Aircraft lift explained – by an Oxford Professor

Professor Holger Babinsky

Professor Holger Babinsky

Do you feel there’s something UNNATURAL about a giant plane being held up by thin air?

If so, you owe it to yourself to discover the very real – and very powerful – force that makes it happen.

I’m talking about ‘lift’.

How can you benefit by having aircraft lift explained to you?

Because if you feel there’s something mysterious about how planes stay in the air, you’ll ALWAYS find air travel Hellish.

Luckily, this interview contains EVERYTHING you need to know about aircraft lift.

And it features a leading expert in lift, Cambridge University’s Professor Holger Babinsky.

As you’ll discover, Holger doesn’t just know his stuff. He’s a GREAT communicator, too.

Amongst other things, Holger explains:

  • What ‘lift’ actually is.
  • How it’s generated.
  • How different wing shapes create different kinds of lift.
  • Why most explanations of lift only tell part of the story.
  • And LOADS more.

To make this stuff EASY for you to understand, Holger refers to the diagrams below. So keep this page open while you listen (or read the interview transcript below).

Play

Explanatory diagrams

Aircraft lift explained

Figure 1

 

Aircraft lift explained

Figure 2

 

AIrcraft lift explained

Figure 3

 

Aircraft lift explained

Figure 4

 

Aircraft lift explained

Streamlines on airfoil

Interview transcript

[STARTS]

Tim Benjamin: If you have a fear of flying, I’m sure you’ve spent loads of time wondering how on Earth a massive aircraft gets into the air – then happily stays there.

The answer is a thing called ‘lift’.

And today, you’re going to find out EXACTLY how it works.

Hi – I’m Tim Benjamin with the Fear of Flying School podcast.

And joining me on the show is Cambridge University’s Professor Holger Babinsky.

Holger is Professor of Aerodynamics at the University’s Department of Engineering.

Holger – welcome.

Holger Babinsky: Hi – Good morning Tim.

Tim Benjamin: Now – before we get started – can you just tell me a little about your area of expertise?

Holger Babinsky: Yes – no problem Tim.

I’m Professor of Aerodynamics – and that means I deal with the science of air in motion.

So – I’m really interested in ANYTHING where air flows around an object.

And because we’re in an engineering department, obviously I’m more interested in the technical aspects of it.

So, the kind of things I work on are aeroplanes – both high speed – like military jets – but also low speed – or typical transport aircraft.

But I also do quite a lot on cars – Formula One cars – trucks.

And I have an interest in things like flapping wings for what we call ‘micro air vehicles’ which are tiny little flapping wing airplanes.

There’s a lot of fundamental science that we’re still trying to understand properly.

And then there is, of course, related areas like wind energy – and even water turbines.

Tim Benjamin: So – a pretty broad range of areas?

Holger Babinsky: Yes – I would think so.

Tim Benjamin: Great – now before we get started, I should point out to anyone who’s listening that you’re going to refer to images throughout our conversation which help explain the ideas that you’re going to be talking about.

Just a message to anyone who’s listening to this: to see those images that Holger’s going to refer to, simply visit ‘fearofflyingschool.com/lift’.

That’s ‘fearofflyingschool.com/lift’.

OK – now with that out the way – Holger can you tell me what does the concept of ‘lift’ ACTUALLY refer to?

Holger Babinsky: Well – we know that if you pick up an object – and you let go – then it drops back to the ground.

And that’s because gravity acts on it.

So, if you want something to fly, then you need to hold it up.

We can do that with our hands – but if you want something to FLY in the air, you need a force that counteracts gravity.

And that’s what we call ‘lift force’.

And that’s what you typically need on aircraft.

Tim Benjamin: OK – well it sounds simple enough.

But how is lift actually generated?

Holger Babinsky: Well – here we’re talking about typical aircraft, so we know that lift is generated by wings.

And, so, you need wings to generate lift.

But wings – alone – isn’t enough.

Because an aircraft sits on the ground – it has wings – it still doesn’t produce any lift.

The other KEY component – for what we call a ‘fixed-wing’ aircraft – is a forward velocity.

So those wings actually need to move forwards through the air in order to create lift.

In contrast, a helicopter – for example – also has wings.

These are the big blades that you have on top.

And it doesn’t need a forward velocity because you make those wings move through the air.

So, what we’re talking about today, really, is how do wings – things that are shaped like wings – generate lift when there is a relative motion of air across the wing.

So, the forward speed of an aircraft, for example.

Tim Benjamin: What does that mean for a commercial jet that somebody listening to this show is likely to have direct experience of?

Holger Babinsky: So in a commercial jet, you need a runway in order to accelerate to some kind of speed.

And that’s what you need the engines for.

That is, to give you that forward velocity.

And then – when you’re in flight – because air actually has friction – it doesn’t seem like it because air seems so light and when we move through the air we hardly feel that friction – but for something like an aircraft there is actually quite a lot of friction.

And so the jets produce thrust which counteracts the friction.

And they’re there to generate that forward velocity.

But the REAL magic of lift is now the SHAPE of the wing that somehow manages to produce lift when the flow is moving.

And I don’t know – should we start with some simple examples?

Tim Benjamin: Yeah – let’s go into some simple examples.

Holger Babinsky: OK – what most people probably have experienced is that you can produce lift quite easily with something very simple like a piece of cardboard.

If you take an A4 piece of cardboard and you sort of move it through the air then you find that sometimes – depending on how you angle it – you can get a force.

The same would be if you stick your hand out of the car window while you’re driving – preferably while somebody else is driving I should say here.

And watch out for lamp posts by the side of the road.

So, I’m assuming we’re in open country.

But if you put your hand out – and you hold it horizontal – you will not really feel much of a force.

But if you now start to angle it so that the forward points upwards – that’s what we call an ‘angle of attack’ – then you will feel a lift force on your hand.

And so, in terms of my sketches that I’ve sent you – and that people can find on the website – if we look at Figure 1 – the first sketch – that is now a flat plate that is aligned with the flow direction.

So, this would be your piece of cardboard that’s perfectly horizontal – and you’re moving it horizontally through the flow.

Or your hand’s sticking out of the car that’s also horizontal.

And there, we don’t have any lift.

That kind of makes sense, really.

But if you now angle it upwards – in a way I’ve drawn it in my second sketch – so this is now a flat plate at an angle of attack – then suddenly we notice there is a lift force.

So, what you see on Figure 2 is that the air is moving from left to right relative to our wing at angle of attack.

And there’s obviously a large number of streamlines.

I’ve just picked out two: one that goes UNDERNEATH the plate and one that goes ABOVE the plate.

And if you were able to see it – which you rarely can because air is transparent – but if you were able to see it – then the streamlines would look roughly like how I’ve sketched them there.

So you see that the streamline on the bottom is sort of being deflected downwards.

It hits the plate and is being pushed down.

And the streamline ABOVE you also see that it doesn’t go straight.

It’s sort of slightly curved.

It’s being pulled back towards the direction that the flat plate is giving it.

Really what I want to highlight now is really the comparison between the flat plate at ZERO degrees angle of attack – which is the first sketch – and at a POSITIVE angle of attack – which is the second sketch.

Tim Benjamin: So, comparing Figures 1 and 2?

Holger Babinsky: That’s right – yeah.

And so somewhere between those two figures is really the secret of lift.

Because the first sketch – Figure 1 – DOESN’T have lift – there is no lift here.

But in Figure 2, there IS.

In fact, I’m going to throw the question back: what do you think is the obvious – the biggest difference between those two flow scenarios?

Tim Benjamin: OK – I think the difference is that in Figure number 1, the streamline at the base is essentially unaffected by the plate, whereas in Figure 2, it’s actually hugging the base of the plate.

Holger Babinsky: Yes – that is sort of really the key point.

And when you say ‘unaffected’ – if I put words in your mouth – in Figure 1 the streamline is straight.

The flow continues as if there were no plate there.

That exactly as you said – it’s unaffected by the presence of the wing.

But in Figure 2, the wing is making the flow change direction.

Notice how the streamline is curved – particularly near the ‘leading edge’ – that’s the front part of the wing.

And whenever an object – air has mass – just like any object, air is not mass-less.

And we know from Newton that when any object in motion changes direction, then there must be a force involved.

And what we see here is that the air is changing direction.

It is being pushed downwards.

And, now, how do we push air?

What is it that makes air experience a force that’s really – primarily – due to air pressure?

And what we see here – if we just look at the lower surface streamline – so that’s the lower of the two lines – it is being pushed downwards – it is being deflected downwards.

And that is done by the airflow – by the flat plate – the wing at an angle of attack.

And if it is pushing the streamline downwards, it means there must be a region of high pressure – just on the surface of that flat plate – that is pushing the air downwards.

And then – according to one of Newton’s other laws – for every action there’s a REACTION.

That – pushing the air downwards – also – in return – pushes the plate UPWARDS.

Does that make sense?

Tim Benjamin: It makes complete sense.

Holger Babinsky: Good.

So, we can see already – in this picture there – that on the lower surface there must be a region of high pressure.

And that’s caused because the air is being deflected downwards.

Now, that alone isn’t enough to give us lift.

Because if there were the same pressure on the other side of the plate, there would be no net force.

So, if we now look at the upper streamline, you see that it is LESS effected by the presence of the plate.

It’s a little bit more straight.

And that’s pretty real for this kind of flow.

But, if we look carefully, you see it still is a little bit curved.

You notice that the airflow changes direction slightly.

It also changes direction a little bit downwards.

It starts off pointing a little bit upwards – that’s quite real.

But then it seems that the presence of the wing is also affecting THAT streamline.

And it is pulling it DOWNWARDS.

It’s pulling it towards the surface.

And I’ve deliberately used the word ‘pull’ here because it’s the opposite of ‘push’.

So, what’s really happening there is that now – in the area ABOVE the plate – there is a region of LOW pressure.

And that low pressure is sucking the air towards it.

And – again – if you’re pulling the air DOWN – the OPPOSITE reaction is something is pulling the plate UP.

And that’s ALSO a force upwards.

Or – if we think in terms of pressures – we have a HIGH pressure on the LOWER surface of the wing.

And a LOWER pressure on the UPPER surface.

And the net result of that is now there is a lift force here.

Tim Benjamin: And – to be clear – the lift force is operating on both sides of the plate: both the underside as well as the top side?

Holger Babinsky: Absolutely right.

And, in fact, you can now – if you start looking at flows like this – you can actually become a little bit more of an expert.

Because – which streamline is more affected?

The top streamline or the bottom streamline?

Tim Benjamin: There is more lift activity taking place ABOVE the wing than below – correct?

Professor Holger Babinsky: Ah – you see.

Well – I’m afraid you’re wrong [laughs].

Tim Benjamin: You’ve destroyed my credibility in one shot.

Holger Babinsky: I have!

In this particular example – if you just look at those two streamlines – I think you would hopefully agree with me that the rapid change in direction is much more pronounced for the lower surface streamline than for the upper surface streamline.

Would you agree with that?

Tim Benjamin: I do agree with that.

Holger Babinsky: Yeah – and so a flat plate actually generates a little bit more lift from the lower surface.

It’s a subtle thing.

It depends on the circumstances but there is more high pressure on the lower surface pushing the streamline down.

The upper surface, on this flat plate, isn’t actually working terribly well.

You see, it’s pulling the streamline down.

But that contribution to lift is not very large.

And that, to me, would be the perfect cue to move on to Figure 3 – the third picture.

What I’ve done there is I’ve sketched a dashed streamline.

And that streamline is one that is much more affected by the flat plate.

Do you see that?

Tim Benjamin: I see that – yep.

Holger Babinsky: And so, now, that streamline – that’s what it would look like if the upper surface was contributing just as much to lift as the lower surface.

Because you now notice that the dashed streamline has just about as much of a kink in it – or a change of direction – as the one on the lower surface.

Now, if we wanted a good wing – a wing that REALLY produces a lot of lift – what we’d want to do is achieve a flow that looks a little bit like the dashed streamline here on the upper surface.

And not one like the real streamline like the one I’ve drawn earlier – which doesn’t want to follow the shape of the wing very well.

That’s one of the reasons why flat plates are not really used on aircraft wings.

Because they would be very easy to manufacture.

But, of course, they’re structurally not very strong.

And I see you’ve had another podcast on that subject.

But also, they’re really not very good airfoils – because you would like BOTH sides of the wing to contribute to lift.

And the upper surface – that ‘sharp leading edge’ as we call it – is just making it really hard for the flow to follow round – to bend round – to be sucked towards the airfoil in the way that the dashed line should do it.

So, how do we do that in REAL aircraft?

Well, this is where I invoke something called the ‘Coanda Effect’ which really would be the subject for another topic.

But – the Coanda Effect is the tendency of a flow to attach itself to a curved surface.

To a smoothly curved surface.

And that’s also the reason – and here’s my last aside – why teapots tend to dribble.

When you pour a cup of tea from a teapot – if you do it slowly – the water actually likes to attach itself to the – just to the very tip of your spout.

And run along the spout rather than come off and flow smoothly into the cup.

So – ah – it shows that fluids in general have this tendency to stick to a curved surface.

But not always.

It doesn’t stick to a curvature if it’s a very SHARP curvature.

If you look at my Figure 3 again – the leading edge of this flat plate is quiet sharp.

And the air that we want to follow that – it’s quite a sudden change of angle.

So, what we want to do in order to make use of the Coanda Effect is change that shape to – and this is my sketch 4.

What I’ve done now is I’ve made the leading edge on the upper surface nice and rounded.

And now, I’m making use of the Coanda Effect.

And now you might see that the streamline – instead of doing what it did before – it now attaches itself to the shape of this.

And we recognise this, of course, as an airfoil.

Tim Benjamin: I understand.

A question that immediately jumps to mind: why are we simply creating a rounded surface on the top side of the wing – not the bottom side?

Holger Babinsky: Because, as we’ve seen from the very early sketch on Figure 2 – and you can confirm that if you do your own experiments – it’s quite easy to PUSH the air out of the way because it hasn’t got anywhere else to go.

It really doesn’t have an option but to follow the bottom of the plate.

But SUCKING it towards the plate on the upper surface – that’s the hard bit.

And, so, that’s why typically we worry more about the shape of the upper surface than the lower surface.

Although I’m now belittling the efforts of airfoil designers – the lower surface is quite important, too.

But aerodynamically, the critical bits are all on the upper surfaces.

Trying to make that streamline STICK to the surface for as long as possible.

OK – so if we now move to the last picture – which is Figure 5 – which is a black and white photograph that I took from one of the wind tunnels in my lab.

And what you see there now is a REAL airfoil at a POSITIVE angle of attack.

And all the white lines are actually lines of smoke.

And I’m basically putting in little blobs of smoke – somewhere to the left of the picture.

You can’t see that.

And the way I set the camera, I get these lines that show where the air is flowing.

So, what you’re seeing here more or less is the streamlines on an airfoil.

And, now, I want you to kind of put together everything I’ve talked about.

And remember that the secret to lift is really how much these streamlines are deflected.

How much they CHANGE their direction.

And if we sort of take the closest streamline to the surface on the UPPER side, you see that – particularly near the nose – there is a HUGE change.

It’s almost going through 90 degrees.

So that’s a VERY VERY effective change of direction.

So there must be a LOT of suction involved to make the air do that.

Which gives you a lot of lift.

But even on the LOWER surface, you see the streamline sort of – first it curves up a little bit.

And it seems to hit the airfoil.

And then it is deflected downwards.

So, in that case, there is also a change of direction.

Which, in this case, is a PUSH – so it’s a HIGH pressure.

And so now you see a case where BOTH sides contribute to lift.

And now we come back to the thing you said earlier: for this picture, the change of direction of the streamlines is MUCH more pronounced on the UPPER surface than the LOWER surface.

So, here, the UPPER surface does most of the work.

Typically, you’d say about two-thirds of the lift now comes from the upper surface.

And one-third from the lower surface.

Tim Benjamin: This all makes sense to me.

But it raises a question.

I remember a number of years ago I saw a very old video of an aircraft known as the Boeing 707 which is very rarely seen around the world these days.

But when it was first launched back in the mid to late 1950s, famously at an airshow to show off the potential of the aircraft, a pilot decided to do a so-called ‘barrel-roll’ in the aircraft where he flew the 707 – very briefly – upside down.

Now, that’s not something that’s going to happen to anybody who’s listening to this program.

But what I couldn’t make sense of was – given what you’ve just told us about the shape of an aircraft wing on a commercial jet liner – how can that shape work when it’s upside down?

Holger Babinsky: If I took this very shape that you have in front of you – and now put it at a NEGATIVE angle of attack so that the nose is pointing DOWN – and the trailing edge – the back – is higher – then you will probably see that the streamlines will now be deflected UPWARDS rather than downwards.

Pretty much in the opposite way.

And that means that you’re now creating a lift force in the OPPOSITE direction.

So, in this case, DOWNWARDS.

But if you’re flying upside down, that’s EXACTLY what you want to do.

In the frame of reference of your aircraft, if you’re flying upside down – if you’re sitting in it – you now want the lift force to be in the direction of the floor of the aircraft.

Because the floor is pointing skywards.

So, it’s exactly the opposite direction to normal flight.

And most airfoils can work in opposite direction.

They just need a NEGATIVE angle of attack.

So, normally we put a POSITIVE angle of attack on – like in the picture here.

But if you make that angle of attack NEGATIVE, you can generate a force in the other direction.

Tim Benjamin: Now – when I look at this image and I think about a large aircraft – whether it’s an Airbus A380, a 747 down to an A320 or a 737.

I mean – they’re very heavy objects.

So presumably, the amount of energy – if you like – that’s being applied to these wings is pretty substantial?

Holger Babinsky: Yes – that’s a REALLY good question.

You always wonder how ENOURMOUS the force must be that keeps the wing in the air.

But it’s all done by air pressure.

As I’ve explained, we have a LOW pressure on the UPPER surface.

And a HIGH pressure on the LOWER surface.

And – actually – you might be surprised to learn that the average distance in pressure between the two sides.

Take the total weight of an A380.

And you divide it by the area of the wing – the wings are really quite large.

Then that gives you an average pressure difference between the two sides.

And that pressure difference is actually very very small.

It’s only a few percent of atmospheric pressure of normal air pressure.

So actually, when you fill up your bicycle tyres with a bike pump, you create much much larger pressures than you see on the two sides of a wing.

Tim Benjamin: Ah – the idea being that the amount of pressure inside a bicycle tyre is substantially greater than that on the OUTSIDE of the tyre – relative to the difference between the underside of a wing and the top side of a wing?

Holger Babinsky: Yes – exactly right – yes.

Tim Benjamin: And yet that small differential between the underside of the wing and the top side of the wing is substantial enough – evidently – for a large aircraft to gain the lift that it needs to get into the air and stay there?

Holger Babinsky: Exactly right.

And that’s because – ah – the force is pressure times area.

And you have such a huge area on a typical aircraft wing that you can generate ENOURMOUS forces from even relatively small differences in pressure.

Tim Benjamin: Your explanations, Holger, have been incredibly straightforward and easy to understand.

Which I thank you for – it’s been a huge learning experience for me.

But something which I’m slightly confused by is that having done some Googling around this whole issue, what I’ve noticed is there seems to be a kind of variation – if you like – in opinions expressed as to how lift works.

Can you just explain to me why it is that there is a slight variation in opinion?

Holger Babinsky: Yes – that’s a VERY good question.

Amongst us aerodynamicists, I think any aerodynamicist knows how lift works.

But we find it’s quite hard to explain.

And especially if you want to explain it properly to the point where you can make calculations.

It can get involved quite quickly.

And so, my explanation as to why there are so many WRONG explanations out there is that – for example – television programs and so on.

They usually have about 30 seconds to explain lift.

And if you just look at the time we’ve already spent – and I apologise for being that slow – it takes MUCH longer to do it properly.

But some of the explanations – particularly the one that I referred to as the ‘difference in distance’ argument.

Where people say it’s longer on the upper surface than the lower surface.

You can string that together in sort of 30-50 seconds.

Which is about as long as a television program has.

Or as along as a hassled teacher has to quickly explain something to students.

And it SOUNDS plausible.

And I think that’s the reason why it has stayed like this.

Whereas trying to make an argument about streamline curvature.

And about the subtleties of flat plates versus airfoils – and so on – just takes an awful lot longer.

Tim Benjamin: It’s possible that somebody listening to this might now start to panic and go ‘Hang on – I hope none of these slight misconceptions have found their way into the design of ACTUAL commercial airline wings.’.

That’s NOT something that’s happened – right?

Holger Babinsky: No – no.

Thank you – that’s a VERY good point to make, actually.

These misconceptions are really just attempts to EXPLAIN to the layman how lift works.

When it comes to actually calculating things on airfoils, we have a SCIENCE that is more than 100 years old.

That is very robust.

And that doesn’t really need the detail of how you explain it in order for it to work.

Tim Benjamin: Very heartening to hear.

Holger – look – this has been a really fantastic session.

I’ve learnt a huge amount.

And I’m sure anybody listening to this has.

Um – on that note – it just kind of really leaves me to say thanks very much for joining me on the Fear of Flying School podcast.

Holger Babinsky: You’re very welcome, Tim.

Tim Benjamin: And to you in the audience, just a reminder that you’ll find the images that Holger has referred to today at fearofflyingschool.com/lift.

That URL again is fearofflyingschool.com/lift.

Thanks for listening.

Bye.

[ENDS]

Got a question?

Now that you’ve listened to Holger’s explanation of lift, is there something you’re still not sure about? There’s no such thing as a dumb question – so leave me something in the comments and I’ll track down an answer for you.

And finally…

Do you know anyone who’d benefit by listening to my talk with Professor Babinsky? If so, please email them a link. It’ll be your good deed for the day 🙂  And your friend will REALLY appreciate it (as will I).

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1 comment

  1. Tim Benjamin

    Good question – I dunno (unfortunately I’m not an engineer).

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