Some DA2's coefficients
I was studying some airfoils, and here are some results I obtained by using Designfoil to analize Dribbler's profile (DA2) from David Aberdeen.
This is the airfoil shape:
And these are the coefficients:
Finally, with this information I calculated the Center of Pressure position as a function of Angle of Attack, using the formula: CoP = 0.25 - (Cm/Cl)
This charts come from an idea I had when reading
values por powered up and powered down angles, as calculated by
David's Surfplan.
Now it's clear to me how far the CoP can go towards the TE when
AoA decreases.
I must take it into account when trying to figure out a kite's
profile AoA.
Consider the formula: AoA = arctan (Xprim - Xcop) / Yprim
where Xprim, Yprim are tow point coordinates, and Xcop is CoP location.
Once the AoA is calculated you can go to last chart (Cop Vs. AoA) and found a new CoP value, and then calculate a new AoA and so on and on! until when? until luffing because AoA keeps going down and down... (it's a consecuence of a negative Cm). This demonstrates that kites can't fly!!!
No. You just know what happens. As AoA goes down so does lift, but drag is still there. As tow point is under the kite the drag force produces a positive moment, keeping nose up, and reaching to an equilibrium point somewhere in the process.
Maybe I'll calculate it next time.
Comments?