Stick shake ?

[asra]

Hello Everyone,Tha last time I setup my blades and flew my machine it developed stick shake at higher speed.I find that 45knots is a good climb and cruise speed for my gyro,everything is fine up to 35knots,at 40knots the shake is quite noticeable and at 45knots its quite annoying.As soon as I load my disc a bit by holding the stick back slightly it stops the stick shake.During the test flights and my first solo the blades run perfectly with no shake at all.The shake is between 2 oclock and an 8 oclock direction with a slight oval.My blades, after testing balance is good,no tracking problem as 3 different pilots observed them as I spun them up.My teeter bush clearence was 0 thou but I opened it up to 2 thou to eliminate posible teeter binding but that made no difference.All if this was done under supervision of my Instructor/TA except the balancing of the blades which I done at home over two days.My instuctor flew my gyro and seemed to think it was a patern or hinging problem.Has anyone experienced this problem before and if they have how do I fix it???Peter Adams

[Brian]

Pete, I'm not a techy but can tell you from my expereince that you can have the rotors flying niely, then take them off and put them back on again and you can get stick shake, very annoying.Try turning the rotors in the forks. Make sure you have the right do nut in the right fork. You might try shimming one side of the pillar block. Its a lot of trial and error unfortunately.Brian

[asra]

We have had the shake you speak of it was found to be out of balance caused by cotton real lengths being different or could be rotor head got smacked in transport try blades on other gyro or try swaping cotton reels from side to sideButch

[asra]

Thanks guys.Sounds like I have got to try a thousand different thing,already tried five hundred so I will get there eventually[]Peter Adams

[NiqueNaque]

Right,A few things to check:0] Are the main spindle bearings sound? Has the bolt / nut come loose? Have any bugs set up camp in your blade vent holdes? Is there any water in the blades - drain them and leave in sun for a while to be sure.1] Run out on the rotor head - does the head turn true on the main spindle? Suggest you take the rotor head assy off which means everything down to the pitch bolt. Stick it in a vice so that it is gently but firmly held, get a dial guage and put it in a fixed position and check that the teeter towers both end up in the same place when you turn it through 180 then 360 degrees, both laterally and longditudinally. Lateral differences can be sorted by adjusting your spindle shims, londitudinal by either getting a new rotor head or adjusting the teeter towers or making new ones with redrilled bush locations.2] Check the teeter block is in the same postion on the hub bar, if it has moved the blades will shake, check the teeter block bolts are not loose.3] Check the teeter block bush has not flogged out.4] Assemble the hub bar into the head complete with teeter spindles, etc as if for flying but without the blades, chock the hub bar so it turns true in the vertical direction, ie, turning the bar through 180 and 360 yields that the blade root connections are both at the same height, then check it horizontally such that a dial guage is no different for each blade root connection point on the radial from the main spindle. Establish the centre of rotation for the head and mark same on the teeter block so you can string line it later.. use masking tape first before you even think about centre punching it!5] Weigh the blades and make sure they are the same weight, you can do this over two scales and make sure they both weigh the same inboard and outboard, ie, both blades weigh the same on both scales, if not this means the effective COG for each blade is at a different centre, and if the blades are the same weight this can mean an imbalance is possible under dynamic conditions.All the above being OK, assemble the blades as per flight conditions.6] String them through the rotor spindle bolt from both the front and back corners, the string must pass over the centre of the bolt [see 4 above], if it doesn't, your blades may have a boomerang shape and this will cause stick shake. Correct this condition by adjusting tension on bolts or return the assembly to manufacturer for correction.7] Check blade angle and make sure they are both the same using a finely graduated inclinometer, one of those torpedo levels with a digital read out is usually quite good, make sure you check its calibration before using it by checking how it reads when you end for end it on a flat surface.Given that your blades flew OK to start with I can't see that you will need to do much of the above, it is a check list for bugs that you can't seem to sort out by more obvious means.Hope this helps,Nick.

[asra]

That was such a complete check list Nick that I have cut and pasted it into my gyro information folder for future reference. Thanks for that.Pete

[Ken Watson]

Just shim the teeter block across between the towers . If going one way makes it worse then go the other way. I went 5 thou at a time and i've got the smoothest I've seen.No shake at all but it looks funny as it is alot closer to the towwer on one side. I mean shim the cotton reals of course. Ken

[Brian]

Getting rotors smooth can be frustrating business, Kens advice is easy to do and will determine if you have to try something else. Also, dont be afraid to shim one side of the pillar block, only thing is you have to expereiment to know which side to shim.Brian

[asra]

Thankyou all for the advice,I found that the rotor head bearing was slightly loose in the housing so I reseated it with retaining compound,might be a bit of a problem when I have to replace it.It only had movement from blade tip to blade tip and none through the chord direction.Found the problem before Nick's post so now I'll have to get the dial guage out and make sure I seated it paralell to the bolt and teeter towers.Peter Adams

[asra]

That's great stuff Nick! In my game it's important to establish a diagnosis before you start treatment or you end up chasing your tail.Under (5) there is another complication I think I've struck with the spring steel hub bar. As the rotor system is now free to adopt a coning angle without interference from a stiff hub bar which is doing its best to be rigid, (it is really just absorbing stresses leading to fatigue) the minor dynamic imbalances between the blades are amplified by their CG now being above the teeter bolt, resulting in shake, thus an adjusment of undersling seems to be appropriate.John EvansThink logically and do things well, think laterally and do things better.