Enjoy fellas, I hope you find this as interesting as I do.


You already have a response from someone at NASA mentioning the fact that the details of the flow separation are dependent on the way the "boundary layer" develops on differing shapes. Sharp leading edges lead to separation at lower angles of attack than happens for leading edges with larger radii.

The result on airfoil properties, at least on lift can be seen in the
attached material which shows the lift versus angle of attack for a NACA 0012 airfoil in forward and reversed flow (figure 14 on page 2-8) and similarly for NACA 63 series airfoils and a double wedge (figure 2 on page 4-2). These are from Fluid Dynamic Lift by Hoerner. The lift versus angle of attack breaks very sharply at a relatively small angle of attack of around 6 degrees. This will lead to a very strong change in the slope of the pitching moment versus angle of attack curve of an airplane using such a sharp leading edge airfoil. The pitch stiffness will be very different at angles of attack above and below the break point. It is also very likely that the flow will be quiet unsteady at angles in the vicinity of this break point and may lead to the phenomena referred to as "hunting". An airplane that has such a sharp leading edge but always operates below this critical angle of attack will behave
conventionally.

The drag will rise quickly for angles of attack above the stall point. This will affect performance, but will not directly affect the pitch behavior of the aircraft.

I also noticed several posts about "bound vortices" with various
comments about this object. The term "bound vortex" in the standard aerodynamic literature refers to an effective vortex associated with a lifting surface. It is not an actual vortex identifiable by a flow pattern in the fluid as in the case of a wing tip vortex. The concept arises from a set of theorems known as the Helmholtz Vortex theorems. One of them asserts that a vortex filament cannot begin or end within the fluid volume. A vortex filament must end at a solid surface, extend to infinity, or form a closed loop. In the standard wing case, the vortex filaments making up the wing tip vortices begin at the wing
surface and extend to infinity. Their imaginary extensions within the wing that connect the vortex filaments attached to the left wing to symmetrically located vortex filaments attached to the right wing make up the "bound vortex".

The "votex" that will usually exist just downstream of a sudden step in a solid surface is an example of a "trapped vortex". It is not in any sense analagous to the "bound vortex" concept. A trapped vortex is an actual pattern in the flow just as is the case for a trailing tip vortex.

Best regards,

Jewel B. Barlow
Director, Glenn L. Martin Wind Tunnel
University of Maryland

NONE OF THIS POST SHOULD BE COPIED OR OTHERWISE DUPLICATED WITHOUT EXPRESS WRITTEN PERMISSION FORM THE AUTHOR!

[email protected] wrote:
> Hello,
>
> Nice website, and great work. My name is Chuck Roseberrry. I am a radio
> Controlled airplane hobbyist. We ghave a debate I'd like to get your
> feedback on and have you join the discussion if/when you have the time
> located at the below link.
>
> The discussion is how sharp leadeing edges affect flight performance.
>
>
> http://www.rcuniverse.com/showthread...504#post640158
> <http://www.rcuniverse.com/showthread...504#post640158>
>
> Thank you for your generous time.
>
> Chuck
> admin/webmaster
> Spadworld.nett