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.
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$\begingroup$ 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. $\endgroup$– AEhere supports MonicaCommented Apr 11, 2019 at 7:06
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$\begingroup$ 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? $\endgroup$– ssmalikCommented Apr 11, 2019 at 7:45
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$\begingroup$ 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. $\endgroup$– AEhere supports MonicaCommented Apr 11, 2019 at 7:55
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$\begingroup$ 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? $\endgroup$– ssmalikCommented Apr 11, 2019 at 11:05
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$\begingroup$ 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) $\endgroup$– slebetmanCommented Apr 12, 2019 at 6:29
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2 Answers
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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.
While you're at it, Google "tail volume RC aircraft" and you'll find some more information specific to RC that'll be helpful.
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$\begingroup$ Agree with the higher coefficient. RC planes fly really slow compared to real planes (100mph is considered really fast). Combined with the small size of the models and we're dealing with very low Reynolds numbers. I'd err more towards .8 than .6 $\endgroup$ Commented Apr 12, 2019 at 6:25
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This is a great opportunity to compare experimental results with mathematical predictions in a scale model aircraft. Once you have chosen your tail planform, you size it until its torque cancels the forward shift of the wing center of lift during pitch up. If it is too small, the aircraft will continue pitching up and stall (try no Hstab at all and you'll see) Naturally, pitching tendency includes pitch components of wing, tail, fuselage, landing gear, nacelles, etc. The Hstab, as AOA increases, will begin to contribute a nose down force to help keep center of all lifts in the same spot. This work can be done "the hard way" in flight, or in a wind tunnel (safer). Better to err on the side of large. Your 1 meter pitch torque arm should make this model very stable and easy to fly. The next step will be area and throw of your elevator for controlled flight.
Comparing tail design of similar models and existing data is a good place to start. Set your Hstab angle so that when it has 0 degrees AOA (neither up or down force) the wing AOA is optimal (usually for cruise).
answered Apr 11, 2019 at 13:53