The NTSB says the manufacturer recommends the addition of power to counteract the oscillation. Good to know stuff!
The 2009 report below issued some information. Transmission mounts as stated above:
This issue was addressed when I was at the Robinson Safety course last year. It only occurs in a certain range of serial numbers of the R-44. They found that the fore/aft oscillation was due to a certain type of rubber transmission mounts that were used and only happens at a certain rate of descent when flying over 100 knots. I can't remember all the details, but that's what it is.
NTSB Identification: ANC09GA040
14 CFR Public Use
Accident occurred Tuesday, May 12, 2009 in Iliamna, AK
Probable Cause Approval Date: 04/22/2010
Aircraft: ROBINSON HELICOPTER R44, registration: N7196H
Injuries: 3 Uninjured.
The National Transportation Safety Board determines the probable cause(s) of this ACC as follows:
The main rotor transmission mount design, which resulted in an in-flight vibration/oscillation, and damage to the helicopter during an emergency descent and hard landing. Contributing to the accident was the lack of information from the manufacturer regarding this known flight oscillation, and loading the helicopter beyond the forward center of gravity limit by the pilot.
The IIC contacted Robinson Helicopter, and asked if they were familiar with the term "chugging" as it related to Robinson helicopters, particularly the model R44.
During a telephone conversation with the manufacturer's accident investigator on May 27, the investigator said that he was familiar with the phenomenon known as "chugging," and that the manufacturer had conducted flight tests related to the phenomena. He said the tests determined that an oscillation may develop during operation of the helicopter at high gross weight, about 90-100 knots, and that the oscillation was more of a "bucking" motion due to the fore-and-aft movement of the rotor mast. He said that the tests showed the tendency to enter the oscillation regime was exacerbated by a forward CG (within the CG envelope) and a 30 degree banked turn to the left. He further indicated that the helicopter may also begin to oscillate in a right turn, but it entered the oscillation regime more easily in a left turn. According to the investigator, the tests also showed that chugging could occur within the normal CG range, and most typically at or near a gross weight condition. According to the manufacturer, it was determined that the oscillation is not divergent (destructive to the helicopter), and that the helicopter can be landed safely.
He said that the oscillation can be mitigated by the application of power, and the condition flown out of. He said that the only damage he had seen to the test helicopter was on one occasion, when the pilot landed while the helicopter was still "bucking," and that the damage was manifested as dents/marks on the cabin-top, made by the fore-and-aft movement of the main rotor-shaft fairings.
The Robinson investigator said that the manufacturer attributes the oscillation to the firmness, or lack of firmness, of the transmission mounts, and that the manufacturer changes the mounts on helicopters that exhibit a tendency toward chugging during post manufacturing test flights. The investigator said he did not know the standard by which mount firmness was measured. He said he believed that approved helicopter service centers were aware of chugging, although currently there are no service alerts/bulletins referencing the phenomena. The investigator said to his knowledge there were no factory provided alerts/bulletins, pilot training, or Pilot Operating Handbook (POH) entries, pertaining to the chugging and the remedies to resolve it. http://www3.ntsb.gov...513X60228&key=1
Edited by iChris, 29 March 2011 - 23:11.