3 Quiz LIFT

The aerodynamic center of a wing is the point relative to which:

• assuming no flow separation, the pitching moment coefficient does not change with varying angle of attack
• the velocity of the relative airflow is reduced to zero.
• the airplane becomes longitudinally unstable when the CG is moved beyond it in aft direction
• the boundary layer changes from laminar to turbulent.

On an asymmetrical, single curve airfoil, in subsonic airflow, at low angle of attack, when the angle of attack is increased, the center of pressure will

• move aft.
• move forward.
• remain matching the airfoil aerodynamic center.
• remain unaffected.

If the aspect ratio of a wing increases whilst all other relevant factors remain constant, the critical angle of attack will:

• increase.
• remain constant only for a wing consisting of symmetrical airfoils.
• decrease.
• remain constant.

The critical angle of attack:

• changes with an increase in gross weight.
• remains unchanged regardless of gross weight.
• decreases if the CG is moved aft.
• increases if the CG is moved forward.

Regarding a positively cambered airfoil section, which statement is correct? I. The angle of attack has a positive value when the lift coefficient equals zero. II. A nose down pitching moment exists when the lift coefficient equals zero.

• I is incorrect and II is correct.
• I is correct and II is correct.
• I is correct and II is incorrect.
• I is incorrect and II is incorrect.

The airfoil polar is:

• a graph, in which the thickness of the wing airfoil is given as a function of the chord.
• the relation between the horizontal and the vertical speed.
• a graph of the relation between the lift coefficient and the angle of attack.
• a graph of the relation between the lift coefficient and the drag coefficient.

Assuming no flow separation and no compressibility effects the location of the center of pressure of a symmetrical airfoil section:

• is at approximately 25% chord irrespective of angle of attack.
• moves backward when the angle of attack decreases.
• is at approximately 50% chord irrespective of angle of attack.
• moves forward when the angle of attack decreases.

Assuming standard atmospheric conditions, in order to generate the same amount of lift as altitude is increased, an airplane must be flown at:

• the same TAS regardless of angle of attack
• a lower TAS and a lower angle of attack.
• a lower TAS for any given angle of attack.
• a higher TAS for any given angle of attack.

The lift- and drag forces, acting on an airfoil:

• are normal to each other at just one angle of attack.
• vary linearly with the angle of attack.
• are proportional to each other, independent of angle of attack.
• depend on the pressure distribution around the airfoil.

When speed is increased in straight and level flight on a positively cambered aerofoil, what happens to the: 1. centre of pressure and 2. the magnitude of the total lift force?

• 1 moves aft and 2 remains constant.
• 1 moves aft and 2 increases.
• 1 moves forward and 2 decreases.
• 1 moves forward and 2 remains constant.