Item 1035
DESIGN: UniCopter ~
Rotor - Blade - General - Twist [Δθ]
Notes:
"A blade designed specifically for speeds above 250 knots would in all probability have no twist."
~ 'Development of the ABC Rotor', May 1971Sikorsky would have reduced the twist from -10º to -8º to improve high-speed performance if they were to produce a second version of the XH59A ABC.
For additional information on twist see; [AH p.262-63]
" .... helicopter and auxiliary propulsion mode. Twist optimization for each would result in selection of a higher twist for a helicopter and lower for an auxiliary propulsion configuration." ~ Advancing Blade Concept ABC High Speed Development, page 3. See [Retreating Blade Consideration] on
DESIGN: UniCopter ~ Rotor - Disk - Advancing Blade Concept (ABC) May switch things around later. Also look at [Split Harmonic Control] if the idea develops into a page.The intermeshing UniCopter may need more twist then the coaxial ABC. This is because, in hover, the downwash from the outer portions of the upper blade pass through a more inner portions of the lower blade Then again, in forward flight this may not be true. See:
DESIGN: UniCopter ~ Rotor - Disk - Downwash Intersection AzimuthsNASA Technical Paper 3675 'A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research'
[I have hard copy], on page 14 discusses hover. It states that for torque balance between the rotor the lower rotor should have a pitch of approximately 1.5º greater than the upper rotor. This appears to be due to the downwash and downdraft between the two rotors and the convergence of the upper rotor's wake before it enters the lower rotor disk. This effect will be even greater on the intermeshing configuration, at least in the overlapping areas at the sides. This is another argument for Active Blade Twist.The cutout on the UniCopter is greater than that of the Sikorsky, therefor there is less blade in reverse airflow at fast forward speed. The reverse airflow at 150 mph is inside 3.33 ft of the blade.
If the blade has twist then having an asymmetrical blade so that one mold will serve both port and starboard rotors no longer applies.
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Twist (negative) is; |
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Good for hover |
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Detrimental for fast forward flight |
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Delays retreating blade stall in fast forward speed [Source ~ RWP1 p.647] (1) |
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Produces vibration in forward flight |
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Bad for autorotation |
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Twist will reduce the length of arm from hub to center of lift.. |
(1) I wonder how strong an advantage this is.
At very high forward speeds, the root of the retreating blade will be in reverse airflow. This will cause a downward force on the retreating blade, which must be compensated for by increasing its pitch. This in turn will increase the negative lift at the root even more. Eventually the tip end of the blade will stall. See information by Paul Cantrell on page
http://www.unicopter.com/1101.html.The increased angle of attack in the center of the retreating blade, due to twist, may delay the stall, but when it does occurs, it will move in from the tip toward the middle much faster.
With ABC, the advancing blade is providing most of the lift. The inner elements of the blade will be experiencing a lower velocity of incoming air therefor their angles of attack will automatically be greater than those of the outer elements.
Higher twists will reduce the downwash (vibration) on the inner elements of the lower blade when it is at azimuth 270º. This twist is detrimental to autorotation but the idea of running the rotors a higher than 100% RRPM during autorotation should overcome this concern.
Re: Vortex Ring State, Particurly relater to Side-by-Side Rotors V-22 etc.:
Without active twist.(variable pitch at root) there cannot be any twist , because the root trailing edge of the upper blade will hit the lower blade.
Active Blade Twist [ABT] (θ1A)
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Last Revised: November 12, 2008