From the creation of the first flying machine, to modern day commercial jets, the flight of an aircraft has always fascinated mankind. How do airplanes fly? Laws of physics and an amalgam of other natural forces act in concert to achieve flight.
One cannot simply associate a single factor, such as the thrust produced by airplane engines, to the flight of an aircraft. There has to be an interaction between the forces acting on an airplane. The question is; how can you, the pilot, make these forces interact with each other?
Forces Acting along the Y-axis: Lift and Weight
Vertical motion of an airplane, and all the associated components of different forces acting along the y-axis, are primarily due to the lift-weight couple. In order to fly, the wings of an aircraft must support the plane’s weight. To do so, these wings generate lift. Lets see how lift is created:
- Aerospace engineers design the wing of an aircraft in a manner such that the air flowing over it is faster than that flowing under it.
- According to the Bernoulli principle, faster-moving air has a comparatively lower static pressure, than slow-moving air.
- This creates a pressure differential over and under the wing; i.e. a region of low pressure over the wing and a region of high pressure under it.
- The area of low pressure literally sucks-in the wing under it, assisted by the high pressure region pushing the wing upwards.
- This vertical movement of the aircraft is due to the force of Lift.
The following is a video showing how air flows around the wing.
Notice how the angle of the aerofoil inside the wind tunnel, relative to the airflow, is steadily increased. This angle is referred to as angle of attack. The relation between the angle of attack and lift is directly proportional, until the pilot breaches the critical angle of attack, beyond which no lift is produced.
Forces Acting along the X-axis: Thrust and Drag
Aircraft need to propel forward, in order to cover horizontal distances. This propulsion is also required to attain a certain speed of relative airflow, necessary for the production of lift. Thrust must not be confused with the airspeed of an aircraft. Airspeed is a measure of aircraft motion through the air, while thrust is the force creating that forward motion relative to the remote airflow.
Generated by aircraft engines, thrust is an airplane’s primary driving force. As an airplane speeds through the air, resistance to its forward motion develops. This resistance to the motion of an aircraft, be it vertical motion (lift) or horizontal motion (thrust), is called drag.
The following video depicts how an object not tuned aerodynamically, disrupts the airflow behind it. A similar effect takes place when an airplane, as a whole (not just the wing), flies through the air. This disrupted airflow represents drag.
Flying an Airplane
To fly an airplane, the pilot must necessarily have the ability to control these four forces acting on the aircraft namely: lift, weight, thrust and drag. This control is achieved via different controllable surfaces, such as ailerons, rudders and flaps, which are installed at key locations in the airframe.
Trevor Thom. Aeroplane General Knowledge and Aerodynamics. Aviation Theory Centre. (2004).
Pilot’s Handbook of Aeronautical Knowledge. Federal Aviation Administration. (2008).
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