How do aircraft fly?

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Marco Ocariza

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How do aircraft fly?
« on: 11/04/2010 11:30:03 »
Marco Ocariza  asked the Naked Scientists:
Hey Chris I'm Marco!

I just wanted to ask you how do aircrafts fly? And why do they stall when their wings reach a certain angle?

This interests me a lot and it would be great if you could answer it!


What do you think?
« Last Edit: 11/04/2010 11:30:03 by _system »


Offline LeeE

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How do aircraft fly?
« Reply #1 on: 11/04/2010 17:16:29 »
When any (nonabsorbent) body moves through the air it must displace the air in front of it i.e. that air must move out of the way of the body, and it does so by flowing around the body.

Although the volume of air through which the body moves isn't actually stationary, in an absolute sense, because of wind and the kinetic motion of the air molecules due to the temperature of the air, we can treat it as though it is stationary with respect to an aircraft wing because of the differences in scale: a wind will affect the entire volume of air whilst the kinetic motion of the air molecules is so small compared with the size of the wing that it's negligible.

Therefore, as we can regard the air as stationary, before it is displaced by the body moving through it, then because some of the air has to be moved from where it originally was, that moved air has had to be accelerated i.e. it wasn't moving until the moving body forced the air to move too.

Now when a sub-region of air (the air displaced by the moving body) is accelerated within a larger body of air (the larger volume of air through which the moving body is passing) the accelerated air ends up at a lower pressure than the surrounding air due to Bernoulli's principle (see's_principle).

If the shape of the moving body is symmetrical e.g. a sphere, then the flow around it will be symmetrical and the resulting regions of low pressure will be equally symmetrical, but if the shape of the body passing through the air is asymmetrical then the regions of relative low pressure will not be symmetrical and this will result in a net force acting upon the body due to the other regions of relative high pressure trying to equalise the pressure.

Aircraft wings are designed to present just such an asymmetric shape to the air as they pass through it and this can be achieved in two main ways: either the cross section of the wing, known as an aerofoil, can be intrinsically asymmetric i.e. it is 'thicker' on the upper side of the centerline than it is on the lower side, and which is known as the camber of the aerofoil, or a symmetrical aerofoil can be presented to the air at an oblique angle, known as the Angle of Attack, or AoA.  In practice, and for nearly all civil aircraft, a combination of both factors is used, with an asymmetric aerofoil being presented to the air at a slightly oblique angle.

An aircraft wing then, works by presenting an asymmetric shape to the air that it is passing though, such that a region of low pressure is formed above the wing, with the result that the wing is lifted by the relatively higher pressure beneath it.

Now although we've been talking about the air flowing around a body as the body passes through the air, this flow isn't perfectly smooth all the way around the body, and especially beyond the point where the cross section of the body starts to decrease and where the air must flow back to fill the empty space left by the body as it moves onwards.  Where the air is forced outwards around the front of the moving body the airflow is smooth because it cannot flow into the body and must closely follow the shape of the body and flow around it but where the air is flowing back to fill an 'empty' space it can flow more chaotically as it is only being 'pushed' back into place by the surrounding air and not by a solid object.  The result is that the airflow becomes turbulent and 'detaches' from the surface of the aerofoil, no longer following its shape, and with 'micro' regions of both high and low pressure, at which point it stops producing lift.

The point along the aerofoil at which the airflow detaches and becomes turbulent depends upon a number of factors, including the shape of the aerofoil and its speed through the air, but the most important one with regard to stalling is the Angle of Attack.  In general, increasing the AoA results in an increase in lift, but as the AoA increases it also means that the air flowing over the top of the wing has further to travel when it needs to flow back after it's reached its maximum displacement, because the upper surface of the wing is now sloping downwards and away from it more steeply, because of the increased AoA, and this means that the point at which the airflow becomes turbulent moves forwards towards the front of the aerofoil.

Eventually a point is reached where the AoA is so steep that the airflow detaches from the upper surface almost immediately, with the result that the wing stops producing any lift at all, which is where you have a stall.
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