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Item 1101

OTHER: Rotor Concept - Active Blade Twist - Overview

[Active Twist Blade] [Active Twist Rotor]

[Controllable Twist Rotor] ~ This was the name used by Kaman

Means of Applying ABT:

Separate Mechanical Pitch Control for the Root and the Tip:

OTHER: Rotor Concept - Independent Root & Tip (IRAT)

Separate Mechanical Root Control and Aerodynamic Tip Control:

OTHER: Rotor Concept - Independent Root & Tip - Aerodynamic Tip Method.

Electro-Mechanical Twisting of Spar

OTHER: Rotor Concept - Active Blade Twist - Electrical Twisting of Spar Diagonal Shrinking of the skin of the blade skin.

Active Blade Profile Morphing:

Piezoelectric materials. Morphing

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

Information from Outside Web Sites: It is only here temporarily.

Double tip vortex

A feature that is evident from both figures is the anomaly of carrying a download over a portion of the advancing side. This is the result of designing the rotor with twist-10¡ in this case-to improve hover performance. The download, of course, penalizes the forward-flight performance. It's a demonstration of an axiom that comes up over and over in helicopter design: "Whatever helps hover, hurts forward flight, and whatever helps forward flight hurts hover."

This download has another effect. Normally, we think of a tip vortex as being shed right at the tip as the lift goes to zero. With the advancing tip, his "tip vortex" is generated inboard on the tip while another spinning in the opposite direction comes from the tip itself. This double vortex phenomenon has been seen in wind-tunnel tests using flow visualization techniques.

A following blade that encounters this double vortex is in for a bumpy ride as the local angles of attack change rapidly. Some aerodynamicists have attributed the vibration that a rotor generates in forward flight to this situation. Indeed, it is well known that the higher the twist, the worse the vibration." ~ Prouty, Rotor & Wing Jan 99

For all of the above see: Rotor & Wing, Jan 1999, Prouty

Quieter Copters:

Looking further ahead, researchers at ONERA and DLR are considering the use of active blade control designs instead of the current passive noise reduction systems. Manufacturers are already working on two concepts: active blade twist and active control surfaces. A few years ago, Franco-German teams started exploratory studies of active control surfaces through the RGA program - rotor with active surface control. "This is an extremely promising concept," enthuses Jean-Jacques Philippe, helicopter and tilt-rotor research director. "But although this type of rotor could make a helicopter very quiet, with a very low level of vibrations and great aerodynamic performance, we won't see it on a production machine for at least another five or ten years."

Looking further ahead, researchers will be working on active methods to improve rotor performance, in particular, active blade twist and active control surfaces.

Design:

In hover;

A large pitch at the root end of the blade (large twist) helps to distribute the lift a little more evenly about the area of the rotor disk.

In fast forward flight;

The retreating blade will be subjected to less negative lift on its inner portion [reversed velocity (flow) region]. This will decrease the need for lift at the tip and therefore less chance of retreating tip stall. In fact, if the twist is enough, then positive lift will be provided and the increased drag (H-force or Profile?), resulting from the swapping of leading and trailing edges will help rotate the rotor.

The advancing blade will be producing less lift on its inner portion and therefore there is less chance of the outer portion being in negative lift.

The vibration will be reduced in forward flight. "it is well known that the higher the twist, the worse the vibration." ~ Prouty from Rotor & Wing Jan 1999

In other words, during fast forward flight the blades will see less extreme angles of attack ( + & -) [Ref: DESIGN: UniCopter ~ Rotor Disk - Advancing Blade Concept (ABC) ~ Retreating Blade Consideration, whereas during hover the distribution of lift over the disk area will be improved.

Note that the rate of twist change is associated with the relatively slow transition in flight mode, not with the faster rotor rpm. This is CBT (or ABT). The Reverse Velocity Rotorcraft Concept (Sikorsky) does more but the pitch will cycle every RRPM.

In autorotation;

The Helicopter program was run with a 0º and -12º twist. It shows the -12º twist requires 2.4% less power during hover. This might imply that utilizing a positive twist of +1º to +3º (as used on gyrocopters) during autorotation might result in a meaning reduction in the rate of descent.

This old NACA report may be of interest. Observations in flight of the region of stalled flow over the blades of an autogiro rotor

The Osprey V-22 has a very high rate of descent when autorotating. I wonder it does/could/should implement Active Blade Twist, to improve its performance slightly in cruise, hover and autorotation?

Vortex Ring State: It is stated that lower twist has a positive effect when operating in the VRS.

Blade Twist Effects on Rotor Behavior in the Vortex Ring State Have hard copy.

From the Conclusions of this article "VRS is not solely determined by the operating conditions of the rotor, such as descent rate and forward speed, but is also dependent on details of the design of the rotor such as the degree of twist incorporated into the blades."

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The spar of the blade must have high torsional stiffness. (See: OTHER: Rotor Concept - Improved L/D Ratio - Independant Root & Tip (IRAT) for alternate design) The skin at the tip is affixed to the spar in a conventional manner. The skin on the inner portion of the blade's span is rotationally flexible about this spar. [The spar's rigidity might be achieved by warping the unidirectional carbon spar in carbon cloth, at a 45º bias. Or, wrap a carbon thread along the span axis.

For the skin, use fiberglass cloth with the thread running at 45º and carbon cloth with the thread running at 0ºand 90º.

The 1P harmonic control (swashplate) will control the rotation of the spar and thereby the pitch of the total blade. The pitch of the skin at the root is modified by a separate actuator [see below for Means of Actuation]. This means that the difference in pitch between the tip and the root is variable [variable twist]

Miscellaneous:

This idea would be somewhat like putting flaps on the inner portion of the blades. At slower speed (hover) the flaps would be deployed for additional lift. The problem with flaps of course as that they will relocate the chordwise center of lift.

Outside Related Information:

Induced velocities near a lifting rotor with non-uniform disk loading

Harry H. Heyson S. Katzoff ~ NACA Report 1319 ~ Jan 1957
http://naca.larc.nasa.gov/reports/1957/naca-report-1319/

http://www.me.sc.edu/research/lamss/pdf/JOURNALS/21_SVD_V32_N5_2000.pdf Have hard copy.

http://www.glue.umd.edu/~bernhard/DOCS/ahs01_v7.pdf Have hard copy.

http://techreports.larc.nasa.gov/ltrs/PDF/NASA-96-asme-wkw.pdf

http://www.isd.uni-stuttgart.de/sfb409/projectA5.html

ACTIVE-TWIST ROTOR CONTROL APPLICATIONS FOR UAVs http://www.asc2004.com/Manuscripts/sessionE/EO-04.pdf Have hard copy.

MAGNETOSTRICTIVE ACTUATORS COMPARED TO PIEZOELECTRIC ACTUATORS http://www.cedrat.com/applications/hardware/doc/ASSET2002_MAGNETOSTRICTIVE_vs_PIEZO_ACTUATORS.pdf Have hard copy.

Aerodynamic Design Study of an Advanced Active Twist Rotor: http://techreports.larc.nasa.gov/ltrs/PDF/2004/mtg/NASA-2004-4dsca-mks.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040027951_2004012240.pdf

COMPREHENSIVE AEROELASTIC ANALYSIS OF HELICOPTER ROTOR WITH TRAILING-EDGE FLAP FOR PRIMARY CONTROL AND VIBRATION CONTROL: http://research.nianet.org/~shenjw/webpage/main.pdf

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Idea: The rotor's RPM is high during hover and so is the blade twist. The horizontal thrust, for forward velocity, is provided by a pusher propeller. As the forward velocity increases, the rotational speed of the rotor is reduced. This reduction causes the blade twist to be significantly be reduced and it allows the craft to fly at faster speeds before the onset of advancing tip compression and retreating tip stall.

Extension-Twist Coupled Laminates: http://www.geocities.com/mschliesman/comp_intro.htm

AEROELASTIC RESPONSE AND STABILITY OF TILTROTORS WITH ELASTICALLY COUPLED COMPOSITE ROTOR http://techreports.larc.nasa.gov/ltrs/PDF/phd-93-nixon.pdf

Large twist is good for hover, but bad for fast forward flight.
High rotor RPM is good for hover, but bad for fast forward flight.

Consider blades that are made of Extension-Twist Coupled Laminates.

In addition, consider an intermeshing helicopter with a pusher prop. The pusher prop provides all the forward thrust. As the craft's horizontal velocity is increased, the rotational speed of the rotors will be decreased. This reduction of rotor RPM results in less centrifugal force and this, in turn, results in less blade twist, due to the Extension-Twist Coupled Laminates.

At the maximum forward speed, the rotor blades will have no twist. The rotors are only required to provide lift, so now these rotors will be operating with minimal engine driven torque, and the rotorcraft will be flying, at high speed, in a semi-helicopter / semi-gyrocopter mode.

For those who like to read themselves to sleep, here are 449 more pages on the subject.

http://techreports.larc.nasa.gov/ltrs/PDF/phd-93-nixon.pdf

From press release about V-22, posted on rec.aviation.rotorcraft on September 24, 2002

Engineers are also studying options to change the blade on the tiltrotor since the current design is a compromise between one designed as a rotor and a prop. Bell and Boeing each have candidate technologies that they say would improve performance in parts of the flight regime. Boeing has explored a piezoelectrically reconfigurable blade that could be optimized depending on whether the tiltrotor is in aircraft or helicopter mode. Designers believe the technology could increase range and lift by 15-20%. A flying prototype isn't expected until 2006. If the promised improvements can be achieved, they would increase V-22 speed to about 300 kt.

Bell Helicopter Textron is working with a slightly more traditional design, by slotting the existing blade. The fixed slot would run about 25-50% of the blade span and is supposed to improve the boundary layer and increase lift coefficients. It borrows a technology that emerged from a NASA/Army study several years ago. The performance improvement would exist in hover mode, particularly at higher power settings. Engineers believe the cruise performance wouldn't be degraded, but that has yet to be proven. So far testing only of a small-scale power model has been conducted. Managers want to see both concepts mature before deciding whether to adopt one for the V-22.

Outside Web Sites:

http://www.asc2004.com/Manuscripts/sessionE/EOP-04.pdf

http://www.wam.umd.edu/~bruck/asme.htm Have hard copy

http://www.resonance-pub.com/actuator.htm

http://www.smalltimes.com/document_display.cfm?document_id=4232

http://www.pkn.co.za/pap-FMLNG.html

http://techreports.larc.nasa.gov/ltrs/PDF/1999/mtg/NASA-99-55ahs-wkw.pdf

http://www.starboardinnovations.com/Downloads/Papers/AHSDerham01.pdf

http://www.starboardinnovations.com/Downloads/Papers/ICAST98.pdf

RECENT ADVANCES IN SMART-MATERIAL ROTOR CONTROL ACTUATION.

http://www.me.sc.edu/research/lamss/pdf/CONFERENCES/C58_ASF00.pdf Have hard copy

MECHATRONICS ASPECTS OF SMART MATERIALS INDUCED STRAIN ACTUATION http://www.me.sc.edu/research/lamss/pdf/CONFERENCES/C62_M2000-250.pdf Have hard copy