Forces in mechanical flight
In this module you will see how various forces combine to get an
aircraft into the air, to maintain flight and to be able to maneuver.
The basic principle used is that of aerodynamics: - the science of air
moving around objects, and the effects this movement has on the air
and the object. Since the Wright Brothers' first flight in a powered
heavier-than-air machine in 1903, the same principles of aerodynamics
have been applied to every aeroplane ever flown.
The four forces
When
an aeroplane is flying, there are four forces in operation - lift, gravitational
force, thrust and drag. For an aeroplane maintaining constant altitude,
and a constant speed, the forces of lift and gravitation, are equal
to those of thrust and drag.
Unbalancing forces of lift and gravitational force causes the plane
to climb or descend, and the other pair of forces, thrust and drag,
controls horizontal acceleration and thus air speed.
Each force will now be considered separately.
Lifting force
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An
aircraft wing is made in a particular cross-section called an
aerofoil. As air passes around the aerofoil the air on top of
the aerofoil travels further than the air below. This means that
the air on top of the wing travels faster than the air passing
under the wing. This is because the wing moving through the air
determines the time taken for the air to move over or under the
wing surface and both must be the same.
So what has this to do with lift? It all goes back to a Swiss
mathematician
named Daniel Bernoulli who, in the mid 1700s, discovered that
the pressure of a gas decreased as the speed of its particles
increased. Now back to the aerofoil where the gas (air) on top
of the wing travels faster than the air below. This means there
is a lower pressure on top, and the difference in pressure created
between the top and bottom surfaces of the wing produces an upward
force on the wing.
As the aircraft speeds along the runway it is the Bernoulli effect
that enables the plane to take off.
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The gravitational
force
The gravitational force is the force that acts between any two bodies
with mass. It pulls every object on the surface of the Earth or near
the Earth towards the centre of the Earth - that's why an aircraft remains
on the ground until a lifting force is applied, and why a continuous
upward force is needed to balance the downward gravitational force.
For a plane to climb a greater lifting force than gravitational force
is needed, then, when the aircraft is at a steady cruising altitude,
only enough force to equal the downward force of gravity is needed.
These two forces of lift and gravity control up and down movements -
but what about speed? You are about to find out.
Thrust and drag
If you have ever travelled in a large passenger plane you will have
experienced the scream of the engines as they drive the aircraft forward.
They are creating the force called thrust that will drive the wings
through the air, so that the Bernoulli effect can operate. At the same
time there is air resistance and skin friction where air passing over
the body of the plane tries to drag it back. Modern aircraft are streamlined
to reduce drag, and more efficient jet engines can create even more
thrust.
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