Engine vs pliot placement relative to neutral pt

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by stankap, Feb 6, 2012.

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  1. Feb 6, 2012 #1

    stankap

    stankap

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    Another question for the group:
    The aircraft design I am working on places the engine a little further foward than necessary for aesthetics, thereby requiring the pilot a little further aft. I realize there is a weight penalty with this (Add'l structure), but what does it do to aircraft inertia realitve to spins, and recovery from them?

    Is there a crtitical Inertia vs tail surface size and location?

    Attached is a sketch of the preliminary design.

    Thanks,
    Stan
     

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  2. Feb 6, 2012 #2

    gordonaut

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  3. Feb 6, 2012 #3

    AV8N247

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    I hate to disagree with another AE with test experience (sorry, Gordon), but inertia about the vertical axis is critical in spins. The Pitts airplanes spin very well (per design) and also come out well, too (also per design). But there are also several documented cases of people offsetting propeller/engine compartment weight by putting lead in the tail section. The results have been flatter spins ... with some unrecoverable. My advice would be to consult with the acro guys in your area (or those you feel comfortable with). It's probably not an issue, but it's always easier to think about it when you're on the ground.
     
  4. Feb 6, 2012 #4

    gordonaut

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  5. Feb 6, 2012 #5

    ultralajt

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    This is more important than moments of inertia:
    If you are worried about spin recovery, move your horizontal tail more aft, so that elevator will hang aft of the fuselage end.
    This will remove blanketing of the rudder by horizontal tail, and help recovering from the spin.
    It is not the best idea to have elevator and rudder hinge line at same station on the fuselage centerline.

    Mitja
     
  6. Feb 6, 2012 #6

    gordonaut

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    Last edited: Sep 26, 2012
  7. Feb 6, 2012 #7

    Aircar

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    The biggest problem with aircraft that place the pilot(s) far aft (or forward for a pusher) is that variation in pilot weight very much affects the Cg --and has a very much large -first order- effect on spin behaviour and handling .
    The tandem tractor aircraft flown from the rear seat (eg Cub) is the classic example where a good tail volume is needed to handle pilot weight range(the second pilot is on the cg) but some two seat side by side aircraft with the pilots both aft of the CG and the engine far forward are a bad combination not just from the 'dumbell' effect of moment of inertia .

    The Alvarez Poliwagen was such an arrangement and sought to reduce the cockpit depth by placing the pilots aft of the mainspar and moving the engine forward to balance (sitting on the mainspar as usual -about on the empty and loaded CG takes the pilots out of the balance equation pretty much --and even the change from two heavies to one light is not critical, but it means the fuselage depth is much greater ) --such tractor aircraft are at their worst with a heavier pilot (unlike gliders or pushers) or pilots which is both heavy and high moment of inertia together --opposite to the case for pilots ahead of the wing .

    The Polliwagen "clone' called Cobra was dynamically unstable with myself and another (much heavier) pilot on board and this experience gives some reason to suspect other 'pilot/s aft -motor forward' layouts -eg the Questair Venture -look at photos of the pilot location... (and Duncan's Razorback might also encounter this problem.
    Incidentally I had not known of either his "inspiration" from the Venture OR aspiration for long distance over water flying until just blundering across an old thread that revealed both (it's in the 'reactived' sidebar from a day or two back ) --so my comments about the similarity to the Venture and the lack of need for exotic Fowler flaps "because you are not planning intercontinental range like the Garrison Melmoth" were both rendered 'redundant' but nobody brought either to my attention . Pilot placement certainly is a prime factor in getting a tolerant aircraft to payload changes --in this case it may be a single seater so having a lower total range of pilot moment and could be accomadated with a moveable ballast weight for example (as do Robinson helicopters or side by side or staggered seating gliders )
     

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