Lift is what keeps an aircraft in the air. The upward force acting at a right angle to the flight path must be sufficiently large to equal the weight of the plane. The fixed wing of modern airplanes is shaped to produce lift in a most efficient way. Sir George Cayley was the first to propose the idea of a rigid airplane for the generation of lift. He gained his wing to a fuselage carrying a distinctly separate propulsive mechanism and added vertical and horizontal tail surfaces for control.
Laying aside buoyant devices such as balloons and airships, all flights, whether natural or mechanical, depends upon the fact that bodies moving through a fluid experience a force called resistance. The major problem of flight with fixed wings is to find means of turning this force to good account by making part of it sustain the aircraft. The modern flying machine consists of a specially shaped hollow body to which are attached rigid supporting surfaces called ailerons, elevators, and rudders, together with a means of producing a thrust, (a force in the direction of motion) by the employment of an airscrew or a high-speed jet.
In aerodynamics the resistance of air is divided into two parts. One component, called drag, acts along the direction of motion; this is the part of the resistance that directly opposes the motion. The other component, called lift, acts perpendicularly to the direction of motion. If the aircraft is flying straight and level at constant speed its drag balances the thrust and its lift, acting along the vertical, opposes the force of gravity and balances the weight of the machine.
Bodies of special shapes, called aerofoils, produce much more lift than drag when moved rapidly through the air. The purpose of the wings (which are aerofoils) is to develop enough upward directed force to overcome gravity. Some of the sustaining force also comes from the tailplanes, but their main purpose is to balance the machine in flight. Aerofoils can produce lift only when air is driven over them or, what is the same thing, they are moved through the air, and it is found that the magnitude of the lift increases rapidly with the speed of the air sweeping past.
When an aircraft is taking off, it is usually pointed into the wind so that, when it moves along the runway because of the thrust provided by the engines, it experiences a very rapid flow of air over the wings, and this enables it to rise after a relatively short run. When airborne the aircraft is guided by a rudder and controlled for stability mainly by the ailerons and elevator and its own inherient properties.
Issac Newton, who flew kites as a young lad gave the world the theory of lift in his Third Law of Motion which states, “For every action there is an equal and opposite reaction.” The simplest way to understand the theory is to comprehend how a kite flies. The wind pushes the kite and produces lift and the kite pushes the wind which is deflected downward. There is no difference whether its the plane that is thrust through the air to make a wind or the kite standing still on its tether with the wind rushing pass it. Lift is produced by the interaction of the forces.
The amount of lift developed can be determined by using the formula:
cL = L
L = CLSq Sq
L = force
S = area
q = force/area
cL = no dimension, cefficient of lift
The cL is determined by wind-tunnel tests for a particular airfoil at various angles of attack in a range of wind speeds.
Certain features have to be built into the aircraft to maintain stability and control when it passes through disturbances in the air known as turbulence. Lift, gravity, thrust, and drag are important when it comes to flying because they are forces which act on an airplane when it is flying. The aircraft is made up of many different parts each having its own unique function and there are many different aircrafts in use today. (See Diagrams 4-6).
The most aircraft in use today are the jets. There are several kinds of jet engines. The most widely used is the turbojet which pulls in large masses of air through the intake and then compress it. The
ompressed air is heated and expanded in a combustion chamber. The expanded air rushes against a turbine and spins it. The spinning turbine turns the compressor and keeps fresh air coming in. The old air, reaching the nozzle, is compressed enough and is moving fast enough to provide pressure to push the plane. There is a nozzle exit at the rear, and the force against the front sends the plane shooting forward.
Another type of jet is the ram jet. It depends upon its own forward speed to scoop up air and ram it into the combustion chamber. The compressed air is heated and expanded to increase pressure. A large nozzle creates a pressure inbalance and gives thrust to the forward end of the engine. The third type of jet engine is the turboprop. There are two turbines. One spins the compressor and the other spins a conventional propeller. The engine uses both a propeller and the direct force of gases (See Diagram 7).
The latest and fastest commercial jet engine aircraft of today is the Concorde, built jointly by Britain and France. The Concorde has a crew of three and its fuselage can carry up to 144 passengers, seated four abreast and their luggage. It stands high off the ground and lands at a very steep angle. It carries 26,000 gallons of fuel. The Concorde’s four engines can deliver over 52,000 pounds of thrust. (See Diagram 8).