Aerodynamics Fundamentals: What Causes Downforce?

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What Causes Downforce?

05.27

00:00	Now we've established the relationship between pressure and velocity, we can begin to discuss how downforce is generated.
00:07	At its core, downforce is generated by a difference in pressure across the two sides of a surface.
00:13	To conceptualize this, let's consider a flat plate.
00:16	On Earth, we have an ambient pressure in the air all around us as a result of gravity pulling the atmosphere down onto the surface of the Earth.
00:24	This pressure varies depending on climatic conditions, but at sea level, it's roughly 101,000 pascals.
00:31	And at higher altitudes, it will drop down to around 70,000 pascals at 3,000 meters and get less again as we go higher.
00:39	Considering our flat plate, this ambient pressure will be applied equally on all sides of the plate.
00:45	Pressure is essentially defined as a force over an area.
00:50	Pascals are just newtons per meter squared.
00:52	So.
00:53	So if our plate had an area of 0.1 m2, and the ambient pressure is 100,000 pascals, then we'll have 10,000 newtons, or almost 1 ton of force, pressing down on its top surface, and 10,000 newtons pressing up on the bottom surface.
01:09	These pressures cancel out, so there's no downforce or lift on our plate, it just sits in space with its forces balanced.
01:16	If we were to create a pressure condition such that we had 1,000 pascals lower pressure on the bottom surface than the top surface, the force on the top surface would remain at 10,000 newtons, but the bottom pressure would drop to 99 ,000 pascals, so our bottom force would become 9,900 newtons.
01:35	So we have 10,000 newtons on the top pressing down, and 9,900 newtons on the bottom pressing up, which gives us a net force of 100 newtons pressing down on our plate.
01:46	This demonstrates how we can create force through our body through our pressure differential.
01:51	So clearly, our aim is to maximise downforce, and to do that, we can create the maximum amount of pressure differential on any horizontal or near -horizontal surface.
02:01	Now the reason I say horizontal or near -horizontal surface is that the direction of the flattest part of the surface is important.
02:08	Pressure can only act perpendicular to a surface.
02:11	If our surface is getting towards vertical, let's call it 45 degrees, and we reduce the pressure on the lower surface, then our force vector will be tilted.
02:21	45 degrees rearwards.
02:23	This means that the amount of this pressure differential going into downforce will be reduced, and the amount going into drag will be increased.
02:31	So to maximise downforce, we want to maximise our pressure differential in the most horizontal part of our surfaces.
02:39	Now when we talk about generating pressure differential and dropping pressure on the underside of a surface, as aerodynamicists we would typically like to use the term suction, as it's relatable.
02:51	But there's a difference as to what we're doing.
02:52	So on a rear wing on a car for example, we'll call the underside the suction surface, and the top side the pressure surface.
02:59	Of course there's ambient pressure being applied to all sides of the body, but we tend to speak with respect to the ambient pressure, not absolute zero pressure as it just makes sense in this application.
03:10	So how do we generate the pressure differential in the real world? Essentially we create a velocity differential, and rely on Bernoulli's principle to lower the pressure.
03:21	So if we have a wing, we want the air to be travelling faster underneath it than over the top, in order to reduce the pressure under the wing versus over the top of it, and thus create downforce on the wing.
03:33	So how do we create this velocity difference? To be honest, the in-depth theory around wings is quite complicated.
03:40	But the most straightforward explanation is that the curvature and angle of the wing, combined with a sharp trailing edge, results in the velocity increasing on the suction side of the wing with respect to the pressure side.
03:51	Where this velocity is highest and the suction is greatest will typically be decided by how much curvature we have on a wing, and where it's located.
03:58	The regions of greatest curvature will typically have the most suction.
04:03	Another example to consider here is the underfloor of a car with a diffuser.
04:07	This is more similar to the pipe example we looked at in the earlier Bernoulli's principle module.
04:12	The underside of our car with a diffuser is just like the pipe we talked about, with a contraction under the car.
04:19	The volume behind the car is much larger than underneath the car.
04:23	So we have that expansion, and as a result, the velocity under the car will end up being much higher than the velocity behind the car, if we assume no significant changes to mass flow.
04:33	This means that the pressure underneath the car is lower than the pressure behind the car, as the velocity is higher.
04:40	The pressure behind the car is essentially ambient pressure, so the pressure below the car is below ambient, so we have suction.
04:48	And this is essentially the mechanism that we use to generate downforce on most of our car.
04:52	With that covered, let's summarise what we've learnt in this module.
04:56	Downforce is generated by creating a pressure differential across a surface, where the pressure on the underside is lower than on the top.
05:04	This pressure difference, achieved through increased airflow velocity under the surface following Bernoulli's principle, results in a net force pressing down.
05:12	In the case of the car, the curvature and angle of the wings, or the expansion underford diffuser, create velocity differentials that reduce the pressure beneath the car, generating downforce to improve performance.