harmonics???

[birdy]

Im not sure if ' harmonics' is the correct term for this thread, but itll do. :)

In my short time in gyros, i'v had the oppertunity to fly many different makes, typs and sizes of rotors with varying loadings and flying rpms. And i'v found that there is one common inconsistancy.
Rotor shake.
Most blades can be set to fly with MINIMAL shake by simple ' tuning', but theres one thing that causes shake that we cant tune out ot them, and thats the differential in cyclic airspeed. [IOW, the advancing blade has greater AS than the retreating one, witch means theres more drag in one section of the cycle, and short of a virtical decent, theres nuthn we can do bout it if we actualy want to go sumwhere.]
But, not all blades seem to shake from this AS diff equally, sum shake like a dog sh........, and sum, like the 30'ers im flyn now, have seemingly no shake at all.
Why, they still have the AS diff.
Then i had a head explosion the other day.
One of the frunt wheels on me bullcatcher is way outa balance, and wen i near bout 50kmh, it trys to shake the steering wheel outa me hands, at 60kmh the hole vehicial is jumpn, but wen i push it past to bout 70, its smooth as silk???? why? its still outa [ mass] balance.
But at that perticular rpm, the stresses [ for want of a better term] dissapear. Are they contained in the actual wheel itself, or would it still be stressn the stub axle? Me think its contained in the wheel, coz if it was still be'n transfered to the stub, then i'd still feel it.
IOW, where dose the effect of the imbalance stop? and wots it do while its ' contained'?
[ and no, its not taken up by the slop in the steering linkage ;)]

If the answer is, that the stress IS contained in the wheel, then with proper smooth rotorblades flying in that 'sweet spot' in rpms, is the stress be'n contained in the hub bar? :help:
Must be.
If so, wots worse, smooth feeln rotors that have unfelt stress in them, or blades that let it out into the airframe?

[Mike Schallmann]

Birdy --while C Beaty hasnt specifically said this -he has eluded to the fact that healthy well built hub bar will go a long way in dampening any inplane harmonics/vibrations/resonance (correct me if Im wrong C B ). this coupled with a kinda/sorta flexible mast or slider head will get the stick shake problem under control.

I flew Vortex/Fleck extruded blades for years with the McCutchen hubbar and had no perceptable stick shake. I now fly Dragon Wings and I dont like the stick shake that I have -- it isnt much but it annoyes the crap outta me. Im working on a new hub bar that will eliminate the inplane resonance at the rotor RPM I fly at--

[JEFF TIPTON]

The Robinson R22 uses a leather bag filled with lead shot mounted to the top of the cyclic to damping their stick shack. Their newer blades are stiffer than older models and this is how they compensate for the lower coning angle.

[birdy]

Wen i say ' no shake', i mean, no shake.
Its not ben dampened by anythn in the machine, flexable mast or anythn magic.
The reason i think its not be'n trandfered is coz if it was, you would see a shake in the head. You can have a shaking head, but no shake in the stick, coz its usualy be'n absorbed by the mast. These blades have NO shake, the head is rock solid, no movement.

[Master Roda]

Hey Birdy,

Ya think maybe at a higher speed your wheel EXPANDS and evens the mass to a harmonically stable position??? Just a thought.

Also, It has a force applied against it (the ground).

What rotors are you flying that are so smooth? Are they only smooth at a certain speed? I have noticed that at a given speed,power and weight combo, you get a set of blades that will fly very smoothly.

Here at Sport Copter, Even though we build the rotors exactly the same. They (each set) seem to all fly a little bit differently. It could be just the temperature or barometric temperature, however, There are other variables that influence the rotor system. Such as a slider system.

Small imbalances can affect stick shake, but most importantly is TRACKING. Most people won't take the time to track their blades until the shake is obnoxious. Otherwise most people out there would be alot happier with their blades, no matter who built them.

Just my 2 nickels

Jon

EDIT: I encourage everyone to check your tracking often.

[Al_Hammer]

Mass imbalance in a wheel generally causes the shake to get progressively worse with higher speeds.
================
RADIAL FORCE VARIATION
Sometimes the problem is neither balance or runout. It is radial force variation (RFV). This is the amount of change in stiffness of the sidewall and footprint when a load is placed against a tire. Subtle differences in the position of the cords and belts in a tire's construction can create stiff spots that make the tire roll unevenly. The stiff spots act like runout to cause vibrations at various speeds.
Vibrations caused by RFV tend to appear at certain speeds, then disappear as the speed changes or increases (unlike vibrations caused by imbalance that usually get worse as the speed increases). In one test, a perfectly round wheel that was properly balanced experienced a vibration that appeared at around 50 mph but vanished at 70 mph. The vibration at 50 mph was caused by RFV in the tire, and produced as much side force as if the tire were out-of-round by .030 inches or out of balance by one and a half ounces.
===============
Tires and rotors may be apples and oranges in this case. Both will vibrate when out of balance, however.
Rotor imbalances and drag imbalances are not so easy to understand, since, as Birdy said, harmonics can be involved. Harmonics are vibration at multiples of the primary "1 per rev" vibration. In other words twice the frequency(2 per rev) or 3, or 4 , or whatever times the main frequency. With 2 blades, 2 per rev tends to be very prominent.

"2 per" is not something that you can remove by balancing the blades(teeter balance), just as you can't dynamically balance a tire by static balance alone. An electronic balancer (commonly used on helos) can tell you how much 2 per rev vibration you have, independent of the 1 per rev.

1 per rev can AWAYS be balanced out. If one blade is heavier, simply add weight on the other blade. Actually, it does matter where you add the weight because you want the cg's of both blades to be at the same coning angle in flight and so the cg's should match pretty closely.

This can be checked by removing the blades and balancing them on a "knife edge" of some sort to find the cg.
I don't know of too many who do this, and presumeably, with blades of good construction, this is taken care of at the factory and will not need to be checked.
Sikorsky used to hang his helicopter blades by the root and measure the exact time of the pendulum swing in order to determine if two blades matched in cg location. If the cg is farther out, it will increase the period of the swing.

2 per rev has various sources. If teeter tower height is not optimum, the blades will shake whenever they flap in forward flight. The reason is that when the rotor disc is on a different axis from the "spindle" you need to make sure the cg of the rotor (which is on an imaginary line drawn between the blades) will pass throough the teeter bolt. Otherwise the rotor mass will wobble at 2 per rev and transmit this vibration to the mast.
In practice the best results seem to be when the teeter height is lower than that predicted above, possibly because it serves to cancel vibration from aerodynamic variations.
Drag variations will tend to push hardest on the blades when they are broadside to the wind, it is assumed.
This is where the trick of having too little undersling may come into play.:
With too little undersling the cg of the rotor is above the teeter bolt. When the rotor tilts back the cg moves back relative the spindle axis. The "equal and opposite reaction" of the rotorhead and mast is to be shoved forward.
If rotor "H-drag" is maximum at this instant, it will tend to counteract the forward motion of the top of the mast and, bingo- vibration is cancelled.
Flapping can only work at the rate of the rotor(1 per rev), but lift itself varies with airspeed squared. As the blade comes around to the advancing side the airspeed starts rising faster than flapping can deal with. What happens is the average lift is equalized on both blades, but it has left over fluctuations at 2 per rev that can't be eliminated. The blade tips dance up at down slightly (as can be seen by a camera mounted to the rotorhead of a gyro.).)
If you have 1 per rev and 2 per rev and you remove the 1 per rev , say by balancing the blades, it often will "unmask" the 2 per rev. When you have only the 2 per rev left, the vibrations may be more noticeable because of the doubling of the perceived rate of shake. The combo of 1 and 2 per looks like a distorted oval if you look at the laser pattern.
Gary K. actually mounted a laser to the mast and captured the trace that was formed by the beam on the inst panel.

If you have no perceived shake in the stick, it could be due to play(slop) in the bushings that effectively isolate the stick.
It could also be a limber mast:
A rod, fixed at one end can be tuned to vibrate such that there is a "node" at the fixed end. the length of the rod (L) has to accomodate an integer number of quarter wavelengths (it is an an integer number of half wavelengths for a string fixed at both ends)
So the bottom, fixed end of the mast is not vibrating, while the top is, if its tuned to the right frequency.

I forgot mention anything about tracking, but obviously that is extremely important, too...

[Al_Hammer]

correction, It wasn't Sikorsky that swung his blades.

Piasecki first realized that the blades on a helicopter must have the same Centers of Gravity. Daland devised a relatively easy way to do this. He would balance the blades dynamically by hanging the blade from its root end and swinging it like a pendulum. Workmen then added or removed trim weights until the set of PV-2 blades swung the same way in frequency and amplitude. Sikorsky only balanced his early production helicopter blades statically, on the assembly line, but this did not cure chronic vibration.

[C. Beaty]

And before Piasecki, Pitcairn. Pitcairns suspended a blade at its root end and measured the pendular frequency.

But none of that stuff helps if you have resonance at the exciting frequency.

[Al_Hammer]

Well, I stand corrected again, Chuck. Truly astonshing how bright the early autogyro guys were.

[birdy]

If you have no perceived shake in the stick, it could be due to play(slop) in the bushings that effectively isolate the stick.
As iv said, im not going by the stick, im lookn at the head, so if there was any shake be'n masked by slop in the control systm, id still be able to see the head shake.

As for the tyres, 99% of the time its mass imbalance[ well, out ere it is anyway, coz i fit them meself and i dont have a balancing gadget. but its usualy a lump of mud or cow crap on the rim].
If i clean the offending mass off, the shake goes.

[Al_Hammer]

Well, I'm wondering just how much the rotorhead actually moves for a typical amount of shake.
There are formulas that apply quite well to other vibrating objects and using them, I come up with only fractions of an inch of predicted peak to peak movement of the head at an rpm of 350(frequency=5.8/sec) and a vibration of 1 or 2 IPS. (You shoot for 0.1 or less when using an electronic balancer.)


IPS is a standard measure of vibration. It represents the peak velocity of a sinusoidal vibration as it passes the midpoint of its travel. The DSS balancer shown below reads out in IPS as you can see.

The bold printed P in the table stands for "Pi" 3.14.

Of course, the stick moves a much larger amount, but it is amplifying the movements of the controls being fed into it and is not a good place to get your "displacement readings from. A sensor at the rotor head is going to pick up far less displacement.

[birdy]

Riteo then, to be more presice, im going by the movement of the rotor tac wires that will shake if theres any shake in the head, unless of course the frequency of the shake is at the point where the wires wont shake.
The wires dont move, and theres no movement in the head, so for me, that means no shake. The point is, iv only had 2 sets of rotors that are this smooth.