You’d think, after more than 100 years of powered flight, and many more years than that if you count human experiments with gliders, that there would be no debate about exactly how an aircraft wing works. Yet the debate continues!

Before you get all freaked-out, let me reassure you… nobody believes that it is “magic”, or that the wing only works properly when OTHER people fly. The wing is a well-proven device that works EVERY TIME, and it is very well understood from a practical “how to use it” standpoint.

The debate that is still occurring is how best to describe the physical processes behind what is going on. You may remember from High School that a guy called Bernoulli has something to do with it… His observation was that a liquid (or gas) moving faster exhibits less pressure. So, the Bernoulli fans would say, it is air moving faster over the top of a wing that causes a low pressure above, hence causing lift.

Others would say that it is because the wing deflects air downwards, thereby pushing the aircraft up into the sky. Like I said, there are different views.

This topic was raised by Paul Tizzard from the excellent Virgin Flying Without Fear blog, when he highlights some recent research into how the wing works which questions they way it has been taught for decades.  Part of the Bernoulli explanation has included the idea that the air moving over the top of the wing has to meet up with the travelling below the wing and, because of the shape of the wing it must therefore move faster (and, therefore, cause lower pressure).  This is the part of the traditional wisdom that has recently been proven to be incorrect…

Here’s the video from the article to which Paul refers…

[youtube]http://www.youtube.com/watch?v=UqBmdZ-BNig[/youtube]

And here is the comment I made on Paul’s blog to his post on this topic…

The interesting thing, I think, is what you do with the knowledge.

There was a fabulous book written in 1933 called “Stick and Rudder” for people learning to fly who wanted to REALLY understand what they were doing, in which the author says the wing simply deflects air downwards, and it is the down-push on the air that gives you the lift (Newton’s Equal and Opposite Force law).

I like that explanation as it helps a student pilot understand that you get more lift by increasing the ANGLE of the wing (thereby deflecting the air MORE DOWNWARD), or you increase the SPEED of the wing (thereby deflecting MORE AIR downward). That’s exactly the way it works in real life.

Irrespective of all of this Bernoulli business and arguments about speed of airflow over the wing and differential pressure above and below, you still end up pushing the air downwards. And Sir Isaac would insist that you have to get an “up-push” because of it.

The downwards movement of air behind a wing is called “downwash”, and it is usually overlooked in any discussion of how the wing works… but it ALWAYS occurs. It may well be that the fancy manipulation of air above the wing has the effect of drawing air more above the wing in a downward direction, rather than just pushing the air below the wing down, thereby increasing the volume of air that the wing pushes down.

The downwash does, occasionally, have an effect that you as a passenger may notice. Air descends behind and below the aircraft at approximately 500 to 1000 feet per minute… sometimes, if you are flying in the opposite direction 1000 feet below another airliner going the other way you will feel a “bump” about a minute later as you pass through this descending (churned up) air.

This is called “wake turbulence” and, like all turbulence, it is no problem at all. It can be surprising to you as a passenger, however, because you don’t see any reason for it (YOU don’t know that another airplane has passed above, but your pilots do).

How the wing works is a fascinating topic (to me at least!), and it is interesting that people are still arguing about the physics of the wing.

That’s not to say it is a mystery… rather that there are several differing viewpoints (all of which lead pilots to the same well-proven correct decisions).

What NO ONE argues about is that it does work, nor is there any argument about HOW to fly the wing — beautifully described in 1933.

 

References:

Dr Holger Babinsky – Cambridge University – Blog Post “How Wings Really Work”
and a paper he published in 2003 on the topic: How Wings Really Work (PDF)