Basic Aerodynamics

Forces Acting on the Aircraft

4 forces, lift, weight, thrust, drag

Once a helicopter leaves the ground, it is acted upon by four aerodynamic forces; thrust, drag, lift and weight. Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.

They are defined as follows:

  • Thrust—the forward force produced by the power plant/propeller or rotor. It opposes or overcomes the force of drag. As a general rule, it acts parallel to the longitudinal axis.
  • Drag—a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.
  • Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. Weight pulls the aircraft downward because of the force of gravity. It opposes lift and acts vertically downward through the aircraft’s center of gravity (CG).
  • Lift—opposes the downward force of weight, is produced by the dynamic effect of the air acting on the airfoil, and acts perpendicular to the flight path through the center of lift.

 Bernoulli’s Principle

Bernoulli’s principle- as the velocity of a fluid increases the pressure exerted by that fluid decreases Pressure and velocity are inversely related.
Bernoulli’s principle is illustrated by the Venturi flow effect.  As air goes through a venturi, or narrowing in a tube, the pressure is lower and the airflow speeds up as seen below.
Imagine half the venturi above.  Half the venturi is similar to an airfoil with lower pressure, faster air, above and higher pressure, slower air below.  These differences in pressure produce lift.

Newton’s Third Law

Newton’s third law : For every action, there is an equal and opposite reaction. The air that is deflected downward through the rotor system also produces an equal and opposite upward (lifting) reaction.

Induced Drag

Induced drag is greatest at slow airspeeds. Induced drag is a by-product of the production of lift.

To decrease induced drag, speed up faster than L/D max (which will vary by aircraft).


 Source: FAA Publication Helicopter Flying Handbook Chapter 2