I would like to know what vertical descent rate a Huey or other helicopter with skids can land safely at. What would be considered a "hard" landing that would require air frame inspection? What would the descent rate be for a running landing? Thanks.
I don't have descent rate numbers, but many years ago I trained on a Bell 206. In the engine/transmission compartment, I recall there being a spike which was attached to the drivetrain, the drive train having some freedom of movement within the compartment. As the drivetrain wiggled around, this spike would move with it, and there was a plate with an opening within which the spike moved. If there was a hard landing, the spike could contact the sides of the opening. This could leave a mark, deform and in extreme cases could even break off. I Googled and found a manual for the 206, it mentions the skids and crosstubes will deform at 2.5G of load. It also mentions if the cabin fuselage reaches the ground, it is considered to have sustained a 10G load. Link to the document: https://www.bellcustomer.com/Bulletins/Download?FileName=206AB-Inspection_and_Airworthiness_Limitations.pdf&CategoryID=139
Neat, I wouldn't have thought the manual for the 205 (aka Huey for the military version UH-1) was also available but it is: See page 95 for the information on hard landing. No specific descent rates mentioned, but rather procedures to check for specific types of damage are spelled out. https://www.bellcustomer.com/Bulletins/Download?filename=205A1-Inspection_and_Airworthiness_Limitations.pdf
A particular issue is a hard landing in which one skid or wheel touches the ground first and the rotor has drag hinges. This can cause the blades to bunch up on one side of the rotor so it develops an asymmetry and starts to wobble. If the frequency of this wobble resonates with the eigenfrequency of the landing gear, ground resonance will develop. What follows can be watched here.
I would be less concerned about the sink rate and more about the symmetry of ground contact. To give only a single number is not sufficient for characterizing a safe landing.
A descent rate of over 240'/min on landing is considered hard. It may damage the skids and is supposed to prompt an inspection of the skids before further use.
Touching down with a descent rate above 720'/min (at 16,700lb GW) risks damaging the airframe as well as the skids.