0422

Item 0422

OTHER: Rotor Concept - Reverse Velocity Utilization - Concerns

Concerns:

Primary Concern:

Will the advantages of utilizing the 'reverse velocity' exceed the disadvantages that are created to the existing 'forward velocity'?

Region of Stalled Flow:
This must be thought through.
Limited Lift from Retreating Side:

It appears that the minimum lift that the retreating blade can provide is when this blade is at 270º azimuth and the advance ratio (tip speed ratio) is 0.5. In this situation the location of zero velocity will be at the center of the blade and the airflow at the tip and the root will be equal, but opposite. The cutout will reduce the lift at the root but this should be offset by the larger chord at the root.

For more information and calculations see;

Initial One:

Will the advantages of utilizing the 'reverse velocity' exceed the disadvantages that are created to the existing 'forward velocity'?

Aeroelastic Instability of the Blade:

At very high forward speeds where the tip is approaching or in reverse velocity;

The pitch axis will probably have to be located further aft on the chord line.
The torque tube will have to be very strong and the flight-control will have to be powered.

See; [Source ~ RWP3, Chapter 19]

Flutter: There is a potential for instability on the retreating rotor blade caused by reverse flow shifting the aerodynamic center from the 1/4 chord to the 3/4 chord. The flexibility of Active Blade Twist may add to this concern.

This may be the biggest concern related to the implementation of reverse velocity rotors. Fortunately, the retreating blade will experience the greatest reverse velocity at its root and this is where the greatest structural strength can be applied to the blade.

Stability:

"It is well known that sharp leading edges promote stalling of the airfoil at very small angles of attack." ~ from Carter Patent Application 20020005458

The inclusion of ABC etc. should mean that the blades would never operate at high angles of attack.

Feathering Axis Location:

As the forward speed increases μ > 1 an ever-increasing area on the retreating side of the rotor disk will be causing the blade to experience reverse flow. A conventional blade in this area will have its chordwise center of mass and pitch reversed but the center of lift will not have moved significantly. How is this problem to be solved?

Initially displayed: June 18, 2007 ~ Latest revision; February 7, 2008