TonyB wrote:Surely there must a few Fluid Mechanics/Dynamics subjects at all unis offering engineering?

\

Hope so! I am very interested in aerodynamics. I wish we had a better opportunity to exploit them in model solar car racing, it wasn't really possible with this years regulations. Everyone went for the rod shape.

Redlands wrote:Hope so! I am very interested in aerodynamics. I wish we had a better opportunity to exploit them in model solar car racing, it wasn't really possible with this years regulations. Everyone went for the rod shape.

Even then, there would have still been some considerable difference between cars.

If you have read Ian Gardner's design hints you'll find various cars from competitions over the last 5 or so years with drag coefficients ranging from about 0.004 to 0.012. These coefficients are not actual Cd figures but are rather figures that have combined all the constants (area, air density and Cd) into one and simply approximate the relationship between the drag force and air velocity squared (newtons per m/s squared). This reduces the air drag equation down to Drag = coeff * velocity squared.

Anyway, when we ran a few wind tunnel test with a couple of the Tas cars before coming up to Melbourne we found a few differences. The test car had a coefficient of around 0.0041 whereas a couple of the other cars were around 0.0065. Try playing around with those 2 values in Ian's simulator.

miseli wrote:Even then, there would have still been some considerable difference between cars.

Yeah, that is true. I guess what I'm saying is that with the current rules there is only one way to orientate the milk carton whereas in 2006 (cans), 2007 and 2008 you had different options because one had to be transverse to the direction of travel.

Speaking of milk cartons, how come you use one to carry your ballast instead of keeping it down low on the floor?

Also - did you get me PM?

Redlands.

I think you are all missing the point. The idea is not so much to create downforce as to eliminate any lift that may be generated. This might not have been a big problem with this years heavy and narrow cars but back when they had to be wide and have a minimum floor area lift was a real big concern. Squeezing too much air under a 300 sq cm floor at 30kph could easily lift a 600gm total car off the track. Never underestimate the power of the wind. Air pressure is relentless. It's not a matter of straight out flying but what is known as Ground Effect. This is what makes hovercraft work. I don't know about model planes but anyone who has ever flown a real plane gets to learn all about ground effect very early on. Like Marc said, most of the additional bits on his car are for streamlining but it never hurts to play safe. Front air dams and side skirts cost nothing in the way of drag, in fact they can reduce it considerably, and can be used as structural elements so there is no extra weight involved. Not all those bits sticking out on an F1 car are for downforce, most of them are to reduce drag. Next year solar cars will again be wide with a big floor and probably weigh less so eliminating lift will be important.

john jeffery wrote:The idea is not so much to create downforce as to eliminate any lift that may be generated.

I think you've pretty much summed it up there John.

Downforce rather than lift, no matter how big or small, will always help keep a car on the track. Again, with the test car the airfoil is there to primarily streamline the front axle and strengthen the car up a bit. It was then shaped to create a slight downforce rather than lift. Even then, as was previously mentioned, this would be next to nothing for the size and shape of the airfoil, particularly in relation to the weight of the car.

Next year's cars, as John has mentioned, will again be a lot wider and require a significantly larger floor area. With the new Electronics vs No electronics formula that will probably be coming in for next year, some of the lower powered cars could potentially weigh only 700g.
As a result, eliminating lift/creating a downforce will be somewhat a little more important since it will now be the shape of the entire car affecting things. Shaping the car to force too much air flow underneath it could cause a few problems.

Redlands, even though I used a milk bottle to hold my ballast, the lead being carried was more or less at the bottom so it was essentially on the floor of the car anyway. This year it was the 2L milk bottle, but in previous years I have used milk cartons or soft drink cans - whatever the rules have been.
There are a couple of main reasons why I use the object that cars need to be capable of fitting to carry the ballast.
1. The first reason is that the cars are designed around these. If positioned securely within the object and then the object placed within the car, the lead ballast is restricted and will not shift around.
2. The second reason is the ease of adding or removing the object. Rather than having to fix ballast to the car using tape, wire ties, etc, a milk bottle, carton or soft drink can can just be placed in or taken out of the car with minimum hassle.

Nowdays they use CFD ( Computational Fluid Dynamics) which is more cost effective solution than a wind tunnel.



http://en.wikipedia.org/wiki/Computatio ... d_dynamics