Guys
I recently re-fitted my SK720 to my Chaos 600 which has a 4 blade head and 4 blade tail fitted to it. I have it running at either 1350rpm or 1400rpm.
During some hover testing I could see the skids vibrating, and the tail was giving a little kick every few seconds (especially bad with a wind gust). I tried tail gains from around 50 up to just over 100. The higher gains seemed to help a bit for the kick, but it certainly did not feel locked in.
When I got back home I looked at the logs and noticed that although the overall number ("Last Vibration Level") reads 0 (is this because I have it set to do a vibration log rather than a playback log ? Looking at the vibration spectrum there are a few spikes that seem higher than desirable.
I would greatly appreaciate some of you more experienced guys having a look and telling me what you think.
The speeds are; electric motor = 19380rpm, main rotor = 1407rpm & tail rotor = 6332rpm. The gears (all are straight cut spur gears except the bevel gears) are; motor pinion = 13T, main gear (meshes with motor pinion) = 170T, tail driving gear (which spins on the main shaft together with the main gear) = 180T, pinion tail gear (meshes with 180T gear) = 40T, and the rest of the tail drive is 20T bevel gears which run at the same tail speed as the 40T gear.
Starting from the left the main peaks are;
23Hz = 1380rpm (roughly main rotor headspeed)
90Hz = 5400rpm (a bit under 4x main rotor speed)
110Hz = 6600rpm (roughly tail speed the smaller spike to the left of the main one is 106Hz = 6360rpm)
238Hz = 14280rpm (roughly 10x main rotor speed)
280Hz = 16800rpm (roughly 12x main rotor speed)
321Hz = 19260rpm (motor speed)
363Hz = 21780rpm
401Hz = 24060rpm (roughly 17x main rotor speed the slightly smaller spike to the right is at 406Hz = 24360rpm)
443Hz = 26580rpm (roughly 4x tail speed)
Obviously the main ones I would like to reduce are 6600rpm, 14280rpm, 19260rpm & 21780rpm vibes. The rest are either small enough or multiples of these so I could probably live with them. I dont have details on the bearings (eg. Number of balls), but they would all be single race ball bearings except for the needle race one-way (acts like a clutch) bearing on the main rotor shaft (it has 11 needles in it).
The main vibrations seem to be up-down and sideways (lateral) movements).
I then started removing parts and re-testing.
I have run some tests this morning with no main blades and the difference was minimal the 10x amplitude dropped a bit, but that was about it.
Then I took the rotor head off (leaving a bare shaft end), and that dropped the motor speed amplitude a bit, but nothing major.
Then I disconnected the tail drive gear (so only the two larger gears spinning with the main shaft) and that obviously reduced the tail frequency amplitude significantly, but actually increased the 10x and motor speed amplitudes.
Then I removed the main gear assembly and main shaft and the 10x and motor speed amplitudes dropped a bit, but still fairly high.
So really, I am not much further forward on this as there was nothing obvious that made a step change improvement. I am struggling to get my head around how this would all average out to a single vibration number, which is what the stabilisation system uses as its criteria for effective stabilising. I dont know how these sharp spikes would affect an overall number are they additive or averaged, or something much more complex than this ?
Any thoughts would be greatly appreciated.
Cheers
Colin
I recently re-fitted my SK720 to my Chaos 600 which has a 4 blade head and 4 blade tail fitted to it. I have it running at either 1350rpm or 1400rpm.
During some hover testing I could see the skids vibrating, and the tail was giving a little kick every few seconds (especially bad with a wind gust). I tried tail gains from around 50 up to just over 100. The higher gains seemed to help a bit for the kick, but it certainly did not feel locked in.
When I got back home I looked at the logs and noticed that although the overall number ("Last Vibration Level") reads 0 (is this because I have it set to do a vibration log rather than a playback log ? Looking at the vibration spectrum there are a few spikes that seem higher than desirable.
I would greatly appreaciate some of you more experienced guys having a look and telling me what you think.
The speeds are; electric motor = 19380rpm, main rotor = 1407rpm & tail rotor = 6332rpm. The gears (all are straight cut spur gears except the bevel gears) are; motor pinion = 13T, main gear (meshes with motor pinion) = 170T, tail driving gear (which spins on the main shaft together with the main gear) = 180T, pinion tail gear (meshes with 180T gear) = 40T, and the rest of the tail drive is 20T bevel gears which run at the same tail speed as the 40T gear.
Starting from the left the main peaks are;
23Hz = 1380rpm (roughly main rotor headspeed)
90Hz = 5400rpm (a bit under 4x main rotor speed)
110Hz = 6600rpm (roughly tail speed the smaller spike to the left of the main one is 106Hz = 6360rpm)
238Hz = 14280rpm (roughly 10x main rotor speed)
280Hz = 16800rpm (roughly 12x main rotor speed)
321Hz = 19260rpm (motor speed)
363Hz = 21780rpm
401Hz = 24060rpm (roughly 17x main rotor speed the slightly smaller spike to the right is at 406Hz = 24360rpm)
443Hz = 26580rpm (roughly 4x tail speed)
Obviously the main ones I would like to reduce are 6600rpm, 14280rpm, 19260rpm & 21780rpm vibes. The rest are either small enough or multiples of these so I could probably live with them. I dont have details on the bearings (eg. Number of balls), but they would all be single race ball bearings except for the needle race one-way (acts like a clutch) bearing on the main rotor shaft (it has 11 needles in it).
The main vibrations seem to be up-down and sideways (lateral) movements).
I then started removing parts and re-testing.
I have run some tests this morning with no main blades and the difference was minimal the 10x amplitude dropped a bit, but that was about it.
Then I took the rotor head off (leaving a bare shaft end), and that dropped the motor speed amplitude a bit, but nothing major.
Then I disconnected the tail drive gear (so only the two larger gears spinning with the main shaft) and that obviously reduced the tail frequency amplitude significantly, but actually increased the 10x and motor speed amplitudes.
Then I removed the main gear assembly and main shaft and the 10x and motor speed amplitudes dropped a bit, but still fairly high.
So really, I am not much further forward on this as there was nothing obvious that made a step change improvement. I am struggling to get my head around how this would all average out to a single vibration number, which is what the stabilisation system uses as its criteria for effective stabilising. I dont know how these sharp spikes would affect an overall number are they additive or averaged, or something much more complex than this ?
Any thoughts would be greatly appreciated.
Cheers
Colin