Understanding Aerodynamics: Arguing from the Real Physics

6

Drag and Propulsion

It is common to define an aerodynamic retarding force on a body moving through a fluid as drag and a propulsive force as thrust. This definition is unambiguous if we apply it to the flight-direction component of the total aerodynamic force on a complete body. But things are not so simple when we try to apply it to separate contributions to the total, as we often do. In practice we are often interested in vehicles made up of multiple parts and having active propulsion, with drag on some parts and thrust on others. Propulsive effort can be applied in a variety of forms: rotating propellers, flapping wings, complicated processes inside engines, and so on. The practical objective is generally to control the total flight-direction force on the vehicle, for example, to make it zero for steady, level flight or to make it a net thrust for climbing or accelerating flight. However, in assessing the aerodynamic performance of such a vehicle we have a natural inclination and some practical incentives to assess the drag-producing parts and thrust-producing parts separately. Thus we often divide the total flight-direction force into a drag on one portion of the vehicle's surface and a thrust on the other portion. This seems like a reasonable thing to do, and we've all seen the ubiquitous elementary force diagrams showing drag and thrust acting simultaneously on an airplane. But resolving partial contributions to the total drag or thrust in this way raises two potentially ...