Homework 3

1. Consider an infinite wing with a NACA 2412 airfoil section and a chord length of 2 ft. The wing is

at an angle of attack of 4 in an airflow velocity of 50 ft/s at standard sea-level conditions. Calculate the lift, drag and the moment about the quarter-chord per unit span. In finding the airfoil drag

coefficient, use 3.1106 as the Reynolds number.

2. Consider an NACA 2412 airfoil with a 2-m chord in an airstream with a velocity of 50 m/s at standard sea-level conditions. If the lift per unit span is 1353 N, what is the angle of attack?

3. The question is often asked: Can an airfoil fly upside-down? To answer this, make the following

calculation. Consider a positively cambered airfoil with a zero-lift angle of -3. The lift slope is 0.1 per degree.

a) Calculate the lift coefficient at an angle of attack of 5.

b) Now imagine the same airfoil turned upside-down, but at the same 5 angle of attack as part a). Calculate its lift coefficient.

Hint: The vertical components of the aerodynamic forces onto the two airfoils below are the

same in magnitude and opposite in direction.

c) At what angle of attack must the upside-down airfoil be set to generate the same lift as that when

it is right-side-up at 5 angle of attack?

4. Consider an airfoil at a given angle of attack, say 1. At low speeds, the minimum pressure coefficient on the top surface of the airfoil is -0.8. What is the critical Mach number of the airfoil?