All Moving tail question

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by berridos, Jan 20, 2011.

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  1. Jan 22, 2011 #21

    Jay Kempf

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    What is the tradeoff and what regimes does this work in. Most of the modern VERY high speed fighters are using all flying tails that are all electromechanically mixed into all function of roll pitch and yaw in servo loops. But there is a lot of FBW divine intervention going on there. So at what speed and for what purpose, like avoiding flutter or PIO using mechanically connected controls with neutral stick force balance points does one avoid an all flying tail. I have been looking at Vtail all flying surfaces. Makes me wonder if the rudder vs. Elevator functions would interfere with each other? Orion?
     
  2. Jan 23, 2011 #22

    flat6

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    the mechanically actuated vtails use mixers to combine the movements. one very nice example is used in the davis homebuilts.
     
  3. Jan 23, 2011 #23

    HumanPoweredDesigner

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    It is starting to sound to me like the higher the percentage of the tail surface that is fixed (vertical or horizontal stabilizer), the more stick free automatic recovery is provided. But the higher the percentage of the tail surface that is moving (rudder, elevator), the smaller, less draggy, and more authority the pilot has during planned moves. I hope there is a way to get both.

    ...

    Maybe the pilot could lock surface/controls with the push of a button. Flying rudder turns into fixed fin at any angle. If you get into a spin and are not sure what to do, put the stick in the center position and flip the lock switch.
     
  4. Jan 23, 2011 #24

    flat6

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    the most common method seems to be the use of anti servo tabs.
     
  5. Jan 23, 2011 #25

    autoreply

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    Any idea why we don't see them more often? Other drawbacks maybe?

    I've noticed two more drawbacks, though I guess both have a lot to do with those particular designs. A stalling stabilizer (Janus-A) and divergent behavior at high speeds (Std Cirrus).
    I'm pretty sure the latter is a matter of improper/insufficient design for flutter, but other have pointed to the large inertial mass (stabilizer has to be balanced around 0.25C) and the way the stick is hinged, which promotes oscillations. Never dared to try it, but supposedly, after you'd let go of the stick for over 2 seconds it started oscillating the whole aircraft violently with completely divergent pitch.
    That stalling stabilizer is a matter of a too large deflection of the stabilizer I think.
     
  6. Jan 23, 2011 #26

    flat6

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    they need anti servo tabs. check this out: ch6
     
  7. Jan 23, 2011 #27

    PTAirco

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    Nobody has mentioned the substantial and fairly sophisticated hinge/pivot mechanism required for an all moving tail. Not exactly as cheap, simple or light as a pair of piano hinges. It needs to be very rigid and free of all play or you'll have some interesting flight characteristics. It would probably more than offset the weight saving you get by using a marginally smaller surface.
     
  8. Jan 24, 2011 #28

    flat6

    flat6

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    like i said its still possible to use piano hinges. check out the jodel d11. in a volksplane the bearings are nylon.
     
  9. Jan 24, 2011 #29

    autoreply

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    I think, PTairco is referring to an ALL moving tail. So a fixed fin/stabilator combination that as a whole can pivot in both directions. An all-flying tail or fin should indeed not be too much harder compared to a normal elevator/rudder.
     
  10. Jan 24, 2011 #30

    flat6

    flat6

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    exactly. piano hinges can actually be used.

    cut off some of the rudder on both sides of quarter chord spar and just bore a hole big enough for the rudder spar to swing around and put a rudder post right next to it. then hook up the rudder and the hinges. then you got a buried fin. you can even let the fin poke up a little and make yourself a neat little dorsal fin.

    i prefer this method than actually using a tube spar and mounting them on big bearings.
     
  11. Jan 24, 2011 #31

    Lucrum

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    If an all moving HZ stabilizer is mounted at it's AC are the anti servo tabs needed? I mean if its really aerodynamically balanced. I ask because I was thinking in terms of springs mounted on the control linkage to provide control force feel.
     
  12. Jan 24, 2011 #32

    flat6

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    im sorry i misread your post!

    I dont know how THAT is going to work. i cant even begin to wrap my head around an all moving u tail.
     
  13. Jan 24, 2011 #33

    flat6

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    i think that can work too. seems to be more effective than antiservo tabs beyond high subsonic...
     
  14. Jan 24, 2011 #34

    orion

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    A symmetrical surface mounted at its ac will of course have no feel at all. Putting springs into the control system will help but could be a contributor to some kind of harmonic response. However, maybe coupled with a damper the system could give you the feel and yet be dampened enough for near normal response. You could also introduce a bit of feel if you move the hinge point just ahead of the ac.
     
  15. Jan 24, 2011 #35

    Lucrum

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    Thanks, that's almost exactly what I was thinking I had read/understood but it's nice to hear it being reinforced, in case I misunderstood.
     
  16. Jan 24, 2011 #36

    berridos

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    Regarding the vertical axis movement (rudder) of the U tail maybe the best solution to distribute the weight along the fuselage would be to install a damper at the tail rotating tube and adjusting the control feel right at the pedals.
    Avoiding antiservo tabs would be crucial.

    More complex would be to find a solution for the antiservo function and the trimming function of the ht section.
    Anyway I think due to the rotating mechanism pushrods are imposible. Everything should be done with cables that adapt to all the rotating stuff.

    I am going thru engineering books to calculate the tube diameter and all the details. I really dont manage to interpret correctly the tail loads according to FAR 23.
     
    Last edited: Jan 24, 2011
  17. Jan 24, 2011 #37

    Jay Kempf

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    The case of an all flying V or U tail is a tough one. The place where it intersects the boom it is attached to could have a gimbel to give it two degrees of freedom and that gimbel could be controlled with pushrods. However, the fact that the tail is not symmetrical about the hinge axis is problematic. For instance an all flying tail is symmetrical about the horizontal plane in through the central chord of the symmetrical airfoil and hinge meaning it has no natural moment about the hinge at attachment. In a V or U shape there is a ton of area asymmetrical to the elevator hinge that would be dragged back by flow and create a non-neutral pitching moment. I have drawn up many of these setups and arrived at a split or scissor tail for that reason. Each side could have a gimbel and be neutral and the combination of the mirrored sides give all of the tail functions and also allow for differential steering in the two tails. Of course this only works on twin or multiple booms.

    There seems to be no way to make the U shaped tail axis neutral in both functions. If it is fixed with movable control surfaces (4) it seems it would be way draggier during most operations than the small amount of interference that it would replace. Course getting the surfaces out of the prop blast is a worthy goal.
     
  18. Jan 24, 2011 #38

    orion

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    Boeing actually tried to propose a "U" shaped tail of sorts on a single gimbal for the JSF program - it didn't make it.
     
  19. Jan 24, 2011 #39

    berridos

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    In a V or U shape there is a ton of area asymmetrical to the elevator hinge that would be dragged back by flow and create a non-neutral pitching moment.

    What problems should I expect in this case? You mean that the generated pitching moment would make the tail very draggy?...resonant at different speeds?

    Boeing actually tried to propose a "U" shaped tail of sorts on a single gimbal for the JSF program - it didn't make it.

    Is a failure on this concept in JSF program relevant for a 140mph plane?


    To make the tail neutral on two of the three axis I would avoid sweep in the vertical surfaces. However if I introduce a little sweep in the fins I could get some passive control feeling.
     
  20. Jan 24, 2011 #40

    Jay Kempf

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    If you project the area of the vertical rise to the vertical plane that intersects the thrust line you get the shape of a rudder. You have two of them with a V or U shaped tail. Much like if you project one wing onto the horizontal plane you have a large draggy item on one side of the axis. So it generates a large torque imposed upon the axis in the case of the tail, pitch down, and increasing in magnitude with speed. The drag spar in a wing is placed on the diagonal to couteract this torque. In an all flying tail this force would always be present in the stick unless cancelled somehow. It doesn't matter whether it is a JSF or an ultralight. In the JSF I would imagine that the control forces are not present at the input device so the computer intercepts that imbalance and then calculates and creates a hydraulic force to balance the force diagram. Hope this makes sense.
     

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