Rotorhead geometery

birdy

Active Member
Joined
Mar 19, 2004
Messages
7,066
Location
Alice Springs-central Oz.
Aircraft
open frame single seat & a 'wasa' RAF, among other types.
Total Flight Time
7000 odd, bout 5000 gyro
Its things like this that make me feel like a simple cow grower. :(

[to cut a long story short] i figured the little annoyn stick shake i used to get with the 28'ers on the wasa at max TOW was coz the teeter bolt was set too high, coz it was worse the more it weighed. [ duh]
Before i test flew it with the new teeter bolt position [ bout 2" closer to the hub bar] i thought the only flyn related thing i mite notice would be the cyclic force required could be higher. Didnt think itd make the slightest difference to performace coz all i did was effectivly lift the center of lift in relation to the head/ pitch and roll pivots.
Things were lookn good as they spun up, smooth as silk, and wen i broke ground it was orgasmic. Not the slightest hint of shake. :) :)

As i backed off to cruise power i noticed it was way outa trimm.
As i was feedn the spring sum slack i though "yeh, that will change too........., but why am i feedn it slack?"
If the COL is further away from the pitch pivot, it has more leaverage over the trimm spring, but it must have less, coz iv had to winde the trimm rite off, and still need to hold the stick forward.
???????????????????

If the COL is moved further from the pivot point, the spring would have less leaverage/ effect, so id should be winding it in, not out.

Shouldnt i ???????

Wot am i missn?

Cant be the RTV, coz its only moved along its own thrust line.
 
Hey Birdy, allow this shade tree engineer to throw a guess at it.

It seems that changing the teeter bolt location would change the rtv-to-pitch pivot relationship because of the blowback angle.........?

If there was no blowback then the rtv relationship wouldn't change.

I haven't thought this through much, and I just woke up, so I could be way off.
 
Birdy did u check your rotor rpm before and after the move ,my guess is that when u changed the bolt hole location it moved the blades down and forward which changed the cg thus a slightly less load on the blades and should have shown up as a few less rotor rpm thus less stick shake, As u said before the move with more weight the shake was worse
 
Birdy, an autorotating rotor flies at constant coning angle. As load is increased, the rotor speeds up and centrifugal force of the rotorblades increases in direct proportion to load, maintaining constant coning angle.

Different loads can indeed affect rotor shake but that has more to do with rotor rpm than anything else. Different rpms change excitation frequencies.

Decreasing the height from teeter bolt to pitch pivot increases the force trying to tip the rotorhead forward.

A picture says it better.
 

Attachments

  • cone.JPG
    cone.JPG
    18.6 KB · Views: 0
If the COL is further away from the pitch pivot, it has more leaverage over the trimm spring, but it must have less, coz iv had to winde the trimm rite off, and still need to hold the stick forward.
???????????????????

If the COL is moved further from the pivot point, the spring would have less leaverage/ effect, so id should be winding it in, not out.

Shouldnt i ???????

Wot am i missn?
Birdy, I think you answered your own question, but perhaps backward?

First, let’s examine the RTV and its 2 components, vertical and rearward. Assuming 10 deg RTV relative to spindle axis, when you moved the CoL 2 inches up along the RTV, you moved the vertical component nearly 3/8 inch closer to the spindle, (.353 to be more precise) thus reducing the effective rotor head offset by that same amount.

Perhaps more significantly, you raised the rearward thrust component (commonly referred to as drag ) a tad over 2 inches further from the pitch pivot. (2.031 be more precise) increasing its leverage against the pitch pivot.

Think that over and see if it makes sense. The RTV relationship crossed my eyes, then the rearward component wrapped my head around the spindle axis.:twitch::eek:
.
Chuck posted while I wuz still figgerin' and stated it much better!
 
Pete, Birdy was confusing me because an RAF rotorhead is laid out bassackwards.

Push on the front or pull on the rear. Where he was decreasing trim spring tension, normal people would be increasing it.
 
The key to puzzling this out is to recognize that "lowering" the teeter bolt height above the bar raises the rotor and moves it very slightly back relative to the rest of the rotor head. Specifically, as you reduce teeter undersling, the rotor rises along its tip-path-plane axis -- whose angle against the horizon is the mechanical spindle axis PLUS the blowback angle.

HOWEVER, if the rotor rises ONLY by shortening the teeter block (Birdy's case, I think), then it simply rises along its own line of thrust. The teeter bolt still attaches to the head at the same point as before (i.e. through the original hole in the "towers"). Therefore, the line of thrust hasn't moved relative to the head, and there should be no change in the arrangement of forces working on the head -- hence no change in head trim.

If OTOH you make taller towers and make the teeter bolt hole in them higher above the torque bar, then there WILL be a trim change. Here's why:

The mechanical spindle axis is normally fixed at 90 deg. to the torque bar, while the typical blowback angle is 2-3 degrees. Therefore, the rotor thrustline (a.k.a. tip-path-plane axis) is tipped 2-3 deg. farther aft than the spindle axis during forward flight. Ths means that the rotor thrust always pulls slightly aft on the spindle during forward flight. Raising the teeter bolt higher above the torque bar is to gives the rotor thrust more leverage to work with -- hence the head wants to tip back. You can call this effect "drag," but it's easy to get confused if you do.*

For some reason, several of the common rotor head adjustments work the opposite way from what you'd expect at first glance. In rotorcraft, we commonly lower the river in order to raise the bridge.

______________________
* No kidding. I managed to confuse myself at first. I used quotes because it's not really "drag" in the usual sense; it's the aft-pulling component of rotor thrust relative to the spindle, caused by rotor blowback. Pete already pointed this out. This aft-pulling bit disappears when blowback disappears -- in a vertical descent, at zero forward airspeed.

NOTE: I edited this post as I realized that it didn't orginally address what Birdy was doing: shortening only the teeter block, not raising the teeter bolt. Chuck Beaty's subsequent posts made me realize the error of my ways. As usual.
 
Last edited:
Geez, i think i feel anatha headache comen on. :(

It seems that changing the teeter bolt location would change the rtv-to-pitch pivot relationship because of the blowback angle.........?
Never even thought of that one Mik, thanx.

Gyro J,
The air here is never smooth enuff to be able to tell small differences like that, even if it did happen.
And moven the bolt moved the blades UP, not down.

Birdy, an autorotating rotor flies at constant coning angle. As load is increased, the rotor speeds up and centrifugal force of the rotorblades increases in direct proportion to load, maintaining constant coning angle.

Thats wot i always believed too CB, but the RRPM hasnt changed, coz the load/AOA is the same, [ and the rotor tac says so] but the shake at high AOAs has gon since i lowered the teeter hight. [ thats why im confused]

Different loads can indeed affect rotor shake but that has more to do with rotor rpm than anything else. Different rpms change excitation frequencies.
So by reducing the teeter hight, iv now got the blades where they shoulda been. Where they were before, they were always on one limit of the sweet spot. Any gain in AOA had them spin'n faster, increaseing the frequency of the irritant.

Decreasing the height from teeter bolt to pitch pivot increases the force trying to tip the rotorhead forward.
I know, but i did the opposite.
I shortened the teeter block.
That lifted the blades COL in relation to the teeter bolt, but the teeter bolt remains the same distance from the pitch pivot.
The COL is now 2" further away from the pitch pivot, so its now got more leverage on the trimm systm...................... but not according to this, coz i had to slacken the trimm sping, not tighten it.
In your bottom picture, you have the RTV NOT alined with the spindle.
I always thought the RTV passed through the rotor bolt.????????

Push on the front or pull on the rear. Where he was decreasing trim spring tension, normal people would be increasing it.
CB, its the same effect.
As i increase the spring tension on the springs that push the frunt up, it has the same effect as increasing the tension on a sprin pulln down the back.
Wether the spring pulls up the frunt, or pushs the back down, the effect on the rotor head is the same.
The RAF trimm springs pull the frunt up. [ same effect as a back spring pulln down]

The key to puzzling this out is to recognize that "lowering" the teeter bolt height above the bar raises the rotor and moves it slightly back relative to the rest of the rotor head.
Yes, but everythn above the pitch pivot tilts with it, at the same rate. So nuthn above the pitch pivot will change, cept for the thing on the other end of the lever, ie; the trimm spring. [ and the cyclic controls, which should be heavier]

whose angle against the horizon is the mechanical spindle axis PLUS the blowback angle.
Ill have to think abit deeper bout this blowback thing. ;) Coz its at a different angle to the RTV init?


the blowback angle is so mall the the primary effect of raising the rotor along the tip-path-plane axis is to give rotor "drag"* more leverage to work with -- hence the head wnats to tip back.
Thanx Doug, now its clear.
If id thought of the blowback thing, this thread wouldnt have been nessesary. :)
 
Sorry, had to cut that post short.
I havnt finished yet. ;)

But i have thought more bout the blow back, and recon thats gota be it, coz other things iv found since is that the stick is 'dead'.
Next to no G load feedback. It never had much anyway [ compared to the ferel] but now its got nuthn. Hit a virtical upper or downer and the stick dont move, and the machine bearly pitches. It just goes with the flow. [ which is rite up my ally ;) ]

Ill have to do a virtical decent now, and if we'er rite bout the blowback bit, it should need alot more back pressure to hold, coz there wont be any blowback to counter the offset and the trimm tension is very lite.

There was sumthn else too, but its sliped me mind for a bit. :(
 
Something here ain't making a lick of sense!

I shortened the teeter block.
That lifted the blades COL in relation to the teeter bolt, but the teeter bolt remains the same distance from the pitch pivot.
The COL is now 2" further away from the pitch pivot, so its now got more leverage on the trimm systm...................... but not according to this, coz i had to slacken the trimm sping, not tighten it.

Of course you would have to slacken it. The additional leverage is applying additional rearward force above the pitch pivot in the same direction as the spring! You would need to slacken the spring to compensate for that.
 
Pete, by lifting the blade's center of lift away from the pitch pivot [ lever hing point], it now has more authority over the trimm spring, so theoreticaly, the spring would need to apply more tension to the other side of the hinge point to compensate.
IOW, the lift component on one side of the pivot has more authority over the equalising force [spring] on the other side of the pivot, coz its further away from the pivot.
BUT, throw in a little bug like blowback, and the tables turn a compleat 180.
 
Last edited:
Geez birdy ... the scg vocabulary has gone out the window somewhere in this discussion! You're giving yourself away here, mate!

"Pete, by lifting the blade's center of lift away from the pitch pivot [ lever hing point], it now has more authority over the trimm spring, so theoreticaly, the spring would need to apply more tension to the other side of the hinge point to compensate.
IOW, the lift component on one side of the pivot has more authority over the equalising force [spring] on the other side of the pivot, coz its further away from the pivot.
BUT, throw in a little bug like blowback, and the tables turn a compleat 180."
 
I sure sum nit picker will be able to find the odd spelln error in there sumwhere mate. ;)
 
A rotor produces a single line of thrust normal (perpendicular) to the rotor plane (tip plane axis). No different than a propeller, a windmill or a rope.

Resolving into rectangular coordinates (lift and drag) compounds the confusion.

Changing the height of the teeter block with no other changes to rotorhead geometry can not change stick force as a first approximation.

But there is a second approximation.

A shaking rotor consumes energy; the stick and airframe don’t shake for free. It can consume a considerable amount with severe shake.

Where does this energy come from?

It comes from the only place it can, the airstream. To extract more energy from the airstream, it must fly at a higher angle of attack in order to intercept more air. That causes the cyclic flapping angle to increase, moving the line of rotor thrust nearer to the pitch pivot of the rotorhead.

A shake free rotor ought to have a higher top speed.
 
A rotor produces a single line of thrust normal (perpendicular) to the rotor plane (tip plane axis). No different than a propeller, a windmill .....


New Year's Resolution about Creative Evolution;

Perform mental masturbation in an attempt to discover if God intended rotating airfoils to operate in axial airflow, only.
 
A shaking rotor consumes energy; the stick and airframe don’t shake for free. It can consume a considerable amount with severe shake.
Id always assumed thatd be the case CB, but initialy, coz theres never any smooth air round ere, i thought the higher efficiancy of smoother blades wouldnt be noticable, specialy ona rotor tac, coz the load is contantly changn.
But the more iv flown these 'new' smoother blades, im thinkn its made a considerable difference. :)

Changing the height of the teeter block with no other changes to rotorhead geometry can not change stick force as a first approximation.
Its so slight a difference in control pressures, it could be just my imagination. ;)

But the 'dead' stick?
Is the blowback now near enuff to equal to the offset leverage, maken it dead??
 
By “dead” stick, I presume you mean one that doesn’t shake, a difficult thing to accomplish with a teetering rotor.

The purpose of undersling is to locate the teeter bolt at the CG of the coned rotor. But that’s not all of it.

A coned rotor has mass both above and below the teeter bolt. If you think long and hard about it, eventually you’ll realize that this mass is forced into rotating in a 2/rev circle in the presence of cyclic flapping. It is a difficult problem in spatial visualization, especially for right-handed people.

Naturally, the above and below mass would rather not go in a circle and instead, shake the rotorhead at a 2/rev rate.

Anything that flattens the rotor cone will reduce shake from this cause; reduce blade incidence and speed up the rotor, added tip weights, etc.

It’s midnight here and past my bedtime but I’ll continue tomorrow with some sketches.
 
Wen i said the stick is dead, i ment as much as in responce to G load changes.
IOW, if i hit an upper, it has nearly no responce [ stick should pull forward.]
It never was as load responsive as the ferel, but theres bearly any responce now.
Float the stick and it hardly responds to g load variations.
Im only thinkn outloud ere.
If the stick, or more precisly, the offset, is less responcive, the g loads im feeln are from horisontal gusts more than virtical ones, coz blowback wouldnt have much of an effect with virtical variations, only more horisontal ones, no?


Midnite??????, bloodyell, im well n truely maken zeds by that time. ;)
 
I realize, Birdy, this is not responsive to all of your dead stick concerns but I've gotta run. I'll continue later.
 

Attachments

  • 2 per rev.JPG
    2 per rev.JPG
    28.2 KB · Views: 0
Top