The airfoil is a 1.75% thick flat plate...the funny and perhaps surprising thing is I wouldn't want it any other way.

When you start looking for airfoil sections for aerobatic aircraft the obvious choice is something of a symmetrical nature simply because you don't bias its force producing capability in a certain direction. When you couple this with low Reynolds number aerodynamics you quickly come to the conclusion that a thin section will be the best from a handling qualities point of view simply because the nondimensional aerodynamic coefficients aren't strong functions of airspeed. (Assuming of course that the size of the airplane is fixed and the kinematic viscosity of air isn't apt to change wildly in the near future).

In other words the airplane's aerodynamics should vary in a linear fashion no matter what speed you're flying.

A good example of this sort of thing is the small balsa hand-launch gliders you can buy at any convenience store. Notice that they have a simple flat (or curved) plate for an airfoil section. Most think that this is done simply because it's cheaper and easier to manufacture but lets suppose that they outfitted their little glider with a standard NACA 4412. If you look at the 2D wind tunnel data you see that the lift curve, drag polar, and pitching moment coefficients vary wildly until you get to a Reynolds (Re) number of greater than 250,000. Actually it only starts to really look good above 3,000,000 which happens to be the Reynolds numbers that it was designed to operate in. If we measure the airspeeds that our little Wal-Mart glider flies at we calculate that it operates from a Re of 10,000-80,000. This assumes that you can throw it at 60mph and it’s trimmed to glide at a min-sink speed of roughly 10 ft/s. Notice that the 12% thick NACA 4412 would be a total disaster in this Re range since the aerodynamics vary so much. Conversely, if you look at the data for flat or curved thin plates you see that the lift curves, polars and pitching moment plots barely change from a Re of 20,000 to 250,000 making it a great choice. In comparison the Tribute operates from roughly 32,000 to 300,000 which also makes it my airfoil section of choice.

The funny thing about aerobatics, especially precision and 3D model aerobatics, is that our pilot opinion, of the airplane’s handling qualities, is governed almost completely by the linearity of the aerodynamics. When we command a certain amount of Tx stick travel we expect a proportional amount of response from the airplane. If this response occurs in a nearly linear manner (or we can setup the curve via exponential or some other mechanism to make it feel linear) the human controller can easily command the model and will often pronounce the handling as good or even exceptional depending on his background and ability.

Simply put “be wise…linearize”

Hope this clears up some of the confusion in the airfoil selection,

George Hicks
Team JR


PS To answer another gentleman’s question, the model is constructed from blue insulation foam.