The shape, or planform, of the wings also has an effect on the amount of lift and drag produced. The four most common wing planforms used on gliders are elliptical, rectangular, tapered, and swept forward. [Figure 3-14]
An elliptical wing is a wing planform shape that minimizes induced drag. Elliptical taper shortens the chord near the wingtips in such a way that all parts of the wing experience equivalent downwash, and lift at the wing tips is essentially zero, improving aerodynamic efficiency. This wing design is difficult and costly to manufacture because of the compound curves in its design. The elliptical wing is more efficient in terms of LD, but the wing’s uniform lift distribution causes the entire span of the wing to stall simultaneously, potentially causing loss of control with little warning.
The rectangular wing is similar in efficiency to the elliptical wing, but is much easier to build. Rectangular wings have very gentle stall characteristics with a warning buffet prior to stall, and are easier to manufacture than elliptical wings. One drawback to this wing design is that rectangular wings create more induced drag than an elliptical wing of comparable size.
The tapered wing is the planform found most frequently on gliders. Assuming equal wing area, the tapered wing produces less drag than the rectangular wing, because there is less area at the tip of the tapered wing. If speed is the primary consideration, a tapered wing is more desirable than a rectangular wing, but a tapered wing with no twist (also called washout) has undesirable stall characteristics.
A swept-forward planform is a wing configuration in which the quarter-chord line of the wing has a forward sweep. Swept-forward wings are used to allow the lifting area of the wing to move forward, while keeping the mounting point aft of the cockpit. This wing configuration is used on some tandem two-seat gliders to allow for a small change in center of gravity (CG) with the rear seat occupied, or while flying solo. This type of planform design gives the glider increased maneuverability due to airflow from wing tip to wing root, preventing a stall of the wing tips and ailerons at high angles of attack. Instead, the stall occurs in the region of the wing root.
Washout is built into wings by putting a slight twist between the wing root and wing tip. When washout is designed into the wing, the wing displays very good stall characteristics. Moving outward along the span of the wing, the trailing edge moves up in reference to the leading edge. This twist causes the wing root to have a greater AOA than the tip, and as a result, stall first. This provides ample warning of the impending stall and, at the same time, allows continued aileron control.
Glide ratio is the number of feet a glider travels horizontally in still air for every foot of altitude lost. If a glider has a 50:1 glide ratio, then it travels 50 feet for every foot of altitude lost.
This explains why minimizing drag is so critically important. Because drag varies with airspeed, the glide ratio must also vary with airspeed. A glide polar shown in Figure 3–15 is a graph, normally provided in a glider’s flight manual, that details the glider’s still air sink rate at airspeeds within its flight envelope. The glide ratio at a particular airspeed can be estimated from the glide polar using:
Airspeed and sink rate must both be in the same units. The example in Figure 3-14 uses knots. The minimum sink speed is the airspeed at which the glider loses altitude at the lowest rate. It can be determined from the polar by locating the point on the graph with the lowest sink rate and reading off the corresponding airspeed. [Figure 3-16]
The best glide speed is the airspeed at which, in still air, the glider achieves its best glide ratio. It is also known as the best lift/drag (L/D) speed. This can be determined from the polar by drawing a line from the origin that is tangential to the curve (e.g., just touching). [Figure 3-17] The point of contact is the best glide speed; the glide ratio at this speed can be calculated as previously described. In still air, the glider should be flown at this speed to get from A to B with minimum height loss.
Increasing the mass of a glider by adding water ballast, for example, shifts the glide polar down and to the right. [Figure 3-18] The minimum sink rate is therefore increased, so as expected, the extra weight makes it harder to climb in thermals. However, the best glide ratio remains approximately the same, but now occurs at a higher airspeed. Therefore, if the thermals are strong enough to compensate for the poor climb performance, then water ballast allows a faster inter-thermal cruise. This results in greater distances being traveled per time interval.
The aspect ratio is another factor that affects the lift and drag created by a wing. Aspect ratio is determined by dividing the wingspan (from wingtip to wingtip), by the average wing chord.
Glider wings have a high aspect ratio, as shown in Figure 3-19. High aspect ratio wings produce a comparably high amount of lift at low angles of attack with less induced drag.
Weight is the third force that acts on a glider in flight. Weight opposes lift and acts vertically through the CG of the glider. Gravitational pull provides the force necessary to move a glider through the air since a portion of the weight vector of a glider is directed forward.
Thrust is the forward force that propels a self-launching glider through the air. Self-launching gliders have engine-driven propellers that provide this thrust. Unpowered gliders have an outside force, such as a towplane, winch, or automobile, to launch the glider. Airborne gliders obtain thrust from conversion of potential energy to kinetic energy.