B479

DESIGN: Electrotor-SloMo ~ Rotor - Disk

Twist:

Aerodynamics - Precone vs. Streamtube Contraction:

Rough Calculations:

Precone of 10º

6ft R = 72" R Area = 72² x 22/7 = 113.1428 sq-ft

cosine of 10º = 0.9848

= 70.9062

= 72" R Area = 70.9062² x 22/7 = 109.73 sq-ft

Reduction in plan view = 109.73/113.14 = 96.66%

Posting on Rotary Wing, April 12, 2010:

This question is directed primarily at the helicopter but it may also apply to the gyrocopter.

The questions is; Will the implementation of a large precone angle (in conjunction with extremely rigid blades) cause a reduction to the Thrust/Power ratio?

For example; a precone angle of 10-degrees reduces the planform area by 3.3%. This reduction, in and of itself, should result in a slight increase of the induced velocity and therefore a reduction of the thrust/power ratio.

Opposing this (perhaps); the coning angle of the blades is attempting to thrust the air outward and away from the vertical. This may delay the contraction of the streamtube slightly and thereby diminish the rate of increase in the speed of the induced velocity.

Subject: Direction of rotation on interleaving and power failure:

A/ Paco on PPRuNe

there is more induced flow at the rear of the disc which increases the difference between the front and rear angles of attack (less at the rear)
.
Note: This is why the nose lifts when you apply collective pitch, or drops when you reduce it, so if the engine fails and you don’t apply rear cyclic to correct this tendency, you could get into an unrecoverable situation.

B/ With non-breaststroke rotation the nose on an intermeshing helicopter will lift when the power is lost.

C/ hihover on PPRuNe;

I think it would be quite difficult for any publication to emphasise bringing the cyclic back on entry into autorotation because it depends very much on the airspeed at which the engine fails and rotor inertia.

C/ Shawn Coyle on PPRuNe;

I'm becoming more convinced that regardless of the situation, the first thing should be to move the cyclic aft (at the same time as lowering the collective).
The few scenarios where this is not the best thing to do are pretty small - I can think of only a few - like at 40 knots or slower and less than 100 feet above the ground; (please feel free to add to the list).
Nearly anywhere else, it's a very good thing to do. There are way too many examples of helicopters hitting the ground with little or no rotor RPM, because the pilot tried to get airspeed but had no rotor RPM for control.
Remember that real engine failures have a huge element of surprise about them - way different than practice ones.

Oggers on PPRune;

I was taught the importance of maintaining Nr AND the effect of loading the disc.

Last Revised: November 12, 2011