Originally Posted by
Capt Hollywood
Sure, the more aware people are of what to expect the better.
A couple of our pilots had reported what they thought was the onset of the 'vibration', both reported it at similar configurations. So I elected to take the aircraft out and see if I could replicate what they felt, I departed with 400lbs of fuel on board. Around the 300lb mark, a slight bounce became evident, you'll see me nod to the camera at the 00:11s mark to indicate I could feel something as I wasn't sure what the camera would pick up. The 'bounce' increased in amplitude so I applied lateral cyclic which, at the time, was a method to correct the phenomenon. The bounce did indeed reduce almost immediately, although it was still present. I continued on to see if it would develop again and this time would wait a little longer to see if it continued to increase in amplitude. I gestured again to the camera at 00:34s mark. When the bounce became uncomfortable, I tried the lateral cyclic again. This time the lateral cyclic had less of an immediate effect. Unfortunately, the video doesn't capture the sound properly as it added to the extremely unsettling feeling!
Longranger with Van Horn main rotor blades.
Capt Hollywood: Thanks for sharing this. I made an attempt to quantify the frequency of the vibration in the video. If it is set to play at quarter speed then it is possible to count the oscillations of your body or the cyclic stick and divide that by the corresponding time on a stopwatch. Converting back to normal speed the frequency is consistently about 4.9 Hz whether for the first portion of bouncing or the second and whether considering your body movement or the cyclic stick movement. The oscillations are also very uniform in frequency. I am not sure if these frequency estimates are corrupted by anything like the video frame rate, but others might wish to comment.
If I understand the 206L data correctly the main rotor 100% speed is 394 RPM? That equates to a frequency of 6.57 Hz, but given there are two main blades, the blade passing frequency becomes ~13.1 Hz (my understanding is that rotor vibration frequency is what the 206L Nodamatic suspension is tuned to damp out). I was sort of expecting it would also be that frequency that might be setting the VH blades up into an undesirable vibration mode. But this blade passing frequency seems to be around 2.7x higher than what the oscillations in the video appear to be.
Has anyone got any further insights into what might be happening?