From The Designee File By Clayton T. King (EAA Designee Chairman) RESONANT FREQUENCY VIBRATIONS
R has caused many engine mount failures. The reports of these failures usually concerned cracks, found ESONANT FREQUENCY vibrations in aircraft engines
in or near welded joints, principally the result of recurrent resonant vibration. Most engines have "rough" spots at certain rpm values at which the engine will vibrate more than usual. The vibration responsible for fatigue cracks in these mounts is due to the engine vibrating about its thrust axis and is produced by the firing impulse frequency from the engine (2 x crankshaft rpm). Many experiments have been tried to eliminate or reduce the engine mount frequency causing these failures, but while the problem can be overcome, the solution results in greater weights, complication and cost. Besides causing failures, vibration results in a general deterioration of the airplane structure and is apt to produce powerplant failure due to fuel and oil lines not working, or malfunctioning of various components. While it cannot be eliminated, vibration can often be greatly reduced by avoiding engine operation at "resonant"
speeds (rpms). In commercial aircraft, the rpm at which this vibratijn occurs is sometimes stated in the owner's manuals. There are of course no owner's manuals for homebuilts, and one cannot compare his aircraft or engine with another as the resonant frequency will be slightly different, depending upon the propeller, size, type and many other factors. How then, can a person determine the "unsafe" rpm of his engine? First, the operator should run the engine while the aircraft is on the ground, starting with the lowest idling speed (which should be no lower than 550 rpm for this purpose), gradually increasing to practically full throttle
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and noting any rough speed encountered. Care should be taken not to overheat the engine. Having noted any rough speed the operator should go above it and check it while decreasing rpm. If this critical or rough speed is in the normal flighl range, further check should be made while the plane is
in the air. Due to tachometer inaccuracies, rough speed readings should be made only from the instrument installed in the airplane, and as we mentioned, never compared with similar makes and models. Rough speed for the engine should be noted on or near the glass face of the tachometer as a warning to the pilot that continued operation within 100 rpm of this speed should be avoided. If a plane has been subjected to excessive vibration far a long time, the engine mount should be inspected for cracks either before further operation or at the next annual inspection. Engine and mount do not have to be removed for this. While in flight, if the pilot notes unusual roughness and cannot eliminate it by changing to another rpm, he should land and remedy the difficulty before resuming flight. Roughness from propeller unbalance will in most cases persist over a wide speed range and tend to become worse only at higher speeds; but resonant vibration is characterized by vibratory motion of the engine, or some component of its installation, attaining a maximum amplitude while in resonance and then diminishing as speed is increased or decreased. If an airplane owner wants to know for certain if the vibration in his aircraft is due to resonance or propeller unbalance, a complete balance check should be made on his prop. In either case, whether excessive vibrations are caused by engine OR prop, AVOID THAT SPEED RANGE!
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