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

[kool kitty89]

The Bell P-59 and Westland Welkin had similar problems at high altitude where there was very little window between shock-stall (compressibility) and normal stall. Although the P-59 could still fly well enough at an impressive 47,000 ft.


Here's an interesting example of a P-38 in compressibility: YouTube - DOGFIGHTS, P38 LIGHTNING VS ME 109 Though it was probably aerodynamic forces and not G's that ripped out the canopy window, any ideas? Also the wing pylons shouldn't have come off with the drop tanks and the props should be rotating outward not inward.... (In fact the pylons were integral to the airframe and not removable iirc, and only the XP-38 had inward rotating props)

[renrich]

A number of WW2 AC, when they got into compressibility, experienced the nose tucking under and attempting to correct using the elevators or trim tabs on the elevators usually was futile and could result in structural failure. The best means of dealing with compressibility and the resulting uncontrollable dive was to throttle back and wait for the airplane to get lower and into warmer air. Since the speed of sound varies only with air temperature, when the AC reached the warmer air it automatically came out of compressibility and became controllable. The problem with the P38 was that a number of them experienced structural failures during dives above 0.65 Mach.

[FLYBOYJ]

Originally Posted by renrich

A number of WW2 AC, when they got into compressibility, experienced the nose tucking under and attempting to correct using the elevators or trim tabs on the elevators usually was futile and could result in structural failure. The best means of dealing with compressibility and the resulting uncontrollable dive was to throttle back and wait for the airplane to get lower and into warmer air. Since the speed of sound varies only with air temperature, when the AC reached the warmer air it automatically came out of compressibility and became controllable. The problem with the P38 was that a number of them experienced structural failures during dives above 0.65 Mach.

The area most prone to failure was the tail - That came from Tony LeVier.

[renrich]

Intuitively, not being an engineer, the tail section(s) of the P38 looks like where a failure would happen.

[drgondog]

Originally Posted by renrich

Intuitively, not being an engineer, the tail section(s) of the P38 looks like where a failure would happen.

The failure mode was in two areas - rudder loads for a roll or turn and elevator for pull out

[FLYBOYJ]

One of the first failures occurred over Burbank in 1940. Lockheed test pilot Ralph Virden was killed when the tail came off a YP-38. If I'm not mistaken part of the wreckage came down in a area known as 5 points which separates Burbank from the city of Glendale. I remember an ole timer at Lockheed told me the tail actually landed in the intersection as day shift was letting out of the Lockheed B-1 facility. Luckily no one on the ground was injured.

[drgondog]

Originally Posted by renrich

A number of WW2 AC, when they got into compressibility, experienced the nose tucking under and attempting to correct using the elevators or trim tabs on the elevators usually was futile and could result in structural failure. The best means of dealing with compressibility and the resulting uncontrollable dive was to throttle back and wait for the airplane to get lower and into warmer air. Since the speed of sound varies only with air temperature, when the AC reached the warmer air it automatically came out of compressibility and became controllable. The problem with the P38 was that a number of them experienced structural failures during dives above 0.65 Mach.

Rich - Strictly speaking I think you meant to say density is a function of temperature and pressure and varies with altitude.

The 'nose tuck' was usually caused by the turbulent flow interfering with elevator control as the Center of Lift moving rearwards during transonic flow conditions.

In most cases the use of elevator trim was specifically recommended against.

[kool kitty89]

Of course the P/F-84 Thunderjet didn't nose down when it exceeded its .82 mach limit at low altitude! It went into a violent pitch-up stall!

[p-51 mustangman]

Chuck Yeager opined once that you had to pay attention to your trim when in a turn with the fuel tank behind the pilot full. However I can't imagine a plane with the mass of a P-38 turning with a Me109 or FW-190. the 38 may have been faster. Also think about the Me-110 the "Zerstorer" twin moter "fighter" which got mobbed during the Battle of Britain. The Luftwaffe had to send fighters to escort fighters.

[renrich]

Bill, are you saying that the warmer the air the less dense it is and therefore the air plane has to go faster in warm air to encounter compressibility than it would in cold air? Was that not the reason the P38 encountered compressibility in the ETO more often than it did in the Pacific? The air was colder over Europe than over most of the Pacific. Would not the air at 30000 feet over Texas in August be warmer than the air over the Antarctic at 30000 feet in August?

[kool kitty89]

Warmer= less dense, colder=denser, lower pressure=less dense, greater=denser

The less dense the air, the lower the speed of sound, however the higher humidity in the PTO may also have had an effect, or the pilots could have been better acquainted with their a/c.

[drgondog]

Originally Posted by renrich

Bill, are you saying that the warmer the air the less dense it is and therefore the air plane has to go faster in warm air to encounter compressibility than it would in cold air? Was that not the reason the P38 encountered compressibility in the ETO more often than it did in the Pacific? The air was colder over Europe than over most of the Pacific. Would not the air at 30000 feet over Texas in August be warmer than the air over the Antarctic at 30000 feet in August?

KK is correct

The formula for density (rho) =1.325x Pb/Ta where Pb is barometric pressure in inches mercury and Ta is degree F (absolute - Rankin)..

The constant works for that lower portion of the atmosphere where the slope of Pressure vs altitude is a straight line..

So, for a surface temp of zero F versus 100 F the relative density is seen by comparing the denominator (460+0) versus (460+100) - i.e more dense at same altitude in colder surface area.

The compressibility then could intuitively occur at lower airspeeds on a cold German winter day than the same altitude over Dallas in summer... conversely the Temperature also affected oil coolers, etc.

[HoHun]

Hi Renrich,

>Since the speed of sound varies only with air temperature ...

Quite right.

Though ...

c = sqrt (kappa*p/rho)

... the basic relationship ...

rho = p / (R*T)

... leads to the elimination of density from the equation ...

c = sqrt (kappa * R * T).

With kappa (isentropic expansion factor) and R (universal gas constant) being constant, the speed of sound in a gas is temperature-dependend exclusively.

Note: Kuchling's Taschenbuch der Physik warns this is only valid "within wide limits", as it relies on the usual idealizations for (a mix of) ideal gases.

Regards,

Henning (HoHun)

[drgondog]

Rich - as your questions are related to the very complicated flow characteristics associated with 'near sonic' velocities the answer is not simple.

First the flow is moving from the theoretical fluid mechanics of incompressible (like water with low viscosity), say at .5 Mach, into the compressible fluids of near sonic flow through increasingly compressible fluid characteristics of free stream flow accelerated over an airfoil to forming a shock wave .

Actually the mere discussion of shock wave formation, boundary layer growth, and boundary layer separation as local velocity over the airfoil moves from just below Mach 1 to just past it was beyond the theoretical knowledge while I was in school.

Point one. The shock wave didn't instantaneously 'start' at Mach = 1.000, it starts a 'little bit' past that for reasons explained below.

Second point. The shock wave at that point doesn't 'penetrate' the local boundary layer at that point, but as the pressure differential changes past the initial shock wave, the complex flow down stream of the shock wave slows below supersonic and it is in this region that there is a 'backwash' if you will, that causes an increase to a separation of the boundary layer, and often the separation location moves forward of the shock.

We were actually looking at Chaos theory to predict the effects as the 'flow tubes' used in the theory exhibited subsonic in one versus supersonic in the other. I won't bore you with the details because I never 'solved' the problem analytically. We even used calculus of variations to attempt to use wind tunnel pressure distributions as the boundary conditions for solving for the theoretical velocity and airfoil (including boundary layer) relationships.

As far as I know this is still very difficult field to try to 'solve' for anything other than thin, symmetrical airfoils in a limited range of Angle of Attack

Net - At the transonic velocity REGION a test pilot will frequently describe a shimmering effect above the wing.

After that is where the next set of complex relationships need to be explored to PREDICT (rather than observe) wing/body interaction, effect to Center of Lift as shockwave moves aft, and interference with elevator as boundary layer starts and proceeds though separation.

Blah, blah blah - The Net-Net is that Local Mach is Temperature dependent at the altitudes you are interested in because given a same surface temp, the density of the air over Germany at 25,000 feet will be the same as over Dallas at 25,000 feet.

It gets considerably more complicated as the altitudes get considerably higher and you have to look at drag characteristics for orbital decay of a low orbit satellite. At that point average mean distance between molecules and surface variations and distribution of the earth's atmosphere over an oblate Spheroid (not a spher) is critical.

[renrich]

This discussion reminds me a little of the discussion about how a wing creates lift. I had always thought it was Bernoulli's Principle until I read "Stick and Rudder" Was in an airliner one day and was sitting next to a pair of(I think) young aero engineers and I mentioned that lift was created by a wing pushing down on the air and the air pushing up. They looked at me like a heretic and started talking about Bernoulli and the air having to speed up over the airfoil and creating low pressure over the top. I then asked them how a wing created lift when the airplane is inverted. We had a lot of fun.