4 Bladed rotor system

67november

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Have any of you made/used them for your gyroplanes, not your models

I would like to see your comments.

thx, newbe gyro person
 
That design isn't used much due to complexity and cost.

You'll find most gyro enthusiast want CHEAP. A four bladed gyro first doubles the price of rotors, then goes up due to a required articulating rotorhead.

a 2 bladed "teetering" system is CHEAP and works...... oh yeah, it's lighter...for the ultralight guys.
 
Bell has been using some 4-blade systems lately that are actually very simple, and might be worth a look for ideas.
 
CHEAP is always the owner way when it comes to their a/c costs.

I would think a semi ridig rotor system would werk on a four bladed system but thats just a guess on my part.
 
A search should turn up a thread or two that talks about what happens when you try a semi rigid setup with 4 blades - it's been done, things start cracking within a few hours if not articulated.
 
I have no doubt that a two bladed semi-rigid rotor would work in a gyro, but to make it work well and be simple enough, it needs a swashplate controlling servo-tabs and a stub mast that tilts in flight. Everything considered, I came to the conclusion that a standard teeter head rotor system is the simplest way to go, although not the best way to go. For more capability and control, I feel that a rigid rotor is the best way, even though it is more complex (simpler than a semi-rigid)….it just depends on what you want in a flying machine.
 
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4 blades

4 blades

I am working on a 3 blade hub for a standard gimble head but I don't see how a 4 blade would work with my design due to the need to have all the hinge lines pass through the center of rotation. If you need 4 blades it may be better to go with 2 teetering rotors 90 degrees apart with each on it's own bearing with a rubber spider to keep them spaced properly but still let them move so you don't break things. This is not something that I have done, I have only drawn it on paper.Chuck Beaty may have some reasons why this would or wouldn't work.
 
First off, criss-crossed teetering rotors need a scissors hinge. In the presence of cyclic flapping, the blades don’t maintain their 90º spacing; if they could flap all the way down, they’d be parallel; tips moving along the path of an orange peel or the longitude lines on a globe.

The most serious problem is resonance. A single see-saw rotor with an anvil bolted to the rotorhead resonates at near 1/rev with the usual hub bar arrangement. This destroys blades in a hurry. The second rotor being the equivalent of an anvil in terms of mass.

The Bell 4-blade rotor is a soft in-plane system, meaning the in-plane resonant frequency is ~60% of rotational rpm. This avoids resonant excitation by the periodic air forces but in most cases, is high enough to avoid ground resonance.

When it comes to rotors, life is never simple.
 
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4 blades

4 blades

would 2 7hp westbend engines on the rotorhead be close enough to an anvil to cause problems, this could be the reason that bensen gave up on that system, when I first saw it I wanted one and then it was gone.
 
Know nothing, just inquiring

Know nothing, just inquiring

First off, criss-crossed teetering rotors need a scissors hinge. In the presence of cyclic flapping, the blades don’t maintain their 90º spacing; if they could flap all the way down, they’d be parallel; tips moving along the path of an orange peel or the longitude lines on a globe.

The most serious problem is resonance. A single see-saw rotor with an anvil bolted to the rotorhead resonates at near 1/rev with the usual hub bar arrangement. This destroys blades in a hurry. The second rotor being the equivalent of an anvil in terms of mass.

The Bell 4-blade rotor is a soft in-plane system, meaning the in-plane resonant frequency is ~60% of rotational rpm. This avoids resonant excitation by the periodic air forces but in most cases, is high enough to avoid ground resonance.

When it comes to rotors, life is never simple.

Criss-crossing teeters occurred to me also. Do you have any photos or sketches of "scissors hinges"?

When mentioning anvils, are you thinking of extra mass above the teeter pivot as increasing resonance? Would the effect be the same if the anvil's mass were at or below the teeter pivot?

The reason I ask is that I could envision a criss-cross teeter, 4-blade rotor which had all its mass at the teeter hinge while in flight, with all four blades tracking the same plane at their tips and at their connection, and with the two pivots both passing through rotational center at the same elevation.

I am sure someone must have envisioned this before, so there's probably something I do not understand.
 
Take a stick of 1/8” welding rod, grasp it between thumb and forefinger 1/4 of its length from one end and give it a thump. It vibrates at its highest possible frequency, the free mode.

Add mass or spring restraint at its center and the frequency is lowered.

The in-plane resonant frequency of a seesaw rotor is just above its rotational rpm. Like the welding rod, mass or spring restraint at center lowers the resonant frequency.

If a rotor is resonant at its rotational rpm, things break pretty fast. It shows up as a 2/rev shake in stick and airframe because of the frequency doubling effect of rotation.

Seesaw rotors must be as stiff in-plane as possible and must be mounted on a soft mast.

Bell had to resort to external stiffening of the rotor in early prototypes. Later models of the B-47 had internal stiffening. The rotor pylon/engine/transmission assembly was mounted on very soft rubber bushings.

Oops! Forgot the picture first time around. Here it is:
 

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Are there any examples of a two rotor see-saw system with four blades? Has it been tried, and can it be researched?

It sounds like you are saying, Chuck, that it could work were the in-plane stiffness sufficient in both rotors, perhaps using very stiff blades as well. Is that a fair assessment?

It seems like such a system, though more costly, would be attractive for all gyros for their smaller diameter while providing for low blade loading.
 
Yes, it’s been tried. The aluminum rotor blades developed trailing edge cracks after a very short time.

Rusty Nance was the test dummy.
 
Kaman_Rotorheads.jpg


This is a sketch of the Kaman 4-blade teetering rotor. The only unique thing here is that the rotor has been split into two 2-blade teetering rotors.

Apparently, the masts absorb some of the lead-lag between the two pair of rotors: as does the mast on the 2-blade Bells to absorb the lead-lag between the rotor and the engine.

In addition, it can be seen that the two blades on the same rotor are connected in the lead-lag direction by a 'turnbuckle' (P19). In the middle of this turnbuckle is an adjustable friction damper.


Dave
 
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I can manage Euler's equations and know about coupled oscillating systems and so on. But it gives me the creeps to realize the complexity of forces and the havoc they can wreak when you design rotors. I'm glad there are some people around who can tell what works from what doesn't by other means than having someone try it out.

I guess that's the difference between applied engineering coupled with experience in the field and a nuclear physicist :)

It's great to have you on the forum, Chuck!

-- Chris.
 
I can manage Euler's equations and know about coupled oscillating systems and so on. But it gives me the creeps to realize the complexity of forces and the havoc they can wreak when you design rotors. I'm glad there are some people around who can tell what works from what doesn't by other means than having someone try it out.

I guess that's the difference between applied engineering coupled with experience in the field and a nuclear physicist :)

It's great to have you on the forum, Chuck!

-- Chris.

I second that opinion. Sometimes cryptic, for good reason, he'll always let you know if he's not 100% sure of something.

I'm thankful to be able to pick your brain Chuck.
 
Chuck Beaty

I was hoping you could supply your expert opinion on the possibility of using two standard teetering gyro rotors layed out in the same configuration as the intermeshed Kaman helicopter for a payload gyro. I assume the main drive transmission (in this case a PP prerotator) would be subject to the same scissors effect in loading between the two rotors but could this not be taken up within the gearbox with a torsionally dampened coupling? I assume each mast would need to be flexible as well to allow for the 2/ rev oscillation. I've been a long time fan of Mr. Kaman and think that there may be a way to move the autogiro out of the "toy" category as you have so rightly appraised it with a workable 4 bladed system coupled with significant power at all times to the rotorblades.

Thanks in advance.

Jeron Smith
Raven Rotorcraft Inc.
 
I can see no reason why teetering rotors wouldn’t work on a synchropter arrangement with torsionally soft couplings in the shafts; Morse Moriflex or the like.

That said, Kaman always used drag hinged rotors on his machines as did Dick DeGraw on his synchropter.

Imagine two flywheels at opposite ends of a shaft and torsional resonance rears its ugly head. I think the trick would be to keep resonance well below 2/rev aerodynamic input which drag hinges do very well. Some IC engines have used pendulum torsional dampers that amount, in principle, to drag hinged rotors.
 
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