M. Guillaume, R. Feitknecht, M. Figliolino, R. Jaccard, G. Mandanis, I. Pfiffner, L. Schmid

The Swiss F/A-18 was redesigned based on the MIL-HDBK-1530 to meet the requirement for a service life of 5000 flight hours of Swiss design usage. In order to validate the extensive Aircraft Structural Integrity Program (ASIP) a dedicated full-scale fatigue test (FSFT) was performed at RUAG Aerospace at Emmen, Switzerland. Fatigue cycling was completed in the fall of2004 after approximately two service lives without major failures. At 3000 flight hours of fatigue cycling already large cracks were found on the lower side of the inboard leading edge flap (ILEF) spar drive lugs. The early fatigue damage came as a surprise as the ILEF spar was redesigned due to early cracking in the U.S. Navy (FT01/ST-16/FT93) fatigue test. The redesign was based solely on ILEF hinge moment spectrum and consisted of parabolic instead of circular fillets and a beef-up of the spar flange. The crack initiation life was predicted to 38000 flight hours. In the Swiss F/A-18 FSFT the ILEF was loaded by two actuators using a two-stage whiffletree. Deflection of the ILEF was fixed at 0°, i.e. fair position. The evaluation of sixteen strain gages installed on the ILEF spar indicated that the ILEF spar is also subject to significant bending. It could be shown by FE analysis that this bending is enforced by the deflection of the inner wing. Further analysis revealed, however, that ILEF deflection results in a beneficial stress relief on the critical location. Therefore, by neglecting ILEF deflection in the Swiss test the early cracking can be explained by over-testing. The parabolic fillets of the redesign emerged as a double-edged sword: On one hand they cause a stress reduction if loaded in hinge direction. On the other hand they result in a stress increase if loaded in transverse (in this case: span wise) direction. The manufacture of parabolic fillets caused further concern, as the poor surface quality of the fillets indicates. Therefore, the use of parabolic fillets in aircraft design is questionable. Cracks from flying aircraft were reported recently, but unfortunately they were located on the upper side of the ILEF spar. This causes some head ache, since the upper side is compression dominated - as far as it is understood today.

23rd ICAF Symposium of the international Committee on Aeronautical Fatique, 2005, Hamburg

Conference Paper

englisch

A5, 12 Seiten

DGLR-Bericht, 2005, 2005-03, 23rd ICAF Symposium of the international Committee on Aeronautical Fatique - Proceedings; S.113-124; 2005; Deutsche Gesellschaft für Luft- und Raumfahrt - Lilienthal-Oberth e.V., Bonn

NA

fatigue tests, f/a-18 aircraft, fatigue, redesign

Bestellbar

2005