A full scale structural fatigue test program

Test methods are described for an experimental fatigue evaluation of a full-scale airplane structure. Specific reference is made to a fatigue test program conducted on the Lockheed P3A wing and gear. A flight-by-flight test loading spectrum comprising 7500 flight hours was constructed to represent the total service experience of the airplane. In the construction of the spectrum, emphasis is directed to the air-ground-air transitions that are provided during a flight-by-flight simulation. Previous studies have shown that these large transitions adversely affect fatigue life. Loads, preprogramed on magnetic tape, were applied to the specimen by hydraulic jacks through a closed-loop electrohydraulic servo system. The program employed 51 servo channels to control the force output of 105 jacks. The test methods devised for this program provided an automated loading system that faithfully duplicated service conditions on the specimen in a relatively short test-time span with a small operating crew. The span for the P3A program was 20 weeks, operating on a three-shifts-per-day basis with a three-man test crew per shift. The methods described can be readily adaptable to supersonic aircraft testing.