Ed, Thanks for a wonderful/thorough review of the aerodynamics of an ercoupe. GREAT PIECE to save. Harry
--- On Fri, 4/10/09, Ed Burkhead <[email protected]> wrote: From: Ed Burkhead <[email protected]> Subject: RE: [ercoupe-tech] Vortex generators To: [email protected], [email protected] Date: Friday, April 10, 2009, 10:14 AM Robert asked: > Does anyone know if there are any Vortex generators approved for the > ercoupe specifically the Alons . Robert, Very interesting question. I don't know of any approved vortex generators for any model of Coupe. It occurs to me that vortex generators are likely to be useless or counterproductive on Coupes. 1. Coupe's minimum flying speed is not a stall speed. 2. In extensive discussions on vortex generators on the Challenger ultralight/eLSA forum, it was agreed that while vortex generators do lower the stall speed, the stall, when it does happen, is more abrupt. 3. The airfoil used on the Coupe's wings is pretty efficient for our gross weight range, speed and minimum flying speed. I have read that this airfoil has a downside of an abrupt stall break. That stall break doesn't affect Coupes as we don't allow the wing to stall (more than a tiny bit at the root). [I'm sorry I can't cite the source of this.] It seems to me that combining an airfoil which has an abrupt stall break with vortex generators which might acerbate the stall break might be undesirable. 4. The Coupes design includes the inability to be held in a spin because it can't be held in a condition where the wing is seriously stalled. The limit on elevator effectiveness prevents the tail from being pushed and held down to give enough angle of attack for that serious stall. With no serious stall, there's no asymmetric stall resulting in autorotation, i.e. no spin. The wing root does begin to stall with full up elevator. This is controlled by the fillet shape between the wing and fuselage and by the small ridge on the leading edge of the wing at the wing root. The stalled air only extends in a fan shape from the wing root a small way out the wing. The turbulent air from this small stalled zone then travels over the empennage, reducing the tail's negative "lift" and helping limit the down push on the tail. Removing this stall could allow the tail to be pushed somewhat farther down which could allow too much of the wing to stall for us to predict the behavior. Summary: I know of no vortex generators for Coupes. It doesn't seem like a good idea for our unique birds. If you are adventurous and wealthy, you might hire an FAA authorized Designated Engineering Representative (DER) who's especially good at aerodynamics and do extensive flight testing to see if any benefit could be obtained. Great question. These are just my humble opinions. Remember that I'm not a mathematical aerodynamicist. Ed
