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FAQ: Fixed Wing Aircraft Vibration
Why be concerned with Aircraft Vibration?
Excess and unwanted aircraft vibration can cause discomfort and fatigue for the flight crew and passengers. The service life of expensive avionics and instruments is shortened considerably when the equipment is operated in a high vibration environment. Excess vibration can also shorten the life of expensive engine components such as vacuum pumps, exhaust systems, engine ducting and many other items.
Can my aircraft maintenance technician troubleshoot Aircraft Vibration?
Yes!!! Aircraft Vibration Troubleshooting is the process whereby an electronic vibration analyzer is used to measure the vibration produced by the aircraft power plant and relate that vibration to one or more unwanted vibrations which are usually felt in the cabin by the occupants. When the actual cause of the vibration problem is known, corrective action can be taken to deal with the problem.
What are the causes of unwanted (excess) cabin vibration?
An unwanted cabin vibration can be generated by a variety of problems:
- Propeller mass imbalance (mass distribution problem)
- Propeller aerodynamic imbalance (blades not pitched equally or airfoils not matched)
- Abnormal engine vibration (combustion problems or unmatched engine components)
- Normal engine vibration transmitted abnormally into the airframe (poor engine to airframe isolation)
- Aerodynamic excitation of airframe by prop wash.
Why go to the trouble of balancing my propeller first?
In each of the situations above, the very first step in troubleshooting an aircraft vibration problem is to dynamically balance the propeller. Because propeller imbalance is by far the most common cause of excess cabin vibration, we eliminate the majority of cockpit vibration problems with this first step.
Okay, so now my propeller is balanced, what comes next?
After the propeller imbalance is eliminated, other engine vibrations can be analyzed and traced to their source. A vibration analysis signature is taken on the engine and then compared to signatures taken
from similar engine models. With some engines, a vibration signature check will point to a combustion problem, perhaps caused by a dirty injector, a dirty spark plug or an ignition problem.
Although rare, sometimes a vibration signature check will point to an internal engine problem. When an engine problem is found this way, the engine can usually be repaired before the problem becomes unmanageable.
My engine vibration is normal, what is causing my unwanted cabin vibration?
If the engine vibration is reasonable, then vibration readings are collected in the cabin so that the "felt" vibration can be evaluated. While the engine may be producing "normal" vibration levels, these normal vibrations may be transmitted abnormally to the airframe due to lack of isolation between the engine and firewall. A vibration check can point the way to detection of isolation problems such as bad engine isolators, hoses which do not have adequate service play, exhaust system to airframe contact, etc.
What about aerodynamic excitation of the airframe?
Even with good mechanical engine to airframe isolation, vibration analysis can be used to detect and identify other unwanted cabin vibration. The air mass which envelops the aircraft can provide a transmission path for unwanted vibration. Any time the propeller is producing thrust, propeller induced air pulses can excite parts of the airframe such as thin or poorly attached windshields. With some aircraft, replacement of the windshield with a thicker aftermarket unit will reduce cabin vibration considerably.
Where can I find a mechanic to check my aircraft's vibration?
Aircraft vibration checks are done in many prop shops, repair stations and FBO's. For a local referral to a maintenance shop which provides vibration analysis and balancing services, talk to Dynamic Solutions Systems.
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