OT: Wing loading at the speed of sound

Just had a chat with a gentleman who related
that his grandfather broke the sound barrier in
a P-47 during WW2. The situation involved
his grandfather's a/c being pursued by more
enemy a/c then the pilot could handle and he
chose to use a power dive to get away from
them. The gentleman said his grandfather's
plane reached and pushed past the sound
barrier during this dive and got away from his
pursuers. I know the P-47 was a remarkably
rugged aircraft and could carry a large store
of bombs and rockets but could its wing handle
the stress of recovering from a 750 mph power
dive? How would the pilot even know that he
had exceeded the barrier? The aforementioned
gentleman delivered the above statement with
the sincere belief that it was valid. Not to dishonor
the memory of his grandfather but some how it
just sounds a little 'pumped up'.
Mike IPMS
Reply to
Mike Keown
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> Just had a chat with a gentleman who related > that his grandfather broke the sound barrier in > a P-47 during WW2. The situation involved > his grandfather's a/c being pursued by more > enemy a/c then the pilot could handle and he > chose to use a power dive to get away from > them. The gentleman said his grandfather's > plane reached and pushed past the sound > barri
Reply to
Boris Beizer
He would certainly have gotten hellacious buffeting and there could have been local transonic flow in places, but it seems like asking a lot to get that shape to go supersonic without having some extra thrust from somewhere. You can get a concrete block supersonic if you want to, but you better have some really big strap-on boosters to overcome all that drag.
Reply to
Jim Atkins
the sound barrier in
to get away from
The fastest speed attained by a Spitfire was ~0.82, by Sqn Ldr Martindale of the High Speed Flight at Farnborough in 1943. Well, that was the fastest speed that the Spitfire went with the pilot being able to walk away and tell the tale; The aircraft itself lost the propeller, reduction gear, and part of the cowling, and, it is said, had a ~1 inch gap torn in the wing leading edges (unintentional increase in wing sweep). The original story may have been a mix up, either in the telling or the interpretation, between the Limiting Mach Number, the Critical Mach Number, and the speed of sound. As I recall (and I'm a bit shaky on this, it's been a while), the Limiting Mach Number is speed at which compressibility starts to become an issue; sonic airflow off the aircraft starts to effect the function of the control surfaces, making it more difficult to control the aircraft, and as such is the speed which the pilot should try not to exceed. Critical Mach No. is the speed at which the sonic airflow across the control surfaces means that all control is lost, and as such usually means that if the pilot exceeds it, there isn't too much they can do to get back below it. But having said that if the dive is started in the rarefied air at altitude; if the power is pulled back, the increase in air density and therefore friction at lower altitudes may reduce the speed enough to give sufficient control to pull up before hitting the ground. If the pilot's lucky, and doesn't overstress the aircraft too much as they pull up and tear the wings off. From a lecture by test pilot Capt.. Eric "Winkle" Brown, the Spitfire had the best Mach No. characteristics of the aircraft tested by the High Speed Flight at Farnborough, next came the Mustang, with the P-38 and P-47 behind them. The problem with the P-47 was apparently that, being a bit on the heavy side, they could pick up speed very easily in a dive, and if the pilot wasn't careful, could accelerate to the point that he didn't have the control authority to pull up. So it may have been that the originator of the story exceeded the Limiting or Critical Mach No, but managed to get away with it and tell the tale.
Jon.
Reply to
Jonathan Stilwell
If I had to pick the aircraft with the best chances of surviving the feat I would pick the Jug hands down. One tough frigging bird. I think they didn't quit flying until they were damn good and ready.
WmB
To reply, get the HECK out of there snipped-for-privacy@earthlink.net
Reply to
WmB
would pick the
I believe that Terminal Velocity for an aircraft is the speed where the pull of gravity and the engine are balanced by the drag/resistance of the airframe so the speed will go no higher. Drag rises as speed goes up so there is a different speed/drag rise curve for each aircraft. No propeller driven aircraft has ever had a curve that allowed it to go supersonic. The propeller itself is part of the problem I believe, but I'm not up on my aero-dynamics that well. Compressibility, the inability of air to flow around a moving airframe smoothly, is a function of the individual aircraft's aero-dynamics. Some aircraft hit it at lower speeds than others. My memory says that the P-47 was one of the first aircraft to hit compressibility problems in high speed dives. After a certain speed, the controls would simply become rigid. Republic Aircraft's Vice-president for Engineering, Hart Miller, deposited the prototype Thunderbolt in the bottom of Long Island Sound finding that out. He also found out that the P-39 type car door cockpit entry was hell trying to bail out of. Result was a re-designed jettisonable cockpit hood and revised control surfaces which delayed the onset of control problems. The P-47's terminal velocity was still below mach-1 but it could go downstairs awfully fast when it had to, ask any Zero or Focke-Wolf pilot. :-)
Bill Shuey
Reply to
William H. Shuey
"William H. Shuey" wrote in news: snipped-for-privacy@starpower.net:
I recently read a book on the P-38 in which there was considerable discussion about compressability. I think one of the conclusions was that no propellor driven aircraft can reach Mach 1. The two limiters are the straight wing and the fact that the blades approach Mach 1 way before the airframe does. I thinl the conclusion was the plane would shed it's prop first.
No expert here, just what I read.
Frank
Reply to
Gray Ghost
Yes! That's what that Spitfire did, it was a Mk.XI PR ship according to "Spitfire, The History". The prop went, took the spinner, hub and reduction gear off the Merlin with it. The pilot dead sticked the Spit and went home to change his pants!
Bill Shuey
Reply to
William H. Shuey
Historically, the P-38 was the WWII fighter that had the most trouble with compressibility in the dive, which wasn't solved completely until they installed dive flaps in the J or L series a/c. Regrettably, I don't know enough to know what this means, but the Spitfire had a higher flow reversal (mach critical?) number for its wing than the P-51 (or any other contemporary piston-engined fighter, for that matter). The issue with propellers was tip compressibility, since the tip of the blade is always moving the fastest. One interesting experiment with the P-47 was the fitting of scimitar-shaped prop blades--I guess the notion was similar to wing sweep--don't know how that experiment turned out. This problem could also be minimized by multiplying the number of blades to diminish the overall length, so you get Spitfires with five and six-bladed props. The big Tupolev turboprops (Bears?) are eight-bladed each, with contra-rotating fours, and I think they also run at very high tip speeds.
One comment on the apocryphal (?) incident that began this thread--I don't know what the speed guage on the P-47 was calibrated to as a max reading, but compressibility could do a number on the pitot data, giving outlandish values when the plane was traveling much slower. That's how one P-38 pilot came back from a power dive to claim he'd gone over 700 mph.
Mark Schynert
Reply to
Mark Schynert
Generally, contra-props are for torque, not speed. IIRC, the big props on the Bears actually turn quite slowly, 800-900RPM I think, yet at 500 MPH airspeed, the tips are running barely under Mach. Most of the noise that we hear from propellers, even on small aircraft today & WWII types at airshows, is the prop tips turning fast enough that the tips are right at, if not exceeding Mach 1. The noise is actually multiple miniature sonic booms. Remember the XF-84H supersonic turboprop? Its prop was nothing fancy in appearance, but on he ground it created inaudible yet irritating noises that caused nausea! Don't recall what all the findings were & what its performance really was.
Reply to
famvburg
Air & Space Smithsonian had a great article about the XF-84 a few months back.... I think the title was SCKREEEEEEEEEEEEEE!!!!!!!! Not a joke.
rich
Reply to
Rich
Thank goodness the thread did not sink into the myth and mythology of reversing controls.... there was even a movie at some point about the Spitfire breaking the soundbarrier, and discovering that if he reversed the direction of his controls, he pulled out to fight another day......
So interesting to read posts that have some genuine information.... much better understanding then this poor CFI knows, albeit a real interest of mine.
rich
Reply to
Rich
The XF84H -- "H" for hybrid, made it into one of my favorite books -- "The World's Worst Aircraft" by Bill Yenne. I worked at Republic and no one there could ever give me a rational answer for why the damned thing was built. It was one of the noisest aircraft ever built -- noiser than most 4 engined airplanes - certainly the noisiest single engine aircraft ever built. The author calls it "dreadful." The air force refused to fly it and all test flights were done by Republic test pilots -- presumably chained into the cockpit. Only twelve flights were made and eleven of them ended in emergency landings. Great book! Worth hunting used book stores for. I'd love to see some model company put out a series of the aircraft in this book.
BOris
Reply to
Boris Beizer
AAaaahhhh, Yes! The Infamous XF-84H "Thunderscreech"! That's one that Republic's people would rather forget. It would give all working on it a headache every time the engine was run. Theory was that the prop tips were actually hitting Mach 1.
Bill Shuey
Reply to
William H. Shuey
It was built in case there was a shortage of jet engines. At least that's what a number of sources say.
Boris Beizer wrote:
Reply to
Ron
Actually, the P-38's problems weren't with compressability but with another high speed phenomenon not completely understood at the time, shifting of the "center of pressure" on a surface at high speed. Essentially, a wing or control surface has something called a "center of pressure". This is the point on the surface where the theoretical sum of all the forces acting on the surface is located. When the aerodynamic forces change with speed changes or maneuvering, the center of pressure moves. On the P-38 it was later discovered that as speed went up the center of pressure on the stabilizer actually moved behind the stabilizer; in effect, the stabilizer/elevator became ineffective. If you were in a high speed dive you suddenly found the plane was not answering to the stick, she didn't want to pull out. If the pilot panicked he either baled out or dug a hole in Mother Earth. With experience, it was found that if the power was reduced and patience and altitude were sufficient, as the P-38 entered denser air at lower altitude and slowed down, control returned. The dive flaps that were added under the wings on later P-38s essentially were a spoiler that disturbed the airflow off the underside of the wing and added some turbulence to the air flowing over the stabilizer/elevator. This altered the center of pressure over the surface to keep it within the chord of the surface, thereby maintaining control. The prototype P-47's early problems were something similar. At high speed the aileron's would lock up and be absolutely un movable by the pilot. Whether Hart Miller would have been able to regain control if he had reduced power and rode her into denser air is something we can only conjecture. I'm not going to second guess what someone else did in a highly dangerous situation flying as they say "at the edge of the unknown".
Bill Shuey
Reply to
William H. Shuey
The prototype XP-47B. She was the only one, cause Hart Miller had such a hard time getting out that re-design of the cockpit canopy was a priority with him, and he was Republic's Vice President for Engineering so he had some pull.
Bill Shuey
Reply to
William H. Shuey
Don't doubt you, don't doubt what the sources believed, but it didn't make a whole lot of sense. Not a whole lot of difference between a turboprop engine and a turbojet. If anything, a turboprop is likelier to have birthing pains than a pure turbojet engines -- none of the big reduction gears and transmission. There was an engine shortage at the time -- mainly the Rolls-Royce Saphire(?) used in the F84, F86 and other jet fighters. It was a production problem. The Brits were building them in one's and two's, with a lot of careful hand tweaking and buffing with crocus cloth in critical spots -- when it was put on a mass production line in the US, they just didn't work. Practically had to redesign the entire engine and the production line to get them in quantities needed for the Korean war. The engine production problems were licked long befoer the Thundershriek had a chance of being operational. Sounds like the kind of solution a politician, rather than an engineer, would come up with.
BOris
Reply to
Boris Beizer
I read somewhere that a P-47 in the Mediterranean theater actually did break the sound barrier once during a steep dive. Admittedly, I read this several years ago and can no longer remember the source, details, or how authoritative it was.
"The world would be a much simpler place if every one could pick and choose their obligations, but we can't and we shouldn't." Major Charles W. Whittlesey
Reply to
Bill Woodier

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