Well, why don't you take ATR's landing weight of, say 18000 kg, approach speed of about 110 KT (55m/s) and calculate back the lift coefficient Cl? As for drag, I can only "guesstimate" that lift/drag ratio in landing config might be approx 1:10, so Cd = 0,1*Cl...
Rough estimate, but better than nothing...
Other idea to estimate Cd in landing config: ATR in on approach in landing config is using approx 20% torque. at 82%Np. According to my "back of tha fag packet" calcs this equals to approx 440Hp per engine. Knowing the power requred you may calculate back the drag coefficient. In these calculations, you should take into account that the following figrues are valid for a 3 deg flight path, and that part of the weight vector is "helping" to overcome the drag... Also, you should make some allowance for propeller efficiency...
P.S. The above is for ATR-42-500, which has bigger engines and is heavier thane the original 42-300
Last edited by Stuck_in_an_ATR; 3rd Jul 2010 at 18:01.