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  1. Nov 17, 2009 #41

    bmcj

    bmcj

    bmcj

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    Topaz' example was just an arbitrary value chosen because of it's familiarity. As for a surface canted more toward horizontal, that would probably be correct since both sides contribute to the projected vertical plane, so each side need only enough vertical to account for half the rudder area. A further advantage to this is that the opposing/cancelling forces I mentioned earlier are reduced with this configuration. However, I can see a situation where the projected area may be sufficient, but the effectiveness is too limited due to being too close to horizontal.
     
  2. Nov 17, 2009 #42

    orion

    orion

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    In discussing V-tail design and vertical requirements it is important to remember that only the net projected area counts - in other words, it does not matter that there are two surfaces. Because of the control behavior and the balance of forces, the two surfaces combine to work as one so in calculating the effective area only the net projected area is used.

    I only know of one airplane that used a 45 degree tail, that being the Bumble-Bee designed by Walt Mooney and his group. Because it was a small (relatively short) airplane that did not need a wide allowable CG range they compromised with the tail configuration since they wanted to provide it with better yaw stability than was generally evidenced with lower angles.
     
  3. Nov 17, 2009 #43

    bmcj

    bmcj

    bmcj

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    Orion, could you clarify this point for those of us with cloudy brains. Do you mean that if each side has a projected area of 4 square feet (each), that the net effective rudder area is only 4 square feet and not 8 square feet?

    Also, if that is true, does that same implication affect twin rudders like the Ercoupe?
     
  4. Nov 17, 2009 #44

    wsimpso1

    wsimpso1

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    Orion has discussed this beforeso he might be reluctant to repeat it AGAIN. The problem is that the two surfaces are close enough to each other that they interact pretty strongly. The flow from one influences the flow from the other. Another way to view it is that a single vertical tail that is skidding or has a deflected rudder would produce high pressure on one side and low pressure on the other. With two near each other, and with the roots right next to each other, think about the pressures in the "inside" faces. High pressure is trying to happen on one, and low pressure is trying to happen on the other, but they are tied together at the root and the low is pulling air from the high... The air pressures on the inside of the V ends up having little difference from one face to the other. Most of the pressure difference is thus between the outsides - so it ends up being the same as having only one times the projected area of vertical. This effect also reduces effectiveness in pitch...

    Remember also that with vertical and horizontal tail inputs mixed together, you can only get full deflection in pitch if you have zero deflection for yaw, and vice versa, so you end up needing more than the amount projected to get the same crosswind and forward CG capabilities as a straight tail.

    By the time you are done "saving" wetted area, you would have as lower drag and better flying airplane with conventional horizontal and vertical tails.

    Oh, and while we are at it, (THREAD DRIFT!) spread the vertical and horizontal tails at least 40% of horizontal tail chord to reduce interference drag and do not sweep the tails unless you are sweeping the wings.

    Billski
     
  5. Nov 17, 2009 #45

    bmcj

    bmcj

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    Thanks Billski, that makes sense. I've always been of the opinion that the compensating factors needed for a v-tail outweigh the benefits. The only reason I can think of for designing with a v-tail is for appearance; any other benefits of the v-tail can be had with a conventional tail and good design work.
     
  6. Nov 17, 2009 #46

    Joe Fisher

    Joe Fisher

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    Back in the early 1980's I flew a Sunburst ultralight. It has an inverted V tail with rudder connected to the pedals. The stick was connected to wing spoilers. The spoilers had no preservable effect on the control of flight. The rudder provided nearly perfect cuppled control of yaw and roll.
     
  7. Nov 17, 2009 #47

    Starman

    Starman

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    thanks billski and orion

    problem solved :)

    in the (inverted) v tail design i was considering the two tails are separated by about two feet at the closest point and the prop is blowing on that area too so most of both vertical projected areas should count, shouldn't it?

    what if you take the prop out of the picture? pressure would still migrate from one side to the other even though they aren't connected, so the amount of separation counts, probably requiring at least as much separation as there is vertical tail height.

    of course even tractor designs have the prop blowing on the tail, but the tails are connected and the prop is far away and the prop blast is messed up (technical term) by the fuselage.
     

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  8. Nov 17, 2009 #48

    zk-jkw

    zk-jkw

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    NACA Report 823 has fairing good coverage of everything discussed

    http://ntrs.nasa.gov/search.jsp?R=960233&id=1&as=false‚ą®=true&qs=Ntt%3D823%2Bvee%26Ntk%3Dall%26Ntx%3Dmode%2Bmatchall%26Ns%3DHarvestDate%257c1%26N%3D0

    In the conclusions:

    The use of a vee tail will probably provide no reduction
    in area unless the conventional vertical tail is in bad canopy
    wake, unless the usually higher location of the vee tail places
    it in a region of greatly reduced downwash or unless the vee
    tail has a higher effective aspect ratio than the conventional
    horizontal and vertical tails.
     
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  9. Nov 18, 2009 #49

    Starman

    Starman

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    considering the pros and cons, i think i'll skip the sloping tails, unless it ends up looking sexy when i get my 3d program
     
  10. Nov 18, 2009 #50

    wsimpso1

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    Airfoils influence the air flow for at least a wing span and/or several chord lengths normal to the foil, and so the foils interact with pressures too. Taking the physical connection between them reduces the interaction, but you will still have strong interactions which weaken the effect of each foil.

    Even biplanes have this effect, which is one of the reasons you use stagger . With one wing forward or aft of the other, your lift is bigger then, but it is never up to what you would expect for the wing area you have...

    You do not want to place any of the foils so that it intersects the axis of the prop. Prop pulses will really be felt by the surfaces... Once you do that, you can pretty well avoid driving powerful resonances.

    So, no Steve, that tail arrangement does not solve it. Cleanest thing to do is to place the horizontal above or below the centerline of the prop plume but within the prop plume, use a vertical off each boom. This minumizes driving resonance while allowing power to increase effect of both tails. It also minumizes the number of interfaces which are draggy too. And it allows you to shove the horizontal and vertical apart some (longitudenally). Staggering the position of max thickness reduces interferences and reduces pressure wrap from one surface to the other.

    Billski
     
    Last edited: Nov 18, 2009
  11. Nov 18, 2009 #51

    Starman

    Starman

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    thanks billski, i understand. i had thought about the prop pulses being felt with the style in my last drawing. i'm still undecided as to whether or not i want a horizontal :)

    it turns out i didn't just sprain my wrist but broke two bones. broken bones are actually better than broken ligaments because they heal 100%, and faster too.
     
  12. Nov 18, 2009 #52

    autoreply

    autoreply

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    As I pointed out before (but not so clear) while doing some simulations (with an angle of 105 degrees IIRC the major problem is the aspect ratio of the "fin". A normal fin has an extremely low aspect ratio making it possible to operate at huge angles of attack, like a delta does. For an elevator (that's normally deflected slightly) this results in quite a lot of extra trim drag. Almost all extra drag came from this extra induced drag on the tail which is the major performance-drawback of a v-tail.

    The interference drag is normally only there during take/off and landing and during single engine operations. Thus for a single engine aircraft a v-tail could work.
     
  13. Nov 19, 2009 #53

    K-Rigg

    K-Rigg

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    what was Leeon Davis obsession with v tail? he used it in every design... was it ease of construction? More efficient to him? cool looking?
     
  14. Nov 19, 2009 #54

    wsimpso1

    wsimpso1

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    There is something to be said for cool factor. Marketing is part of the process of selling airplanes.

    At one time, the story of it being more efficient and thus lower drag was promoted and/or bought into by many. Remember, even Beechcraft got into the act, and they did have their issues with the V-Tails.

    Not surprising that someone else might have fallen into it...

    Billski
     
  15. Nov 19, 2009 #55

    autoreply

    autoreply

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    There is one real advantage, one tailsurface less which slightly lowers complexity.

    Aside from that I agree with Billski that the cool thing is probably 90% of it.
     
  16. Nov 19, 2009 #56

    bmcj

    bmcj

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    I think Billski hit the nail on the head!
     
  17. Nov 20, 2009 #57

    K-Rigg

    K-Rigg

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    in looking at the DA-2, i noticed the v tail looks small compared to what i would expect for a v tail on a short tail of an airplane. But reading a article, i noticed a picture of the tail and how it is controlled. Davis has the whole vtail movable for a control surface with a tab on the back. how does this help the vtail design?

    untitled.JPG
     
  18. Nov 20, 2009 #58

    K-Rigg

    K-Rigg

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    also what are the tubes at the top of the tails for? Mass balance?
     
  19. Nov 20, 2009 #59

    bmcj

    bmcj

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    The tubes are the Laser Blasters! :gig:

    Actually, I suspect they are mass balances.

    Full moving surfaces will give you more control authority, but it does nothing for static hands free stability. Surface area is the only solution for that.

    As for being small, only a proper analysis can confirm that.

    Bruce :)
     
  20. Nov 20, 2009 #60

    Topaz

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    And a lot of older homebuilts had rather 'relaxed' stability by our current standards. Small tails were quite common.
     

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