How do planes fly?

17 July 2018

Interview with 

Dave Ansell

Plane Wing

Plane Wing

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It’s incredible that we can fly so high. In fact, how do planes get off the ground, and stay in the air? Chris Smith was joined by gadget whizz and Naked Scientists veteran Dave Ansell in the studio to tell us, or should we say, show us...

PA ANNOUNCEMENT - We’ll be cruising at an altitude of 60,000 ft and a speed of 1,000 mph. In a few moments time, we will be moving through the cabin to offer some refreshments.

Chris - Oh good. I am feeling rather thirsty. Right on cue.

Izzie -  I think some snacks have just arrived.

Chris - And a very beautiful stewardess

Stewardess - Champagne, sir? Caviar, madam?

Chris - Have you got anything better than that?

Stewardess -  No.

Chris - The thing about flying is that you have to actually know how your aeroplane works in the first place. It is amazing that aeroplanes even fly. You think of a fully laden A380, for example, there are 800 people plus on there. So we thought we would do an experiment to explain how flight actually works and how aeroplanes and wings work. So who better to ask than kitchen science veteran and actually whizz making science experiments to show how things actually work and that’s Dave Ansell. Welcome to the program Dave.

Dave - Hello!

Chris - What have you brought in?

Dave - I’ve got a lot of gadgetry here. I've got a big fan and model wings and things. Start off thinking about how planes fly is how you stay up in the first place. Everything is being pulled down by gravity and the fact you're not falling through the floor must mean there's a force pushing you upwards. And at the moment you're achieving that by applying a force downwards onto the chair and then Newton's laws mean that every action has an opposite reaction. The chair is pushing upwards with exactly the same force and holding you up so you don't fall to the center of the earth very quickly.

Chris - Right. So how does the wing of an aeroplane create that force, which then leads to the airplane being able to stay up in the air and combat gravity accelerating downwards?

Dave - So it must be pushing down on something and the only thing a plane has got to push down on is the air around it. And the part of the plane which holds it up is of course the wing. So I have a model wing here, which is made out of a bent piece of card, and we’re going to be wanting to see how the wing is affecting the air moving past it. Now to get any lift from a wing, you need the wing to be moving through the air. Now to see that while it's running around is very difficult so I’ve got a big fan which I can turn on and produce air flying past the wing.

Chris - Right. So we have a bent piece of card which is curved in the shape of a wing. You have a screwdriver with a ribbon on it, which is going to reveal where the air is going. So talk us through what will happen when we put the wing in front of this large fan. What should we be looking for?

Dave - In order to stay up, the wing should be pushing the air down, and we should to see that by the ribbon behind the screwdriver being pushed downwards.

Chris - Now I can understand how that will work with the bottom side of the wing because the wing is higher at the front than the back so air hitting the wing is going to be deflected downwards. So if you push the air downwards, it's going to push the wing upwards, as Newton's law tells us. What about the top of the wing though, does that contribute to the lift?

Dave - If you get the aerodynamics right, which is important part designing a plane, then air will tend to stick to a smoothly curving surface. So with any luck you will also get the top of the wing deflecting the air downwards and also producing lift.

Chris - So you get lift from the top of the wing and because you're pushing down with the bottom of the wing you get lift there too.

Dave - Exactly right.

Chris - Let's do the experiment then. This is noisy everyone at home so we apologize in advance. Here we go. So Dave’s turned on his large fan.

Dave - So at the moment I've just got the streamer ribbon moving in the air and it's is going horizontally. Now if I move the wing down towards it from the top you should see that that stream is being deflected downwards even though it's not actually touching the wing.

Chris - Yes, indeed. So the streamer is nowhere near the wing it's underneath the wing but the streamer is curving downwards just like the same shape as the wing. So there's obviously air being pushed downwards by the lower surface of the wing.

Dave - Exactly right. And similarly if we bring the wing upwards towards the ribbon from underneath the air starts to stick to the wing. So the air go over the top is also being deflected downwards and so pulling up the wing. So if I let go of the wing it moves upwards. It’s producing lift.

Chris - One last question for you Dave then. What about when a plane flies upside down when a stunt pilot goes upside down the plane still flies. How does he do it?

Dave - It's exactly the same principle. The plane is at an angle so the air hits the wing of the angle and you have to float you downwards so air pushes the plane upwards.

Chris - Are you saying the pilot basically has to modify his flying technique or her flying technique so the wing is still pushing downwards.

Dave - Basically the nose of the plane is pointing up a bit more if you're flying upside down than if you're flying horizontally because they're designed to fly right way up.

Chris - High angle of attack I think is the correct parlance, isn’t it?

Dave - It certainly helps.

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