# How do I calculate the tail size of my rc plane?

I'm in the conceptual design phase and my plane is supposed to be of 1.7kg. Wing dimensions are 1m*0.3m and AoA that it will fly at will give a pitch down moment of 0.03. The moment arm between CG and tail is 1m. How do i calculate the size of the tail. It's supposed to be a conventional tail.

• What have you tried so far? It looks like you have everything you need there except the $C_l$ of the airfoils and area of the tail. Apr 11 '19 at 7:06
• Haven't done much yet, was still researching on it. Cl of the wing airfoil at the set AoA is 1.4. How do i calculate the area of the tail? The tail volume ratios and tail volume coefficient seem confusing to me. Do i have to consider these things in my first ever rc plane? Apr 11 '19 at 7:45
• What is your intended speed range? It seems you are setting $\alpha$ and $C_l$, which will give you only one flight condition, and therefore one particular HTP solution, which may be in the middle or on the extreme of your flight envelope. And no, you don't need to consider these things, you can make anything fly, even a folded A4 sheet. But since you are here, I assume you want to consider them. Apr 11 '19 at 7:55
• By plugging in the values of Cl, wing area and lift, I've calculated the stall speed from the lift equation which comes out to be 8m/s. I'll fly it at low speed, maybe not more than 20m/s, since it'll be my first flight. And yes i am setting alpha and Cl from the airfoil graphs, isn't this the way you proceed while designing? Apr 11 '19 at 11:05
• I'm not sure about calculations but the general rule of thumb I know is for small free-flight planes you'd want the h-stab to be around 1/2 the span of the wings. For RC planes of up to 1m wing span you'd want it to be around 1/3 the span of the wings. Larger planes can have stabs as little as 1/4 the span of the wings (indeed, that's what you'll typically see smaller full-scale planes) Apr 12 '19 at 6:29

There's a formula for calculating minimum tail volume coefficient (tail area X distance from the wing) for light aircraft. Not sure if the same value applies to RC but it's a good starting point. This is from David B Thurston's book Design for Flying (Thurston was an engineer on numerous Grumman programs going back to the Hellcat).

(Tail Arm ÷ main wing MAC) X (Tail surface area ÷ wing area) = .55 or greater

Tail Arm is the distance from the C of G to the 30% chord point of the horizontal tail.

.55 minimum is for a symmetrical airfoil section tail typical of light planes like Cessna 150s. I would use a higher number for a flat sheet tail surface, and being RC, so maybe .6 to .8.