Effectiveness of the P-38

GregP said:

I've always thought the center of pressure moves aft when you go supersonic while the center of gravity doesn't move

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That's why the F-106A had an automatic fuel transfer system, activated when you hit Mach 1.05, to change the CG.

GregP said:

Hi Bill,

I've always thought the center of pressure moves aft when you go supersonic while the center of gravity doesn't move, thereby causing a nose-down pitching moment.

If the tail gets blanked by the shock wave, it becomes impossible to pull up until the airflow reattaches.

If you aren't supersonic, and you speed up, a stable aircraft will nose up in order to maintained trim airspeed.

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Greg, I mistakenly believed the same cause and effect. What actually happens is that lift in a dive is Near zero to slighty negative at -2degrees AoA with occasional forward pressure on stick required to maintain sufficient negative lift to remain neutral in the dive. The Moment Coefficient for a NACA 2015 airfoil is slightly negative pitch down throughout the angle of attack range until the stall break where it goes severely negative. In normal flight slightly up elevator trim is required.

What I believe is that when the shock wave first occurs on the 23015 wing section, the pressure distribution and lift (near neutral) is reduced aft of the shock wave. As a result, the constant negative CM for all angle of attack generate the pitch down moment and the shock wave wake turbulence blanks the elevator and trim.

My conclusions are that a.) the flap generates an immediate Pitch up and positive angle of attack, b.) the associated lift distribution in front of the shock wave increases, giving positive lift to the wing/body, c.) begins the degradation of the airspeed (and shock wave) by increased lift and drag, d.) enables natural dive pullout before the elevator is unblanked, slowing the P-38 in the dive until the airflow dive speed reduces below Mcr due to increase in temperature as f(altitude).
We know Lift is reduced behind the shock wave as major separation occurs. Logically the lift/pressure distribution center would move forward - but even if not so, the CMac remains negative for the NACA 23015.

Go look at the 2315 and 23015 section data and see if you draw the same conclusions.

Or my brain has been irreparably addled by drink and Wolfhound slobber.

Wolfhound slobber is one of those mysterious liquids that can affect life, love, and sometime aerodynamics.

I think it is, overall, due to the FM Effect (Freaking Magic).

GregP said:

I think it is, overall, due to the FM Effect (Freaking Magic).

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Knew it. Folks, all those diagrams, charts and formulae are really just incantations. There is no science.

And, you have to hold your tongue in the right position for it to work.

Obviously, the person below is thinking ... as should be intuitively obvious to the casual observer ...

drgondog said:

Or my brain has been irreparably addled by drink and Wolfhound slobber.

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I'm a few Shiners out of a helluva birthday party. Not sure why you're slobbering around with wolves but hey, we got no judgement here, Bill. You do you.

Frog said:

The trouble with the P-38 is it was initially conceived as an interceptor, used as a long range fighter and then switched to the fighter bomber role in the ETO...
All its imperfections, coupled with the pilots inexperience notwithstanding, it could fly at heights inconceivable for the P-39 and P-40, farther than both and the P-47, at a time when the Merlin P-51 was yet to be.
By the Way, in the rough climates of PTO or Aleutians, it did not met the troubles it had in the ETO.
By the way, my father was almost strafed by a P-38 in 1944...

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Although I'm new to this discussion, I agree with Frog on the Lightning's conception and its ultimate use. We need to realize that the plane was developed in the mid-1930s, when the possibility of Germany developing a long range bomber was feared. A fast climbing, heavily armed interceptor was needed and the P-38, although not needed in its original role, was used wherever it was needed until better planes were built.
Just my two cents worth.

MarkS said:

One thing about the P-38 is that it was recognizable at long distance due to its distinctive layout. And this was a war where fighter pilots' ability to distinguish friend from foe, at the limits of their vision, was important.
Built-in, can't be fixed.
But the Lightning was (obviously) an excellent fighter plane; its record proves that. And the USAAF found ways to use its superior characteristics, and fix its issues, so that it was effective. Remember that everyone was using what they had and modifying their aircraft, strategy, and tactics as the campaigns wore on… Kelly Johnson didn't know in 1939 what combat conditions would be, in different theaters, five years hence. He had an idea, it was a good one, and combat experience helped them improve it.
This hindsight stuff… in 1950 every Allied air commander, if asked, would have preferred to have had F-86 Sabres in 1943. But they fought and won with what they had. And certainly the P-38 did its job well.

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Mixed blessing. Being easily identified meant it was far less likely to be shot down by "friendly fire". IIRC it was the only fighter that flew over the D-Day landing beaches for that very reason. How much quicker could a gunner on a bomber differentiate a P-38 from a Bf-109, vs a P-51? And of course-once we realized how much friendly fire was impacting our fighters, particularly singe-engine ones-we painted great big honking black and white stripes on them-so they could be identified quickly at long range anyway, negating that disadvantage.

IdahoRenegade said:

..-we painted great big honking black and white stripes on them...

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But they really looked cool.

SaparotRob said:

But they really looked cool.

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That they did.

The other point to make-about the P-38 being DESIGNED as an interceptor. I've read Bodie's book a few times (granted it's been a while) and I recall a discussion about this-don't remember if it quoted Johnson or (then Lt) Kelsey. Ben Kelsey wrote the specification package for what became the P-38, knew what he wanted in a fighter, in terms of firepower (a gun and ammo loadout unheard of in "pursuits" of the time), range, rate of climb and straight line speed. But what he felt was necessary flew in face of conventional USAAC thinking at the time (tied in with the "bomber mafia"-they didn't want anyone thinking bombers needed a fighter escort), and he understood he'd never push it through as a fighter. Fighter (pursuit) production was also a low priority at a time when so much focus was on bombers. So they built a plane that did everything he felt a fighter needed to do (and the only way to do so in 1937 when he wrote the spec was with a twin engined plane). And the only way to get that through procurement was to "market" it as an "interceptor", not a "fighter". So Kelsey wanted/spec'd a fighter, Kelly designed and built a fighter-but called it an interceptor as a marketing tool. Of course-if you're going to "market an interceptor" it better do interceptor sh*t too-fast climb rate, heavy firepower to take out bombers, decent range-which of course it did. Part of the reason the "money guys" were more willing to fund an interceptor than a fighter came back to the bomber mafia-they proclaimed the capability of the heavy bomber so much that Congress (and USAAC brass) understood that they probably should have some way of addressing a potential enemy bomber threat against the US.

The other feature and example of Kelsey's and Johnson's forward thinking. They had a pretty good idea that bombers WOULD need fighter escort and understood the range implications. So in addition to a big internal fuel load (for the day) they anticipated a future need for drop tanks (effectively "banned" at the time by USAAC leadership, for the very reason of not producing an "escort fighter") and "blocked in" a provision for them in the design. In early 1942, the question of self deploying from the US to England was posed (by Arnold IIRC, perhaps partly in jest) and the answer came back sure, we can do that. Drop tank plumbing and hardware was quickly installed and tanks developed, IIRC by around May 1942, in support of Operation Bolero. 165 gallon tanks initially (and most commonly used throughout the war) along with some 310 gallon "ferry" tanks.

Another point to discuss-the wing leading edge fuel tanks in the outer wings (replacing the intercoolers) of later (J and L models) did add to the polar moment of inertia-impacting roll rate. But like the rear fuselage tanks on the P-51, they were intended to be burned first and would be empty by the time they (were planned to) engage in combat, so it was somewhat of a moot point.

Now-I'm getting older and the brain cells ain't a firin' like they used to, I'll try to check this out in the book in the next few days.

IdahoRenegade said:

Another point to discuss-the wing leading edge fuel tanks in the outer wings (replacing the intercoolers) of later (J and L models) did add to the polar moment of inertia-impacting roll rate. But like the rear fuselage tanks on the P-51, they were intended to be burned first and would be empty by the time they (were planned to) engage in combat, so it was somewhat of a moot point.

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You can't have used up both your drop tanks and your leading edge tanks and still have enough fuel to get home.
Main fuel tanks = 300 gallons
Leading edge tanks =110 gallons
Drop tanks =330 gallons

You used 440 gallons to get to the target and have only 300 gallons of fuel to fight at maximum power and still get get home.

Reluctant Poster said:

You can't have used up both your drop tanks and your leading edge tanks and still have enough fuel to get home.
Main fuel tanks = 300 gallons
Leading edge tanks =110 gallons
Drop tanks =330 gallons

You used 440 gallons to get to the target and have only 300 gallons of fuel to fight at maximum power and still get get home.

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Easy. You come in really high, drop tanks, fight, and dive to catch the tanks before they hit the ground ...

The things one learns here.

Does a plane like a P-38 consume fuel on the return flight at the same rate as the outbound and combat? Is the lighter load from consuming 300 to 400 gallons of fuel in the noise with respect to fuel consumption?