How aircraft fly-Gravity,drag,lift and thrust relations.

How aircraft fly?

                All fixed wing aircraft use the same principal to fly. They simply travel their wings through an air mass, trying to develop enough lift force to “defy” gravity. I mention gravity because gravity and the drag are the 2 prevailing forces that a flying machine has to overcome in order to attain and sustain controlled flight.

                In this picture is illustrated the lift creating process of the wing. Imagine 2 identical air molecules Alpha and Beta that must travel from the front side of the wing(leading edge) to the back side of the wing(trailing edge) tacking 2 totally deferent paths. Alpha must travel the upper surface of the wing, while Beta must travel along the bottom surface. Well here is where the magic starts. Both molecules are part of the same air mass, so they are forced to comply with the laws of hydrodynamics that dictate that both Alpha and Beta must meet at points A1 and B2 at almost the same time. But clearly the Alpha molecule has to travel a greater distance due to the curvature of the wing on the top side of it. So the alpha to make it in time for the appointment with Beta must travel with greater speed, thus creating a lower air pressure on top of the wing. Now comes the big thing, the lower air of the wing with the higher pressure, wants to travel to the upper side of the wing of lower pressure and tries to puss trough the wing surface from the lower surface to the upper surface. This force that is exerted on the bottom side of the wing directioned to the top, is called lift . The same principal is used also to the automobile industry in car spoilers, that use that principal in reverse to create downforce.
 

But that’s only the half of the story. The aircraft in order to attain and maintain speed has to overcome also the force called drag. Drag is and fluid drag (sometimes called air resistance or fluid resistance) refers to forces that oppose the relative motion of an object through a fluid. It takes many forms but in the aviation the most crucial are 3:

A)Friction drag-The drag of the tires and any surface of the aircraft to the ground and is only to be considered at takeoff and landing.

B) Induced drag- The drag generated as a result of lift, and is greater at lower speeds when higher angle of attack are required to maintain flight.

C) Parasite drag-This drag also called skin friction drag is the tenancy to the aircraft itself wanting to resist resist to fly through the airmass at any speed. As the airspeed over the aircraft is increased the parasite drag will increase, unless the aircraft changes shape(retraction of flaps,slats,landing gear,spoilers etc).

In order to overcome drag then the aircraft need a force. The name of the force?

                     THRUST

Thrust is a mechanical power , created by devices engines-motors in form of either horsepower that is applied to a propeller and create thrust by pusing/pulling air mass with its blades, or by jet propulsion, like turbofan engines do, pushing a pillar of air backwards thus propelling the engine forward. If the amount of thrust is equal to the amount of drag the aircraft can maintain horizontal and stable flight. If he total amount of the drag is greater than the thrust then the excessive drag will force the aircraft to slow down and lose lift. If the thrust is greater than total amount of drag, then the excessive thrust  can be used in order to increase air speed and increase lift in order to climb higher.

*Also in some advanced aircraft the thrust can be used in order to increase the drag of the aircraft and bring it to a halt or even go backward in the ground. Some aircraft that are not feeted with spoilers use reverse thrust in the air in order to slow down as the turboprops

 ATR 42/72 and the supersonic  passenger aircraft Concorde(using thrust reversers at engines 2/3)