Question

A NACA 22104 airfoil is operating at AOA = 3°. The lift-coefficient at this AOA is 0.15. Determine the value of angle of attack at designed lift co-efficient.

(a) 4.36°

(b) 4.36rad

(c) 0.05 rad

(d) 0.05°

I have been asked this question in an interview for job.

Enquiry is from Airfoil Selection-3 in chapter Airfoil and Geometry Selection of Aircraft Design

Answer 1

The correct choice is (a) 4.36°

For explanation: Given NACA 22104 airfoil.

AOA = 3°, Cl = 0.15 at 3°

Based on NACA -5 digit series design Cl = first digit*0.15 = 2*0.15 = 0.3

Now, for thin airfoil lift curve slope = 0.11 per degree

0.11 = (design Cl – Cl at 3°) / (AOA –3°)

0.11 = (0.3-0.15) / (AOA -3°)

0.11*(AOA -3°) = 0.15

Hence, at designed Cl, AOA = (0.15/0.11) + 3° = 4.36°.