Effectiveness of the P-38 (2 Viewers)

[MIflyer]

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

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

 

[drgondog]

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.

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.

 

[GregP]

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).

 

[SaparotRob]

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

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

 

[GregP]

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 ...

 

[Thumpalumpacus]

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

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.

 

[Tiger11]

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...

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.

 

[IdahoRenegade]

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.

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.

 

[SaparotRob]

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

But they really looked cool.

 

[IdahoRenegade]

But they really looked cool.

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.

 

[Reluctant Poster]

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.

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.

 

[GregP]

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.

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

 

[SaparotRob]

The things one learns here.

 

[pinehilljoe]

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?

 

[J.D.]

Spitfire didn't do very well in Australia over Darwin, either versus Zeros. That didn't mean Spitfires were bad fighters. It meant the Spitfire pilots were unfamiliar with their Japanese opponents, their mounts, and their tactics.

Now that's an interesting topic, if a bit O.T. in the context of this thread. I read Anthony Cooper's excellent, forensic examination of this last year. To say there was a bit more to it would be something of an understatement but you are on the right track. In fact, this was the best book I read last year and that's saying a lot. Highly recommended:

Darwin Spitfires

Darwin Spitfires tells this little-known story, and in so doing, recovers this important aspect of Australian history.

unsw.press

 

[drgondog]

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.

The spec was written for both the XP-38 and XP-39 on the basis of meeting Interceptor as you state, but the Circular Proposal CP37-608 for the future twin engine interceptor which resulted in the P-38 was truly for an interceptor anticipating an enemy capability equivalent to the fast high-altitude B-17. With the internal fuel that remained constant until the 55 gal leading edge tanks were installed in the P-38J in mid 1943, the combat radius was approximately 140mi - more than the XP-39 but nothing to plan long range escort around. Kelsey as Lt. was subordinate to Cpt. Schlatter Fighter Projects officer, reporting to Echols, reporting to Brett at Mat.Cmd. Wright Field. I have zero knowledge regarding Kelsey's inputs to the design, but have a reaallly hard time seeing him a a 'prime influencer' at the bottom of the totem pole of the money decisions.

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.

See above and perhaps re-think the politics. "Kelsey wanted' is irrelevant to what Westover/Arnold/Andrews and Emmons visualized in 1937, but there is no dispute that he was very important as the Lockheed projects officer. But to assign Kelsey as a leader in 'marketing' the P-38 as a multi-role fighter seems to be a stretch. IMO Kelsey's influence did materialize in 1940 when he was assigned along with Spaatz, Brett as US 'Observers' to the Brit/France war with Germany and carried back to Republic and Lockheed to 'prepare for long range ferry requirement' as the U-Boat threat escalated. He did, indeed successfully prod Lockheed to modify the P-38 to carry both ordnance and fuel tanks beginning in late summer of 1940. He was unsuccessful with Republic. That is the genesis of P-38 multi-role capability as the B-10 racks were capable of 2000 pound bomb/torpedo/165-gal drop tank.

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.

See above. Lockheed began studies for both P-49 and P-38 in 1940, delivering in December 1941. First with 75-gal tanks developed for P-40 and P-39. What is true, however, is that the conflict in AAF domestic policy regarding flying with pressurized external tanks was in effect in 1941 and hindered Lockheed's development of pressurization scheme for external drop tanks until the delivery of the P-38Js in early 1944. They were field modified with same scheme as P-47 and P-51 by VIII ATS led by Cass Hough.

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.

P-38E/F-4 kits were installed in December 1941, in production for P-38F in January 1942' in time for Arnold's "Increase range of all tactical aircraft in March 1942. The 75-gal tanks were ready for initial Bolero migration and the 150/165-gal were first delivered to UK in September 1942.

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.

Not really - internal fuel remaining after combat is the God of surviving the trip home. The 55-gal tanks, unlike the P-51B fuse tank, was located pretty close to the CG and within the forward cg limit. Boosted ailerons were the solution to roll rate beginning with the J-25. The P-51B instruction from Mat.Cmd. was to burn 40 gal to achieve normal maneuver cg limit. That was an operational decision at the tactical level and depended on the mission range requirement. Standard SOP was either take off with 65-gal and burn during climb before switching to external tanks, or take off with 85-gal and burn down to 45 before switching.

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.

Highly recommend that you obtain Reel A2069 from USAHRC. It contains both the Case History Droppable Fuel Tanks and FAREP. It was the key source for Boylon's USAF Study 136 Development of the Long Range Escort Fighter.

 

[Shortround6]

Maybe I am misinterpreting things that I read in books (not original documents) so I could be wrong on several paths.

The spec was written for both the XP-38 and XP-39 on the basis of meeting Interceptor as you state, but the Circular Proposal CP37-608 for the future twin engine interceptor which resulted in the P-38 was truly for an interceptor anticipating an enemy capability equivalent to the fast high-altitude B-17.

From what I remember the fundamental difference between the two original requirements was that the twin engine had twice the endurance that the single engine requirement wanted.
Things are not made easier in that performance specifications, at least in books and not USAAC documents, seem to have changed. Like from 1936 minimums to 1939 minimums AND 1939 DESIRED, 1939 XP-38 Guarantees, and 1939 XP-39 Guarantees.
Book accounts can also be a little selective (more than a little) in listing the specific performance limits. Like XP-38 was supposed to do 1.75 hours at "operating speed" and the book in question does not list either the altitude or the operating speed. XP-38 was supposed (promised/guaranteed) to have a range of 1386 miles but again, no altitude or speed given so suitability for bomber escort is very hard to judge/guess. XP-39 (with turbo?) was supposed to have an endurance of 1.0 hour at operating speed.
Obviously the XP-38 has more endurance/range than the XP-39 but also obviously not enough to escort B-17s at high speeds at altitude very far.

With the internal fuel that remained constant until the 55 gal leading edge tanks were installed in the P-38J in mid 1943

This is a minor quibble but the XP-38, YP-38s and the 30 (?) P-38s had 400-410 gal of internal fuel. When the P-38Ds got self-sealing tanks the internal fuel went down to the 300 gal where it stayed until the P-38J as you have stated.
This quibble makes no difference to what happened between the summer of 1940 and the summer/fall of 1943. It might only make a difference to what some people may have thought in 1938 through early 1940. The YP-38s and early P-38s had 400 US gallons of fuel internal but no drop tanks. 400 US gallons was not enough to escort even old (new in 1939-40) B-17s.
The early P-38s had 1150hp engines. The P-38Fs got 1325hp engines according to most books. Ability to lift heavier loads and cruise with drop tanks was at least somewhat improved.
However this was sort of a retro-grade power rating. It took from March 1942 to October 1942 to actually get the 1325hp rating approved due to problems with the leading edge intercoolers. The 1325hp rating was only good to 4,000ft and the pilots were supposed to limit the rpm to 2800 to keep the power to 1240hp up to 25,000ft.

Attempts to blame the "bomber Mafia" for the actual capabilities of 1938-1941 aircraft (actual engine power, lack of protection, etc) seems to be rather slanted.
Unless somebody has actual documentation stating that the USAAC or even a few officers considered the XP-38, YP-38 or plain P-38s as potential escort fighters in 1938-1940
it seems like revisionist thinking.

 

[drgondog]

Maybe I am misinterpreting things that I read in books (not original documents) so I could be wrong on several paths.

From what I remember the fundamental difference between the two original requirements was that the twin engine had twice the endurance that the single engine requirement wanted.

The XP-39 and XP-39B actually had 200 gal, whereas the XP-38 had 400 gal. Both unsealed tanks. 200 gallons per engine. P-38 much heavier and had more induced drag, whereas the P-39 was a very draggy airplane due to parasite drag that removal of the turbo and several trips to NACA to clean up the airplane. .

Things are not made easier in that performance specifications, at least in books and not USAAC documents, seem to have changed. Like from 1936 minimums to 1939 minimums AND 1939 DESIRED, 1939 XP-38 Guarantees, and 1939 XP-39 Guarantees.
Book accounts can also be a little selective (more than a little) in listing the specific performance limits. Like XP-38 was supposed to do 1.75 hours at "operating speed" and the book in question does not list either the altitude or the operating speed. XP-38 was supposed (promised/guaranteed) to have a range of 1386 miles but again, no altitude or speed given so suitability for bomber escort is very hard to judge/guess. XP-39 (with turbo?) was supposed to have an endurance of 1.0 hour at operating speed.

During the FAREP program, the P-38 through H had a stated straight line range of 950mi on 300 gal at TO and cruise at 10,000 ft. I don't have figures for the XP-39 but with the same engine basically running at 60% there is no reason why it would not fly much farther than the P-38H/early J in a straight line ferry on internal fuel

Obviously the XP-38 has more endurance/range than the XP-39 but also obviously not enough to escort B-17s at high speeds at altitude very far.

It isn't obvious to me that the XP-38 with 1/2 the fuel per engine, would fly farther than the XP-39. All P-39, and IIRC, all P-38 tests for cruise were at 60% throttle at 10K.

This is a minor quibble but the XP-38, YP-38s and the 30 (?) P-38s had 400-410 gal of internal fuel. When the P-38Ds got self-sealing tanks the internal fuel went down to the 300 gal where it stayed until the P-38J as you have stated.

When the P-39D cut down to a 120gal self sealing tankage the straight line range reduced to 620mi pr FAREP.

This quibble makes no difference to what happened between the summer of 1940 and the summer/fall of 1943. It might only make a difference to what some people may have thought in 1938 through early 1940. The YP-38s and early P-38s had 400 US gallons of fuel internal but no drop tanks. 400 US gallons was not enough to escort even old (new in 1939-40) B-17s.
The early P-38s had 1150hp engines. The P-38Fs got 1325hp engines according to most books. Ability to lift heavier loads and cruise with drop tanks was at least somewhat improved.

However this was sort of a retro-grade power rating. It took from March 1942 to October 1942 to actually get the 1325hp rating approved due to problems with the leading edge intercoolers. The 1325hp rating was only good to 4,000ft and the pilots were supposed to limit the rpm to 2800 to keep the power to 1240hp up to 25,000ft.

Until the J all the upgrades to the Allisons were virtually unused at high MP due to the Intercooler issues. It wasn't until the J-15 that the issues to both intercooler and oil cooler were largely solved.

Attempts to blame the "bomber Mafia" for the actual capabilities of 1938-1941 aircraft (actual engine power, lack of protection, etc) seems to be rather slanted.
Unless somebody has actual documentation stating that the USAAC or even a few officers considered the XP-38, YP-38 or plain P-38s as potential escort fighters in 1938-1940
it seems like revisionist thinking.

IMO - it took the Spanish Civil War and very early WWII results for Arnold to not only revise thinking about long-range escort, but prioritize the 1500 mile fighter as number 1 when he revised the 1939-1940 Emmons Board recommendations. Arnold so states in his memoirs but it is clear that AWPD-1 reflected that consideration. It was clear to Arnold, Emmons and Armstrong that the role of escort would be required as Interceptor/pursuit technology was maturing faster than the B-17 defensive systems. It is equally true that the AAF went down two rat holes with the YB-40 and XP-75 before the FAREP initiatives pushed the P-51B, P-38J and newer models of the P-47D, all equipped with drop tanks, extended ranges to point of target escort feasibility in ETO.

In fairness to Kelsey, he DID view the P-38 as a suitable escort fighter and Was a sponsor for internal systems to pressurizes drop tanks, but was over-ruled in 1940 at Wright Field when Lockheed raised the question.