p-47, p-51, p-38......

Discussion in 'Flight Test Data' started by billswagger, Mar 12, 2009.

  1. billswagger

    billswagger Member

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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
     
  2. drgondog

    drgondog Well-Known Member

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    Bill - sometimes structural failure preceeded terminal velocity. The 51's highest recorded controlled dive in a Test scenario was .84-.85M in an RAF test.

    It could have gone a little bit faster - maybe - but I'm reasonably sure the pilot wanted to have dinner later.
     
  3. billswagger

    billswagger Member

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    yeah, i know its not an easy fact to dig up. The 47 reached compressibility before it reached terminal velocity as well.

    Also the 51s wings would fall off, if it tried to dive like a 47.

    P-47 Compressibility Dive - 510th Fighter Squadron
    this gives a good pilots account of a couple compressibility dives in a 47.

    I'm really looking to see which plane was able to accelerate in a dive the fastest, not necessarily the highest top speed. I thought knowing the terminal velocity of each plane might shed some light on this.
     
  4. drgondog

    drgondog Well-Known Member

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    So, do you have any facts from say test runs which specify comparative performances in a dive between a P-47 and P-51?

    The 51 tended to out accelerate the Jug initially, with Jug catching and moving ahead but I have never seen the limit dive Mach number for the 47 so have no idea what it was... so hard to know for sure whether the 'wings would come off for the dive at max permissible speed.
     
  5. Negative Creep

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    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
     
  6. drgondog

    drgondog Well-Known Member

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    no - nowhere close

    The 51 D made .85 Mach in an RAF test before cutting power back - but the 38 still had early comprssibility issues at .7-.74 M. I have never seen any data on the max dive for a 47 but I am sure it was in the range of a 51 and lower than the Spit with its thin wing.
     
  7. marshall

    marshall Member

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  8. drgondog

    drgondog Well-Known Member

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    Good article - first mention of .83Mach dive that I have seen of the P-47.

    I know the limit dive recommendation for the 51 was .75 Mach but it routinely did .83 and at least one test run at .85 M.
     
  9. davebender

    davebender Well-Known Member

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    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.
     
  10. drgondog

    drgondog Well-Known Member

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    Dave - What I have read is that in comparative tests the 51 out accelerated, the Jug caught up and forged slightly ahead until compressibility was encountered.
     
  11. davebender

    davebender Well-Known Member

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    How long did this require?
     
  12. drgondog

    drgondog Well-Known Member

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    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."
     
  13. billswagger

    billswagger Member

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    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.

    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..
    Some of these 47 pilots killed themselves attempting to dive in a 51 the same way....yikes..simply because the 51s would crumble.
    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.
    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."


    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.
    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.

    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.....

    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
     
  14. red admiral

    red admiral Member

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    An initial dive rate is down to the cleaness of the airframe and the power of the engine which is why the P-51 pulls ahead. Later in the dive, at a steeper angle, the mass of the aircraft itself comes into play more, and the P-47 will start to accelerate faster.

    The Tempest is probably the fastest diving piston-engined fighter of the war. Its a relatively heavy airframe coupled to a powerful engine that accelerates quickly (high rpm = fast engine response (usually)) and a nice thin wing giving a fairly high Mcrit.

    Mcrit for the P-38 is around 0.69 and its not much better for the P-47 due to the thick wing. The actual speeds that could be attained could be higher in practise but have some measure to risk attached to them.
     
  15. drgondog

    drgondog Well-Known Member

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    All Jug drivers should have easily (and safely) outdived a 38.
     
  16. billswagger

    billswagger Member

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    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. 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.


    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.

    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/

    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




    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???

    This is where TAS would be valuable...and maybe even knowing terminal velocity.


    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
     
  17. drgondog

    drgondog Well-Known Member

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    The USAAF report I cited from Jeff Ethell's book Mustang indicated that the 51B-1 tested against 47D-10 (first paddle blade production model) out accelerated the Jug and then kept the lead.

    Proportionately speaking the D was 10% heavier than the B-1 and had 15-20% more Hp depending on altitude (the supercharger on the 1650-1 and -3 were set differently from 1650-7 in late model B/C and all D/K's

    As you noted earlier there was a huge increase in drag in all prop systems the faster you went... it was not a friend to either the 51 or the 47,

    Trying to calculate the terminal velocity without very good data regarding drag, prop activity factor, exhaust thrust and even possible Meridith effect of 51 radiator cooling system - that is why the dove the damn things until somebody died - then backed off a notch.
     
  18. billswagger

    billswagger Member

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    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
     
  19. drgondog

    drgondog Well-Known Member

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    Bill - the problem with anecdotal personal accounts is that they are neither comparative in actual and verified metrics but they depend heavily on personal predjudice (frequently).

    The simple facts are a.) the Mustang did a better job air to air against the Luftwaffe than the combined P-38 and P-47 (in half the time), and b.) did a far better job of destroying German aircraft on the ground than the combined P-38 and P-47, and c.) had better ratios of enemy a/c destroyed (air and ground) to losses than both the P-47 and P-38.

    At the end of the war the USAAF had the opportunity to evaluate all three fighters and made a decision to retain the Mustang at the expense of the P-47 and P-38 when future production costs were not a factor - just based on inventory on hand and perception of value going past WWII at the dawn of the jet age.

    So far we have on this thread one USAAF comparative report that says the P-47D-10 did not even outdive a P-51B-1 (10% lighter than a D and 15%less horsepower than a D), a test pilot report that the P-47D could achieve .83 M in a dive and an RAF report of a .85 M test dive in a Mustang IV, as well as a personal anecdote of a pilot who claimed 700mph and 14 reasons why the P-47 was a better fighter.

    Here is the RAF test of a Mk IV which went all the way to .85 M

    Note on Dive Tests on ‘Mustang IV’


    Absent another, and contrary, test report specifically comparing the Mustang to the Thunderbolt in a dive and stating the P-47 superior - why would you stiil believe the P-47 conclusively could dive faster?
     
  20. billswagger

    billswagger Member

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    :D cause i do....

    there is no doubt that the mustang might have been a superior air to air and escort fighter, but i'm not convinced that the 51 could dive faster than the 47.

    You can spit numbers and facts out of a flight test manual, where the planes are tested in optimum flight conditions. I'm talking normal everyday performance.


    Did they ever use the P-51 to see if it could break the sound barrier....(post WW2) with the Curtis experimental props/??

    Did they ever use the P-47 with the experimental props???

    I'll have to look that up too.

    Thanks for your input...I do get a lot out of these dissenting opinions that i have. People such as yourself throw a lot of information my way.


    BTW, i haven't looked at actual numbers, but i've read several articles that say the p-47 was the most successful fighter, when you talk kill ratios and targets destroyed. It could've been the sheer numbers of them, 15,000+, that were deployed in the war that might generate this opinion

    .......and there was also the p-47M and P-47N, that saw combat late in the war. The P-47J was an awesome plane, but was never massed produced because it was later decided it would save time to use the C and D airframes at a cost of a fraction of the performance....hence the M and N were born.

    You can have your opinion, as do I, i still believe the p-47 out dove the 51.

    You certainly are correct about a pilots account having some biased, but if the man flew both planes and still says the 47 could dive faster, i think their is some merit there. I'll see if i can't dig up another pilots account. :)
     
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