcame the fin and rudder with polar moment of inertia issues, but there could also be other variables affecting it. Having 4 feet cut off each wing combined with 2,850 HP on tap can seriously affect the ability to conrol the aircraft. I have almost no real data about the plane as flown that day. Most of it is heresay and supposed witnesses can't agree on major things, much less minor details. What we know is Mike hit the horizontal stabilizer when he departed the aircraft and didn't manage to survive the ensuing events.

 

[tomo pauk]

The wing of the P-39 was certainly not that 'fast' as it was the case for the P-51 or Spitfire, but it was not that 'slow' either - 15% at the roort, 9% at tip. That is as thin (in percentage, or relative terms) as Fw 190, and we know that P-39 was as fast or faster on same or similar power when compared with P-40, Ki-61 or Spitfire V. Or the host of smalish fighters like Bf 109E/F, MC.202, Yak-1/7/9 or Lagg-3, even when they got the fit finish right.
The less we say of the P-63's wing, the better - far thicker, yet without capacity to have the BMG inside, let alone the 20 mm, with too low amount of fuel until the too late redesign. There was still plenty of space for fuel in the P-39's wing.
Even with its 7 guns, the P-39N was still good for 385 mph; the P-39Q without pods and with a bit better engine was still faster, in 400 mph range.

 

[drgondog]

The slipper tank on the P-63 was at the CG - which is where you want to put All external loads if possible. Placing a 'saddle' extra fuel tank between back of pilot's seat and in front of the Allison is impossible without alterin airframe. The P-39 and P-63 ultimately were as good as they would ever be.

 

[tomo pauk]

It might be interesting to note that a modified P-39 won the Thompson trophy race in 1946. In 1947, it came third, after the two modified Super Corsairs. Races were done at low level, though.
Re P-63 - lack of internal fuel was one of major things that turned down any USAF desire for it, so it was not as good as realistically possible. It was also much draggier than the earlier P-51 - 20% greater Cd0, and with barely smaller Cd0 than the P-39. Plus - external HMGs despite such a thick big wing?

 

[drgondog]

Re P-63 - lack of internal fuel was one of major things that turned down any USAF desire for it, so it was not as good as realistically possible. It was also much draggier than the earlier P-51 - 20% greater Cd0, and with barely smaller Cd0 than the P-39. Plus - external HMGs despite such a thick big wing?

Tomo - there was no place to put more internal fuel in the P-39. Only opportunity to improve performance in any (practical) way, short of re-design, was to improve the engine - which did materialize with the P-63.

Thin airfoil, all things being equal, is useful relative to Mcr. The actual airfoil properties, thick or thin, apply to L/D and Cdo. The P-39 wing was a NACA 23015 which was the same as the FW190 IIRC. The P-38 had the 23016 for 16% T/C. Pretty much the same L/D between all of them as far as the airfoil section component is concerned. Both the P-39 and FW 190 had less parasite drag (by far) than the P-38 primarily because of size/area/friction drag difference.

For example, The CDo of the Mustang wing was far lower than the Spit which had an appreciably lower T/C but the L/D for the NACA 45-100 (Mustang A/B/C/D/K wing) ratio was significantly better.

 

[FLYBOYJ]

Don't make combat performance comparisons to the post war Thompson Trophy Racers. These aircraft were modified (no guns, armor and probably radios) and a lot of the end results had to do with pilot skill in flying a perfect line around a circular course without cutting a pylon as well as engine performance for the days of the races.

 

[tomo pauk]

Stating the racing qualities was to reinforce the opinion that P-39 was not a slow machine in the 1st place

Whether the P-39 was or was not 'fuel upgradable' should have nothing to do with P-63 having such a small fuel quantity - half of what vast majority of Merlin Mustangs had. Vs. the wing of the P-39, the one on the P-63 was greater in both in thickness and area. That there was enough of room for better fuel quantity was shown with the too late P-63D, with ~30 to 40 gals more internal fuel than with other P-63s, plus the tank for ADI fluid already from mid -A models.

Wing of P-39, -D to -N (similar on the -C and -Q), the '6-cell' fuel tank is visible, as is the place for LMGs and their ammo boxes; ditch the LMgs and ammo and put some fuel tanks there?
IMO, either the USAF or Bell or both 'overdid' the armament package on the P-39, that in weight was comparable with what P-38 carried with help of more than twice the engine power. Reducing it down to 3-4 HMGs, or 20 mm + 2 HMGs only shoud improve both speed by a bit, and RoC.

Wing size/area comparison between P-39 and P-63 (the bigger one), never mind the cyrilic:

 

[GregP]

I have to give it to you, Tomo. You are definitely dedicated to this.

 

[Piper106]

Wing of P-39, -D to -N (similar on the -C and -Q), the '6-cell' fuel tank is visible, as is the place for LMGs and their ammo boxes; ditch the LMgs and ammo and put some fuel tanks there?

Thanks for the overall cutaway and the wing drawings Tomo.

As I recall when Bell employees prepared Cobra I and II for the Cleveland air races, in the machine gun and ammunition bay of each wing, they were able to put a 50 gallon fuel tank, I am sure that was a thin wall sheet metal tank, a self sealing tank would have been less capacity.

Question about the P-39C. The drawing shows two 85 gallon unprotected tanks for 170 gallons total. I have always read that the P-39D and later with self sealing tanks only had 126 gallons capacity. Was the rubber bladder of a self sealing tank really that thick that you lost that much capacity?? Or were there other changes made that cut the fuel capacity of P-39 after the P-39C???

 

[tomo pauk]

The P-39, at least the -D and subsequent, have had 12 fuel bags (divided by ribs), 6 bags each comprising a tank, decreasing the fuel carried vs. what could be with a single tank per side, like the P-51 for example. The bags were connected by short pipes. Some P-39s were even shipped with total fuel capacity reduced, and there were kits to restore the capacity back to 120 gals.

Majority of the P-63 used two 'unitary' fuel tanks, total capacity either 126 or 136 gals, not sure about the P-63D

Design analysis of the P-39:

 

[Just Schmidt]

I absolutely agree that the P-39 wasn't slow, in fact at its entering service it must have been one of the fastest fighters of its day.

On the other hand it had problems with climb rate in the Pasific, often enabling Japanese bombers to bomb unopposed from medium attitude. In fighter vs fighter it probably suffered the same initial problems with tactics as most other allied fighter8forces) in the initial many months of the pasific war. I comparison with the P-40 it was also a more untried design (airframewise), and the caothic situation in the early part of 42 combined with primitive and partly improvised logistics will have made it extra challenging to overcome theething troubles.

On the other hand an extensive redesign could also be problematic. The moving forward of the pilot might not only create trouble for the fuselage mounted armament, but also for the nose wheel and the transmission from the engine drive shaft. I don't have my books 7cutaway here, and haven't tried modeling a solution, but I am not optimistic on that count.

More basic alterings as removal of armament to reduce weight seem the easy way to save weight, later incorporated at production level. I believe that 1x20 and 2x12.7 would actually be sufficient in the first years of the pasific war. The Russians did that and spent much effort on familiarising pilots with the new aircraft, and seemingly got good results, countering snags as they went along. Though i don't remember in which post, i base this primarily on a link supplied in this forum last summer. A resent work by a russian it has its limitations (fx recounting the claim of He-113's), but other parts seem solid enough. IIRC it dosn't delve into the issue with tuning the Allisons.

I believe the USA could have achieved similar quite immediate results had they expended the same effort, but didn't because the P-40 came close in performance and probably seemed as a safer bet. Even then the P-39 served for quite a while in numbers with the western allies. The early catastrofic losses by the VVVS, and yet to be mass use of La's and Yaks arguably made the russians more eager to spend a lot of effort on getting results by the best performer then supplied by lend lease.

Of course a lot of redesign would have been needed to make the P-39 into a good performer at higher attitudes over western europe, efforts probably better spent on the aircraft for that role that historically was designed. Here redusing the armament would be less attractive too, though getting rid of/replacing the 7.7's probably wouldn't hurt much, also simplifying issues of supply.

In a hypothesised lack of alternatives for the Pasific (and eastern asian fronts), I believe that the USA could and would have made better use of the P-39 in an easily improved form.