Rotor blade vibration

[Arnie Madsen]

I should also mention that the Bell Nodamatic System gives a nice ride to the pilot and passengers by isolating the 2 per rev vibration. However a secondary problem arises because the rotor and transmission move up and down almost two inches twice per revolution and the splined input power shaft compensates for the movement. The grease in the splined coupling tends to pump out and has been known to enter the turbine intake causing all kinds of problems .... always something .....!!!

Arnie

 

[joe nelson]

Hey Arnie,

It sounds like you're a serious rotorhead like me. I've flown both the "nodamatic" and "live" systems from Bell and from the pilot's perspecive works good (occasional hemorhoid not withstanding). Doing some "gyroneering" here, the nodamatic principal would work well on larger (cabin-class) gyros but the weight penalty for smaller ones might be a problem.

The wife and I went to Mentone last week and I had my nose in every gyro I came to. What I found was that a large number of our "roto-buds" uses lord mount/dynofocal type mounts for vibration control. Other use absolutely nothing and just live with the shakes...I'm too old for that! What impressed me the most was the craftmanship and care each person used in their aircraft.:hail:

 

[rustynance]

Chuck,
Is the feathering and flapping then more responsible for the 2/rev shake? In other words, if a Delta3 hinge was incorporated into the mainrotor teter block, would that reduce the amount of 2/rev shake due to reduced flapping which would decrease the total deflection of the circle you described? I'm well aware of the down side in terms of rotor response to using Delta3 in the main rotor.

Rusty

 

[C. Beaty]

A considerable amount of delta-3 would reduce 2/rev shake but at a penalty: reduced rotor damping and control crossup.

Rotor damping arises from the lag; something that disturbs the airframe is resisted because the rotor lags and supplies a restoring moment about the CG. Delta-3 imposes a kind of aerodynamic spring between rotor and airframe, tying them more tightly together.

With a normal rotor, force/displacement has a 90º relationship; with delta-3, the rotor resonant frequency is higher than rotational speed because of the aerodynamic spring and displacement is less than 90º. When the stick is pushed forward, there will be some sideways tilt of the rotor with delta-3.

I’ve played with delta-3 using model rotors running in front of a fan. With a seesaw rotor, delta angle can be set by skewing the teeter axis. At angles near 45º, things start getting squirrelly; the rotor will sometimes get into a nutational mode like a wobbling top.

The A&S 18-A has a huge amount of delta-3; it looks to be about 30º because it is the mechanism by which collective is lowered following a jump. As the rotor slows and coning angle increases, pitch is automatically pulled as a result of the coupling. The resulting cross control is partly compensated by swash plate phasing but can only be exact at a fixed flight condition; at others, the control crossup creeps back in.

Delta-3 is great for tail rotors but I’m not so sure it’s the magic bullet for main rotors.

But to answer your original question, Rusty, the violent shakers are the result of inplane resonance being excited by the 2/rev aerodynamic input. A rigid rotor pylon will almost always bring the rotor inplane resonance down to the 2/rev input. The cure is a limber mast combined with the stiffest possible rotor inplane.

The attached photo shows what Bell had to do to solve 2/rev on one of the B-47 precursors. The production model had the rotor stiffened internally.

 

[layada]

Is part of the 2 per rev vibration caused by the difference in drag caused by the cross airframe retreating and advancing blade positions (vs. the longitudinal fore and aft blade position that has equal lift and drag)

Cannot say I've really thought this through but would a simple drag increasing device of two flat plates mounted atop the rotor head either side and 90 degrees to the main rotor enable a more constant angular drag to be achieved - mitigating the vibration (at the expense of performance)?

 

[rustynance]

Yep, I agree completely Chuck. I've flown rotorheads that had 30 degrees of Delta 3 and can attest that it does create a few problems. Any mechanical flapping will cause the rotor to momemtarily tilt roughly 1/2 the amount of mechanical flapping 90degrees later in rotation. I often thought this was the reason the 18A would roll a little left after a jump. This doesn't just happen with cyclic changes but collective changes will cause the same effect providing you have some airspeed.
The original Carter Copter had 30 degrees of Delta 3 in the rotor, very stiff blades in plane, and an air cylinder that allowed the mast to rock back and forth gently ( some engineering is required in the air cylinder as the pressure required at high speed is much greater than at low speed). As a result it had absolutely no 2/rev vibration.
Thanks again Chuck.
Any possibility of getting you to write an article for the magazine on what we discussed a couple months ago? (Height Velocity) I cringe when I see people fly very slow at 30 feet off the ground.

Rusty

 

[C. Beaty]

Rusty, I occasionally think about that article you asked me to write and give myself a mental kick in the rump for not having done so; but I will.