B311

DESIGN: UniCopter ~ Vibration - Rotor Induced - Analysis

Overview:

An attempt to determine what will cause rotor induced vibrations in the UniCopter.

Primary index of ID's can be found on pages; 1020.html & 1089.html

ID 19/

Lateral Dissymmetry of Lift and Drag:

The UniCopter's blades must intersect at the sides (advancing at 90º & retreating at 270º). One advantage that the UniCopter has over the coaxial, if the breast-stroke (inside-forward) is used, is that the distance between the advancing blade's center of thrust/drag and the centerline of the craft is less. This is due to the lateral offset of the rotor hubs, This should result in less rolling and yawing moments. The current thinking is the use the outside-forward rotation and this will increase the vibration.

I think that the major vibration will be a 3P roll moment, caused by an alternating thrust at the sides (2P?) (and/or the rotor-rotor aerodynamic interaction at 90º and 270º azimuths. See: UniCopter ~ Rotor - Disk -Lateral Dissymmetry of Lift (for 3-blade rotor), UniCopter ~ Rotor - Disk -Lateral Dissymmetry of Lift (for 4-blade rotor), and OTHER: Aerodynamics - Vibration - Rotor Induced - Analysis Re: Sikorsky ABC.

It can probably be significantly reduced by HHC, specifically DESIGN: UniCopter ~ Control - Flight - Swishring - Fixed Azimuth Overview.

ID 17/

Rotor to Rotor (downwash & downdraft):

How to eliminate the rotor to rotor induced vibration, in helicopters with overlapping twin-rotors by somehow temporarily increasing the pitch [and thereby maintain a consistent angle of attack] on the blade (or its individual elements) as it (they) passes through the downwash from an upper blade and through the downdraft from a lower blade.

In other words, the blade section aerodynamically pitches up when it starts to experience a decrease in thrust and it aerodynamically pitches down when it starts to experience an increase in thrust.

For general information see: OTHER: Aerodynamics - Vibration - Rotor Induced

The RRPM during hover will be fairly fast, however during forward flight and using the slowed rotor concept, the RRPM will be much slower. The web page OTHER: Aerodynamics - Vibration - Rotor Induced - Overview shows the Vibration Acceptability. I suspect that the rotor to rotor vibration would be most noticeable during hover if the RRPM was the same for hover and cruise, therefor the slowing of the rotor during cruise may not be a detriment.

This can probably be significantly reduced by IBC, specifically 1087, 1112 and 1095.

ID 18/

Blade to Blade (BVI):

This may not be very pertinent during forward flight because;

The relatively slow RRPM combined with the fast forward velocity, due to the pusher prop, may result in the vortices being aft of the following blades.
The contribution of the pusher prop will reduce the disk loading.
Vortices are likely to be struck by the other rotor's opposite rotating blades before the following blade from the same rotor.

ID 1, 3 & 4/

Downwash on the Fuselage

This may present a problem. The thinness fuselage may help.

	Hover	None
	Forward Flight	Pitching moment
	"	Yawing moment

ID 2/

Downwash on the Tail Boom:

A problem is that the blades, and particularly their tips, are in very close proximity to the boom.

An advantage is;

	Hover & Forward Flight	Pitching moment	HS will help dampen.
	"	Yawing moment	VS will help dampen.

A potential disadvantage is that the blade crossing of the boom will cause the thrust to more forward along the boom.

Dihedral at the blade tips may reduce the size of the horizontal stabilizer and the tail boom slightly.

ID 14/

Gusts:

Gust will have a greater effect on this rigid rotor, but probably it will be no worst that which is experienced in a plane.

ID 15 & 16/

Blades to Pilot Controls via Pitch Links:

Hydraulic controls may already be necessary due to large pitching moments and forces.

Length of Moment Arms:

The pilot is located so close to the UniCopter's center of gravity that the moments may not be a problem.

The vertical linear motion may be a concern. Why. Its not a problem on the ABC.

Frequency:

UniCopter I ~ At 600 RRPM the six blades will intersect 60 times per second. This is the frequency of AC electricity in North America. Perhaps the high frequency combined with the inertia of the rigid blade will significantly reduce the vibration. Can HHC or IBC cycle at anywhere near this frequency? UniCopter II ~ At 550 RRPM the six blades will intersect 55 times per second.

Resonance:

At appears that the aerodynamic sources of the N-per-rev vibration, which are summed at the hub, are only one half of the problem. The other half is the dynamic resonance(s) that amplifies this vibration. If it is assumed, for this argument, that the sources of this vibration cannot be reduced, then it would appear that a reduction of the resonance is preferable over that of offsetting the vibration.

Much is done to assure that the components within the fuselage are 'out of tune' with the vibration. The blade is the primary source of aerodynamic vibrations, so what is done to assure that the blade does not resonate?

In this regard, I would think that a highly rigid blade would be advantageous since its frequency will then be far above those of the sources of the vibration. Lu previously mentioned " ... undamped sinusoidal wave that travels down the blade from the tip to the root.". A rigid blade should result in a local disturbance at a blade element being greatly dampened by the other elements of the blade.

And even More Notes:

1/ All current vibration analysis appears to be directed at the single rotor, plus its interaction with the fuselage. It also appears that vibration may have been the primary objection to the coaxials and intermeshers.

2/ The vortices that come off the trailing edge of a blade and particularly from its tip and root appear to consist of many small random events. As these vortices strike another 'flexible blade' they probably cancel each other out on occasion and compliment each other on other occasions. I feel that absolute rigidity will disburse each small protuberance over the whole 'rigid rotor assembly'.

Notes re Linear Vertical Motion or Oscillation about the X-axis:

The blades will overlay each other at the sides at 6P. This will alternate laterally at 3P.

4-blade vs. 2-blade Rotors: The Prewitt testing of the Flettner Fl-282 showed a relatively high level of rotor induced vibration. If this vibration was rotor-rotor induced between the two disks then the use of four blades per rotor should offer a significant reduction in this vibration.

Each blade of a four-blade rotor is supporting half the load of a comparable two-blade rotor.
The blades are interacting four times more per revolution, which means a higher frequency and reduced amplitude.

This may result in a vibration reduction to (1/3) * (1/2) = 25% of the 2-blade rotor, and also, and will result in a frequency increase of 200%. This may be a primary reason why Kellett tried to implement three blades per rotor.

Note that the required rotor stiffness to have three blades per rotor will create a poet/starboard vibration.

Wild thought: Could something tweak the whole collective up & down and also change the azimuths of up & down as the forward velocity changes? See: OTHER: Aerodynamics - General - Higher Harmonic Control

A thought re ABC: The UniCopter should have significantly less rotational vibration about the X-axis than the initial Sikorsky ABC had. This is because the Sikorsky's 3-blade rotors appear to impart a severe 3P lateral oscillation the craft, where as the UniCopter's 4-blade rotors should produce no similar vibration. .

Another thought. The fact that the horizontal stabilizer is not in the downwash of the rotor should result in less vibration. There may be two reasons for this. The obvious one of no cyclical downwash hitting the stabilizer and secondly, the stabilizer may dampen some of the vertical motion imposed on the end of the boom.

My Thoughts re the UniCopter and Oscillatory Roll:

There should be symmetry in vibration because both blades are high when in the advancing position. This may, hopefully, be similar to the lesser vibration situation above.

The intermeshing Unicopter has an offset overlap to the side and the advancing (high lift) blade is on the inside (breaststroke). This means that the moment arm from the center of lift to the center of the craft will be less than the arm length of a comparable coaxial.

See: OTHER: Aerodynamics - Vibration - Rotor Induced

Also : [Source ~ RWP4 ch. 8]

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Last Revised: October 18, 2005