The airliner or military jet passing majestically overhead seems to move almost magically through the sky, so effortless is the apparent motion. Yet these aircraft weigh tons, sometimes many tons. The one of a kind An-225 Mriya, designed to lift extremely heavy cargoes for the Russian space program, has an incredible maximum takeoff weight of 640 tons.
What gets these giants into the air, and what keeps them there?
There are four principal – and many secondary – forces at work when any conventional aircraft takes flight. These are known as lift, thrust, weight or gravity, and drag. All are intimately woven together and are collectively known as the forces of flight.
Lift is the force which helps the 640-ton laden Antonov giant take leave of the runway and make its way into the sky. Lift occurs when the aircraft achieves forward motion through the power of its engines, causing the wings to force their way through the air.
Wings are designed in such a way that the distance from the leading edge to the trailing edge is longer at the top of the wing than at the bottom. This causes the air at the top of the wing to travel faster to reach the trailing edge than does the air beneath the wing. A volume of air traveling faster over a steeply curved surface applies less pressure to the wing than does the air at the bottom.
The higher pressure air beneath the wing simply “lifts” the wing into the lower pressure air above.
The aircraft merely goes along for the ride.
Weight is the term given to the force generated by gravity on the aircraft that must be overcome before the plane can take flight. It is conceived as passing through the center of gravity of the aircraft and pointing directly to the center of the Earth, or the Earth’s center of gravity.
Weight and lift work in opposition to determine an aircraft’s vertical motion. To ascend, lift must exceed weight. To descend, weight must exceed lift, and for level flight the two forces must balance out.
Thrust is the force which moves the aircraft through the air to generate the lift that is required to achieve flight. It is most generally generated by engines turning propellers, usually pulling the aircraft, or by jets, pusher propellers or, in some cases, rockets which push the aircraft.
The center of thrust is considered to pass through the center of the aircraft.
Drag is the element or force of flight that directly opposes thrust and therefore, by extension, lift. It derives from the air pressure built up in front of the aircraft and from the shape and construction of the aircraft itself. The latter greatly influences the amount which friction will add to overall drag, which determines the amount of thrust needed to generate the lift needed to overcome a particular weight.
Drag works in the opposite direction of the line of thrust.
It all works marvelously together and seems quite simple, but of course there is more to flight than an illustration as simple as is this one can possibly address. Changing the angle of attack or the attitude of the aircraft relative to horizontal changes everything; banking an aircraft into a turn similarly alters the equation.
These are topics for another day.
For the present, keep the basics of lift, weight, thrust and drag in mind and you will know just how those giant and not-so-giant aircraft perform their aerial magic.
