So, you are really interested to know how does an aircraft fly? Lets begin our journey…..
The inspiration behind flight
Wilbur Wright rightly said” There is no sport equal to that which aviators enjoy while being carried through the air on great white wings”.
Have you ever imagined how bird (or an airplane) flies????
As the French proverb goes “There is no flying without wings”. Of course the flight of a bird or an airplane is just because it has wings.
In Sir James Matthew Barrie’s words “The reason birds can fly and we can’t is simply that they have perfect faith, for to have faith is to have wings.” Wright Brothers had the same faith. They believed that they could invent a machine with wings that could make humans capable of flying too. Thus one of the miracles of science was developed.
Observe when a bird flies, it moves swiftly through the air with its wings stationary. The wings of the birds are at a slight angle; they are never straight, so they are able to deflect the air gently downward which causes a reaction force to occur in the reverse direction. This reaction force acts in the upward direction and in aeronautical term is referred to as the “Lift”.
Now the question comes why the birds never fly straight forward??? They tend to leans a little downwards. Well, the answer to this question is that the bird needs to lose some height in order to overcome the drag that it faces during flight and in order to maintain its speed.
The same basic theory is applied for the flight of an aircraft.
Forces on an aircraft: How does an aircraft fly?
Physics defines “A force is any push or pull in a specific direction. “Before going into the detailing of flight , we must first know what are the forces which acts on an aircraft.
Generally, there are four forces which act on an aircraft. These are:
Weight
Weight is a force that always acts toward the center of the earth.
Lift
To overcome the weight force of an airplane, an opposite force is generated in the upward direction called lift. It is generated when the airplane moves through the free stream air; mostly the wings. It acts perpendicular to the direction of airflow.
Drag
The resistance force due to the motion of an airplane in the air is called drag. As
the aircraft moves through the air, it is resisted by this force. It acts
parallel to the direction of airflow.
Thrust
To overcome the drag force, airplanes use a propulsion system (the engines) to produce a force called thrust. In steady flight level conditions, the thrust must be greater than drag to accelerate the aircraft forward. The performance of an airplane depends on the thrust that it develops.
In a straight and level condition;
Lift=Weight
Drag=Thrust
The Bernaulli’s theorem of Lift
The fundamental theory of flight is derived from Bernoulli’s Principle.Before going into the aerodynamic description of flight, let me explain what actually is an airfoil.
“Airfoil is any body which when placed in a free stream medium (lets say airflow) would tend to produce more lift than its drag.To fulfill this requirement , the airfoil is shaped in such a way as shown in the figure or we may say they are cambered.
As the airfoils are so shaped, the air beneath the wing are slightly at a high pressure than those above the wings. But why is it so?????
The reason is : For airflow to reach at the end point of an airfoil, the airflow at the upper surface would take more time to reach than the lower portion because of its shape. Hence it has to cover its distance at a faster pace than the air that is lying beneath the surface. The slow moving air thus creates high pressure at the lower surface of the airfoil and the fast moving air creates a low pressure at the upper surface of the airfoil.
It is this difference in pressure between the upper and lower surface of the wing that produces lift. The lift causes the airplane to rise up above and hence flight is possible.
Newton’s theory of Lift production
Newton’s 3rd Law states that “To every action there is always an equal and opposite reaction”.
The wings of an airplane are attached to the airframe structure at a slight angle tilting downwards. Hence the air passing over the wing or an airfoil (considering a small cross section of the wing) is pushed downwards at the bottommost portion of the wing. This downward movement of air causes an equal and opposite reaction force to be created. This reaction force acts in the upward direction and is referred to as lift.