Yaw moments are produced by the differential cyclic tilting of the rotors (i.e. one side forward, the other back).

The PA-97 was built under a 1980 U.S. Navy contract for the Forest Service to demonstrate a heavy vertical airlifter for harvesting timber from inaccessible terrain.

Their tail rotors were removed, their fuselages shortened and they were attached to a crude tubular aluminum framework beneath the helium-filled envelope.

[3] Test flights were made from the Naval Air Engineering Station Lakehurst in New Jersey, making use of the long-established hangars for handling large airships.

[3] The vibration was sufficient to cause a structural failure as the starboard rear helicopter broke off its mounting, its rotors cutting into the gasbag.