Are thin airfoils more difficult to fly?

I tend to prefer "thick" arfoils (10/11 %) to the "thin" ones (6/7%), because it takes a much better pilot than me to extract the possible extra performance out of these.

From Don Stackhouse :

This is a common misconception, but has very little basis in fact. The ability of a model to help its pilot "extract the possible extra performance" depends on far more than just airfoil selections. Moment arms, area ratios, mass distribution, flying surface incidences, c/g, planforms, twist ("washout") distribution, control surface design and travel setup including differential all play important roles in aircraft handling. If ALL of these factors (plus some others) are properly considered in the design, the model will have good performance and handling.

At the Reynolds numbers typical of most models there is a huge performance penalty inherent in thicker airfoils. Lift and drag both suffer significantly. In fact, at extremely low Reynolds numbers (somewhat below where most r/c models operate) a flat plate can have a higher max lift coefficient and l/d than most cambered airfoils. In my experience the max lift and the stall behavior of typical 10-12% thick airfoils is often WORSE than a well designed thinner airfoil. I have designed and kitted quite a few models with airfoils less than 3% thick on portions of the wing, which have earned themselves a reputation for extremely easy handling and superior performance. I have one design, a Q-40 racer, where the root airfoil (because of minimum thickness rules) was more than 11% thick, but the tip was less than 3.5% thick. The handling was excellent, and the root stalled first, even though that wing used less than 1.0 degree of washout.

If the designer does his job right, a well designed model with a "high performance airfoil" setup will perform better for even inexperienced pilots than a model with a fat, low performance airfoil will perform for an experienced flier. It will also be more fun to fly.

There are some models that use thinner airfoils and have demanding flying characteristics. In most cases this is because the stability of the entire aircraft has been tailored for maximum performance at the expense of handling. It isn't the airfoils that cause these models to be demanding, it's the entire design, particularly the tail sizing and c/g setup. There are also models with thicker airfoils (typically "previous generation" models) that are also intended for a competition environment and that also have demanding flying characteristics. There is nothing inherent in the airfoil selection alone that can pre-determine handling characteristics.

We have a lot of very successful experience with models intended for beginners, that use airfoils in the neighborhood of 6% thick on large portions of the wing. They are extremely stable, have good control response, give lots of feedback to the pilot, and have excellent performance without requiring extraordinary flying skills. In the R/C hand-launched glider class, airfoils over 10% thick are virtually unheard of these days, and even 9% thick airfoils are on the decline, even for beginner models. The performance just isn't there, regardless of pilot skill level, and there are NO handling benefits for the thicker sections either.

Don't be afraid to try a model with "thin" airfoils, just be prepared to do your homework and/or do a lot of flight testing. Or, you could just buy a kit with thin airfoils from a company that does their homework!

A final note, don't get overly confident with performance predictions that use the overall camber and thickness of an airfoil to try to characterize its performance. It's very possible for two airfoils with the same camber and thickness to have a huge difference in their performance and handling. Even the old rules of thumb about thicker airfoils having a gentler and higher-angle stall can be hugely in error (note the example above of my Q-40 pylon racer design). Performance prediction programs that use just the thickness and camber of the airfoils to predict performance will be gross approximations at best. They are still better than just using "gut feel", but they will not give you a reliable or exact answer. Ultimately, if you want to know the true performance of the model, you will have to build it and fly it!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/