Aerodynamics ~ One more attempt at Reverse Velocity Utilization
 

The Preamble:

An objective of the new Sikorsky ~ X2 ABC is to increase the forward speed over that of the previous Sikorsky ~ S-69 ABC by 30 to 40 knots. Much of this is to be done by reducing the profile and induced drag of the blades. Specifically, to reduce the drag and downward thrust resulting from the negative angle-of-attack airflow over the blade roots on the retreating side.

However, significantly higher forward speeds cannot be achieved by drag reduction alone. Lift must be produced from this Reverse Velocity Region. Extracting more lift from the rotors means that they can be smaller, and this in turn, means that they will generate less total drag.


The Concept:

The X2 is using blade roots that have rounded profiles at both the leading and the trailing edges. This limits the increase in its maximum forward speed.

The concept on this thread is based upon the very opposite. Essentially, this concept has blade roots that have sharp profiles at both the leading and the trailing edges. The aerodynamic advantage of sharp trailing edges is obvious. The problem with sharp leading edges is that they are subject to leading edge stall at small angle of attacks.

However, if the roots of the blades are not subjected to angles of attack that exceed a specific amount, they should not stall.


The Method:

The rotors have Independent Root and Tip Control. The blade tips are controlled directly by the pilot, perhaps without the need for power assist. However, G-forces, forward airspeed, plus the pilot's collective and cyclic actions etc. are input to a computer. This computer then controls the pitch of the blade roots through actuators, perhaps at rates greater than 1/P.

The rotors have Blades, with Sharp Leading & Trailing Edges Near the Blade Root.

All of this will allow for additional reductions in the RPM of the rotors, and this allows for greater forward velocities without experiencing excessive compression at the tips of the advancing blades.

_________

Tear the idea apart; or line up for the initial share offering. :)

Dave

Dave,

It would seem at high speeds that generating lift is not the problem. There is the whole V^2 thing going on. Also, would you really want small rotors? That might be good for high speed but bad for hover and autorotation.

-- IFMU

I am beginning to picture DJ as the wacky professor... :8

In my mind, reverse velocity is the fart coming out of the mouth of CAA's inspectors!

IFMU,Good point, but .... http://www.unicopter.com/Think.gif
At high speeds the air velocity will be greater on the advancing blades, but it will be less on the retreating blades. It will also be reduced on all azimuths as the rotational speed of the rotors is slowed to assure that the advancing tips do not get too close to Mach1.

Perhaps the generated lift will be less at high speeds. http://www.unicopter.com/NoIdea.gif

The thinking (or hope) is that the diameter of the rotors can be reduced somewhat by having an airfoil that goes all the way to the root of the blades, plus having a larger solidity ratio due to a substantial taper.

For hover it might be possible to have very large active negative twist on the blades.
For cruise, at tip speed ratios in excess of 1 or 2, the very slow turning rotors may result in the blades acting as airplane wings (wings similar to those on the F-104 ~ but a greater aspect ratio)

________________

Heli-Ice,

Wacky ~ Yes
Professor ~ No


Dave

An airfoil can be sheared off near the trailing edge and still be lower drag than you might think. Like the shape of my Honda CRX or the Prius. So maybe an airfoil that is blunt on both the leading and trailing edges with a little rounding of the corners would lift in both directions.

I think this applies to body shapes like cars, may not apply to wing sections.