Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
Time and Flight Expired Items
A number of items on the airframe will have either time or number of flight limits on them before they should be serviced or replaced.
Most obviously, any internal combustion (IC) engine will need regular servicing by appropriately trained staff closely following the manufacturers’ manuals. We carry out minor services such as valve clearance adjustments ourselves but prefer to use specialist companies for major overhauls. It is very easy, for example, when using four-stroke IC engines with very small valves to reach a situation where the exhaust valves are not seating correctly. If an engine is flown in such a condition, it is possible for very rapid valve seat wear to occur and the engine to fail in flights as short as 10 min duration. On a single-engine aircraft, this might lead to the total loss of the airframe.
Servos and Control Surfaces
Control surface servos are the next most obvious items that can suffer significant wear during flight. Consequently, we tend to replace these after specified numbers of flying hours. If one is using aero-modeler-grade items, or if the servos are very heavily loaded, they should probably be replaced at intervals of 10 h or less. Certainly, no servo should be used beyond 100 h flying time, even if only very lightly used. The attrition of on-board vibration can of itself lead to the gradual degradation of any complex electromechanical part. Attention should also be paid to the linkages and hinge points that are used to locate all control surfaces. It is very easy to accumulate wear in any bearing surfaces that will gradually allow play in the control mechanism, which will make flight control less easy and can induce control surface vibration and flutter.
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