Why so few V-Tails?

Discussion in 'General Experimental Aviation Questions' started by Algoa, Aug 17, 2016.

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  1. Aug 17, 2016 #1

    Algoa

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    Why do so few designs have V-tails? To my simple mind they look like they would weigh less and have 1/3 less wetted area.
     
  2. Aug 17, 2016 #2

    Swampyankee

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    They tend to weigh more, because there needs to be a mixer assembly to separate the pilot's pitch and yaw inputs and have about the same wetted area. To a first approximation, a V-tail needs to have a projected area on a horizontal plane the same as the horizontal stabilizer and a projected area on the vertical plane the same as the vertical stabilizer.
    "The V-tail may perform the longitudinal and directional trim role satisfactorily, but it has deficiencies in maintaining the aircraft longitudinal and directional stability. In addition, the V-tail design is more susceptible to Dutch roll tendencies than a conventional tail, and total reduction in drag is minimal." (Sadraey M., Aircraft Design: A Systems Engineering Approach, 2012, Wiley Publications)

    "V-tails combine functions of horizontal and vertical tails. They are sometimes chosen because of their increased ground clearance, reduced number of surface intersections, or novel look, but require mixing of rudder and elevator controls and often exhibit reduced control authority in combined yaw and pitch maneuvers." (Tail Design and Sizing)

    See also http://www.homebuiltairplanes.com/f...technology/10013-confusion-about-v-tails.html and http://www.homebuiltairplanes.com/f...-aerodynamics-new-technology/6407-v-tail.html


    They're not often used because they show little or no benefit. Certainly, the drag benefits are largely illusory: I believe no competition sailplane uses a V-tail, and these guys would do anything to keep the lift demons happy.
     
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  3. Aug 17, 2016 #3

    TFF

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    Most are sized too small; you get the V tail shuffle. Once sized correctly they look big.
     
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  4. Aug 17, 2016 #4

    larr

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  5. Aug 17, 2016 #5

    BoKu

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    The trouble is that it's the area that does the work of providing stability and control.

    Some practical observations from having owned and operated V-tailed sailplanes, and having manufactured and distributed parts for same:

    * The mixer weight and complexity is kind of a red herring. The HP-18 mixer in particular has surprisingly few parts.

    * The big advantage of the V tail is that it leaves you one less surface to build. But only if you consider each half of the horizontal stabilizer as a surface.

    * As another poster points out, when you size them properly they work great. But then they tend to look a bit larger than pleases the eye.

    * V-tails fell out of favor in the glider world mostly because of composite construction and awkward packaging. With a T-tail, the moments of the right and left halves of the horizontal stabilizer cancel each other out, so the removal attachment really only has to handle lift loads with a modest amount of margin for transient assymetric loading. Once removed, the horizontal stab is a relatively flat piece that is easy to find room for in the trailer. With the V-tail, you can't really have it be one piece, since that would be a huge awkward part to lug around and stow. And if you go to fold it, you have to react a bunch of concentrated bending loads of the sort that are awkward in composites.

    * Dick Schreder loved putting V-tails on his legacy HP glider designs because they worked well enough, were relatively simple, had low parts count, were easy to build, and very efficient in terms of materials. Case in point: The standard HP-11 through HP-18 ruddervator skins use almost exactly a 4' square of aluminum. When sheared to template, the scrap is a 3/8" x 48" strip of aluminum (less than 1% scrap).

    * Having good stability and control harmony usually dictates having different relative areas of elevator to horizontal stabilizer and rudder to vertical stabilizer. But with a V-tail, you are kind of locked into the same ratio. You can juggle the relative vertical and horizontal areas by adopting an included angle other than 90 degrees, but that can't address the ratios of fixed to hinged areas.

    That said, I think V tails are pretty and have a certain visual economy to them. I think they'll come back into style when we have elastomeric surfaces and fuselages that bend without discrete hinge lines.

    Thanks, Bob K.
     
  6. Aug 17, 2016 #6

    Himat

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    Most points against V-tails have been mentioned, but there are at least two more.
    - An upright V-tail does have a secondary adverse roll. That is, the rudder will tend to fight the ailerons in a turn. This is in most cases an unwanted feature.
    - It places a constraint on the sizing on the horizontal and vertical tail surfaces. On a V-tail the size of the two surfaces relates to each other and is constrained to a certain range of relative size. If the size relationship between vertical and horizontal area is outside this range, a V-tail is sub optimal.

    And V-tails have been a popular subject on this forum, here is another two threads:
    https://www.homebuiltairplanes.com/...ontal-tail-twin-boom-pusher-propwash-out.html
    https://www.homebuiltairplanes.com/...t-advantages-disadvantages-v-tail-design.html
     
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  7. Aug 17, 2016 #7

    nerobro

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    I read that the biggest benefit is the reduction in interference drag. Everything else more or less cancels out.
     
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  8. Aug 17, 2016 #8

    Topaz

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    After all the back and forth I've heard over the years, yeah, that's my impression as well. As Bob notes, there are some small construction advantages.
     
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  9. Aug 17, 2016 #9

    Highplains

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    Little if any drag reduction with a V-tail. Worse yet your rudder action is much worse. The angle of the V depends on what the relative areas of a conventional tail would be. Mostly it come down to solving the equation cosine squared plus sine squared equals one. So to check your math skills, set the vertical to 1/3 the area of the horizontal and if your angle comes out to be 120 degrees you got it.
     
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  10. Aug 17, 2016 #10

    nerobro

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    "Less of it." I really like how davis handled it. He's got a simple, seemingly robust, mixer, and then the tails are held on through one weldment.

    Now.. a real disadvantage of a V-tail is is one of the control rods fails. You lose control almost entirely.
     
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  11. Aug 17, 2016 #11

    BoKu

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    There's one other disadvantage of V-tails that doesn't get a lot of coverage. Depending on where the mixer lives, V-tails can have a lot more control inertia and friction than conventional tails, especially in pitch. At issue is that ruddervators are often sized to give enough authority for crosswind takeoffs and landings, and their drives are provisioned for the loads commensurate with those sizes at Va. So the surfaces tend to be on the large side, and they tend to have larger counterweights and larger push-pull tubes. And not only that, but for any pitch input you have to move both surfaces.

    In practical terms, that means that for an HP-18 or similar sailplane, when you move the stick you are moving 24 feet of 3/4" push-pull tube through ten guides in the aft fuselage. And since the HP-18 is a (relatively) low cost kit aircraft, those guides are simple plates of nylon with holes that the push-pull tubes bear on. And since the ruddervators have to be pretty large, their counterweights have to be pretty large to mass balance them. So there you are pushing a relatively large mass of push-pull tube through ten far-from-frictionless guides to move fourteen pounds of ruddervator and counterweight. Can you see where there might be a lot of inertia and slop and friction involved? Especially with the stock side stick and its meager 4.25" pitch travel, the friction and inertia can tend to overwhelm what little static stability there is.

    My experience with the HP-11 and HP-18 impelled me to follow the European model when it was time to design the empennage and pitch controls:

    * Given a decent tail arm, the elevator itself (and consequently its mass balance) can be relatively tiny.

    * The tiny elevator develops relatively little hinge moment, so its drive circuit can be relatively lightly built.

    * In order to preserve control feel, all push-pull tube guides are of the linear ball bearing type.

    The result is light (and low fatigue!) pitch control, with good feedback from the trim system and from the aerodynamic forces.

    Thanks, Bob K.
     
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  12. Aug 17, 2016 #12

    Victor Bravo

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    Nerobro, Leeon Davis' "one weldment" at the back of the airplane (DA-5) looks like it requires enough time and effort to build an entire other airframe ! If you aren't scared to death by looking at it, my hat's off to you :)

    All the previously mentioned intersection drag stuff is all well and good, but a well designed T-tail has one less intersection than a V-tail, with lower overall drag, and none of the aerodynamic problems that come with the V-tail.

    More recently, I took a look at Michel Colomban's magnificent Luciole design, and it has just about the same number of intersections as a T-tail but without any of the structural and mechanical disadvantages.

    It should be noted that Piper abandoned the T-tail and Beech abandoned the V-tail, both after full certification cost/effort and having gone into full production.

    To my amateur eyes, the Colomban method seems to be the best overall solution for most "normal" uses .
     
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  13. Aug 17, 2016 #13

    fly2kads

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    I think one of the reasons why we don't see as many V tails is simply that we tend to think of things in linear, X, Y, Z coordinates. That's our frame of reference, so that's how we conceptualize and solve our problems. Our reference images of airplanes, real and imagined, are commonly presented in 3 view outline drawings, after all. When we learn to fly, we're taught about controlling the airplane around three separate axes. Stability and control is presented in textbooks in neatly divided chapters on longitudinal and lateral derivatives.

    Nature, however, doesn't act in neatly separated spatial planes. Gusts that so rudely disturb us from our precise altitude and heading don't come strictly from above or below. They come roiling at us from any direction. Making coordinated turns that hold altitude requires simultaneous control input from all three flight controls.

    An alternate way of thinking about V-tails (or any other tail type, for that matter) is to think of that surface area following dutifully behind us as a net potential force. If you take all of your combined tail surface area, at a given moment arm and lift coefficient, it represents a potential force available to trim and stabilize the aircraft. As a designer, you get to choose how to deploy that force. For a conventional tail, you physically divide it into horizontal and vertical panels. For a V tail, you fold it upward. You determine the total amount of stabilizing force you need, and divvy it up as required.

    The relatively recent attention to complex wing shapes and winglets has, perhaps, given us a better way to view and analyze V tails. These complex wings are generically called "non-planar lifting systems." That's precisely what V tails are. I have only been able to fiddle with it a little bit, but it appears that the analytical approaches developed for winglets and such are directly applicable to V tails, and would allow a much more precise understanding of their aerodynamics than we can get from older methods. This would, in turn, enhance our ability to tailor V tails to get the flight behavior we want.

    Food for thought...and yes, I am a geek. ;)
     
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  14. Aug 18, 2016 #14

    rbrochey

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    Think late 1950's Buddy Holly... The Big Bopper... Ritchie Valens V tail Bonanza...
     
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  15. Aug 18, 2016 #15

    Swampyankee

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    T-tails look cooler than V-tails, but, were I to do a light aircraft design, I'd most likely use a conventional tail, as it makes analysis and design of the empennage and control system easier. With a T-tail, I've got to get the elevator control up the vertical stabilizer and make sure the vertical stabilizer is strong and stiff enough for safety. For a V, I'd need to sort out the mixer, and pitch trim.

    About the only time I think I would go with an unconventional tail is if the configuration is unusual, like a pusher, or there is a problem with something blanketing a conventional vertical fin. There's enough work to do getting a design right; an unconventional empennage would significantly, possibly double, the design work on that system.
     
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  16. Aug 18, 2016 #16

    Dan Thomas

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    The DA-2's mixer was elegant in its simplicity and was light. But as VB pointed out, the weldment at the tail adds weight and complexity.

    T-tails look neat, too, but the vertical fin has to be much stronger to take the stab and elevator loads, including assymetric loading caused by turbulence and rotating prop blast. You end up with a heavier tail. I have worked on a couple of cruciform tails, on the Lake and the Rockwell 114, and both have had issues with cracking at various points. The stab on the Lake is two pieces, connected to the fin spar by fittings that have an AD against them. They crack. The 114's forward fin spar cracks at the stab junction. When a horizontal stabilizer is mounted to the fuselage, the mount points are farther apart and the spars can usually be made as one-piece affairs, much lighter and with fewer places to crack. And cheaper to build.

    Esthetics can cost more. Ugly, ordinary-looking airplanes tend to be easier to maintain. That said, all airplanes have their weaknesses. You want to choose designs that have fewer problems.
     
  17. Aug 18, 2016 #17

    Swampyankee

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    As an engineer working for a for-profit company, part of the job is to help the company turn a profit (another part is to make sure the products don't kill people).

    If a V-tail or a T-tail can increase sales numbers or the asking price enough to overcome the cost of design, it's a winner. If it can't, it's a design failure.
     
    Last edited: Aug 18, 2016
  18. Aug 18, 2016 #18

    djschwartz

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    Another disadvantage of V tails that has not been mentioned here is trim drag. If the aircraft is balanced such that there is little load on the tail to maintain level flight then the V tail will have very slightly less drag than the conventional tail due to the reduction in interference drag mentioned above. However, when the tail must produce a balancing lift force, either up or down, to trim the aircraft, the angled deflection produces a lift force that is not vertical. The vertical component of this lift force must be sufficient to trim the aircraft, which means the total magnitude of the lift force must be greater than that. While the horizontal components of the lift force cancel out, the added drag from producing them does not and the total drag of the tail is greater than what the conventional tail would produce. This is especially true if you want the V tail to provide the same level of controllability and stability as the conventional tail and also want a C.G. range wide enough to make for a practical and useful aircraft.

    Like so may of the other design ideas that perennially bring up the "why don't we see more of this?" question, the V tail was the result of a hyper focus on one aspect of aircraft design and one specific flight regime without due consideration of the impact on the overall capabilities of the aircraft throughout the full range of operational conditions it would encounter. This is why real aeronautical engineering is more than a sketch-up and spreadsheet effort.
     
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  19. Aug 18, 2016 #19

    Matt G.

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    Here's another very complex, yet fairly ingenious V-tail structure (Schempp-Hirth Austria/SHK):

    IMG_0006.jpg

    The trim is integrated into the linkage connection for the anti-servo tabs. In theory I'd still have pitch control via the trim in a control linkage failure. NOT looking forward to the day I have to replace or adjust the cable for the trim system, (wound around the little aluminum drum at the rear).
     
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  20. Aug 18, 2016 #20

    fly2kads

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    Thanks for posting that, I've always wanted to see what their mixer looked like.
     
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