magknight;n1135000 said:
This question is to all of you rotor balance geniuses. We need some additional brain power to think through an issue. We installed new rotor blades (GyroTech) on the Cavalon today. They were dynamically and statically balanced at the factory with the teeter towers included. We installed the teeter tower and rotor system, all seems good.
On spin up, it’s very solid with no vibration, but on test runs / short flights down the runway we begin to get a strong cabin hop. Rotor RPM is about 330-350 and the hop is about 5-6 hops per second (which would point to a blade balance issue I think).
The dilemma is that the stick is really smooth, but the cabin hop is pretty extreme.
Thoughts?
Some thoughts I collected from from a while ago when building our Hornet.
Blade shake is cause by many things. Cone set, static balance of the blades, chordwise balance dynamic balance, mast stiffness. These can be tackled one by one. Start with undersling or height under the teeter bolt, then spanwise balance, weight either side, check chordwise balance, achieved by stringing the blades with the string passing through the centre of the teeter bolt, then check dynamic balancing or blade tracking. Make sure your hub bar is in the middle of the rotor head. Use feeler guages to centre hub bar then shim teeter bearing race.
Stick shake is fore and aft probably teeter height/undersling needing adjustment, or chord balance/stringing.
The machine hoping up and down is tracking or dynamic balance. A round symetrical shake possibly static balance.
The COM of a coned rotor should lie at the height of the teeter bolt, which is controlled by the undersling of the rotor. This does not affect stability per se, but affects 2 per rev shake. Undersling can be found in the rotor peformance sheet and will vary with the weight of the gyro and diameter of the rotors.The best teeter height depends on lengths of the blades, mass of the blades, CG locations of the blades, the blades' lifting qualities and other variables. But a given rotor at a given pitch should have the same steady-state coning angle despite varying loads.
Moving the teeter bolt position in relation to the center of lift of the blades or cone set, and you will get shake. One way to find out if your cone set is causing blade shake is to load and unload the rotors. This moves the COL or centre of lift above and below the teeter bolt. If it shakes more in the high g situation, the teeter bolt is too high. If it smooths out under high g's, the teeter bolt is too low. Holding the machine in a constant high g approximates straight and level because it gives the blades time to catch up and straighten out to normal. It is the response in the stick as one loads and unloads the blades and they are spinning up or down that gives you the information you need to know.
If you have correct cone set but after some time start to get get blade shake begin by checking if there there any friction in the cyclic set up. If so remove and tighten it up. Check your balance springs, have you recently adjusted them? If neither of these remove the rotor. Remove the blades from the hub. Keeping all the bolts nuts and washers from each side separate, weight them to ensure each side is equal. If not adjust to achieve good static ballance. Reassemble the blades then string them to achieve good chord balance. Put them back on and track them to dynamicaly balance them. If tracking looks to be out check bushings and hardware on the teeter bolt. Is there a shim or shims inserted between teeter block and hub bar if so this was from tracking adjustments. Add a piece cut from a thin piece of aluminium coke can then see if tracking gets worse or better. If worse remove the shim you added and the one there before then check again to see if it improves. If needed start adding to the other side. If when you added it improved but still not quite there add some more until tracking is as good as you can achieve. A tendency to roll can be caused by not enough side to side play play in the teeter bearing. Movement should be between teeter bushings and teeter towers. With teeter bolt tight up to .010 side to side