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The force created by propellers and jet engines is called thrust, and is also an aerodynamic force (since it acts on the surrounding air). The aerodynamic force on a powered airplane is commonly represented by three vectors: thrust, lift and drag. [3]: 151 [1]: § 14.2
Lift-induced drag (also called induced drag) is drag which occurs as the result of the creation of lift on a three-dimensional lifting body, such as the wing or propeller of an airplane. Induced drag consists primarily of two components: drag due to the creation of trailing vortices ( vortex drag ); and the presence of additional viscous drag ...
If a powered aircraft is generating thrust T and experiencing drag D, the difference between the two, T − D, is termed the excess thrust. The instantaneous performance of the aircraft is mostly dependent on the excess thrust. Excess thrust is a vector and is determined as the vector difference between the thrust vector and the drag vector.
If speed decreases, drag decreases, and the aircraft will accelerate back to its equilibrium speed where thrust equals drag. However, in slow flight, due to lift-induced drag , as speed decreases, drag increases (and vice versa).
The low speed region of flight is known as the "back of the power curve" or "behind the power curve" [7] [8] (sometimes "back of the drag curve") where more thrust is required to sustain flight at lower speeds. It is an inefficient region of flight because a decrease in speed requires increased thrust and a resultant increase in fuel consumption.
For example, a Savonious wind turbine is a drag-based machine, while a Darrieus wind turbine and conventional horizontal-axis wind turbines are lift-based machines. Drag-based machines are conceptually simple, yet suffer from poor efficiency. Efficiency in this analysis is based on the power extracted vs. the plan-form area.
As the lift generated by a sail increases, so does lift-induced drag, which together with parasitic drag constitutes total drag, (D). This occurs when the angle of attack increases with sail trim or change of course to cause the lift coefficient to increase up to the point of aerodynamic stall , so does the lift-induced drag coefficient .
In flight a powered aircraft can be considered as being acted on by four forces: lift, weight, thrust, and drag. [1] Thrust is the force generated by the engine (whether that engine be a jet engine, a propeller, or -- in exotic cases such as the X-15-- a rocket) and acts in a forward direction for the purpose of overcoming drag. [2]