P-38 Lighting: Thinner Wings?

[Shortround6]

While one cannot make wholesale changes in the wing cord at will, adjustments or tweaks can be made and have been made.

The F-86 Sabre jet 6-3 wing being an example. Cord was extended 6 inches at the root and 3 in at the tip and many existing aircraft were modified.

As Tomo has said, the guys at NACA (with the wind tunnels) were suggesting an extension of the wing leading edge between the fuselage and the engine nacelles. not the entire wing.
The P-38 suffered from an anomaly in that the air from the sides of the fuselage and the engine nacelles was forced through the area occupied by the that inner section of wing which meant that was about the highest speed airflow on the whole plane (or at least the wing).Changing the cord on that section of wing to change the critical mach number may have worked even leaving the rest of the wing alone.

 

[Elmas]

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.

That can be certainly done, experiments in aviation are not forbidden, and often tests are made to push a design "to the limit".
But one thing is a test, another is modifing assembly lines, jigs etc. when a war is going on, with a modification of doubtful effectiveness and certaily not thorougly tested.

 

[tomo pauk]

That can be certainly done, experiments in aviation are not forbidden, and often tests are made to push the design "to the limit".
But one thing is a test, another is modifing assembly lines, jigs etc. when a war is going on...

Tests are necessary IMO.
NACA suggested the addition to the wing (and not the application of a whole new wing profile) to the P-38 as the best solution exactly due to production reasons - far easier to do it, than to introduce a new wing profile, even if it is just for the inboard wing.

 

[Elmas]

While one cannot make wholesale changes in the wing cord at will, adjustments or tweaks can be made and have been made.
The F-86 Sabre jet 6-3 wing being an example. Cord was extended 6 inches at the root and 3 in at the tip and many existing aircraft were modified.
omissis
Changing the cord on that section of wing to change the critical mach number may have worked even leaving the rest of the wing alone.

Yes, of course, it may have worked. With a little bit of luck.
But while on P-38 a lot of modifications were made during the war, this modification was not implemented. That clearly means that designers were not at all sure if that modification was really an improvement, not were not at all sure how this modification could have altered other parameters of the airplane, or simply they had not time and resources to test it thorougly, both in wind tunnel and on prototipes ( and I think that this was certainly done in the wing modification of F-86).
"If you are not sure, let things how they are...." is one of aeronautical engineers mottos.

 

[Elmas]

(Already published in another 3d).
Of course modifications to the wings can be feasible… with what today may seem fantasy and ... foolishness.
The prototipe and the first batch of Macchi MC 200 had a very sharp leading edge

that caused bad stall and instability problems, expecially handling the planes "the italian way"...
Some rumors say that it was an Italian test pilot, Adriano Mantelli, ace of the Civil Spanish War, that, worried by the instability of the new monoplane, by his own initiative, grabbed some balsa boards that were in the hangar, glued them to the leading edge, shaped them with a rasp and covered the whole with doped fabric, all in just one night.
Very early the following morning, without telling anyone, he took off to test the plane, with great success.
Of course the designers and some military test engineers of the Reparto Sperimentale di Volo (that knew Adriano Mantelli personally, and that I personally interwieved in early '80s) denied (obviously) completely this story.
I can't imagine how all that could have happened today, with the mountains of paper work necessary just to change a bulb lamp in an aeroplane.
In the following series the profile was changed (not too difficult to do, the Italian fighters were practically hand-built, and so no complex realigning of heavy machinery was needed...) and proper wash-out introduced. After these changes the wing was so successful that it was retained in both MC 202 and 205.

 

[EverKing]

As Tomo knows, I have done some pretty deep research into this topic. There is a lot to unpack in this discussion but the basic idea, of using a thinner wing on the P-38, poses a few problems, not least of which is the afore mentioned reduction in fuel capacity. There is also potential structural concerns: the center-section (inner-wing) needs to support the entire load of the engines, nacelles, turbos, main gear, radiators, and the boom structures not to mention the entire empennage and the various control loads associated with it. Or, looking at it another, the main spar and two sheer beams (forward and aft) in this area provided the main structural support to tie the two halves of the aircraft (booms) together and support the cockpit + armament compartment). While there was a tendency at the time to overbuild many aircraft structures I think expecting the engineers and more importantly the brass at the AAC to accept a reduced structure and still expect it handle the stresses of combat, etc, is a bit of a stretch.

Another concern would also be take-off/landing and stall speeds. The reduction in CL of a thin, high-speed, airfoil could theoretically increase the top-end performance through reduced drag (ignoring the limiting drag of the center gondola--it's what Hal Hibbard called it, so that's what I stick with--and canopy) but the trade off would be poor low-speed handling especially considering an aircraft with a dry weight in of roughly 13,000 lbs and a combat load of around 18,000 lbs. While the Fowler flaps would help this it would still increase TO/L and Stall speeds above what the P-38 ended up with. This, especially at TO/L, would also impact the required gear strength and the ability to operate off short, underdeveloped, or unimproved strips.

The better option(s) are those explored by NACA in their March 1942 report. The main three options were replacing the inner wing with a 10% longer chord 6-Series airfoil with the same maximum absolute thickness (therefore maintaining the internal structures), the same but using a 20% longer chord 6-series--this was explored and tested on the Swordfish--but the best recommendation was a 20% forward chord extension with a reprofiled leading edge applied entirely afore the forward box spar. This latter maintains the internal structures, fuel space, and most of the wing structure. The change in the location to the center of pressure that this final option would produce would be balanced by installing leading-edge coolant radiators into this space, therefore removing the need for the bulging remote radiators in the mid-booms. Regardless of the changes to the inner-wing, NACA recommended also extending the gondola three feet rearward and re-profiling the canopy to improve the critical flow in these areas and encourage convergence of the boundary flow sooner, before the horizontal stabilizer; as well as extensive filleting added to the wing/fuselage confluence (the only recommendation from the report that was applied to the actual P-38).

Incidentally, where fuel source selection is concerned, the P-38 was pretty forgiving regardless of which tanks were full or empty. The POH for the plane stated that start-up, warm-up, Take-Off, and the first 15 minutes of flight were to be run from the Reserve (forward, 60 US Gal) tank to free up space for the vapor return from the carbs. The recommend sequence was then to start with the Reserve for a little while, switch to external if carrying drop tanks (cross-fed one-at-a-time and dropped when empty), run the Outer Wings dry (if equipped...you would want to unload the outer wings as much as possible for better maneuvering in combat), then run the Mains (93 US Gal, behind the main spar but in front of the rear shear beam/flap mounts) dry, before finally switching back to RES to finish off, if needed.

 

[tomo pauk]

<snip>

Cheers, man, long time no see

 

[EverKing]

Cheers, man, long time no see

Yeah, been busy with offline life...house projects, moving, etc. I saw the link to this discussion on that other site we're on and figured I'd pipe in. About to join the discussion over there as well.

 

[Jugman]

Investigation of Diving Moments of a Pursuit Airplane in the Ames 16-Foot High Speed Wind Tunnel

 

[Zipper730]

Wing loading of Spitfire and P-38?

Quite a bit different

 

[EverKing]

Investigation of Diving Moments of a Pursuit Airplane in the Ames 16-Foot High Speed Wind Tunnel

That is one, but not the original report. The original report and investigation was "Guryansky, Eugene R., and Preston, G. Merritt: Full-Scale Wind Tunnel Investigation of Buffeting and Diving Tendencies of the YP-38 Airplane. NACA MR, March 1942." It appears the old link I had is no longer available and I have not been able to find it on the updated NASA historical documentation library (they changed it in the past year). Luckily, I had previously downloaded and saved a copy so I will try upload it here.

 

[EverKing]

And in looking at that report again I realized I mis-spoke in my previous post. Tomo was indeed correct that the three options were .1c extension, .2c extension, and the 66-115 "glove" at 1.2c rather than a .2c extension and 66-115 at 1.1c and 1.2c as I mistakenly said.

The final conclusion is the same, however, that the final NACA Recommendation for the modification with the best return was the .2c leading edge extension and moving the radiators into the newly extended leading edge. All of the details are in the report I attached above.