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Thread: Confusion About V-Tails

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    Registered User Toobuilder's Avatar
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    Question Confusion About V-Tails

    I've searched various forums, read some articles and books and drawn some general conclusions concerning V-Tails - There seems to be two opposing "universal truths" concerning their use:

    1. V-tails don't work very well. Despite their theoretical aerodynamic and structural efficiency, if the Bonanza and Sonex/Waiex series has shown us anything, V tails turn out heavier, more complex, less stable, and without any speed advantage.

    Yet -

    2. UAV's frequently feature V tails. Vehicles used for long endurance intelligence, surveillance, and reconnaissance (ISR) missions need all the efficiency they can get, so the theoretical advantage of V tails is a logical (and popular) choice.

    My question is this: If V tails generally are a poor choice for light aircraft, then why does this design feature show up on so many UAV's large and small?

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    Registered User djschwartz's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Toobuilder View Post
    I've searched various forums, read some articles and books and drawn some general conclusions concerning V-Tails - There seems to be two opposing "universal truths" concerning their use:

    1. V-tails don't work very well. Despite their theoretical aerodynamic and structural efficiency, if the Bonanza and Sonex/Waiex series has shown us anything, V tails turn out heavier, more complex, less stable, and without any speed advantage.

    Yet -

    2. UAV's frequently feature V tails. Vehicles used for long endurance intelligence, surveillance, and reconnaissance (ISR) missions need all the efficiency they can get, so the theoretical advantage of V tails is a logical (and popular) choice.

    My question is this: If V tails generally are a poor choice for light aircraft, then why does this design feature show up on so many UAV's large and small?
    In order to achieve their theoretical drag reductions a V-tail needs to be operating at near zero lift. Visualize it this way. Suppose due to its loading the aircraft needs some nose down trim at cruise. With a conventional tail the elevator is deflected down, creating a small lift force. This does cost a bit of additional drag but with good design that amount is small. With a V tail both ruddervators must be deflected; but, they don't just deflect down, they also deflect out. (or in if it's an inverted V). So the along with creating the small lift force, which causes the same drag increment as with a conventional tail, the V tail is also creating two opposing sideways forces. These two cancel so no yaw is created; but, the incremental drag from those sideways forces remains. The same effect happens in climb when a significant nose up trim is required.

    UAVs can be designed with carefully placed payloads that result in the need for minimal trim. They also have on board flight computers that can provide stability, both it pitch and yaw. Aircraft designed to carry people need to be able to operate over a range of CG locations to be practical. So, as is often the case, the "best" solution for an aircraft isn't necessarily the one with that offers the "best" possible performance; it's the one that offers good performance over a useful range of operating conditions.
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    Registered User Joe Fisher's Avatar
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    Re: Confusion About V-Tails

    I don't think the up right V works very well. I have occasionally flown Bonanzas and the straight tail fly better except the early models that are just lighter airplanes. I flew a Sunburst ultralight with an inverted V tail. It had spoilers for ailerons and they were completely worthless. But the rudder worked like cupled rudder and aileron. It was extremely nose heavy and flew level with half up elevator.

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    Moderator autoreply's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Toobuilder View Post
    out heavier, more complex, less stable, and without any speed advantage.

    Heavier? That really depends. Some V-tails are lighter as their conventional counterparts, others heavier. Complexity? The steering is a lot simpler and mixing is very easy and doesn't require any more parts. Total parts count goes down and you can basically use one mold for both tailsurfaces. Two fully-moving ruddervators are the simplest tail you get get. The structural analysis and design though is indeed more complex and quite a bit. That might also result in a pretty large weight penalty in strengthening the tail. As for the drag advantage, indeed none in reality.
    As for the trim drag disadvantage, most V-tails have a higher aspect ratio than the tail they replace. That offsets the extra trim drag.
    My question is this: If V tails generally are a poor choice for light aircraft, then why does this design feature show up on so many UAV's large and small?

    Because they are better in some aspects. Lower complexity and parts count is one of them. Not that I would bother for a full-sized aircraft since for me the advantages don't outweigh the disadvantages, but they certainly have their merits as a valid and safe choice if properly designed.
    Further on, aviation has a lot of old wives tales. V-tails will kill you the instant you look at them. V-tails cannot be stable. And so on. Those old wive tales probably kill more people than any other safety problem, because they prevent so many safer/better designs...
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    Re: Confusion About V-Tails

    The airplanes built with the most drag attention are competitive sailplanes. For the most part, they have T tails, but part of that need probably has to do with being able to easily remove the horizontal tail to get it into the trailer... But if there were a serious advantage to V tails, you can bet that these guys would be running them.

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    Registered User djschwartz's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by wsimpso1 View Post
    The airplanes built with the most drag attention are competitive sailplanes. For the most part, they have T tails, but part of that need probably has to do with being able to easily remove the horizontal tail to get it into the trailer... But if there were a serious advantage to V tails, you can bet that these guys would be running them.
    Same goes for Formula One Racers. And it should be noted that even these highly specialized aircraft still need good performance over a range of trim conditions. Sailplanes must perform well at both minimum sink and penetration speeds, and racers both on the straights and pulling g's on the turns.

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    Moderator autoreply's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by wsimpso1 View Post
    The airplanes built with the most drag attention are competitive sailplanes. For the most part, they have T tails, but part of that need probably has to do with being able to easily remove the horizontal tail to get it into the trailer... But if there were a serious advantage to V tails, you can bet that these guys would be running them.
    Well, there have been several sailplanes with a V-tail.

    Confusion About V-Tails-js1.gif
    The angle of a V-tail is almost proportional to the ratio of V-stab area and H-stab area. Twice as much H-stab as fin and your ruddervators are at roughly 30 degrees from the horizontal. The picture is a small glider with only 50 ft span. H-stab area goes with mean chord and stays roughly constant if you increase the span. Fin area on the other hand goes up dramatically.
    Even this glider would probably need ruddervators that are 70 degrees or so from the horizontal. That would make their use as elevator/h-stab pretty inefficient because it's almost vertical. If you go to the gliders with 100-ish ft of span, they have the same elevator, sometimes a smaller one. Their fin/rudder is 2/3 times as large as this one...

    Thus, V-tails simply don't work on aircraft with high aspect ratio, unless you have an oversized elevator, like the Glasflugel Salto for example. Reason for the T-tail by the way is induced drag. One less intersection is worth the extra weight and the extra complexity. (A removable conventional H-stab is less complex to mount, since you don't need the top attachments and the various elevator bellcranks)

    Basically, V-tails only work if you have a comparable H-stab and V-stab area, which shouldn't differ more than half their area or so. Not that you're likely to find a drag advantage their though..
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    Registered User Toobuilder's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by autoreply View Post
    ...Thus, V-tails simply don't work on aircraft with high aspect ratio, unless you have an oversized elevator, like the Glasflugel Salto for example...
    ...Yet many UAV's such as the Global Hawk are pretty high aspect ratio. I understand the variable payload requirements of a passenger aircraft vs. a UAV (missed that point before my post), but the instability of a Bonanza is repeated enough that it can't be completely a "wives tale", can it? That said, the Vans RV-8 is known to exhibit a yaw instability in turbulence, and that is as about as "conventional" a design as one can find. Perhaps the famous "Bonanza Boogie" is simply more of a function of a poorly sized empennage than of the V tail configuration itself?

    Is it reasonable then to conclude that structural or systems concerns, rather than some clear cut performance advantage, should dictate the configuration? In other words, a given platform may lend itself structurally to a V tail, but from an aerodynamic standpoint, that configuration is likely to perform "roughly" the same as a conventional tail?

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    Moderator autoreply's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Toobuilder View Post
    ...Yet many UAV's such as the Global Hawk are pretty high aspect ratio. I understand the variable payload requirements of a passenger aircraft vs. a UAV (missed that point before my post)
    Yepz, and by doing the (ball-parked math), I think the H-stab "area" is way too large. I guess (but that's really just a guess) they've done so to accommodate for a large range in CG, given the position of the possible payloads. Stealth might also have played a role (no vertical surfaces)
    but the instability of a Bonanza is repeated enough that it can't be completely a "wives tale", can it? That said, the Vans RV-8 is known to exhibit a yaw instability in turbulence, and that is as about as "conventional" a design as one can find. Perhaps the famous "Bonanza Boogie" is simply more of a function of a poorly sized empennage than of the V tail configuration itself?
    And from my point of view, that's the clue, though I don't know whether the above is valid or not. Many times those "old-wives-tales" take anecdotal evidence to "prove" something is good or bad. The C337 is a famous one. It climbs better on the rear engine, thus, pushers are way more efficient. While that's not necessarily untrue, it's a nonsense argument if you know what happens. (massive flow separation with the rear engine shut off)
    Is it reasonable then to conclude that structural or systems concerns, rather than some clear cut performance advantage, should dictate the configuration? In other words, a given platform may lend itself structurally to a V tail, but from an aerodynamic standpoint, that configuration is likely to perform "roughly" the same as a conventional tail?
    I'd say that's a big, fat yes. The differences in drag between all tail configurations are pretty slim and only worth choosing them when you're looking for the nec plus ultra, like in sailplanes.

    I just noticed that we didn't mention one advantage of a v-tail; vibrations. If you have a conventional-tailed pusher (like my design), the fewer wakes of control surfaces your prop sees, the better (vibrations, noise). I guess that's a good reason for the Vmax Probe and the Mini-imp to choose a V-tail.
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    Moderator Topaz's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Toobuilder;97533...My question is this: If V tails generally are a poor choice for light aircraft, then why does this design feature show up on so many UAV's large and small?[/SIZE
    [/FONT][/FONT]
    Most of the genuine issues with V-tails are related to pilot feel. All the pireps I've read say that they tend to "hunt" a little on the pitch and especially the yaw axis, particularly in turbulence. Most of the benefits AR is talking about (especially structural and manufacturing) are real but, pilots being what they are, things like "slight hunting" get blown out of proportion and V-tails are disliked.

    The autopilot on a UAV doesn't care about "slight hunting", which it can automatically dampen out anyway. So you see manufacturers taking advantage of the V-tail's plusses on an airplane where they don't have to worry about the minuses.
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    Moderator Dana's Avatar
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    Re: Confusion About V-Tails

    One more [minor] point... the "complexity" of a V-tail is in the mechanical mixer required to mix rudder and elevator inputs. On a UAV, it's all done electronically, so all it needs is one actuator per surface with a simple connection; no mechanical mixing.

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    Registered User Dan Thomas's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Dana View Post
    One more [minor] point... the "complexity" of a V-tail is in the mechanical mixer required to mix rudder and elevator inputs. On a UAV, it's all done electronically, so all it needs is one actuator per surface with a simple connection; no mechanical mixing.
    And even the mixer as found in the Davis DA-2 is astoundingly simple (simpler than the Bonanza's) and wouldn't add much to the build time. Its tail surfaces are all-moving affairs, pushrod-operated off the mixer. Much simpler than four cables, stab and elevators, fin and rudder, and their hinges. Leeon Davis knew how to design simple, very light, and fast airplanes that ran on minimal power. They just didn't glide too far with their short wings.

    Dan

    Confusion About V-Tails-davisda-2a_1j.jpg

    Edit: Here's a ruddervator mixer from a Moni. The Davis' mixer is made of flat 4130 and is easier, I think, to make, but you get the idea. The view here is looking aft; the cables at the bottom come from the rudder pedals and they pivot the vertical shaft. The two pushrods at the top go to the ruddervators. When the rudder cables pivot the vertical shaft, the ruddervators act differentially. The pushrod on the left comes from the stick and tips the mixer fore and aft on the two bolts at the bottom (which are in line with the rudder cable attachment so that the cables aren't affected by stick movement) and when it tips the ruddervator pushrods move together and move the surfaces up and down together.

    Simple, huh?

    Last edited by Dan Thomas; April 17th, 2011 at 07:23 PM.

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    Registered User Toobuilder's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Dan Thomas View Post
    ...Here's a ruddervator mixer from a Moni. Simple, huh?

    Simple, yes, but I sure don't like the apparent angles that the rudder cables will be forced to endure right at the swaged connection during a pitch input (unless there are spherical bearings in there). If the forks are limited to single axis (as on a rudder horn), then the cables will kink (and it looks like they have).

    Anyway,

    Thanks everyone for more food for thought!

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    Registered User Dan Thomas's Avatar
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    Re: Confusion About V-Tails

    Quote Originally Posted by Toobuilder View Post
    Simple, yes, but I sure don't like the apparent angles that the rudder cables will be forced to endure right at the swaged connection during a pitch input (unless there are spherical bearings in there). If the forks are limited to single axis (as on a rudder horn), then the cables will kink (and it looks like they have).

    Anyway,

    Thanks everyone for more food for thought!
    Take a look at the rudder cable attachment on any swept-fin 172 (about 1962 and on). The cables attach to the bellcrank at something of an angle, and as the rudder swings the angle gets worse. It's really primitive, but we've sold one old 172 with over 13,000 hours and another with over 11,000, and those rudder attach points were still OK. All it takes is a bit oversize on the hole. The movement is minimal, loads aren't great, and inside the fuselage as on the mixer is better than outside in the dust as on the 172.

    One could use spherical bearings if he wanted, but the average homebuilt will accumulate only a few hundred hours in its lifetime and it would be a waste of time, weight, and money.

    Dan

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    Registered User fly2kads's Avatar
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    Re: Confusion About V-Tails

    With most of man's problems, there is usually more than one way to solve that problem. (I saw a great quote the other day, "All problems are soluble with beer." Sorry, but I digress.) Same with aircraft stability. To simplify a bit, you can think of the designer as having to determine the worst case loadings the airplane will have to deal with (e.g.: landing flare with full flaps and power off at most forward c.g.; sideways gusts of a certain velocity) and size the tail to balance that load. The length of the tail boom gives the lever arm to work with. Each square foot of tail area can be assumed to generate a certain amount of force, so this balancing load is achieved by solving for the area needed at the given tail boom length. (There is obviously a bit more to it than that, but that's the gist of it.) The vertical and horizontal areas can be arranged in any number of ways and still provide the needed balancing loads. There are "conventional" tails, cruciform tails, T tails, V tails, Y tails, H tails, triple tails, among others. There are some subtleties involved with each type, but one is not inherently more or less stable than any other. A conventional tail can yield poor stability just as easily as any other type.

    To expand on what Autoreply said earlier, in a V tail, the total tail area is combined into a pair of surfaces. The available balancing force is "divided up" among the horizontal and vertical axes by setting the dihedral angle of these surfaces. While it is true that a higher aspect ratio wing requires a greater amount of force to provide yaw stability, this doesn't have to come from the dihedral angle alone, as the areas can be adjusted as well. It's the combination that counts.

    The analysis of V tails has improved over the years, which I think has contributed to a rise in interest in the configuration. In the past, some V tails were sized by making an imaginary box with the length and height being given by an "equivalent" conventional tail. The V tail was simply bent and stretched up into the corners of that box. Another method was developed to use the combined area of the equivalent tail, adding a fudge factor for mutual interference, and solving for the required dihedral angle. Now, computer codes using techniques like vortex lattice methods can be used to find the forces and moments generated by a V tail with a much higher degree of certainty. To some extent, we have to give thanks to Dr. Richard Whitcomb for his invention of winglets for these recent developments. A lot of smart people have spent a lot of energy figuring out how to analyze and optimize "non-planar lifting systems" (a.k.a. winglets). It turns out that the V tail is, you guessed it, a non-planar lifting system, too.

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