Item 1112

DESIGN: UniCopter ~ Vibration - Rotor Induced - Control - Lower Surface Flap(s)

Intent:

To provide a means of rapidly (and momentary?) increasing the lift of a rotor blade.

Abstract:

Rotor induced vibration is one of the primary problems of rotorcraft (helicopters). Very likely, this is particularly true of helicopters with twin aerodynamically interacting main rotors. The direct confrontation methods of Higher Harmonic Control and Individual Blade Control, currently employed or under consideration, cannot attain high enough frequencies to result in a significant improvement. The reason for this is the time lag between the controllers signal and the resultant change in the blade's lift.

Individual Blade Control is the faster of the two methods, but it requires the following activity to sequentially take place. The control signal must be sent to the actuator. The actuator must then physically reposition the tab. The tab must then aerodynamically reorient the blade's pitch angle over a portion or its entire span. In addition to the forgoing, the initial actuation of the flap may impart a moment on the blade, which is the opposite of the desired result.

Alternative A:

By locating a single small flap at the aerodynamic center of the lower surface, it should be possible to have a shorter delay between the controller's instigation of the lift change and the actual lift change. Approximately 95% of the blades mass does not have to be rotationally moved and the instigation of vertical movement is immediate, in the desired direction.

Alternative B:

By locating a small flap and plenum at the front of the lower surface and another at the trailing edge, it should be possible to have a shorter delay between the controller's instigation of the lift change and the actual lift change. Approximately 95% of the blades mass does not have to be rotationally moved and the instigation of vertical movement is immediate, in the desired direction.

Potential Advantages:
    1. The flap and the passage to it from the plenum does not have to pass through, and weaken, the lower flange of the spar.
    2. The two flaps might be smaller than a single flap for the same lift.

Preamble:

There is no way that a rotor blade will be able to rotate about its feathering axis at rates approaching 60 Hz, but it may be possible to move a small leading and a small trailing edge at this rate.

This concept has only two positions. When the actuators are in the 'home' position the profile is a conventional NACA 00xx. When activated, the flap moves down. This should causes an increase in lift (and drag) without changing the primary feathering angle of the blade (creating a moment about the pitch axis). It is intended that the increase in lift is of very short duration, and there might even be a spike to take advantage of.

The amplitude change is a constant and the effectiveness of the change is governed by the duration.

Overview:

Locate a small flap on the lower surface of the blade at the center of chordwise lift (approx. 25% of chord). When activated this flap bends down slightly to temporarily increase the lift of the blade.

Inside the blade just above the flap is a chamber (plenum) that contains only air. The default pressure of this air is between ambient air pressure and that of the pressure on the outside (lower) surface of the flap. The chamber receives its air from the leading edge of the blade, from the root of the blade or from the tip of the blade.

If the tab exists at only one location on the blade and the rotor turns at a fairly constant rpm then the delivery tube, from input port to chamber, might be tuned so as to create a higher pressure at the time when the flap is opened. The blade azimuth may vary as the forward velocity varies but the time between openings does not vary.

The intent of the chamber and its related components is to minimize the vacuum that would be created behind the flap, if there were no air feed.

Drawing:

Notes:

How fast can this idea cycle? 60 Hz+??

For control see: UniCopter ~ Vibration - Rotor Induced - Control - by Computer 1095

Dynamic Overshoot:

See pages 397 to 407 in Helicopter Performance, Stability and Control on this subject. It can probably be utilized at high angles of attack and may be beneficial at lower angles of attack, as well. Its short duration may be excellent for accepting the downwash and tip vortex from the upper advancing blade.

Also see: [Source ~ RWA II p.108]

Drag - Profile & Induced:

The drag may not increase when the tabs are activated. This is because the downwash of the passing upper blade has temporarily reduced this blades angle of attack, and therefore at least reduced its induced drag.

Limitation:

This concept may be limited to a frequency that is too slow for that which is required.

Spanwise Location of Flaps:

The greatest downwash on this lower blade will be in the middle of its span, when it is at 270-degrees azimuth. The further out the blade the flaps are, the greater the thrust, because the air speed is greater. A problem with this is that there will be an upward force at the tip of the blade, a downward force at the middle of the blade and a resultant very downward force at the hub.

Perhaps one or two upper blades, near the lower blade, should 'jump up' at the same moment as the lower blade is receiving the downwash.

Alternate Method; from which this one was derived:

DESIGN: UniCopter ~ Vibration - Rotor Induced - Control - High Frequency Leading and Trailing Edge Flaps

Means of Activation:

Aerodynamics - Vibration - Rotor Induced - Individual Blade Control (IBC) ~ Piezoelectric Actuators:

Additional Relevant Information:

DESIGN: UniCopter ~ Rotor Induced - Control - Closed-Loop Controller

Gurney Flap:

An aerodynamic 'dam' on the trailing edges of helicopter stabilizing surfaces

Part 1 by Prouty: http://www.avtoday.com/reports/rotorwing/previous/0200/02rwaero.htm

Part 2 by Prouty: http://www.avtoday.com/reports/rotorwing/previous/0300/03rwaero.htm

Related Patents:

Search:- "rotorcraft" AND "helicopter" ~ August 26, 2002

6,425,553 ~ Piezoelectric actuators for circulation controlled rotorcraft

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Initially displayed: September 5, 2002 ~ Last Revised: September 11, 2004

The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent.