Monday, May 14, 2012

Thrust, lift, drag

I have had a smattering of questions lately about the roles of lift and drag in animal flight. An extensive review would require a book, but here are some basics:

1) Lift is the component of fluid force that is directed perpendicular to flow. This need not always mean that lift is directed upwards. For example, thrust in animal flyers is actually a component of the generated lift. By angling the flight stroke such that the power stroke sweeps down and forward, flying animals point some of their lift forward as thrust. The distal part of the wing produces more thrust and proportionately less weight support. As you move further inboard (proximal) on the wing, weight support becomes more important and thrust contribution diminishes.

2) Drag is the fluid force component parallel to the fluid flow. Most flying animals fly at a lift:drag ratio above one. This will typically mean that they use lift as the primary source of weight support and propulsion, and that minimizing drag improves propulsive efficiency. However, the situation can be complicated. Drag can contribute to weight support, and very tiny insects fly at L:D ratios less than one - as such, they paddle through the air rather than use true, lift-based flight.

In the aquatic realm, both drag-based and lift-based propulsion is common. The former is particularly used in fast starts. For flyers, the same principle applies during their equivalent of a "fast start": namely, takeoff. I'll be writing about that basic derivation later.

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