V-tail angle

[Jim Poor]

I've got a v-tail that I am going to put an a 30 inch slope wing (one of the wind busters) but I need some advice on the angle. As built, it is about 130 deg. I've heard that to provide stability, it needs to be closer to 90. I don't plan on any v-tail mixing so it's elevator only back there. The plane has a long tail moment which the builder says will provide some stability.

Any advice??

tks
Jim

[FlightofSong]

130 is a bit on the shallow side... Most vtails tend to be around 110 degrees or so. 90 tends to bias it too much toward yaw stability and not enough pitch. But if the AR is low, then it's common for Vtails to be flatter.

V-tails are one of those controversial isses that battle lines tend to get drawn for. The best method is just to try it out and see...

[Sparky Paul]

Flatter angles have more response in pitch, steeper in yaw. With ailerons, go flatter.

[Jim Poor]

Here's a pic, my 130 angle was just a guess.

[banktoturn]

Jim,

Even though you will use the tail as an elevator only, it still needs to be configured to give sufficient yaw stability. Assuming that you have about the right total area, I would guess ( and its only a guess ), that you would want closer to 110 deg. to get enough yaw stability.

banktoturn

[Jim Poor]

You mean:

-------------------------------------------------------
To convert a conventional tail into a V-tail:

A_vtail = A_vertical + A_horizontal

angle = arctan[ sqrt( A_vertical / A_horizontal ) ]

Notes: "A_vtail" is the area of both halves together, rotated flat. "angle" is the V-tail's dihedral angle from the horizontal.

To convert a V-tail into a conventional tail, use the reciprocal formulas:

A_horizontal = A_vtail * [ cos(angle) ]^2

A_vertical = A_vtail * [ sin(angle) ]^2

------------------------------------------------------------


<<<Scratches head :confused:


Thanks for the link though. It seems that's mostly for convertingone to another. I think I'll just use the TLAR method, and experiment.

[nuevo]

Wanna read (a lot) about it...

http://www.fmsg-alling.de/vtail.htm

[Sail 'n Soar]

Originally posted by Jim Poor
You mean:

-------------------------------------------------------
To convert a conventional tail into a V-tail:

A_vtail = A_vertical + A_horizontal

angle = arctan[ sqrt( A_vertical / A_horizontal ) ]

Notes: "A_vtail" is the area of both halves together, rotated flat. "angle" is the V-tail's dihedral angle from the horizontal.

To convert a V-tail into a conventional tail, use the reciprocal formulas:

A_horizontal = A_vtail * [ cos(angle) ]^2

A_vertical = A_vtail * [ sin(angle) ]^2

------------------------------------------------------------



The above equations are easily derivable and right on the money. Assuming you can calculate the trig functions, use them. The trouble with the original question is that it asked for a single standard angle. Unfortunately, the required V-tail angle is highly dependent on wing aspect ratio, everything else being the same. Models with high aspect ratio wings will have V-tail included angles approaching 90 degrees. That is, use 90 degrees on planes where you would use equal conventional tail horizontal and vertical stab areas. With lower aspect ratio wings the vertical to horizontal stab area ratio drops - less vertical and more horizontal - pushing you more toward 120 degrees included angle.

The best thing to do is to determine the horizontal and vertical stab areas you would use for a conventional application and then just use the equations to calculate the required areas and angles.

Here's an example based on "Homer" from RCM plans (AR = 6.44):

A_horizontal = 87 sq in
A_vertical = 25 sq in.

A_V-tail = 87+25 = 112 sq in.
(Each vertical = 112/2 = 56 sq in.)

Angle = Atan(sqrt(25/87))
= Atan(sqrt(.287))
= Atan(.536)
= 28.2 degrees from horizontal
Converted to included angle by subtracting twice the angle from 180 degrees:
= 180-2*28.2 = 123.6 degrees included angle

If your 30" "wind buster" has a wing aspect ratio on the order of Homere's 6.44, then between 120 and 130 degrees seems quite reasonable.

[HELModels]

That 30" wind buster is about the same size as what I have been trying to build and experiment with. My wing aspect ratio is about 6:1 and the included V angle is about 120 degrees. Responds nicely to elevator. The problem and pain came when I tried to turn. It flipped violently into the ground. Just the thought of a turn and slightest pressure on the stick results in uncontrollable roll. The problem seems to be that there is WAY too much dihedral in the main wing. The wing came directly from an old free flight model and has polyhedral with alot of total dihedral. Crash survivability has been the only successful portion of the design process.

What I wonder is if the V angle and tail area and moment provides so much torque then would reducing the wing dihedral to about 3 degrees be enough to tame the beast?

[Sail 'n Soar]

That 30" wind buster is about the same size as what I have been trying to build and experiment with. My wing aspect ratio is about 6:1 and the included V angle is about 120 degrees. Responds nicely to elevator. The problem and pain came when I tried to turn. It flipped violently into the ground. Just the thought of a turn and slightest pressure on the stick results in uncontrollable roll. The problem seems to be that there is WAY too much dihedral in the main wing. The wing came directly from an old free flight model and has polyhedral with alot of total dihedral. Crash survivability has been the only successful portion of the design process.

What I wonder is if the V angle and tail area and moment provides so much torque then would reducing the wing dihedral to about 3 degrees be enough to tame the beast?


Sounds like a tip stall sensitivity flying near stall speed.

[HELModels]

Sail 'n Soar,
I respect your opinion and input, but how could you possibly guess how close to stall the beast was flying at?

Harry

[HELModels]

Sail 'n Soar,
Still respect your opinion. I am really more puzzled about Why a tip stall then your guess that tip stall was the problem. Thinking about it and examining the tail brings up another question:

For a right turn, how should a V tail be set up to move?

Assume I had it backwards and I can see How a tip stall occurred.
Flying slowly as you guessed, it started drifting left; I gave it a blip of right and it actually turned more left; getting concerned, I gave it alot more right and it went hard left until the Tip Stalled. Sound about right?

[Sail 'n Soar]

Sail 'n Soar,
I respect your opinion and input, but how could you possibly guess how close to stall the beast was flying at?

Harry


Harry,

Just a hypothesis based on the extreme sensitivity to rudder inputs. May not be the reason, but something to consider. The 120 degree included angle sounds just about right for something with a 6:1 aspect ratio. Is you tail moment, total tail area and CG similar to the free flight model it came from? How much dihedral did it have? I guess that flying a large dihedral model at a high speed/low CL could generate the sort of extreme roll response you indicate. If you release rudder, does the bird recover or continue turning in? Three degrees sounds too small for something with the proportions you describe. If you've read a couple of my other posts, a little wash-out is something like asprin - cures many many ills. Being from a free flight, I would guess your wing has a lot of film-covered open framework, lending itself more to a heat-gun washout adjustment alot more than a knife to the structure dihedral reduction. Sound like you have little to lose. Give it a try.

Gerry

[Sail 'n Soar]

For a right turn, how should a V tail be set up to move?

If the V is open up, then imagine how the ailerons would deflect and have the V surfaces deflect just opposite, e.g., to turn right the right V-tail surface will deflect down and the left up.

[HELModels]

Waiting for sunshine and calm to test it again. The tip panels on wing have a bit of washout. I've got to do some digging to check how the proportions compare to the old Free Flight, but I remember trying not to change things as I was building. I did increase the semi spans on the horizontal to make the v.

Another way to look at how the surfaces should move on a V is to imagine folding the V panels together into a vertical stab and should move like a rudder, not like ailerons. I had it the opposite and hope that correcting the motion makes it easier.

Funny how you can find alot of info on correcting adverse yaw and fancy mixing schemes, but very little direct info on which way surfaces should move.