What is Aerodynamics?

Aerodynamics is concerned with how the motion of air or other gaseous fluid interacts with a moving object (Wikipedia, 2009 4). NASA (2008, p. 1 2) defines aerodynamics as "the science that deals with the motion of air and other gaseous fluids and with the forces acting on bodies when the bodies move through such fluids or when such fluids move against or around the bodies".

Any object that moves through air reacts to aerodynamics and hence aerodynamics explains how an aircraft is able to fly (NASA, 2008b 3).

External and Internal Aerodynamics

  1. External Aerodynamics studies the flow of air around solid objects and the way objects move through air (Wikipedia, 2009 4). For example, evaluating the forces interacting on a moving airplane.
  2. Internal Aerodynamics studies the flow of air through passages in solid objects (Wikipedia, 2009 4). For example, evaluating airflow through a jet engine.

Understanding the motion of air around an object enables the forces acting on the object to be calculated (Wikipedia, 2009 4).

The Four Forces of Flight


(image embedded from How Stuff Works on 09 Aug 2009)

There are four basic aerodynamic forces in effect on any moving aircraft. These four forces of flight are weight, lift, drag and thrust (NASA, 2008b 3). Thrust and drag are opposing forces and lift and weight are opposing forces. The amount of each force applied in comparison to the opposing force determines how an aircraft moves through the air.

  • If the amount of drag acting on an airplane increases while thrust stays constant then the airplane will decelerate
  • If the amount of thrust acting on an airplane is greater than the amount of drag then the airplane will move forward or accelerate
  • If the amount of lift decreases below the weight of an airplane then the airplane will descend
  • If the amount of lift applied is greater than the airplane's weight then the airplane will climb (Brian & Adkins, 2009 1).


Weight is the natural downward force acting on an aircraft that is generated by gravity or "g force" that pulls objects to the earth's surface. Weight is a function of the amount of gravity multiplied by an object's mass (NASA, 2008b 3). The mass of an aircraft consists of the weight of the aircraft in addition to the weight of passengers, crew, baggage and cargo. For example, a Boeing 747 aircraft has a maximum take-off weight of 394, 625 kilograms (Brian & Adkins, 2009 1). This weight must be counteracted by lift to achieve flight.


Lift is the upward push force that is required to hold an aircraft in the air, mostly generated by the wings of an airplane (NASA, 2008b 3). A helicopter's lift is achieved by the motion of curved rotor blades through the air that move the helicopter upward. A hot air balloon achieves lift because the hot air inside the balloon is lighter than the heavier air around it causing the hot air to rise and carry the aircraft with it (NASA, 2008b 3).

Airplane Wings and Lift

The airfoil shape of an airplane's wings promotes airplane lift and makes it possible for the airplane to fly. As an airplane's wings are curved on the top and flatter on the bottom, air flows over the top of the wing faster than below the wing resulting in less air pressure above the wing (NASA, 2008b 3). Lower pressure causes the wing and airplane to move in an upward direction (NASA, 2008b 3). Helicopter rotor blades are also aerodynamically designed with a curved shape to alter air pressure and produce lift.

Additionally, wing angle of attack is important in controlling the amount of lift that the wing generates through downward acceleration of air by the airfoil (Wikipedia, 2009c 6).


Drag or air resistance is an aerodynamic force that resists or pulls back on the movement of an aircraft through air by acting opposite to the direction of movement (Wikipedia, 2009b 5). As an aircraft moves, air molecules are pushed aside. Drag is the result of resistance from these air molecules (NASA, 2008b 3). Drag must be counteracted by propulsion or thrust to maintain forward motion.

The amount of drag created by an aircraft depends on the size and shape of the aircraft, velocity of the aircraft and density of the air. Drag decreases as an aircraft slows down. Similarly, retracting landing gear after take-off reduces the size of an airplane and therefore reduces drag.


Thrust is a forward force or push force required to overcome the opposite drag force and move an aircraft forward. More thrust than drag must be created to keep an aircraft moving forward (NASA, 2008b 3).

Airplanes create thrust using propellers and jet engines. A glider on the other hand does not have thrust. A glider can only fly until drag causes the aircraft to slow down and land (NASA, 2008b 3).

Airplane Stages of Flight and the Four Forces of Flight

Take-off and climb

Throughout take-off and climb lift is greater than the weight of the airplane and thrust is greater than drag.


During the cruise phase of flight an airplane moves forward at a constant airspeed and altitude in a straight and level manner. Lift is equal to weight to hold the airplane in the air and thrust is greater than drag to keep the airplane moving forward.


During descent airplane weight is greater than lift but thrust is still greater than drag to keep the airplane moving in a forward direction.

1. Brian, M., & Adkins, B. (2009). How Airplanes Work. Retrieved August 08, 2009 from the World Wide Web:
2. NASA (2008). Scientific and Technical Information (STI). Retrieved August 08, 2009 from the World Wide Web:
3. NASA (2008b). What is Aerodynamics?. Retrieved August 08, 2009 from the World Wide Web:
4. Wikipedia (2009). Aerodynamics. Retrieved August 08, 2009 from the World Wide Web:
5. Wikipedia (2009b). Drag (physics). Retrieved August 08, 2009 from the World Wide Web:
6. Wikipedia (2009c). Lift (force). Retrieved August 08, 2009 from the World Wide Web

Want to know more?

Aviation Knowledge - How do Aircraft Fly?
This Aviation Knowledge page offers further information about how aircraft fly including lift and propulsion
How Stuff Works - How Airplanes Work
This article provides further information about lift creation, wings and other airplane parts in relation to aerodynamics
Wikipedia - Aerodynamics
This website provides further information about aerodynamics including the history of aerodynamics and incompressible and compressible aerodynamics

Contributors to this page

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Stability and Control
Stick Free Characteristics
Hinge Moments
Each control surface on an aircraft has a hinge of some sort. By deflecting the control,
there is an aerodynamic moment about that hinge. The pilot (or some power augmentation
system) must provide the moment to counter that hinge moment if s/he is to be able to deflect the
surface. So the study of hinge moments is important to be able to predict the forces (moments)
required by the human pilot or the hydraulic or electric actuator system. Further, the feedback of
this required force or moment to the pilot is an additional cue to help her/him to fly the aircraft.
Here we will be primarily interested in the elevator hinge moment, but the procedure is
the same for the aileron or the rudder (or most other flapped control).
where the elevator (control surface) area behind the hinge line = the mean aerodynamic chord of the elevator (control surface) of the same area
Generally we assume that the hinge moment depends linearly on the tail angle-of-attack, the elevator deflection, and the deflection of an additional surface at the end of the elevator called the
where are defined accordingly.
Reversible and Irreversible Controls
At this stage we must introduce the concept of reversible and irreversible control systems.
In a reversible system, the pilot controls are directly connected (using pulleys, cables, and pushrods)
to the control surface. Therefore if the pilot controls are deflected, the corresponding
control surface is deflected. Also, if the control surface is deflected, then the pilot controls will
be deflected (one of the standard pre-flight checks).
On the other hand, in an irreversible system, the controls may be directly connected, but
there is an additional boost system the supplies additional forces to the controls. Therefore when
the pilot moves the cockpit controls, the control surface moves. However, if an attempt to move
the control surface is made, it will not move, or if it does move, the pilots controls will not move.
Generally, the boost system will hold the control surface in a fixed position once it is set at that

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