In keeping with the recent interest with Vee tail setups, I have been reworking the plans for a Butterfly. (It was RC single channel, which only operated one surface in the rear.)

I have the formulas etc. for sizing a standard Tee tail model, but what is the sizing for a Vee tail design?

Wm.

This works well...
http://www.charlesriverrc.org/articl...tailsizing.htm

More

http://www.rcuniverse.com/forum/V-ta...1297280/tm.htm

Simplisticly... 20% more area than the flat horizontal and then mount the halves at 110 to 120 deg from each other. (120 is easy to hit exactly. 110 is better)

It works.

I THINK the ideal is 110 deg... can't remember. It will fly anywhere from 90 to 130 deg. If the V is too narrow (90) then elevator function acts a lot like an airbrake. If too wide (130) the rudder function gets poor and you have trouble getting out of a spin.

FH - I did a lot of research on V-tails after building the first prototype of Great Gonzo. I learned a lot, but I don't remember any of it now. I know one of the problems of getting the angle wrong was Dutch roll. I didn't have that problem, but had some other problems and cut off the tail twice and built new ones. Mine is at 110 and it works very well.

Yes, dutch roll is a function of improper vertical stabilizer area.

It is possible that the 20% increase in area would be too much and induce a bit of dutch roll. But that is MUCH prefered to having inadequate area, and an unstable aircraft. Dutch roll tends to only be a problem within one portion of an aircraft's speed range (a lot of models have some show up durring take-off climb)

More urban legend I see.
"airbrake"...
The vee-angle is dictated by the control system.
A rudder-elevator plane needs less angle between the surfaces than an elevator-aileron plane to get effective turning action, which may or not have rudder as a seperate control.
90° works well in the first case, 110° to 120° for a plane with rudder function, and 135° performs quite well with elevator-aileron only.

Not Urban legend... fact. Especially for up-elevator on an upward angled V.

I had a Coroplast mdel built by plans from the SPAD website. This particular model has a V tail.
The method for getting the V angle is by heaing and deforming some PVC to make the angle brace. As the PVC cooled... it sprung back toward 90 deg. I ended up with appx 95 deg angle V.

Up elevator at full throttle slowed me down like I'd killed the engine. Down elevator was not as bad, but it was noticeable.

I cut the bad V tail off... and put on a conventional tail. The same elevator deflection angle resulted in almost no noticable loss of speed. (of course a high climb angle always slows you down... but it was a HUGE performance change.)

****
BTW... ideal V angle is NOT dependent on control function... it is an aerodynamic constant. You are making the V surfaces perform the function of a vertical and a horizontal. you need to geve the effect of the vertical area of a standard Fin/rudder and the horizontal effect of a standard flat stabilizer. To do this... the projected shadow of the V in top view would need to be slghtly larger than the flat horizontal originally was (the angle creates a little loss of effect) and the side proection needs to be just a bit more than half the original Fin/rudder. You get these proportions at...110 deg.

As with anything that flies, there is NO single attribute that MUST be "thus-and-so".. Not wing area, weight, power, surface configuration, c.g. whatever.... Any airplane is a series of design compromises, all intended for a specific purpose, which is why they look different, when the purposes are different.
For the current instance, which involves tail areas and angles, the areas for the tail itself can vary easily from least acceptable to more acceptable by a factor of 2 or so, and the aircraft will be flyable for whatever purpose it is sized for, when other areas of the design (compromises) are included in the design.
And if the areas are too small/large, other accomodations (compromises) in other parts of the design can be utilized.
It may be better to fix too-small/too-large, but neither need be fatal. A compromise can be designed.
Nor are they "required" or "best"..
As the areas can vary, so can the angle between the surfaces of a vee-tail..
A steep (90°) seperation between vee'd surfaces could be selected for a rudder-elevator plane which will be turning a lot.. as a sloper, for instance. The turning quality overrules any other quality which might be present in the configuration.
(Turning quickly is the most important attribute a sloper can have.)
The same identical airplane as a thermal duration flier could use a more relaxed angle.. greater than 90°, as the flight requirements are different.. rapid response is NOT a prime flight consideration; ease of control and stable flight is.
An extreme relaxed angle, (135°) on an aileron, vee-tail elevator airplane, again as a sloper, makes use of the ailerons for the turning quality, and the relaxed angle on the elevator for more pitch authority.
Such planes make excellent flat-spinners for one thing.
Anywhere in between is useable for the intended purpose, AND the control configuration of the airplane.
The angles seen on thermal gliders are there because they work well, for thermal gliders.
An aerobat would go for something different.
A pylon racer....
All these design qualities depends (compromise) on what manuvers will be intended for THAT airplane.

Well, the sweep between the upper surfaces of the Vee tail according to the plan is about 112 degrees. Presume that is within the parameters already noted.

The Butterfly design looks to be a replica of some small home built of the early 1960's

Wm.

Hey everyone.

I'm having a heck of a time converting a plane to v-tail. I used autocad to figure out the current area of the vertical and horziontal stabs (with and w/o control surfaces) and when I add everything up and try to size a v-tail for it, the porportion of tail to wing visually is rediculous. The tail looks huge on the plane. I look at pictures of other v-tail gliders and the tails dont look much larger than the elevator would look normally.

I've read somewhere on the forums that you can simply size the elevator 20% bigger and that works too. Anyone have any luck with this?

I'm this close ----> <----- to abandoning the v-tail idea, but I really am stubborn to making it work!

Thanks in advance!
Rico

KLRico - It's not that critical. Just draw a front view of the stab and fin and then draw a box around them. Draw in your V-tail at 110 degrees so that a box drawn around the v-tail is roughly the same area as the box around the original tail.

After you build the thing, you may have to cut it off and build a new tail to adjust it so it performs better. That's the way design goes. Even if you have the "perfect" formula, it won't be perfect. A good design requires a lot of tweaking and adjusting. I've never designed anything that I didn't cut parts off of and make new ones to improve it.

- Paul

I've done the box method and the mathematical method. Both work.
I prefer the larger tail surfaces of the math method.

Thanks guys. That sounds pretty good. I'm sure I'll find a happy medium somewhere. I'll use the math method as a starting point and go from there....

I would consider building the tail a seprate piece and screwing it onto the plane so I could change it out easy but it's going onto a small HLG, any other suggestions on attaching the tail so I could remove it fairly easy if needed? Would simply using CA to attach and the solvent to remove it work? Just make it normal, and cut it off? What do you think?

Rico

I cut a V-groove block on my table saw and remove whole block. Sand off the old glue and attach a new tail. I would use a bolt-on method too, but the system would have added too much weight.

It's important to note that the V tail angle is not related to any "Golden" angle but rather it's only function is to distribute the horizontal and vertical area in a ratio similar to what exists on a conventional model. The fact is that we NEED more horizontal than vertical area in most cases and that is the only reason the angle is more open than 90. Mark Drela and Don Stackhouse (at DJ Aerotech) have gone to great lengths to show that you need to look into the whole thing more than just looking at the conventional flat areas and transforming them into projected areas of the V tail. The angle required in the end has a lot to do with the ratio between the required vertical and horizontal tail volume coefficients. I think that if the final angle comes out as being in a fairly narrow range of angles then that is more good luck than good planning. There's GOBS of stuff written to Don at his site about Vtails and he make reference to some of the material that he worked from as well in his answers...

http://www.djaerotech.com/dj_askjd/

FH, I think what you have with your rule of thumb is a system that will work great as long as you're not trying to cut the areas really fine to the bone like with some slope racers or F3B models where the designers try to cut the tail areas to the minimum to limit the drag. At that point I think it would be important to do the math and use what the numbers say. But for someone trying to mod a Gentle Lady or Spectra it sounds fine.

A couple of more items....

The bit about the upward elevators acting like a drag brake is more related to the angle of throw. I have heard that the up elevator on a V tail often has more authourity than the down elevator because the often used clearance V closes and contains the air better with each surface acting like an end plate for the other. The effect does not working with the same strength when the angle opens during down elevator. I believe this last bit was on the DJ Aerotech site in the same questions area. FH, I suspect you were using the same angles for up and down. If you had tuned the angles such that the inside and outside looping was the same radius perhaps the up elevator angle would have been less and the braking effect reduced.

FH, you also said....
I think you got spirally unstable and dutch rolling confused in that bit. Dutch roll comes from too small a vertical area which. Too much area in the vertical shows up as a model that is spirally unstable.

Rico, CA and debonder may work or what about trying Ambroid or Sigment with light fiberglass reinforcing? If you do this I suggest the Ambroid as it's thinner and will soak into the surface's end grain better. Double glue the area by coating the boom with a thin layer and force some into the end grain of the surfaces and coat the butt ends for about 3/8 to 1/2 inch out on the outer surfaces. After it dries glue the surfaces on with a second coat of wet glue and let dry. Then reinforce the joints with some 3/4 wide strips of glass or carbon cloth using more Ambroid as the "resin" You may need two or three coats to fill the weave. Once it dries completley for a couple of days it'll be super strong but if you want to soften it to move things you can just lay acetone or dope thinner on the Ambroid until it sofens and either bend it into the new angle if the change is minor or keep softening the glue until it releases all the way so you can install new surfaces or radically modify the angle if needed. Sigment works the same way but it comes out of the tube quite a bit thicker. For this use I'd thin it with some acetone to a thick brushing consistency instead of the jelly like thickness that is stock. Otherwise it tends to dry too fast and ball up as you are trying to spread it.

Thanks for all the help guys.

I'll just have to give it a go and see how it all works out. I think I'll go for the ambroid as far as attaching. I'll keep you guys posted on how it turns out. The wing is finished now, and waiting for some things in the mail before I can really go on (I live in Germany)

Thanks again for the assistance!
Rico

Here's the moded tail I have on my GP Spirit...
Charles River method.

Nice pictures there! How does she fly, in comparison between the tails?

Directional control is a bit better with the vee... I use the plane to carry a camera and maintaining a good heading is important.

I'm going to dig this up in the hopes I can get some input and help.

I'm currently converting my ARF Senior Telemaster to a V tail. I have removed the stock stab in one piece and used one of the ribs as a template for my new stab. I scatch built two halfs with the same cord and length as the stock horizontal stab at 17". Today at the field a guy warned me that my surfaces may not be big enough and that was confirmed by BMathews via a PM. I have the ability to extened each side about another 3 inches which is roughly 20% more surface.

Now I consider myself a smart guy but I can't figure out the actual formula.

Any help or advise would be great.

You know the span of the originals. Use a ruler and a triangle and with the triangle set to the same 1/2 span on the ruler as the stock stabs lift the ruller through a 35 degree angle from horizontal and see what measurement the ruler is at where it meets the triangle. That's about how much more you need.

This is very rough and the glider guys would tar and feather me for it but in this case where the ST is such a conservative design this will be close enough.

This advice is probably too late, but keep in mind that the Vtails that are at 45degree angles naturally give more rudder area than needed, or less stab area, depending on how you size them. OK, that wasn't very clear was it.

Most airplanes need far more horizontal tail area than they need vertical tail area. And a Vtail with the two sides at 45degrees provides equal areas for rudder/stab. One way to deal with that is to flatten out each side of the V. Instead of a 90degree angle between the two halves, you go to 120 or so. That gives you more horizontal surface effect and less vertical surface effect.

Since the topic subject is Vee Tail sizing, I figured I'd throw this in for people who're designing to consider.

I figure you aren't planning on rebuilding the whole empennage, but there always is a next time.

I was thinking that the Telemaster uses a lifting horizontal stablizer? That might make it interesting.

Formulas:

Measure the areas of the original design's vertical and hoizontal surfaces.

A lot of sport models use about 1/3 of the horizontal area for the vertical area. To figure the angle the following example shows the basic math for such a ratio.

Once the ratio of vertical to horizontal surface area is determined, it becomes a simple math problem to calculate the angle of the V, as well as the area.

The basic trigonometric function is : Sin^2  + Cos^2  = 1

{This should read Sine squared Theta + Cosine squared Theta equals one}

If we want the ratio to be 1:3, then Theta () needs to result in:
0.25 + 0.75 =1

The square root of 0.25 is 0.5, and the square root of 0.75 is .866

The arc-sine or inverse of the sine function of 0.5 gives 30 degrees.
The arc-cosine or inverse of the cosine function of .866 also gives 30 degrees.

Thus, for an equivalent ratio of 1:3 for the vertical to horizontal area, the angle of each half of the V-tail need to be 30 degrees above the horizontal plane. Since the angle between the V is typically called out, it becomes 120 degrees (180 – 2(30)).

There is some disagreement as to the area required. They range from:
a) using the same size as the existing horizontal stab area
b) using the projected stab area of the V the same as original
c) using the same total area as the combined area of the original tail
d) using the square root of the sum of the squares of areas of the vertical and horizontal

Gliders often use very acute angles (90-110 degrees), because they need a lot of rudder authority.
Pylon racers often use (110-120), I’ll reserve comment as to why.

Seems to me that you would design a V tail so that whem you looked down on it, it would look the same as a horizontal stab and elevator, and when you look at it from the side, it looks like a regular vertical rudder, more or less.