kool kitty89
Senior Master Sergeant
Found an interesting article on the P-38's compressibility problems, haven't finished it yet. Service Testing Compressibility | Air Classics | Find Articles at BNET.com
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"The only time the dive flaps would pitch the P-38 nose upward would be at Mach number under critical. Every time I dove a P-38 at critical Mach number I had to pull, sometimes hard as hell! During the final dive test that I made in about July 1943, I started pulling out of my 60-degree dive angle at 20,000-ft. At 19,000-ft 1 had only gained one degree. 1 finally got it under full control at about 10,000-ft but all six red warning lights were on and 1 thought the plane's parts were just barely holding hands!
"I don't think another pilot in the history of the P-38 has had more experience with compressibility as I have had. From the time MiIo and I started doing the dive tests together, I didn't stop doing them as long as I had those special dive flaps, including doing them with my own P-38.
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"The P-47 Thunderbolt and P-51 Mustang had trouble with compressibility except they both had a higher critical Mach than the P-38 and got away with it much better. However, when I was in England flying all over the place demonstrating the dive flaps, the high command of the 8th Air Force ordered me to conduct dive tests with, or rather against, both the Thunderbolt and Mustang.
"The tests took place at Nuthamstead in East Anglia, home of the 55th FG. I ran the test which required us, one at a time, to climb to 30,000-ff and do a series of dives.
"The Mustang was first, but his Merlin engine wouldn't kick into high blower at 22,000-ft so we just played around down on the deck. I beat his ass good and proper right in the front of the whole 55th.
"The P-47 flight went much better. He was directed to climb to 30,000-ft and, with me in trail at full speed, to push over into the maximum dive angle he dare go to. I specifically ordered him not to exceed his limits. He did what I said 011 my command and dove almost straight down to 20,000-ft and started to pull out of the dive. I was on his tail all the way, but he was a little faster because I dove with power off. I had no trouble pulling out and came right up to his right wing.
"Next was a split-S from 30,000 and this is where the fun began. Again in the lead, he split-S'ed and I followed. He pulled away, but I was on his tail all the way. At 20,000-ft, he didn't pull out so good but finally made it. Again, I pulled up to his right wing.
"One the ground, he told us of the horrible trouble he had. In his pull out, he had aileron oscillation which caused the control stick to flail and beat his legs black and blue. He said it scared the hell out of him. He further stated that, after the first dive, he figured he'd never see me again. He was absolutely amazed to see me on his wing!
"At the beginning of the war, I don't think anyone knew much about Mach number, except Mr. Mach who at the time was up there looking down laughing like crazy at us all. In 1938, I heard about an aerodynamic term called 'Solidified Burble.' I think that was dreamed up to describe what became known as 'shock stall.' I think it was a term originated in Europe.
"It was Ward Beman who discovered what was going on and he came up with the compressibility dive flap configuration. That's when I was asked to share the dive testing with MiIo.
"When the P-38 wing developed a supersonic enclosure at the critical Mach number, the center of pressure moved aft, causing the nose to tuck down. The shock-stall aft of the shock wave caused a turbulent wake which impinged on the tail and caused buffet effect and reduced down load helped to tuck more and reduce control effectiveness.
I'm somewhat perplexed by your insistance that the dive flap functions primarily as a speed brake given that all the literature states that it works by restoring lift to the wing.
B- Believe what you will. I say it restores lift to the wing by slowing it down under Mcrit. I say that it, by itself is designed as a Brake, not a lift device.
I SUSPECT that many of the anecdotes trotted through this discussion are interpretations by an author or a pilot repeating what they thought they heard. When I hear these words I keep finding questions - but simply it is designed to a.) be deployed before the dive, and b.) it is designed to alleviate compressibility entry.. it would cause a pitch up if deployed sub mach crit after entering the dive
I'm certainly no aerodynamicist, but I have some grasp of Newton's 3rd law of motion, and it seems to me that if the airstream encounters the forward chord (ahead of the most violent compressibility-induced turbulence) mounted dive flap at at 30 degree angle, the air will be pushed downward...and the wing upwards, with the result that the a/c will pitch up (a 4-G hands-off pitch-up, from what I've read) and the tailplane will return to a negative AoA to the airflow, thus restoring elevator control.
The 'brake' is Located at 30% chord - which as a structures guy, suggests that it was attached to the primary load carrier - the main spar..
The airflow should experience two forms of flow. The first form is to come to complete 'stop' in which case it would be like coming to rest on a 90 degree angled flat plate... the resultant force there is parallel to the bottom of the wing, offset vertically to the center of the brake and roughly be 1/2 rho*V>>2 times the area of the brake. If that point was above the CG, it would contribute to 'pitch up' (and I am guessing it is.)
If deployed after the dive but before Mcrit it could cause a pitch up
The secondary flow stream would bypass the mess around the brake and create its own 'aero shape' and maybe (Maybe) re-attach downstream just before the trailing edge. Between the lower wing and that secondary flow is the brake/the stagnated flow, the turbulence and huge boundary layer behind the brake.
The only place the flow stream is remotely attached to the wing is in front of the 30% chord/brake.
It is possible that the resultant 'higher' pressure of the disturbed flow below the wing could act at a point forward of the brake, but I doubt it.
If it (higher pressure section) acts aft of the 25% chord line it will only re-inforce the pitching moment issue I described before.
Care to suggest alternative physics for it to act at or forward of the 25% m.a.c. to help me understand your point?
While the dive flap necessarily increases drag, it does not prevent the P-38 from reaching its Mach limit. Here's a quote from Tony LeVier's 'Pilot'...
"Although I had completed my test on that flight I decided to dive to low altitude at the critical mach number, for no reason except that 'Nosey' dove so well.
This time I really hung on. I held the plane right at the mach limit at an extremely steep angle, reaching a top speed of 530 miles indicated, which was 100 over the maximum allowed for that airplane at low altitude."
This would seem to indicate that the flap allowed control to be maintained regardless of rate of speed...
So, help me out - did he deploy the brake? Did he just wait out the dive to a lower altitude? Was he ever pulling back on the stick or was he forcing it to stay in the dive? When did the airflow over the wing slow below Mcrit, at Which altitude? How does he know when he is at critical Mach at mid to low altitudes? was he able to stay at Mcrit and for how long? What was he using to convert fro IAS to TAS and what did the flight test profile reveal?
Got to know TAS and altitude to figure out just where you are in the Mcrit profile. Got to know whether the dive slowed below Mcrit, then accelerated past it (believeable but show me).. all the instruments started showing departures due to air compressibility effects above M.> 0.5
In other words, B - what do you Know about the test profile described above? And from what you Know, what conclusions do you draw?
Last I agree it by and of itself is too small to Prevent entering Mcrit at a dive angle steeper than the 45 degrees recommended by Lockheed. In that example it is not a pure break. But in no case is it intended as a lift device... other wise it would be used for manuever and landing/take off
Here's another link on the subject:
A Complete Waste of Space Forums-viewtopic-Compressibility Thread--Corky Meyer flys the P38 at JFC
JL
"The only time the dive flaps would pitch the P-38 nose upward would be at Mach number under critical. Every time I dove a P-38 at critical Mach number I had to pull, sometimes hard as hell! During the final dive test that I made in about July 1943, I started pulling out of my 60-degree dive angle at 20,000-ft. At 19,000-ft 1 had only gained one degree. 1 finally got it under full control at about 10,000-ft but all six red warning lights were on and 1 thought the plane's parts were just barely holding hands!
After hundreds of hours of hard work, Kelly Johnson came up with the most logical answer to Virden's crash when, after examining the wreckage and data, he concluded that linkages to the larger elevator tabs failed, causing them to go into extreme deflection which probably resulted in a sudden, and catastrophic, force traveling through the tail boom. This, in turn, caused an immediate structural failure of the units.
Langley's answer to the P-38 dive problem was the addition of a wedge shaped dive recovery flap on the lower surface of the wings. Aerodynamic refinement of the dive recovery flap was continued in a coordinated program with Lockheed engineers and the new Ames Aeronautical Laboratory, just south of San Francisco, in the latter's new 16-foot high speed tunnel. The dive recovery flaps ultimately saw service on the P-47 Thunderbolt, the A-26 Invader, and the P-59 Airacobra, America's first jet aircraft.
I had heard that Republic test pilot Parker Dupouy-another decorated AVG ace I had met at Republic in 1943-had dived the P-47 vertically to its maximum Mach number of .868 and made a very successful dive recovery-flap pullout. His dive also disproved several USAAC combat pilot's reports that they had dived the jug supersonic. Dupouy's dive was 61mph faster than the P-47's compressibility entry Mach number. To be sure of their operational availability, I checked out the extension and retraction of the dive-recovery flaps several times during the climb. I then fearlessly pushed over into a 60-degree dive and ran the combined Mach number/airspeed indicator rapidly up to .80-well past its compressibility limit Mach number. The stick expectedly became immovable, and the aircraft rapidly pitched nose-down. Extending the dive recovery flaps provided an instant drag increase and a stick-free 4G pullout that brought the aircraft back below its airspeed limit within a few easy seconds.
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. That action would rapidly pitch the aircraft up to 4G and enable the pilot to regain full control. Although Lt. Kelsey evaluated and approved this dive-recovery flap in February 1943, Lockheed did not incorporate it into production for another 14 months! By that time, 5,300 P-38s-more than half the number eventually produced-had been delivered to the USAAF.
...
In 1943, I experienced compressibility in a Hellcat; I wonder how many of those P-38 pilots in the pursuit of the enemy dived too steeply-well beyond the critical Mach limit and into compressibility-in the heat of combat and disappeared into oblivion. At the Joint Army/Navy Fighter Conference on October 16, 1944, I tested the P-38L dive-recovery flap well in excess of its 0.65 Mach-number limit. Upon actuation, they instantly provided a smooth, 4G recovery without pilot effort. Immediately after I evaluated these "jewels," they were installed on all Grumman 17817-1 Bearcat fighters.
You further realize that the two statements have nothing to do with each other?Your condescension doesn't bother me much; I'm quite aware that my understanding of aero is trivial compared to your ownThat said, it doesn't follow that you are necessarily a priori correct in your assessment.
I agree on the latter comment - my prior comments are really that you challenge my thoughts with anecdotal evidence - and sometimes when you do, the 'evidence' is out of context, or incomplete and you expect rebuttal from me to 'prove' my points.
My Points are all about applying my knowledge to suggest the cause or design approach - and They are absent flight test data also.
I am not trying to be condescending but confess irritation when you grab anecdotal references, frequently absent context, to present proudly as proof of your opinion. I apologise for the irritation that I experience and will try to be 'better'
Re; LeVier: In answer to your queries, all I can answer with are the his own words following the previous quote... "A recording camera in the nose of the airplane had faithfully photographed the test instrument panel throughout my dive. We watched the altimeter drop at an extremely high rate, with the speed going up and the altitude going down until it dropped under ten thousand feet and suddenly i realized what I had done. Milo (Burchham) started to take on a queer look and make unpleasant glances at me, and when the film showed I dove to one thousand feet and pulled 71/2 G's getting out, he blew his top."
LaVier was testing the dive flaps at the time, so I think it's safe to assume he was using them. As for his TAS, I have no idea. In any case, use of the dive flaps and the addition of the fillet to the fuselage wing juncture (to reduce buffet, not 'flutter')
This is a great example of your tendency to make statements without full context - then correct me on mine?
There is evidence, anecdotal and factual, that the wake turbulence behind the early P-38 wing body design caused BOTH. If you have been following the discussions regarding aeroelasticity you may recall the connection, then not understood, regarding the phoenomena of Flutter' when immersed in high speed turbulent flow. It is usually a resonance condition of the airframe part being investigated. The buffeting experience is caused by the variation in vertical components of velocity on the horizontal tail - causing deflections positive and negative of the elevator - but not a t Resonant frequency
raised the Mcrit of the P-38 from .65-.68 to .725. That being the case, it seems improbable that the dive recovery flaps (I call them that because that is how the literature almost unanimously designates them) function by reducing speed below the clean Mcrit of M .65-.68...
Call them what you wish - make the distinction for the purpose and location and we can communicate
"...later called the dive recovery flaps, increased the P-38's trim lift coefficient by 0.55 at a Mach number of 0.725."
What were they called 'formerly'?
"The reduced lift curve slope of wing center section also reduced the rate of change of the downwash at the tail with AoA, accounting for the static longitudinal stability increase."
You realize that the latter statement has nothing to do with the Dive brakes (and or dive 'flaps') and that the primary wing Flap has in fact a 'section' inboard of the Nacelles' attached to traing edge? And that deployment of That 'airfoil/fowler flap' at Mcrit should tear off the wing?
These two quotes are from taken from a book on the history of aerodynamics, complete with charts and figures, available in pdf from this link:
Airplane Stability and Control: A ... - Google Book Search
JL
And on the servo tabs, they did allow pull-out with normally very high stick forces, but as mentioned could cause structural failure. (namely with Virden)
However the failure wasn't due to the reason I'd thought. I had initially thought the failure was caused by pulling too hard or too quickly on the stick while using the servo tab, thus overstressing the tail, however this seems not to be the problem. According to the article, the conclusion was that the coupling to the spring loaded servo tab must have failed, and it then rapidly deployed to full deflection. This in turn resulted in tremendous leveredge on the elevator deflecting it very quickly and thus overstressing the tail assembly.
Service Testing Compressibility | Air Classics | Find Articles at BNET.com
Also, I never mentioned the fabric surfaces on the P-51 I was refferring to the P-47. (which only the B model ever had, and it was only used for testing and training iirc)
And I was refferring to the limiting factor of the early P-47's diving ability being due to the aileron flutter pummelling the pilots thighs to black and blue. This was improved with metal ailerons, but still a significant problem until the "blint nosed" ailernons were introduced on the P-47D line. (flutter effectively limited the models with the early metal ailerons to ~.73 mach where it got really bad, the blunt nosed ailerons increased this to ~.83 iirc, though the late D models also got "dive recovery flaps" increasing max recovering speed further, such speeds were above the limiting mach number)
I think the term "dive recovery flap" refferred to it as they were flap like devices in apearance, and could be deployed at high speed to recover. (actual "dive brakes" ie on dive bombers, were to be deployed only at loer speeds, before entering the dive)
Als they deployed at angles much different from most contemporary "dive brakes."
ch3-5
And further info on the operation and actual effect of the device:
jug and I flying the P-47, The | Flight Journal | Find Articles at BNET.com
and
P-38 the legend explained | Flight Journal | Find Articles at BNET.com
And there are also the 2 NACA doccuments (.pdf) to look at on the topic.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.
I havent't read through those articles yet. Just skimmed over the points.
Could you be specific about which ones those statements were in? (the fron view with 4x dive flaps, and the DC-4)
Airplane Stability and Control: A ... - Google Book Search
Figure 11.2 for the pitching moment change by CL by Mach Number
Figure 11.3 for the 'illustration'
The article goes on to talk about introducing boosted ailerons 'a further contribution to high Mach No control (which is silly).. the author seems to be wandering from facts to some confusion (IMO) and this is another example.
Using Boosted ailerons in a high Mach (.7-.75 as in compressibility) for a P-38 seems like a great way to make many small pieces out of one big one.
Boosting ailerons was about getting better and faster a.) initial turn momentum and .b) lighter stick forces resulting in much better aileron respons/roll - not about 'compressibility control'
And also in the quotes I referenced, it seems that the dive flaps were extended to recover only, not before entering the dive. (inless the pilot wanted to limit the dive speed in the first place, probably helpful for dive bombing, especially with the flap already allowing a hands-free pull-out)
P-38 Dive Flaps | Air Classics | Find Articles at BNET.com
Has excerpts taken from the Lockheed operating hand book - this one refers to 58" 'dive flaps' and discusses the advice to put them out before starting the dive, going on to explain why it will pitch up as it deploys if activated after starting the dive. Note - it will pitch up either way if stick free but the advise is put the stick forward after deployment to control the pitch better then go into dive and chase with steady forward pressure - than than say midway through a chase and try to hold the guns on a target while you start the flap/brake deployment
and on the actual dementions of the dive flaps (from my previous quote)
P-38 the legend explained | Flight Journal | Find Articles at BNET.com
And there are also the 2 NACA doccuments (.pdf) to look at on the topic.
BTW I found all those articles on a google search for "dive recovery flap"
That diagram of the P-38's flaps (nose on view) apears to be mistakenly placed, and actually depicting the trailing edge flaps.
Particularly seeing as the outer flaps are labeled 11 1/2' and the dive recovery flaps spanned only 40 in. (the inboard ones are labeled 8 1/2')
"A recording camera in the nose of the airplane had faithfully photographed the test instrument panel throughout my dive. We watched the altimeter drop at an extremely high rate, with the speed going up and the altitude going down until it dropped under ten thousand feet and suddenly i realized what I had done. Milo (Burchham) started to take on a queer look and make unpleasant glances at me, and when the film showed I dove to one thousand feet and pulled 71/2 G's getting out, he blew his top."
Or a combination of slowing down and reaching warmer air at low altitude where the speed of sound is higher.
But in the case of the P-38 out-diving the P-47 (obviously early metal ailerons) must have had the dive flaps retracted untill the pull-out.
And that account form Buzzard about the 72 G??? pullout at 1,000 ft, while lacking pertinance, is something interesing on its own.
I believe we can't compare this two planes
because the P-38 served longer than the P-51 (in the WWII),
but the p-51 was a pretty good fighter but came late in the war