1087

Item 1087

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

This page may later be broken up into a general page, with separate pages for each of the considered methods.
Objective:

A means of minimizing the blade vibration at the blade.

A means of rapidly and momentarily increasing, or decreasing, the lift at one or more of the elements of the rotor blade.

A two position (fixed amplitude) motion.

A variable high rate pulse modulation.

Increased lift is achieved by having the leading flap move up at the same time as the retreating flap moves down. A morphing of the airfoil's profile.

To utilize Lift Overshoot. (see; Vertiflite, Winter 2001 ~ 'Dynamic Lift', p. 30, by Prouty.) and Adaptive Airfoil Dynamic Stall Control

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 under consideration, cannot attain high enough frequencies to result in a significant improvement. One reason for this is the time lag between the controller's signal and the resultant change in the lift of the blade. To significantly reduce vibration, the frequency must be significantly faster than 1 or 2 per rev.

Individual Blade Control may the faster of the above two methods, but it requires the following activity to sequentially take place. The control signal will be sent to the actuator. The actuator will then physically reposition the trailing tab. The tab will then aerodynamically reorient the pitch angle of the blade. In addition to the forgoing, the actuation of the flap may impart an initial moment on the blade that is the opposite of the desired result. In addition, this method imparts loads on the pitch mechanism.

By locating an imbedded flap at both the leading edge and the trailing edges, it should be possible to have a much shorter delay between the controller's instigation of the lift change and the actual lift change. Approximately 95% of the blade's mass does not have to be moved. In addition, the change of the vertical force is almost immediate in the desired direction. This action should not change the chordwise aerodynamic center of lift and it should not attempt to rotate the blade about the pitch axis.

In addition, overshoot may supplement the amplitude of this short duration activity.

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 flap at this rate.

The flaps employed in this concept have only two positions. When the actuators are in the 'home' position the profile is a conventional one. When activated, the leading edge goes up and the trailing edge goes down. This should causes an increase in lift (and drag) without changing the primary feathering angle of the blade (i.e. without 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 of the flap is a constant. The effectiveness of this action will be partially governed by its duration.

This is an attempt to obtain additional short-term lift from individual blades, at a number of irregular and changing azimuths around the disk. Specifically the intent is to minimize the vibration when this blade is in the downwash or upwash from a blade in the other rotor of an intermeshing configuration.

The only mass to be moved is a small amount at the leading and at the trailing edge of the blade. When activated, the new profile may have a slightly higher drag but it must have a higher lift, without rotating the main mass of the blade about the blades feathering axis.

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

It might be noted that Kaman's initial rotor blade had both leading and trailing flaps. They were mounted at a distance from a very flexible blade.

Generalized Detail of Activity:

The blades airfoil is NACA 0015, +/- depending where on the span of the blade this device is located.

Extended airfoil for the leading and trailing edge is NACA 0012.

They have the same thickness.

Pitching Moment:

Because there is a flap at the leading and the trailing edge, it should be possible to change the blade's thrust, without creating a moment about the feathering axis.

Amplitude, Rate and Duration:

The thinking is to have a fixed amplitude. In other words, there are only two positions. The rate of change is to be as fast as is possible. The azimuths of change (durations) are the variable. Can it cycle at 60 Hz+?

Dynamic Overshoot:

" The overshoot can be related to the change in angle of attack during the time required for the airfoil to travel one chord length." ".... oscillations entirely below stall ..... have only small dynamic effects." 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.

Location - Spanwise:

Probably between the cut-out and the tip of the other rotors blade, when the blade is at 90º azimuth. This is approximately 4-feet of tab and the longer the tab the less movement that is required for the same effect. This is where the blades x-section is the greatest and therefor there is the most room. Another advantage of locating it here is that the weight will not add significantly to the centrifugal force. The disadvantage is that the additional lift is required when the blade is low, on the retreating side and unfortunately, the airflow over the blade will be quite nominal. The advancing blade is low and will be the one subjected to the downwash.

Retreating Blade:

The leading and trail flaps are to be located at the location of the greatest chord, which is 48" radius. With a 480 RRPM and a forward velocity of 150 knots, these flaps will be experiencing a -52 ft/sec velocity when the blade is at azimuth 270º. In other words, the flaps will be in the reverse velocity region from approximately 225º to 315º azimuth, They will have no or possibly a slight negative effect, because of the reverse air flow, in this region. It will be the upper blade at this azimuth, therefor there should be no need to activate the flaps in this quadrant.

Specific Methods for Consideration:
Eventually put the proposed methods on separate web pages

Method C, w/ modification, looks the most promising, at this point in time.
Method A:~ Hydro-mechanical

Activator:

The plunger can have a small amount of leakage around the piston without causing a problem, if it spends more time in the 'home' position than the activated position. This is because the leakage will be predominately toward giving positive seating in the 'home' position.

Plunger motion chordwise:

~ or ~

Plunger motion spanwise:

The activator could be a series of interconnected plastic plungers with the one at the root end (to minimize) centripetal force) being the permanent magnet. Centripetal force moves the plungers to their home position and the electromagnet causes the device to have increased lift. Large ports and low viscosity oil is used.

The plunger could be relocated to 75% of chord (behind the spar) if it conflicts with the spar and its strength.

Input:

Blade stress sensors and a CPU to determine the azimuths and amplitudes of the pulses. See: B1095.html

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.

Limitations:

This concept may be limited to a frequency that is too slow for that which is required.
The movement of the fluid between the leading and trailing edges will cause a slight chordwise shifting in the CG. Will this be detrimental; or might it be advantageous?

Potential Problem:

The movement of the fluid will affect the chordwise balance.

If this blade is to be used on high speed rotorcraft the following two points represent problems;

On the advancing side the embedded flap may experience a very high Mach number and be subjected to Transonic Flow. This may result in the trailing edge flap have little of no effect. Reference the change from fixed horizontal stabilizer and elevator to rotating stabilizer when they were starting to brake the sound barrier with fixed wing craft..

On the retreating side if the embedded flap may experience reverse velocity. Is the flap(s) capable of working with reverse velocity?

Idea:

If the actuator encompasses a lot of the blade's span (like 3 or 4-ft) consider Individual Blade Element Control.

Safety:

Should this device fail, the amount of change in the angle of attack caused by this device may incur a high level of vibration but it could not be great enough to cause a fatal problem.

Method B:~ Electrohydrodynamic

This is an alternative and perhaps better way then method A of moving the fluid between the leading and trailing flaps. It consists of a linear induction motor wrapped around a tube, which connects the chambers of the two flaps. The tube may be located on the tip side of the chambers so that the magnetized material, which will be heavier than the fluid, will remain in the area of the motor. Alternatively, the 'armature' might be a solid plunge floating in the fluid.

~ The following is copied from; A SURVEY OF MICRO-ACTUATOR TECHNOLOGIES FOR FUTURE SPACECRAFT MISSIONS http://www.robotstore.com/download/Actuator_Methods_Survey.pdf ~ I have Hard copy.

Electrohydrodynamic (EHD) motion arises when the particle of a polar fluid are subjected to a strong electric field. The resulting motion can be used to generate a fluid pressure, and create flow or fluid circulation.

Examples:

An as-yet fictional macro scale EHD application is represented by the "caterpillar drive" depicted in the movie "The Hunt for Red October". It shows a submarine using unspecified voltages to pump huge quantities of sea water through tubes to drive the craft without the noise of rotating propellers. On the factual side, a micro scale ethanol pump has been demonstrated that develops a pressure of up to 2480 Pa at 700 Volts and a flow rate of up to 14 ml/min. [31] The charged grids were constructed from etched silicon.

Figure 20 - Electrohydrodynamic effect

Benefits / Drawbacks:

Electrohydrodynamic devices have an inherently simple design, providing direct conversion of electricity to fluid flow. They produce the fluid motion without moving parts that can stick or wear. They require high operating voltages, but at low currents. Their operation depends greatly on the electric properties of the working fluid. They can produce a high volume of flow compared to piezoelectric or thermally driven pumps, and have the potential to can act as drivers for pumps to move other types of fluids unsuitable for direct EHD flow.

Alternative Arraignment:

Have the sealed leading and trailing edges within the profile of the blade. In other words; the spar and internal diagonal wraps of carbon are within the profile and they provide all the structural strength. The trailing edge and leading edge are flexible fiberglass?? See additions to actual drawing.

The device could be a 3-position one, with the 'home' position being the central one. Why?