Tall paul> Is there a method of determining the real-life performance of any model, and comparing that to a predicted value with any degree of certainty?

Hey, you cant answer a question with a question, that’s illegal! <g>. I guess I was hoping someone would say, I was bored at work & truncated airfoils just so happen to fascinate me, so I put 6 identical airfoils into Xfoil, which is a top notch program written by a real smart guy, at varying truncation amounts 100%, 95%, 90%, 85% … etc & attached are the polars which show you blah blah blah…



banktoturn > Why do you say truncation is inevitable? If performance is really important, you could choose to use harder wood for your trailing edge, and just make it sharp.

I’ve miked a few what I would call ‘representative’ all composite high end racers, F3D, F5D, performance soaring wings & the like made from CNC milled molds & using all the cool composite materials & bagging techniques to ensure structural rigidity in its thinnest practical form. The range seems to be between 0.015” – 0.025”. I was involved in a related project & I can tell you with a bit of certainty its actually quite a bit of work to achieve this thickness from the mold milling, layup preparation technique for final product. Someone, somewhere down the line must have figured all this pain was worth the effort because a plain jane 1/16” roundover is dead easy by comparison.


banktoturn > If the trailing edge is blunt, the flow will separate. If there are sharp corners (a squared-off trailing edge), the flow will separate right at the corners. If it is rounded, the flow will stay attached partway around the curve, and then separate. The main effect will probably be slightly higher drag…. If you look at a plot of upper and lower surface pressure toward the trailing edge, you will see that the pressure difference is getting pretty small in that area. Since the amount of area lost is small, and the pressure difference is small, the amount of lost lift would probably be negligible

Great! This is the verbage I was looking for. Thanks.



Tall Paul> I have yet to see a jet with a blunt trailing edge, or an Expert class pattern plane…

You’re right about the jets but I can tell you that my buddies latest ZN design which he flew at the last F3A WC’s have big fat TE’s in the order of 3/16 – 1/4” thick & that is the ‘in’ thing now. I doubt very much it has much to do with eeking out the n-th degree of airfoil performance like in a racer. Ballistic pattern manevers went out in the 80's with the introduction of turnaround. Drag is what they want for downhill breaking & constant speed maneuvers. He tells me it softens/dampens the control response (or something to that effect) & that is a function of their airframes, performance & task at hand. Anyway, different application to what Im talking about.


Bmathews> I'm not sure if I read it in Soartech 8 or elsewhere but Michel Selig commented on trailing edges and model performance at some point and the end result was if you want to go fast or have lower drag in sailplanes then keep it thin, keep it sharp and keep it stiff.

Maybe that’s where I saw it referenced. Did he quantify it in any way? I dont have that publication anymore.