Rear Pylon Design & Optimisation
Now that the basic design of the rear wing surfaces have been determined, the next step is to work out how to attach it to the car without the wing falling off at high speeds! There are several different ways to do this but I have gone for the "swan neck" style of pylons/hangers, similar to those found on LMP and top-end GT cars. If you look at the coefficients of static pressure around the wing you'll see why.
To quickly explain what's going on in this graph; from Bernoulli's principle we can say that Coefficient of Static Pressure + the Coefficient of Dynamic Pressure (1/2pv^2) = the Total Pressure = 1. By looking at the coefficients around an aerofoil we can see how the wing is behaving and compare it against other profiles without having to worry about scale, allowing us to use scale wind tunnel models to accurately predict full size aero performance.
What you're looking at (from left to right) are the static pressure coefficients across the wing from the leading edge to the trailing edge. Where the airflow stalls at the leading edge (v = 0m/s), the static pressure jumps up to 1 indicating very high pressure and in this case drag. You'll then see that the coefficients split into two plots- one above the zero line and one below. These represents the upper and lower wing surfaces respectively. As expected, we can see that the upper surface raises the static pressure (all coefficients greater than zero) whilst the lower surface reduces the pressure, before both plots reattach at the trailing edge. We've got downforce- no big surprise there!
With the aero loads for the stress analysis worked out earlier, we can then FEA the pylon design with the intention of reducing as much weight as we can without them snapping!
The loads were applied to the FEA model (yellow = downforce & red = drag) and analysed. Finite Element Analysis (FEA) is a very useful tool that allows engineers to load a structure and evaluate the behaviour and resulting stresses that occur. From this you can then tell if the structure will fail or not and how to optimise the design further to save weight.
Pylon 1 (4.8kg each) was evaluated as shown. The red areas are where it's highly stressed and likely to break if we've got our sums wrong and the aero loads exceed what we've predicted! Hopefully this is unlikely, but there's a small safety factor of +10% in there just in case.....
Conversely, all the blue areas are the lowly stressed areas. It's these areas in which the material is doing nothing structurally and is merely coming along for the ride. We can therefore get rid of weight here and design a more efficient structure- hence Pylon 2. Pylon 2 is capable of carrying the same aero loads as Pylon 1, yet weighs almost half as much - only 2.6kg each!
The final step is to work out how to attach the pylons to the car. Remember to mount them directly to something solid like the chassis and not the light weight / flexible boot lid. There's no point going to all that effort designing an efficient wing if you're just going to waste some downforce bending whatever it's mounted to!
As you can see from the FEA videos, there's still plenty of weight to come out of Pylon 2, but I think I'll leave the aero design for a while and get back to building the car.
For more information on any aero stuff, check out Joseph Katz's Race Car Aerodynamics or any of Simon McBeath's books and articles.
| || |