Birds & Insects - Natural Flyers

Gliding :

Gliding occurs when the resultant force, R, of the lift (L) and the drag (D), is exactly vertical, thus balancing the gravitational pull.

The glide ratio is the same as the lift to drag ratio. (Adapted from Alexander, D. E, 2002. Nature's Flyers)

R, makes angle with L. Hence,
tan θ = D/L

The glide ratio is defined as the ratio of horizontal distance covered to the vertical distance covered.
Hence, glide ratio = cot θ = L/D

A higher aspect ratio indicates higher value of L/D, thus, a smaller θ. This implies, that, birds with a high aspect ratio can tilt a little bit, and start to glide.

As they do so, they also cover a longer horizontal distance as compared to others with lower aspect ratios.

Some interesting values for aspect ratio and the corresponding L/D ratio are as follows:

Flyer	Aspect Ratio	Ratio of Lift to Drag
Fruit Fly	5.5	1.8
Bumble bee	6.7	2.5
Sparrow	5.3	4.0
Swift	11.0	10
Albatross	15.0	19

(Adapted from Alexander, D. E, 2002. Nature's Flyers)

As R balances weight in case of gliding, the speed remains constant, but the motion is generally directed downward.

Soaring :

Soaring occurs when a bird glides but maintains a constant height, or, actually, increases its height! Hence, it does not flap its wings but continues to rise. This can happen, for instance, if the bird glides through a region of rising air. Then, if its downward speed due to gliding is smaller than or equal to the speed of the rising air, soaring takes place.