# What area is used to determine the horizontal and vertical tail volumes of a vtail?

What area is used to determine the tail volume of a 45 deg vtail?

Is the same entire area of the vtail used to calculate vertical and horizontal tail volume?

or do you use the projected areas in the x and y directions for horizontal and vertical tail volumes, respectively?

or other?

Depends on what the tail volume is used for.

For control authority and trim: Use the projected area.

For flight dynamics and damping: Reduce that by the cosine (horizontal) rsp. sine (vertical) of the V-angle, in your case multiply the projected areas by 0.707.

The control effectiveness of a control surface is proportional to the cosine of the V-angle, but the damping characteristics go down with the square of the cosine.

• I wouldn't worry too much about controllability. Initial sizing is all about stability (ie damping). So tail volume should be computed with cosine and sine squared. In any case, mixed control tend to overwhelm one side anyway, so it's not like you'll have too much controllability.
– JZYL
Commented Sep 18, 2019 at 19:23
• So... which area should be used when using standard horizontal and vertical tail sizing ratios? I assume these recommended sizing ratios for GA aircraft take care of everything in a standard flight envelope. I'm just asking, as I want to convert the tail volumes of the Lazair to a standard configuration as a comparison for my ultralight design which I want to design with a landing speed of 25mph, with slats, slotted flaps and blown wings. The Lazair specs note a landing speed of 18 mph.
– Fred
Commented Sep 18, 2019 at 19:28
• @Fred Use a longer tailboom to boost damping versus controllability. This will keep areas small and maintain damping. Play with the numbers until both controllability and damping look close enough to what the Lazair has. Commented Sep 18, 2019 at 21:03
• Is this the main reason for the tail wag problems of airplanes like the V tail Bonanza? Commented Sep 19, 2019 at 0:32
• @PeterKämpf I don't see why there would be too much controllability. I would be worried about lack of controllability, especially for multi-engine configuration. On a past project, combining OEI takeoff with even minor crosswind has been troublesome for V-Tail. On top of that, cross axis control can become nonlinear for large control deflections.
– JZYL
Commented Sep 19, 2019 at 2:07