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Forces on an Airplane in Flight
The four aerodynamic forces that act upon an airplane in flight are lift (the upward acting force),
weightgravity, 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. (or
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
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 neutral, normal lift
aileron lowered, increased lift
aileron raised, decreased lift
Aileron Position
(As viewed from the end of the wing)
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 raised, reduced lift, tail goes down, nose goes up
elevator neutral (centered)
elevator lowered, increased lift, tail goes up, nose goes down
Elevator Position
(As viewed from the side)
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 left, tail right
rudder neutral (centered)
rudder right, tail left
Rudder Position
(As viewed from above)
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.
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
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
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
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 airstream 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
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.
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 airstream entering the inlet to subsonic
speeds before combustion, a scramjet combusts the supersonic airstream without slowing it.