Originally posted by David Gladwin
having ply skins and mine weigh a little over 2 ounces. The center of mass of these elevators is about 1 3/4 inches fom the hinge line so that the torque resisting the servo power is 2x 1 3/4 or 3.5 ounce inches when the model is static.
I know from flight tests with my JetCat GPS equipped F15 that the F15 pulls a peak of 10.9 g in moderate aerobatics and upto 12.9 G in hard turns. ( I spoke to Horst Lennerz of JetTronics about this at the JWM in South Africa and he agrees that these values are realistic). I am sure that the big, thick wing of the BobCat can easily attain load factors of 10 and in a 10 g manoevre the torque generated by the mass of the unbalanced stab. is now 35 oz/ounces, so more than half of the servo power is used just to hold the elevator against G forces.
Hi David,
Interesting analysis, however I feel that you may have a few errors. I just took some measurements from my set of drawings and I can not get to the 1.75“ moment arm you speak of for the center of gravity. The geometric center is only 1.4” from the hinge line, and since there is more structure forward of this point than aft, the CG should be forward as well. Could you have meant 1.25”? If so, for your 10 g case this gets the hinge moment due to inertia down to 25 ounce inches. Now that load must be transferred to the servo shaft. To do this the moment is multiplied by the ratio of the arms (servo arm divided by the control arm). For the as drawn configuration this ratio is 0.5625 (.45”/.8”), also note that since this ratio is less than one it is the maximum that will exist. As the servo moves its effective arm reduces faster than the control arm on the surface so it gains more advantage the higher the deflection. Multiplying 25 ounce inches by the transfer ratio of 0.5625 gives us the torque the servo must produce to hold the surface, 14.06 ounce inches. Again, this the maximum, if the model is pulling 10 g’s the elevator is deflected a fair amount and the servo gains advantage thus lowering the torque a bit.
Respectfully,
Steven Ellzey