0944

Item 0944

DESIGN: UniCopter ~ Rotor - Blade - General - Overview

Notes:

Require low drag at high Mach number near tip. See drag divergence Mach number

Desire high lift at low Mach number near root.

I do not think that a high maximum coefficient of lift near the tip is of any advantage since the retreating blade is partially unloaded (by having a reduced angle of attack). This argument may not be valid when considering autorotation since there is the desire to extract the maximum amount of energy out of the rotors during the flare.

Many requirement maybe to reduce compressibility at the tip; regarding the advancing blade.

Forward Flight:

During high speed forward flight, an obvious conflict exists in the blade characteristic requirements on the advancing and retreating sides Because of the combination of very low section-lift coefficients and high Mach numbers encountered by the outer sector of the advancing blade, one would like to see airfoils exhibiting high drag divergence Mach numbers and moment divergence Mach numbers around c_l= 0. This, of course, would favor either symmetrical or slightly chambered thin airfoil sections. ~ [Source ~ RWA p.356]

It looks as if the NACA 0006 will be preferable to the VR-9. ~ DBJ

To satisfy high-speed flight, the twist is usually less and the blade area usually more than is the optimum for maximum hovering performance. ~ [Source ~ RWP1 p.26] It looks like the UniCopter's aspect ratio is to small, aprticularly for ABC and RRR.

Means to Reduce the Length of the Moment Arm:

The desire is to reduce the weight of the rotor's components. The following will reduce the weight by bring the blade's center of lift in closer to the root and the thicker profile will allow a lighter blade construction.

Increases taper
Increased twist (but this is not an advantage in fast forward flight)
Decreased RRPM. This will decrease the difference in air speed between the blade's tip element and elements closer to the root, and therefore the difference in lift between these two locations. The forgoing statement applies to the blade at 90º azimuth. It will have no affect at 0º and 180º azimuth. At 270º azimuth, i think that decreased RRPM will help, particularly if the blade has active twist and can therefor take advantage of reverse velocity.

Camber:

IMO ~ The UniCopter will not require camber for the retreating blade during fast forward flight due to the use of the Advancing Blade Concept. However, camber might be an advantage during hover due to a higher than normal pitch angle resulting from the slow speed of the rotor.

From Tech-Tips - Rotorcraft,

Posting to someone wanting to make model blade. ~ The other problem with using most pultrusions is that all the fibres are running along the pultrusion and as such the section has very little torsional rigidity as well as being prone to splitting. If you are really serious about building flying blades, email me at sms@performance-composite.com. Before I set up Performance Composites Ltd. I was an engineer at Westland Helicopters, developing composite rotor blades since 1975.

Outside Web Page:

http://www.helis.com/howflies/maxspeed.htm Light coverage regarding what limits the speed of the helicopter.

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Last Revised: February 15, 2005