Could you have designed a better P-39?

[Readie]

Bell P39Q-15BE 44-2911 Airacobra

Have you seen this?
Cheers
John

 

[P-40K-5]

With hindsight (ain't hindsight a great thing?) could you have made the P-39 a truly great plane?
Perhaps scaling-up the basic design for greater fuel capacity, better turbocharger inter/aftercooler arrangement, armament, etc?
The mid-engine layout appears to be an efficient way to incorporate a turbocharger into a single-engine fighter/interceptor (all the ducting of the P-47 is not necessary.)
The mid-engine layout also lends itself to impressive nose armament.

I'm not suggesting improving an existing P-39, rather how the basic mid-engine turbocharged design concept could have been.

a two-stage supercharger would have been a blessing for the Allison engine ( high alltitude escorts). 2 50cals in the nose a 20mm cannon firing through the prop hub
would have been sufficient to bring down almost anything. plus it would keep the plane light(er), or to improve the armor for the engine cooling system.

 

[drgondog]

Shortround summed it up - "what coulda happened with the P-39 - did happen! It became the P-63" end of story.

Small airframe limited the P-39/P-63 from ever being effective long range fighter, engine limited in power at altitude interesting to western allies but fine for USSR... crappy 37mm gun. The Sovs made it into a good low altitude performer by putting in a good gun and stripping the useless .30 cals out.

 

[tomo pauk]

I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.
2nd, Indeed, 4 x .30cals were never looked as an asset in air combat, up to the point of discarding them by Soviets. The 350lbs worth of those guns ammo, and the volume occupied by those, might have been better used as fuel load (2 fuel tanks + 2 x 25 gals maybe; 1 gal = 6lbs) for 170 gals total. I've attached the wing cross section to make myself more clear.

As for Soviet use of the P-39: no new gun was installed (other than US .50cal under wing of late model P-39s; wonder if they deleted that one too?). The 37mm was never regarded as crappy by Soviets. If the basic design was a bad one, it would've never been a good performer, at any altitude.

 

[Shortround6]

I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.

You have to wonder why that was, did they just not think of it? or did they think of it, do some quick calculations of the back of an envelop or napkin and decide it wasn't worth taking any further?

2nd, Indeed, 4 x .30cals were never looked as an asset in air combat, up to the point of discarding them by Soviets. The 350lbs worth of those guns ammo, and the volume occupied by those, might have been better used as fuel load (2 fuel tanks + 2 x 25 gals maybe; 1 gal = 6lbs) for 170 gals total. I've attached the wing cross section to make myself more clear.

Long, flat protected tanks are heavy. Fuel system for a P-38J weighed 506lbs for a capacity of 410 gallons. P-39 was around 270lb (or more ) for 120 gallons. Both P-47 and P-51 use less weight per gallon. The P-39 was a ground loving airplane. The only single seat US fighter that needed more runway was the P-47. The P-39Q (the best P-39 at taking off) needed 1350ft of pavement at 7600lbs at sea level on a ) degree day. that is with 87 gallons of fuel on board and the four .50s. Adding either a 500lb bomb or a 75 gallon drop rank requires another 300ft of pavement. Making it over the 50 ft barrier adds 450 of distance over the plane when clean.
1150-1325hp just isn't enough to lift large external loads without very long runways. Long runways came along later in the war but by then the P-39 was on the way out.

As for Soviet use of the P-39: no new gun was installed (other than US .50cal under wing of late model P-39s; wonder if they deleted that one too?). The 37mm was never regarded as crappy by Soviets. If the basic design was a bad one, it would've never been a good performer, at any altitude.

From what I have read they did delete the wing guns, in fact some of the last delivers may have been made with guns never installed. The 37mm took some sorting out. There was a problem with the ejection chute that caused many jams, you can't get new rounds into the gun if the old ones can't get out. This was sorted out and the gun didn't seem to have too many other problems.

The design wasn't necessarily "bad" but it did have limits. I had a couple of old Triumph sports cars, they were pretty good at somethings and terrible at others, like taking 5 friends to a party or trying to move a refrigerator.

 

[tomo pauk]

[about wing drop tanks]You have to wonder why that was, did they just not think of it? or did they think of it, do some quick calculations of the back of an envelop or napkin and decide it wasn't worth taking any further?

Still wondering; care to share some info?

Long, flat protected tanks are heavy. Fuel system for a P-38J weighed 506lbs for a capacity of 410 gallons. P-39 was around 270lb (or more ) for 120 gallons. Both P-47 and P-51 use less weight per gallon.

Fuel system + fuel of real P-39 weighted 990 lbs. We have weight bonus of 350lbs (LMGs + ammo deleted), giving us around 35% more fuel - 42gals? Not the +50 gals I've claimed, but 160 gals internal never the less.

The P-39 was a ground loving airplane. The only single seat US fighter that needed more runway was the P-47. The P-39Q (the best P-39 at taking off) needed 1350ft of pavement at 7600lbs at sea level on a ) degree day. that is with 87 gallons of fuel on board and the four .50s. Adding either a 500lb bomb or a 75 gallon drop rank requires another 300ft of pavement. Making it over the 50 ft barrier adds 450 of distance over the plane when clean.
1150-1325hp just isn't enough to lift large external loads without very long runways. Long runways came along later in the war but by then the P-39 was on the way out.

I'm not suggesting that we fill the P-39 with 400 gals of fuel, along with 1 or 2 thousands lbs of bombs. Carrying 160 gals of internal fuel doesn't add up a single pound when compared with P-39s with 4 x LMGs.
Further about runaways: the P-39 with more fuel can put a decent runaway into a good use (and a plane itself). The one with scarce fuel cannot.

From what I have read they did delete the wing guns, in fact some of the last delivers may have been made with guns never installed. The 37mm took some sorting out. There was a problem with the ejection chute that caused many jams, you can't get new rounds into the gun if the old ones can't get out. This was sorted out and the gun didn't seem to have too many other problems.

Agreed.

The design wasn't necessarily "bad" but it did have limits.

Ditto.

I had a couple of old Triumph sports cars, they were pretty good at somethings and terrible at others, like taking 5 friends to a party or trying to move a refrigerator.

Those are among the reason why I just love my VW Transporter

 

[Lighthunmust]

I'll just point into some things that either a costumer (USAAC) or a producer (Bell) did not think that were worth pursuing.
1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.

You have to wonder why that was, did they just not think of it? or did they think of it, do some quick calculations of the back of an envelop or napkin and decide it wasn't worth taking any further?


The 37mm took some sorting out. - Shortround6


The USAAF was dominated by the "Bomber Mafia" (a term probably coined post-WW2 and also true in the USAF). Much of the rest of the world's air forces also believed "The Bomber will always get through".

In the USAAF the doctrine of a strong defensive armament on bombers would insure "The Bomber will always get through" was sacrosanct. To even hint that long range fighter escorts for bombers were needed was sacrilege. How do you justify to Congress spending depression era dollars on expensive bombers that are less than invulnerable? The suggestion of the need for drop tanks would not be tolerated. Even "Hap" Arnold would not tolerate the idea, although if I recall it was also due to a belief it would degrade fighter performance. This short explanation is far from comprehensive, but I hope gives some idea of why no one was considering drop tank installation capability.

If I recall correctly, the Soviets did field modifications to get the 37mms functioning better. Genius knows no geographical boundaries.

 

[Shortround6]

[E=Lighthunmust;791008]QUOT1st, P-39 was never issued with wing drop tank installation. That's hardly a fault of basic design, so either USAAC didn't ordered it, and/or Bell never proposed it.[/QUOTE]

They did use a 75 gallon belly tank. And an even bigger ferry tank 175 gallons. total fuel with the ferry tank 295 gallons. They probably had a very good idea of the take-off performance (or lack thereof) and range.

While ditching the wing guns saves weight it is not enough to mount large under wing stores. Even 52 gallon drop tanks are going to go over 360lbs installed. What does it get? 104 gallons under wing and a belly tank of 75 for 179 gallons or one belly ferry tank of 175 gallons?

Even with two 500lb under wing stations you get two 75 gallon tanks. 270 gallons total compared to 195 gallons with the belly tank. Is an extra 75 gallons going to turn it into an escort fighter? An extra 75 gallons is worth about 321miles at max cruising speed (750hp) but that is in clean condition. Wings will have to be stressed for the under wing loads.

The plane is not going to carry all three stations loaded. Not without one looooong runway.

[E=Lighthunmust;791008]
In the USAAF the doctrine of a strong defensive armament on bombers would insure "The Bomber will always get through" was sacrosanct. To even hint that long range fighter escorts for bombers were needed was sacrilege. How do you justify to Congress spending depression era dollars on expensive bombers that are less than invulnerable? The suggestion of the need for drop tanks would not be tolerated. Even "Hap" Arnold would not tolerate the idea, although if I recall it was also due to a belief it would degrade fighter performance. This short explanation is far from comprehensive, but I hope gives some idea of why no one was considering drop tank installation capability.
[/QUOTE]

What is strange about this is that a number of US biplane fighters used either drop tanks or auxiliary external tanks or external tanks that could be dropped in an emergency? Both Curtiss Hawk biplanes and Boeing Biplanes in both Army and Navy service. The idea of drop tanks was neither new or untested.

 

[Lighthunmust]

I don't think Bell is responsible for much in the way of design faults for the P-39 regarding range or weapons package. They responded to conditions established by USAAF. I am aware that there was some use of auxiliary tanks prior to the war. This certainly eliminates any possibility that USAAF leaders did not know they could be a viable option. They simply did not want them. They also made little effort to create anything more than glorified ferry tanks unsuitable for high altitude if memory serves me well. The most recent source of my information are Warren Bodie's books.

 

[tomo pauk]

Turbo 39. Radiators relocated at leading edge, with intercooler at another. Compressor is under engine. Exhaust manifolds meet under hull, a feet or two prior entering the turbo. Wing guns ammo deleted, fuel tanks instead (optionally; 2 x 20 gals). Fuel tank in front part of wing central section (30-40 gal). Under-wing drop tanks (2 x 75 gals).
At 20-25K perhaps 380mph (1941-42; 1150 HP), 400 (1942-43; 1325 HP), 410 (1943, 1425 HP), 420 (1600 HP @ WER); P-63 was managing 400-410 mph with 1150-1200HP @ 25-20K (exhaust thrust comes to aid).

(red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)

 

[MikeGazdik]

I'll take mine in sand and spinach, over light grey please!!! Matter of fact, think its time to change my avatar again to my favorite underdog!!

 

[Shortround6]

Turbo 39.

(red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)

For an idea of what you are trying to deal with, here are some pictures, diagrams of a B-17 turbo installation, 1200hp so the airflow requirements shouldn't be too different.

Warbird Information Exchange • View topic - b-17 wing intercooler removal

 

[wuzak]

Turbo 39. Radiators relocated at leading edge, with intercooler at another. Compressor is under engine. Exhaust manifolds meet under hull, a feet or two prior entering the turbo. Wing guns ammo deleted, fuel tanks instead (optionally; 2 x 20 gals). Fuel tank in front part of wing central section (30-40 gal). Under-wing drop tanks (2 x 75 gals).
At 20-25K perhaps 380mph (1941-42; 1150 HP), 400 (1942-43; 1325 HP), 410 (1943, 1425 HP), 420 (1600 HP @ WER); P-63 was managing 400-410 mph with 1150-1200HP @ 25-20K (exhaust thrust comes to aid).

(red box - turbocharger; blue box - intercooler; orange box - fuel tank in central wing section)

Apart from the intercooler (and maybe the fuel tank, you have just described the XP-39.

http://aerofiles.com/bell-p39b.jpg

Or at least you have put the turbo in the same place. The radiators are fed from leading edge inlets, but the radiators themselves are in the fuselage.

I'd suggest placing the turbo behind the engine with the turbine axis pointing down and to the rear with an exit for the turbo and wastegate below the rear fuselage. There wouldn't be any space for an air to air intercooler, so I would look at using a liquid to air intercooler, like on the two stage Merlins. The radiator required for that may be small enough to fit with the engine coolant and oil radiator group. If not, we could place that radiator under the chin, as in 2 stage powered Mosquitos.

 

[tomo pauk]

For an idea of what you are trying to deal with, here are some pictures, diagrams of a B-17 turbo installation, 1200hp so the airflow requirements shouldn't be too different.

Thanks for the link
Maybe I was not crystal clear: the exhaust-to-turbo ducting in mostly external, as is the intercooler-to-carb. The turbo-to-intercooler duct is now in place of wing part of oil cooler duct.

Apart from the intercooler (and maybe the fuel tank, you have just described the XP-39.

http://aerofiles.com/bell-p39b.jpg

Or at least you have put the turbo in the same place. The radiators are fed from leading edge inlets, but the radiators themselves are in the fuselage.

Yep, I know about the XP-39.
IIRC, the exhaust ducting there was too short, so any increase in power would've damaged the turbo? The intercooler was either under-sized, or at bad place, or both? The radiators in fuselage (actually, in central wing section) and their ducts are eating too much of precious space, hence they're in front of the wings. Of course, we can put them under front part of wing, for looks a-la later Spitfires.

I'd suggest placing the turbo behind the engine with the turbine axis pointing down and to the rear with an exit for the turbo and wastegate below the rear fuselage. There wouldn't be any space for an air to air intercooler, so I would look at using a liquid to air intercooler, like on the two stage Merlins. The radiator required for that may be small enough to fit with the engine coolant and oil radiator group. If not, we could place that radiator under the chin, as in 2 stage powered Mosquitos.

With turbo behind the engine, there is 300+ lbs some 5 feet behind CoG - not the best thing for P-39. Too bad Bell wasn't producing P-63s as turbo, 'stead of mech two stage.
The chin radiator would've interfere with front wheel system?

OTOH, we could contemplate a razorback P-39 - glycol oil tanks moved above engine, making weight allowance space for turbo intercooler.

 

[Shortround6]

Thanks for the link
Maybe I was not crystal clear: the exhaust-to-turbo ducting in mostly external, as is the intercooler-to-carb. The turbo-to-intercooler duct is now in place of wing part of oil cooler duct.

Sticking bits and pieces on the outside increases the drag. Same problem as as Bell's attempt's at an external turbo add on. You may get your 380-400mph at 20-25,000ft in 1941-43 but if you cut the sea level speed from about 310mph to 270-280mph and the speed at 10,000ft from 350mph to 310-320mph you wind up with a very specialized airplane. It will also climb worse than a standard P-39 up to about 15,000ft or so. Pulling the wing guns may not be enough (especially if you stick in extra fuel). It is only when the 1425 hp engine becomes available that this starts to look attractive and by then the need for it is fading. You do need bigger radiators and oil coolers for the higher powered engines and even the intercooler needs to change from the early version to the late version. The diagrams and pictures were to give you an idea of the size of the pipes/ducts you are dealing with. You can shorten them and change them form square to round or vis versa but you can't make them smaller in cross section (especially forhe higher HP engines)

Yep, I know about the XP-39.
IIRC, the exhaust ducting there was too short, so any increase in power would've damaged the turbo? The intercooler was either under-sized, or at bad place, or both?

The intercooler was both. They also had sever problems with both the radiator and oil cooler.

 

[wuzak]

With turbo behind the engine, there is 300+ lbs some 5 feet behind CoG - not the best thing for P-39. Too bad Bell wasn't producing P-63s as turbo, 'stead of mech two stage.
The chin radiator would've interfere with front wheel system?

OTOH, we could contemplate a razorback P-39 - glycol oil tanks moved above engine, making weight allowance space for turbo intercooler.

I'd suggest that the wings could move, or other equipment, to get the weight balance back.

 

[Shortround6]

Sort of what they did on the P-63. look at were the cockpit is in relation to the wing leading edge on both planes.

Since the P-63 was what the Bell engineers thought they needed for a two stage supercharged Allison engine fighter ( mechanical drive instead of exhaust drive for the auxiliary stage) I am not sure why some people think they can stuff the same amount of equipment (or more) into the P-39 airframe. While they didn't cut metal on on anything except the two mockups of the add-on turbos we have no idea how many (or how few) sketches they did of other arrangements. If these are the arrangements that made it to the shop I can only conclude that they were considered the best options at the time. Granted they may have been constrained by the need for minimum changes to the P-39 in order to keep up production after the P-39C/D was adopted. Moving wings, and wholesale re-arrangements of internal components and ducts requires new tooling and assembly procedures (new in the sense of what parts get added first for best access and things like that).

If the rework is supposed to take place at the time of the XP-39 before the P-39 goes into even limited production you are faced with the same problem the airforce was faced with historically. The turbo simply wasn't ready for squadron service in any airplane at the time in question. Delaying the start of production by about a year (what the USAAF estimated as the time needed to bring the turbo up to usable standard) meant an unacceptable loss of production.

To illustrate this they had completed 208 P-38s by the end of 1941, 128 of them in Nov and Dec. at that time they had completed 939 P-39s. Delaying the P-39 program by, about year could have resulted in 600-620 fewer P-39s available in Jan of 1942, this keeps going, it means 600 fewer P-39s available in June of 1942.

While about 3 times the number of P-40s had been built the P-39 in Jan of 1942 outnumbered all the P-38s, P-51s, F4Fs built with enough to spare to cover most of either the P-43 production or Buffalo Production, take your pick.

Bell started work on the Model 33 in Feb 1941, US Army orders two prototype XP-63s on June 27 1941. I would think they had a pretty good idea how much room they needed for what they wanted in the aircraft.

 

[davparlr]

What would I do?

1. Get someone else to do the aero. As those who have read my post on the P-59 know, I am not a fan of Bell aerodynamics. A look at the P-39 and one cannot be anything but impressed by the clean design with imbedded radiator with leading edge air inlets and its overall smoothness. With its small wing it should have been faster. Its CD0 was higher than the P-47! This should not have been true. I think the problem was in the details. Cleaning up the details would have improved airspeed, especially at low level.
2. Better supercharger. Perhaps installing the F4F-3 supercharger which seems to have had a higher operating range (1000 hp at 19k). This would have improved high altitude performance.
3. I like the two 50cals and one (good) 20mm and no wing guns.
4. More fuel. Added wing tanks may have helped here. Overall gross weight for combat must not increase.
5. Improve roll rate to P-40 levels.

This would have made the P-39 much more formidable.

 

[Shortround6]

What would I do?

2. Better supercharger. Perhaps installing the F4F-3 supercharger which seems to have had a higher operating range (1000 hp at 19k). This would have improved high altitude performance.
3. I like the two 50cals and one (good) 20mm and no wing guns.
4. More fuel. Added wing tanks may have helped here. Overall gross weight for combat must not increase.
5. Improve roll rate to P-40 levels.

This would have made the P-39 much more formidable.

2. F4F-3 supercharger was a two stage with inter-cooler. back to the weight and space problems of the turbo/P-63
3. This one is a good idea.
4. Might work but increase will be small and is only going to work if wing guns are left out.
5. getting weight out of wings may help but his may mean no extra tanks way out in the wings.

 

[krieghund]

According to Dean's "America's Hundred Thousand" the Cdo's fall out this way but one must also take into account the equivalent flat plate area;
CDo......................EFPA sq ft.........DRAG AT 250 MPH
P-51D = .0176......................4.1............................. 845
P-39N = .0217..................... 4.63........................... 864
P-47B = .0213..................... 6.39..........................1465
F6f-3 = .0272......................9.08.......................... 1673

The following chart shows the real speed of the prototype at 20000ft (340 mph) and the speed it could have attained if the NACA recommendations were applied (390 mph) The Cdo originally was .03 and after the NACA mod would have been .021