XP-39: pros cons

[Shortround6]

1st picture shows that 2 air intakes were in each wing, other 2 (feeding glycol cooler) were in wing central section.

And on the XP-39 the oil cooler and radiator were in the wing root area ahead of the wheel, but to have good airflow you also need an exhaust duct behind the the radiators/coolers.

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).

Maybe it was never tried because that location is behind the CG. P-39 already changed handling just by having empty ammo magazines in the nose.
The P-39 was loved by good pilots but was considered tricky to fly by average or below average pilots. Making it tricker to fly probably is not a good idea. Granted you can use the rear wing tanks first (just like a P-38 ) but take-off, climb-out accidents may increase. Then there is the question of weight. The fuel system in a 120 gal P-39 went about 290lbs. Let us say you can get 40 gallons behind the wing spar (wing is getting thinner) and that your fuel cells only add another 100lbs (they won't, fuel cell/tank weight varies with the surface area of the tank, not it's capacity) empty. Another 240lbs for fuel. P-39s, while small, were heavy. One reason some of the Q's had the fuel capacity cut to 87 gals, increase in performance wasn't worth the loss of range.

The reduction of armament into hull-only guns can add another 40-50% of fuel. Never tried.

It was, sort of, post war. Some of the Thompson Trophy racers put fuel tanks in the wing gun/ammo bays. They managed 100 gal but were using thin wall fuel bladders. They had also replaced the self sealing tanks with thin wall fuel cells/bladders. Two planes were prepared by a group of Bell engineers/employees in their off hours.

While a break down of the CG isn't available the list of changes and weight differences is.

They did install a 25 gallon fuel tank in the rear of the fuselage but considering the amount of weight they took out of the plane and the amount of weight they added that doesn't really mean much. Not only did guns, ammo and armor go but so did 109 lbs of radio, 70 lbs of communications equipment, 10lbs worth of window winders, 40lbs worth of instruments, 47lbs worth of engine starter and other items.
Things added include a P-63 25gallon water injection system in the wing, a 75 gallon water tank in the nose, a 4 bladed propeller from a P-63 at 110lbs more than the stock propeller and more.
When they got done the racer/s weighed 47.2lbs more for take-off (clean) than the P-39Q-10 they started with.

 

[tomo pauk]

Maybe it was never tried because that location is behind the CG. P-39 already changed handling just by having empty ammo magazines in the nose.
The P-39 was loved by good pilots but was considered tricky to fly by average or below average pilots. Making it tricker to fly probably is not a good idea. Granted you can use the rear wing tanks first (just like a P-38 ) but take-off, climb-out accidents may increase.

The hull fuel tanks on P-40, P-51 and some Spitfire variants were located even more away from CoG than ones I propose. Despite that, benefits out-weighted the shortcomings.
For P-39, the decrease of frontal weight by expanding the hull ammo is nicely canceled out by fuel burned from additional tanks.

Then there is the question of weight. The fuel system in a 120 gal P-39 went about 290lbs. Let us say you can get 40 gallons behind the wing spar (wing is getting thinner) and that your fuel cells only add another 100lbs (they won't, fuel cell/tank weight varies with the surface area of the tank, not it's capacity) empty. Another 240lbs for fuel.

Too bad we haven't continued this discussion at the thread about possible US fighters for mid 1943. I've proposed there that P-39's have either 5 x .50 cals, or cannon + 2 x .50 cals; we save 100-150 lbs on empty/equipped weight.

P-39s, while small, were heavy. One reason some of the Q's had the fuel capacity cut to 87 gals, increase in performance wasn't worth the loss of range.

Basic weights, from US 100K, in lbs, rounded to nearest 10lbs:
P-39D1: 6290 ; D2: 6420; 39Q-1: 6420
P-40B: 5990; P-40E: 6700; P-40N-25: 6720
P-51A: 6890
Apart from light-weight P-40B, P-39 is lightest.

It was, sort of, post war. Some of the Thompson Trophy racers put fuel tanks in the wing gun/ammo bays. They managed 100 gal but were using thin wall fuel bladders. They had also replaced the self sealing tanks with thin wall fuel cells/bladders. Two planes were prepared by a group of Bell engineers/employees in their off hours.

While a break down of the CG isn't available the list of changes and weight differences is.

They did install a 25 gallon fuel tank in the rear of the fuselage but considering the amount of weight they took out of the plane and the amount of weight they added that doesn't really mean much. Not only did guns, ammo and armor go but so did 109 lbs of radio, 70 lbs of communications equipment, 10lbs worth of window winders, 40lbs worth of instruments, 47lbs worth of engine starter and other items.
Things added include a P-63 25gallon water injection system in the wing, a 75 gallon water tank in the nose, a 4 bladed propeller from a P-63 at 110lbs more than the stock propeller and more.
When they got done the racer/s weighed 47.2lbs more for take-off (clean) than the P-39Q-10 they started with.

Thanks about the info re. racers

 

[Shortround6]

The hull fuel tanks on P-40, P-51 and some Spitfire variants were located even more away from CoG than ones I propose.

Why did I know you were going to bring those planes up

Please note that by the time the Mustang and Spitfire got rear fuselage tanks they also had engines that were over 300lbs heavier than the engines they were first built with, they had propellers that were 80-100lbs heavier than they were first built with (in the case of the Spitfire it could be 300lbs heavier, MK I Spits needed about 70lbs of ballast in the tail when they switched from the 2 blade wood prop to the 3 blade constant speed prop).
And a fair amount of other equipment added or moved around. Trying to stick a rear fuselage tank in a MKI or II Spitfire with no other modifications could well lead to disaster. It will fit in the space though

P-40 was designed from the start to have that tank, it wasn't added later. Most planes have a CG range in which they are safe to fly. The CG depending on load can move a bit fore or aft. once you get outside that range things get squirrelly in a hurry. Think of some 4 passenger light planes, some have a baggage compartment aft of the cabin with a certain weight limit. You are not allowed to put more weight in that compartment even if you are only carrying 3 people and your total gross weight is within limits.
P-40 was designed to be safe to fly with that tank full even if some maneuvers were restricted.
P-39s were operating closer to the aft CG limit than some other planes to begin with. They had a no warning stall, bad spin characteristics and very effective elevator control. Not much input needed for a lot of results, great for really good pilots. Not so good for the more ham fisted. a little to much up elevator in certain flight conditions and the plane stalls and goes into a flat spin.

Basic weights, from US 100K, in lbs, rounded to nearest 10lbs:
P-39D1: 6290 ; D2: 6420; 39Q-1: 6420
P-40B: 5990; P-40E: 6700; P-40N-25: 6720
P-51A: 6890
Apart from light-weight P-40B, P-39 is lightest.

Great, now figure that the P-39 also had an almost 10% smaller wing than the P-40 and P-51 and that you aren't trying to beat the P-40 or P-51 but Me 109s or Zeros or Tony's and figure out the power to weight ratios.

The drive shaft on the P-39 added at least 50lbs to the weight of the plane while the stiffer fuselage required another 50lbs or more.

Birch Mathews book "Cobra" says that it wasn't possible to add rear fuselage tanks to the P-39 or P-63 and while you aren't proposing rear fuselage tanks both planes had CG problems. The P-63 water injection system used a 25 gallon tank in leading edge of the left wing and not out board of the landing gear like you propose.

 

[tomo pauk]

Why did I know you were going to bring those planes up

Please note that by the time the Mustang and Spitfire got rear fuselage tanks they also had engines that were over 300lbs heavier than the engines they were first built with, they had propellers that were 80-100lbs heavier than they were first built with (in the case of the Spitfire it could be 300lbs heavier, MK I Spits needed about 70lbs of ballast in the tail when they switched from the 2 blade wood prop to the 3 blade constant speed prop).
And a fair amount of other equipment added or moved around. Trying to stick a rear fuselage tank in a MKI or II Spitfire with no other modifications could well lead to disaster. It will fit in the space though

Oil tank (full) weights some 70-80lbs. We can move it adjacent to engine (for non-turboed P-39), or under pilot's seat (for turboed ones). The oil armor plate can be moved forward, too. The pic attached shows plenty of room between angine and oil tank it's armor plate. Also plenty of room under cockpit.
Addition of fuel tanks 1-2 ft away from CoG makes less CoG issues than adding it 3-5 ft away.

P-40 was designed from the start to have that tank, it wasn't added later.
Most planes have a CG range in which they are safe to fly. The CG depending on load can move a bit fore or aft. once you get outside that range things get squirrelly in a hurry. Think of some 4 passenger light planes, some have a baggage compartment aft of the cabin with a certain weight limit. You are not allowed to put more weight in that compartment even if you are only carrying 3 people and your total gross weight is within limits.
P-40 was designed to be safe to fly with that tank full even if some maneuvers were restricted.

As said, benefits overweighted the issues.

P-39s were operating closer to the aft CG limit than some other planes to begin with. They had a no warning stall, bad spin characteristics and very effective elevator control. Not much input needed for a lot of results, great for really good pilots. Not so good for the more ham fisted. a little to much up elevator in certain flight conditions and the plane stalls and goes into a flat spin.

P-39 had issues, main ones being performance at altitude range. CoG issues were present, allocation of P-39s for more experienced pilots seem like remedy.

Great, now figure that the P-39 also had an almost 10% smaller wing than the P-40 and P-51 and that you aren't trying to beat the P-40 or P-51 but Me 109s or Zeros or Tony's and figure out the power to weight ratios.

If I tried to propose US Zero, I'd say: 'delete the armor, install non-protected fuel tanks, go with 3 HMGs all the time; good, we've saved 1000 lbs'. But I'm not. The proposal is US-way: plenty of armament, fuel, armor, power; travel fast, make fast attack, avoid turning battle. Worked well historically.
Glad you've mentioned power to weight ratio.
The non-turbo P-39 I've proposed has same weight with same fuel aboard, as historical ones had.
The turboed one is heavier 5% for take off, little more drag, while having 50% more power already at 15K.

The drive shaft on the P-39 added at least 50lbs to the weight of the plane while the stiffer fuselage required another 50lbs or more.

Already counted in 'basic weight' figures.

Birch Mathews book "Cobra" says that it wasn't possible to add rear fuselage tanks to the P-39 or P-63 and while you aren't proposing rear fuselage tanks both planes had CG problems.

As above: comparing CoG issues with fuel tank 1-2ft away from CoG with the one 3-5 ft away is apples oranges.

The P-63 water injection system used a 25 gallon tank in leading edge of the left wing and not out board of the landing gear like you propose.

Allocation of fuel tanks to leading edge was beyond of designer's scope, unfortunately

BTW, P-63 have had fuel tanks at location similar to my proposed additional tanks for P-39, aft central spar. Since it was okay for P-63, I'd reckon they would've worked just fine for P-39

 

[Shortround6]

Oil tank (full) weights some 70-80lbs. We can move it adjacent to engine (for non-turboed P-39), or under pilot's seat (for turboed ones). The oil armor plate can be moved forward, too. The pic attached shows plenty of room between angine and oil tank it's armor plate. Also plenty of room under cockpit.
Addition of fuel tanks 1-2 ft away from CoG makes less CoG issues than adding it 3-5 ft away.

They usually tried to keep the top of the oil tank and the coolant header tank above the top of the engine to help get air bubbles out. They may have tried to space the oil tank from the engine for temperature reasons. You have attached a schematic diagram and not a photograph, it does not show all pipe connections, brackets and small pieces of equipment in the area. I don't have a picture either so I don't know what is in that area. I do know that in a similar discussion concerning the P-40 somebody had a similar diagram that showed space behind the engine and claimed that something could go there. In the real plane that space was partially occupied by the sidesway brace of the engine mount. That is probably not the case here but these diagrams should not be take as the last word on available space. You may also want to leave room for the mechanics to work on the back of the engine, carburetor, starter, generator, gun synchronizers and any other accessories are on the back.
Some planes were more tolerant of CG issues than others, either they weren't flying normally with their CG quite as far aft as the P-39 or they had different stall characteristics or they had different spin recovery characteristics or combinations of the these.

As said, benefits overweighted the issues.

P-39 had issues, main ones being performance at altitude range. CoG issues were present, allocation of P-39s for more experienced pilots seem like remedy.

Increasing the likelihood of spin/crashes for extra range calls for a very careful balance. There perhaps were ways to gain a slight performance increase for the P-39 that did not involve making the flight characteristics anymore dangerous. In the early part of the war the P-39 was the second most produced fighter. By the end of 1942 over 2800 had been built. That is a lot of "experienced pilots" to come up with by then and it tends to leave the P-38 and P-40 squadrons with more than a fair share of "green" pilots.

If I tried to propose US Zero, I'd say: 'delete the armor, install non-protected fuel tanks, go with 3 HMGs all the time; good, we've saved 1000 lbs'. But I'm not. The proposal is US-way: plenty of armament, fuel, armor, power; travel fast, make fast attack, avoid turning battle. Worked well historically.

It didn't work historically for the P-39 and that is why the P-39 has it's bad reputation. Pilots, group commanders and theater commanders knew it couldn't do the all round fighter job although it could be useful in some roles. I am not saying turn it into a Zero but that extra weight is one reason it can't perform like other fighters using similar powered engines. Over coming a 15-30% weight handicap is going to take more than minor tweaking. Making a fast attack is a little difficult if the enemy is 5,000ft higher than you are.

Something to consider when using book performance figures to evaluate combat is that book performance charts are established using standard atmospheric conditions. 59 degrees F or 15 degrees C and standard sea level pressure. Planes operating in colder areas got a bit of a boost in performance (denser air) while planes operating in the tropics/desert took a performance hit from the less dense air. It was a double wammy. The engines provided less power while the wing provided less lift.
Because of Engine boost and wing loading not all aircraft suffered exactly the same performance loss per degree of temperature.

Glad you've mentioned power to weight ratio.
The non-turbo P-39 I've proposed has same weight with same fuel aboard, as historical ones had.
The turboed one is heavier 5% for take off, little more drag, while having 50% more power already at 15K.

Well then, the non-turbo won't perform any better, but it might not perform worse at best? not exactly an endorsement.
Just 5% heavier? only a little more drag if you don't use an intercooler. While you may get 50% more power (using WER rating from a P-38 J engine? 1600 vs the 1125hp for a P-39Q engine is 42%) you might be getting only 13% more power 9000ft.

As above: comparing CoG issues with fuel tank 1-2ft away from CoG with the one 3-5 ft away is apples oranges.

BTW, P-63 have had fuel tanks at location similar to my proposed additional tanks for P-39, aft central spar. Since it was okay for P-63, I'd reckon they would've worked just fine for P-39

Talk about apples and oranges

P-63, aside from using a 37mm cannon and the Allison extended shaft engine set-up had no interchangeable parts with a P-39. A new fuselage, a new wing of different airfoil (laminar flow with maximum thickness much further back along the cord, which is why the fuel tanks were back there) different area and different construction, it used a main spar and a rear spar instead of the spar arrangement of the P-39. While it had a longer fuselage please note that the wing was further back on the fuselage than on the P-39 which means the engine was further forward in relation to the wing, center of lift. Cockpit was in front of the wing instead of above the leading edge. It's landing gear was wider tracked and it had 10 more inches between the nose gear and the main wheels.
Where they could put fuel on the P-63 has very little to do with where they could put fuel on the P-39.

However, with both the P-39E and the P-63 as attempts to improve the concept of the P-39 (37mm gun firing through prop hub) they are indications that more "stuff" could not be crammed into the existing P-39 airframe.
While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of, sketches made with preliminary calculations and given up. Considering some of the other stuff that shows up on paper sketches (Merlin powered P-63s with massive airscoops in the wing center section) it doesn't seem likily that this was totally over looked.

 

[tomo pauk]

They usually tried to keep the top of the oil tank and the coolant header tank above the top of the engine to help get air bubbles out.

Okay, then we'll rotate it 80 deg upwards, so it's parallel to the prop shaft; it's own CoG mowes forward too. Under it goes the turbocharger. We will locate inter-cooler just aft the front wheel well, while also moving radio forward. The oil-tank armor can loose it's lower half to address CoG issues, too.

They may have tried to space the oil tank from the engine for temperature reasons. You have attached a schematic diagram and not a photograph, it does not show all pipe connections, brackets and small pieces of equipment in the area. I don't have a picture either so I don't know what is in that area.

You can check the schematics in The Book.
Plenty of space between engine oil tank, with couple of pipes to connect the tank engine, plus some parts of starter system.

I do know that in a similar discussion concerning the P-40 somebody had a similar diagram that showed space behind the engine and claimed that something could go there. In the real plane that space was partially occupied by the sidesway brace of the engine mount.

Yep, looking at P-40 diagram the turbo is a non-starter with current set up, aft the engine.
(soon in theaters near you: P-40 with turbo intercooler mounted in chin, with glycol oil coolers relocated in front of wings)

That is probably not the case here but these diagrams should not be take as the last word on available space. You may also want to leave room for the mechanics to work on the back of the engine, carburetor, starter, generator, gun synchronizers and any other accessories are on the back.

Not very maintenace friendly, I agree. But perhaps comparable with P-47, or twice as good as P-38 (maintenace-wise)?

Some planes were more tolerant of CG issues than others, either they weren't flying normally with their CG quite as far aft as the P-39 or they had different stall characteristics or they had different spin recovery characteristics or combinations of the these.
Increasing the likelihood of spin/crashes for extra range calls for a very careful balance. There perhaps were ways to gain a slight performance increase for the P-39 that did not involve making the flight characteristics anymore dangerous. In the early part of the war the P-39 was the second most produced fighter. By the end of 1942 over 2800 had been built. That is a lot of "experienced pilots" to come up with by then and it tends to leave the P-38 and P-40 squadrons with more than a fair share of "green" pilots.

You have a good point there.
Would you check out the sketch of wing for P-39 in thread about hypothetical US fighters for P-39. I've tossed some ideas above, that should've helped to keep CoG at it's place.

It didn't work historically for the P-39 and that is why the P-39 has it's bad reputation. Pilots, group commanders and theater commanders knew it couldn't do the all round fighter job although it could be useful in some roles. I am not saying turn it into a Zero but that extra weight is one reason it can't perform like other fighters using similar powered engines. Over coming a 15-30% weight handicap is going to take more than minor tweaking. Making a fast attack is a little difficult if the enemy is 5,000ft higher than you are.

If we have turbo on board, the P-39 is the one 5000ft above.

Something to consider when using book performance figures to evaluate combat is that book performance charts are established using standard atmospheric conditions. 59 degrees F or 15 degrees C and standard sea level pressure. Planes operating in colder areas got a bit of a boost in performance (denser air) while planes operating in the tropics/desert took a performance hit from the less dense air. It was a double wammy. The engines provided less power while the wing provided less lift. Because of Engine boost and wing loading not all aircraft suffered exactly the same performance loss per degree of temperature.

I'm trying all the time to ram more air in engine, so we can have more power

Well then, the non-turbo won't perform any better, but it might not perform worse at best? not exactly an endorsement.

The non-turbo (main changes are in reduced weight of armament [either 37 + 2x .50 cal, or 5 x .50 cal] to cancel out the weight of new tanks) will have the same weight. If on e wants range, he can cram more fuel in it. Or attach a bomb, and have decent range. So it's better performer.

Just 5% heavier? only a little more drag if you don't use an intercooler. While you may get 50% more power (using WER rating from a P-38 J engine? 1600 vs the 1125hp for a P-39Q engine is 42%) you might be getting only 13% more power 9000ft.

300 lbs for "supercharging with accesories" - half of what is stated for same item(s) of P-38J in The Book. The drag is price to pay, Supermarine paid it when developing Spit IX from Mk.V. The comparison between two V-1710s makes all the point: we want a plane for USAAC that can do hi-alt work as early as mid-42, while being available affordable.

Talk about apples and oranges

P-63, aside from using a 37mm cannon and the Allison extended shaft engine set-up had no interchangeable parts with a P-39. A new fuselage, a new wing of different airfoil (laminar flow with maximum thickness much further back along the cord, which is why the fuel tanks were back there) different area and different construction, it used a main spar and a rear spar instead of the spar arrangement of the P-39. While it had a longer fuselage please note that the wing was further back on the fuselage than on the P-39 which means the engine was further forward in relation to the wing, center of lift. Cockpit was in front of the wing instead of above the leading edge. It's landing gear was wider tracked and it had 10 more inches between the nose gear and the main wheels.
Where they could put fuel on the P-63 has very little to do with where they could put fuel on the P-39.

No apples and oranges.
If it's bad to have plenty of expandable stuff aft the CoG, it doesn't make any difference what plane is subject.
If you point at impossibility to install fuel tanks in front of main spar, because of lack of space, that's not likely. The wing thickness in front of main spar, but inboard, was greater than where fuel tanks were located. The P-38 have had fuel tanks in leading edge - a space much less generous than front half of P-63's wing.

However, with both the P-39E and the P-63 as attempts to improve the concept of the P-39 (37mm gun firing through prop hub) they are indications that more "stuff" could not be crammed into the existing P-39 airframe.

The only 'stuff' P-63 was carrying extra was auxiliary blower (not even intercooler). Despite designing new wing, it was inferior to, older, P-51's (less fuel, no internal HMGs, ammo for only for 1 HMG). P-63 can hardly be used as a proof how some people are good designers/engineers, dismissing a premise that only P-63 is a way to make better P-39.

While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of, sketches made with preliminary calculations and given up. Considering some of the other stuff that shows up on paper sketches (Merlin powered P-63s with massive airscoops in the wing center section) it doesn't seem likily that this was totally over looked.

Well put.
Is the part "with rear fuselage/wing tanks" referring to drop tanks, among other, or to additional inner wing tanks? I'd like to know more anyway

 

[Shortround6]

Tomo, First you have my apologizes for the last sentence in my post.

"While we "know" that P-39 was flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of,...."

It should have read "While we "know" that P-39 was NOT flown with rear fuselage/wing tanks we do not "know" one way or the other that the idea wasn't thought of,...."

Sorry for the confusion.



something to consider while thinking of all of these "let's just squish it here" plans, sometimes "empty" space on some of these systems drawings isn't empty.

Why no leading edge tanks on the P-39? Maybe that is because that is where the Aileron control runs were.

Why is there "nothing" in the space in the wing center section under cockpit and between the radiator intake ducts? Maybe because that is where the the bottom of the control stick was with the aileron and elevator connections. the "stick" had a hole in it and rotated around the drive shaft but it went through the floor of the cabin and and made it's connections in the space below.

Why was the oil tank in the rear fuselage? In addition to the possible reasons I have already given maybe there wasn't room for it in the engine compartment. The space available between the fuselage beams may not have been great enough without redesigning the fuselage beams.

Considerations for turbo location include the distance from the engine, space was needed to allow the exhaust to cool somewhat before it hit the turbine buckets (blades). Blade life was significantly shortened if incoming exhaust gas was too hot.
Allison was experimenting with turbo compounding in 1944. they found that even using massive amounts of ADI they could still exceed safe inlet temperatures on the turbine. They resorted to injecting ADI fluid straight into the exhaust manifolds.

Short, sharp bends in internal ducts increase drag even if they save space. Usually not a good trade.
Oil coolers, radiators and intercoolers need careful designing. There are many trade offs and sometimes they do get it wrong. But consider, the drag of the airflow through an oil cooler, radiator or intercooler goes up with the square of the speed of the air going though it. Doubling the size of a radiator can reduce the airflow speed for the same cooling power ( same mass of air) such that the bigger radiator has 1/8th the drag of the smaller radiator. Against this you have to balance the weight of the bigger radiator and the bigger duct/s needed.

I have been looking for the source and can't find it at the moment but I remember reading that a rule of thumb for a second stage super changer system (turbo?) was on the order of 10 cu ft for a 1000hp engine. Thar is 60cm X 60cm by 75cm. Even if this is a bit off and you can get the system for 1500hp into 12cu ft. that is 60cm X 60 Cm by 90cm. Granted it doesn't have to be all in one place (it is better if it isn't) but the more spread out the more ducting and volume needed.

As for a few specific points.
On the 1600hp turbo Allison (50% more power than the regular P-39).
"we want a plane for USAAC that can do hi-alt work as early as mid-42, while being available affordable"

The Engine with 1600hp potential doesn't show up until March of 1943 with the P-38H, It was severely limited by the intercoolers which you can possibly fix earlier but the basic engine may not be quite 1600hp. We may want it but available and affordable may not be there. The Us Army did not approve WER settings for just about all of 1942. An earlier acceptence of such power ratings may have helped US planes in combat even if it doesn't do anything for altitude performance.

The apples and oranges does apply. With the different wing, different wing location and different center of gravity relationships, the P-63 was a different airplane not only in construction but to fly. It provided more stall warning, it didn't enter a spin as easy as a P-39 and it was easier to get out of a spin. The different location of the wing and equipment (the engine may be a foot or so further forward in relation to the wing than a P-39) make it hard to judge weights and balances with the information we have available. I would suggest you to read "The Book" a bit more, like top of page 209

"It has been stated the P-39 was the single exception to the rule that all US World War II fighters had good safe characteristics." Granted that may be just a few peoples opinion but read the section on the P-63.

There are a number of comments about how improved the P-63 was in general handling compared to a P-39.

I am OK with the P-63 being not as good as the P-51 but that in no way proves that the engineers were incompetents who could have "fixed" the P-39 with a few simple, cheap tweaks if only they had thought of them.
The 37mm cannon and midship engine layout imposed penalties that the P-51 didn't have to put up with. With North American guys may have had a better idea to begin with or executed it better but they didn't have to deal with the 37mm cannon and the 10 ft extension shaft.

 

[tomo pauk]

I can certainly agree that slapping parts around don't make an eagle from turkey; I've felt that P-39 was feasible to upgrade with proper functioning turbo, and tried to find the possible 'solutions' for space CoG issues. While I agree that you've made many fair points, in the same time many of mine were not off the mark, too

I was not inquiring about absence of fuel tanks in front of main/central spar of P-39, but of P-63 (tomo pauk: The P-38 have had fuel tanks in leading edge - a space much less generous than front half of P-63's wing.

About the power ratio (1600 hp vs. 1125): I agree that we are almost in 1944 here.
Let's then compare 1325 hp @ 25K (MIL for P-38F) with 1150 hp 12K (MIL for P-39s of same era). The MIL value @ 25K for earlier P-39s should've been 900-950Hp (100 % of power is at 12 kft, zero HP at 55-56Kft, meaning a 10% loss for each 5.5kft above those 12kft) - only 60% of what was possible in same time frame, but with turboed engine. Quite a difference. At 17.5kft we can talk about 30% more hp for turbo.

 

[Zipper730]

1. There was a potential problem with drive-shaft vibration that called for a redesigned heavier drive shaft to be fitted

To start off, are you talking about the turbocharger or the crankshaft?

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.

What is the full RPM?

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.

Not very impressive, the Spitfire would perform better.

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.

So it would overheat?

The XP-39 needed a much larger intercooler to perform properly and this larger intercooler would not fit in the airframe.

1. Would it have fit in a plane the size of a P-63? It was about two feet longer...

2. Would it have been possible to remove the door, and used a sliding canopy? It could allow the linkages to be reworked and placed on the side rather than underneath the pilot (I just contacted an other aviation buff who came up with the idea).

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.

Just out of pure interest, do you have drawings of these layouts?

 

[Zipper730]

While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?

1. 360 mph in level flight, preferably 400
2. Climb to 20,000 feet in 6 minutes or less
3. Carry 1000 pounds of armament including a cannon
4. Preferably have a nose-gear

 

[wuzak]

To start off, are you talking about the turbocharger or the crankshaft?

The extension drive shaft to the propeller.

What is the full RPM?

It was in the region of 2800-3000rpm. I think for an early V-1710 as in teh XP-39 it may have been 2950rpm.

1. Would it have fit in a plane the size of a P-63? It was about two feet longer...

The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.

2. Would it have been possible to remove the door, and used a sliding canopy? It could allow the linkages to be reworked and placed on the side rather than underneath the pilot (I just contacted an other aviation buff who came up with the idea).

Yes, since later P-63s did.

 

[wuzak]

While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?

1. 360 mph in level flight, preferably 400
2. Climb to 20,000 feet in 6 minutes or less
3. Carry 1000 pounds of armament including a cannon
4. Preferably have a nose-gear

Lighter weight and better aero.

Which is what the P-39 had over the XP-39. The P-39 certainly met 1 and 4, maybe 3 and I'm not sure about 2.

 

[Zipper730]

The extension drive shaft to the propeller.

Gotcha...

It was in the region of 2800-3000rpm.

Okay

The P-63 used the 2 stage V-1710 and still no intercooler (at least for production versions). Instead it used ADI.

That was to avoid the intercooler?

Yes, since later P-63s did.

Didn't know that...

 

[wuzak]

That was to avoid the intercooler?

Yes.

 

[Zipper730]

Yes

I take it the intercooler was preferred over ADI?

Lighter weight and better aero.

Let's start with strength and ruggedness

1. How would you propose increasing overall strength?
2. How did the P-39 compare in strength to the F4F, F6F, F4U, Fw190, Hurricane, P-40, P-47, P-51, and Me 109?

 

[GregP]

The P-63 wasn't inferior to the Mustang. It would give a P-51 all it could handle at 25,000 feet. It certainly rolled better and the climb should have been comparable if not a bit better, too. It did come up some 15 mph slower, but that's nothing in combat. It makes a big difference in a protracted race, maybe. But if you were fighting with someone, you'd not notice 15 mph since neither of you is at top speed or anywhere even close.

Go ahead and make that high-speed 5 g break. You won't be at top speed when you come out of it.

It also wan't designed as a long-range fighter. And I get the idea that a long-range fighter is what they were looking for when the P-63 was ready.

I am under the distinct impression that the P-63 was not bought for one primary reason. And that is because the P-63 was about as good as a P-51 and the P-51 was already in production. If you are not getting a demonstrably better plane, there is really no compelling reason to start a production line that also requires training and a logistics chain. Also the P-63 did not have long range, and if they weren't getting SOME benefit from tooling up for it, there was no compelling reason to do it.

That is was produced for the Soviet Union was probably due to the USAAF wanting all the P-51s it could get for the U.S.A. and UK, so they let Bell Aircraft make the P-63, and only actually procured a handful themselves, mostly for evaluation purposes and experiments. Perhaps they didn't want Bell to make P-51s because they didn't want the Soviet Union to get P-51s in quantity. I bet we never really find out if we haven't to date.

I would like to have seen a 2-stage Merlin-powered P-63, but that apparently wasn't in the cards.

I like the P-63, but probably would not have bought it either, unless there was an issue with P-51 production, had it been my choice. As it turned out, there was no issue with P-51 production, and they'd have been better off having Bell build P-51s rather than chase the P-39 concept except for possibly not wanting to give Mustangs to the USSR in quantity.

I'm with Shortround when he says the P-39 airframe was too small to have a lot of improvements worked on it, and that it really wasn't big enough for a turbo system.

They also couldn't get the P-39 to tumble in a wind tunnel during the war. After the war was over, around 1949 or so, they did a wind tunnel test with the model ballasted as if the ammunition were empty ... and the CG was a bit more rearward, as when flying it in combat and most of your ammunition is gone. It tumbled beautifully in the wind tunnel. During the war they always tested it as though it had full ammunition, and the CG was more favorable, causing it to not even be ABLE to tumble. That sort of hints at a possible design weakness, and that is very limited CG range.

That only proves that you need to test at both forward AND aft CG limits for full aerodynamic results to be obtained, which any good test engineer SHOULD have been able to tell them anyway.

 

[tomo pauk]

While a little off topic: If Bell had designed the P-39 different from the start, what would have allowed it to have met the baseline requirements that Saville and Kelsey dictated?

1. 360 mph in level flight, preferably 400
2. Climb to 20,000 feet in 6 minutes or less
3. Carry 1000 pounds of armament including a cannon
4. Preferably have a nose-gear

The 2 and 3 are in conflict, no single V12 before 1942 will satisfy both requirements. Since it is USA we're talking about, the R-2800 is needed to meet all the criteria, or go with two V-1710s. Lockheed managed it with P-38.

In the old continent, the IMAM Ro.58 and Westland Whirlwind should fit the requirement, bar the tricycle gear.

 

[Zipper730]

The 2 and 3 are in conflict, no single V12 before 1942 will satisfy both requirements.

Maybe true, but at least it would be decent to try and bring them into the closest degree of alignment

 

[Shortround6]

Maybe true, but at least it would be decent to try and bring them into the closest degree of alignment

MK I & II Spitfires carried under 500lbs of armament, so did Bf 109s (except gun boats) , As did most Russian fighters with V-12 engines.

You can't put 1000lbs of armament in a 5-6000lb aircraft. Not had have a powerful engine or a strong airframe.

 

[Zipper730]

MK I & II Spitfires carried under 500lbs of armament, so did Bf 109s (except gun boats) , As did most Russian fighters with V-12 engines.

How much did the P-39 actually carry?