billswagger
Airman 1st Class
- 256
- Mar 12, 2009
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Terminal velocity is when the force of the air pushing against the airplane is equal to the force of gravity pulling it during a dive. Its about 600mph true airspeed, well above the point of compressibility for the 47. But i was wanting to know what the terminal velocity was for the 51 and the 38, for comparison.
Bill
Did any of these aircraft break or get close to breaking the sound barrier in a dive? I've read of a PR Spitfire doing it, but nothing else
Maybe you will be interested in this:
Pushing The Envelope With Test Pilot Herb Fisher
There's a nice graph with dive speeds for P-47, though not standard but with some experimental Curtiss propeller.
I suspect this was a lot more important then max dive speed. An aircraft that accelerated quickly would be long gone before the Jug got up to speed.
How long did this require?
Non specific observation..I re-read the report made by AAF Board Project no. (M-1) 50 (page 49 and 50 of Jeff Ethell's Mustang). November, 1943
"The P-51B dives away from all other fighters excepth the P-47D against [which] the P-51B jumps several hundred feet ahaed in the initial pushover and then holds that position, apparently neither gaining nor losing distance"
"Dive, rate of climb and acceleration of the P-51B-1 are superior to the P47D-10. The P-51B also holds its high speed longer than the P-47D in level flight after a dive, because it decelerates much slower."
Its a shame they don't discuss the dive rates, but also 1943 would've been earlier versions of both models. It would also be nice to know how the 51d and the 47D-22 or 27 compared. If they stayed inside of what engineers called "safe" then these findings couldn't be relayed to actual use of the planes. I haven't read the article, so forgive me if it explains this.
No observation based on airspeed indicator was worth anything due to the calibrattion error w/o tempertature recordings. The P-51H was the ultimate Mustang with the slightly thinner wing, more horsepower and a slightly cleaner airframe
But looping back to my other post.
The pilot claims the 47 could out dive the 51, and level out at much higher speeds...of course slowing down as it extended.
P-47 Compressibility Dive - 510th Fighter Squadron
This link, i'll repost it, seems to suggest that pilots would ignore what the manual would call safe airspeed, in a 47 and push it up to 650-670..
In reality they were doing close to .83 M and NEVER close to 650mph
Some of these 47 pilots killed themselves attempting to dive in a 51 the same way....yikes..simply because the 51s would crumble.
What sources are you using?
The pilot also claims no prop plane ever broke the sound barrier...which i tend to agree with because we still make a big deal over the first guy to do it.....uhhhh Chuck Yeager.....i remember him too.
You can take this to the bank
Although airspeed indicators probably exceeded 700IAS, the actual speed of the aircraft was hard to know because of the technology used then.
He says it better than i can...
"I'm certain of one thing, though. I've heard pilots who claim to have reached Mach 1 in a P-47, but no propeller-driven airplane ever reached Mach 1 - not even the mighty Thunderbolt. It's just not possible, because, at that speed, the propeller no longer delivers thrust, becoming instead a drag which prevents further acceleration."
Agreed
I was in a heated debate with a friend about which plane fell faster...the 47 or the 51.
I think its evident the 47 was faster in the top end (fastest part of the dive), but i would also speculate its weight and horse power let it out accelerate the 51 in a dive.
Why is it "evident"? the 51 was a much cleaner airframe requiring a lot less THP to achieve (and exceed) P-47 performance. More drag on the entire system (prop plus CD0 of airframe and wing) with less THP as the velocity increases leads you to believe the 47 is faster? Why
but this is only speculation...hence why i asked the question. Maybe the 51 would initially get that couple hundred feet, but compressibility and durability, or lack there of, made the 47 a superior diver.
The RAF flight tested the D (Mark IV) to .85 M, well into compressibility. What eveidence do you have that the 47 of any dash number met or exceeded that?
I've pretty much dropped discussing the 38 because it was a faster diver than both planes, but its ability to function effectively and safely in a dive was not as apparent as the 47.....
It had a limit placard of .69 M, the 51 and 47 were at .75 M. The issue with the 38 was a.) fat wing, and b.) severe nose tuck due to Cm change as shock wave move the aerodynamic center aft during early stages of compressibility. It could not dive safely with a 109, a 190, a 47 or a 51 and required dive brakes to keep it out of near instant compressibility in a dive at 25K
side note: there are some 47 aces who knew the 47 well enough (brave enough) to out dive a 38, but again "capable" and "likelihood" probably does depend on several other factors.
Bill
All Jug drivers should have easily (and safely) outdived a 38.
Originally Posted by billswagger View Post
Its a shame they don't discuss the dive rates, but also 1943 would've been earlier versions of both models. It would also be nice to know how the 51d and the 47D-22 or 27 compared. If they stayed inside of what engineers called "safe" then these findings couldn't be relayed to actual use of the planes. I haven't read the article, so forgive me if it explains this.
No observation based on airspeed indicator was worth anything due to the calibrattion error w/o tempertature recordings.
True
>The P-51H was the ultimate Mustang with the slightly thinner wing, more horsepower and a slightly cleaner airframe
I'm aware of the 51H, but i wanted to focus on the d models because they were used in combat in ww2. I'm not sure the H ever saw combat (in WW2), from the material i've read.
It did not - all deployed to stateside interceptor squadrons from late Feb 1945. only 500+ built by VE day
But looping back to my other post.
The pilot claims the 47 could out dive the 51, and level out at much higher speeds...of course slowing down as it extended.
P-47 Compressibility Dive - 510th Fighter Squadron
This link, i'll repost it, seems to suggest that pilots would ignore what the manual would call safe airspeed, in a 47 and push it up to 650-670..
>In reality they were doing close to .83 M and NEVER close to 650mph<
...i realize the flaw of using TAS, but this was a figure based on the pilots account.
And simply impossibe - just like corresponding Mustang Encounter reports of dives >600+ mph
Some of these 47 pilots killed themselves attempting to dive in a 51 the same way....yikes..simply because the 51s would crumble.
>What sources are you using?<
I was using the pilots account. The 51s may have exceeded the speed of the 47 in the dive, but weren't able to hold together....not sure/
Two things working here - actually three. First is the structural design ultimate loads as Gross ~ both were 12g aircraft but over/under gross design weight would be a factor. Dynamic Pressure (Q) is another factor and represents the primary loads on the airframe at max velocity. Last, is the Moment Coefficient change due to compressibility moving the aerodynamic center from ~25% chord to 50% ------> resulting in pitch down moments the horizontal stabilizer could not manage to correct from until the a/c slowed down.
The 47 apparently had the problem (as the 38) and entered near vertical dive before stabilizing. The 51 did not but you had to restrain yourself from any lateral manuever and let the airplane slow down by backing off the throttle until you reached 'denser' air.
The pilot also claims no prop plane ever broke the sound barrier...which i tend to agree with because we still make a big deal over the first guy to do it.....uhhhh Chuck Yeager.....i remember him too.
You can take this to the bank
Although airspeed indicators probably exceeded 700IAS, the actual speed of the aircraft was hard to know because of the technology used then.
He says it better than i can...
"I'm certain of one thing, though. I've heard pilots who claim to have reached Mach 1 in a P-47, but no propeller-driven airplane ever reached Mach 1 - not even the mighty Thunderbolt. It's just not possible, because, at that speed, the propeller no longer delivers thrust, becoming instead a drag which prevents further acceleration."
Agreed
I was in a heated debate with a friend about which plane fell faster...the 47 or the 51.
I think its evident the 47 was faster in the top end (fastest part of the dive), but i would also speculate its weight and horse power let it out accelerate the 51 in a dive.
Why is it "evident"? the 51 was a much cleaner airframe requiring a lot less THP to achieve (and exceed) P-47 performance. More drag on the entire system (prop plus CD0 of airframe and wing) with less THP as the velocity increases leads you to believe the 47 is faster?
but this is only speculation...hence why i asked the question. Maybe the 51 would initially get that couple hundred feet, but compressibility and durability, or lack there of, made the 47 a superior diver.
The RAF flight tested the D (Mark IV) to .85 M, well into compressibility. What eveidence do you have that the 47 of any dash number met or exceeded that?
I've pretty much dropped discussing the 38 because it was a faster diver than both planes, but its ability to function effectively and safely in a dive was not as apparent as the 47.....
It had a limit placard of .69 M, the 51 and 47 were at .75 M. The issue with the 38 was a.) fat wing, and b.) severe nose tuck due to Cm change as shock wave move the aerodynamic center aft during early stages of compressibility. It could not dive safely with a 109, a 190, a 47 or a 51 and required dive brakes to keep it out of near instant compressibility in a dive at 25K
side note: there are some 47 aces who knew the 47 well enough (brave enough) to out dive a 38, but again "capable" and "likelihood" probably does depend on several other factors.
Bill
Isn't the mach reading how fast the air is moving over the wing, and not an indication of how fast the plane is actually moving???
The instruments are connected to the pitot tube extending out in front of the leading edge of wing (now nose) and records the stagnation pressure of the freestream air (undisturbed by airflow over wing at higher velocities) brought to a stop within the tube.
This is where TAS would be valuable...and maybe even knowing terminal velocity.
As you noted earlier the pitot tube and instrumentation needed Temp corrections (and sensors) to obtain a reasonable TAS
I could see...51 = less drag, lighter airframe......47= more drag, but a much heavier airframe. I wouldn't know how to factor in horse power. Hard to say if it evens out, or where the advantage is....but i agree the 51 would be superior in overall performance, but i'm still not convinced it was a faster diver.
Bill
I found this to be an interesting read as well.
P-47 THUNDERBOLT
The second half is a WW2 vets account on both planes that he had the honor of flying. He gives 12 reasons why the Jug was better than the 51, also admitting the 51 was also a great fighter plane.
One of the reasons goes into dive speeds,
"4. The Jug could out dive the Mustang. As a matter of fact, it could out dive any enemy fighter, and at 7.5 tons loaded, it dove fast! I have personally been in a dive at what we called the "state of compressibility," at nearly 700 mph indicated air speed. I was scared to death, but with a tiny bit of throttle, I pulled it out at about 2,000-foot altitude, literally screaming through the sky."
I find pilot accounts rather convincing.
Give it a quick read, and look over the other 11 reasons.
thanks for the reads,
Bill