Item 1094

OTHER: Aerodynamics - Vibration - Rotor Induced - Tip Vortex Interference:

The changes in the local [segments of the blade's span] induced velocities due to the proximity of the vortex cause large discontinuities in the angle of attack distribution. ~ [Source ~ RWP1 p.55]

Effectively the advancing blade tip vortices introduce a powerful in-plane velocity component at the tail rotor. ~ [Source ~ HFD p.169]

Photographs of the tip vortex made on a humid day at Sikorsky .... have shown that in some cases the tip vortex remains in the tip path plane until the next blade actually strikes it, as sketched in Figure 1.32. ~ [Source ~ RWP1 p.55]

Under certain conditions of powered decent or vigorous maneuvering, rotorcraft blades pass through the wake and trailed tip vortices from previous blades. This interaction of the rotor blade with the tip vortex (my bold) of a preceding blade has been identified as a significant source of noise and vibration in rotorcraft. ~ from outside web site below.

My thoughts:

For an intermeshing configuration with ARR, the force(s) of the vortex will be distributed along a larger portion of the blade span because of the high rigidity. This should mean that more blade mass is resisting the BVI force and that a larger area is acting as an aerodynamic damper. This might mean that less of the BVI vibration would be propagated, as a wave, to the root of the blade.

Web Sites:

http://www.ae.gatech.edu/research/windtunnel/vortorgn/vortex.htm

http://www.aero.gla.ac.uk/Research/LowSpeedAero/Oldbvi.htm

http://www.psu.edu/dept/rcoe/ppt/review/ppframe.htm ~ This discusses the possibility of having two pair of blades on the same hub. "The two sets of blade can be vertically offset on the shaft, have differential droop, pitch and radius, etc. For example, for differential radius, tip vortices of the longer blades may not interact with shorter blades. Tip vortices from shorter blades would strike the longer blades inboard - not as harmful." This is the same thinking as my thinking below, where the tip vortices from the advancing blade will strike the mid point of the retreating blade.

Hovering Helicopter Rotor Wake ~ From this Particle Image Velocimetry, it appears that in hover the tip vortex moves down by 7% of R and in by 12% of R (0.98R to 0.86R) in one rotor revolution.

Patents: (Search ~ blade vortex interaction)

US 5,735,670 Rotor system having alternating length rotor blades and positioning means therefor for reducing blade-vortex interaction (BVI) noise

US 5,620,304 Rotor system having alternating length rotor blades and positioning means therefor for reducing blade-vortex interaction (BVI) noise

US 5,620,303 Rotor system having alternating length rotor blades for reducing blade-vortex interaction (BVI) noise

_____________________

Teasing Nick,

Sikorsky has three patents for a ~ 'Rotor system having alternating length rotor blades for reducing blade-vortex interaction (BVI) noise'. Is the intent to offer this as an Optional Extra?

On the intermeshing helicopter (synchropter), alternating offset tips come as Standard Equipment. :)

Posting:

To: rec.aviation.rotorcraft ~ June 22, 2002

Others have said that a gust causes a relatively small perturbation in a helicopter's body, due to the flexible rotor. If a hypothetical helicopter had an absolutely rigid rotor, would not this gust causes a greater perturbation in the body, similar to that experienced in a plane?

Is it correct to assume that a gust perturbs the total rotor whereas a tip vortex interaction from a previous blade only perturbs a portion (1 or 2 elements) of the following blade?

If the above are correct then is it also correct to believe that a localized BVI will cause less perturbation in a helicopter with an absolutely rigid rotor then in a helicopter with a flexible rotor? This belief is based on the premise that a flexible blade will transmit much of the localized BVI to the body, whereas the inertia and aerodynamic damping of the total absolutely rigid rotor should cancel much of the perturbation before it is transmitted to the body.
--
Dave J.

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