There are two main ways for wings to create lift; The Longer Path, (Bernoulli or equal transit time) explanation and the Newtonian, (momentum transfer or deflection) explanation.
The longer path explanation works on the theory that if an object hits air, all particles of air passing around the object will meet again on the other side.
This means that if an object is shaped in a particular way so that the top side of the object is longer from front to back than the underside, then when particles of air go above and below the object the fact that particles above the wing are travelling faster than the ones below it means that the two pressures are different. The low pressure above the wing and high pressure below it cause lift.
This explanation is flawed in several ways however, although in practice it normally works, the theory isn’t completely right. For example; when a group of air particles hit an object and go all different ways around it, there is no logical reason to assume that they should rejoin on the other side.
The alternative explanation to that is the Newtonian explanation, originally worked out by Isaac Newton as a way to make more efficient, faster boats. However, since water and air are both fluids, the theory also works in air.
In the late 1600’s Newton theorized that air molecules behave like individual particles, and that the air hitting the underside of a wing behaves like shotgun pellets hitting a metal plate. Each individual particle bounces off the bottom of the wing and due to Newton’s third law which states that every action has an equal and opposite reaction, the bouncing particles push the wing slightly, and with sufficient speed these tiny pushes would give significant lift.
This explanation is also flawed. For example, in this theory the top side of the wing is not considered at all, so therefore any lift calculations made are extremely inaccurate. Also about 100 years after Newton found this, a man named Leonard Euler proved Newton’s theory about air particles acting like shotgun pellets wrong due to the fact that a shot gun fires a minute amount of particles over a relatively wide area in comparison to the millions of air particles that a wing might hit, and the closeness of particles of air. However at hypersonic speeds (over 5 times the speed of sound), and at very low air pressures this theory becomes true, and air particles act more like shotgun pellets.
RADAR
RADAR is an acronym for “RAdio Detection And Ranging”. Radar is primarily used for detection of aeroplanes and ships. However, it is also used for weather prediction, using it to detect large rainclouds and storms, information like this is very useful for pilots to know about, as they may have to alter flight paths and schedules accordingly.
Radar works by sending out short, high intensity burst of high frequency radio waves. By detcting their echo to return it gives the ability to determine an objects location, distance (range), speed and in some cases, identity. The distance and therefore location are found by the time it takes for the beam to return to the receiver, in early radar machines, this information would then be projected onto a cathode ray tube.
The speed of the object can be determined by the doppler shift of the beam. For example, if the object is coming towards the radar, the beam would have been ‘squashed’ and the wavelegnth decreased and the frequency increased, and if the object was moving away the wave would be ‘stretched’ and the opposite would occur.
Doppler shift can be calculated with a simple calculation:
Doppler Shift = 2x the Relative speed between reciever and transmitter
Wavelength of transmitted signal.
This means that if you know the doppler shift (the amount that a wave has been shifted), by knowing knowing the wavelength of both transmitted and received (shifted) waves you can work out the relative speed of the object.
For example, if a radar picked up a doppler shift off an aeroplane of 420, using a radar beam with a 1.2m wavelength, you can multiply these numbers together to get 504. you can then half this to get the speed of the plane, which is 252m/s, which is approximately 567mph, or 907.2 km/h, which is approximately the cruising speed for a Boeing 747 Jumbo Jet
The same principles of Radar are also used in Sonar, and Lidar.
Sonar (a sound wave equivilant of radar), has very little use anywhere except underwater, due to the fact that sound waves don’t travel very far in air, and is therefore used mainly on ships and submarines.
Lidar, (a light wave equivilant) which is being used in parts of the world by police as a replacement for speed radars.
Bibliography
Images
http://www.shorehamairport.co.uk/
Self Drawn
http://www.yesmag.bc.ca/focus/flight/flight_science.html
Self Drawn
Modified from 2 Images
www.centennialofflight.gov/essay/Dictionary/radar/DI90G1.htm
and
http://www.hk-phy.org/iq/stealth/stealth_e.html
Information
History
http://ourworld.compuserve.com/homepages/BMLSS/Airport.htm
http://www.shorehamairport.co.uk/
Flight
http://www.howstuffworks.com/airplane.htm
http://wings.avkids.com/Book/Flight/
Encyclopedia
Multimedia Encyclopedia
Class notes
Radar
http://electronics.howstuffworks.com/radar.htm
http://www.radarworld.org/works.html
Encyclopedia
Multimedia Encyclopedia
Class Notes
