Dive Brake Sound file?

[Ack-Ack]

So calling the P-38 "dive recovery flap" a speed or dive brake is not all that unreasonable. It was designed to assist in recovery from exceeding critical mach but it functions as a speed brake as well and I would imagine it was used regularly as a speed brake by P-38 pilots.

No, it wasn't used as a speed brake by P-38 pilots because that's not how the dive flaps functioned in the P-38. 

ack-ack

[Dawger]

The NACA High Speed Wind Tunnel team under John Stack's direction had been working on this problem and had devised a small pair of 6x40-- inch, electrically operated dive-recovery flaps to be installed on the P-38 wing's underside and outboard of the engine nacelles; they could be extended to 40 degrees.

The dive recovery flap on the P38 is a small panel that extends to 40 degrees located on the underside of the wing. It works EXACTLY the way a speed brake works.

The Phantom uses something very similar although it is not as far forward because the Phantom had other means to deal with Mach Tuck.

The P-38J25-LO and P-38L's were terrific. Roll Rate? Ha! Nothing would roll faster. The dive recovery flaps ameliorated the "compressibility" (Mach limitation) of earlier Lightnings. An added benefit of the dive recovery flaps was their ability to pitch the nose 10-20 degrees "up" momentarily when trying to out turn the Luftwaffe's best, even when using the flap combat position on the selector.

It also caused a pitch up moment because of its location on the wing. Its location was the reason it was a "dive recovery flap" not its design.

Any surface extended into the airflow is going to cause drag and act as a speed brake. There is no way the dive recovery flap on the P-38 did not act as a drag producer and thusly a speed brake.

Mach tuck or compressibility as it was called is well understood now but it wasn't then. When the airflow over the top of the wing exceeds mach 1 a shock wave develops that physically moves the center of pressure aft (pitching the nose down)until it overpowers the ability of the tailplane to overcome. The dive flap on the P38 created lift well forward on the airfoil to counteract this downward pitch moment. Move the same dive flap aft on the wing and it makes the mach tuck worse instead of better.

Essentially the same design is used on the upper and lower surfaces of airplanes. They all deflect air, create lift and produce drag. They all act as speed brakes. Where they are placed on the wing are what determines how the lift and drag they produce affect the aircraft.

Not all speed brakes are designed this way. There is no one method for achieving the goal. The A-10 has symmetrical flap panels on upper and lower surfaces. This provides plenty of drag and there is no pitch moment associated with deployment. Many aircraft use a vertical panel with lightening holes. No pitch moment with this method. The F-15 uses a large fuselage flap for a speed brake. The BAE 146 has a clam shell speed brake on the tip of the tail.

The dive recovery flap on the P-38 creates drag and is a speed brake. Its purpose on the airplane is to create a pitch up moment. It does that too (although not as dramatically in AH as maybe it should). Its placement on the wing creates the pitch up moment, not its design. The same flap in other positions will create the same drag and lift but its effect on aircraft pitch moment will be different.

[Dawger]

Just for clarity sake, while the P-38 dive recovery flap creates drag and acts as a speed brake it is not this quality that aids the pilot in recovery from critical mach tuck dives. It is the ability of the dive recovery flap to create lift and create a pitch up moment that aids the pilot in recovery.

[gyrene81]

Nevermind, noticed there are 2 convo's going on. If the dive flaps or dive recovery flaps exist, I suppose there should be some sound associated with their deployment.

[Ack-Ack]

Just for clarity sake, while the P-38 dive recovery flap creates drag and acts as a speed brake it is not this quality that aids the pilot in recovery from critical mach tuck dives. It is the ability of the dive recovery flap to create lift and create a pitch up moment that aids the pilot in recovery.

This discussion should be it's own thread, however, you are still incorrect about the P-38's dive flaps being speed brakes.  It was not designed with the intent to be used as speed brakes, the dive flaps did not work that way.  The design of the dive flaps on the P-38 is a very inefficient design for speed brakes.


ack-ack

[Chalenge]

Just for clarity sake, while the P-38 dive recovery flap creates drag and acts as a speed brake it is not this quality that aids the pilot in recovery from critical mach tuck dives. It is the ability of the dive recovery flap to create lift and create a pitch up moment that aids the pilot in recovery.

Really? I think Philips disagrees with that. I guess I should read that section again just to be sure.

[Dawger]

The flight spoilers on the CRJ are very inefficient as speed brakes but that doesn't mean we don't use them as speed brakes. I see no reason why the P38 dive brake would be treated any different, especially since they would provide an extra pitch up moment.

Just because a device is not intended to be used in a specific manner does not mean that pilots will not use it in a manner not originally intended.

In fact, I think you may find that pilots tend to innovate new uses for gadgets pretty regularly.

(PS: your efficiency argument falls apart once you consider that the dive recovery flap increases AOA, increasing lift and induced drag. I suspect the P-38 dive flap was a pretty darn good speed brake since it pitched the airplane up.)

[Chalenge]

I am afraid you dont fully understand the effect of the dive recovery flaps of the P-38 and especially the problem the flaps were designed to correct. Dont worry though its a common mistake and this one in particular has been discussed on this very BBS many times before.

As Warren Phillips discusses in his book "Mechanics of Flight" the P-38 suffered from inadequate tail boom stiffness and when the aircraft met with compressibility (in the case of the P-38 it didnt even need to meet with compressibility) the tail booms both flexed to a point that the pitch stability and the elevator power were reduced. When this happens the nose of the airplane tends to 'tuck' which places the plane into an even steeper dive. At the point of compressibility the flow of air creates a shock wave which prematurely seperates the flow of air which increases drag and decreases lift. Since the downwash of an aft tail is directly related to the lift of the wing the compressiblity effects drastically reduces the downwash on the tail which in turn increases the angle of attack for the tail and creates more nose down pitching moment (making the tuck worse).

With the dive recovery flaps engaged the lift is restored to the wing which increases the downwash to the tail and allows the pilot to recover.

The pitch up you refer to is induced by a pilot that is attempting to trim his way out of the dive which could very well cause larger problems when lift is restored. If the pilot leaves the trim set for the initial dive the plane should not pitch up unless the control column is back due to the pilots input. The dive recovery flaps themselves will not cause a pitch up.

[Ack-Ack]

I am afraid you dont fully understand the effect of the dive recovery flaps of the P-38 and especially the problem the flaps were designed to correct. Dont worry though its a common mistake and this one in particular has been discussed on this very BBS many times before.

As Warren Phillips discusses in his book "Mechanics of Flight" the P-38 suffered from inadequate tail boom stiffness and when the aircraft met with compressibility (in the case of the P-38 it didnt even need to meet with compressibility) the tail booms both flexed to a point that the pitch stability and the elevator power were reduced. When this happens the nose of the airplane tends to 'tuck' which places the plane into an even steeper dive. At the point of compressibility the flow of air creates a shock wave which prematurely seperates the flow of air which increases drag and decreases lift. Since the downwash of an aft tail is directly related to the lift of the wing the compressiblity effects drastically reduces the downwash on the tail which in turn increases the angle of attack for the tail and creates more nose down pitching moment (making the tuck worse).

With the dive recovery flaps engaged the lift is restored to the wing which increases the downwash to the tail and allows the pilot to recover.

The pitch up you refer to is induced by a pilot that is attempting to trim his way out of the dive which could very well cause larger problems when lift is restored. If the pilot leaves the trim set for the initial dive the plane should not pitch up unless the control column is back due to the pilots input. The dive recovery flaps themselves will not cause a pitch up.

Actually, the dive flaps on the P-38 did create a positive pitch.  When deployed at level flight, pilots would report a sudden nose "pop up".   I think it's the loss of energy that occurs after the pop up that make people think its a dive brake but don't realize that the loss of speed isn't due to the flaps but the positive pitch.  Which is why when those Lightning drivers that use to deploy the dive flaps in high speed turns would immediately retract the flaps after the turn to minimize the sudden loss of speed.

ack-ack

[Dawger]

The pitch up was induced by the lift produced by the flap. All lift creation also creates drag. Induced drag is what it is termed. Any object in the slipsteam will also produce parasitic drag.

The reason for the pitch up moment is the fact that the dive flap is forward of the center of gravity on the under side of the wing. Increasing lift at that point would cause the nose up pitch.

The nose up pitch does not increase drag. It is the lift induced drag component of the dive flap as well as some parasite drag.

[Dawger]

I am afraid you dont fully understand the effect of the dive recovery flaps of the P-38 and especially the problem the flaps were designed to correct. Dont worry though its a common mistake and this one in particular has been discussed on this very BBS many times before.

As Warren Phillips discusses in his book "Mechanics of Flight" the P-38 suffered from inadequate tail boom stiffness and when the aircraft met with compressibility (in the case of the P-38 it didnt even need to meet with compressibility) the tail booms both flexed to a point that the pitch stability and the elevator power were reduced. When this happens the nose of the airplane tends to 'tuck' which places the plane into an even steeper dive. At the point of compressibility the flow of air creates a shock wave which prematurely seperates the flow of air which increases drag and decreases lift. Since the downwash of an aft tail is directly related to the lift of the wing the compressiblity effects drastically reduces the downwash on the tail which in turn increases the angle of attack for the tail and creates more nose down pitching moment (making the tuck worse).

With the dive recovery flaps engaged the lift is restored to the wing which increases the downwash to the tail and allows the pilot to recover.

The pitch up you refer to is induced by a pilot that is attempting to trim his way out of the dive which could very well cause larger problems when lift is restored. If the pilot leaves the trim set for the initial dive the plane should not pitch up unless the control column is back due to the pilots input. The dive recovery flaps themselves will not cause a pitch up.

I'm afraid you are the one who doesn't understand high speed aerodynamics but I'll have to start a new thread in the help and training section to explain it.

[Dawger]

Here is a good article explaining some of the theory of compressibility and Mach Tuck.

http://findarticles.com/p/articles/mi_qa3897/is_200108/ai_n8957348/?tag=content;col1

Written by a test pilot flying Grummans.

Some excerpts including the P38.

On April 7, 1944, during my first Hellcat demo flight in F6F-3 Bu. No. 26101, I pushed over from 28,000 feet to what I had estimated to be a 60-degree dive angle and totally concentrated on the build up of airspeed rather than on my rapidly decreasing altitude. With full power, the aircraft's descent rate soon exceeded 38,000 feet per minute! I estimated that I would attain 485mph indicated airspeed just as I went through 10,000 feet, and I planned to make the easy, 2.5G pullout at that point. To maintain the 60-degree dive angle during the speed buildup, I had to continually retrim the elevator with more nose down to overcome the aircraft's natural tendency to pitch up as speed increased.

I was about to start the simple pullout when I noticed with great alarm that the aircraft didn't require any more nosedown trim. The nose was quickly going down of its own volition, and that rapidly increased the dive angle without any retrimming, push force, or desire on my part! I was no longer flying the plane; I was a passenger!

He said that the Hellcat generated supersonic shock waves over its full wingspan at .75 Mach. (On my first dive, I reached .77 Mach!) This was because the airflow speeded up to supersonic speeds so that it would go around the wing's very thick airfoil. Simultaneously with the formation of the shock wave, the wing's center of lift instantly moved several feet backward beyond its designed rear limit! In spite of my Herculean efforts, these effects had given my aircraft its strong buffeting, stuck-in-concrete stick forces and unnatural pitch-down direction.

When they exceeded their critical Mach numbers, almost all of the Allied fighters of the WW It era exhibited the same disastrous diving tendency-with the buffeting and the "frozen" stick-as the Hellcat encountered. Ralph Virden and several other test pilots died during the Lockheed P-38 demonstration program, and its entire aft fuselage and tail structure were redesigned to withstand compressibility effects.

To alleviate the symptoms of compressibility, NACA developed small, dive-recovery flaps that were installed on the underside of combat aircraft wings; when extended, they immediately counteracted nosedown pitching and stick freezing. The extended flaps also increased drag to decelerate the aircraft below its critical Mach number. When the flaps were extended, the dive brake's natural nose-up pitching power greatly helped the pilot to make an immediate pullout. The flaps were installed on P-47s and P-38s in production-but not until too many combat pilots had lost their lives to this too often fatal phenomenon. Sad to say, the Navy never required their installation on any of the 12,500 Hellcats it procured.

[Chalenge]

I'm afraid you are the one who doesn't understand high speed aerodynamics but I'll have to start a new thread in the help and training section to explain it.

The little bit of information I gave comes directly from Dr. Warren Phillips book "Mechanics of Flight"... arguing against that is like disputing that the sun will continue to rise.

[Dawger]

The little bit of information I gave comes directly from Dr. Warren Phillips book "Mechanics of Flight"... arguing against that is like disputing that the sun will continue to rise.

I get the feeling you misinterpreted what Mr. Phillips was talking about but you would have to provide a direct quote from the text.

Mach Tuck is well understood now and causes nose down pitch in  every airplane that gets the flow over the wing supersonic.

Once the shock wave develops on top of the wing, the center of pressure moved aft resulting in the nose down pitch. The center of pressure moves so far aft that the elevator usually does not have the required aerodynamic authority to raise the nose. In addition, the tail itself quickly will enter transonic flow regions shortly after the wing, exacerbating the problems for the pilot. The issue with the P38 was that nearly all of the bad stuff happened simultaneously. Once into a Mach tuck situation the loads on the aircraft become extreme very quickly. The role of the horizontal stabilizer is to provide down force while the wing provides up force.

All of this pivots on the center of gravity. The shock wave moving the center of pressure aft, away from the CG puts tremendous, increasing force in opposition with the down force of the horizontal stabilizer. The wing over powers the tail. It is the stronger structure by far and is capable of generating a lot more aerodynamic force. And the situation only gets worse.

High speed causes the nose to tuck, causing higher speed, causing the nose to tuck more until the aircraft hits the ground or the tail breaks off.

Here is an excerpt from an accident report. Sounds a lot like a P-38.

If a pitch upset occurs near M the airplane can accelerate rapidly
into a region where the flying qualities are unacceptable. Consider, for
example, any  type of nose down pitch axis malfunction (such as trim
runaway, pusher hardover, autopilot hardover, etc.). In this case, if the
pilot restrains the control column, the pull force can go as high as
50-60 lbs. (80 lbs. for pusher malfunction.) Because of pilot reaction
time (3 seconds according to 8110.10), -10/ the speed will have increased beyond the limit Mach number. If the pilot follows the AFM procedure for overspeed and deploys the spoilers (which is instinctive), the required
pull force will increase an additional 50-80 Ibs.

Also, because of the pitch instability due to Mach tuck, the pull force will continue to increase as speed increases. Adding the maneuvering stick force to required to pull 1.5 g, the total pilot force required for recovery can be
as high as 150-200 lbs.

 The stick puller was installed to prevent Mach overspeed, but in the
event of a nose down pitch axis malfunction, and/or deployment of the
spoilers, its 18 lb. pull becomes insignificant. At some Mach number beyond M the elevator effectiveness will
decrease due to shock wave formation. Additionally, stretch in the longitudinal control system at very high control forces can negate any further elevator deflection in the recovery direction.

From a 1981 Lear 24 accident.

http://www.airdisaster.com/reports/ntsb/AAR82-04.pdf

The major issue seems to be what is the cause and what is the effect.

The shock wave created by supersonic flow over the wing moves the center of pressure aft resulting in the nose down pitch and difficulty raising the nose using a conventional elevator. This is the initial causal factor.

The results of this "Mach Tuck" bring ever increasing stress on the airplane, especially the tail, resulting in a bad situation getting much worse for conventional airplanes designed in the 30's and 40's. Many things were tried to correct the problem.

To this day aircraft are designed to avoid mach tuck to varying degrees because ALL aircraft experience a nose down pitching moment when the wing passes through the transonic region. On subsonic jets critical mach is avoided to the extent most aircraft have automated systems to prevent over speed.

[Chalenge]

No what I wrote is nothing like what you describe. Mach tuck from aeroelastic flexibility is different from the Lear incident. You cannot assume (like so many others) that the P-38 was a rigid inflexible structure because it wasnt. And what I wrote from Phillips book holds.