If you could get a built up and fully sheeted wing the same weight as a foam wing, would the decrease in airfoil accuracy be worth the ability to get ellipses (like the Chrysalis wing)?

From : Don Stackhouse

Paul,

Good question, no easy answer (sometimes it seems like almost all engineering questions are like that). The answer to this one depends very much on the specific situation and the candidate designs being compared. All of the differences listed in your question result in fairly small differences in performance, so you can probably make the answer fall either way depending on how well you optimize each design.

In actual practice, a built-up wing with an elliptical planform would be very difficult to sheet smoothly without introducing airfoil distortions and warps due to oilcanning of the sheets. This could be improved by making some restrictions in the original airfoil selections, but this may involve additional compromises. This would be particularly critical for the forward section of the wing, where the curvature and non-linearity are greatest, and where the airflow conditions the rest of the wing has to live with are initially established.

In the case of a built-up open frame structure like the Chrysalis, the ability to use non-linear planforms and airfoil distributions does offset to some extent the detrimental effects of covering sag. The corrections to the rib shapes to reduce the effects of covering sag that we use on the Chrysalis add further improvements. Still, there is covering sag, and the performance is still not as good as a smooth wing surface would deliver. The main reason we chose this construction on the Chrysalis was to make the model light and simple for beginners to build (sheeted frame structures are something that beginners historically have trouble with). We came close to the performance of a sheeted foam wing, but if I was flying at the Nats I would still choose a Monarch 'D' or a Wizard, not the Chrysalis. If I were to build a Chrysalis with the sag corrections removed and the wing fully sheeted, I expect that it would still have slightly lower performance than the Monarch 'D'. After all the work involved it building it I also might be afraid to fly it!

Folks seem to make a big deal about planforms, and also about airfoils. People go on and on about the pros and cons of the current "best planform" and the "highest performance airfoil". I'll let you in on a secret: THERE IS NO SUCH THING IN EITHER CASE !!!

The fallacy is in looking at each of these as isolated items. Airfoil distribution, planform, and twist distribution all interact, and cannot be determined individually without regard to the others. Changes or restrictions in one of these can be compensated for in the other parameters. Structural and manufacturing considerations play a part too. With proper tailoring, a foam wing can be made economically that has few if any real compromises compared to the "theoretically perfect" ellipse. In fact, for models in the real world, the ellipse isn't even an optimum shape. A more or less elliptical lift distribution is desirable, but there are many ways to achieve this, and some of them are much better than specifying an elliptical planform.

Overshadowing all of this is the fact that your design and analysis tools must be efficient and accurate for you to optimize any of this. Without those tools you are merely guessing and hoping for a happy accident. That's why we are constantly looking for ways to improve our design tools.

As far as planform goes, if it's going to be a big enough factor that a smooth-skinned non-linear shape is justified, I would mold the wing in composites, like we did on our Spitfire 22 Q-40 pylon racer. Yes, the molds are an unbelievable ordeal to make, but if you're planning to make more than one it's the only way to go. In the long run it will be a lot easier than trying to sheet a bunch of built-up non-linear wings.

Don Stackhouse @ DJ Aerotech