XP-39: pros cons

[tomo pauk]

The prototype of future Aircobras was featuring a turbocharger, providing an excellent all-altitude performance for the era. The turbo installation was deleted shortly after XP-39 flew, so serial-produced P-39 were lacking performance, above 12 kft especially.

If someone has a more detailed info re. XP-39 pros* cons (esp. connected to the turbo installation), please post here

* not counting already-covered high performance

 

[Shortround6]

Tomo, this has been gone over in other threads, but basically, The XP-39 did not meet the performance numbers promised.

There is no real evidence that it ever came close to the numbers given for in many books/web sites. And there is evidence that it couldn't have done what is claimed during the time period it is claimed it flew those numbers.
1. There was a potential problem with drive-shaft vibration that called for a redesigned heavier drive shaft to be fitted, This was not done until after the NACA wind tunnel tests and until fitted the engine was restricted to 2600rpm. No where near full power.
2. XP-39 was plagued with both oil and coolant overheating problems which called for redesign of both the oil cooler and radiator ducts.
3. General Arnold was making arrangements less than a month after it's flight to have the XP-39 put into the full size wind tunnel at Langley.
4. Contract weight was 5,550lbs, when weighed at Wright field during "expedited" acceptance trials it weighed 6,104lbs. about 10% over weight.

Many people claim that army generals and the NACA "ruined" the P-39. They never go into any details except to say they removed the turboPlease note the above was before the NACA got their hands on the P-39.

In their report they (the NACA) claim the XP-39, as they received it was good for 340mph at 20,000ft and just under 280mph at sea level.
There were severe problems with the airflow for the radiator, oil cooler and intercooler.
For the last, US practice of the time was that the intercooler should remove 1/2 of the heat added by the turbo-supercharger to the intake air before it entered the engine carburetor. In the XP-39 the NACA estimated the intercooler (based on airflows) was removing only 25% in high speed flight and about 12% during climb. This significantly affected power output. The inter cooler was small in size to fit the airframe and had a high pressure drop across it. this meant high drag. The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.
Bell did at least two mock-ups of turbo/intercooler units in 1941 to be 'added' to the P-39. the extra drag of these units caused 40-45mph speed loss at the lower altitudes.

 

[tomo pauk]

Thanks for clearing it up; I've read (P-39 Detail Scale; Americas 100K - guess they believed Bell ) the figure of 390 @ 20K.
Quite a disappointment re advertised vs. achieved :\

 

[tomo pauk]

After reading that power @ military rating was just 1150 HP @ 20kft for the XP-39, the 390mph figure looks achievable only in shallow dive

 

[tomo pauk]

FWIW, here is the turbo installation of our plane:

 

[davebender]

I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.

 

[Jabberwocky]

I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.

Depends on how you define the term "successful turbocharged fighter".

I'd call the P-30A a minor success, proving that a turbosupercharged engine gave significant performance gains at altitude. Despite being built in only small numbers, and being somewhat heavy and ungainly due to the two person requirement, the fighter's altitude performance was much better than the P-35 and P-36 which followed it.

 

[Shortround6]

I don't doubt the GE turbocharger salesman made such a claim. It's unfortunate the U.S. Army Air Corps believed this claim even though nobody had yet produced a successful turbocharged fighter aircraft.

Are you perhaps referring to Dr. Sanford Moss?
Try defining successful please.
In addition to the 50 P-30s built there were small batches (1,2,5) of turbocharged Biplane fighters from about 1925/26. These were single stage systems (no engine supercharger) but they certainly gave the Army a rather good idea of what was possible even though the early installations had some practical problems. Experience going back 10 years with 60-70 (more?) airframes before the the Airacuda and P-37 make the scene?
Turbos might not have been ready for squadron service in wartime until 1942 but the Army had a rather good idea of what they were getting, both in benefits and costs (weight, bulk, cost of the equipment.

That is unless you can show documents that say otherwise?
edit>
And what other "salesmen" should the Army have believed?

The one with the two speed supercharger drive?
First used on the Armstrong-Siddley Tiger VIII in the mid 30s it doesn't make the US until about 1937 on a few Wright Cyclones, P&W soon follows but most (all?) P-36s with P&W engines have single speed superchargers.

The one with the mechanical drive two-stage supercharger? That would be P&W With two, count them-two, fighters equipped with them in the 1939 fighter trials. It wasn't really ready for squadron service at the time either.
<edit

 

[Shortround6]

FWIW, here is the turbo installation of our plane:

Thank you Tomo.

I have no idea why (not enough cooling airflow front to back-not enough cooling area for the bottom to top charge air?) but this set up provided only about 1/2 the needed intercooling for the performance that was needed. that is for an 1150hp engine, as power grew to 1325hp or even 1425hp (P-38 engine) the intercooler would have had to get even bigger.
The turbo installation of the P-39 (turbine hanging out the bottom for cooling and the four exhaust pipes) also left something to be desired drag wise.

 

[davebender]

The manufacturers of the two most successful V12 fighter engines during the late 1930s. Daimler-Benz and Rolls-Royce.

 

[tomo pauk]

About low cooling performance of the inter-cooler on XP-39: perhaps it was the position, combined with small size of the intercooler (compared with what, even early, P-38 were equipped with) that was hampering it? My take is that during the climb the inter-cooler was in 'aerodynamic shade' of the wing, since the plane experiences increase of angle-of-attack. Mounting of intercooler of greater capacity* to a position akin of D.520/Hurricane would've been better IMO. The relocation of 4 exhaust pipes' exits to the back side of wing fairing would've been cool too.

Having a P-39 with such changes with 1325HP @ 25kft makes a nice asset for second half of 1942.

*we pay the drag penalty/loss of speed under cca 12kft altitude, P-40s non-turbo P-39 cater for that during 1942

 

[Milosh]

The manufacturers of the two most successful V12 fighter engines during the late 1930s. Daimler-Benz and Rolls-Royce.

D-B and R-R were trying to sell engines to the Americans?

 

[Shortround6]

About low cooling performance of the inter-cooler on XP-39: perhaps it was the position, combined with small size of the intercooler (compared with what, even early, P-38 were equipped with) that was hampering it? My take is that during the climb the inter-cooler was in 'aerodynamic shade' of the wing, since the plane experiences increase of angle-of-attack. Mounting of intercooler of greater capacity* to a position akin of D.520/Hurricane would've been better IMO. The relocation of 4 exhaust pipes' exits to the back side of wing fairing would've been cool too.

Having a P-39 with such changes with 1325HP @ 25kft makes a nice asset for second half of 1942.

Tomo, when climbing many planes flew at a speed of 160-200mph. with top speeds of 320-400mph you are going to get twice the airflow through the intercooler per minute.

*we pay the drag penalty/loss of speed under cca 12kft altitude, P-40s non-turbo P-39 cater for that during 1942

cater how? provide more airplanes? Non-turbo P-39s PLUS turbo P-39s? how many more P-39s do you plan to build and where do the pilots and ground crew come from?

 

[tomo pauk]

Thanks for pointing to the speed difference.

I don't plane to build more P-39s; the idea is that, as number of turboed P-39s produced increases, the number of non-turboed decreases. Eventually in some time in 1943 P-39 is built as turboed exclusivelly, along with other changes as stated in other thread.

 

[Shortround6]

D-B and R-R were trying to sell engines to the Americans?

Rolls -Royce and Diamler-Benz Superchargers were better than Allison super chargers in 1937-39?

 

[Shortround6]

Thanks for pointing to the speed difference.

I don't plane to build more P-39s; the idea is that, as number of turboed P-39s produced increases, the number of non-turboed decreases. Eventually in some time in 1943 P-39 is built as turboed exclusivelly, along with other changes as stated in other thread.

No matter how we gloss over it the P-39 was a plane with definite limitations. Unless you can figure out how to get 1 1/2 liters in a 1 liter bottle it was never going to do what was originally promised. Bell had a rather long history of innovative planes that never lived up to their initial promise or advertising.

 

[tomo pauk]

I've described what things could've been done to address main shortcomings of P-39 (performance beyond 12kft combat/ferry range) here:

http://www.ww2aircraft.net/forum/aviation/fighters-made-usa-mid-1943-how-would-you-do-28623.html#post776073

 

[Shortround6]

I know you have made a number of proposals. I don't believe most will work or will require such major redesign you might as well build a new plane. For instance you say just move the radiator and oil cooler to the wings leaving center section free for fuel tanks. Fuel tanks are already in wings just out board of the air intakes. They are between the two wing spars. behind the the second wing spar is the landing gear. You can either move the radiator/s oilcooler even further out board or hang them under the wing like a Spitfire but that means more drag. That or all you have done is change the fuel tanks from the wings to the center section for not much change.
Oil coolers and radiators were in the wings on the XP-39 but suffered very bad airflow from ducts trying to get through the landing gear area and the spars. XP-39 had cronic and sever cooling problems which were solved by putting the cooling systems in the center section. Under wing loads require beefed up wing structure.

The P-39 was the smallest US fighter and had the lowest drag next to the Mustang. There is a reason for that and a price, people want to take the benefit of the small size and low drag but don't want to pay the price, less room for fuel and systems. Especially with the engine taking up a lot of room at or near the CG.

One book on Aircraft powerplants published in 1943 estimates a 1000hp powerplant needs about 10 cu ft of space for a turbo and inter cooler installation. A more powerful engine will need more volume.

The Bell engineers tried to improve the P-39 themselves. It was called the P-39E and was going to be produced as the P-76 because it had so little in common with the P-39. It helped a lot with the other "improved" P-39, the P-63. I am not an engineer but I figure that even with the mistakes the Bell guys made with the P-39 they still knew more than I ever will and had reasons for making the successor aircraft bigger to solve the problems rather than just rearranging some items in the existing airframe.

 

[krieghund]

This my conception of what the armed turbo version would look like with NACA's proposed revised ducts.

 

[tomo pauk]

1st picture shows that 2 air intakes were in each wing, other 2 (feeding glycol cooler) were in wing central section.
2nd picture shows the wing of P-39. Note the wing part of oil cooler duct in front of wheel well. Also note that at least 3 fuel cells might have been installed outboard of U/C leg attachment point - a simple upgrade that would've increased internal fuel tankage by some 40-50%. The idea never dawned at Bell designers (not trying to be harsh on people).
The reduction of armament into hull-only guns can add another 40-50% of fuel. Never tried.

edit: Neither of those two modifications increases drag, second one reduces it. (end edit)

Installation of wing racks for drop tanks, for historical P-39s, was also never accomplished, unlike for P-40.