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

kool kitty89 said:

Yeah, the roll rate was a major limiting factor (which had continually degraded up to the J model, particularly with full LE tanks, until the boosted ailerons were added)

And Bill, you do understand I was talking about dive flaps (dive recovery flaps) which are mounted outboard of the nacelles under the main spar. They obvioulsy would have a significant drag effect, but the main purpose was to prvent the pitch-down behavure iirc. Granted that earlier statement about shock-wave reduction/delay doesn't make sense.

And AFIK the P-38 never had actual airbrakes/dive-brakes.

Wikipedia seems to have gotten it right:

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kool kitty89 said:

I know the "tuck under" was a result of the shockwave/tyrbulent flow causing the tailplane to lose downward pressure ("inverted lift"), but I also think that the dive flaps forced pitch up if control had been lost by increasing lift w/out changing AoA. (granted they would also have a braking, or speed limiting effect)


See what was mentioned here: http://www.ww2aircraft.net/forum/aviation/elevator-trim-during-combat-13155-3.html


buzzard's comments also said that the tailplane actually exhibeted a positive lift, forcefully pitching the a/c down, and attempts to pull up on the elevator (if you had the strength) resulted in further pitch down, a kind of "control reversal."

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The general aeronautics community was suddenly awakened to the realities of the unknown flight regime in November 1941, when Lockheed test pilot Ralph Virden could not pull the new, high-performance P-38 out of a high-speed dive, and crashed. Virden was the first human fatality due to adverse compressibility effects, and the P-38, shown below, was the first airplane to Suffer from these effects. The P-38 exceeded its critical Mach number in an operational dive, and penetrated well into the regime of the compressibility burble at its terminal dive speed, as shown by the bar chart on page 80 .35 The problem encountered by Virden, and many other P-38 pilots at that time, was that beyond a certain speed in a dive, the elevator controls suddenly felt as if they were locked. And to make things worse, the tail suddenly produced more lift, pulling the P-38 into an even steeper dive. This was called the "tuck-under" problem. It is important to note that the NACA soon solved this problem, using its expertise in compressibility effects. Although Lockheed consulted various aerodynamicists, including Theodore Von Kármán at Caltech, it turned out that John Stack at NACA Langley, with his accumulated experience in compressibility effects, was the only one to properly diagnose the problem. The wing of the P-38 lost lift when it encountered the compressibility burble. As a result, the downwash angle of the flow behind the wing was reduced. This in turn increased the effective angle of attack of the flow encountered by the horizontal tail, increasing the lift on the tail, and pitching the P-38 to a progressively steepening dive totally beyond the control of the pilot. Stack's solution was to place a special flap under the wing, to be employed only when these compressibility effects were encountered. The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly. This is a graphic example of how, in the early days of high-speed flight, the NACA compressibility research was found to be vital as real airplanes began to sneak up on Mach one.36

Indeed, it was time for real airplanes to be used to probe the mysteries of the unknown transonic gap. It was time for the high-speed research airplane to become a reality. The earliest concrete proposal along these lines was made by Ezra Kotcher, a senior instructor at the Army Air Corps Engineering School at Wright Field (a forerunner of today's Air Force Institute of Technology). Kotcher was a 1928 graduate of the University of California,
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36. The "tuck-under" problem, and its technical Solution, is described in John D. Anderson, Jr., Introduction to Flight (New York, NY. McGraw-Hill Book Co., 3rd ed., 1989), pp. 406-08.

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kool kitty89 said:

Blanking would make more sense (I haven't seen mention of control reversal elswhere either, except for accounts of pilots mistaking the "tuck under" for control reversal)

I haven't seen this but could be true prior to compressibility airflow, ditto flutter.

I do believe the dive flaps forced a pitch op though.

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The general aeronautics community was suddenly awakened to the realities of the unknown flight regime in November 1941, when Lockheed test pilot Ralph Virden could not pull the new, high-performance P-38 out of a high-speed dive, and crashed. Virden was the first human fatality due to adverse compressibility effects, and the P-38, shown below, was the first airplane to Suffer from these effects. The P-38 exceeded its critical Mach number in an operational dive, and penetrated well into the regime of the compressibility burble at its terminal dive speed, as shown by the bar chart on page 80 .35 The problem encountered by Virden, and many other P-38 pilots at that time, was that beyond a certain speed in a dive, the elevator controls suddenly felt as if they were locked. And to make things worse, the tail suddenly produced more lift, pulling the P-38 into an even steeper dive. This was called the "tuck-under" problem. It is important to note that the NACA soon solved this problem, using its expertise in compressibility effects. Although Lockheed consulted various aerodynamicists, including Theodore Von Kármán at Caltech, it turned out that John Stack at NACA Langley, with his accumulated experience in compressibility effects, was the only one to properly diagnose the problem. The wing of the P-38 lost lift when it encountered the compressibility burble. As a result, the downwash angle of the flow behind the wing was reduced. This in turn increased the effective angle of attack of the flow encountered by the horizontal tail, increasing the lift on the tail, and pitching the P-38 to a progressively steepening dive totally beyond the control of the pilot. Stack's solution was to place a special flap under the wing, to be employed only when these compressibility effects were encountered. The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly. This is a graphic example of how, in the early days of high-speed flight, the NACA compressibility research was found to be vital as real airplanes began to sneak up on Mach one.36

Indeed, it was time for real airplanes to be used to probe the mysteries of the unknown transonic gap. It was time for the high-speed research airplane to become a reality. The earliest concrete proposal along these lines was made by Ezra Kotcher, a senior instructor at the Army Air Corps Engineering School at Wright Field (a forerunner of today's Air Force Institute of Technology). Kotcher was a 1928 graduate of the University of California,
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36. The "tuck-under" problem, and its technical Solution, is described in John D. Anderson, Jr., Introduction to Flight (New York, NY. McGraw-Hill Book Co., 3rd ed., 1989), pp. 406-08.

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kool kitty89 said:

What about the NASA article I posted? (the one buzzard posted on the elevator trim thread)

Research in Supersonic Flight and the Breaking of the Sound Barrier

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kool kitty89 said:

What about the NASA article I posted? (the one buzzard posted on the elevator trim thread)

QUOTE]

KK - I read the article, what I would like to find is the actual report to make sure it was translated correctly - Having said that, two things are clear. John Stack is the father of compressibility, and what he says about 'reduced lift is coupled to reduced downflow' is also true.

Having said that I want to parse the statements in Bold

[And to make things worse, the tail suddenly produced more lift, pulling the P-38 into an even steeper dive. This was called the "tuck-under" problem.

The guy that wrote that probably assumed that Lift at the elevator was 'positive' (up) in the same direction as wing Lift is up.. this is not correct. the 'lift' force on the tail is Down (pull down at tail aft of CG and the nose comes up from neutral state.. what really happens as I try to explain, is that the reduced downwash resulted in a change of relative AoA at the elevator - Reducing the download on the tail, not increasing Lift at tail. The net effect is that the P-38 needed a bigger Download than it was getting at that speed and local angle of attack and at those speeds the aerodynamic forces on the elevator were too great to move in any direction

Next he said - "It is important to note that the NACA soon solved this problem, using its expertise in compressibility effects. Although Lockheed consulted various aerodynamicists, including Theodore Von Kármán at Caltech, it turned out that John Stack at NACA Langley, with his accumulated experience in compressibility effects, was the only one to properly diagnose the problem." He goes on to say-

" The wing of the P-38 lost lift when it encountered the compressibility burble. As a result, the downwash angle of the flow behind the wing was reduced."

This is true for Separated flow over that particular wing section and area

He quotes Stack further

"This in turn increased the effective angle of attack of the flow encountered by the horizontal tail, increasing the lift on the tail, and pitching the P-38 to a progressively steepening dive totally beyond the control of the pilot".

This is partially true and the reason I want to see Stack's actual report. But I comment on the 'Lift of the tail' above and again below

The author further states Stack's solution was to place a special flap under the wing, to be employed only when these compressibility effects were encountered. The flap was not a conventional dive flap intended to reduce the speed. Rather, Stack's idea was to use the flap to maintain lift in the face of the compressibility burble, hence eliminating the change in the downwash angle, and therefore allowing the horizontal tail to function properly

IMO - This last statement is simply wrong and I don't believe Stack said something like this for the following reasons.

The inboard wing 'experienced reduced lift' due to the flow separation occuring during shock wave formation (compressibility in this case)..The out board wing was still under high lift, shock wave not yet forming, and increasing lift with that speed as it increased, until it later also experienced transonic effects...

as the inboard wing separates there is an increase in drag rise as well as much turbulence behind the wing (resulting in the initial flutter problem)

Next, the 'compressibility burble' was Stack's nomenclature for transonic shock wave effects - namely turbulent flow and boundary layer separation at the shockwave interface... that occurred when the local velocity at that point approached Mach..which occurred first on the inboard section of the wing between the engine nacelles.

The Dive Brake was positioned outboard of the wing nacelle (pg 26 P-38 Lightning at war) at the 30% chord. My guess is that 30% was chosen so it could be attached to a wing spar. You'll notice that if it was INBOARD of the nacelle it would interfere with the inboard pylons... and worse, separate flow at 30% rather than close to 50%

Next, placing it outboard of nacelles has nothing to do with the Compressibility burble occurring first over the higher thickness airfoil section inboard of the nacelles - which is primary problem to be solved, until it is deployed and increases drag to point of slowing the P-38 down enough to regain control.[/B]

And last, as the flow is separating in that section of the wing between the nacelles, it is absolutely plausible that the remaining 'reduced lift' forward of the shockwave in turn results in reduced downflow from the remaining lift and results in a reduction of relative angle of attack of the downwash on the horizontal stabilizer. That is plausible and I would agree the conclusion and it is better than my original idea.

The questions in my mind are three fold.

First, was Stack aware at the time he wrote the report that 'transonic to supersonic flow' moves the aerodynamic center from .25 (+/-) to .5 (+/-) of the mean aerodynamic chord, which in turn changes the Pitching moment of the airframe. Probably not because they were just at the stage of theory development and Schlieran photography were just mature enough to detect the shock wave but probably not far enough to understand the aft movement of the A.c. from ~.25 to ~.50 (speculation on my part)

This would have two effects..First the Pitching moment woul increase by 25%more negative, making the airplane MORE stable.

Second, the increase in negative pitching moment means larger control forces - usually implying either larger control surfaces, or greater deflection for more required 'lift' (positive or negative) - are required at the tail to change the pitch of the airplane.

So, if this is happening (which it does in transonic flow) then the elevator needs to move to compensate (can't grow the horizontal stabilizer/elevator in flight) - but stick forces were too great in this flight profile to adjust the elevator - leading to experiments in hydraulic and electrical boost for trim tabs.

I buy this explanation. It also says my initial theory about blanking the horizontal stabilizer came from some place where the sun may not shine brightly... because If I had thought about more, it would be clear there is SOME control back there or the ac continues to nose over and soon reach catastrophic failure. This happened to Me 262's occasionally.

So, I don't believe the tuck was unconstrained - I believ a more plausible explanation is that the dive angle was increased until perhaps terminal velocity wher the aero forces stabilized for one dive angle equilibrium - and remained at that angle until the lower altitudes were reached and the pilot regained control of (small at first) his elevaotors.

Remember, at that point, as he pulls back on the stick - the elevator deflects 'up', pushing the tail down and the nose up.

The turbulent flow definitely caused the Flutter issue and the improved fillets at the wing/fuse interface pretty much solved it.

Next -

Dive flap/brake - whatever you want to call the retractable device at 30% chord - was strictly to create enough drag to keep the P-38 contollable and prevent immediate drop into compressibility.. that it was designed to 'create lift' seems silly. The aircraft was already past the local velocity/lift threshold to get the shock wave - why try to increase lift?

Lift is created when the airflow velocity increases over a wing at a specific AoA. At some point near compressibility you do Not want to do that with a device which is designed to keep you out of that velocity range.

Summarize - I believe the report accurately summarizes what Stack said about downwash but I also believe the primary issue is the change in Pitch moment (down) combined with huge stick forces required to make the elevator deflect - which is why the electrically boosted trim tab was introduced before the dive flap - and it was still dangerous from a structural load standpoint. Also, by definition I was wrong about 'blanking' being the cause of elevator ineffectiveness... I should have looked up the reason rather than speculating.

I do not believe the dive brake was designed in any way to 'create lift'.. manuever flap - yes, dive brake -no. If you think about it - if it ever increased lift you would see references made to deploy at take off and landing

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otftch said:

I was under the impresion that in order to supply an aircraft under lend-lease,that the aircraft had to be in production for the US.I remember reading somewhere that the powers that be in the Army Air Force did not really want another fighter(they were commited to the P-40) but by designating the Mustang as a ground attack aircraft they could get it into production and thus into the hands of the RAF.Sounds kind of wierd but these were the same people who took the superchargers off of the P-39.
Ed

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On March 11, 1941, the Lend/Lease Act was passed by Congress, permitting the "lending" of American-built aircraft to nations deemed "vital to the security of the United States". On September 25, 1941, the US Army ordered 150 Mustangs under the provisions of Lend-Lease for delivery to Britain. All previous RAF Mustangs had been direct purchases by Britain. These Lend-Lease Mustangs were designated Mustang Mark IA by the RAF and NA-91 by the factory. The RAF serial numbers assigned to this lot were FD418/FD567. For contractual purposes, these aircraft were assigned the US designation of P-51, and the Allison V-1710-F3R engine was given the US Army designation V-1710-39. The P-51s were assigned the USAAF serials 41-37320/37469.

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kool kitty89 said:

That makes a lot more sense, particulalr on the tail "lift" issue. (that was a particularly confused part)


But don't you think by the angle of deployment, and the positioning under the wing the dive flap would cause a pitch up? (not act as a high-lift device, but forse the wing to a higher AoA if elevator remained neutral, and I don't think it would have been used at takeoff because it wouldn't act to increase lift, in fact it would probably disrupt airflow and decrease actual lift at a given AoA)

The fundamental reason for the brake had to be to increase drag - immediately and slow the airplane down..

from my perspective, over the span of the dive brake, flow would be disrupted immediately aft of the brake (~3 feet?) , probably causing separation there. Conceivably, that would cause a 'local increment' of slightly higher pressure under the wing, thereby increasing net lift over the top of the wing assuming the velocity and AoA remained the same.

Below transonic speeds, the aerodynamic center shouldn't change, so the pitching moment shouldn't change, and that implies no change (up or down) to angle of attack.. I would have to see something like a real report to understand if there was an attempt to get more lift

Other dive brakes had similar effects, so as to continue a dive pull-out after the pilot had blacked out, the a/c pitching up with stick neutral, so the plane had to be held in a dive. (most dive bombers for example)

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kool kitty89 said:

The Mustang wasn't inspired By the Brits, NA already had it on the drawing board as a private project when they were asked to build P-40's, that's why the prototype was made so fast. The political issue with the USAAC/AAF was that it was a private project, not built to compete in one of there specifications, so it was ignored at first. It was the Brits' request for export P-40's which spurred development though, but they had nothing to do with the actual design of the airframe.

See: North American P-51 Mustang

And the A-36 wasn't the first version of the Mustang to see service with the USAAF and the A designation wasn't a political move. The A-36 was a dedicated GA aircraft, it even had dive breaks for dive bombing! It was capable of dogfighting below 15,000 ft though. The P-51/F-6A (NA-91, Mk.1A in British service) North American P-51/F-6A Mustang armed photo-recon a/c was the first to see service and they were direct conversions from the Mustang I.

And before that several Mustang I's (NA-73's) were taken by the USAAF as XP-51's North American XP-51 Mustang

And also on that page the political/bureaucratic problems:

At that time, the Army was overloaded with other test programs, the Lockheed P-38 Lightning, Bell P-39 Airacobra, and Republic P-47 Thunderbolt being thought to meet all the Army's requirements for fighter aircraft. Furthermore, the Mustang was a "foreign" type not built to any American specification, and was therefore way down on the Army's list of priorities. ... The Mustang may have been the victim of the "Not Invented Here" (NIH) syndrome, in which the Army looked askance at an upstart aircraft which had not been designed in response to any of its official requirements.

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buzzard said:

drgondog,

The AoA issue as it relates to the P-38's compressibility problem seems to me to concern the altered flow of air coming off the wing, and it's interaction with the tailplane.

I'll try to explain it as I understand (or perhaps, mis-understand) it:

In normal flight, the air-flow from the wing goes straight back to the TP, or possibly with a slight downwash, so that raising the elevator forces the tail down, increasing the AoA of the wing.

Recognizing that I am not looking at the engineering my thoughts are about what I think is happening. What you are saying above is the way it should be behind the wing/centerbody

- and the horizontal stabilizer incidence should be slightly 'up' so that at cruise, the downwash for the zero elevator deflection is still enough to put just enough 'down lift' at cruise to offset the natural pitching moment of the wing. When that downwash angle gets smaller, it approaches the slight 'up' iincidence of the horizontal stabilizer in relative angle - meaning you have to trim it or even small up elevator to increase the 'down lift'

In the compressibility 'burble', the turbulent flow off the wing actually strikes the TP from underneath, resulting in a positive angle of attack as the TP relates to the disturbed airflow.

Turbulent flow behind a shock wave causes total separation of boundary layer in that region and by definition it would have unpredictible flow in the Burble - hitting the aft section with positive and lateral and negative velocity vectors- but other flow would mix in and stream normally over the horizontal stabilizer.

What I visualize happening at the HS is that the very high speed flow that is running parallel to the reduced downflow is running over both top and bottom surfaces of the HS as to make even small delflections of the elevator difficult from a stick force standpoint.

IMO - The 'mixing turbulent flow' is what cause the flutter issues that improved fillets mostly solved by reducing

This lifts the tail and steepens the dive. Even when the elevator is raised, it is still at a positive AoA to the airflow. The dive flap forces the air downwards, increasing the lift to the wing, and the airflow on the upper surface, no longer compressed upwards by the disturbed flow from beneath, restores the negative AoA to the TP/elevator, and pitch control of the a/c is regained.

I don't think they could control the elevator while in the compressible dive - and only by trim tabs exert enough force to get a small deflection -until they got into the lower altitude, denser air and were able to slow down enough to get increasing elevator movement

On second thought, since the dive flaps are outboard of the TP, it's more likely that the increased lift of the mainwing simply overpowers the lift generated by the TP. And of course, the dive flap also adds considerable drag, slowing down the a/c. But the most vital function is to restore lift to the wing, increasing its AoA without also increasing the AoA of the TP . At least as I semi-understand it

JL

EDIT: I guess I should have read your post first...

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kool kitty89 said:

I don't think he was talking about the maneuvering flaps Bill. He said "dive flap"

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