Lesson 6.5 - Spanwise Flow, Vortices, Downwash

7/29/2019 Lesson 6.5 - Spanwise Flow, Vortices, Downwash

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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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Lesson 6.5 - Spanwise Flow, Vortices, Downwash