Question on Servos
12-21-2013, 04:18 PM
#1
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Question on Servos
How do you find out what size of servos to use, (meaning torque) on any aircraft? How do you size a servo to fit the requirements. Is it wing loading, weight, ? If no recommendations are listed in the aircraft instruction manual, which most Chinese ARF'S fail to do, then is there a chart or anything to find this out?
12-21-2013, 06:28 PM
#3
This is a good question, and I believe for the majority of us there is no technical answer. Some guys have gone as far as to measure loads applied to control surfaces and corresponding current draw and servo performance. The rest of us use 'rules of thumb' or use whatever works for our friends.
I personally would find a similar airplane made by a reputable company and see what they recommend.
I fly ~65-78" wing span planes. My 'bare minimum' on glow or electric for aileron torque is 80oz-in, rudder is 120 oz-in, elevators is 45oz-in per side. I honestly don't know if I could get away with less, but the servos I've used were recommended by friends, they work well, and are not terribly expensive.
The other thing you can do is look at the servo mfgr's web page and see if they have recommendations. You might see an 'aileron servo' that is recommended for 1.20 - 1.60 size planes or something.
The technical answer: would have to take into account the load (air pressure acting over control surface, ounces/in2 x in2 = oz) x distance (control geometry, factoring the position of the horns, length of the horns, etc, in inches) = oz-in. You would want something greater than the max possible load so you don't stall the servo. This would ensure the servo can always move the control surface. In addition to all this is the dynamic load (only talking about static loads so far). I haven't thought a ton about dynamic loads but it's safe to say that you need some margin to handle dynamic loads ( for example think holding some left rudder, then to full right rudder, maybe back to a little left rudder) where there might be some momentum or maybe the high current draw lowers the voltage (potential) to the servo.
If the servo is not sized to handle dynamic loads you can get into flutter, which is an oscillation of the control surface. When the servo is close to being maxed out it might be able to move the control surface to a point, then the load on the control surface becomes too much and gets pushed back, and as the geometry straightens out the servo regains control, this goes on as if there was a spring in the control linkage and bad things happen. So you want a little margin.
How would a higher power (more torque) servo compare to a 'sized right' servo? Maybe it doesn't get bogged down when it is close to max load, so you would have a 'crisper' feeling control surface. Maybe this is why 3D guys use servos with tons of torque (in addition to the fact the control surfaces are big).
For the 2M aerobatic planes I am most familiar with, a common rudder servo might be 200 oz-in. Some guys have measured the actual loads and say it's nowhere near that. 200oz / 16oz/lb = 12.2lbs. It would seem that a 200 oz-in servo would be strong enough to power more weight than the entire airplane itself (11lbs). But you have to remember to factor in the geometry and/or distance. I haven't thought too much about that at the moment...
So the point is that most of us don't know the true 'minimum requirements' for sizing a servo, but we do have friends in these forums with decades worth of experience to learn from. There are also reputable ARF manufacturers who hopefully are recommending the right size servos for their airplanes, and we can make good guesses as to what similar planes might need.
Edit: Well I got beat to the point by the tool in the above link
. I would personally want to see what goes into the code before completely trusting it, but it's hopefully going to get you in the right playing field. Note that you have to put in your own 'factor of safety' into that tool. At 70mph, a 2"x15" aileron deflected 40 degrees would require only 22oz-in. That sounds low, but maybe that's all we need? Only a few ways to find out ...
I personally would find a similar airplane made by a reputable company and see what they recommend.
I fly ~65-78" wing span planes. My 'bare minimum' on glow or electric for aileron torque is 80oz-in, rudder is 120 oz-in, elevators is 45oz-in per side. I honestly don't know if I could get away with less, but the servos I've used were recommended by friends, they work well, and are not terribly expensive.
The other thing you can do is look at the servo mfgr's web page and see if they have recommendations. You might see an 'aileron servo' that is recommended for 1.20 - 1.60 size planes or something.
The technical answer: would have to take into account the load (air pressure acting over control surface, ounces/in2 x in2 = oz) x distance (control geometry, factoring the position of the horns, length of the horns, etc, in inches) = oz-in. You would want something greater than the max possible load so you don't stall the servo. This would ensure the servo can always move the control surface. In addition to all this is the dynamic load (only talking about static loads so far). I haven't thought a ton about dynamic loads but it's safe to say that you need some margin to handle dynamic loads ( for example think holding some left rudder, then to full right rudder, maybe back to a little left rudder) where there might be some momentum or maybe the high current draw lowers the voltage (potential) to the servo.
If the servo is not sized to handle dynamic loads you can get into flutter, which is an oscillation of the control surface. When the servo is close to being maxed out it might be able to move the control surface to a point, then the load on the control surface becomes too much and gets pushed back, and as the geometry straightens out the servo regains control, this goes on as if there was a spring in the control linkage and bad things happen. So you want a little margin.
How would a higher power (more torque) servo compare to a 'sized right' servo? Maybe it doesn't get bogged down when it is close to max load, so you would have a 'crisper' feeling control surface. Maybe this is why 3D guys use servos with tons of torque (in addition to the fact the control surfaces are big).
For the 2M aerobatic planes I am most familiar with, a common rudder servo might be 200 oz-in. Some guys have measured the actual loads and say it's nowhere near that. 200oz / 16oz/lb = 12.2lbs. It would seem that a 200 oz-in servo would be strong enough to power more weight than the entire airplane itself (11lbs). But you have to remember to factor in the geometry and/or distance. I haven't thought too much about that at the moment...
So the point is that most of us don't know the true 'minimum requirements' for sizing a servo, but we do have friends in these forums with decades worth of experience to learn from. There are also reputable ARF manufacturers who hopefully are recommending the right size servos for their airplanes, and we can make good guesses as to what similar planes might need.
Edit: Well I got beat to the point by the tool in the above link
. I would personally want to see what goes into the code before completely trusting it, but it's hopefully going to get you in the right playing field. Note that you have to put in your own 'factor of safety' into that tool. At 70mph, a 2"x15" aileron deflected 40 degrees would require only 22oz-in. That sounds low, but maybe that's all we need? Only a few ways to find out ...
Last edited by Jetdesign; 12-21-2013 at 06:36 PM.
12-25-2013, 02:50 AM
#4
As Joe stated,
It's something you just learn over time.
Meanwhile, rely on the advise of those, where you fly, that you respect their building ability until you gain experience enough to "just know" on your own.
Many USA based Kit and ARF companies don't make servo recommendations either,
Tell us what plane it is and we'd make some recomendations
It's something you just learn over time.
Meanwhile, rely on the advise of those, where you fly, that you respect their building ability until you gain experience enough to "just know" on your own.
Many USA based Kit and ARF companies don't make servo recommendations either,
Tell us what plane it is and we'd make some recomendations
12-25-2013, 03:53 AM
#6
The old rule of thumb prior to all the 40% plus planes and 3D flying was 1 ounce of torque for every square inch of control surface. That still works well today for planes other than previously mentioned. IMHO we tend to "over" servo our planes, that is not altogether bad but using some good common sense can save us a lot of cash over the years.
12-25-2013, 05:46 AM
#7
Senior Member
How the system is constructed makes a big difference. Well balanced, both static and aerodynamic, surfaces can be safely moved by very small forces. Case in point, full scale ailerons were controlled with a regular RC type servo just to prove the point by Burt Rutan on the Vari Vigan. The servo operated a "boost tab" which then moved the aileron. I've done the same on some quite large 1/4 scale planes using a boost tab on both the elevator and the rudder. A 40 ounce/inch servo handled them very well.
