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The Wings The ratio of the overall wing span (length) to the average chord (width) is
known as its aspect ratio.
Simple experiments confirmed that high aspect ratio wings produced a
better ratio of lift to drag than short ones for flight at subsonic speeds.
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The Wings High aspect ratio wings have a good ratio of lift to drag, they are used on
aircraft intended for long range or endurance.
Very low aspect ratio wings, such as those of Concorde, produce less drag
in supersonic flight
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Lift Generation by Wings The wing produces a circulatory effect; behaving like a vortex
English engineer F. W. Lanchester reasoned that if a wing or lifting surface
acts like a vortex
A theory of vortex behaviour indicated that a vortex could only persist if it
either terminated in a wall at each end, or formed a closed ring
More lift =
strong vortices
Danger behind
large aircraft
Turbulence
Flow downward and
outward.
Bernoullis
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Lift Generation by Wings From Bernoullis equation, we can relate pressure and speed of the air.
The air speed in the centre of the vortex is high and the pressure is low.
The low pressure at the centre is accompanied by a low temperature.
This causes any water vapour in the air tends to condense and become
visible in the centre of the trailing vortex lines,
The vapour trails frequently seen behind high-flying aircraft are normally
formed by condensation of the water vapour from the engine exhausts, and
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Span wise flow
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Vortices Wingtip vortices are circular patterns of rotating air left behind a wing as
it generates lift
The wing's main purpose in life is to produce a pressure difference between
the top and bottom surfaces. The contours of the wing force air to
accelerate over the top surface, dropping pressure relative to the bottom,
and providing a net upward force on the airplane, allowing it to fly.
At the wingtips, high-pressure air on
the bottom spills over to the top
surface, swirling around in a horizontal
vortex at each wingtip. The vortex
influences the air travelling over to the
wing, pushing it down and reducing the lift.
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Vortices Suppose that the tip of the wing curved up for the last few feet instead.
There would still be some pressure difference between the outboard andinboard sides of the wingtip (or winglet), but since the vertical section
itself isn't producing lift, it would be less than in the winglet-free case. Wingtip vorticies are less intense and further away from the main, lifting,
section of the wing when winglets are present, boosting wing lift andallowing an airplane to carry more payload further for the same sizewings.
the airplane now has to carry two surfacesthat weigh something and add some drag.
The optimum size winglet is that whichproperly balances the drag reduction frommoving tip vortices away from the wingswith the drag increase from the extrasurface area and the fuel penalty.
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Vorticity and Horseshoe System The wing-bound vortex, together with the trailing vortices, form a kind of
horseshoe shape, and this is sometimes called the horseshoe vortex system. It
forms three sides of the predicted closed ring. The circuit is completed by
the starting vortex.
A strong starting vortex is formed when the aircraft rotates at take-off.
More vorticity is produced and left behind
when the aircraft produces an increase in
wing circulation.
An additional starting vortex is formed,
when an aircraft starts to pull out of a dive.
The counterpart of starting vorticity is stop
ping vorticity, which rotates opposite and is
shed every time the circulation is reduced,
landing.
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Trailing Vortices & Downwash The trailing vortices are not just a mildly interesting by-product of wing lift.
Their influence on the flow extends well beyond their central core,
modifying the whole flow pattern.
In particular, they alter the flow direction and speed in the vicinity of the
wing and tail surfaces.
The trailing vortices thus have a strong influence on the lift, drag and
handling properties of the aircraft.
Downwash, is apparent not only behind the wing, but also influences the
approaching air, and the flow over the wing itself. It causes the air to be
deflected downwards as it flows past the wing.
The angle of attack relative to the modified local airstream direction, is
reduced. This reduction in effective angle of attack means that less lift will
be generated at a given AoA.
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Downwash
Increase AoA,
increase drag
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Trailing Vortices & Downwash A strong starting vortex is formed and left behind just above the runway
when the aircraft rotates at take-off. More starting vorticity is produced and
left behind whenever the aircraft produces an increase in wing circulation.
An additional starting vortex is thus formed, when an aircraft starts to pull
out of a dive.
The counterpart of starting vorticity is stopping vorticity, which rotates
in the opposite sense, and is shed every time the circulation is reduced, as on
landing.
In level flight, the amount of circulation required reduces as the speed
increases, so stopping vorticity is shed when an aircraft accelerates in level
flight.
Strong starting and stopping vortices can be generated during violent
manoeuvres, and may significantly affect the handling.
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Downwash
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Trailing Vortices & Downwash Trailing vortices are by-product of wing lift, their influence changes the air
flow pattern.
They alter the flow direction and speed in the vicinity of the wing and tailsurfaces.
The air behind the wing is drawn downwards, and this is called downwash.
Downwash also influences the approaching air, the flow over the wing, and
causes the air to be deflected downwards as it flows past the wing.
Due to downwash, the angle of attack relative to the local airstream isreduced. This means that less lift will be generated at certain angles.
It also produces trailing vortex drag
Trailing vortices also produce a large upwash outboard of the wing tips. The
upward momentum change thus produced cancels out the downward
momentum change of the downwash.
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The End
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