Rotor inertia.

birdy

Active Member
Joined
Mar 19, 2004
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Location
Alice Springs-central Oz.
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open frame single seat & a 'wasa' RAF, among other types.
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7000 odd, bout 5000 gyro
Ok, the 'happy season' is over, and we can forget bout be'n warm n fuzzy and get back to sum serious learning.;)

The ' affects of ......... on rotorblades' thread has gon to sh1t, lost in a boghole of egos, BS n facts, but like most lost causes, it surfaced a few other points, like inertia.:D

Now, DF [ Dennis Fetters] started one tangent by listing the benifites of lighter glass blades over heavier extruded alloy ones.

I replied with a counter list of benifits of the heavier ones over lighter, and it started go'n down hill from there.

But, we did agree on one point, that be'n, a heavier blade will possess higher resistance to cyclic pitching [ stick shake] than the lighter blade, simply coz of the mass of the baldes providing the resistance, twice per rev, to the longditudional torque involved in the cyclic pitching.
Thats logical and even a SCG can savvy. :bored:

But then i kinda agreed to dissagree with DF on the point of the rate at which a tip weighted blade of equal inertia to a uniforely weighted blade would respond in RRPM AFTER the disc started pitching and gaining load.
He said, basicaly, coz.
I didnt argue coz i didnt know, but i couldnt understand how 2 blades of equal inertia and identical profile could have different rates of rpm responce.
Then CB [ Chuck Beaty] jumped in and explained in great detail that they do actualy have the same respose rate, with facts n figures backed by laws of phisics.
Even a SCG knows who to take seriously.:p

But this thread is not bout taken sides, its bout FACTS.

Fact one;
Heavier blades will provide greater resistance to cyclic inputs.

Fact 2;
blades of identical inertia and profile will have identical rrpm response rates to load changes, and identical flight parths, regardless of total balde weight.

How am i go'n so far?:help:
 
Birdy, I'm just an uneducated country bumpkin but I can state beyond doubt that rotor blade weight was really noticeable between the R22 and the Bell 204. Really noticeable....:lol:
 
Let me toss a monkey wrench into your machinery, Birdy.

In forward flight, with the rotor disc flapped back from the rotor head axis, there are two ways of viewing the rotor.

1) Viewed from the rotorhead axis, the rotor undergoes cyclical flapping but not cyclical feathering;- the blade pitch is always fixed relative to the teeter bolt. That being the case, how can feathering axis moment of inertia shake the stick?

2) Viewed from the disc axis (tip plane axis), the rotor doesn’t flap but there’s a cyclical pitch variation. Can this shake the stick even though there’s no feathering motion relative to the teeter bolt?

Mass forces do produce shake but that’s for a later episode.
 
1; That being the case, how can feathering axis moment of inertia shake the stick?
Didnt say it did ....................... did I ?????


2; Can this shake the stick even though there’s no feathering motion relative to the teeter bolt?
Yes, i woulda thought so.
I woulda thought this is wot caused the inevitable 2bl shake we have. [2 per from the drag pulses].
But thats not the shake im thinkn of, im talkn bout high input related shake.
Where am i stuffn up here??

Iv only been ina R22 C E, so ill have to take your word for it. ;)
 
Tell me what you think of this sketch, Birdy. The perspective isn’t right on the second sketch, having to make do with MS Paint, but you’ll figure it out.

Imagine a barbell fitted with a teeter pin and installed on a rotorhead.

Given a spin and without friction, it would spin forever with its axis fixed somewhere in space. As the Earth rotated, it would run into the teeter stops; the Earth rotates 15º/hour.

The second sketch shows some additional weights to simulate chordwise mass.

Would their addition make the rotorhead shake?

Imagine both were slid all the way inward until they merged with the teeter bolt. Wouldn’t that be the same as making the teeter bolt a little longer and attaching all 4 sliding weights to it? Would it shake?
 

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Given a spin and without friction, it would spin forever with its axis fixed somewhere in space. As the Earth rotated, it would run into the teeter stops;
Yup, i savvy.

Would their addition make the rotorhead shake?
No.

Wouldn’t that be the same as making the teeter bolt a little longer and attaching all 4 sliding weights to it?
Yes.

Would it shake?
No.

[I get the feeln we'er not on the same road ere sumhow.]
 
Birdy, I've got some questions.

First, what do you mean by this statement of stick shake?

But, we did agree on one point, that be'n, a heavier blade will possess higher resistance to cyclic pitching [ stick shake] than the lighter blade, simply coz of the mass of the baldes providing the resistance, twice per rev, to the longditudional torque involved in the cyclic pitching.

Second, does the tip weighted rotor accelerate and decelerate , at the same rate as the unweighted, if their moment of inertia are the same?

Last, does the tip weighted rotor cone at the same angle as the unweighted, if their beam strengths are the same?

Phil.
 
Episode 1.75: chordwise mass.

A rotorhead with extended teeter bolt holding weights shakes while being tilted; a 2/rev gyroscopic vibration identical to the shake from tilting a 2-blade propeller.

To summarize, chordwise mass of a rotor causes no shake under steady conditions but produces 2/rev shake proportional to tilt rate and chordwise MOI.

[I get the feeln we'er not on the same road ere sumhow.]

We’re sneaking up on it.
 
But then i kinda agreed to dissagree with DF on the point of the rate at which a tip weighted blade of equal inertia to a uniforely weighted blade would respond in RRPM AFTER the disc started pitching and gaining load.
He said, basicaly, coz.
I didnt argue coz i didnt know, but i couldnt understand how 2 blades of equal inertia and identical profile could have different rates of rpm responce.:

David,
I don't know where you just came up with the above statement out of our discussion. I said, in fact, that the light and heavy set of blades of equal inertia and identical profiles "do have the same rate of RPM response change."

What I said was the the lighter set of blades would have a faster maneuvering response time, which at first you agreed, but now I don't know what you believe after Chuck disagreed.
 
Chuck - looking at your barbell sketch, I realized I haven't considered the centrifugal force before. Maybe I get it wrong but it appears to me the barbell axis will align itself with the rotorhead axis due to centrifugal force. Think of the extreme case of the barbell having only one weight. The weight will rest low without any turning motion and will rise as a function of the speed of your drill, or whatever is turning it. Same thing will happen with two opposing weights.

If this observation is correct, then rotor blowback is being opposed by a centrifugal force. In other words, aerodynamic forces are keeping the disc tilted vs. the rotorhead axis, while inertial forces are trying to bring it back.

Looking back at your barbell with chordwise mass - unless the chordwise weight distribution is even and centered, wouldn't the chordwise weights shake the rotor (pitch wise) due to the transition from fore/aft, where they are not exactly opposing each other to left/right, where they are?

I don't have this picture completely clear in my head but I have a feeling there are some chordwise inertial forces that do not necessarily balance each other.

Udi
 
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Here’s a simple trick you can do, Udi.

Take a butter knife of similar slender object and tie/tape a string around it where it balances. Suspend from the string, give it a spin in a plane that isn’t horizontal. (I just tried and it works best if snapped between thumb and forefinger to give the spin).

It maintains its plane of rotation even though its axis is not concentric with the string.

With a 2-blade rotor, the teeter hinge is all the universal joint we need.

Centrifugal force is along the plane of rotation, not at right angles to the axis of the string.
 
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We’re sneaking up on it.
Yup, now we is on the same road. :)

So, to be sure CB, now we both know we aint talkn bout a steady rotor, are me socalled facts in line?

First, what do you mean by this statement of stick shake?
Phil, CB answered it for me here, in much better terms than i could ever dream of. ;)

To summarize, chordwise mass of a rotor causes no shake under steady conditions but produces 2/rev shake proportional to tilt rate and chordwise MOI.


I said, in fact, that the light and heavy set of blades of equal inertia and identical profiles "do have the same rate of RPM response change."
Honestly D F, i cant recall zactly wot we dissagreed on, and im not go'n back top 'that' thread to find out, but i do know we agreed to dissagree sumwhere.
If iv stuffed up sumwhere and misquoted you, it wasnt intentional.
I put ;But this thread is not bout taken sides, its bout FACTS. in the opening post for good reason and i mean no offence.

What I said was the the lighter set of blades would have a faster maneuvering response time, which at first you agreed, but now I don't know what you believe after Chuck disagreed.
Sit tight mate, im getn to that, i just need to get things streight in me own head before we dig any deeper. [ you gota remember, your dealn with a SCG's mind here, it takes time. ;) ]
 
Here’s something else to chew on.

The teeter bolt needs to be located at or near the CG of the coned rotor.

If you took a stick of welding rod, bent it in the center to mimic a coned rotor then laid it on its side on top of a stick of straight welding rod, the CG would lie along the line where the bent welding rod balanced.
 

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Coriolis effect is in the eye of the beholder.

Viewed from the tip plane axis, there is no radial movement of blade mass.

A shallow sheet metal cone, driven from its apex by a CV joint, has no particle of metal moving nearer or farther from its axis of rotation.

I suppose the confusion arises as to just what its rotational axis happens to be. A sheet metal cone driven as described rotates about its axis of symmetry, not the driveshaft axis.

If the cone is slit into segments to mimic a rotor, nothing changes and we have an analog of a floating hub rotor, first used by Doblhof in his tip jet helicopter and then by Doman in his floating hub helicopter. McDonnell also used a hingeless floating hub in their various compound helicopters.

In a Cierva rotor with 3 or more blades, drag hinges are a kinematic rather than a dynamic necessity. Cierva utilized Coriolis effect to make the math work when the rotorhead axis is the reference axis but realized it was simply a math trick.

No rotor will shake in a vertical descent so long as the blades are tracked, balanced and reasonably well matched as to pitching moment, etc.

All seesaw rotors shake in forward flight to a greater or lesser extent, depending upon softness of mounting, rpm, etc. And that’s without exciting inplane resonances.
 
A gyro rotor flies at constant coning angle, Justin.

With an increase in load, the rotor rpm increases as the square root of the load ratio. The coning angle, the ratio of centrifugal force to load is therefore constant (rotor centrifugal force varies as the square of RPM).

A rotor, viewed from the tip plane axis does not flap. Therefore, Coriolis force can not exist relative to the tip plane.
 
There could be some misunderstanding here.
With regard to the flapping.... the blades are most definitely flapping about the teeter bolt. If they did not, the helicopter would roll in forward flight.[/QUOTE]


I agree, but possibly you didn't understand what he said;
A rotor, viewed from the tip plane axis does not flap. Therefore, Coriolis force can not exist relative to the tip plane.

The tip plane axis of a teetering head gyro in forward flight is inclined behind the spindle axis of rotation. Flapping as we call it, is only in relation to the spindle axis. Viewed from tip plane axis it discribes a perfect circle. Viewed from the spindle axis, the tips describe an elipse. A matter of perspective.
 
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I have a question. Is the 2/rev still there when you cut power and come down vertically?
I often do virtical decents to eliminate any 'forward fl;ight' related imbalances wen tryn to figure a shake in sumones blades.
If the blades are perfectly tuned, the only thing that could still make them shake in a virtical power off is the turbulant air from a big HS, but itd be un noticable for the average jo.

The teeter bolt geometry is there to allow the rotor to, as closely as possible, maintain the same center of mass as the blades flap.
??????????????
The teeter bolt is only a hing for the blades to freely share load [ flap] wen the airflow becomes inconsistant [ forward flight]

I do not know of any rotorcraft that flies at a constant coning angle. Most 2 blade rotors have a "Pre cone angle", but during the dynamics of flight, the coning angle changes constantly.
Iv got 2 that do.
Till i change sumthn that will change load/rrpm, then they will shake. The greater the change from the preset 'constant', the greater the extent of the shake.
Yes, itd be impossable to have a blade that will not shake in any condition, but its possable to have a blade that wont shake, IN A CERTAIN CONSTANT.

When flaring, the initial RPM boost you see is due to coning.
The initial rrpm boost , in fact all the boost is due to load increase. The coneing is due to the lag in rrpm boost.

You see the effects before the higher induced flow has a chance to accelerate the blades.
Wen you pull the stick back Justin, the first thing that happens is the cyclic pitch reaction will lift the 'nose of the disc', this provides the indervidual blades with an increase in AOA, which increases the load, which increases the cone angle [ coz of the increased load for same rpm], THEN the blades begine to speed up. The reason why they cone up faster than they rev up, is coz it takes more time for the rpm to increase, it takes much less time to flex up.
 
So, CB, could you rewrite my 2 socalled FACTS so they actualy become facts please. So's everyone is on the same road.
 
......... Viewed from tip plane axis it discribes a perfect circle. Viewed from the spindle axis, the tips describe an elipse. A matter of perspective.


I understand the perspective part.....but the flapping is still there. Where it exists, ( the teeter bolt) it is subject to all the forces that would act if the center of mass where to move. Maybe I am missing something.....

Birdy,

I have real trouble trying to understand your posts and what your point is.????? You seem to be confirming much of what I am saying???? ....But trying to nit-pick things.... like coning angle is constant....until you change the load etc.....Obvious to my point but, you seem to be implying that I am saying different. Same with your statement about the teeter bolt.... Or when I say the rpm increase is due to coning.....and you say.... No, it is greater loading, leading to coning, leading to rpm increase. I think that is obvious and you seem to think you have caught me saying otherwise.
 
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I have real trouble trying to understand your posts and what your point is.?????
I havent gotn near the point yet Justin, im still tryn to get everyone on the same road to avoid any confusion. [ apparently unsuccesfuly so far].

You seem to be confirming much of what I am saying????
Am I ??
When flaring, the initial RPM boost you see is due to coning.
How can coneing boost RRPM?
Im not nit pickn, your confused.
 
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