# How do drag and downforce in a rear spoiler of a car vary with angle of attack?

I ran an two aerodynamic simulations, one for a spoiler angle of 0 deg and the other for the same spoiler declined at 30 deg. The results showed a considerable increase in downforce, but only a slight change in drag. Is this the expected behavior of a spoiler?

• Check out naca profiles, check out how wings work on aeroplanes, also check out stall, so much information on this topic. Mar 22 '19 at 5:52
• Read, to start, this : oppositelock.kinja.com/… or this : nas.nasa.gov/About/Education/Racecar/physics.html Mar 22 '19 at 6:19
• It would be good if you could include a drawing or photo as it isn't clear if you are using a flat plate spoiler or an inverted airfoil. Aug 13 '20 at 20:27
• Need actual numbers, geometry and the details of your simulations - software, meshes, graphical output. We don't do hand-wavy stuff here. Aug 13 '20 at 20:34

It isn't clear from the question whether you are using a flat plate spoiler like in NASCAR or an inverted airfoil like in Formula 1. For flat plate spoilers a big part of what they are doing is to inhibit aerodynamic lift from the car's body. Lift always causes induces drag so spoiling the lift reduces the induced drag. This is why they are called spoilers.

• only a slight change in drag.

The spoilers cross sections are made by the aerofoils/airfoils. These are called "Streamlined bodies" in which you tend to get more lift than drag. (In the other way, they produce less drag than bluff bodies like sphere/cylinder etc.)

To explain a bit more,

Assume your car moves at 100 km/hr ~ 28 m/s, and the spoiler (chord) size is 0.3m and also made up of NACA 0012 aerofoil (Google for this profile!). Then the $$Re$$ number of the flow will be roughly ~ 5$$\times$$10$$^5$$.

At this Reynold's number, the NACA 0012 aerofoil produces the lift of ($$C_l$$ - coefficient of lift) 1.5 for ~15$$^o$$ of angle of attack (remeber you have mentioned angle of spoiler not the angle of attack. They are very different. pls. check this!) at the same time, the drag coefficient ($$C_d$$) is 0.03!. So the ratio between these two is called efficiency of the aerofoil $$C_l / C_d =$$ 45.

The Drag force is less for streamlined bodies.

Note: Please check your angle, the effieciency is a strong function of angle of attack. At very high angle of attack the flow tends to seperate and can create adverse effects.

• It’s really good to fit a spoiler to lift the backend of the car the faster you go... :) Mar 22 '19 at 19:59
• It is not at all clear the OP is using an airfoil. Many car spoilers are simple plates set at an angle (see any NASCAR car). The reduction in drag is from reducing the induced drag due to the lift of the car's body. Dec 17 '19 at 23:14

This is a typical graph for $$C_L$$ and $$C_D$$ with respect to angle of attack. As you can see, up to 30 degrees, $$C_D$$ increases very slowly, while $$C_L$$ reaches its maximum and then drops. So this is pretty standard. The ratio between $$\frac{C_L}{C_D}$$ can go from 10 to 50 depending on the specific airfoil that you use.

So your simulation results seem to be in the right track.