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Old 01-18-2019, 06:57 PM
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I'm building a 1/4 scale Sig Piper Cub and I was wondering what size servos I should be using. How much pulling strength would a particular servo need to have to safely fly a plane of this size?
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Old 01-18-2019, 08:02 PM
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What engine do you plan to use? The reason I ask is that servo requirements are not only flight loads but also the vibration level plays a part as well.
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Old 01-19-2019, 05:10 AM
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I plan on using the OS 160 twin cylinder Gemeni. The plane will also be on floats.
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Old 01-19-2019, 05:37 AM
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You can go here and calculate the servo torque you'll need. Servo Torque

Sig recommends "heavy duty servos" (whatever that means) for their 1/4 scale Cub. I'd see what the calculator has to say but something in the 75 ounce plus torque range will probably be fine. Depending too if you're doubling up anywhere like one for each aileron, etc. But why stop there? You can get some pretty powerful servos today for near pennies.

Servo City is a good source for checking specs on a lot of servos at once. Servo City

Last but not least, I'm working up a Hostetler Piper Cherokee. 108 inch wing. Here's what I worked up for the servos:
Aileron - 100 oz x 2
Flaps - 200 oz x 2
Rudder - 50 oz x 1
Flying Stab - 150 oz x 1

The calculated torque is far less than what I elected to go with. Example: The flaps at 40 degrees deflection calculated to 147 oz at 85 MPH. I don't think this model will do 85 MPH without the help of gravity. And I doubt I'll ever try to deploy flaps at full speed. The ailerons at 30 degrees deflection (that's a lot) calculate to 50 oz at 85 mph.

So try the calculator. It was a learning experience for me. You'll need the dimensions of your control surfaces. Difficult on the tail surfaces but a good estimate will work. The area of the surface is what you're after.
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Old 01-19-2019, 05:55 AM
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Believe it or not but bog standard Futaba 3003s running on 6 volts will do the job just fine.

If you are fitting flaps I would use two servos one in each wing. Same for ailerons. This is more for convenience than anything else.

The only time you would run out of control authority is if you try to fly knife edge at full throttle. A most un Cub manoeuvre.
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Old 01-19-2019, 06:38 AM
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The Hitec 645 servo would be a popular choice and fairly inexpensive. Depending on how much precision you want out of the servo you could also opt for the digital version 5645. These two would work very well for scale like flight. If you were looking at some aerobatics with the model you may want to look at some of the 79xx servos that have more power and better cerntering. One of my favorites is the 7955, a solid performing servo but a very good value for the dollar.

I might also add that your linkage setup is equally important. A good rule of thumb is to always use the shortest servo arm and longest control horn possible that provides desired throw. If you find yourself reducing throw via your TX then you can improve upon your mechanical setup.

Last edited by speedracerntrixie; 01-19-2019 at 07:24 AM.
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