Could you have designed a better P-39?

[Shortround6]

GUYS THIS ENGINE IS RATED AT 1150HP @ 21400 FT MILITARY AND ONLY USED IN THE XP-39E AND IN THE P-63 PROTOTYPES

IT PROPELLED THE XP39E TO 386MPH AT 21500 FT JUST THINK WHAT IT COULD HAVE DONE IN THE P-51A



MY PREVIOUS POSTED PICTURE IS A P-39C NOSE ATTACHED TO THE XP-39. ALSO IN PLACE OF THE WING .30 CALS AND AMMO ARE INTERNAL FUEL TANKS TO ROUND OUT THE INTERNAL FUEL LOAD AT 180 GALS INSTEAD OF 120 GALS.

AND WITH THE B-5 TURBO ADJUSTED PROPERLY IT SHOULD DO 380 MPH ABOVE 18000 FT. THE ONLY FIGHTER IN 1940 TO CATCH IT IS THE HE100D.

And with the extra length and weight it just wouldn't fit in a standard P-39 fuselage which is why the XP-39 was over a foot longer.

Why do you think the P-39 could have hit anywhere near 380mph at 18000ft? the orginal XP-39 from what evidence there is never actually flew at the numbers claimed for it and the Guys at Langley with the full size wind tunnel estimated it was 40mph below the Bell calculated figures. Now if had really flown 390mph at 20,000ft as claimed and the guys at Langley said it would only do 350-360 don't you think there might be more dispute over the Langley results?

By the way, just what weight do you estimate for your super P-39?

 

[Colin1]

Whoops
my bad, I stated that the V-1710-47 powered the P-51A (post #50), which it didn't, the -81 did

Sorry!

 

[Clay_Allison]

And with the extra length and weight it just wouldn't fit in a standard P-39 fuselage which is why the XP-39 was over a foot longer.

Why do you think the P-39 could have hit anywhere near 380mph at 18000ft? the orginal XP-39 from what evidence there is never actually flew at the numbers claimed for it and the Guys at Langley with the full size wind tunnel estimated it was 40mph below the Bell calculated figures. Now if had really flown 390mph at 20,000ft as claimed and the guys at Langley said it would only do 350-360 don't you think there might be more dispute over the Langley results?

By the way, just what weight do you estimate for your super P-39?

I guess it would be about halfway between a P-39 and a P-63.

 

[krieghund]

The story of the test flight maximum speed of the XP-39 being at 390 mph has been reprinted so many times it has become somewhat of an urban legend.
The flight history (all in Dayton Ohio) is thus;
6 Apr 1939, retired Navy Lt. James Taylor takes it up for 20 minutes and says handling is pretty good. (legend says this test flight he did 390)
7 Apr 1939, he flew it again for Gen Hap Arnold but had to return early due to oil pressure and oil temperature problems.
22 Apr 1939, he flew it two times this day for a total of 47 minutes. He recorded all his findings and observations in his dairy. (This would be a crucial find for this subject)
Daniel D Whitney's "Vees for Victory - The Story of the Alison V1710 Aircraft Engine 1929-1948" pp79-988 has a bit to say on the development of the P-39 and the removal of the turbo.

> During 1937 when Alison had sold only 15 V1710 they pushed to develop altitude rated rather than turbo versions as they felt that their sales problems were partly due to Air Corps focusing on inadequately developed turbo charging.
>XP-39 Flight testing was delayed several months while Alison fixed vibration problems, resulting from torsional vibration of the extension shafts, found during ground running tests. This had been only partially alleviated by first flight so certain RPM had to be avoided.
> He says "It is clear from recently located NACA test reports on the XP-39 that it was not meeting the contracted performance guarantees. While it has been reported that the aircraft was able to climb from takeoff to 20,000 ft in five minutes, and that the maximum speed at that altitude was 390mph, with the airplane weighing 5550 lbs the data does not show it.
...
Furthermore, given that General Arnold was hurriedly arranging to have NACA put the airplane in its wind tunnel for drag reduction tests only a month after the first flight, suggests that all was not well."
> The XP-39 was 10% overweight at 6104 lb on official weigh in versus a contracted 5550 lb.
> Flight testing had shown inadequate cooling of oil, Prestone and inter-cooler. NACA found for example the radiator required 10,250 scfm and that during climb at 160mph only 7,880 scfm was provided.
> NACA tests showed that the various air inlets were badly arranged with internal constrictions from the way the aircraft had been packaged (wing spars, landing gear obstructions etc) reducing air flow.
> A table of drag breakdown is given showing turbo charger related drag (Turbo + waste gate + intercooler) was about 16.4 % of the aircraft total.
> Intercooler air flow was only 1,600 scfm at high speed while 5,000 to 7,000 was required if the engine was to achieve full power without detonation.
> External arrangement of air inlets was causing asymmetrical stalling with the left wing root stalling at several degrees angle of attack before the right.
> As received by NACA they estimate speed to be 340mph at 20,00ft with 1150bhp
> Following testing of modifications recommended by NACA drag was reduced to a level where it was estimated that 392mph at 20,000 if 1150 bhp was provided by the engine.
> NACA considered an altitude rated V1710 and with the other changes this was expected to give about 402mph at 13,200 feet with the elimination of the drag from the turbocharger and intercooler even before allowing for the reduction in weight from the removal of the turbo. NACA estimated that the performance at 20,000 feet would be "about the same" as the turbo version
> NACA recommendations were
- Improve streamlining of wheel well doors
- Lower the canopy
- Remove turbosupercharging due to high installation drag.
- Relocate carburetor scope from left side of fuselage to just behind the canopy.
- Install the altitude rated engine.

To me this indicates the 390 mph at 20,000 ft is a probably a myth, being the NACA predicted value if the sum of all recommendations had been applied and that the V1710 provided 1150 bhp at that altitude.

I am presently working up the computer model of the XP-39A to show the performance of the aircraft if it had proceeded to the development stage as the P-39C.

I will show that the combat weight will be similar to that of the production P-39D but with over 230 BHP available at 20000ft and 443 BHP at 25000ft at military power.

 

[Piper106]

Shortround6 hit the nail on the head. Too much space and weight taken by armament.

Four .50 machine guns would have be enough. (note 1 below) Three would be mounted to fire synchronized through the propeller disc (200 rpg), and one would be fire through the propeller hub (270 rounds). (Note 2 below) Compared to the 37 mm + two .50 + four .30 guns carried by the D through N models, this new four .50 gun arrangement would have weighed 175 pounds less. Compared to 37 mm + .50 + .30 ammo, there would have been at least 50 pounds less ammo on board at takeoff.

With the .30 wing guns removed, the empty gun bay in each wing could be used to hold an additional 30 gallon fuel tank, allowing up to 186 gallons internal fuel for longer missions. (note 3 below).

I would combine those changes with some serious arm twisting at Allison to get more power. NOW!!!.
They needed to get their 9.6:1 high allitude blower gears (or the bigger late war G series 10.25" diameter supercharger OR a license built Merlin 45 blower grafted onto the V-1710 engine) plus clearance to operate at 3200 rpm for war emergencies at lot sooner then history tells us.

Piper106

Note 1: By early 1941, British reports from the Battle of Britian would have shown reasonable people that .30 caliber machine guns were ineffective, and that the hitting power of the 37 mm cannon was overkill for anyting the Axis powers were putting into the air.

Note 2: The P-39C had two .50s and two .30 in the nose in addition to the 37 mm cannon. On the P-39D the two nose .30 guns were removed to make room for more 37 mm ammo. With the 37 mm cannon out of the way, three synchonized machine guns would have been practical.

Footnote 3; Post-war, the two P-39s converted for air racing, Cobra I and Cobra II had a 50 gallon fuel tank in each gun bay. Agreed, these were tanks made from thin sheet metal, but even allowing for the thicker rubber used for self-sealing tanks, it would seem reasonable that 30 gallons could have fit into the space available.

 

[Wavelength]

The first thing I would have done was get rid of the car doors.

How did Diamler Benz get okay altitude performance out their 600 series motors?

This is just a what if, but what would a P-39 with a DB motor have been?

 

[Milosh]

The first thing I would have done was get rid of the car doors.

How did Diamler Benz get okay altitude performance out their 600 series motors?

This is just a what if, but what would a P-39 with a DB motor have been?

By using a variable speed supercharger. If the DB's supercharger had been fitted to the Allison engine......

 

[Shortround6]

By using a variable speed supercharger. If the DB's supercharger had been fitted to the Allison engine......

it wouldn't have changed much. The DB supercharger wasn't providing as much boost as the Allison supercharger. The DB engine didn't need (and wouldn't stand) the same level of boost as the Allison.

 

[Wavelength]

What about employing the DB design concept for driving their supercharger to drive the Allison's supercharger impellers?

 

[Shortround6]

What about employing the DB design concept for driving their supercharger to drive the Allison's supercharger impellers?

Allison did use the " DB concept" to dive the 1st stage of their later 2 stage engines. There is nothing "magic" about using a variable speed drive. It will allow for more efficient use of the supercharger at LOWER altitudes than a single speed or two speed supercharger and it has no NOTCH effect as do 2 and 3 speed superchargers. It is however, a bit heavier and more complicated (expensive)than a 2 speed drive and it does heat up the engine oil when it is 'slipping' at lower altitudes requiring a slightly larger oil cooler. The 'DB concept" is actually fairly close to the torque converter in most automatic car transmissions.

 

[Markus]

Shortround6 hit the nail on the head. Too much space and weight taken by armament.

Four .50 machine guns would have be enough. (note 1 below) Three would be mounted to fire synchronized through the propeller disc (200 rpg), and one would be fire through the propeller hub (270 rounds). (Note 2 below) Compared to the 37 mm + two .50 + four .30 guns carried by the D through N models, this new four .50 gun arrangement would have weighed 175 pounds less. Compared to 37 mm + .50 + .30 ammo, there would have been at least 50 pounds less ammo on board at takeoff.

I did some math on that.

The weight of the guns is 1*238lb, 2*70lb and 4*25lb = 478 lb
The weight of the ammo is 60*1.72lb, 400*0.3 and 4,000*0.065lb = 482lb
That makes a total of 960lb!

Your proposed armament would weight 280lb(4 guns)+261lb(870 rds of ammo), makes 541lb, saves 419lb.

Though 200 to 270 rounds per gun is not much. The four-gun Wildcats had 430 rounds per gun and the USN pilots were not happy when the six-gun version reduced that to just 240. I prefer three cal.50s in the nose and 400 to 450 rounds per gun. That gives the pilots at least twice the firing time and still saves at least 355lb.

 

[Clay_Allison]

Alternately, the U.S. Military was aware of the Gast Gun and a compact two-barrel setup capable of 1600 RPM might have been been made to fire through the prop hub of the P-39. A tight center-line stream like that could make for very easy shooting.

 

[V-1710]

I think the real 'problem' with the P-39 was the U.S.A.A.C., not the aircraft itself. The original mission of the P-39 was that of a high altitude bomber interceptor, powered with a turbocharged Allison V-1710 (similar to the YP-37) and armed with the Oldsmobile built 37mm. cannon. The aircraft didn't need to have a long range, and of course less fuel kept the weight down for a fast climb. The 37mm. cannon would have been very effective against bombers. High wing loading would not have been an issue as the P-39 was orginally designd to combat large bombers, not other fighters. The turbocharged Allison, though complex and tempermental, had fine high altitude performance characteristics. During development, the U.S.A.A.C. basically informed Bell that the interceptor P-39 was not needed, and directed Bell to develop the P-39 as a ground attack aircraft. In that role, the turbocharger was not needed. Armor and additional machine guns were added, and coupled with the less powerful single-stage supercharged Allison, performance suffered. However, the Russians proved time and time again that the P-39 was quite effective in the ground attack role, so I think it is safe to say the transition was successful. The P-39 was simply not ever intended to be a pure fighter.

 

[Colin1]

The P-39 was simply not ever intended to be a pure fighter

Your source?
The P-39 was faster than the A6M in some regimes, it was more heavily armed than the P-51. It was undeniably manoeuvrable, the Bf109 could not compete with the P-39 in a turn. Even with the Bf109 on the P-39's tail to begin with, the P-39 would reverse the advantage within two turns.

NACA had a big say in the deletion of the turbocharger, not that Bell were complaining about it; US turbochargers of the time were unreliable. And bulky, even with the negation of the ducting facilitated by the position of the powerplant, it was still a job and a half to shoehorn it into the dainty proportions of the P-39. The point is that notwithstanding the absence of the turbocharger, adequate supercharging would have made the P-39 a superb fighter, possibly constrained to the interceptor role.

 

[V-1710]

My sources were 'Vees for Victory' by Daniel D. Whitney (THE definitive work on the Allison 1710) and 'the American Fighter' by Angelucci/Bowers. 'Vees' has interesting commentary of NACA's role testing and improving the P-39's aerodynamics. It appears to be the opinion of the author that the decision to remove the turbocharger had been made by "the powers that be" before NACA had begun testing. In fact, it would seem that NACA was given the task of redesigning the intake system for the single-stage V-1710. That would be consistant if the U.S.A.A.C. had decided to change the P-39's role to ground attack from interceptor. Further, remember the U.S.A.A.C. held the belief in the mid-30's (and probably up until the time of the Battle of Britain) the air-to-air combat between fighters would not be much of a factor in any envisioned military action to defend the U.S. (read isolationism). This mindset lead to the emphasis of heavy long range bombers over every other type of combat aircraft, as such aircraft would be the most effective method of repelling enemy naval attacks on the continental U.S. (the only thing the U.S. military was concerned with at the time). The FM-1, YP-37, P-38, P-39, P-40, and P-43 were all born out of this 'original sin', as each aircraft was either designed as an interceptor or a ground attack/support fighter. Only the P-38 had the ability to function as a world class air superiority fighter at the time.

 

[Shortround6]

What I find amazing about this "ground support" theory is that none of those planes were fitted with bomb racks in their early forms.

It is not because this was a new idea, even in US service, because many US biplanes could carry small bombs and the P-26 could carry a respectable bomb lad for a plane of it's size and power. The P-35 was rated for something like a 300lb bomb load.
Export P-36s could be fitted to carry a respectable bomb load so it is not like Curtiss didn't know about it or promote it.

I have read that the army didn't want what few fighters they had being drawn off to ground support missions or perhaps it was an attempt to get more money from congress for more planes? each type having a role so more total aircraft were needed rather than getting fewer multi-mission aircraft.

The original P-40 was a particularly strange plane to be considered as ground support. A liquid cooled engine, which the army already didn't want for attack planes (see attack planes of the period or early thirties) and just a pair of .50 cal Mgs for armament with NO bomb racks? A very expensive and inefficient way to get a pair of .50s into strafing position.

Considering that the Army had planes like this: Factsheets : Northrop A-17A

Both in service and on order for ground support the ground support theory for US fighters doesn't seem to hold water very well. Mid to low altitude fighter to control that airspace FOR the attack planes may make more sense.

 

[V-1710]

Good points. In addition to the A-17A, there was also the A-24. In any event, the U.S.A.A.C. seemed convinced most fighter operations would take place at low to medium altitudes.l

 

[Markus]

In any event, the U.S.A.A.C. seemed convinced most fighter operations would take place at low to medium altitudes.l

If so, why did they spend two decades to get a turbosupercharger working and designed planes like the FM-1 and P-38? I think the USAAC´s infatuation with heavy, high flying bombers convinced them they needed high flying fighters to shoot down the enemy´s heavy bombers.

 

[V-1710]

If so, why did they spend two decades to get a turbosupercharger working and designed planes like the FM-1 and P-38? I think the USAAC´s infatuation with heavy, high flying bombers convinced them they needed high flying fighters to shoot down the enemy´s heavy bombers.

I agree. Back to the original point that the P-39 was designed as an interceptor, as was the P-38 and FM-1. I think the U.S.A.A.C. made a destinction between the roles of a 'fighter' (as in P-40 or P-43) and an 'interceptor' (P-38 and original P-39).

 

[Piper106]

I did some math on that.

The weight of the guns is 1*238lb, 2*70lb and 4*25lb = 478 lb
The weight of the ammo is 60*1.72lb, 400*0.3 and 4,000*0.065lb = 482lb
That makes a total of 960lb!

Your proposed armament would weight 280lb(4 guns)+261lb(870 rds of ammo), makes 541lb, saves 419lb.

Though 200 to 270 rounds per gun is not much. The four-gun Wildcats had 430 rounds per gun and the USN pilots were not happy when the six-gun version reduced that to just 240. I prefer three cal.50s in the nose and 400 to 450 rounds per gun. That gives the pilots at least twice the firing time and still saves at least 355lb.

First none of the P-39s carried 60 rounds of 37 mm ammo, the standard references indicate that the capacity of the ring magazine in the D and later models being 30 rounds. Second, while the ammo boxes may have been able to hold 4000 rounds of .30 ammo, hopefully they were never filled. A more realistic accounting would be that the ammo weight at take-off would have been reduced about 120 pounds.

References indicate that the .50 Browning fired at about 600 rpm when syncronized, so 200 rpg would have given about 20 seconds worth of fire.
Same time of fire for the centerline mounted 50 with 270 rounds.
But I could buy into your concept also, three .50s with 400 to 450 rpg.

Piper106