content_image/576408.jpg
Although the effect of wind is not covered in this section, it is a very prominent force acting on the gliding distance of the aircraft in relationship to its movement over the ground. With a tailwind, the aircraft glides farther because of the higher groundspeed. Conversely, with a headwind the aircraft does not glide as far because of the slower groundspeed. 

Variations in weight for an aircraft with a rigid wing do not affect the glide angle provided the pilot uses the correct airspeed. Since it is the lift over drag (L$_{D}$) ratio that determines the distance the aircraft can glide, weight does not affect the distance. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft glides. The heavier the aircraft, the higher the airspeed must be to obtain the same glide ratio. For example, if two aircraft having the same L$_{D}$ ratio but different weights start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed arrives at the same touchdown point in a shorter time. Both aircraft cover the same distance, only the lighter aircraft takes a longer time. 

However, the WSC aircraft has different characteristics because it has a flexible airframe. As more weight is added to the WSC wing, it flexes more creating more twist in the wing decreasing aerodynamic efficiency, as discussed in chapter 2. For example, a pilot is accustomed to a glide ratio of 5 to 1 flying solo; a passenger is added, and this glide ratio may decrease to 4 to 1. This decrease in glide ratio for added weight is true for all descent speeds. The amount of decrease in glide ratio varies significantly between manufactures and models because each wing flexes differently. The more flexible the wing is, the greater the decrease in glide ratio. Pilots should become familiar with glide ratios for their aircraft at all speeds and all weights. 

Although the propeller thrust of the aircraft is normally dependent on the power output of the engine, the throttle is in the closed position during a glide so the thrust is constant. Since power is not used during a glide or power-off approach, the pitch attitude must be adjusted as necessary to maintain a constant airspeed. 

The best speed for the glide is one at which the aircraft travels the greatest forward distance for a given loss of altitude in still air. This best glide speed corresponds to an angle of attack resulting in the least drag on the aircraft and giving the best lift-to-drag ratio (L$_{DMAX}$). [Figure 6-16] 

<img src='content_image/576408.jpg'>

Any change in the gliding airspeed results in a proportionate change in glide ratio. Any speed, other than the best glide speed, results in more drag. Therefore, as the glide airspeed is reduced or increased from the optimum or best glide speed, the glide ratio is also changed. When descending at a speed below the best glide speed, induced drag increases. When descending at a speed above best glide speed, parasite drag increases. In either case, the rate of descent increases and the glide ratio decreases. 

This leads to a cardinal rule of aircraft flying that a student pilot must understand and appreciate: the pilot must never attempt to “stretch” a glide by applying nose up pressure and reducing the airspeed below the aircraft’s recommended best glide speed. Attempts to stretch a glide invariably result in an increase in the rate and angle of descent and may precipitate an inadvertent stall. 

To enter a glide, the pilot should close the throttle and obtain the best glide speed. When the approximate gliding pitch attitude is established, the airspeed indicator should be checked. If the airspeed is higher than the recommended speed, the pitch attitude is too low; if the airspeed is less than recommended, the pitch attitude is too high. Therefore, the pitch attitude should be readjusted accordingly by referencing the horizon. After the adjustment has been made, the aircraft should be retrimmed (if equipped) so that it maintains this attitude without the need to hold pitch pressure on the control bar. The principles of attitude flying require that the proper flight attitude be established using outside visual references first, then using the flight instruments as a secondary check. It is a good practice to always retrim the aircraft after each pitch adjustment. 

