GLIDERS: SOARING

In Figure 4.1 the forces (resolved again as in Figure 1.4) on a gliding model are shown. From the geometry of this diagram it is found that the angle of glide, a, is the same as the angle between the total air reaction force, R, and the resolved lift component, L. From this it follows that the ratio of height lost to distance covered in the glide is exactly the same as the ratio of lift to drag. (The various equalities of triangles are marked in the diagram.) For this reason the glide ratio is often quoted as the Lift to Drag or L/D ratio.

Flight at minimum sinking speed is slow at a high Cl. The minimum rate of descent occurs when the ratio of Cl1’3 to Cd is highest (The derivation of this is given in Appendix 1.) This ratio is often termed the power factor for a model, since it also indicates the trim condition for level flight with minimum motor power. The ratio is written in a number of ways which are all equivalent

afThe last gives a value which is equal to the power factor squared.) The power factor Should not be confused with the maximum L/D ratio. The flattest glide, covering greatest distance over the ground in still air, is not the best trim for minimum sinking speed, which requires slower flight at higher Cl – Depending on the wing profile, the minimum sinking speed may occur at angles of attack fairly close to the stall, or, in many cases, at a flight speed about 75% of that for the best L/D. This is the trim to be sought when a glider is soaring.