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I'm a student of mechanical engineering and my graduation project is basically an RC aircraft powered by a propulsion system and It is used for general transportation. The aircraft doesn't require much maneuverability. The problem is that when I was designing the ailerons using the design equations included in the reference 'Aircraft Design A Systems Engineering Approach by Sedraey', the roll rate calculated was roughly 12 rad/sec, which is probably a large number for an aircraft that doesn't require much maneuvering.

My question is of course a sanity check; is this roll rate reasonable ? because I used MATLAB for optimization of the geometric parameters of the aileron and the output geometry seemed logical when I visualized on SOLIDWORKS.

Update : I looked up other reference which are specialized in aircraft stability and control and I have finally managed to get reasonable results. From my experience I would never recommend using 'Sedraey' as a reference for learning about aircraft design. Thank you all for helping me.

enter image description here

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  • $\begingroup$ I’m voting to close this question because it belongs on Drones.SE. $\endgroup$
    – Ralph J
    Nov 9, 2021 at 19:40
  • $\begingroup$ Something is wrong in these formulae. How do they willy-nilly assign [m/s] to $y_D$, while it is clearly just [m]? If $L_A$ is the roll moment from ailerons ([N m]), then (12.37) will indeed be [1/s], but 1) randomly picking "drag moment arm" at 40% while the main resisting force, roll damping, is proportional to the square of wingspan, seems wrong. And 2) this $L_A$ will decrease with the roll rate (simply because the local angle of attack will partially negate aileron deflection). Very suspicious source. $\endgroup$
    – Zeus
    Nov 10, 2021 at 8:58
  • $\begingroup$ Roll rate will be dependent on the airspeed. A model airplane will generally be flying at around 20 knots. The V$^2$ is used to calculate the roll torque based on the lift difference of aileron up and down. Roll rate stabilizes when drag torque equals roll torque. Weight placement in wings is also critical. 1 radian per second is more like it. Comparison with existing R/C models may be helpful, as would speaking with their pilots. $\endgroup$ Nov 10, 2021 at 11:34
  • $\begingroup$ I agree with you that these equations are quite suspicious. It would be helpful if you referred me to a better resource, especially if it is related to RC airplanes. @Zeus $\endgroup$ Nov 10, 2021 at 17:53
  • $\begingroup$ I think talking to RC experts and pilots would give me a better insight and intuition. @Robert DiGiovanni $\endgroup$ Nov 10, 2021 at 17:58

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Twelve radians is nearly two full turns -- per second!

I've got a couple hundred hours of stick time in R/C, ranging from two-channel sailplanes to four-channel mildly aerobatic sport models. The most aerobatic models I ever had, a Swizzle Stick and a Terrier, had a lower roll rate than you propose; few pattern aerobatic models have higher (maybe a Zlin or Cap-21 -- and maybe not). Two rolls in a second is asking to become disoriented, which leads to "planting balsa trees" (an old phrase when I flew in the 1980s, perhaps obsolete with modern materials and methods).

If the main goal is to fly like a transport aircraft -- that is, mainly wings level, just enough bank to make turns to stay within sight and control range -- you could do very well with a fourth that roll rate and probably get away with an eighth. However, I'd recommend you build to have at least twice the control authority you believe you'll need, and adjust your control throws near their minimum for maiden flights, then increase them once you know what's needed. Also strongly recommended is to have an experienced pilot at least do the first takeoff and hand over the transmitter once you have some altitude, especially if you have little experience flying R/C aircraft.

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  • $\begingroup$ Thanks you for taking the time to reply to my inquiry. I know the number doesn't seem reasonable. I was wondering if this number is a result of any mathematical error I could have made and probably I didn't pay attention to. But I guess from your reply you mean that the issue can be solved if I informed the pilot with this high roll rate and that the aircraft still maintains lateral stability, right ? $\endgroup$ Nov 9, 2021 at 19:25
  • $\begingroup$ I certainly wouldn't want to try a first takeoff with 12 radian/s roll rate unless I also had exponential control in the transmitter (that was a very expensive option when I flew a lot). Then again, I used to set up my control throws with "TLAR" rule -- That Looks About Right -- and all my models flew very well. $\endgroup$
    – Zeiss Ikon
    Nov 9, 2021 at 19:33
  • $\begingroup$ Well then if I wanted to redesign the aileron so that I can achieve a decent roll rate, what is the first thing I should look at ? $\endgroup$ Nov 9, 2021 at 20:54
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    $\begingroup$ I'd start with control throws. A reasonable size aileron can produce hyper-roll rates, or easily controllable rates, depending on how many degrees it's deflected. Control throws can also be easily adjusted (on the ground, or in flight with some radio sets) to give transport or aerobatic flight characteristics. $\endgroup$
    – Zeiss Ikon
    Nov 10, 2021 at 12:12
  • $\begingroup$ I will definitely talk to the pilot, and he has to be a professional one, and see how we can probably solve this issue. Thanks alot @Zeiss Ikon $\endgroup$ Nov 10, 2021 at 18:00

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