Elevator trim during Combat

[kool kitty89]

And something else interesting is that this P-38 manual LOCKHEED P-38 LIGHTNING does suggest the use of the elavator trim tab if necessary to aid in dive pull-out.

 

[FLYBOYJ]

Then what were you talking about here?

I now don't think the early P-80s had any type of wing mounted dive brakes, just the belly mounted speed brake and wing flaps.

 

[FLYBOYJ]

And something else interesting is that this P-38 manual LOCKHEED P-38 LIGHTNING does suggest the use of the elavator trim tab if necessary to aid in dive pull-out.

It does say that but it also says to reduce speed prior to using the elevator trim tab.

A USAAF pilot testing an early P-38 got out of a high speed dive using elevator trim. Lockheed test pilot Ralph Virden tried it and pulled the tail off.

 

[kool kitty89]

That was when they were testing the servo taps though, a bit different from the trim tab. (if this article is correct)

By November 1941, many of the initial assembly line challenges had been met and there was some breathing room for the engineering team to tackle the problem of frozen controls in a dive. Lockheed had a few ideas for tests that would help them find an answer. The first solution tried was the fitting of spring-loaded servo tabs on the elevator trailing edge; tabs that were designed to aid the pilot when control yoke forces rose over 30 pounds, as would be expected in a high-speed dive. At that point, the tabs would begin to multiply the effort of the pilot's actions. The expert test pilot, 43-year-old[14] Ralph Virden, was given a specific high-altitude test sequence to follow and was told to restrict his speed and fast maneuvering in denser air at low altitudes since the new mechanism could exert tremendous leverage under those conditions. A note was taped to the instrument panel of the prototype underscoring this instruction. On 4 November 1941, Virden climbed into YP-38 #1 and completed the test sequence successfully, but 15 minutes later was seen in a steep dive followed by a high-G pullout. The tail unit of the aircraft failed at about 3,000 ft (910 m) during the high-speed dive recovery; Virden was killed in the subsequent crash. The Lockheed design office was justifiably upset, but their design engineers could only conclude that servo tabs were not the solution for loss of control in a dive. Lockheed still had to find the problem; the Army Air Corps was sure it was flutter, ordering Lockheed to look more closely at the tail.

 

[FLYBOYJ]

OK....

 

[kool kitty89]

I now don't think the early P-80s had any type of wing mounted dive brakes, just the belly mounted speed brake and wing flaps.

Got it.

So when the F-80A manual refers to "dive flaps" is it just referring to the belly airbrake?

 

[FLYBOYJ]

Got it.

So when the F-80A manual refers to "dive flaps" is it just referring to the belly airbrake?

Yep....

 

[kool kitty89]

And the manual refers to the dive flaps in plural. So 2 separate flaps located side by side in similar under fusalage position?

From the F-80A manual:

7. DIVE FLAP CONTROLS.
The dive flaps are controlled by a switch (9, figures 6 and 6A) which operates an electrically actuated hydraulic valve. It is not possible to stop the flaps in any intermediate position; they must be either "full up" or "full down."

 

[FLYBOYJ]

Exactly - and that's exactly how I remember them operating when I've flown in T-33s. They are either retracted or deployed. I never did landings in the T-33 but in the L-29 and in the Jet Provost I liked to deploy them once the MLG touched the pavement.

 

[kool kitty89]

From what I've read the P-38's operate in a similer manner.

 

[FLYBOYJ]

From what I've read the P-38's operate in a similer manner.

I believe so, but in the case of the P-38 you really didn't need them on landing. A recip aircraft accelerates and slows down pretty rapidly. A jet (especially an early jet) could take a lot to "spool up" and a lot to slow down, that's why speed brakes are so critical. The speed brakes on the P-38 were there mainly to get the pilot out of trouble if he found himself hurling towards the ground.

 

[kool kitty89]

Do the Shooting Star's dive flaps/airbrakes caue it to pitch up at all when deployed? (without elevator imput to counter such movement)

 

[buzzard]

Here's some info on the rather counter-intuitive idea that the tailplane's lift could overcome the reduced lift of the wing in the 'compressibility burble' regime.

"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"

And the link to this very interesting site: Research in Supersonic Flight and the Breaking of the Sound Barrier

Weird stuff...

JL

 

[kool kitty89]

buzzard you might want to look at the argument on this topic here: http://www.ww2aircraft.net/forum/polls/p-38-lightning-vs-p-51-mustang-better-fighter-3867-19.html

 

[FLYBOYJ]

Do the Shooting Star's dive flaps/airbrakes caue it to pitch up at all when deployed? (without elevator imput to counter such movement)

In the flights I took in the T-33 we never deployed the speedbrakes at high speed. When they were deployed it was done so to slow us down during pattern entry to a landing, I think around 250 knots. When deployed the aircraft shook with a washboard effect and airspeed diminished rapidly. One pilot I flew with would pop them open on final to slow down a bit, especially if a long final was flown. In essence I think they were deployed more to slow the aircraft down during landing than anything else.

 

[FLYBOYJ]

Just another comment on all this...

I knew Tony LeVier pretty well. He once told me that when the P-80 became transonic the ailerons would buzz and the nose would "tuck down." The same thing would happen with the F-94 except it wasn't as abrupt. Tony told me that he exceeded the speed of sound several times in the F-94 (without tip tanks) and even beat Chuck Yeager in a diving race, he flying the F-94, Yeager in an F-86.

 

[kool kitty89]

That would be the F-94C Starfire, almost a completely redesigned a/c from the earlier F-94's, with a much thinner wing (10% chord instead of 13%) and a much more powerful afterburning J48 (lisence built afterburning Nene) engine. (8750 lbf, opposed to 6,000 lbf of the earlier J33)

It was seriously redesigned from the original F-94 and thus was initially designated F-97.

(the contemporary straight winged F-89 and CF-100 could also break Mach 1 in a dive)

Lockheed F-97/F-94C Starfire

I have thought it would have been interesting if the F-80 had been modified with a similar wing and given fully boosted controls.

 

[buzzard]

buzzard you might want to look at the argument on this topic here: http://www.ww2aircraft.net/forum/polls/p-38-lightning-vs-p-51-mustang-better-fighter-3867-19.html

Hmm...looks like maybe I oughta' change my moniker to 'kopy kitty 89'

JL

 

[kool kitty89]

No, I meant there were some differences in the discussion.

Most notably on the excess lift generated by the tailplane you mentioned and the "blanking" effect Bill (drgondog) was referring too.

 

[kool kitty89]

And if you notice buzzard I posted that link to the other thread a while back, and you posted the quote from the NASA site before I did. Also I mentioned that I had gotten that quote from your post on this thread.