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Servo torque selection for swashplate


Old 02-10-2015, 09:42 AM
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Question Servo torque selection for swashplate

Good Morning, This is my first post in this forum, and I hope you can help me.
Actually, I am designing a helicopter of three blades of 700mmX79mm asymmetrical, with 7Hp motor of two cylinders. My problem appear when I need to select the servos for the Swashplate; I will use a configuration of three servos at 120° each other, but I don’t know how to determinate the torque of these servos.
I begin calculating the pitching moment at the 25% of the chord (the screw to support the blades are at this position), but the moment that I can calculate is very low 1.55 Kg-cm per blade at 2000 Rpm with moment coefficient of - 0.019 value obtained with Javafoil. For the three blades I will required around 4.5 Kg-cm.
If I consider the double of torque because the friction, deformation, or any other reason I will need 9 Kg-cm, but I will use 3 servos so each servo needs to give 3 kg-cm of torque.
Now, I find that similar helicopters of 700 mm uses three servos at 120° of at least 18 Kg-cm of torque each one, so I do not understand the reason or how I need to calculate it, because I don’t want to use only a bigger servo, I want to understand and justify the use of a specific servo.
I hope someone can help to solve this.
Thanks a lot for your time
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