Video #2: How Lift Is Created

Had a few people email me about the last lift video (you can see the old lift video on YouTube here), so I thought I would redo it. What service at profpilot.co.uk.

If you have seen the other one already, it doesn’t matter – this one is completely new (with a couple of old pictures sprinkled in for good measure).


Script (with weird spellings and lots of comma’s to make the helpful American gentleman narrator speak properly!)

Hello! Welcome to number two of the profpilot dot co dot ewe kay flight training video series. This video will be looking at lift, and how it is created.

Lift is quite an important force when it comes to aeroplanes, otherwise they would just be some very inefficient, people moving, ground scrapey tubes. Getting stuck behind one in a traffic jam would be a particularly jolting experience. {{Pause=2}}

Despite 100 years of aviation, there is still no concrete explination that everyone agrees on as to why a wing actually generates lift. When you are next in an aeroplane, keep in the back of your mind the fact that no one really knows how the hell this big heavy metal thing actually gets off the ground. Perhaps it’s just god.

On the fairly safe Assumption that it isn’t god, this video will be looking at how a wing creates lift, based on what I was taught for my European Airline Transport Pilot Licence Theory Exams. This focussed on two things, conservation of mass, and the work of two men, Sir Isaac Newton, and Daniel Bernoulli.

Let’s start by looking at conservation of mass. a. {{Pause=3}} Don’t worry, I’m not about to go all hardcore physics on your ass. Basically this is saying that in a steady state process, what goes in, must come out, at the same rate.

This means that if I have a sealed wind tunnel, like this wonderfully illustrated example, and I start sending through some air at the front, then the same amount of air must exit through the back, at the same rate as the front. Well that’s pretty obvious isn’t it. But what happens if a fat man sits on my wind tunnel?

The air coming out of the wind tunnel must still match the rate that the air is entering it. However, there is now an obesity induced dent in the middle, and this changes proceedings somewhat. Lets plonk in some lines to represent the airflow. As you can see, the air here, now needs to converge, and because there is less space, it must speed up going into this gastric band to ensure the same amount of air is going through the tunnel. Once through, it can slow down again, until finally the speed of the air at the end of the tunnel matches the speed of the air at the entrance.

That’s all well and good, but what has all that got to do with a wing? Well, here is a wing, and here are the streamlines around the wing. Within these streamlines are stream tubes. You can think of these streamlines and stream tubes as being little windtunnels like before, as air cannot move from one stream tube into another.

These are the two stream tubes we are interested in, the upper and the lower. The upper stream tube makes a great voyage along the top of the wing, whereas the lower stream tube undertakes it’s magnificent expedition along the bottom. You can see that the upper stream tube is constricted at the leading edge of the wing, and you remember what happens when there is a narrowed part of a windtunnel, thanks to the kindly obesity gentleman. The air speeds up.

Along the bottom, the stream tube actually opens out a little, and the air will actually slow down. So we now know that a wing has faster moving air on top of it, and slower moving air underneath it. How does that create lift though?

Enter again, the fascinating team that is Newton and Bernoulli. What a party.

Bernoulli first. Here is bernoulli’s formula. a. {{Pause=4}} You will be pleased to know simplicity ensues. He is basically saying than when a fluid is sped up, the static pressure is lower, when compared to a slower moving fluid, when it’s static pressure will be higher. In this case, the fluid in question is air. Since it is going faster over the top of the wing, a patch of low pressure exists here.

Couple this with the higher pressure air under the wing, and you have this situation. Since air always wants to move into the low pressure zone, this causes the wing to be sucked upwards, and we get the wonderment of lift.

Which brings us onto newtons contribution to proceedings. Newton said with his third law of motion that for every action, there is an equal and opposite reaction. If we have a look at the aerofoil when it is still, and plonk what the air looks like when there is no movement, you can see very little excitement follows.

If however, we introduce some movement between the aerofoil and the air, then you can see that in front of the wing there is some upwash, and behind the wing there is downwash of the air. I won’t go into why, but it involves violent vortex eey whirlwinds around the wing. While the upwash cancels out some of the downwash, the net effect is an overall downwards deflection of the air.

Again, every action has an equal and opposite reaction, and this air that is forced downwards by the wing, will in turn force the wing upwards. This also adds to the lift.

I certainly hope that was not too confusing for you all. For more written articles on flight training, and videos, check out w w w dot prof pilot dot co dot ewe kay.

Cheers!

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One Response to “Video #2: How Lift Is Created”

  1. Sheikh akil says:

    Very good work

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