How Airplanes Fly

Take off!

The following information may be used to supplement your classroom lectures on aviation and aerodynamics. The diagrams and explanations are rather simplistic, but they help get the point across.

Forces on an Airplane in Flight

The four aerodynamic forces that act upon an airplane in flight are lift (the upward acting force), weight (or gravity, the downward acting force), thrust (the forward acting force), and drag (the air resistance or backward acting force). These four forces are continuously battling each other while an airplane is in flight.

	LIFT
THRUST		DRAG
	WEIGHT

Gravity opposes lift, thrust opposes drag. In order to take off, the aircraft's thrust and lift must be suffucient to overcome its weight and drag. In level flight at constant speed, thrust exactly equals drag and lift exactly equals the pull of gravity. To land, an aircraft's thrust must be reduced safely below its drag, as its lift is reduced to levels less than its weight.

How an Airplane Generates Lift

Lift is the aerodynamic force that counteracts gravity and holds an airplane in the air. Most of the lift required by an airplane is created by its wings, but a certain portion is also generated by other parts of the aircraft, such as the fuselage. But what actually causes the lift to be created?

First, understand that air is a fluid, just like water, and that all fluids adhere to the same physical and mathematical principles. Next, realize that lift can only be generated when a fluid is in motion. For example, a wing must be passing through the air or the air must be moving around a stationary wing, one or the other. (The way it usually happens is that the wing is doing most of the moving, although the air may be moving too, at the same time.)

Cross-section of Airplane Wing

Most airplane wings have a special, basic shape as viewed edge-on: their upper surfaces are curved and their lower surfaces are flatter. This shape is what works with the fluid motion of the air to create lift. As air moves around a wing, some goes over the top and some goes underneath. The air that goes over the curved upper surface undergoes two important changes: it is reduced in pressure (by the centrifugal force of flowing across the curved surface) and it is accelerated downward (as it leaves the trailing edge of the wing). The wing is forced into the region of reduced air pressure above the upper surface of the wing by the higher air pressure beneath the wing. Also, the downward acceleration of the air (downwash) at the trailing edge forces the wing upward.

Since lift is dependant on the motion of the air, it increases as the speed of the air increases. Lift also increases (to a point) as the angle that the wing makes with the airflow (known as the angle of attack) increases. Past a certain point, however, increased angle of attack will cause the wing to suddenly lose its lifting ability, or stall.

Control Surfaces and Maneuvering

An airplane in flight moves around three axes of rotation: longitudinal axis, lateral axis, and vertical axis. These axes are imaginary lines that run perpendicularly to each other through the center of gravity of the airplane. Rotation around the longitudinal axis (the line from the nose of the plane to the tail) is called roll. Rotation around the lateral axis (the line from wingtip to wingtip) is called pitch. Rotation around the vertical axis (the line from beneath to above the plane) is called yaw. The pilot guides and controls the aircraft by controlling its pitch, roll, and yaw via the control surfaces. These include the ailerons, elevators, and rudder.

Basic Control Surfaces on an Airplane

Ailerons

The ailerons on an airplane's wings control roll around the longitudinal axis. They work together, simultaneously, tied to the control wheel, or stick, in the cockpit. When the control wheel is turned left, the aileron on the left wing goes up and the one on the right wing goes down. The opposite occurs when the wheel is turned right. But how does this make the airplane roll?

The ailerons alter the lifting ability of the wings slightly. When an aileron is lowered, the lift on the outer portion of that wing increases, causing that wing to rise a little. When an aileron is raised, the lift on the outer portion of that wing is decreased slightly, causing that wing to drop a little. Since the ailerons on an airplane work together, their action causes the airplane to roll.

Aileron Position (As viewed from the end of the wing)

Neutral

Aileron lowered, increased lift

Aileron raised, decreased lift

Elevators

The elevators on the horizontal portion of the tail of an airplane control the pitch of the plane, or its motion around the lateral axis. They are also tied to the control wheel in the cockpit. When the wheel is pulled back, the elevators move upward, causing the tail of the plane to move downward and the nose to pitch upward. When the wheel is pushed forward, the elevators move downward, causing the tail of the plane to rise and the nose to pitch downward.

The elevators work like the ailerons on the wings, in that they cause changes in the lift generated by the tail of the plane. Also, the elevators work together, simultaneously, like the ailerons, but they do not work in opposition to one another. Both go up when the control wheel is pulled back and both go down when the control wheel is pushed forward.

Elevator Position (As viewed from the side)

Elevator raised, reduced lift, tail goes down, nose goes up

Elevator neutral (centered)

Elevator lowered, increased lift, tail goes up, nose goes down

Rudder

The rudder on the rear edge of the vertical fin on the airplane's tail controls yaw around the vertical axis. It is connected to the pedals at the pilot's feet. Pushing the right pedal causes the rudder to deflect to the right. This makes the tail of the airplane move toward the left, causing the nose to move to the right. Pushing the left pedal makes the rudder deflect to the left, the tail moves to the right, and the nose points to the left.

Rudder Position (As viewed from above)

Rudder left, tail right

Rudder neutral (centered)

Rudder right, tail left

Although the rudder pedals and control wheel in the cockpit are not linked together, they must be used simultaneously to control the plane. The pilot guides the airplane by careful and precise movements of the control wheel and rudder pedals, as well as adjusting the thrust of the aircraft.

How Does an Airplane Produce Thrust?

Thrust is the force created by a power source that overcomes the airplane's aerodynamic drag (its resistance to passing through the air) and gives it forward motion. This force can either "pull" or "push" the aircraft forward, depending on the type of power source used. Common types include reciprocating (piston-powered) engines driving propellers, and jet engines.

Reciprocating Engines with Propellers

A reciprocating engine is an internal-combustion engine in which pistons moving back and forth act upon a crankshaft to create rotational movement. (This is the same type of engine that powers most family cars.) A mixture of fuel and air is compressed by the pistons, an electric spark causes the mixture to explode, driving the pistons downward. This motion is transferred to the crankshaft by connecting rods. The rotating crankshaft turns the propeller.

A propeller is a type of airfoil (similar to a wing) that turns and accelerates air. As the blades of the propeller rotate they create lifting forces just as a wing does, only working in the horizontal plane instead of the vertical as with wings. Thus, the propeller creates a propulsive force perpendicular to its plane of rotation that moves the aircraft forward as a reaction. Props can either "pull" the aircraft from their position on the front of the wings or fuselage, or "push" it from behind, or both.

Jet Engines

A jet engine is any engine that ejects a jet or stream of gas or fluid, thereby obtaining thrust in reaction to the ejection force. A jet aircraft engine obtains oxygen from the atmosphere for the combustion of its fuel, creating thrust in reaction to the rapid exhaust of the combustion products. There are several types of jet engines. Some are briefly described below.

Turbojet

A turbojet engine is a jet engine that incorporates a turbine-driven compressor to take in and compress air for the combustion of fuel. The exhaust from the combustion drives the turbine and creates the thrust-producing jet.

Basic Turbojet Engine

Turbofan

A turbofan engine is a turbojet engine in which additional thrust is gained by extending a portion of the compressor or turbine blades outside the inner engine casing. These extended blades propel bypass air around the engine core, between the inner and outer engine casings. This air is not combusted but does provide additional thrust since it is compressed by the blades.

Basic Turbofan Engine

Turboprop

A turboprop engine is a turbojet engine in which a portion of the exhaust energy is used to drive a propeller. The engine's thrust is therefore generated by a combination of the propeller's thrust and the jet exhaust from the engine.

Basic Turboprop Engine

Ramjet

A ramjet engine is the simplest type of jet engine since it has no moving parts. The engine is basically a specially-shaped duct open at both ends, with the air necessary for combustion being compressed by the forward motion of the engine. Fuel is sprayed into the air stream and the mixture is ignited. The high-pressure air coming into the combustion chamber keeps the reaction from going back toward the inlet.

	Combustion Chamber
Air Intake		Exhaust nozzle
	Fuel injectors/ignition grid

Basic Ramjet Engine

Ramjet engines cannot operate under static conditions. In order to function, they have to already be traveling through the air at slightly over the speed of sound. (The speed of sound is somewhat over 740 miles per hour at sea level.) This means that the aircraft using them must first get up to the required speed using some other type of propulsion, then start the ramjets. They can operate at up to five times the speed of sound.

Scramjet

A scramjet, or "supersonic combustion ramjet", engine is similar to a ramjet, but is designed to operate at well over five times the speed of sound, or at hypersonic velocities. As with ramjets, aircraft powered by scramjets must first be brought up to required speed by some other means of propulsion. Unlike ramjets, which slow the supersonic air stream entering the inlet to subsonic speeds before combustion, a scramjet combusts the supersonic air stream without slowing it.

Octave Chanute Aerospace Museum Web Site "How does an Airplane Fly”
Retrieved 10/9/2005 from http://www.aeromuseum.org/Education/Lessons/HowPlaneFly/HowPlaneFly.html