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Tuesday, March 25, 2008, 04:20 PM - PULL TESTS PERFORMED
Posted by Administrator


The FAR covering the design requirements for propellers says that all parts of the propeller shall have a safety margin of twice the calculated stresses. I now quote CAM 14.

"14.151 Centrifugal load test. The hub and blade retention arrangement shall be subjected to a centrifugal load equal to twice the centrifugal force to which the propeller is to be subjected in normal operation. Either one of the following two test methods shall be acceptable:

A one-hour whirl test, or
A static pull test."
OK, so FAR 35 replaces CAM 14. Again I quote, "Sec 35.35 Blade retention test.

The hub and blade retention arrangement of propellers with detachable blades must be subjected to a centrifugal load of twice the maximum centrifugal force to which the propeller would be subjected during operation within the limitations established for the propeller. This may be done by either a whirl test or a static pull test."

We will now go into the technicalities and then present the results of my testing.


A typical 0-85 blade exerts a total load of about 19,000# of centrifugal force. The sum of all side loads add up to less than 500# for a total of about 19,500#.

The screws have a minor diameter of 0.199 to 0.203" They are heat treated to tensile strength of 160,000 to 180,000 psi. Taking the worse case of .199 x 160,000 yields a breaking point of 4,976# each. With 18 screws in the 0-85 blade that equals 89,575# for destruction. Therefore we divide 89,575 by 19,500 we get 4.59 safety factor where only 2.0 is required by the FAR. That is taking the worse case screw diameter and the minimum heat treat. If we take the maximum screw diameter and maximum heat treat we get 5,826#. Multiply that by 18 and it yields 104,864#. That is a 5.37 safety factor.

All screws have 4.5" of threads, some have socket heads and others have hex heads. The function is the same regardless of the type of head and the length.

THE TEST RIG & HOW IT WORKS. This test rig can pull up about 40,500 pounds.

Hydraulic fluid is pumped from the reservoir via the hand pump via the high pressure line into the hydraulic cylinder above the piston. Pressure is applied via the hand pump causing the cylinder to retract the ram creating the pull force. The pressure gauge reads the hydraulic pressure. By observing the pressure at the time the test article fails allows the total force to be calculated. For example. The bore of the cylinder is 4 inches. The area of the shaft is subtracted from the area of the cylinder and the result is 11.43 sq/in. Therefore if the pressure gage reads 2000 psi, by multiplying 11.43 times 2000 you get a total force of 22,860#. That's a bunch.

These photos show the hand pump, reservoir, calibrated digital pressure gage, 0 - 5000 psi and some plumbing.

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