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## The Four Forces of Flight

by | Mar 30, 2018

How can a 700,000 pound 747 possibly fly?

There are four forces that act on a flying airplane:  Weight (gravity), Lift, Thrust and Drag.

Four forces of flight

For an airplane to fly it must win the tug of war between the opposing forces of lift vs. weight and thrust vs. drag.  During take off and ascending, thrust must be greater than drag and lift must be greater than weight/gravity so the airplane can become airborne.  For descending and landing thrust must be less than drag and lift must be less than weight.

Lift occurs when a solid object turns (changes) the flow of a gas (or liquid). According to Newton’s Third Law (for every action there is an equal and opposite reaction), lift is generated in the opposite direction that the flow of gas is turned. SO – if the flow of air is turned down by the plane, it will create lift for the plane to rise. The wings create the vast majority of the lift and both the upper and lower surfaces of a wing contribute to the flow turning. According to MIT Physics Dept: “lift is created by the flow of air over an airfoil. The shape of an airfoil causes air to flow faster on top than on bottom. The fast flowing air decreases the surrounding air pressure. Because the air pressure is greater below the airfoil than above, a resulting lift force is created.”

In other words, lift is produced by a wing because it diverts air’s momentum and creates a “downwash”. Think of the plane moving thru the air and the wing coming along and pushing a pillow of air beneath it (from the plane’s perspective it is as if the wing is fixed in space and the air is rushing towards it at hundreds of miles per hour).  The wing forces air downward which creates a relative void above the wing.  The differential of the higher pressure below the wing and the lower pressure above the wing is what gives the wing its lift.

The amount of lift generated by a wing depends on the power with which the plane is being forced through the air, the size of the wing and the “angle of attack”.  The angle of attack is basically the angle at which the wing is meeting the oncoming air.  The wing must be somewhat tilted upward as compared to the air so that it can force the air downward and create the pressure differential.  The lift of a wing is proportional to the amount of air diverted times the vertical velocity of the air – so that as the plane increases speed more air is diverted (assuming the same angle of attack).  Thus, bigger wings create more lift.  A greater angle of attack creates more lift.  When a plane comes in for landing and puts its flaps down it is increasing the angle of attack and surface area so more lift is created.

angle of attack

Drag is caused by friction and differences in air pressure. Weight is caused by gravity. The more drag and weight, the more thrust and lift is needed for flight. Thrust is created by the airplane’s propulsion, usually via jet engine or propeller.

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