3.10 Types of Stall

Figure 3.17. Aerodynamic center – invariant near quarter-chord

(Fractional chord position about which moment is taken)

are ignored in this book), it is noticed that the moment is invariant to the angle of attack until stall occurs. This point at the quarter chord is called the ac, which is a natural reference point through which all forces and moments are defined to act. The ac offers much useful information that is discussed later.

The higher the positive camber, the more lift is generated for a given angle of attack; however, this leads to a greater nose-down moment. To counter this nosedown moment, conventional aircraft have a horizontal tail with the negative camber supported by an elevator. For tailless aircraft (e.g., delta wing designs in which the horizontal tail merges with the wing), the trailing edge is given a negative camber as a “reflex.” This balancing is known as trimming and it is associated with the type of drag known as trim drag. Aerofoil selection is then a compromise between having good lift characteristics and a low moment.

3.10 Types of Stall

Section 3.3 describes the physics of stall phenomena over an aerofoil. It is essential that designers understand stalling characteristics because wing stall is an undesirable state for an aircraft to enter. Figure 3.18 shows the general types of stall that

Figure 3.18. Stall patterns