P-38 Lightning vs P-51 Mustang: Which was the Better Fighter?

chuckn49 · 02-28-2008, 09:44 PM

For what it is worth, my brother frequently talked about a P-38 pilot who put a 38 through sound barrier in a dive and lived to talk about it. The plane was a total loss because of structural damage to wings and tail. Also, instrument panel was pushed in due to pilot's feet on it.

He was an aeronautical engineer and believed the P-38 could exceed sound barrier and survive. I don't know how many others did that but this is the only one I ever heard of.

drgondog · 02-28-2008, 11:05 PM

Quote:

Originally Posted by chuckn49

For what it is worth, my brother frequently talked about a P-38 pilot who put a 38 through sound barrier in a dive and lived to talk about it. The plane was a total loss because of structural damage to wings and tail. Also, instrument panel was pushed in due to pilot's feet on it.

He was an aeronautical engineer and believed the P-38 could exceed sound barrier and survive. I don't know how many others did that but this is the only one I ever heard of.

Chuck - there are a lot of tribal legends about the first a/c and pilot to break the 'sound barrier'... all mostly from "visible evidence" of an airspeed indicator.

The airspeed instruments worked on stagnation pressure readings and did not account for compressibility - said issue becoming noticeable near .75-.80 Mach and increasingly divergent from freestream airspeed until it was worthless.

Beyond that point, the me 262 was probably the cleanest aerodynamic airframe of all the notable aircraft until the F-86.

The Me 262 in most likelihood came as close as maybe .88-.9 in a dive before the transonic shock wave moved the Center of lift too far aft or disrupted airflow over the horizontal stabilizer to the point that it couldn't recover from a nose down pitch.

The F-86 flown by Welsh probably exceeded Mach 1 in a dive and my guess that it was the 'actual' first.

As to the P-38? It was far less likely than say a Spitfire with a very thin wing or even a P-51H... both faster in a dive and far behind a Me 262.

Most ships that exceeded the mfr's recommended limits by a little too much ultimately were not able to bring it back for a refund

A claim was made for a Spit also and my belief systems are strained entirely too far to contemplate a prop driven a/c whose blade tips can't go supersonic, much less create sufficient thrust through the blade plane of the rotating prop, or have an elevator system capable of pitch control after transonic flow separtion ultimately occurs on the wing.

I would be shocked if the P-38 could get to .82 Mach straight down at full power, and even more shocked if the wings didn't come off in the dive if it made it past that point.

Regards,

Bill

chuckn49 · 02-29-2008, 08:03 AM

I have no proof one way or the other and, from my own experience in the military, I know how legends get started. I am trying to recall conversations I had with him from 30 or 40 years ago so, it is possible that my memory is faulty but, somehow, I don't think so as to what he believed because it impressed me so at the time. Anyway, as I remember the story, the dude in question was an aeronautical engineer type who, for some reason, wished to be a pilot. The legend goes he was transitioning into 38s and, of course, they were warned about the compressibility problem in a dive, etc. I know my brother said it was a not unheard of disaster for a pilot to not be able to pull out of a dive in the 38. Long short, he calculated the 38 could be pulled out of a dive through some maneuver or other and, without authority, deliberately went beyond the limits imposed for safety in a dive. Result, he exceeded speed of sound, allegedly but, managed to get aircraft out of dive by standing on control panel.

drgondog · 02-29-2008, 10:58 AM

Quote:

Originally Posted by chuckn49

I have no proof one way or the other and, from my own experience in the military, I know how legends get started. I am trying to recall conversations I had with him from 30 or 40 years ago so, it is possible that my memory is faulty but, somehow, I don't think so as to what he believed because it impressed me so at the time. Anyway, as I remember the story, the dude in question was an aeronautical engineer type who, for some reason, wished to be a pilot. The legend goes he was transitioning into 38s and, of course, they were warned about the compressibility problem in a dive, etc. I know my brother said it was a not unheard of disaster for a pilot to not be able to pull out of a dive in the 38. Long short, he calculated the 38 could be pulled out of a dive through some maneuver or other and, without authority, deliberately went beyond the limits imposed for safety in a dive. Result, he exceeded speed of sound, allegedly but, managed to get aircraft out of dive by standing on control panel.

Chuck - he could easily be an aero and at that time not have much insight on compressible flow. Practicing aeros at the leading/bleeding edge of high speed aeo were just getting clues with the compressibility issues and the P-38/P-47 were leading indicators of a problem they didn't understand..

Wind tunnels weren't doing supersonic studies either.

So, in his mind I suspect he looked at the evidence, high IAS at high altitude (and they could do Temperature, density calculations based on altitude) based on his airspeed, the compressibility evidence on based on stick force, and the relative shambles of wrinkled skin and popped rivets.

What they didn't have is a pitot tube/airspeed system that also measured temperature and therby missed on the effect to Density in the Pressure calculations leading to freestream airspeed.

The reality for that airplane is that you could start out at 500,000 feet in a terminal dive, go well past the surface speed of sound, but during the coarse of flying that P-38, never exceed .80+ mach. The aerodynamics of first the wing, then the wing center body, then the prop fan interface are going to (all) act like drag brakes.. I suspect the first big Drag component would be the huge spike in wake drag during the transonic transition.

But simplistic as the concept is, all those drag factors added up to a drag chute effect at the high mach number to prevent it from going much past .75-.80 Mach. The reason (I suspect) that Lockheed posted the Dive Limit on the website I referenced - is they wanted to prevent the P-38 from going compressible - Period and the dive brakes on the P-38L did just that..

kool kitty89 · 02-29-2008, 01:28 PM

Sorry the .75 mach figure was the wind tunnel speeds the P-38 was tested in that finally allowed Lockheed to find a solution. Another note is that very early on there was also buffeting problems around the cockpit but wing fillets solved this.

I,m not sure but I think the P-47 was able to better withstand the aerodynamic forces at high speeds (600 mph, possibly more, depending on altitude) than the P-51 or F4U, especially the later models with stronger wings and improved "blunt nosed" ailerons introduced on the D model and recovery flaps were also added to later D models.

There was a study on the Me 262 and it was found (with mathematical models) that it could exceed the sound barrier and recover under the right conditions. (a very steep dive from high altitude was required) The 262 had a trimmable tail-plane that would allow sonic dive recovery, though speed would have to decrease significantly before pulling up to prevent mechanical failure. And even so it was more than likely the wings would be warped to hell.

See: http://www.ww2aircraft.net/forum/avi...ier-10422.html (The sound barrier)

The most accurate way to document braking the sound barrier is a sonic boom, but in a war zone this could be confused with an explosion. Welch's dives (one several weeks prior to Yeager and another less than an hour prior to Yeager's flight in the XS-1) resulted in sonic booms being heard by the locals, the second one actually shattering some windows, this was followed by the much more docile boom of the XS-1 ~30 min later. Of course many thought the loud boom was from Yeager... (according to The Amazing George Welch: Part two )

drgondog · 02-29-2008, 04:12 PM

[quote=kool kitty89;328378]

I,m not sure but I think the P-47 was able to better withstand the aerodynamic forces at high speeds (600 mph, possibly more, depending on altitude) than the P-51 or F4U, especially the later models with stronger wings and improved "blunt nosed" ailerons introduced on the D model and recovery flaps were also added to later D models.

The failures associated with 'aerodynamics' would be lift loads on pullout, or rudder loads causing torque on the epennage/fuselage area. You could get a catastrophic failure from either of those situations when exceeding manufacturer's Limit Loads..

The F4U and P-47 and P-51B/C/D were designed using same structural engineering philosophy, and each were probably slightly stronger than the P-51H as all were designed to 12 G Ultimate versus 11G for P-51H

There was a study on the Me 262 and it was found (with mathematical models) that it could exceed the sound barrier and recover under the right conditions. (a very steep dive from high altitude was required) The 262 had a trimmable tail-plane that would allow sonic dive recovery, though speed would have to decrease significantly before pulling up to prevent mechanical failure. And even so it was more than likely the wings would be warped to hell.

The problem that I don't believe Messerschmidt ever solved, was the drastic 'pitch down' moment caused when the transonic shock wave turbulence blanked the elevator, as well as movement of the Center of Lift Aft. The drag coefficients were probably close if that 'little problem' had been solved... but the a/c was long gone before the shock wave moved to the trailing edge.

QUOTE]

Airframe design has at least three theoretical components. Aerodynamics, Stability and Control, and Structures.

The a/c may have been designed properly as far as Drag requirements, but the wing may be too thick to delay and reduce the effect of transonic shock wave on the airflow aft of the shock. The operation was a succes but the patient died - in this case the me 262.

That is one reason the typical supersonic wing has a very low thickness to chord ratio...

renrich · 02-29-2008, 04:51 PM

Don't know where the numbers you all are quoting on P38 L are coming from but I have two sources that give Vmax of the L as 414 mph at 25000 feet. Dean says that it was placarded in the cockpit that dive speeds were restricted to the equilavent of Mach 0.65. That was when the AC began to get into compressibility and buffeting commenced at Mach 0.675

kool kitty89 · 03-01-2008, 02:37 AM

I've seen the 440+ figure several plases around here, aparently the 414 figure was at normal WEP, but 100/150 Avgas with increased boost and improved strength of the engines of this model aparently allowed these higher speeds. I honestly havent seen documentation of this though...

But one thing that doesn't make sence is that the P-38L's figure is slower than the P-38J's figure (422 at 25,00 ft mph iirc) at the same altitude. The L wasn't that much heavier and had more power.

The P-47D could do 444 mph at 23,400 ft (critical altitude for 70" hg for the D's turbo) with 2,600 hp at 2,700 rpm and 100/150 fuel.

See: P-47 Performance Tests
http://www.wwiiaircraftperformance.o...ess-report.pdf
http://www.wwiiaircraftperformance.o...-47-66inch.jpg

renrich · 03-01-2008, 02:15 PM

I just found a third source(these are all books in my library) that says P38 L Vmax was 414 mph @ 25000 feet. Another source, one I quoted earlier has the performance figures on P38J and it it has Vmax of 414 mph also. This source says the additional HP given the L was diluted by the additional weight(500 lbs) To me the P38 fits in that category of AC that at a certain point additional HP cannot yield additional speed. I don't know the technical term for that but it looks like the P38 being as large as it is there is a lot of drag and the faster it goes the more drag is generated so you reach the point of diminishing returns.

kool kitty89 · 03-01-2008, 11:51 PM

I found out where those guys got the speed figures: http://home.att.net/~ww2aviation/P-38-3.html (just search for P-38L + 441 or 442 or 443 mph on the forum and you'll find several threads mentioning it)

It demonstrates the 414figure at Millitary power and ~442 mph figure at WEP. And this is very close to the P-38's limit.

FLYBOYJ · 03-02-2008, 12:36 AM

I was told by some old timers when I was at Lockheed that because WEP speeds were so close to maximum dive speeds at certain altitudes that speeds at WEP were just not advertised. Some of them also said that the top speed of the P-38L was actually 425 mph. Again take it with a grain of salt.

renrich · 03-02-2008, 09:42 AM

Flyboy, I have a tablespoon of salt ready but your point is well made. According to Dean the Mach number do not exceed of 0.65 of the P38 corresponded to 440 mph TAS(290 mph, IAS) at 30000 feet. Reminds me of a story that the P80 had a problem at 40000 feet in that the stall speed was almost as high as it's Mach number. A little slower and it stalled, a little faster and it was in compressibility.

FLYBOYJ · 03-02-2008, 10:22 AM

Quote:

Originally Posted by renrich

Reminds me of a story that the P80 had a problem at 40000 feet in that the stall speed was almost as high as it's Mach number. A little slower and it stalled, a little faster and it was in compressibility.

Actaully that is true with the U-2 - a 9 knot stall/ Vne window.

drgondog · 03-02-2008, 12:33 PM

Quote:

Originally Posted by FLYBOYJ

Actaully that is true with the U-2 - a 9 knot stall/ Vne window.

Confirmed - The pilots I talked to described the flying the U2 at extreme altitude as a razor thin edge between flight and stall.

Also true that the P-38L was on ragged edge of compressibilty at Vmax and 30,000 feet so when the 38 pilot pushed the wheel forward he needed to quickly deploy dive brakes

chuckn49 · 03-02-2008, 09:33 PM

It is clear from this site and other research I've done that a P-38 could not have broken the sound barrier as my brother thought, though there were lots of claims, apparently, that it had or had come close.

I did learn, however, that in a dive wind did go supersonic over the upper wing surface of the P-38. Also, allegedly, its aerodynamics made it experience much worse compressibility problems than the P-47 or P-51 since many pilots, to their eternal dismay, learned that a P-38 could dive like a bullet all the way into the ground despite reaching denser air in which the 47 or 51 could recover.

As I read this, I remembered another conversation with my brother, albeit, imperfectly, I'm sure, in which he indicated that they were taught not to fight the controls in such a dive but, rather, to allow the plane to go inverted slightly or something of that nature.

I'm sure others here will understand and explain that better than I can. All I remember is that he mentioned the fear when trying to pull out only to have the plane fall back into the dive each time he pulled back on the control. I wish he were still alive so I could explore this more fully with him.