Saturday, 11 February 2012

Lift theory queried

This blog is for the more technical of you!

An article was published debunking current lift theory.

Here is one of our pilot's reply to it:

I'm really struggling to accept this theory that the Bernoulli effect has no role in creating lift. It is true that two adjacent particles of air at the leading edge of the wing probably will not reach the trailing edge of the wing at the same time however, the fact that as air accelerates the pressure drops is true. There are many complicating factors to airflow patterns as air moves over lifting surfaces. One such effect of air not "meeting" is the creation of vortices; the cause of wake turbulence.

There are several interacting effects including pressure distribution across the wing leading to span wise flow. The theory put forward in the video does not, in my opinion, undermine the Bernoulli Theorem. The problem has always been keeping explanations simple for most to understand. As in all things, life is not so simple.
As I have always said there are two effects at work in lifting bodies. One is the Bernoulli effect and the other is Newton's Law of action and reaction.
This is potentially complicated and we should continue to keep things simple. For a hundred years pilots and aeronautical engineers have understood how wings work. That doesn't mean I don't accept there are alternative theories out there but the fact that innovation and wing design has evolved using current theory must mean there is some truth in it.

Take care,

Paul Tizzard

1 comment:

  1. 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.