High speed sailplane airfoils...

[skyzking]

Hi everyone. Ive been working on an airfoil for a high speed sailplane which will be used for F3B competitions. All i want is pure speed and good turns out of it. It doesnt have to be a high lift airfoil at all.

Ive uploaded the airfoil as txt, could you guys please test it out and tell me what you think?

Thanks,
Abdullah...

[BMatthews]

You may not need high lift as in slow flight thermal sort of performance but you'll want to try to max out the lift for the high speed turns.

Ideally what you want is an extended lower drag "bucket" where the vertical part of the Cl/Cd curves are as straight and as long as possible with the lower corner right at or just a hair below the Cl=0 line. This way it'll still show up with minimum drag at the silly low Cl's you'll get during a speed run pass but you'll have the most lift with least drag during the high G turns where the Cl can climb more than you'd think. For example do the math or load the numbers into Foilsim for a 4 lb model pulling 15 G's at around 80 to 100 mph in a tight reversal turn. I think you'll find that there IS a need for a "high lift" airfoil. But one that still minimizes the drag down at Cl=0 to 0.1 where you'll be operating on the flat runs.

Another option is to generate some coordinate numbers for what the airfoil would look like with 1.5, 2.0, 2.5, 3.0 and 3.5 degrees of flap. Do the Xfoil thing on that and see what the Cl/Cd charts look like and compare them to the 15G turn Cl value range mentioned above. If it looks good then figure on using some coupled flap and drooping ailerons in the turn coupled in with the elevator.

[skyzking]

Thanks, im currently using Profili Pro. Ill run the numbers on there and post them back up here. Its really hard to get an airfoil which has low drag and high lift for turns. So i was hoping that if i design an airfoil with low drag, i could then when flying use snap flap to increase lift in turns.

Thanks for the reply...