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
> 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
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.
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.
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
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
"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
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
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
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.
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
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
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
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
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
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
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
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