V-tail

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bmcj

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I've never heard that and don't see any reason for that to be true. However, I suspect that a V-tail may affect spin recovery (good or bad?).
 

K-Rigg

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I've never heard that and don't see any reason for that to be true. However, I suspect that a V-tail may affect spin recovery (good or bad?).
from what i read is that the placement of the horizontal tail is crucial in that when a plane is in a flat spin, the horizontal blocks air from getting to the rudder on the vertical tail. In a v tail, the horizontal is the rudder so in a flat spin, the control surface has air.

Thats why a t-tail is known to be easier to spin recover (piper tomahawk)
 

Dan Thomas

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Topaz

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Huh. Wasn't at the time, IIRC. There was a big to-do about "the trainer you couldn't spin." That's my memory of the situation, though, and perhaps I'm wrong. Or perhaps there's some difference between the Tomahawk I and Tomahawk II. I mostly flew the former. I know that we did spin one (instructor and myself), with a caveat from him to "...not tell anyone back at the school." Quite a ride.

Thanks for the update, Dan.
 
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Dan Thomas

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Huh. Wasn't at the time, IIRC. There was a big to-do about "the trainer you couldn't spin." That's my memory of the situation, though, and perhaps I'm wrong. Or perhaps there's some difference between the Tomahawk I and Tomahawk II. I mostly flew the former. I know that we did spin one (instructor and myself), with a caveat from him to "...not tell anyone back at the school." Quite a ride.

Thanks for the update, Dan.
Some schools probably prohibited spins in their Traumahawks because the recovery could get exciting. I've never flown one but ours guys that have say that it likes to drop off into a spin the other direction when you recover from the first spin. And they said it wasn't as bad as some made out, but that may be since we Canucks teach spins all the time and are familiar with them, while many US schools stay away from them altogether. The FAA doesn't mandate spin training, while Transport Canada does.

Dan
 

bmcj

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We Canucks teach spins all the time and are familiar with them, while many US schools stay away from them altogether. The FAA doesn't mandate spin training, while Transport Canada does.
Yet another reason to move to Canada. :gig:
 

Mac790

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skier said:
...but I was wondering if anyone knew of anyplace to find out how the rudder and elevator controls are mixed such that there are only 2 movable surfaces on the tail?
pic 1,2,3 Moni motor glider, pic 4 Monerai glider, hope it helps.

Seb
 

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Topaz

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Some schools probably prohibited spins in their Traumahawks because the recovery could get exciting. I've never flown one but ours guys that have say that it likes to drop off into a spin the other direction when you recover from the first spin. And they said it wasn't as bad as some made out, ...
Well, I've done one spin in a Tomahawk, and was a passenger for the one immediately before that. My limited experience is that she sure likes to roll over going from the stall into the first turn (more so than others I've experienced), and flatten out just a bit if you let the turns develop. Not a lot of flattening, nor dangerously, but it was an exciting ride. Exit from the spin was non-eventful for me. Seems like you'd have to be pretty ham-fisted to break into a spin the other way upon exit, but again, I have very limited experience with this airplane in that regard.
 

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Not necessarily. Billski made that point about total area requirements. Just because they are dual-function doesn't excuse you from needing the requisite amount of vertical and horizontal projected area:
Originally Posted by wsimpso1
By the time you get projected areas equal for tail volume, you have the same total wetted area. And then there is control authority and interference between the foils, which actually drives you to more wetted area than with a three surface tail.
thanks bruce, i glossed over that the first time =)

i'm afraid the first part of the statement doesn't make sense because a diagonal is shorter than horizontal plus vertical, which should be better, or less overall area, so the losses must come from interference at the center or too much proximity of the two tail surfaces

for example considering one side of the v tail a 7 ft diagonal has the same projected area as a 5 ft horizontal plus a 5 ft vertical so that saves 3 ft on each side.
 

vortilon

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I owned a Bonanza for a spell and did not mind the bit of fish tail. It went fast enough to diminish it in cruise.
 

wsimpso1

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There are aero studies on this... All sorts of folks think that you should be able to carry less total tailplane wetted area and one less set of interfaces, so it should have less drag.

I forget the whole explanation, but the upshot is this: Anytime you build with less total tailplane wetted area than what you would use on a conventional tail, it's stability is poor and its control authority at low airspeeds is inadequate... And when you build with just that amount, you will not have enough control authority if you are carrying some rudder, like landing in a crosswind. So you end up having to go bigger than with a conventional tail.

You can run your own search. I believe that Kitplanes had an article on it, as has Sport Aviation. Even Wikipedia carries the comment that you end up with more tail area with a V-Tail to get equal stability and control...

Billski
 

Starman

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aha, so the 'projected' area stuff doesn't apply. thanks billski, i'll take your word for it. i do like efficiency so guess my plan will probably have no sloping tails.
 

autoreply

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There are aero studies on this... All sorts of folks think that you should be able to carry less total tailplane wetted area and one less set of interfaces, so it should have less drag.
Which is because it's true in theory. If you simply look at the formulas your total tail area will go down.
I forget the whole explanation, but the upshot is this: Anytime you build with less total tailplane wetted area than what you would use on a conventional tail, it's stability is poor and its control authority at low airspeeds is inadequate... And when you build with just that amount, you will not have enough control authority if you are carrying some rudder, like landing in a crosswind. So you end up having to go bigger than with a conventional tail.
I did a design project for a 737-sized airliner and we chose for a V-tail, despite the fact that we were warned that it wasn't more efficient in reality. Some of the issues:
*A fin needs a really low AR to be effective in crosswind. The elevator needs a high one to minimize drag.
*You need redundancy, thus each tail had two movable surfaces. One was driven from halfway in the tail, leading to a serious weight- and complexity penalty.
*The negative roll/yaw coupling is rather big. You need considerable more roll authority.
*One engine out operations is a nightmare..
*Total aircraft weight and drag ended up around 2% higher compared to a t-tail and a conventional tail.

The only valid reasons to have a v-tail in my opinion:
*Ground clearance (taildragger with a v-tail)
*Engine positioning
*Deep-stall risk (part of the elevator is always out of the wing wake)
*Lower wake (pusher prop)
 

bmcj

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Autoreply mentioned some of the reasons that may drive the choice of a v-tail. I think these two are probably the strongest:

*Deep-stall risk (part of the elevator is always out of the wing wake)
*Lower wake (pusher prop)

Most designs, however, are best designed with a conventional tail, and can even be designed to address the issues that might call for a v-tail. For example, you can put some space between the tail and a pusher prop to mitigate wake disturbance, and you can add small fixed ventral fins (like Learjet did) to address deep stall issues.

Other disadvantages of the v-tail include opposing (cancelling) forces and structural complexity. When the v-tail creates any amount of downforce (as is the case with trimmed flight), there are horizontal components of force from each side that point inward and cancel each other out, but create drag in the process. Structurally, a conventional horizontal stab can have a carry-through spar for strength (hence, no torsion), whereas the v-tail requires the small bulkhead to carry all torsional loads generated by the v-tail (and some of that load comes from the opposing force mentioned earlier). Yes, the rudder of a conventional tail can create torsional loads, but rudders loads are typically much less than elevator loads.
 

Topaz

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...I forget the whole explanation, but the upshot is this: Anytime you build with less total tailplane wetted area than what you would use on a conventional tail, it's stability is poor and its control authority at low airspeeds is inadequate... And when you build with just that amount, you will not have enough control authority if you are carrying some rudder, like landing in a crosswind. So you end up having to go bigger than with a conventional tail. ...
The best (albeit non-scientific) explanation I ever heard was from an old aero professor at CalPoly. Very practical guy. He said that, fancy math aside, all you're doing with a V-tail is rotating a conventional tail 45° and redistributing the area. Same as rotating the fletchings on an arrow 45°. "Why would simply rotating the tail like that result in less required tail area for a given amount of stability?" Good question. It won't. And since a conventional tail usually has a little more area in the horizontal tail than the vertical, distributing the total area into two equally-sized fins spaced at 45° will result in the pitch axis being 'shortchanged' a bit - which means more total area required to make up for that.

A few V-tail designs have 'opened up' the angle between the fins to more than 45° to compensate, but that usually messes with yaw effectiveness pretty quickly - resultants of forces and such - so that there's little you can really do in that regard that won't make things worse.
 

K-Rigg

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tin response to Topaz, i have read that people are having the v tail at 35 degrees from horizontal, but in my mind, you woul want them at 45 degrees.. whats best?
 

Hot Wings

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tin response to Topaz, i have read that people are having the v tail at 35 degrees from horizontal, but in my mind, you woul want them at 45 degrees.. whats best?
What is best? Depends on what you want to do. Years ago I started messing around with X-plane and one of the things I tries was to put a "V" tail on the little plane I'm building. I started with the assumptions mentioned in this thread - that is 45 deg and total equal area. Turns out, in X-plane anyway, that just slightly less area at 30 deg gave me the desired balance between dutch roll and spiral stability while still maintaining the original control authority. Real world might be different? Decided to stay "per plans"
 
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