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Aug 16, 2004, 03:53 PM
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Ib Jensen's Avatar
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Servo Sizing


Can anyone point me to a link that will help me determine the torque requirements of a servo for a given control surface size and expected speed range? I am sure that I saw something along those lines a few years ago but I have been unable to locate anything via Google.

Ib
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Aug 16, 2004, 05:27 PM
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Quote:
Originally Posted by Ib Jensen
Can anyone point me to a link that will help me determine the torque requirements of a servo for a given control surface size and expected speed range? I am sure that I saw something along those lines a few years ago but I have been unable to locate anything via Google.
repost:

Quick Servo Moment Prediction
-----------------------------

Xfoil is a reliable means for predicting a flap hinge moment.
But it can be closely approximated by the following formula:

M_hinge = K * Lift * flap_chord * (flap_area/wing_area) / CL

Multiplying by the servo/flap "gear ratio" then gives the servo torque:

M_servo = M_hinge * (servo_arm/flap_horn_arm)

The constant K depends on the airfoil camber and the flap deflection.
Conservative upper-limit values are:

K = 0.30 40 deg flap (landing)
K = 0.25 15 deg flap (launch)
K = 0.15 5 deg flap (snap-flap turn)

Example 1: Aegea 130" glider on strong winch or two-man tow launch.

K = 0.25
CL = 1.2
Lift = 150 lb
flap_chord = 2.4 in
flap_area/wing_area = 0.067
servo_arm/horn_arm = 0.75

M_hinge = 5.0 in-lb = 80 in-oz
M_servo = 60 in-oz

In this case, the flap is deployed before launch.
So the servo merely needs to hold this torque
without breaking and without too much compliance.

Example 2: 3m slope racer in turn.

K = 0.15
CL = 1.0
Lift = 90 lb (6 lb weight at 15 g's)
flap_chord = 2.0 in
flap_area/wing_area = 0.060
servo_arm/horn_arm = 0.75

M_hinge = 1.6 in-lb = 26 in-oz
M_servo = 19 in-oz

In this case, the flap is deployed under load.
So the max servo torque must exceed this 19 in-oz
torque by an adequate margin.
Aug 16, 2004, 06:19 PM
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Ib Jensen's Avatar
Thread OP
Thanks Mark,

I am trying to decide if I want to add flaps to 4M scale ship and didnít know what I would need to install. The formula should give me a good idea since the flaps would be longer than any I have done in the past.

Ib
Aug 16, 2004, 09:34 PM
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seaotter's Avatar
hi ib,

although dr. drela more than adequately answered your question, here's a quicky, plug-in servo calculator.

http://www.csd.net/~cgadd/eflight/calcs_servo.htm
Aug 17, 2004, 02:14 PM
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nuevo's Avatar
I used to use a spreadsheet published by Craig Tenney several years ago. A copy of it is available on this page:

http://www.rrcc.org/downloads.htm labeled RC Calculator

I do not know if the results from this spreadsheet correlate with Mark's equations.
Aug 17, 2004, 05:34 PM
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Ib Jensen's Avatar
Thread OP
Thanks for the links.

This will help confirm my guestamate.

Would you use Xfoil to calculate the torque requirements for pitherons?

Ib


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