V-tail

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Topaz

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There is one real advantage, one tailsurface less which slightly lowers complexity...
Not to be persnickity, but you really aren't building 'one less surface.' You're still building the same area and weight of tail surface, but you're just distributing it differently and putting the mounting point of both of them on the end, instead of a conventional horizontal where the mount is in the middle.

In the end, you're actually building exactly the same amount of structure. I guess you're building one less tip, but that's not much of an advantage.
 

autoreply

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Not to be persnickity, but you really aren't building 'one less surface.' You're still building the same area and weight of tail surface, but you're just distributing it differently and putting the mounting point of both of them on the end, instead of a conventional horizontal where the mount is in the middle.

In the end, you're actually building exactly the same amount of structure. I guess you're building one less tip, but that's not much of an advantage.
Well, you are a pit picky :)

A conventional tail or t-tail really are constructed as 3 surfaces. I was only referring to the complexity, two surfaces to actuate, only two (straight) leading edges, only two spars (a t-tail still has a concentrated load in the middle as does a conventional) and so on.
 

Topaz

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Understand that I mean this in good fun, but no, most small-plane horizontal tails are built in a single piece that is then attached to the airframe. More often than not, the elevator is really just one piece with a cutout for the rudder.

Sure, there are minor build advantages to V-tails, but it's definitely a lot less than "you only have to build two surfaces instead of three", and as BMCJ points out, that is largely negated by the necessity to build a control mixer.

When you factor in the tendency of most V-tail aircraft to 'wander the nose' a bit in turbulence, I'm pretty sure there's only one good reason to go with that configuration:

It sure looks cool.

That's not a trivial concern. "Looks cool" sells more airplanes than you might expect.
 

mikemill757

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The more an elevator is away from horizontal, the less efficient it is (rudders make lousy elevators) and so need to be larger (more drag/weight) in V-tails. Same for non-vertical rudders.
On an upward, aft mounted V-ail, a rudder input will add roll counter to normal turn co-ordination, ala V-tailed Bonanzas. A better design might be a) a downward, aft mount that would help avoid prop strikes on rotation, b) a upward, forward mount, or c) one from each twin tail boom angled up and inboard, joined the middle. Any of these would aid in turn co-ordination.
IIRC, the best compromise was a 34 degree angle from horizontal.
Still, there is that "cool" factor....
Mike
 

Michealvalentinsmith

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So In theory an inverted V tail should give the correct rolling moments coupled with ailerons? I knew an Ultralight Lazair pilot who claimed it rolled quite well on rudder alone and it had a large inverted V tail - which if I recall was more to position the tail dragger wheels that for control. There wasn't much dihedral for yaw roll coupling either.

I know in the past there have been aircraft with rolling tails - that still got adverse yaw (which makes you wonder about the drag only explanation).
 
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bmcj

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So In theory an inverted V tail should give the correct rolling moments coupled with ailerons? I knew an Ultralight Lazair pilot who claimed it rolled quite well on rudder alone and it had a large inverted V tail - which if I recall was more to position the tail dragger wheels that for control. There wasn't much dihedral for yaw roll coupling either.

I know in the past there have been aircraft with rolling tails - that still got adverse yaw (which makes you wonder about the drag only explanation).
Dihedral is not the only feature that feeds into the roll coupling effect. Wing sweep will because the wing on the outside of the yaw will have a more effective span (it sticks further out into the airstream than the inboard wing, so it generates a greater rolling force). Also, just the fact that the wing is generating lift will give you a little roll because the outside wing is moving faster than the inside wing, so it generates a little more lift.

This has no real relationship to tail design though.
 

Michealvalentinsmith

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The Lazier had no wing sweep either but I take your point about their being other factors than dihedral.

If so though you would expect some skid into the turn from rudder only and a non dihedral straight wing before yaw coupled roll occurred.

I can't be sure, and never flew one, but the pilot didn't mention any skid, so something else may have been at work. I can't imagine the roll forces from a V tail being significant - or it would not of had ailerons. But it seems the effect was there.

I expect those aircraft will rolling tails had large tail volumes - now I think about it they may have been military jets with all moving, anhedral tails and high speeds - I think.
 

bmcj

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We did a lot of flying in the two-seat Quicksilver (MX2). It had the rudder connected to the stick and the differential spoilers connected to the pedals. With the high-lift wing and high dihedral, we never used the spoilers (except to deploy them together to descend faster). Any movement of the stick translated to almost instant roll. Sure, there was also a yawing motion, but the roll response was so closely coupled that the yaw went largely unnoticed.
 

robbienick

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Re Vee tails , very interesting the various fors and against vtails. What are the opinions with regard to a twin boom fuselage with an upward/ inward facing veetail joined in the centre. Stiffer rear end ? fewer vortices( no rudder, fin or tail plane tips), well positioned to avoid missiles from the wheels.Giving a lower fuselage for a pusher, besides I like this layout and it looks cool.
 
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K-Rigg

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Re Vee tails , very interesting the various fors and against vtails. What are the opinions with regard to a dual fuselage with an upward/ inward facing veetail. Stiffer rear end ? fewer vortices( no rudder, fin or tail plane tips), well positioned to avoid missiles from the wheels.Giving a lower fuselage for a pusher, besides I like this layout and it looks cool.

or how about an x tail that has the lower surfaces on the fuselage to be half as big as the top to allow for ground clearances, but have some of the benefits of an x tail.

Taking 1/3 of the area off the v tail and putting it down on the bottom of the fuselage to form an x tail.
 

Mac790

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The Davis DA-series homebuilts all have the V-tail, and it works well. The mixer is an astoundingly simple device inside the tail. I have the blueprints somewhere. They have a diagram of the setup. If I can find a few minutes, I'll unroll them and post the diagram. Should be no copyright issues with that...
Hope you are right about copyright issues.


Seb
 

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Topaz

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Hope you are right about copyright issues. Seb
You're fine, at least in the USA. Can't say how it works in other countries.

You're showing a fragment of the original work, for a definite illustrative purpose, in a non-commercial setting. Falls under the "Fair Use" clause of the Copyright Act. That clause, thanks to the music industry, is probably the most endangered species on the planet, but you're still good in this case.
 

rtfm

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Twin tails vs V-tails

Hi,
Rather than start another thread on a very closely related topic, allow me to ask this question here instead...

Do twin tails (eg like those used on the F/A-18 Hornet) suffer from the same drawbacks as V-tails? I noticed the other day that the Aquaglide 5 WIG also used almost vertical twin tails. I would imagine that one would simply treat these as two vertical tails (ie no need to mix the rudder movements) since the horisontal component of rudder deflection is so much less than the vertical component.

Would this be fair comment?

Of course, one would then still need a h-stab - but that's another issue.

Duncan
 

Bart

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An alternative with low interference drag mighty be U tail--with proper radius of horizontal and vertical junctures.

Bruce Carmichael advocated this in his pod and boom pusher concepts, so the thrust column from the mid-mounted propeller goes between the vertical fins.

Just a thought.
 

autoreply

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Re: Twin tails vs V-tails

Do twin tails (eg like those used on the F/A-18 Hornet) suffer from the same drawbacks as V-tails? I noticed the other day that the Aquaglide 5 WIG also used almost vertical twin tails. I would imagine that one would simply treat these as two vertical tails (ie no need to mix the rudder movements) since the horisontal component of rudder deflection is so much less than the vertical component.
The main problem is interference between those V-stabs. Jet fighters need it because of stealth and vortex tail blanketing. When those 2 v-stabs are close enough to influence each other their effectiveness is reduced, and in most cases considerably.
That's the reason you barely ever see a tail configuration as in the aquaglide, one single tail or an H-stab is lower-drag and probably more effective.
 

rtfm

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Hi, and thanks for the input. This is very disappointing, because my preliminary estimates indicate that twin tails with an h-stab attached to the tops (ie two t-tails, if you will) would actually weigh only a little more than a more beefy single t-tail. Construction would be a bit more involved, and there would be additional interference drag - but it would make the tail feathers so much more rigid.

I'm intrigued by the loss of v-stab effectiveness you mention, however, since this is not intuitive.

How close is "close"? What type of interference are we talking about here? Under what conditions is this interference experienced?

Can you point me to references which quantify these concerns? I checked Raymer (Conceptual Approach textbook) who says (pg 79) "Twin tails are usually heavier than an equal-area centreline-mounted single tail, but are often more effective"

As I said, I'm attracted to this design both from the fact that it will be easier to make the tail feathers rigid in this configuration, and also because of the aesthetics of the design. But I'm not wedded to it, and obviously will have to weigh up the pros and cons before committing to it.

Thanks for your help,
Duncan
PS Some sketches of this configuration attached.
 

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rtfm

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Aha! I think I've answered my own questions... I found the following post from Orion
The tunnel effect I mentioned is the phenomenon where the verticals' direct the air flow between them, preventing one of the surfaces from reading the full characteristics of the free-stream flow. Let's take the extreme case and assume that you're yawed to one side. The result of the airplane being turned is that one of the surfaces will be slightly forward of the other WRT the prevailing flow. As the airstream approaches the leading surface, the flow will turn and align itself with the fin. The stream will complete the turn around that fin's leading edge and then flow parallel to its surface. But, because of the physics of the flow, that turning of the stream will be accomplished even a substantial distance away (entrained flow) from that leading surface.

So, if we now look at the second fin - since it's somewhat behind the first one, much of the flow it sees is already turned in the direction of the first fin. As a result it does not see as much yaw as the first fin sees and as a result its centering (or correcting) force vector is significantly smaller than that of the first fin (wind tunnel testing has shown that this is also true, although to a lesser extent, for even small yaw angles). For this reason and for the purpose of staying a bit conservative, when designing twin tailed aircraft it is good practice to ignore the second fin and just use the net projected area for the design and for all the stability calculations.

And this is especially important for "V" tailed aircraft where the situation is a bit more complex.

If you want to save a bit of weight, yes, provided you do the math you probably can get away with making the fins smaller, but to be on the safe side I'd probably recommend making that reduction relatively small.

The full text can be found here:
https://www.homebuiltairplanes.com/forums/design-structures-cutting-edge-technology/3515-single-versus-twin-tail.html

I guess this means bigger tails than I had intended. But I still like the rigidity it provides, and of course, the cool look...

Duncan
 

lr27

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You probably get more drag with all those intersections, too.

I just noticed that your stab looks like might drop into the wing wake at high angles of attack. Have you evaluated that? Seems like if it was down there on the tailwheel strut, it would be in free air all the time. But I suppose that would be too close to the ground.
 
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