Teeter height ?

[asra]

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[Tim]

There was a misprint in the article (Calculating Coning Angle) in the last Gyro News. Something got lost in the conversion.The left hand scale on the Graph should read - "Centrifugal Force X 100". Apoligies for any inconvenience.Yes the correct teeter height should be at the centre of mass of the spinning disc. (on a straight line between the C of M of each individual blade - coned in normal flight). This ensures minimal control forces because you are not displacing the mass of the disc with control movements.

[asra]

Hi Tim,
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[Brian]

Just a thought from left field.Now, there can be other variables that will influence rotors and cause shake. Nearly everyone will already be way ahead of me here but it could be worth mentioning.Around the end of 06, start of 07 I wasnt impressed with my blades smoothness and blamed various things like new hub bars etc as it seems no two hub bars are made or perform the same.I had Rosco replace the mast in May 07 and noticed a marked improvement in the rotors, in fact the rotors were nearly smooth, so on checking my log book, it seems the mast was a bit overdue.Just before the end of the year I had Rosco put in all new rod ends in the control system and the result was very pronounced, a lot of the shake that was left after the mast re placement had nearly gone. Its worth noting that the rod ends up at the stick end were original and had done some 5000 hours.

[asra]

Brian, I changed from bushes to bearings in the pivot between mast and rotor head a few years back and after that I had stick shake in blades that had been dead smooth. The set that had some shake with bushes now had too much shake to fly on. Last year I got that set out to find out what was causing the shake. I felt the balance and track were right so I adjusted the teeter height. If I went too high I got a fore aft shake or bounce, too low I had a rear left to forward right shake. By adjusting the height a couple of mm at a time I found a point that was very smooth in level flight but any loading or unloading of the blade would bring on both these shakes. I have noticed a few references to friction in the rotor head to mast pivot and as the shake had appeared after I put the bearings in I decided to replace them with bushes to add some friction. The shake from loading and unloading the rotors disappeared. I have found if there is play in the controls, shake caused by a low teeter point will not be as noticeable, but having play in the controls when the teeter point is high will allow the shake or bounce to intensify.

[asra]

Please, please continue, this is really interesting and it explains a lot of the things we have sat around the fire and debated..All things equal (wel sort of..) which will be better, a "flexable" mast or a stiff mast?

[asra]

Gyrofan, just my thoughts. The mast needs to be flexable in the air yet strong in case of accident. The hubbar then needs to be designed to transfer residule lead lag to the mast without fatigue for the required number of cycles

[Tim]

The percentages I was referring to is the amount of mass above and below the teeter line of the coned blade to neutralise lead lag of the full blade. In the advancing rising blade the mass above the teeter line is decreasing radius and increasing rpm or leading. The mass below is increasing radius and decreasing rpm or lagging. I take it that when the inertia of the leading and lagging section in each blade is in balance this would try to rotate the blade on the axis point of the teeter line- cone intersection but neutralise lead lag of the full blade. G"day Peter.The blades actually do not individually raise or lower above and below the teeter height, but rather the tips spin in a perfect circle, albeit on their own axis, which can be different to the rotorhead spin axis. Their own axis is always perpendicular to the tip plane path so in relation to their own axis, they do not increase or decrease radius, height or circumferential speed as they turn.Gyro fan, a fully flexible mast (a cable for instance) would be best but it would have control and structural issues. There is a happy medium that allows enough flexability to absorb inherent rotor vibrations and yet strong enough not to raise any structural issues. There are too many factors to consider to give a black and white answer as to the flexability of the mast material, including all up weight, mast free length, material specs, intended manner operated, mast angle etc, etc.

[asra]

Thanks Tim,

[asra]

Tim, I just checked scale drawings of an underslung rotor at different stages of teeter and the radius certainly seems to change.

[asra]

Im just starting to remember things I worked on months ago. The radius of the teetering blade can only remain constant if the teeter height is equivilant to the radius of the coned blade.

[Brian]

Amazing isn"t it, when I changed to bearings, I didnt notice much difference however, in your case, it might have been worth trying a new set of bearings as not always are they perfect.I wouldnt go back to bushes either as the bearings give me a far better run which is what I need.While we are talking shake etc, something I have slowly come to learn is that there are so many variables out there. One idea that has merit is that if you have the opportunity, swap rotors with someone you trust and see what a different gyro does to those rotors. I sold a set of 28 ft AK"s to a bloke who on his first flight with them, they shook so bad he skun his knuckles on the dash yet on my old girl they smooth as silk and on trying them on another gyro again, were smooth as silk so it proved beyond doubt that the head set up of the bloke who bought my rotors was way out and needed fixing bad !!Another thing, if your rotors have some shake, putting extra weight up on the head like a electric pre rotator will most likely lead to more shake and the lighter your gyro, the less shake you will get [most of the time].

[asra]

Brian, I think correctly set up rotors will fly well on any machine so long as the balance, track and teeter height are correctly set. If any of these are slightly out a difference in weight or controls may cause shake.Tim, sorry about all the posts back there. When I look at scale drawings of the rotors teetering in forward flight the radius from the axis is constantly changing causing lead lag.

[chook]

Peter, Hi. How were you able to easily change the teeter height? Were you replacing the pillar blocks, shimming between the hub bar and the hub bar teeter block or replacing the teeter block?Regards......Chook.

[asra]

Hello Chook, there was a 25mm spacer under the teeter block which I removed. I started testing there and worked up in10-12mm steps. I was able to go about 12mm higher by removing the teeter stops. This is when I first noticed the distinctly different shakes between high and low teeter point. I tested this a few times and the shake always changed when I went past the ideal height. The rotors were 28ft AKs. I will add a bit here that I should have put in my answer to Brian. The first set of AKs I bought off Jeff were 26ft with 1 degree pitch and they were very smooth. I then bought a set of 28s for the summer. Jeff set these at .75 degree pitch as his longer rotors develped stick shake at a coarse pitch setting. The pitch setting was too fine for mustering in the summer and unusable so I asked Jeff to make me another hubbar with 1.5 degree pitch. He warned there would be stick shake but agreed to make it.