A wing designed for efficient high-speed flight is often quite different from one designed solely for take-off and landing. Take-off and landing distances are strongly influenced by aircraft stalling speed, with lower stall speeds requiring lower acceleration or deceleration and correspondingly shorter field lengths. It is always possible to reduce stall speed by increasing wing area, but it is not desirable to cruise with hundreds of square feet of extra wing area (and the associated weight and drag), area that is only needed for a few minutes. Since the stalling speed is related to wing parameters by:
It is also possible to reduce stalling speed by reducing weight, increasing air density, or increasing wing CLmax . The latter parameter is the most interesting. One can design a wing airfoil that compromises cruise efficiency to obtain a good CLmax , but it is usually more efficient to include movable leading and/or trailing edges so that one may obtain good high speed performance while achieving a high CLmax at take-off and landing. The primary goal of a high lift system is a high CLmax; however, it may also be desirable to maintain low drag at take-off, or high drag on approach. It is also necessary to do this with a system that has low weight and high reliability.
This is generally achieved by incorporating some form of trailing edge flap and perhaps a leading edge device such as a slat.
Figure 1. Flap System Geometries
Figure 2. The triple-slotted flap system used on a 737.
Figure 3 shows a double-slotted flap and slat system (a 4-element airfoil). Here, some of the increase in CLmax is associated with an increase in chord length (Fowler motion) provided by motion along the flap track or by a rotation axis that is located below the wing.
Figure 3. Double-Slotted Flap and Slat System
Modern high lift systems are often quite complex with many elements and multi-bar linkages. Here is a double-slotted flap system as used on a DC-8. For some time Douglas resisted the temptation to use tracks and resorted to such elaborate 4-bar linkages. The idea was that these would be more reliable. In practice, it seems both schemes are very reliable. Current practice has been to simplify the flap system and double (or even single) slotted systems are often preferred.
Figure 4. Motion of a Double-Slotted Flap
Flaps change the airfoil pressure distribution, increasing the camber of the airfoil and allowing more of the lift to be carried over the rear portion of the section. If the maximum lift coefficient is controlled by the height of the forward suction peak, the flap permits more lift for a given peak height. Flaps also increase the lift at a given angle of attack, important for aircraft which are constrained by ground angle limits. Typical results are shown in figure 5 from data on a DC-9-30, a configuration very similar to the Boeing 717.
Figure 5. DC-9-30 CL vs. Flap Deflection and Angle-of-Attack
Slotted flaps achieve higher lift coefficients than plain or split flaps because the boundary layer that forms over the flap starts at the flap leading edge and is "healthier" than it would have been if it had traversed the entire forward part of the airfoil before reaching the flap. The forward segment also achieves a higher Clmax than it would without the flap because the pressure at the trailing edge is reduced due to interference, and this reduces the adverse pressure gradient in this region.
Figure 6. Maximum Lift Slotted Section.
The favorable effects of a slotted flap on Clmax was known early in the development on high lift systems. That a 2-slotted flap is better than a single-slotted flap and that a triple-slotted flap achieved even higher Cl's suggests that one might try more slots. Handley Page did this in the 1920's. Tests showed a Clmax of almost 4.0 for a 6-slotted airfoil.
Figure 7. Results for a multi-element section from 1921.
Leading edge devices such as nose flaps, Kruger flaps, and slats reduce the pressure peak near the nose by changing the nose camber. Slots and slats permit a new boundary layer to start on the main wing portion, eliminating the detrimental effect of the initial adverse gradient.
Figure 8. Leading Edge Devices
Slats operate rather differently from flaps in that they have little effect on the lift at a given angle of attack. Rather, they extend the range of angles over which the flow remains attached. This is shown in figure 9.
Figure 9. Effect of Slats on Lift Curve. Dotted curves are slats extended; solid curves show slats retracted.
Today computational fluid dynamics is used to design these complex systems; however, the prediction of CLmax by direct computation is still difficult and unreliable. Wind tunnel tests are also difficult to interpret due to the sensitivity of CLmax to Reynolds number and even freestream turbulence levels.
Figure 10. Navier Stokes computations of the flow over a 4-element airfoil section (NASA)