Fan car: how a suction fan glues a car to the road
Switch on a fan that sucks the air out from under a sports car, and see how a small pressure difference over the car's floor gives a huge downforce, more grip and faster cornering. Then blow air in to make the car float, and fire thrusters to see when thrust beats weight and the car flies.
Core concept
Force = pressure difference × area: a fan that lowers the pressure under a car presses it onto the road
Topic: Physics of new inventions
The air around us presses on everything with a pressure: How much force presses on an area. A sharp knife has high pressure at its edge. All key words of about 100000 Pa. Usually it presses just as hard under a car as on top of it, so the two pushes cancel. Fan cars, like the McMurtry Spéirling racing car, use a fan to suck air out from under the floor. The pressure under the car drops a little, so the air above pushes down harder than the air below pushes up. Because the floor is large, even a small pressure difference gives a big force: A push or a pull. Forces can start, stop or turn things. All key words: force = pressure difference × area. This downforce presses the tyres onto the road, so they grip better. The next Tesla Roadster has even been talked about as a car that could fly, using thrusters. A thruster blasts gas downwards, and the gas pushes the car up: this push is the thrust. The car only leaves the ground when the thrust is at least as big as its weight: The pull of gravity on something, measured in newtons. It is less on the Moon. All key words, and it speeds up as it rises only when the thrust is bigger. In this lab you can blow air in under the car to make it float, and fire thrusters to make it fly.
What you will learn
- Calculate the force from a pressure difference using force = pressure difference × area.
- Explain how a suction fan increases the grip of a car's tyres.
- Explain what is needed for a car to float on air, and why it then has no grip.
- Compare thrust with weight to decide whether a car stays down, hovers or rises, and calculate its upward acceleration.
Before you start
- Know that pressure = force ÷ area, and that air presses on everything around it.
- Know that friction grows when two surfaces are pressed together harder.
Materials
- A sports car with a sealed floor (side skirts), a fan at the back and thrusters underneath
- A dry road with a bend of radius 50 m
Equipment
- A pressure gauge for the air under the floor
Procedure
- With the fan off, record the grip of the car's tyres.
- Switch the fan to suck air out from under the car, set the pressure difference and record.
- Use a heavier car and find a fan setting whose downforce is bigger than the car's weight.
- Switch the fan to blow air in under the car and make it float.
- Fire the thrusters and find the thrust that lifts the car off the road and makes it rise.
How this simulation is simplified
- g is taken as 10 N/kg.
- On a dry road the tyres can grip with a force as big as the force pressing them down (coefficient of friction 1).
- The skirts seal the floor, so the fan's pressure difference acts over the whole floor area.
- The bend has a radius of 50 m; the fastest speed round it is when the grip just gives the turning force.
- The thrusters push straight up and the car's weight stays the same (the fuel they burn is ignored).
- Figures for real cars are their makers' own; this lab uses simple round numbers.