The force that resists the movement of the glider through the air is called drag. Two different types of drag combine to form total drag: parasite and induced. The various types of drag are explained in greater detail in the Aeronautical Knowledge Section.
Parasite drag is the resistance offered by the air to anything moving through it. The aircraft surface deflects or interferes with the smooth airflow around the glider. The wing of the sailplane alone has very low parasite drag, but when the total drag of the glider is added to it, the amount of drag becomes significant. This is apparent particularly at high speeds since parasite drag increases with the square of speed. Simply put, if the speed of the glider is doubled, parasite drag increases four times. [Figure 3-3] Parasite drag is divided into three types: form drag, skin friction, and interference drag.
Form drag results from the turbulent wake caused by the separation of airflow from the surface of a structure. [Figure 3-4] Any object moving through the air has to push the air in front of it out of the way. This causes a buildup of pressure in front of the object. Similarly, the object leaves a low-pressure void in its wake. This difference in pressure between the front and back surfaces of the object results in the force called form drag. Form drag can be reduced by reducing the object’s cross-sectional area or by streamlining it.
Skin Friction Drag
Skin friction drag is caused by the roughness of the glider’s surfaces. Even though the surfaces may appear smooth, they may be quite rough when viewed under a microscope. This roughness allows a thin layer of air to cling to the surface and create small eddies or areas of lower pressure that contribute to drag. As air flows across a wing, friction brings the layer of air molecules directly in contact with the surface to a standstill. Air is a viscous fluid, hence the stationary layer of air on the wing’s surface slows the layer above it, but not as much as the layer above. This layer then slows the layer above it, but again not as much, and so on. Therefore, the velocity of the flow increases with distance from the surface until the full speed of the flow is reached. This layer of decelerated air is called the boundary layer. The frictional forces that create the boundary layer [Figure 3-5] create an equal and opposite skin friction force on the glider. When the surface area is reduced, the amount of skin friction is reduced.
The boundary layer can take on two distinct forms: the laminar boundary layer and the turbulent boundary layer.
- Laminar boundary layer—each layer of air molecules slides smoothly over its neighbors. [Figure 3-6]
- Turbulent boundary layer—dominated by eddies and irregular turbulent flow. [Figure 3-7]
Turbulent boundary layers generate 5 to 10 times more skin friction drag than the equivalent laminar boundary layer. [Figure 3-8] Therefore, glider designers try to maintain laminar flow across as much of the aircraft as possible. Figure 3-9 shows why this turbulent transition occurs.
There is a point that is referred to as the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the positive pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point.
These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a turbulator. Turbulators are aerodynamically positioned in a spanwise line along the wing and are used to trip laminar flow air into turbulent flow air at a desired location on the wing. This is beneficial because the turbulent boundary layer contains more energy, which will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possible eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.
In gliders, the turbulator is often a thin zig-zag strip that is placed on the underside of the wing and sometimes on the fin. [Figure 3-10] For a glider with low Reynolds numbers (i.e., where minimizing turbulence and drag is a major concern), the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the farthest for a given height.
The boundary layer can also be tripped into a turbulent flow at any point by discontinuities on the wing’s surface. It is important to keep wings clean and avoid rain and icing to prevent premature transition, and the increase in drag that it causes. As the boundary layer is only 1.0 millimeter thick at the leading edge, objects, such as rivets, splattered insects, rain drops, ice crystals, and dust, are all large enough to cause localized turbulent transition to occur. [Figure 3-11]
Interference drag occurs when varied currents of air over a glider meet and interact. Placing two objects adjacent to one another may produce turbulence 50–200 percent greater than the parts tested separately. An example of interference drag is the mixing of air over structures, such as the wing, tail surfaces, and wing struts. Interference drag can be reduced on gliders with fairings to streamline the intersection of air.
Induced drag is generated as the wing is driven through the air to develop the difference in air pressures that we call lift. As the higher pressure air on the lower surface of the airfoil curves around the end of the wing and fills in the lower pressure area on the upper surface, the lift is lost, yet the energy to produce the different pressures is still expended. The result is drag because it is wasted energy. The more energy the glider requires to fly, the greater the required rate of descent is to supply sufficient energy to convert into thrust to overcome that unnecessary drag. The energy that produces the vortices is wasted energy. The object of glider design is to convert all of the energy into useful lift and the necessary thrust. Any wasted energy translates into poorer performance. [Figure 3-12] Glider designers attempt to reduce drag by increasing the aspect ratio of the glider. The greater the aspect ratio of the wing is, the lower the induced drag is. Wingtip devices, or winglets, are also used to improve the efficiency of the glider. There are several types of wingtip devices and, though they function in different manners, the intended effect is always to reduce the aircraft’s drag by altering the airflow near the wingtips. Such devices increase the effective aspect ratio of a wing, without materially increasing the wingspan.
Total drag on a glider is the sum of parasite and induced drag. The total drag curve represents these combined forces and is plotted against airspeed. [Figure 3-13]
L/DMAX is the point at which the lift-to-drag ratio is greatest. At this speed, the total lift capacity of the glider, when compared to the total drag of the glider, is most favorable. In calm air, this is the airspeed used to obtain maximum glide distance.