Definitions

Glide ratio

Glide ratio, also called, Lift-to-drag ratio, glide number, or finesse, is an aviation term that refers to the distance an aircraft will move forward for any given amount of lost altitude (the cotangent of the downward angle). Alternatively it is also the forward speed divided by sink speed (unpowered aircraft):

$\left\{L over D\right\}=\left\{\left\{Delta s\right\} over \left\{Delta h\right\}\right\}=\left\{v_\left\{forward\right\}over v_\left\{down\right\}\right\}$

The terms glide ratio and lift-to-drag ratio are interchangeable. This is true because the force vectors also determine the direction of travel with the engine off. Glide ratio is the preferred term for unpowered aircraft, and lift-to-drag ratio the preferred term for aerodynamics literature and powered flight. This parameter effectively describes the efficiency of the airframe.

Importance of the glide ratio in gliding

Although the best glide ratio is important when measuring the performance of a glider, its ability to achieve a good glide ratio at high speed determines its success when racing (see article on gliding). However all soaring aircraft need to be able to climb effectively in the available thermals. This normally limits the maximum tolerable sink rate. A sink rate of approximately 1.0 m/s is the most that a practical hang glider or paraglider could have; sailplanes have an even better performance. At higher sink rates soaring would be difficult because air rising at higher rates is less common. Consequently an airliner may have a better glide ratio than a hang glider, but it will not be able to thermal because of its much higher stall speed and so its much higher sink rate. (Note that the Boeing 767 in the Gimli Glider incident achieved a glide ratio of only 12:1.)

During landing, a high lift/drag ratio is desirable. Experiments with lifting bodies show that a lift/drag ratio below about 2 makes landing very difficult.

The loss of height can be measured at several speeds and plotted on a "polar curve" to calculate the best speed to fly in various conditions, such as when flying into wind or when in sinking air. Other polar curves can be measured by loading the glider with water ballast. When ballast is carried, the best glide ratio is achieved at higher speeds (the glide ratio is not increased).