I’m going to cover aerodynamics over a series of videos as I think it’s best to get a
real understanding of the fundamentals before getting our teeth into aerodynamic effects
and tricks.
So in this video that’s what we’re going to do – we’re going to look at the building
blocks of aerodynamics.
A lot of it will – at first - seem removed from directly what you hear about happening
across the wings and body of an F1 car but these fundamental aspects will soon bring
us neatly back to racing aerodynamics.
We’re discussing how we can affect the way air moves about.
So we’re going to look at pressure – what it is, and how it moves air around and creates
forces.
We’re also going to look at a key law of physical forces and we’re going to look
at air flow.
This video will set you up for everything else we’re going to look at, aero-wise.
Yes, it’s a bit of a physics lesson, but I brought a balloon!
So it’s fun!
Stick with it, it’ll all come together.
Pressure then.
Air pressure.
What is it?
You’ve probably got an intuitive sense of what air pressure is but I’m going to explain
it in terms of what’s actually going on so hopefully you can visualise it and understand
it fully.
So, air is made up of lots and lots of molecules all flying around inside the space the air
occupies.
We can think of pressure are the ‘busyness’ of all the molecules present in any particular
volume of air . Pressure is basically the total momentum of all the little molecules
flying around.
To imagine this more clearly, let’s think of a balloon.
If we fill a balloon with air, the pressure of the air inside keeps the balloon inflated.
Why?
Well, all the particles in the gar are flinging around all over the place and they naturally
bang against the skin of the balloon [SFX].
Every time a particle hits the edge of the balloon it pushes against it with some force.
Now, multiply this force by all those millions of molecules colliding against the skin constantly
and it’s enough to hold the balloon inflated!
There are three main qualities that contribute to just how much pressure there is.
One is how many molecules there actually are flying around in the space.
More molecules means the air is much more busy and there are more collisions.
We also have to think about how much space the molecules have to move around in.
The air needed to hold a balloon inflated will certainly not work on something the size
of a blimp!
The number of molecules per volume is key here – we’re asking how densely packed
is the air.
Thirdly, pressure is affected by the temperature of the air.
Temperature actually refers to how much energy each particle in the air has.
Higher temperatures mean the particles are more energetic and they’re flying around
faster.
In our balloon, this means they’ll be slamming against the inside with MORE force with each
collision if the air inside is hot.
Aha, you say – but there is also air on the outside with its particles banging against
the outside of the balloon.
Why don’t these particles push the balloon in, deflating it?
The answer is of course that the air inside the balloon must be of higher pressure than
outside.
The forces pushing the balloon outward overpower the forces pushing the balloon inward.
If the forces inside are significantly greater, the balloon inflates.
Now we are starting to see how forces can be generated when air is at different pressures.
So now let’s talk about what happens when we open the neck of the balloon.
All the air rushes out, right?
Bearing in mind what we just discussed, it’s easy to see why – the air pressure inside
the balloon is stronger so the air forcing itself out of the balloon is stronger than
the air trying to force itself into the balloon.
The result?
The air rushes out.
In fact, the air rushes out really fast because its forced through a tight gap.
There’s a principal of air flow called the Venturi effect that notes the smaller a region
you force air to flow through, the faster the air will travel because you’re still
forcing exactly the same amount through – it just doesn’t have as much space so it HAS
to go faster.
That’s why squeezing the end of a hose makes all the water squirt out much faster – you’re
forcing it through a tighter space.
As a quick aside – in the balloon case there is an extra element at play here.
The balloon is elastic so there is a natural tendancy for the stretchy skin of the balloon
to want to deflate.
But, even if the air wasn’t in a balloon but in something more robust like a can, if
we opened a hole between the high pressure inside and the low pressure outside, the air
would all rush out.
In fact, I just described how aerosol cans work.
So, now we’ve just described how air flows from high pressure areas to low pressure areas
if there’s a space for it to flow through.
We’ve just described air movement – air movement is aerodynamics.
Moving air deliberately is all about creating differences in pressure.
If we want to blow a candle out you create a sudden high pressure area inside your mouth
which rushes out through the small gap you make in your pursed lips to the low pressure
outside of your mouth.
The result: fast, controlled air flow.
Congratulations – you’re an aerodynamicist.
Now, before we move on, I’m going to make sure we’re all on board with Newton’s
Third Law of motion.
Some of you will know what this already.
If not – it’s very simple.
It’s the one that goes, “every action has an equal and opposite reaction”.
But what does this mean in the real world?
It means if you push against something, it pushes back with equal measure.
If I shove a Sumo wrestler, I get pushed backwards, too.
If a car crashes into a barrier, the barrier pushes car on the car and sends it ricocheting
back onto the track.
If a bird pushes its wings down on the air, the air pushes back on the bird, causing it
to stay in the air instead of falling.
In the balloon, the particle pushed against the balloon skin, pushing it outside.
The balloon skin also pushed back on the particle, which is why the particle bounces off.
Now, this may seem obvious, you may be saying.
Why haven’t I seen an F1 car yet in this video, you may be asking.
OK, well here we go.
Here’s your F1 car.
Now, let’s look at the rear wing.
The rear wing’s job is to force air upwards.
But if the wing forces air upwards there must be an equal and opposite reaction, right?
When the wing forces the air upward, the air forces the wing downwards.
And that’s where downforce comes from.
It’s really that simple.
Shove a load of air up and the air will shove you down.
OK, but how are we shoving the air upwards?
How does the wing achieve this?
Let’s bring this all together.
The wing profile is shaped a bit like this.
Obviously in real life it’s in three dimensions and doing all sorts of clever things but in
simple terms, it looks like this.
Now at high speeds in an ideal world, the air is going to come rushing at it in pretty
much a homogenous straight line.
In fact, we can think of it as these nice layer of air.
When the air hits the wing, some of if tucks over the wing.
These layers of air will scoot along following the curved surface of the top of the wing
and exit out the back.
The rest of the air that hits the wing will go underneath the wing.
This air has to take a much more aggressive approach to get under the wing and you’ll
see here it gets squeezed between the wing and the wall of fast moving air below.
And, as we discussed, squeezing air into a tighter space makes it go faster, so this
air accelerates and travels under the wing at a much higher speed, following the curve
of the wing and exiting at the back tip.
So two things are happening here.
One: the air under the wing is going much fasters than the air over the wing.
This means the air under the wing is a lower pressure.
Why?
Well let’s imagine particles again for a second.
When the air is accelerated through the squeeze the particles separate, much like the concertina
effect you see with a train of cars leaving a slow corner.
The faster moving air is more spread out and – as we covered – having less air in a
space means that air has lower pressure.
So with low pressure under the wing and high pressure over the wing we know – like with
the balloon, the air above will be pushing harder on the wing than the air below so the
overall force will be downwards.
Secondly – and more importantly – look at the direction the air is going after it
leaves the wing.
It’s going more upwards than when it met the wing.
The wing has forced the air upwards so, in response (a la Newton’s 3rd Law), the air
had forced the wing downward – a downward force pushing on the wing.
Downforce.
Naturally, of course if you tilt the wing further against the air stream (at an angle
referred to as the ‘angle of attack’, you’ll force the air further upwards and
get a greater downforce… to a point.
We’ll get onto that next time.
These then, are the basics of fast flowing aerodynamics.
But as you know that’s not the whole story.
We haven’t yet looked at WHY the air flow follows the shape of the wing… and when
it doesn’t.
This will lead into how wings can be inefficient, how air can stall and where drag comes from.
What is turbulence?
And what are these vortices that you hear so much about?
How do they arise and how can they be in any way useful if we want nice smooth airflow?
In our next video we’ll go a little more advanced and look at more complicated airflow.