Aeroplane Construction and Operation

by John B. Rathbun

CHAPTER V. AERODYNAMICS OF LIFTING SURFACES (AEROFOILS).

General Wing Requirements. The performance of flat plates when used as lifting surfaces is very poor compared with curved sections or wing forms. It will be remembered that the greatest lift-drag ratio for the flat plate was 6.4, and the best Ky was 0.00294. Modern wing sections have a lift-drag ratio of over 20.0, and some sections have a lift coefficient of Ky–0.00364, or about 60 per cent higher than the lift obtained with a flat plate. In fact, this advantage made flight possible. To Langley, above all other men, we owe a debt of gratitude for his investigations into the value of curved wing surfaces.

Air Flow About an Aerofoil. To distinguish the curved wing from the flat plane, we will use the term "Aerofoil." Such wings are variously referred to as "Cambered surfaces," "Arched surfaces," etc., but the term "Aerofoil" is more applicable to curved sections. The variety of forms and curvatures is almost without limit, some aerofoils being curved top and bottom, while others are curved only on the upper surface. The curve on the bottom face may either be concave or convex, an aerofoil of the latter type being generally known as "Double cambered." The curves may be circular arcs, as in the Wright and Nieuport wings, or an approximation to a parabolic curve as with many of the modern wings.

Fig. 1-b shows the general trend of flow about an aerofoil at two different angles of incidence, the flow in the upper view being characteristic for angles up to about 6 degrees, while the lower view represents the flow at angles approximating 16°. At greater angles the air stream breaks away entirely from the top surface and produces a turbulence that greatly resembles the disturbance produced by a flat plate. It will be noted in the top figure (At small angles) that the flow is very similar to the flow about a streamline body, and that the air adheres very closely to the top surface. The flow at small angles is very steady and a minimum of turbulence is produced at the trailing edge.

Figs. 1a, 1b. Aerofoil Types and Flow at Different Angles.

When increased beyond 6°, turbulence begins, as shown in the lower figure, and a considerable change takes place in the lift-drag ratio. This is known as the "Lower Critical Angle." The turbulence, however, is confined to the after part of the wing, and little or no disturbance takes place in the locality of the lower surface. We observe that an increase in angle and lift produces an increased turbulent flow about the upper surface, and hence the upper surface is largely responsible for the lift. Below 10° the trend of the upper portion of the stream is still approximately parallel to the upper surface.

Fig. 2. Showing How Lift Is Obtained When an Aerofoil Is Inclined at a Negative Angle, the Line of Flight Being Along X-X.

From 16° to 18°, the stream suddenly breaks entirely away from the wing surface, and produces an exceedingly turbulent flow and mass of eddies. The lift falls off suddenly with the start of the discontinuous flow. The angle at which this drop in lift takes place is known variously as the "Second Critical Angle," the "Burble Point," or the "Stalling Angle." Any further increase in angle over the stalling angle causes a drop in lift as the discontinuity is increased. With the flat plane, the burble point occurs in the neighborhood of 30° and movement beyond this angle also decreases the lift. In flight, the burble point should not be approached, for a slight increase in the angle when near this point is likely to cause the machine to drop or "Stall." The fact that the maximum lift occurs at the critical angle makes the drop in lift at a slightly greater angle, doubly dangerous.