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Author: Smile Khatri Date: November 11, 2024
Servo selection
To size the servos for the elevator and the rudder an online calculator is used: RC Airplane Calculator
The chord length, and span of the control surfaces (identified as ‘length’ in the calculator) are input into the calculator, along with servo deflection and surface deflection. An assumption is made that a 30 degree deflection of the servo maps to a 30 degree deflection of the control surface, hence the input and output response is linear. For elevator and rudder, this is the best practice (RC Basics: The importance of good linkage geometry).
100 km/h is the maximum speed we expect this plane to fly at, so the servo selection is being made at the most extreme case. The cruising speed would actually be about 50 - 60 km/h.
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From the results, the following servo is selected
2.5kg Goteck Metal Gear Micro Servo - RobotShop
Key specifications:
Stall torque: 2.3 kg-cm(4.8v), 2.5 kg-cm(6.0v)
Operating voltage: 4.8 - 6 V
Weight: 14.7 g
Size: 23 x 12.1 x 28.8 mm
Required torque for the rudder at maximum speed is 1.81 kg-cm, the stall torque at 6V is 2.5 kg-cm for the servo, giving us a safety margin of 0.69 kg-cm (1.38 safety factor)
For the elevator, 2 servos would be needed. The required torque is 3.34 kg-cm and the total stall torque from 2 servos is 5 kg-cm. This gives a safety margin of 1.66 kg-cm (1.5 safety factor).