Figure 3.19. Comparison of three NACA aerofoils

can occur. This section describes how these different types of stall affect aircraft design.

3.10.1 Gradual Stall

This is a desirable pattern and occurs when separation is initiated at the trailing edge of the aerofoil; the remainder maintains the pressure differential. As the separation moves slowly toward the LE, the aircraft approaches stall gradually, giving the pilot enough time to take corrective action. The forgiving and gentle nature of this stall is ideal for an ab initio trainee pilot. The type of aerofoil that experiences this type of stall has a generously rounded LE, providing smooth flow negotiation but not necessarily other desirable performance characteristics.

3.10.2 Abrupt Stall

This type of stall invariably starts with separation at the LE, initially as a small bubble. Then, the bubble either progresses downstream or bursts quickly and catastrophically (i.e., abruptly). Aerofoils with a sharper LE, such as those found on higher-performance aircraft, tend to exhibit this type of behavior.

Aircraft stall is affected by wing stall, which depends on aerofoil characteristics. Section 3.19 addresses wing stall (see Figure 3.40).

3.11 Comparison of Three NACA Aerofoils

The NACA 4412, NACA 23015, and NACA 642-415 are three commonly used aerofoils – there are many different types of aircraft that use one of these aerofoils. Figure 3.19 shows their characteristics for comparison purposes.

The NACA 23015 has sharp stalling characteristics; however, it can give a higher sectional lift, Cl, and lower sectional moment, Cm, than others. Drag-wise, the NACA 642-415 has a bucket to give the lowest sectional drag. The NACA 4412 is the oldest and, for its time, was the favorite. Of these three examples, the NACA 642- 415 is the best for gentle stall characteristics and low sectional drag, offsetting the small amount of trim drag due to the relatively higher moment coefficient. Designers

Figure 3.20. Flap and slat flow field (see

Figure 3.43 for slat and flap effects)

must choose from a wide variety of aerofoils or generate one suitable for their purposes.

Designers look for the following qualities in the characteristics of a 2D aerofoil:

1.The lift should be as high as possible; this is assessed by the CLmax of the test results.

2.The stalling characteristics should be gradual; the aerofoil should able to maintain some lift past CLmax. Stall characteristics need to be assessed for the application. For example, for ab initio training, it is better to have aircraft with forgiving, gentle stalling characteristics. For aircraft that will be flown by experienced pilots, designers could compromise with gentle stalling characteristics and better performance.

3.There should be a rapid rise in lift; that is, a better lift–curve slope given by dCL/dα.

4.There should be low drag using a drag bucket, retaining flow laminarization as much as possible at the design CL (i.e., angle of incidence).

5.Cm characteristics should give nose-down moments for a positively cambered aerofoil. It is preferable to have low Cm to minimize trim drag.

An aerofoil designer must produce a suitable aerofoil that encompasses the best of all five qualities – a difficult compromise to make. Flaps are also an integral part of the design. Flap deflection effectively increases the aerofoil camber to generate more lift. Therefore, a designer also must examine all five qualities at all possible flap and slat deflections.

From this brief discussion, it is apparent that aerofoil design itself is state of the art and is therefore not addressed in this book. However, experimental data on suitable aerofoils are provided in Appendix C.

3.12 High-Lift Devices

High-lift devices are small aerofoil-like elements that are fitted at the trailing edge of the wing as a flap and/or at the LE as a slat (Figures 3.20a and b). In typical cruise conditions, the flaps and slats are retracted within the contour of the aerofoil. Flaps and slats can be used independently or in combination. At low speed, they are deflected about a hinge line, rendering the aerofoil more curved as if it had more camber. A typical flow field around the flaps and slats is shown in Figure 3.20. The entrainment effect through the gap between the wing and the flap allows flow to remain attached in order to provide the best possible lift.

Figure 3.21. High-lift devices

Considerable lift enhancement can be obtained by incorporating high-lift devices at the expense of additional drag and weight. Figure 3.21 lists the experimental values of the incremental lift coefficients of the Clark Y aerofoil. These values are representative of other types of NACA aerofoils and may be used if actual data are not available.

Higher-performance, high-lift devices are complex in construction and therefore heavier and more expensive. Selection of the type is based on cost-versus- performance trade-off studies – in practice, past experience is helpful in making selections.

3.13 Transonic Effects – Area Rule

At high subsonic speeds, the local velocity along a curved surface (e.g., on an aerofoil surface) can exceed the speed of sound, whereas flow over the rest of the surface

Typical flat upper surface with aft camber for rear loading

Figure 3.22. Transonic flow (supercritical Whitcomb aerofoil)

remains subsonic. In this case, the aerofoil is said to be in transonic flow. At higher angles of attack, transonic effects can appear at lower flight speeds. Aerofoilthickness distribution along the chord length is the parameter that affects the induction of transonic flow. Transonic characteristics exhibit an increase in wave drag (i.e., the compressibility effect; refer to aerodynamic textbooks). These effects are undesirable but unavoidable; however, aircraft designers keep the transonic effect to a minimum. Special attention is necessary in generating the aerofoil section design, which shows a flatter upper surface. Figure 3.22 depicts a typical transonic aerofoil (i.e., the Whitcomb section) and its characteristics.

The Whitcomb section, which appeared later, advanced the flight speed by minimizing wave drag (i.e., the critical Mach-number effects); therefore, it is called the supercritical aerofoil section. The geometrical characteristics exhibit a round LE, followed by a flat upper surface and rear-loading with camber; the lower surface at the trailing edge shows the cusp. All modern high-subsonic aircraft have the supercritical aerofoil section characteristics. Manufacturers develop their own section or use any data available to them.

For an aircraft configuration, it has been shown that the cross-sectional area distribution along the body axis affects the wave drag associated with transonic flow. The bulk of this area distribution along the aircraft axis comes from the fuselage and the wing. The best cross-sectional area distribution that minimizes wave drag is a cigar-like smooth distribution (i.e., uniform contour curvature; lowest wave drag) known as the Sears-Haack ideal body (Figure 3.23). The fuselage shape approximates it; however, when the wing is attached, there is a sudden jump in volume distribution (Figure 3.23). In the late 1950s, Whitcomb demonstrated through experiments that “waisting” of the fuselage in a “coke-bottle” shape could accommodate wing volume, as shown in the last of Figure 3.23. This type of procedure for wing–body shaping is known as the area rule. A smoother distribution of the crosssectional area reduces wave drag.

Whitcomb’s finding was deployed on F102 Delta Dragger fighter aircraft (see Figure 3.23). The modified version with area ruling showed considerably reduced transonic drag (see Figure 4.29). For current designs with wing–body blending, it is less visible, but designers still study the volume distribution to make it as smooth as possible. Even the hump of a Boeing 747 flying close to transonic speed helps with the area ruling. The following subsection considers wing (i.e., 3D body) aerodynamics.