Servo selection for 38" span 90 mph plane
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Servo selection for 38" span 90 mph plane
I'm scratch building a 38" span plane. I'm targeting 90 mph for speed. Aerobatics are not important factors... just speed. I'm planning to use one E-Flite S-75 per control surface. The elevator has about 50% more surface area than the other control surfaces, though. It makes sense to me that the combination of control surface area, control surface throw, and speed (air pressure) determine servo torque requirements. Without knowing how much throw will be required (certainly not 3D aerobatics levels... I'm guessing +/- 20 degrees or so), should I bump up to a stronger servo (HS-81) on the elevator purely because of the increase surface area?
I've also read that having a strong servo and a stiff, slop-free linkage can help delay the onset of flutter. Is this also an argument for increased servo strength on a large (relatively speaking) control surface?
Or should I chill out and just use the S-75 servo since it saves weight? The elevator has an area of 15 square inches, while an aileron has about 10.
(Sorry to post yet another n00b question. I do my best at searching the archives before posting... I swear!)
I've also read that having a strong servo and a stiff, slop-free linkage can help delay the onset of flutter. Is this also an argument for increased servo strength on a large (relatively speaking) control surface?
Or should I chill out and just use the S-75 servo since it saves weight? The elevator has an area of 15 square inches, while an aileron has about 10.
(Sorry to post yet another n00b question. I do my best at searching the archives before posting... I swear!)
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RE: Servo selection for 38" span 90 mph plane
First off you won't need that much elevator. Assuming that the span of the stabilizer is 12 inches you only need the elevator to be about 3/4 inch wide. And 20 degrees is more than enough too.
There's links to a couple of online control surface to servo load calculators in the FAQ threads at the top of the listings. Find out what the load on the servo will be from one of those and then plan on using a servo with 1.5 to 2 times that much torque. You need this safety factor to avoid stalling the servo in use. Also I'd use a speed of closer to 130 mph so you don't overload it in a dive and find that it doesn't want to pull out... [X(]
PS: Sorry, it's over in Aerodynamics. Check down through [link=http://www.rcuniverse.com/forum/m_2867994/tm.htm]THIS THREAD[/link] for links to the load calculators.
There's links to a couple of online control surface to servo load calculators in the FAQ threads at the top of the listings. Find out what the load on the servo will be from one of those and then plan on using a servo with 1.5 to 2 times that much torque. You need this safety factor to avoid stalling the servo in use. Also I'd use a speed of closer to 130 mph so you don't overload it in a dive and find that it doesn't want to pull out... [X(]
PS: Sorry, it's over in Aerodynamics. Check down through [link=http://www.rcuniverse.com/forum/m_2867994/tm.htm]THIS THREAD[/link] for links to the load calculators.
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RE: Servo selection for 38" span 90 mph plane
Wow, those calculators are nice!
My control surfaces are that large for reasons of scale accuracy. (I can't help myself!) Those little tiny E-Flite servos can drive them at +/- 15 degrees up to 190 mph and still have 2x the required torque, though. At +/- 20 degrees I can go 140 mph and have the same safety margin. I'll probably shoot for +/- 15 since I'm not trying to make an aerobatic plane.
Thanks very much for the links and the advice... in this thread and other ones as well. You are a great help!
My control surfaces are that large for reasons of scale accuracy. (I can't help myself!) Those little tiny E-Flite servos can drive them at +/- 15 degrees up to 190 mph and still have 2x the required torque, though. At +/- 20 degrees I can go 140 mph and have the same safety margin. I'll probably shoot for +/- 15 since I'm not trying to make an aerobatic plane.
Thanks very much for the links and the advice... in this thread and other ones as well. You are a great help!
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RE: Servo selection for 38" span 90 mph plane
If you want surfaces that size for scale reasons then just reduce the throw. Figure on + 3/16 inch at the trailing edge of the elevator to start with and re-calculate the angle that results and go for that much. Best of luck with the speedster.