Quest for Performance: The Evolution of Modern Aircraft
Chapter 10: Technology of the Jet Airplane
High-Lift Systems
[271] Increases in the capability of high-lift devices have always accompanied the use of higher wing loadings. This trend has been particularly evident in the evolution of the modern jet transport aircraft. Data given in chapter 3 of reference 176 show that airplane maximum lift coefficients of about 3 are being obtained in flight on modern operational jet transport aircraft. The corresponding two-dimensional airfoil maximum section lift coefficients for the flapped sections are probably somewhat in excess of 4. By comparison, the data in figure 7.5 show that airplane maximum lift coefficients slightly in excess of 2 were being achieved by the end of World War II. The technology for achieving two-dimensional maximum lift coefficients, without boundary-layer control, of about 3.2 existed at the end of World War II, as shown by the comparative data in figure 5.3.
The high-lift system employed on modern jet transport aircraft consists of an assortment of various types of leading- and trailing-edge devices. A number of these devices and the manner in which they are mechanically actuated are described in reference 197. Although the detail design and relative effectiveness of the different devices vary, the basic means by which they increase the maximum lift coefficient remain the same. Trailing-edge devices are designed to increase the effective angle through which the flow is turned and thus increase the lifting capability. Leading-edge devices are basically designed to assist the flow in negotiating the sharp turn from the lower surface, around the leading edge, and back for a short distance on the upper surface, without separating. Modern high-lift devices as used on large transport aircraft form the subject of the next few paragraphs.
Two typical high-lift configurations are shown in figure 10.25. A wing section equipped with a leading-edge slat and a triple-slotted [272] trailing-edge flap is shown in figure 10.25(a). The trailing-edge flap deploys rearward and downward and separates into three components. The slots in the flap allow flow from the lower surface to the upper surface. The flow through the slots energizes the boundary-layer flow on the top surface, which is negotiating a positive pressure gradient, and prevents separation and subsequent loss of lift. The detail design of the slot contours is very critical and must be carefully worked out in wind-tunnel studies. Both the leading- and trailing-edge devices are completely retracted in cruising flight and are only deployed for landing and takeoff.
A wing section equipped with a leading-edge Krueger flap and a trailing-edge double-slotted flap is shown in figure 10.25(b). The Krueger flap is somewhat less effective than the slat but is probably simpler in mechanical design. Some aircraft employ slats on the outboard portion of the leading edge, where more powerful flow control is required, and Krueger flaps on the inboard portion of the leading edge. The double-slotted trailing-edge flap is not as powerful as the triple-slotted flap but is mechanically simpler and easier to implement than the triple-slotted flap. The simple single-slotted flap is often used as a trailing-edge device. This flap consists of a single unsegmented....

wing section design drawing
(a) Airfoil with triple-slotted flap, slat, and spoiler.
wing section design  drawing
(b) Airfoil with double-slotted flap and Krueger flap.
Figure 10.25 - Typical flap systems employed on jet-powered aircraft.

[273]....element that is deployed by moving rearward and downward. Although less effective than either of the other two types of trailing-edge devices described, it is by far the most mechanically simple of the three, and the aerodynamic design is the simplest. Many other types and combinations of high-lift devices may be used on jet transport aircraft. The types shown in figure 10.25 are only intended to be representative of typical installations.
Also shown on the upper surface of the wing in figure 10.25(a) is a spoiler in the deployed position. The spoiler is flush with the wing surface when retracted. The action of the spoiler in the deployed position is to "spoil" or separate the flow downstream. The lift of the wing is therefore reduced and the drag increased. These two aerodynamic effects are utilized in several ways. When deployed on only one wing of an aircraft, they cause that wing to drop and thus serve as a lateral-control device. The wings of many jet transport aircraft employ several spoiler elements on each wing. These elements may act simultaneously or in reduced-number, depending on the flight condition and the function they are intended to fulfill. Some elements of the spoilers are frequently used in combination with conventional ailerons to assist in lateral control. The mix between ailerons and spoilers varies with the flight conditions under which the aircraft is operating. For example, the dynamic pressure corresponding to cruising flight at 35000 feet and a Mach number of 0.8 is 223 pounds per square foot, whereas that for an approach speed of 135 knots at sea level is 60 pounds per square foot. The need for additional lateral-control devices for flight at low speeds, as compared with cruising flight at high Mach numbers, is clearly shown by the difference in the dynamic pressure for the two flight conditions.
The spoilers are also used to reduce lift and increase drag when deployed symmetrically, that is, in the same manner on each wing. The spoilers are usually deployed in this way immediately after touchdown on landing to assist in stopping the aircraft. The increased aerodynamic drag serves as a braking function for the aircraft, and the reduction in lift increases the percentage of the aircraft weight on the runway and thus increases the effectiveness of the wheel brakes. Many aircraft also utilize symmetrical deployment of the spoilers in flight to increase the rate of descent, for example, to comply with air-traffic-control requirements during the transition from high-altitude cruising flight to flight in the terminal area.

view of extended wing flap
[274] Figure 10.26 - Lower-surface view of triple-slotted flap on Boeing 737 airplane. [NASA] [Original photo was in color, Chris Gamble, html editor]


view of fully extended wing flaps
Figure 10.27 - Upper-surface view showing triple-slotted flap and spoilers on Boeing 737 airplane. [NASA] [Original photo was in color, Chris Gamble, html editor]

[275] Two views of a triple-slotted flap installed on a Boeing 737 aircraft are shown in figures 10.26 and 10.27. The large fairing shown on the lower side of the wing and flap in figure 10.26 houses the mechanism for deploying the flap. The four segments of the spoiler system employed on each wing are shown in the deflected position in figure 10.27. The leading-edge slat is shown in the deployed position in figure 10.28.

view of extended leading edge slat
Figure 10.28 - Lower-surface view of leading-edge slat on Boeing 737 airplane. [NASA] [NASA] [Original photo was in color, Chris Gamble, html editor]