# How do you know the maximum load a control surface can withstand?

This is a separate strength testing (load testing) of the elevator of an airplane, how was the load determined? How did the engineers determine the maximum elevator load?

• – ymb1 Oct 3 '18 at 16:55
• Which of the 3 questions are we answering here? Basically, I'm going with "test to destruction", and when it fails the maximum load has been determined. – CrossRoads Oct 3 '18 at 17:56
• @CrossRoads - Your proposed method would test how much a control surface can take, but not how much it will experience in-flight. – ymb1 Oct 3 '18 at 18:45
• That wasn't the question tho of the 3 questions. That answer requires more info than is available to answer. That would have to come from the designer. How much is needed? Enough to provide full elevator authority when coming in engine out and be able to safely flare. – CrossRoads Oct 3 '18 at 19:30

Elevator loads are the function of the design and flight envelope. Looks like a small plane, so look up for example CS-VA 391 (or the correct national code for your category) how to calculate the loads. Base air force distribution on the horizontal tail can usually be assumed to be triangular, and increment from control surface deflection need to be added, also usually approximated as a triangular distribution acting from the hinge line.

The elevator must be able to withstand aero loads at $$V_{NE}$$ and maximum deflection, using a safety factor of 1.5 at least.

The aero load = the hinge moment $$H_M$$ is:

$$H_M = C_h \cdot ½ \rho {V_h}^2 \cdot S_s \cdot {\bar{c}}_s$$

with

• $$½ \rho {V_h}^2$$ = dynamic pressure at the control surface
• $$S_s$$ = control surface area
• $${\bar{c}}_s$$ = mean aerodynamic chord of the surface behind the hinge axis.
• $$C_h$$ = factor depending on angle of attack and surface deflection. The figure below depicts the coefficients for the Fokker 27.