Calculating the VNE ?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Pops, Feb 10, 2014.

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  1. Feb 10, 2014 #1

    Pops

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    To open a can of worms. How do you calculate the VNE of an aircraft? When the test pilot chickens out. :gig:
    Dan
     
  2. Feb 10, 2014 #2

    stol

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    Find a new test pilot that wears a parachute....:gig:
     
  3. Feb 10, 2014 #3

    Himat

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    Remote Control?
    Make it a real fishtail.

    More serious, when any gust or control input do over stress the wings?
     
  4. Feb 10, 2014 #4

    bmcj

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    Vne can be based on any number of criteria, and depends on which factor is the slowest. By 'any criteria' I include structural limits, flutter limits, controllability, etc. I believe we have discussed that here before... a search should come up with something.
     
  5. Feb 10, 2014 #5

    dcstrng

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    There's an EAA (I think) Spreadsheet tha does a decent estimation (with charts), but I guess is too large to attach -- can google it...
    [TABLE="width: 320"]

    [TD="colspan: 4"]Air Loads Spreadsheet for Sport Aviation[/TD]


    [TD="colspan: 5"]-Based on FAR 23 Appendix A and CAM 04[/TD]

    [TD="colspan: 4"]-Written by Neal Willford 12/16/02[/TD]


    [TD="colspan: 5"]-Simplified design loads for aircraft under 6,000 lbs[/TD]
    [/TABLE]


    See: http://tinyurl.com/l9u7d4o
     
  6. Feb 10, 2014 #6

    Pops

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    Bob Barrows and I had a long discussion about this subject a few years ago and wondered what every ones answer would be. Dan

    Added -- Did a search before I started this thread and came up with zero.
     
  7. Feb 10, 2014 #7

    Dana

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    You can calculate a Vne based on air loads and the aircraft's structural strength, but you wouldn't want to reach it... the Vne that goes in the operating limitations is the fasted speed actually tested.

    Himat, when a gust overstresses the aircraft, that's how maneuvering speed, not vne, is determined (in simplest terms, the speed at which Clmax imposes the limit load).

    Dana

    I've noticed that the press tends to be quite accurate, except when they're writing on any subject I know anything about.
     
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  8. Feb 10, 2014 #8

    nerobro

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    An airframes VNE is defined by the part that fails at the lowest airspeed. There are many examples of this, here's a few of them.

    1. SWAG: A number determined by the builder, or designer that only relates to something in thier mind. AKA: "They made it up"
    2. Gust response: If your expected wind gusts are enough to push your airframe past it's design loads, you need to set your VNE to a number less than that.
    3. Flutter: As speeds go up, the torsional rigidity of surfaces matter, and your surfaces can flutter. Your VNE may be determined by testing, as (FAA says 10%) some number less than the speed flutter begins.
    4. Engine RPM: If you have a fixed pitch propeller, your VNE may be a speed at which the engine overspeeds.
    5. Other structure issues: If your use thin, or light coverings, there may be a speed at which those will tear, crush, or rip.
    6. Compressibility: Mach effects change where lift is made on an airfoil. They also make trailing edge edge devices less useful, so you may lose elevator authority, or roll authority.

    I'm sure I can come up with others.
     
  9. Feb 10, 2014 #9

    autoreply

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    VNE is only an operational speed. For certification you mostly use VD. VNE usually can't be more than 90% of Vd.
    A few criteria for Vd. The lowest of these sets Vd and thus limits VNE
    *Highest test flight speeds, full controlability there etc.
    *Gust load (7.5 m/s) @ Vd. Especially for planes with a wide speed range this is often one of the most severe load cases. For strutted planes (multiple struts, fabric) it can also be a PITA due to the very rearward loading.
    *Flutter free up to Vd for all modes.
    *CS 22; drag with spoilers >0.707*MTOW @ 0.9*Vd (thus VNE). PITA at higher density altitudes, I think it's now strictly IAS.
    *VNE<WOT speed at least draggy condition. A major problem with microjets. In some regulations, 1.2Vh<Vd
    *Chute deployment speed, or the highest chute deployment loads your airframe can survive. Good for banging your head in a wall. a LOT.

    Note that neither gust loads, nor flutter scale with the IAS (high-altitude operation). Flutter speeds are halfway between IAS and TAS. Gust loads may be assumed to linearly decrease above a certain altitude.
     
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  10. Sep 10, 2019 #10

    Charles_says

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  11. Sep 10, 2019 #11

    Charles_says

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    Link is dead...
     
  12. Sep 10, 2019 #12

    Dana

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    That's because the thread is 5 years old...
     
  13. Sep 10, 2019 #13

    Charles_says

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    Yep! Thank you.
    Just wanted to defray the frustration of others.... since it was quoted recently.
     

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