Teeter height ?

[Tim]

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.G"day again Peter. When the blades are tilted from joystick inputs the axis of rotation tilts also and stays perpendicular to the tip plane path, meaning that the C of M of the individual blades remains the same distance from the axis of rotation. Compare it if you will to the fromt wheel of a front wheel drive car. When the steering wheel is turned the wheel is induced to turn on a different axis to the drive axle. Any position on the tyre does not lead lag or flap but rather just scribes a perfect circle on its own axis and is happy to remain that way unless it is acted upon by an outside force. In the car, the outside force to "steer" the wheel is directly mechanical and in a gyro, the outside force is to "tilt" the rotor is aerodynamic.I think the common misunderstanding stems from the assumption that the blades teeter or flap in forward flight, which is actually incorrect. The tendency to treeter is (unknowingly) compensated for by the pilot with forward stick. The greater the forward speed, the more forward stick is required. Forward stick gives less cyclic pitch to the advancing blade and more to the retreating blade, equalising lift on each blade so that they still fly level

[Brian]

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. Rotors are amazing things, I have had [still have] a set of Ricks 27"s, Rob Patroneys 27ft6inch, AK"s 27 ft & a set of AKs 28"s.The Ricks run out of hours but were doing the job really well but due to not being able to get a new hub bar for them, bought a set of 27ft 6in from Rob which are now on their 3rd hub bar. The AK 28 fters, seemed to be okay but I didnt like the way that when you got slow, they seemed to loose lift so I tried a stet of 27"s which despite assurances, couldnt keep me level without using a lot more power so I sent the hub bar back and Jeff stuck a full 2* into them, now they fly but they have this shudder that comes up through the seat although I havent tried them since getting all new rod ends on the controls so they may well be okay now. The 28"s had the .75* pitch and are super smooth and pretty quick I found. The last hub bar Rob made for me, he put more coning angle into it and I reckon it performs better.Anyways, hope you find some information that helps as there doesnt seem to be too many hard and fast rules.Birdy loves AK"s, I love my Patroneys and there you go !!

[asra]

Good morning Tim. For a theory to be correct it must explain what happens in practice. In thinking about your last post I have found something that if I have understood it properly would account for the distinctive shake I noticed when the teeter point was low, but in other areas there appears to be holes in the theory so I would like to ask a number of questions. I hope you will keep answering them. I will start with two.Quote: I think the common misunderstanding stems from the assumption that the blades teeter or flap in forward flight, which is actually incorrect.Question 1; I have found that if there is any friction in the teeter bushes or bearings stick shake will occur in straight and level forward flight and when forward speed is increased the shake also increases. For that to happen the rotor must be pivoting back and forth on the teeter bolt axis.Question 2; A coned two blade rotor with the teeter axis at the point of the cone would lead and lag in forward flight. If the teeter axis is then moved slowly up through the cone it would move from the lead lag theory to the balanced spinning disc theory. I take it this change would be gradual rather than suddenly changing from lead lag to balanced spinning disc. This would mean there would be an exact distance from the point of the cone to the teeter point axis to achieve perfect balance.

[Tim]

Question 1; I have found that if there is any friction in the teeter bushes or bearings stick shake will occur in straight and level forward flight and when forward speed is increased the shake also increases. For that to happen the rotor must be pivoting back and forth on the teeter bolt axis.I will try my best Peter. Even though the rotors fly laterally level in forward flight, they are subject to cyclic aerodynamic forces due to the differing centres of lift on the advancing and retreating blades (the centre of lift is further outboard on the retreating blade). This requires a smidgen more pitch on the advancing blade to compensate. Any extra "lift" on the advancing blade manifests itself at the front of the disc (precession laws) and the front flies slightly higher than it would otherwise (sometimes called blow-back angle). This places the rotor-disc axis slightly out of line with the rotor-head axis, meaning that to rotate on two different axies there has to be movement on the teeter pivot. The faster you go, the greater the blow-back angle and the greater the teeter movement. Stiff teeter bushes will stand out as stick shake which would get worse with speed. This can again be compared to the front wheel drive car. When it is on full lock, the CV joint has its maximum movement, which would shake the steering wheel if it got too stiff. (See attached diagram "rotor axis")Question 2; A coned two blade rotor with the teeter axis at the point of the cone would lead and lag in forward flight. If the teeter axis is then moved slowly up through the cone it would move from the lead lag theory to the balanced spinning disc theory. I take it this change would be gradual rather than suddenly changing from lead lag to balanced spinning disc. This would mean there would be an exact distance from the point of the cone to the teeter point axis to achieve perfect balance.I do not quite understand this question. The disc axis is always perpendicular to the tip plane path of the blades, so any lowering or raising of the teeter height would always follow the disc axis, and the disc C of M should always remain on that axis. I do take your point however that any out-of-balance in the rotor system would manifest itself as stick shake, and the further the disc C of M is away from the head pivots, the greater the shake. (See attached diagram "Teeter height")

[asra]

Tim thanks for that, the answer to question one has given me a very clear understanding of your earlier post. Just one question for now;

[asra]

Tim, as I said earlier theory must fit what happens in practice. I am presently looking into something loosely termed hooks joint effect which may explain the different shakes I felt going above and below a certain teeter height. Although the questions may seem irrelevant the answers are important to me.

[asra]

Forget the last one Tim. The problem is dependent on blow back angle and cone angle. The higher the cone angle and the greater the blow back angle, the more critical the exact teeter height becomes.

[Tim]

Spot on Peter - teeter height is definately more critical on larger blades. That is why larger diameter blades are always harder to get to run smooth than smaller blades. The larger blades generally turn slower and run a higher cone than smaller blades and the higher cone means that they have greater mass further away from (above and below) the teeter bolt that can create or amplify out of balance/track problems. Small fast turning blades running a low cone always have their C of M mass close to the teeter height. On larger blades, any abrupt stick movements can also throw the cone further out than on smaller blades, creating further stick shake, which should stop when the gyro returns to "normal flight".

[Mark Horan]

That would explain why my 28 footers on the tandem get a bit "grumpy" when I stick in a fairly hard, fastish turn!Mark.

[asra]

Hello tim, for this system to remain perfectly stable the rotor or tip circle must remain mathematicaly perfect. For this to occurr the blade pitch must be mathematicaly perfect and there must be absolutely no air turbulence to allow the mathematical perfect symmentry of lift which is a prerequisite for a perfect circle. At the slightest deviation from the perfect circle angular momentum must be allowed for.

[GraemeMonro]

Hello tim, for this system to remain perfectly stable the rotor or tip circle must remain mathematicaly perfect. For this to occurr the blade pitch must be mathematicaly perfect and there must be absolutely no air turbulence to allow the mathematical perfect symmentry of lift which is a prerequisite for a perfect circle. At the slightest deviation from the perfect circle angular momentum must be allowed for.There are very few things in this world that are perfect, and me being one of them you would think that I would have known what you was on about Graeme.

[asra]

Howdy Graeme, this is certainly not a perfect world. My missus tells me I drink too much whisky. That makes it very imperfect world.

[Tim]

There are very few things in this world that are perfect, I don"t know Graeme - I know of a bloke who is a perfect idiot. He is depriving a village somewhere.

[asra]

jeeze Tim, I hope that last post dosent apply to to me

[asra]

Tim scince I have been talking to you on this forum I have considered you as a wealth of knowlage not an expert. Please do not dissapoint me now.