Thanks for the info. Assuming that what you say is true, I have a question: If such airfoil is so stall resistant for the model size, why it is never used on the trainers?
Hi Roberto, your welcome. I just hope I got everything reasonably accurate. Like I said, I am not an expert by any means, I've just spent a bunch of time looking into this sort of thing just for the sake of learning as much as I can.
Now, as for why they don't use airfoils like that for a trainer, well, the study of airfoils can be very complex. While an airfoil may behave on way when slow, it may be somewhat different at high speeds. Also, an airfoil that is hard to stall, may not be stable. This all comes down to something called the "pitching moment". Basically, all airfoils have a tendency to want to pitch nose down, or nose up, or some are fairly neutral on there own. What designers do, is they will pick an airfoil that will work for their design. For a trainer, you want an airfoil with a gentle stall, able to slow down, and stable. Usually they choose on with a forward pitching moment, and then put the CG a bit more forward than an aerobatic type plane. Then, they design the horizontal stabilizer to produce a down force rather tha lift in order to keep the nose from dropping. This all makes an airplane that is dynamically stable, meaning it will right itself when you get into trouble. Typically, stability sacrifices manueverability. You can experiment with these planes by moving the CG aft further and further, and you will see that it will become less and less stable, especially in pitch, but you will gain a bit of aerobatic ability.
Now, with the symetrical airfoils we talked about previously, many of these are fairly neutral. The CG is adjusted accordingly so that the aircraft is contollable without being twitchy. What this means is that the airplane will be very manueverable, but it will not be "self righting" like the last one. If you get into trouble, it will just keep pointing whever it was headed before, and will not stabilize. This is what is called neutrally stable.
There is one more thing you could run into, an airplane with such a wing that is dynamically unstable. These airplanes, if you get into trouble, it makes itself worse!!!
All of these can have the same slow flight and gentle stall, but for the trainer, it is that dynamic stability that you want. The self-righting ability to give the student more time to react and correct. The symetrical airfoils on those fun fly types do not give that enough, which is why those airplanes can do the tricks that they can.
Hope this helps a bit again
PS, if there is anyone else out there who knows this stuff better than me, or can explain it better, please jump in and help, or correct me!!! BMatthews where are you