P-38 Lighting: Thinner Wings?

[Zipper730]

I remember in Warren Bodie's book on the P-38, it was said that the P-38's wings would have been as thin as the Spitfire had ferry range, and auxiliary fuel not been a problem: I'm not sure if that was an honest statement or a 20/20 hindsight view, but a P-38 with slimmer wings would have been quite a design, and might have avoided a lot of the problems the actual P-38 would have faced.

There'd have been range penalties, provided they didn't use an overall different planform, but it would have been a hell of a performer, in overall

 

[pbehn]

I remember in Warren Bodie's book on the P-38, it was said that the P-38's wings would have been as thin as the Spitfire had ferry range, and auxiliary fuel not been a problem: I'm not sure if that was an honest statement or a 20/20 hindsight view, but a P-38 with slimmer wings would have been quite a design, and might have avoided a lot of the problems the actual P-38 would have faced.

There'd have been range penalties, provided they didn't use an overall different planform, but it would have been a hell of a performer, in overall

It didn't really have a fuselage so if you remove a lot of fuel from the wings you have a plane with a shorter range than a Spitfire, apart from stellar performance in mock dogfights what would you use it for?

 

[Elmas]

With a thinner wing, what about take-off and landing speeds?

 

[pbehn]

With a thinner wing, what about take-off and landing speeds?

It is so light it can be hand launched and caught in a fishing net to land.

 

[Elmas]

It is so light it can be hand launched and caught in a fishing net to land.

I would suggest a rubber band for take-off.




and/or some additional power



An airplane is always the result of many compromises, like a suit tailored by skilled tailors: if you lift shoulder, you have to lenghten sleeve ...

 

[pbehn]

I would suggest a rubber band for take-off.


View attachment 575437

That was the Navalised version. Winkle Brown was impressed.

 

[Zipper730]

It didn't really have a fuselage

I suppose it sort of had three. There were the booms which were fuselage-ish, except they had no cockpit; then there was the gondola, which had the nose, the guns, and the nose-gear, which was fuselage like.

if you remove a lot of fuel from the wings you have a plane with a shorter range than a Spitfire

Actually, how much fuel did the XP-38 and P-38 variants prior to the -J model have in the wings and booms? How much of a reduction would you have if the T/C went from say 15-16% to about 13%?

With a thinner wing, what about take-off and landing speeds?

I'm not sure the exact effects on thickness/chord to stall speeds, but the Spitfire flew pretty good...

 

[tomo pauk]

I remember in Warren Bodie's book on the P-38, it was said that the P-38's wings would have been as thin as the Spitfire had ferry range, and auxiliary fuel not been a problem: I'm not sure if that was an honest statement or a 20/20 hindsight view, but a P-38 with slimmer wings would have been quite a design, and might have avoided a lot of the problems the actual P-38 would have faced.

There'd have been range penalties, provided they didn't use an overall different planform, but it would have been a hell of a performer, in overall

As thin as the Spitfire wing - are we speaking about relative value(thickness to chord ratio) or about absolute value (wing being so many inches thick)?

It didn't really have a fuselage so if you remove a lot of fuel from the wings you have a plane with a shorter range than a Spitfire, apart from stellar performance in mock dogfights what would you use it for?

There is a lot of space in outer wings that can accept fuel tanks.

With a thinner wing, what about take-off and landing speeds?

P-38, along with Fairey Firefly, was the only in-service ww2 Western fighter that sported Fowler flaps.

Actually, how much fuel did the XP-38 and P-38 variants prior to the -J model have in the wings and booms? How much of a reduction would you have if the T/C went from say 15-16% to about 13%?

Up to the H model, 300 gals in protected tanks, 400 gals if the tanks were not protected.
Reduction of the TtC by 20% will reduce the fuel tankage by more than 20%, since the thickness of the s-s tanks will not be reduced. So we are looking at perhaps 70-80 gals of fuel less on H model and prior, unless we find the place to install additional fuel tank(s). There was no fuel in the booms.

 

[Elmas]

I suppose it sort of had three. There were the booms which were fuselage-ish, except they had no cockpit; then there was the gondola, which had the nose, the guns, and the nose-gear, which was fuselage like.
Actually, how much fuel did the XP-38 and P-38 variants prior to the -J model have in the wings and booms? How much of a reduction would you have if the T/C went from say 15-16% to about 13%?

I'm not sure the exact effects on thickness/chord to stall speeds, but the Spitfire flew pretty good...

Wing loading of Spitfire and P-38?

As thin as the Spitfire wing - are we speaking about relative value(thickness to chord ratio) or about absolute value (wing being so many inches thick)?

There is a lot of space in outer wings that can accept fuel tanks.

P-38, along with Fairey Firefly, was the only in-service ww2 Western fighter that sported Fowler flaps.


.........
.

Exactly. You can't install Fowler flaps on a plane that already has, can you?

 

[tomo pauk]

Exactly. You can't install Fowler flaps on a plane that already has, can you?

What is the point you're trying to make, and I don't seem to get it?

 

[swampyankee]

The P-38 used (according to the Incomplete Guide to Airfoil Usage,
The Incomplete Guide to Airfoil Usage) the NACA 23016 at the root and the 4412 at the tip. For an aircraft with the design speed of the P-38, there is very little reason to go thinner than 12% (as an aside, the C-130 uses a NACA 64A318 at the root and a 64A412 at the tip, while the P-3 uses the NACA 0014-1.10 at the root and a 0012-1.10 at the tip; the P-3 operates at speeds comparable to or greater than the P-38, and the C-130 is faster than many WW2-era fighters).

What a thinner wing will get you is a heavier wing with less usable internal volume, and it's also unlikely that there will be a significant increase in maximum speed. The thin-wing P-38 may have reduced Mach tuck tendencies, but fixing that (not just the dive flap crutch) would likely require redesign of the fuselage* and engine nacelles. It also wouldn't solve the P-38's other failings, related to engine installation, roll acceleration (it may make that worse, as thinner wings may have less torsional rigidity), and cockpit heating that restricted its utility in a major combat theatre.

Some other airfoils used by US fighters of the era:

• Most models of the P-51 used a NAA/NACA airfoil, the 45-100.
• The P-51H used a NACA 66-(1.8)15.5 at the root and a 66-(1.8)12 at the tip
• The P-40 and P-36 used a NACA 2215 at the root, and the NACA 2209 at the tip
• The F4U used a NACA 23015 at the root and a 23009 at the tip
• The F6F used a NACA 23015.6 at the root and a 23009 at the tip
• The F4F used a NACA 23015 at the root and a 23009 at the tip
• The P-47 used a Seversky S-3 airfoil.
• The P-39 used a NACA 0015 at the root and a 23009 at the tip.
• The F2A used a NACA 23018 at the root and a 23009 at the tip.

And some non-US fighters

• German
  • Bf109 Root: NACA 2R1 14.2, Tip 2R1 11
  • FW190 Root: NACA 23015.3, Tip NACA 23009 (the Ta152 used the same airfoils)
  • Bf110 Root: NACA 2R1 18.5, Tip 2R1 11
• British
  • Hurricane: Root Clark YH (19%), Tip Clark YH (12.2%)
  • Spitfire: Root NACA 2213, Tip NACA 2209.4
  • Typhoon: Root NACA 2219, Tip NACA 2213
  • Tempest: Root H/1414/37.5 (14%), Tip H/1410/37.5 (10%)

One interesting observation is that a lot of non-US aircraft used NACA airfoils (I wasn't cherry-picking). I suspect that this is because a) NACA had started a systematic investigation of airfoils between the wars and b) published the data on a lot of them. This gave designers an excellent catalogue to choose from, with a lot of high-quality airfoils with high-quality data. Some of these weren't used often -- as far as I could tell nobody except Messerschmidt used the NACA 2R1 airfoils -- but they were used by every nation.

A second is that people designing for the USN really liked 230xx airfoils. While I'm not a particular fan of that airfoil series, there isn't anything particularly wrong with them (despite Riblett's rantings about the 23012), there do seem to have been better options at the time.


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* Some people argue that the P-38 didn't have a fuselage; it had three nacelles: two for the engines and one for the cockpit. I really don't care what one calls that thing where the pilot sits.

 

[Shortround6]

I believe the the early P-38s (up until the "D") had 400-410 gallons of fuel in the inner wing, The installation of self sealing tanks cut that to 300 gallons. Using a thinner wing root/inner wing between the "fuselage" and the engine nacelles would cut that further. As Elmas as so correctly pointed out, changes in one area affect other areas. Yes there is empty space in the outer wings. However in the P-38J & L they used the space forward of the spar for the fuel tanks, that is just forward of the CG. on the P-38D-H that is where the intercooler is (poor as it may have been) and putting fuel tanks behind the spar

puts them behind the CG, how much this would affect handling I don't know. However, the P-38 was already noted as not one of the best rolling fighter planes going and sticking hundreds of pounds of fuel outboard of the engines was certainly not going to improve things. At least until the power boosted ailerons show up.
For the Fowler flap fans note the distance from the rear of the wing to the attachment point/s for the Fowler flap. The thin green line behind the main tanks in the inner wing and continuing on through the engine nacelle and out to the aileron where it steps back to the aileron hinge point.
Fowler flaps can do wonderful things but they have to be designed in from the start and not added later. They also help dictate the rear spar placement and the available volume in that part of the wing.
On the P-38 the main tanks behind the spar were 90 US gallons while the tanks in front of the spar were 60 gallons each and labeled reserve tanks. The main tanks would normally be used for starting, warming up and taking off. Normal practice for most planes in WW II seems to have been to use up fuel from the tanks in certain sequences to maintain an acceptable CG. I don't know of any cases where fuel was pumped from one tank (or set of tanks) to another to maintain the CG as normal procedure. The more complicated fuel management becomes the great the risk of accident.

I will note on another subject that the P-38s cockpit heating problem was actually fairly easy to solve. Once the bean counters were forced to accept a 2nd electrical generator in the aircraft (one on each engine) there was sufficient electrical power for a cockpit heater. Having just one generator on what was a fairly electric dependent plane for so long was just stupid. Not only all lights, most instruments and all the radio gear (including the IFF) but the plane used electric propellers, electric armament control, auxiliary fuel pumps, turbo regulators, oil cooler and intercooler exit flaps, and a few others.
The 2nd generator did not show up until after the first of the J models were built (I believe the P-38J-10s got them?) and so did not show up until after the P-38 was thoroughly and justly condemned for poor cockpit heating in NW Europe, what difference few months makes

 

[Elmas]

To make short a long story, if you install a thinner wing in a plane, all other things unchanged, you will have (generally) at the same (low) speeds an inferior Coefficient of Lift.
So, to have the same overall lift necessary for take-off, you have to increase speed (or wing surface).
Coefficient of lift can be increased by the use of flaps, and in particular of Fowler flaps, that increase both camber (increasing CL) and wing surface, but in a plane that has already Fowler flaps there's nothing you can do from this side.
But, of course, Fowler flaps increase dramatically Drag Coefficient, so more power is needed. I remember a war time Lockheed movie of the series "How to fy... etc... " (very interesting) with a Tony leVier that stated that with or without flaps the lenght of take off of a P-38 was practically the same. I think it is still on youtube.
If I remember well, there were even some protests form (young) Pilots that considered P-38 take-off a little bit too tricky.

Hurricane, and other planes, expecially at the start of the war, where concrete t/o - landing strips were inexistent, had their reason to have such a thick airfoil, that was one of his strenghts and at the same time one of his weaknesses.

P.S.1 : sorry for the very poor drawing.
P.S. 2: I am a P-38 fan....

 

[swampyankee]

I believe the the early P-38s (up until the "D") had 400-410 gallons of fuel in the inner wing, The installation of self sealing tanks cut that to 300 gallons. Using a thinner wing root/inner wing between the "fuselage" and the engine nacelles would cut that further. As Elmas as so correctly pointed out, changes in one area affect other areas. Yes there is empty space in the outer wings. However in the P-38J & L they used the space forward of the spar for the fuel tanks, that is just forward of the CG. on the P-38D-H that is where the intercooler is (poor as it may have been) and putting fuel tanks behind the spar
View attachment 575453
puts them behind the CG, how much this would affect handling I don't know. However, the P-38 was already noted as not one of the best rolling fighter planes going and sticking hundreds of pounds of fuel outboard of the engines was certainly not going to improve things. At least until the power boosted ailerons show up.
For the Fowler flap fans note the distance from the rear of the wing to the attachment point/s for the Fowler flap. The thin green line behind the main tanks in the inner wing and continuing on through the engine nacelle and out to the aileron where it steps back to the aileron hinge point.
Fowler flaps can do wonderful things but they have to be designed in from the start and not added later. They also help dictate the rear spar placement and the available volume in that part of the wing.
On the P-38 the main tanks behind the spar were 90 US gallons while the tanks in front of the spar were 60 gallons each and labeled reserve tanks. The main tanks would normally be used for starting, warming up and taking off. Normal practice for most planes in WW II seems to have been to use up fuel from the tanks in certain sequences to maintain an acceptable CG. I don't know of any cases where fuel was pumped from one tank (or set of tanks) to another to maintain the CG as normal procedure. The more complicated fuel management becomes the great the risk of accident.

I will note on another subject that the P-38s cockpit heating problem was actually fairly easy to solve. Once the bean counters were forced to accept a 2nd electrical generator in the aircraft (one on each engine) there was sufficient electrical power for a cockpit heater. Having just one generator on what was a fairly electric dependent plane for so long was just stupid. Not only all lights, most instruments and all the radio gear (including the IFF) but the plane used electric propellers, electric armament control, auxiliary fuel pumps, turbo regulators, oil cooler and intercooler exit flaps, and a few others.
The 2nd generator did not show up until after the first of the J models were built (I believe the P-38J-10s got them?) and so did not show up until after the P-38 was thoroughly and justly condemned for poor cockpit heating in NW Europe, what difference few months makes

I don't think it was the bean counters; I think it was a decision made within Lockheed's design office to save weight, probably over 100 pounds. Given the rest of the aircraft, I think minimizing complexity was low on the list of concerns, although having two generators would make the electrical system much more complex.

 

[Shortround6]

Perhaps, but loosing aircraft due to a generator failure while both engines are still running seems a poor risk/benefit ratio.
Build long range airplane that cannot make it back from a mission if the generator fails near the limits of the radius?
If the battery fails (goes dead) the pilot cannot control the props or the oil cooling system. He is forced to turn off electrical items (including the radio) to save battery power to keep the airplane flying and turn things off and on periodically to check things (like his location?). Heaven forbid that he being flying at night and need cockpit lights on occasion

Granted in the late 30s very few other multi engine aircraft had redundant systems, some Halifaxes (in the early part of the war)were truly terrible, with 3 generators on four engines each generator powered part of the aircraft with no crossover. One generator powered the front and one the rear, I don't remember what the 3rd covered, If you lost a certain generator or engine the cockpit was fine but your tail turret and dorsal turret no longer worked, or vice versa. Simple wiring but I don't imagine the crews thought much of it. two working generators and your two most important defensive turrets still won't work. Or two working generators and the pilot can't see the instruments (those that are still working).

 

[Zipper730]

As thin as the Spitfire wing - are we speaking about relative value(thickness to chord ratio) or about absolute value (wing being so many inches thick)?

Thickness/Chord ratio almost certainly based on the statements in the book.

 

[Shortround6]

I believe Tomo had referred to an NACA study about extending the P-38 wing leading edge and trailing edge in order to change the thickness/cord ratio. This being the easiest way to do such a thing. It keeps the majority of the structure between the fuselage and engine booms the same. Keeping the wing leading edge and trailing edge the same and using a thinner structure basically means redesigning the whole wing structure. Or at least the parts until you get down to the 23012 or so airfoil on the way to the 23009 at the tip.
However extending the wing leading edge makes the downward view from the cockpit (none too good to begin with) even worse. If I am doing the math correctly (big if) going for a 14% airfoil while keeping the max wing thickness the same at the wing root means you need another 16-17 inches (400-430mm) of cord. How much on the leading edge and how much on the trailing edge I don't know.
How much extending the trailing edge is going to mess up the existing Fowler flaps I don't know.

 

[tomo pauk]

...
However extending the wing leading edge makes the downward view from the cockpit (none too good to begin with) even worse. If I am doing the math correctly (big if) going for a 14% airfoil while keeping the max wing thickness the same at the wing root means you need another 16-17 inches (400-430mm) of cord. How much on the leading edge and how much on the trailing edge I don't know.
How much extending the trailing edge is going to mess up the existing Fowler flaps I don't know.

The suggestions included just 'adding' to the wing in front of the existing leading edge, not to the trailing edge. Just inboadr part of the wing, ie. between the pod and nacelles. There were two suggestions:
- 10% increase of chord
- 20% increase of chord; such extension was found to move center of lift too much to the front, so it was further suggested that cooling system is relocated into the newly acquired volume (= from behind the CoG to in front of CoG), so the plane is ballanced out.

The 3rd suggestion included implementation of the newly-fanged 'laminar flow' to the inboard wing, with a note that such a change will require much more of a modification to the existing production lines. Also, the suggestion was for the windscreen to be more slanted, as well as for the aft part of the PoD, all so the critical Mach number is increased.

The downward view from P-38 was probably far better than on P-47 or F4U to begin with, especially to the front-down.

 

[Elmas]

It is not possible, in an particular aeroplane, to increase or decrease the chord "at will" without completely re-design that aeroplane.
1st - Every wing profile has:
Coefficient of Lift
Coefficient of Drag
Coefficient of Momentum

Aerodynamic Lift, Drag and Moment Coefficients | AeroToolbox

If you increase or diminuish the chord of the wing, the Center of Pressure will move along the chord and so the Pitching Momentum will vary: but, as Center of Gravity and Center of Pressure are in close relationships between each other, if you move the Center of pressure you'll have to move consequently the Center of Gravity, moving all the masses of the aeroplane.
A nightmare.
2nd – Wing profiles derived from Clark Y, one about 19% and one 12%

A cantilever wing is subjected to
Bending
Torsion
Shear
In bending, the resistance depends from the thickness of the wing and, in particular, from the Moment of Inertia. Using for example a longeron with an H shape, the formula is the following:

being the formula of the Moment of Inertia non-linear, even small increases or decreases in the thickness of the wing involve big increases or decreases of the bending stiffness, if we use the same materials.
For Torsion, as the stiffness depends from the area A of the D-box (Bredt formula)



also in this case a decrease in area A means a decrease in torsion stiffness and vice-versa.
Shear…. That involves the Center of Shear, and that involves Torsion, things get a little bit complicated without advanced Maths.
So, if you, even slightly, decrease the thickness of a wing, mantaining (practically) the same weight of the structures (same longerons, stringers, thickness of plates etc.), you will have much less resistance and less stiffness: to mantain the same resistance and stiffness of the thicker wing there will be an increase of weight.
Much better a new piece of paper.

 

[tomo pauk]

People at NACA were of opinion that you can increase the chord at a part of the wing of P-38, they adressed the change of lift; people at Lockheed flew the P-38 'Swordfish' with a modified wing section. So I'd say that proof is in pudding.