1038

Item 1038

DESIGN: UniCopter ~ Rotor - Blade - General - Tip

Tip Shape:

Should not be square, so as to reduce noise and delay compressibility effect. [For more info. see ~ PHA p.204]

If the leading edge is radiused back and also if the trailing edge is extended back then possibly;

The center of lift will be a little further back and therefore the feathering axis will be a little further back. This will make the mass infront of the axis a little closer to the mass behind the axis. This may allow for a little more unidirectional spar material as well.

Current tip is 2" long. Its radii are 2" spanwise and 3" chordwise.

Consider adding dihedral at the blade tip. See: DESIGN: UniCopter ~ Rotor - Disk - Dihedral at Blade Tip . No; pre-cone has been added instead.

Should reduce tip vortex. This should make for less vibration and noise?

Comparison NACA 0006 vs. VR-9:

It appears that the NACA 0006 has a better lift to drag ratio than the VR-9. The NACA 0006's center of lift may also be better to balance the center of lift of the other elements of the airfoil.

Excerpt from Reports:

Application of this analysis capability permits the practical optimization of subtle refinements in rotor blade tip shape, twist, taper, sweepback, and anhedral to reduce rotor power requirements. Typical results for an Apache-sized helicopter indicate a 1000-lb increase in hover thrust for the same installed power.

3.1.4 Blade Tip Modification

Modification of the tip of main rotor blades has two potential payoffs: improved aerodynamics and reduced noise. Aerodynamic improvements include lower drag, delay of transonic flow, reduced tip loading, and less tip loss. The primary acoustic benefits include mitigating "de-localization" effects at high forward airspeeds, hence HSI noise, and diffusing the tip vortex which in turn lessens the interaction between blades and shed vortices, hence BVI noise.

Excerpt from:

Revolutionary Concepts for Helicopter Noise Reduction — S.I.L.E.N.T . Program

~ http://techreports.larc.nasa.gov/ltrs/PDF/2002/cr/NASA-2002-cr211650.pdf

Blade Tip Geometry:

Rectangular

Tapered

Swept

Subwing ~ http://www.enae.umd.edu/AGRC/Aero/AHS2002_Martin.pdf

BERP ~ http://techreports.larc.nasa.gov/ltrs/PDF/1997/tm/NASA-97-tm4754.pdf ~ Have hard copy There does not seam to be any advantage in using BERP.

Swept Ogee

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I wonder how effective a series of decreasing subwings would be?

Idea re Minimizing Tip Vortices:

My thoughts. The following may reduce tip vortices and thereby reduce rotor induced vibration.

taper (smaller blade area at the tip)
active twist (reduce the thrust coefficient at the tip).
anhedral tip twist.

Radius the leading edge for the outer 3" of the blade.
put anhedral (downward curve) in the trailing edge of this radiuses tip.
The intent is that at the very tip there will be an attempt to rotate the vortex in the opposite direction from that of the vortex.

Blade tip droop.

From Nick on PPRuNe ~ The downward anhedral tip on the Sikorsky 3rd gen (S-92) and 4th gen blades are specifically to control the tip vortex and create extra lift at low speed without adding drag at high speed. They add between 5 and 7% more hover performance to the aircraft. The drooped tip adds downward speed to the shed vortex so that it pushes the vortex away from interfering with the next blade, the aero guys said. It also minimizes the vortex energy to some degree. It is akin to the upswept tips of fixed wing jets.

The tapered blade tip sweeps back and angles downward to reduce noise and increase lift. See pictures here.

Idea re Safety of Blade Crossing:

Having dihedral at the wing tips may also provide an additional level of safety when the blades are crossing at the front of the craft. This is because the dihedral may act as a ramp will help assure that the blades cross properly during a server perturbation. The tip will always cross a constant specific location on the other blade therefore it should not be a great difficulty to make the skin of the blade a little stronger at that location.

DESIGN: SynchroLite - Rotor - Blade - Composite - VR-7b - Drooped Tip (anhedral)

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Last Revised: August 31, 2007