Why does the Wizard use such large control surfaces?

I am curious about the rationale for cutting the surfaces so deep on the wizard. I usually cut the surfaces on my 2 meter and open class planes as close as possible to the ideal reflex point ( usually 20-25% ahead of the TE) .

From : Don Stackhouse

Ah, yes, but what makes it "ideal"? Remember, a lot of the empirical "common knowledge" about this sort of thing is based on full scale practice, and may (or DOES) NOT apply at model Reynolds numbers. Just as airfoil design doesn't translate 1:1, neither does tail surface design or control surface size.

I know the rage in kits these days seems to be billboard surfaces but I've read in the literature by Martin Simon that increasing the surface chord from 20 to 30% only results in in about a 2% improvement in effectiveness while it clearly has the potential for messing up the wing profile in reflex and perhaps camber as well.

Blanket statement by Martin, but it cannot be blanketly applied across the entire range of model sailplane classes. In particular, hlg's are in a Re region that contains some dramatic changes in behavior, and much of what holds true in the other classes will not work for hlg's. We were one of the first to fully appreciate this, and this is one of the keys to why the original 1993 Monarch was such a huge improvement over the other hlg's of that time. Likewise, we were one of the pioneers of what you call "billboard surfaces". We started out in our flap development program in 1994 with conventional size surfaces.

They didn't work.

We then re-thought the aerodynamics of the application, and began experiments with wider surfaces, which eventually led to the Monarch 'CX'. Since then we have continued our experiments in control surface chord, and come repeatedly to the same conclusion, that wider surfaces work better on hlg's.

Think of it from the air molecule's point of view. At hlg Re's, the air would rather be laminar and separated than turbulent and attached. Even if you try to turbulate it, it will try to transition back to laminar. Because of this, the boundary layer is not getting fresh infusions of energy from the layers above, and must fight the adverse pressure gradient from the high point to the trailing edge using only the kinetic energy it started with. This supply of kinetic energy is being progressively used up as the air decellerates against the adverse pressure gradient, and it is also being simultaneously syphoned off by skin friction. By the time the air approaches to the trailing edge it is almost completely drained, and has little or no reserve left to negotiate discontinuities in the surface, such as kinks from hinges on deflected control surfaces. You will not be able to deflect that surface very far in the angular sense before the flow just gives up and separates.

If the hinge line is further forward, much less angular deflection is required to achieve the same % change in camber, and it occurs at a point where the air has a lot more energy available to negotiate the corner and get itself settled down again. You can get more lift from this arrangement before reaching the point of separation.

Don't think of it as a control surface, think of it as a new airfoil shape. If the optimum place for maximum curvature in the camber line for low Re's was at the 80% chord location, the best hlg's would have their camber line high points back there! Actually, I think the optimum hinge location may be even further forward than where we have it now, but the structural design becomes very tricky if you try to do that.

Another bit of corroborating evidence is turbulator design. While there are a few airfoils (notably the RG15 when used at open class and F3B Re's) that benefit from turbulators aft of 70% chord, most airfoils do better with turbulators forward of the upper surface high point. BTW, Joe Wurts, one of the more vocal proponents of narrow chord control surfaces (in fact, he has quoted that same passage out of Simons to me in discussions of this), has most of his experience with flapped hlg designs with the RG15 airfoil. No great surprise there!

We tried some narrow chord flaps on a Wizard recently as part of another experiment. Yup, you guessed it, it was measurably worse!

Don Stackhouse @ DJ Aerotech