Limits on ship motion pitch and roll are developed for helicopters on ship decks to ensure peak loads at airframe tie-down points are not exceeded.
However, the cyclical loading on the airframe due to ship motion and the effect on aircraft fatigue is generally not taken into account.
During any one day a ship may pitch and roll around 8,000 times and the effect this has on accumulative fatigue damage can be of concern, particularly for helicopters not designed for maritime operations.
Ondeck is capable of using simulated or experimental ship motion data covering a range of ship speeds, relative wave headings and significant wave heights.
Knowing the usage of the ship and helicopter and the probability associated with experiencing a given sea state allows tie-down and undercarriage component load spectrums to be derived.
By computing the stress levels in each component and using a S-N curve for the material the number of allowable cycles may be determined. Detailed knowledge of the structure is required.
Top: Simulated image from the Back Hawk fatigue analysis.
Bottom: Typical Load Spectrum Plot
Left: Black Hawk lashed to the deck of a RAN ship.