Sunday, July 17, 2011

Methods of Lift

Aircraft fly because they produce lift. There are many different ways in which lift can be generated, and they will be examined here.

Fixed-wing aircraft

The forerunner of the fixed-wing aircraft is the
kite. Whereas a fixed-wing aircraft relies on its forward speed to create airflow over the wings, a kite is tethered to the ground and relies on the wind blowing over its wings to provide lift. Kites were the first kind of aircraft to fly, and were invented in China around 500 BC. Much aerodynamic research was done with kites before test aircraft, wind tunnels and computer modelling programs became available. The first heavier-than-air craft capable of controlled free flight were gliders. A glider designed by Cayley carried out the first true manned, controlled flight in 1853.

(Right below): A size comparison of some of the largest fixed-wing aircraft: The Airbus A380-800 (largest airliner), the Boeing 747-8, the Antonov An-225 (aircraft with the greatest payload) and the Hughes H-4 "Spruce Goose" (the aircraft with the greatest wingspan). (Left): Some of NASA's fixed wing test aircraft.

Besides the method of propulsion, fixed-wing aircraft are generally characterized by their wing configuration. The most important wing characteristics are:
  • Number of wings – Monoplanes, biplanes, etc.
  • Wing support – Braced, cantilever, rigid, flexible, etc.
  • Wing planform – The aspect ratio, angle of sweep and any variations along the span (including the delta wings class).
  • Location of the horizontal stabilizer, if any.
  • Dihedral angle of the wing – positive (tilted upwards), zero (flush with the fuselage) or negative (tilted downwards, towards the ground) A variable geometry aircraft can change its wing configuration during flight.
A flying wing has no fuselage, though it may have small blisters or pods. The opposite of this is a lifting body which has no wings, though it may have small stabilising and control surfaces. Most fixed-wing aircraft feature a tail unit or empennage incorporating vertical, and often horizontal, stabilising surfaces.

Some people consider wing-in-ground-effect vehicles to be fixed-wing aircraft, others do not. These craft "fly" close to the surface of the ground or water. An example is the Russian Ekranoplan (nicknamed the "Caspian Sea Monster"). Man-powered aircraft also rely on ground effect to remain airborne, but this is only because they are not powered by engines — the airframe is theoretically capable of flying much higher.

For more information of how wings generate lift, visit this link:


Rotorcraft, or rotary-wing aircraft, use a spinning rotor with aerofoil section blades (a rotary wing) to provide lift. Types include helicopters, autogyros and various hybrids such as gyrodynes and compound rotorcraft.

Helicopters have powered rotors. The rotor is driven (directly or indirectly) by an engine and pushes air downwards to create lift. By tilting the rotor forwards, the downwards flow is tilted backwards, producing thrust for forward flight.

Autogyros have unpowered rotors, with a separate power plant to provide thrust. The rotor is tilted backwards. As the autogyro moves forward, air blows upwards across the rotor, making it spin. This spinning increases the speed of airflow over the rotor, to provide lift. Rotor kites are unpowered autogyros, which must be towed to give them forward speed or else be tether-anchored to a static anchor in a high-wind situation for kited flight. (Right): A gyroplane.

Compound rotorcraft have wings which provide some or all of the lift in forward flight. Compound helicopters and compound autogyros have been built, and some forms of gyroplane may be referred to as compound gyroplanes. They are nowadays classified as powered lift types and not as rotorcraft. Tilt-rotor aircraft (such as the V-22 Osprey) have their rotors/propellers horizontal for vertical flight and vertical for forward flight.

Some rotorcraft have reaction-powered rotors with gas jets at the tips, but most have one or more lift rotors powered from engine-driven shafts.

For more information of how rotors generate lift, visit this link:

Other methods of lift

A lifting body is the opposite of a flying wing. In this configuration the aircraft body is shaped to produce lift. If there are any wings, they are too small to provide significant lift and are used only for stability and control. Lifting bodies are not efficient: they suffer from high drag, and must also travel at high speed to generate enough lift to fly. Many of the research prototypes, such as the Martin-Marietta X-24, which led up to the Space Shuttle were lifting bodies (though the shuttle itself is not), and some supersonic missiles obtain lift from the airflow over a tubular body. The flat bodies of some jet fighters also produce lift, as in the F-14 Tomcat's "pancake". (Right): The X-24B's lifting body.

Powered lift types rely on engine-derived lift for vertical take off and landing (VTOL). Most types transition to fixed-wing lift for horizontal flight. Classes of powered lift types include VTOL jet aircraft (such as the Harrier jump-jet), among others. For more information on the Harrier, click here:

No comments:

Post a Comment