WW2-fighter and critical Mach speed

Discussion in 'Flight Test Data' started by delcyros, Mar 10, 2005.

  1. drgondog

    drgondog Well-Known Member

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    #81 drgondog, Nov 9, 2015
    Last edited: Nov 9, 2015
    I suspect by the icon displayed under your username, that Joe has eased your way from this community. As a parting observation your rants remind one of Sam Kennison, the former 'comedian'. Maybe we will reconnect on some other forum and continue the dialogue?

    I am really curious regarding your body of work and the experiences that led you to the rants you have posted?
     
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  2. drgondog

    drgondog Well-Known Member

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    Greg - not a problem on the 'name'. We use our full names so seldom.. as to P-51 time 'and not in a two seater', well it was a two seater, which is how I learned to fly the damn thing. My father would not have given me a check out standing on the wing and wave 'bye'. Momma would have kicked his ass.

    The value of in-air instruction, observation and coaching tips can not be overestimated for relatively low time pilot transitioning from AT-6..
     
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  3. GregP

    GregP Well-Known Member

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    #83 GregP, Nov 10, 2015
    Last edited: Nov 10, 2015
    Hi Bill MARSHALL,

    I figured Mr. Relax might needed a dose of reality and took some poetic license. I think I was somewhat of a hair trigger some 2 - 3 years ago myself. It probably had a lot to do with being unemployed. Not so anymore. Busy teaching electronics at a college and find myself much more, if you'll pardon the expression, relaxed ... nothing whatsoever to do with Mr. Relax.

    I don't have anything to prove at this time and I really DID get aeronautical training ... but haven't used it much since 1971. And I moved from a 2800 sq foot house into a 1000 sq foot apartment and only have two books out on aerodynamics, and neither one is a detailed treatise ... sort of basic intro stuff. I promise you I have 'em buried away, but I also don't exacctly recall the exact sequence of things or the entire set of concepts exactly since I haven't looked at it much over the last 40 years.

    I'll probably just take your word for aerodynamics at this point until and unless I can get somewhere to unpack and study for awhile. When that happens, I'll probably just wind up agreeing with you anyway, Bill.

    I started to do an aerodynamics spreadsheet from a Navy aerodynamics text and just ran out of time. Each and every class I'm teaching for the next year will be completely new to me and I'll probably spend most of my time just staying ahead of the classes. After the first time, it gets markedly easier, as I'm sure you know.

    And way back when we DID do some FEA, but we called it "Numerical methods" and it took a large deck of punchcards to get what we can do these days on a spreadsheet, so it probably didn't go quite as far as today's software. It was the best we could do at the time with "double precision floating point numbers!"

    The PC I just bought today has a 6th-gen Core i7 processor that would probably put all the computers in the world, all put together, in 1969 to shame. We've come a long way, baby ... and we can now get the wrong answer much faster, and with far more precision.

    Sorry about the "Senior moment" there, Bill.



    Senior moments:

    View: https://www.youtube.com/watch?v=Xv1tMioGgXI
     
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  4. drgondog

    drgondog Well-Known Member

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    Greg - you have zero to apologize for, now and in the past.

    As to FEA using punched card stacks to define a model much less solve one boggles the mind. The numerical relaxation methods were barely emerging in your time at Purdue.

    My Computer use career started during a co-op stint at NASA to help develop a low orbit atmospheric degradation algorithm using Bessel functions in the mid 60's - Fortran II and IBM 600 series with (IIRC) 16K of memory.
     
  5. GrauGeist

    GrauGeist Well-Known Member

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    16K memory in the 60's? Lord have mercy, that must have cost a fortune!!

    My late 80's i386 computer had a whopping 8mb (when typical systems had 2mb or 4mb) system memory with a Seagate ST225 hard drive (25Mb capacity) and this beast cost a paltry $2,750! :lol:
     
  6. drgondog

    drgondog Well-Known Member

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    Dave - I have no idea how much it cost, but it was somewhat self contained and you loaded the JCL, the data and the Fortran application to initiate the job. Sub routines and dimension statements had to be carefully examined for potential memory issues - either capacity or overlay.
     
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  7. GregP

    GregP Well-Known Member

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    #87 GregP, Nov 11, 2015
    Last edited: Nov 11, 2015
    That's hilarious. The first computer I used in 1968 at Purdue was an IBM 1130 programmed in Moron (no kidding .... LIBF $READ, etc .... which was library file read ...). The first language I ever used that was "high level" was Fortran II! We moved on to the PUFFT (Purdue University Fast Fortran Translator) programmed by Jerry Rubin, who I believe at the time was 16 years old and had just gotten his PHD. We still used punchcards since that's what the students were allowed to use at the time. I was in hog heaven when I got one of the first HP 67 calculators that would read little strips of magnetic tape and run a 200 or so line program! At least it didn't have nixi-tube displays. It was a red 7-segment LED display.

    I seriously doubt that the calculations we did amount to a full-fledged FEA, but they seemed like a LOT of calculations at the time. Of course, Neil Armstrong, when he landed on the Moon in 1969, had 16k of CORE memory! My first "home PC" was a Commodore Vic 20. It was a real POS but we felt like kings when we made the TV set do some rudimentary programming at home. Later I got a Commodore 64. We called it a Commode Door 64. but at least you could play a game or two on it.

    Enough of this thread hijacking.

    None of those damned PCs had a critical Mach number anyway. Most would only travel faster than stationary-and-nailed-down if you dropped them or hurled them across the room in frustration. I dropped one out of a Cessna 172 and was immensely pleased with myself at the time. As I recall, my circular error of probability was huge since I didn't even aim except to be sure I was away from people and homes, and was way out in the Arizona badlands.
     
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  8. MiTasol

    MiTasol Active Member

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    I agree totally

    At least you only have an occasional "senior moment" which is far better than suffering from CRAFT :<(
    (Can't Remember A Flipping Thing)
     
  9. GregP

    GregP Well-Known Member

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    Well, I sincerely hope I don't get there but, if someone does, at least they can't remember it.

    I always called it CRS (can't remember sh*t).

    I miss flying now (with the exception of the occasional warbird flight) but I absolutely don't miss paying for it. I'm planning on building an ultralight when I retire, but you never know. It might turn into a tractor to work the land. I DO plan on putting a CNC end mill in the shop for fun, but that could change, too, since I might not want to make airplane parts for restoration until I die. It might turn into a laser engraver or maybe a CNC router. I HAVE used one to make a beautiful instrument panel for an Embraer Tucano.

    That might be a reasonable hobby with the potential for some income.
     
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  10. Zipper730

    Zipper730 Member

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    This doesn't look right, I know a Spitfire VIII was tested up to Mach 0.891, though that was pushing the airplane into a territory that was beyond the placard limit (0.85)

    Was that the MMO or placard limit?

    Was this when supersonic flow appeared on the wing, when mach effects started to take hold (heaviness on the control column), or the maximum operating mach number?

    I wouldn't be surprised if that's the speed supersonic airflow started to form over the wing, but from what I was told around 0.68 was when control-problems began, with a total loss of control at Mach 0.74.

    [quoteP-47C: 0.69[/quote]I remember the prototype P-47's had significant problems in dives, and numerous changes were made to the elevators and stabilizer. I'm pretty sure the P-47D was tactically useful to 0.72 mach (I'm not sure how the C compared)

    I assume that's the MMO, right?

    Actually, from what I remember there was a narrow set of conditions where the Me-262 could actually exceed Mach 1, and some recent tests corroborated it. It might very well put the plane very close to structural damage or worse (it'd likely require both stab-trim and elevator application which can sometimes overstress the aircraft): I can't say whether Mutke managed to do it.

    From what I remember the placard limit of the P-51D was 0.75, and was dove once to Mach 0.84 or 0.85 with skin buckling occurring. I'm not sure how much different the dive speeds were with the P-51B and D, though the -D might have had more drag due to it's canopy.

    Was this the MMO or placard limit? I'm actually quite fascinated about the placard speeds for the Bf-109/Me-109 and Fw-190 variants: I do remember something to the effect that either the stabilizer or the rudder balance imposed some kind of flutter restriction that was eventually fixed and allowed dive speeds faster than the P-51.

    To some degree the thickness was made necessary because of the high aspect ratio: High aspect ratios produce more flexing of the wing, which require more thickness or more internal support members.

    I'm not entirely sure why they used such a high-aspect ratio to be honest.

    I think you might be mixing up several different scenarios.

    1. Spitfire Mk.VII/VIII's were fitted with a stronger wing than the previous designs: The placard limit was around 0.85, though tests were done up to 0.891. During at least one of these tests, the propeller came off and the pilot managed to glide the plane and put her down somewhere.

    2. During the development of the Miles M.52, they fitted a Spitfire PR variant with a stabilator: Dives were successfully done all the way up to 0.92 mach

    3. In 1952, an atmospheric sampling flight went awry and a maximum speed of 0.94 was achieved, and the plane barely recovered. I'm not sure to what degree the plane took damage.

    The kink didn't affect the dive speed?

    I thought loss of control was predominantly related to the shockwave strength, turbulent flow, and loss of the downwash until you were nearly sonic, when the loss of the upwash shifted the C/L to 0.50% MAC

    I didn't know spanwise flow would occur under these conditions. In fact I thought it was mostly an issue related to stalls.

    I didn't think the tail produced positive lift, merely it stopped producing most of it's negative load.

    What defines a safe dive-speed? Is this related to control surface deflection, g-load or both?
     
  11. fubar57

    fubar57 Well-Known Member

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    I'm grabbing some popcorn..........
     
  12. drgondog

    drgondog Well-Known Member

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    Too many questions and repetitive assumptions to comment on past this point.
     
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  13. Zipper730

    Zipper730 Member

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    drgondog

    Wait, if the plane couldn't even do Mach 0.83, why was that speed listed?

    Just to be clear, do you know what mach number the plane would either suffer control heaviness of a degree that recovery would be impractical/impossible, or that the tail would get blanked and tuck-under would occur?

    That makes sense because I was responding to several people :)
     
  14. drgondog

    drgondog Well-Known Member

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    Recommended reading:
    1. Aerodynamics for Engineeers - Bertin and Smith
    2. Introduction to Flight - Anderson
    3. Fluid Dynamic Drag - Hoerner
    4. Aircraft Design - A conceptual Approach - Daniel P. Raymer
     
  15. Zipper730

    Zipper730 Member

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    Firstly, if you were able to identify which book would be the most valuable, what would you recommend?

    Secondly, I'm not sure I'd be able to provide an answer if I read those books as math was never my strong suit (B/C average)
     
  16. pbehn

    pbehn Well-Known Member

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    Why ask a question if you cannot understand the answer? On most aviation subjects drgondog can lose 90% of this forum, on his real specialities he (and some others) can lose 99% of the forum. You are asking for answers to very involved questions about aerodynamics which you admit you dont have the math to follow, when given a simplified answer for a laymans benefit you demand more detail, there is no short cut to knowledge, you cannot know the subject without the math(s), if you don't have the math(s) just accept a potted version because it isnt witchcraft.
     
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  17. Zipper730

    Zipper730 Member

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    I'm not sure that's entirely accurate

    I'd like to re-frame it this way: Why would a person list Mach 0.83 as a number if it was beyond the critical mach number, the placard limit, a point where a loss of control would occur (if applicable at this speed) or in fact the aircraft's maximum mach number without structural failure? As I see it, this number is completely meaningless unless it was based on the shape without factoring in things like structural strength, which is purely theoretical and not data that reflects the facts.

    Yet some of what he's saying I can actually follow to some extent: There was an earlier thread I posted, then momentarily lost track of, and now I found again

    WWII A/C: Maximum Mach Number & Airspeed in Dives

    Some of what he was saying got me lost a little, but the basics of it I could actually follow on some level: A thinner wing tends to do better at a higher mach number, a lower camber wing tends to do better at a higher mach number, a wing with the crest further back tends to yield a higher mach number for the same t/c ratio.

    Based on what he explained there is a point where the airflow starts to go supersonic on the wing and that's the critical mach number, and on older aircraft, the placard dive speed was a little bit over that to the point and outlined the maximum safe dive speed. The maximum mach number that was allowed was the maximum speed you could dive at without risking the destruction of the aircraft (failure of the frame or component).

    i suppose it would be more accurate to say that if given the mathematics equation I'd probably require guidance on the more complicated stuff as mathematics has never been my strong-suit.

    As for the textbooks: I usually would not refer a person to a textbook if I was capable of answering the question; if I could not answer it any other way, I'd list the books and upon request at least, tell them which one I thought was the easiest to grasp and/or the most comprehensive.

    It would be wise to point out that it's preferable to read through one textbook over four, especially when you consider that it would be cheaper and I have a substantial book list :D
     
  18. pbehn

    pbehn Well-Known Member

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    Please dont suggest that some people here cannot explain what you are asking, the problem is you cannot follow the explanations without knowing all the theory behind it and so just ask more questions.

    For my tuppence, the limits are for the whole aircraft and while the wings are important there are other factors, a poster here did the calculations on a spitfire and it is the windscreen that is its worst feature.
     
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  19. drgondog

    drgondog Well-Known Member

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    Anderson's Introduction to Flight
     
  20. Zipper730

    Zipper730 Member

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    I didn't say they couldn't

    That would have never occurred to me...

    Thanks
     
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