If you found my lift article 'What Goes Up' in the December 2010 issue of RCM&E interesting then here are a few videos you might enjoy. They demonstrate various things to do with the Bernoulli effect which we discussed and many of which you could try yourself at home. Watching them will, I hope, help build a firmer picture in your mind of what is happening to a wing in flight. I’ve written a few notes to go with each video pointing out features to look for. I hope you enjoy them.
This is a very basic demonstration of the Bernoulli effect. The ball is attracted to the flowing water because the pressure in the moving fluid is lower than that in the stationary air, just as it is lower on the top of the wing than the bottom.
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This is a classical aerodynamics lecture’s party piece! Take one vacuum cleaner, reverse the motor (or the fan) so it blows instead of sucking. Start the blower up and place a ball so that its centre is just below the jet and let go. Hey presto magic levitation!
The faster moving air over the top of the ball is lower in pressure than the stationary air below – and so the ball experiences a pressure difference between top and bottom and so experiences lift. In case you are wondering the ball spins largely due to the friction force of the air flowing over the top, as there is no air flowing over the bottom the result is an overall torque on the ball and it spins. This is side effect and is nothing to do with Bernoulli. While it is impressive with a large beach ball (as shown in this video) I think its even better with a table tennis ball because you can stabilise the ball a long way out from the air jet and get some surprising large angles!
This is one of my favourites and a stunt you can win a few wagers with at your next club meeting. It's a two-stage trick; firstly, “How can you put the coin in the cup without touching it?” Easy, blow over the top of it! This lowers the pressure above the coin (Bernoulli effect), up it lifts and then the air current blows it in. Remember you have to blow over the top of the coin – not at it – or it wont work! There is now a nice follow-up that works in this way…once you’ve taken their wager over whether you could do it at all – bet double or quits that they still can’t do it even though they have seen you do it. The odds are they won't be able to because they will assume that you did it by blowing at the coin which of course you didn’t, you blew over the top of it.
This is the only way the trick will work. If you just blow directly at the coin it wont lift from the table it will just slide along and you will not be able to blow it into the cup which you will notice is in a slightly raised position. Could keep you in lemonade shandies all night…..
This is a wind tunnel video made by Cambridge University showing flow over a wing section. The streamlines are made visible by injecting thin streams of smoke into the flow. At the beginning of the video we see the wing section as it would be in normal flight with the air going above and below it. There then follows a clever demonstration showing that the air going over the top of the wing speeds up compared to that going below.
This is done by pulsing the smoke flow – producing short bullets of smoke. This is first shown at full speed then slowed down. It can be clearly seen that although the bullets approach the wing vertically aligned, as they move passed the wing the ones on the top gradually get in front of those below. By the time the smoke bullets reach the trailing edge of the wing the ones that have travelled over the top are well ahead showing that they have travelled faster.
The video then goes on to show us what happens as the angle of the wing to the airflow is increased. At first the flow continues around the wing. The path length difference between the top and the bottom gets bigger and bigger and so the lift increases. This goes on until, all of a sudden, the flow separates off the top surface when it can no longer make the highly cured trip.
This is a classical stall. At this point all the low pressure lift from the top surface of the wing instantly disappears and our model falls out of the air. Notice that at this point the high pressure flow over the bottom of the wing is still intact, but this would not be enough to stop the bottom falling out of world at this point! Also notice that the stall is not a gradual effect – the wing goes from maximum lift to stall in an instant. Its the suddenness of the stall which is one of its dangers.
This is a very old American film and yes it is a bit corny – but if you can get past the beginning it does contain some interesting explanations and relates to what happens in the wind tunnel to the full size aircraft in flight very well. It also touches on some issues we will discuss more in more detail in the January issue of RCM&E. There is actually one point in which they over-simplify things a bit too much which leads to a point where they say something that isn’t true. For advanced students…see if you can spot what it is!
In Pt.2 of 'What goes up' in the Jan 2011 issue of RCM&E, I'll be looking at the various factors affecting lift and hope you'll join me and, in the meantime, if you have any comments or thoughts then feel free to join me in the feedback forum thread (link below).
