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Re: avian flight - glide ratio



Glide ratio is the performance measure for gliding flight.  It is the ratio of
lift to drag -- which is also the ratio of horizontal travel to vertical
descent and of airspeed to descent rate.  Energy for maintaining gliding
airspeed is dervived from the potential energy gained and converted to kinetic
energy as altitude is decreased.  A pigeon's glide ratio is about 6:1,
therefore its drag is about one sixth of its weight.  When not flapping and in
neutral sink, it descends about 1 foot for every 6 feet of forward travel.
Glide ratio is also a measure of the amount of power required to maintain
level flight.  For example, if an animal has a glide ratio of 28:1 and weighs
336 pounds, then it is generating 12 pounds of drag.  Let us assume for
illustration that this is happening at an airspeed of 46 mph (67.47 fps).
Then the animal will be descending at 2.40 fps (67.47/28=2.40), and it would
require the input of 1.47 horsepower (2.4*336/550=1.47) to maintain level
flight.  Soaring is an extension of gliding in which the energy required to
maintain level flight is extracted from one of several sources of energy often
available in the atmosphere.

Please note that weight changes do not affect the glide ratio or range of a
glider.  If you take a one pound animal with a 6:1 glide ratio and release it
from a height of 1000 feet into straight gliding flight, it will touch down
6000 feet away.  If you then feed it one pound of lead weights (I don't
recommend performing this experiment in life since most animals would object,
not to mention animal lovers) and release it again from a height of 1000 feet,
it will still touch down 6000 feet away.  However, it will arrive at the
touchdown point in only 70.7% of the time it took at the lighter weight.  When
gliding, extra weight (higher wing loading) is an advantage in that more
terrain can be covered per unit time without adversely affecting range..

David Marjanovic wrote:

> Erm... what's glide ratio? :-]