Chuck Beaty - I need a clarification on your flap-wise stiffness statement please….

RotoPlane

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Chuck, to move forward I need some clarification on your below statements (in bold text).

Ed, with a Hiller servo rotor, you can slow response enough that the counterweight can catch up with the main rotor blade in small models.

Also, no one really cares how much a model vibrates so long as the radio doesn’t shake apart.

Teetering is commonly locked out in RC models to permit inverted flight. Vibration would be intolerable in a full size machine with a hingeless 2-blade rotor.

If you’re going to dispense with flap hinges in a full size machine, you’d better have at least 3 blades.
Ed, with a 2-blade rotor, any flapwise stiffness results in vibration.

Look at all the reports of shake from just sticky teeter bearings. Those were mainly tilt head machines where the pilots muscles supplied the spring restraint against flapping when the teeter bearings stick.

In one instance a number of years ago before universal internet, a Mini-500 helicopter owner in South Africa contacted me about lateral 2/rev shake he was experiencing and it was sticky teeter bearings.

With 3 or more blades, spring restraint against flapping becomes a steady force. You can sorta view it as single-phase vs.3-phase power. With 3-phase power, you can generate a rotating magnetic field and motors don’t need starting capacitors; with single-phase power, the magnetic field just pulsates and you need something to provide a phase shift for starting. But single-phase motors always buzz and vibrate more than 3-phase motors.
The first quote seems to say to me that an articulated two-bladed rotor with a hinge on either side of the center of rotation would not vibrate intolerably with a spring restraint, where a flexing fiberglass non-hinged rotor would…..yet the second quote tells me both would vibrate intolerably.

It seems to me that both of these rotorheads could be subjected to a spring restraint either by the bending of glass or by the weight of a tilted airframe and vibrate due to the "T-bar" effect. I really hope I'm wrong and the articulated hinged head would work just fine with two blades.

When you said "hinged", were you referring to a head hinged on the center of rotation, like a teetering head?
 

C. Beaty

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Ed, I expect you’ve been around long enough to remember phonograph records and turntables.

Imagine you have a turntable and to the pilot post in the center, you’ve attached a swivel joint and a stem. The red bars are springs.

When you lean the stem in a direction that doesn’t deflect the springs, there is no resistance. You get resistance when you lean in a direction that extends or compresses the sprngs.

With the stem leaned and the turntable rotating, you get a 2/rev oscillating force in the stem.

With 3 springs evenly spaced at 120º, the force felt in the stem becomes steady rather than oscillating.

That’s why you can’t tie a 2-blade rotor to an airframe if you’re going to carry human passengers. An RC model carrying lab mice might be OK.
*********
Bell once experimented with spring restraint on a seesaw rotor, -They used torsion bars instead on normal teeter bearings. They were able to cancel 2/rev shake due to the torsion bars by reducing undersling sufficient to cause an equal but out of phase shake.

They published a paper in the AHS journal; I have a copy around here somewhere.
 

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RotoPlane

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Very good visual illustration….so with a two-bladed rotor, you're saying there is no way in hell one can safely use any rotorhead other than freely flapping or teetering rotors hinged on the center of rotation. Got it…hate it….but I got it.

Thank you Chuck! Did you know it is difficult, and sometimes impossible, to find definitive answers to questions like these in books. I love you long time…..;).
 

C. Beaty

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2/rev shake in a 2-blade rotor is fairly simple, Ed.

Things get more complicated with articulated 3-blade rotors.

We owe Cierva for the complications of Coriolis force.

My plain old Random House dictionary has it right:
Image1Coriolis.JPG

A fictitious force that permits an artillery shell to travel an apparently curved path without violating Newton.

When the Germans shelled Paris from a distance of 100 km or whatever toward the end of WW I, they had to correct for Coriolis effect. But an artillery shell does not and can not fly a curved path when viewed from inertial space. Viewed from the Earth’s surface, it appears to.
 
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Rotor Rooter

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Ed

I agree with Chuck. An extremely flatwise stiff 2-blade rotor will result in a 2/rev vibration when the rotor disk is not normal to the mast. A 3-blade rotor will significantly reduce the vibration and a 4-blade will reduce it even more.

An example of this is the 2-blade Hiller X-2-235. Nick Lappos said that ground testing of the craft all most destroyed the building. :)
The 3-blade Sikorsky -69 (XH-59) ABC was much better but still had high vibration.
Finally, the 4-blade Sikorsky ~ X2 TD was even better.

The coaxial configuration of these 3 craft probably added to the vibration.

This graph may be of interest. It shows the the worst frequency for the pilot is 15 cycles per second. This is a rotor rpm of 15 x 60 / 2 = 450 rpm.

On a very small and fast rpm coaxial rotor such as Schoeffmann's the vibration was not noticeable


Dave
 

Alan_Cheatham

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I wonder what, if anything, holds the center of the x-axle in position.
Nothing, if you were to remove one blade grip the shaft would slide right out, it just seeks it's own center.

I've run r/c helis strapped down to a stand, with cyclic input you can see the rotor disk tilt in the dampers and form an angle to the rotor mast (drive shaft).

.
 

Rotor Rooter

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Alan,

Thanks.

Some thoughts;
On a manned helicopter their bushing would not likely handle an imbalance of centrifugal forces between the two blades.
However an elastomeric bearing, perhaps even a homemade one, should handle the axial load while allowing teeter and lead-lag movement.

Dave
 

kolibri282

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It seems to me so far that the 2/rev shake is mainly due to the 1/rev sinusoidal control input. Wouldn't it be possible to diminish vibration by a higher harmonic control input?
 
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Rotor Rooter

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It seems to me so far that the 2/rev shake is mainly due to the 1/rev sinusoidal control input. Wouldn't it be possible to diminish vibration by a higher harmonic control input?
Juergen.

This is just my opinion, but assuming that Ed is considering a single rotor configuration, i do not think that higher harmonic control will help.

Perhaps an compromise between a teetering rotor and a really rigid rotor could be of interest. One, adjustable, example is a Hub Spring. Another, non-adjustable, example is a Teetering Rotor with Offset


Dave
 

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I was considering moving on to an EAB so that I could use my flex rotorhead with four blades and still fulfill my desire to have some control over the airframe. But as Chuck said in post #4, the shake in a 2-blade rotor is fairly simple….things get more complicated with articulated 3-blade rotors, so I decided to stay with a UL for now. Later I can go EAB, add two more blades and the flex head….no changes to the airframe or controls will be required.

Before the idea of using the flex head, I had toyed with an articulated 2-bladed head that is hinged on the center of rotation….yeah; I cannot bring myself to build clones…just buy them if I need to. I think it is a personality disorder ;). The arm travel angle right now is +/-10° (3° coning + 7° for control) but I'm going to add ~5° more that is filled with a rubber bumper and that should give me a fairly soft warning to ease up on the cyclic. I'll insert the drawing below so you will have a better idea of the ideas….that bring on the "Why's?". BTW….this head is mounted on a non-turning mast that angles in pitch when need by the rotor disk….like take-off.

Dave - I don't believe this rotor will have as much vibration as a gimbaled head because the cyclic controls the blades with servo-flaps….not by directly moving the head. In my opinion, any rotorcraft should have conventional gear, like the X2 or the H-34 or the Apache or that one Kaman that I can't remember the name of.

Alan - I had no idea those R/C rotorheads were that simple….if only something like that would work on full sized choppers….

Juergen - You may have noticed….that I don't have a clue about harmonic stuff ;).
 
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RotoPlane

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Those are (4) 5/16" dia. studs Alan and they thread into the hub 1.25". Their main job is to carry the spin-up loads (via a plate and shaft that goes down though the mast) and to hold the (2) bellcrank brackets for the swashplate control rods (not shown). The cross-section of the hub shows the side view of them….if it is clear enough on your screen.
 

Alan_Cheatham

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Thanks for your reply Ed but I don't think were on the same level, my fault. Try this new picture, the bolts that the blades pivot on appear to be thread in studs as well. If they are I would consider that an unacceptable design practice from a stress concentration/fatigue stand point.

.
 

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RotoPlane

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Okay...now I know what you wanted....my fault too ;). Those are high tensile (~125,000 psi) shoulder bolts and are mainly used as a shaft for bearings. The 1" dia. shoulder (this may go to 1.25 because of a different bearing I may be forced to use) contacts the hub wall (by the end of the threads) and is highly torqued....so much of the shear load on the ¾" threads is born as tension by this hub wall/shoulder torque.

Each shoulder bolt feels about 1/2 of the gross weight (plus stressed to 5G's or 1510 lb each) and 1/2 of each blades centrifugal force or 6000 lb for a total of 12000 lb max, in shear.

Unless I missed something, which is possible, the shoulder bolts should easily take that and the 6061-T6 hub should be able to handle the 1510 lb each….or closer to 2G's of 604 lbs on each bolt.

But I am all ears ;).
 

RotoPlane

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Hopefully with a white background and after the forum does its thing, this drawing will be clearer.

Okay, the white helped a little bit.....
 
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kolibri282

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Juergen - You may have noticed….that I don't have a clue about harmonic stuff
The question was partly academic, Ed, I wanted to check whether I had the right notion about the cause of the problem. Suppose the pitch horns driving your flaps would not ride along a swash plate but along a sort of roller coaster whose track you could vary in any manner you like during flight. I thought that then you could tilt the disk sideways a bit more over a smaller arc both sides of the 90/270 degree blade position. The net disk tilt would be the same but the hub would be unloaded from the spring force before it reached the for and aft position. In practice though the only way to implement this might be a fly by wire control where you would program the blade deflection as a suitable (and fairly arbitrary) function of blade position, so you'd build the most advanced gyro control currently in use....;-)

I keep my fingers crossed that someone comes up with a solution that's easier to implement!
 
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Vance

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I am confused.

I am confused.

Okay...now I know what you wanted....my fault too ;). Those are high tensile (~125,000 psi) shoulder bolts and are mainly used as a shaft for bearings. The 1" dia. shoulder (this may go to 1.25 because of a different bearing I may be forced to use) contacts the hub wall (by the end of the threads) and is highly torqued....so much of the shear load on the ¾" threads is born as tension by this hub wall/shoulder torque.

Each shoulder bolt feels about 1/2 of the gross weight (plus stressed to 5G's or 1510 lb each) and 1/2 of each blades centrifugal force or 6000 lb for a total of 12000 lb max, in shear.

Unless I missed something, which is possible, the shoulder bolts should easily take that and the 6061-T6 hub should be able to handle the 1510 lb each….or closer to 2G's of 604 lbs on each bolt.

But I am all ears ;).


What does “highly torqued” mean?

How do you manage a shear load with tension?

I don’t understand your process of calculating the stress on the bolts.

Why five Gs?

Where does your 6,000 pound number for the blades come from?

What safety factor are you using and where is it represented?

Thank you, Vance
 

Rotor Rooter

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I assume that the objective is to obtain a greater control authority over a rotor that has only two blades, while not creating additional vibration.

Skip this posting if the above assumption is incorrect.
Also, skip this posting if one does not want to consider an electric prerotor / partially powered rotor.
Also, skip this posting if development and machining etc is of little interest.
:eek:
__________________________________________
If anyone is still reading ----
__________________________________________

The control authority of the 2-blade rotor can be increased by the inclusion of hub springs or by using offset teetering hinges. In forward flight this will give a 2 per rev vibration at 180-degrees and 360-degrees.

The addition of a high speed electric motor in the hub, for prerotor and partially powered rotor, will give a 2 per rev vibration at 90-degrees and 270-degrees.

The combining of these 4 pulsations will (should) eliminate the vibration, as would a 4-bladed rotor.

It's all here for free for the experimenter who likes to try new ideas. Electrotor - Plus


Dave
 
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