Flaps and Slats

If you've ever sat towards the rear of an aircraft and looked out the window, you might have noticed large sections of the wing that move downward during takeoff and landing. These portions of the wing are called flaps, and their purpose is to generate greater lift. The reason a pilot wants more lift at these portions of flight is to reduce the speed at which the aircraft lands or takes off. This relationship can be understood by looking at a basic equation for lift:

In a previous question about how wings generate lift, we discussed the dynamic pressure, which is represented by the quantity "q" above. The term "S" is the area of the wing (called reference area), "rho" is the air density, "V" is the velocity, and C L is a quantity called the coefficient of lift. As flaps are extended, this coefficient is increased. Since an aircraft needs only enough lift to balance its weight, we can consider the lift "L" to be a constant over the brief time periods we are discussing. So, we can rearrange the above equations to show that as the lift coefficient increases, the velocity needed to remain airborne decreases:

If the aircraft can generate enough lift to become airborne at slower speeds, the amount of runway space needed to takeoff and the time needed to reach takeoff speed are decreased. Similarly, landing at slower speeds reduces the length of the landing roll as well as time the needed to stop the aircraft.

Now you might be wondering why extending or "dropping" a flap causes the lift to increase. Returning briefly to how wings generate lift, recall that the Bernoulli theory says that lift is generated because of the difference in pressure between the upper and lower surfaces of the wing. This pressure difference is created because the air flows faster over the upper surface than the lower, and this difference in velocity results from the curvature of the wing. This curvature, known as the camber of a wing or airfoil, is directly related to how much lift the wing produces. By dropping a flap, the wing's curvature increases, so its camber increases as well. As a result, the lift also increases, and this is quantified by an increase in the lift coefficient. A simple flap will generally increase the maximum lift coefficient of a given airfoil section by about 50%. The flap does significantly increase drag, but this is usually beneficial during landing since it helps slow the aircraft more quickly reducing the landing roll.

So why did I spend all this time dicussing flaps when you asked about slats? The reason is that a slat is simply a flap on the leading edge of a wing rather than the trailing edge. The slat typically extends forward and downward from the leading edge to increase camber thereby increasing lift in a similar manner to a flap. The simplest slat generally increases the maximum lift coefficient by about 40%, but it does so in a slightly different way. While flaps increase the maximum lift coefficient, they also usually lower the maximum angle of inclination of the aircraft, or the angle of attack. For a typical airfoil, the maximum allowable angle of attack is about 15 degrees--beyond this point the air flow no longer curves around the airfoil but separates from the surface creating a turbulent wake. This separation is marked by a sudden decrease in lift that can be very dangerous when close to the ground during takeoff or landing. Simple flaps may decrease this angle to 12 or less. Slats, on the other hand, actually increase the maximum angle of attack to about 20. Thus, the wing can generate more lift by going to a higher angle of attack when a slat is used. Unfortunately, the slat also causes the aircraft nose to pitch upward, which is an unstable condition, and they produce higher drag at higher speeds which is undesirable. Because both the flap and slat offer great advantages in generating high lift, most aircraft today use a combination of both to maximize the maximum lift coefficient as well as the maximum angle of attack while minimizing the effect of pitching moment.
- answer by Jeff Scott, 7 January 2001

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