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Item 0793

OTHER: Aerodynamics - Vibration - Rotor Induced - Higher Harmonic Control (HHC)

Overview:

Higher harmonic control (HHC) is an active disturbance rejection technique, which suppresses the vibrations due to periodic aerodynamic effects.

To reduce vibration by controlling rotor activity at frequencies higher than 1P. Probably, at a frequency equal to the number of blades in the rotor. ~ Who said this? Me? I do not think that it is a valid statement.

HHC may be related more to vibration in the non-rotating portion of the helicopter and not the individual blade.

See also Active Blade Control and Individual Blade Control.

A thought:

I wonder if HHC (2P & >) will provided any significant improvement to increased rotor efficiency or reduced rotor-induced vibration? This question is even more valid if the helicopter was already equipped with an Independent Root & Tip Control Concept

Another Idea:

Using the hub-mounted electric motor, and then giving the individual (groups) of stator coils the ability to independently vary their electro-magnetic driving force.

See; Electrotor-SloMo - Control Flight - Cyclic by Torque-Pitch

Notes:

One of the first flight tests was done on the four-bladed OH-6 BY Hughes Helicopters. This helicopter with a hydraulic control system (not normal in the OH-6) and a set of high speed actuators to apply a 4-per-rev pitch variation on top of the normal one-per-rev cyclic pitch. [Source ~ RWP4 p.86] <--HHC is a complete chapter in this booklet.

The SynchroLite and UniCopter will have a higher cycle rates because of the faster RRPM. Is this too fast for hydraulics?

Idea:

An idea may be to have a 'Y' coupling. The angle between the two upper legs of the 'Y' will be very small. The bottom leg will be connected to the cylinder at the swashring. One of the upper legs will be connected to the cyclic stick and the other will be connected to a device than gives a short-term pulse at the desired rotor azimuth. Hopefully, the pulses will be directed toward the cylinder and not 'equally' down the other incoming line. Reference: DESIGN: UniCopter ~ Control - Flight - Hydraulic Schematic

Related Information:

This Web Site:

OTHER: Aerodynamics - General - Individual Blade Control: (IBC)

OTHER: Aerodynamics - General - Vibration - Rotor Induced

DESIGN: UniCopter ~ Control - Flight - Swashring

Outside Web Sites:

http://celi.umd.edu/Res/HHC.html

EFFECTS OF TWO/REV HIGHER HARMONIC CONTROL ON ROTOR PERFORMANCE. Comment from this report: The results show that properly phased 2/rev inputs can reduce the rotor required power by up to 3.8%. The improvements are very small.

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The following pages are from a few years ago.

http://atrs.arc.nasa.gov/atrs/97/jacklin/977183/977183_jacklin.pdf

HHC by Dr. Roberto Celi @ University of Maryland ~ No significance

Performance Considerations in the Control of Helicopter Vibration

Hover Testing of a 1/6th Mach-scale CH-47D Blade with Integral Twist Actuation

Reviewing the Aerodynamics Applied to Helicopter Rotor Aeroelasticity in Hovering Flight

Active Control of helicopter Rotors

http://www.aoe.vt.edu/flowdata/flowdata.html

http://www.glue.umd.edu/~osho/repo.html

http://www.dlr.de/EA/Abt_TA/hhc1

http://www.engin.umich.edu/dept/aero/people/faculty/old_faculty_pages/Friedmann/publications.html

http://celi.umd.edu/pubs/AHS2rev.pdf

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The following page is available on the Web as at November 2, 2005. From Google search on ["Higher Harmonic" Stall Control]

http://celi.umd.edu/Pubs/2revAHSJ.pdf

Printed Material:

Even More Helicopter Aerodynamics, Ch. 26

Patents:

6,283,406 Use of flow injection and extraction to control blade vortex interaction and high speed impulsive noise in helicopters

Concerns:

The biggest concern is as follows. Let's assume that a blade must increase its pitch by one or two degrees and then return to the original pitch setting, all within 30 degrees of rotor revolution. The blade-flap can probably cycle within this time frame, but is it possible for this flap to aerodynamically overcome the blade's inertia about its feathering axis, within this time frame? [I.e. Frequency x Amplitude x Inertia]

Also see: Effect of Rapid Pitch Changes (overshoot) at [RWP1 p.115]

My posting to PPRuNe March 18, 2002

3 D

This post follows the 'air down the blade' theme of your thread but differs in the application of this air.

Vibration is a problem to helicopters and a reasonable amount of research is currently being conducted in an attempt to reduce it. This effort is referred to as Higher Harmonic Control, Individual Blade Control and Active Blade Control. It appears that all or most approaches have a mechanical device move a tab or change the shape of the trailing edge.

I, in ignorance, believe that the time required to activate the mechanical device, which will change the angle of attack, which will change the lift of the blade (and then return it all) Is TOO long for anything higher then the lowest harmonic(s). A pulsed discharge of air immediately changes the angle of attack without having to overcome the blade's inertia about it pitch axis. And in addition, nothing has to be done to return the angle of attack to is original attitude.

rotorque has mentioned the probable limitation of being able to supply a sufficient amount of air.

A tuned exhaust is used on two-stroke engines to increase the compression ratio. It is a simple concept and only requires the opening and closing of a single port. The frequency of the pulses is only dependent on the length of the tube. I may do the calculations to see how the length of the blade (tube) relates to the RRPM (pulse rate).

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I am very concerned about rotor-to-rotor vibration on my intermeshing helicopter projects so if you patent the idea, please give me rights of use. :)

My posting to rec.aviation.rotorcraft, May 7 2002

Higher Harmonic Control or Individual Blade Control

It seems that there must be upper limits to the frequency x amplitude of pitch that can be achieved by blade control at rates higher than the swashplate's one per revolution.

In other words; the force required to rotate (cycle) a blade about its feathering axis is going to exceed the ability of its elevator or the strength of its root mechanism at some ([frequency] x [blade inertia] x [pitch amplitude]).

There may be some correlation between light and heavy helicopters since the heavy helicopter will have greater blade inertia to overcome but its slower RRPM will mean a lower frequency.

Can anyone offer comments on the maximums that HHC or IBC is capable of, or direct me to sources of this information?

Related Patents:

5,655,878 ~ Higher harmonic control apparatus for a helicopter rotor

5,314,308 ~ System for controlling higher harmonic vibrations in helicopter rotor blades

6,102,659 ~ Swash-plate device for controlling the pitch of rotor blades with cylindrical runners sliding on non-rotating pillars

6,074,168 ~ Swash-plate device for controlling the pitch of the blades of a rotor with rotary guided parts on slides translated along the rotor axis

6,033,182 ~ Swash-plate device for controlling the pitch of rotor blades with non rotating plate stop track and pin

5,314,308 ~ System for controlling higher harmonic vibrations in helicopter rotor blades

Last Revised, August 11, 2010