This month’s question comes from a long time member and glider guider Bob Film. This type of thing is near and dear to my heart!

Since there are 100’s of airfoils, and each one is usually efficient at an air speed, I would assume that the thicker the airfoil & under camber airfoils, would be for slower air speed, so is there a simpler formula to determine each one’s angle of incidence (not dihedral). So, for instance, if you put different wings on a plane; how would you set its incidence for max performance. I assume that angle is always in reference to elevator being zero degrees. Since I am a sailplane “NUT”, this becomes more critical. Thanks – Bob Film

Thanks for the question Bob. You have hit the nail on the head here. Airfoil selection and setup (incidence, planform, washout ect.) is the heart of how the plane flies. A simple incidence change sometimes can turn a dog into a thoroughbred. Unfortunately there is no simple formula to figure this out, but there are some guidelines that I like to use. There is much, much more, that can be added to this discussion, so I’ll try to keep it in the context of your question. Please feel free to contact me for further discussion if you wish. Up front I’d like to state that I generally am not a big fan of using incidence and/or decalage to trim an airplane. These are compromises to make a plane behave a certain way at a certain airspeed, and my feeling is airfoil selection plays a big part here and if done correctly, you won’t need the incidence bandaid. More often than not, my designs will be setup with zero incidence. They certainly have their place, Trainers and Old Timers for instance, but since you asked about max performance, I’m going to assume you equate that to efficiency. It’s all about trim drag. If you need up or down elevator trim at certain airspeeds, it means your incidence, CG or both are wrong and there is unnecessary drag from trim detracting from performance. Climbing under full power is a classic symptom of this.

First, you must define the purpose of your airfoil selection and its use. An Old Timer type aircraft with an undercambered section will likely be flown more slowly and benefit from some positive incidence. All airfoils have a natural pitch down tendency, but UC sections react more than most. This positive incidence coupled with a necessary more forward CG makes for a very sedate flying aircraft. For a high performance TD (Thermal Duration) sailplane, Racer or even an aerobatic plane you’ll find that incidence needs to be at or a fraction of a degree of within zero.

So how do you measure it? You are correct that on a sailplane the reference line, “0” degree or datum is often the elevator, and the wing is measured in degrees positive from there. Power planes are a different story, with their thrust angle, positive incidence, decalage (tail incidence), they often use a line through the center of the fuselage for reference. For most airfoils you can find a published set of ordinates that will include useful information such as chord line, camber line, zero lift line and other fun things like lift vs. drag polars and laminar bucket information. In most instances you can use the chord line to measure from which is often simply a line from the leading edge to the trailing edge. For highly cambered sections, the chord and camber line can actually exit the bottom of the airfoil making measurement a bit trickier. There are many online and printed references out there. For printed references I like Airfoils at Low Speeds (Soartech series) available from Carstens Publications SoarTech, and online there is the UIUC’s webpage from Professor Michael Selig. With this reference material it will tell you specifically at what angle of incidence it will likely perform best.

Bottom line for me is I will set up a plane with the least amount of incidence possible (often zero) and move the CG aft incrementally until the trim change with speed goes away and I still have acceptable stability.