Carl Fausett

Mother,

actually there is a spoiler design similar to that allowed at Bonneville. But, its not allowed to have an adjustable control surface (which we felt we needed) and it doesn't lay out very well on the curved tail of a 928. It seemed to me to lend it self to squared-off contours like the car it is mounted on.

Look at the wind tunnel video and you will see that the air flow clings to the rounded sides of the 928 and tries to re-join the air that has gone over the roof at the hatch. Then it tries to exit at the back of the car normally. We felt that this design doesn't match the rounded rear end of the 928 as well.

Good idea, tho, thanks for posting.

atb

Carl,

I showed a friend of mine your wind tunnel video. He used to work for Ford as an aerodynamics engineer. His group provided the factory's input to the NASCAR teams racing Ford Products. They would do computer sims and windtunnel testing for the NASCAR teams racecars.
I showed him a picture of what your 928 looked like in Landspeed trim, then showed him your wind tunnel video. When I showed him your video he immediately nodded in approval when he saw that you had rounded out the front end with the card board and boxed in the splitter. He also had concern over the trip fences (he called them side plates) on your spoiler. He said when they experimented with them in the windtunnel, they found that the air traveling around the sides of the car does not have an equal, uniform effect simutaneously on both sides of the car. Meaning that as the wind travels along the side of the car, the car is subjected to high and low pressure areas on a vertical plane (we tend to only be concerned with these on a horizontal plane because it affects lift and downforce). But, as the air goes around the side of the car, it tends to push and pull laterally on the car. This action isn't simultaneous in all cases, so you can have an unequally timed pulsing effect on both sides of the car. They found that large side plates like what your car had, in some cases, not all, created lateral instability in the car because it provided a large surface for these pushing/pulling surfaces to act on.
I thought it was pretty interesting. I asked him if he had any suggestions for your design, and like any engineer, he said test, test, test.
When he saw the pic of your car, he did mention that it looked like you had a lot of testing to do.
He didn't mean this in a deragatory or judgmental way, just that you have so many elements in your design that you need to be able to control, that the only way to know if they will all work as a unit is to test them. He said that when he was at Ford, they would input all variables and designs in a computer sim first, and the computer would make a calculation based on the design. They would then view the results and sharpen the design and re-input it. When the computer finally determined that they had the best combination design, they would then build it and wind tunnel test it. As a "hat's off" to you, he did say that one design sim would take the computer overnight to calculate, and they would go through dozens of these before even getting to a wind tunnel. Once at the windtunnel, they would then have to pick up where you are now - but with a design that had been sim-tested to perfection- before subjecting it to the real world wind tunnel.
At any rate, he wanted to make sure that my shared comments from him don't dissuade or persuade you one way or another, since the only answers are really going to come from your own testing and experience.

Carl Fausett

Well said, Adam. Of course, there are two costs involved, the time budget and the fiscal budget.

You might remember how we tried to get a succesful point cloud from our 928 so we could flow-model it in the comoputer before doing our designs. I think I posted about this in 2010.

Unfortunately, the computer program did not bear fruit in time, and I could not wait for it any longer. So I made my aero designs the old-fashioned way: I read books, I went to Bonneville and crawled around similar cars, and I guessed. We did pretty good for a first shot at it, and I know now a) a lot more about aero than I did, and b) how much more there is to go!

Tell your firend he is spot-on and I appreciate your/his input. We hope to do much better next year. Bonneveille for us will be on Sept 3rd I'm told. It's a process.

Here are a few pics of taking the data points from the 928. We used lasers and measured more than 1,600 x,y and z data points. The other pics are the first point cloud the program put up from the data points.

bronto

Carl;

Did you happen to run the 928 in the tunnel backwards, to see if the old myth is true?

Carl Fausett

The point cloud information we collected in 2010 (Post 63 above) is now bearing fruit.

The point cloud is fully formed in Flow Works, and the data model adjusted to show the Bonneville car at the correct (1.75") ride height, the required Bonneville rear spoiler, and the parachute.

This was all done by my son Taylor, who has been in aerospace engineering since he graduated with his engineering degree. he currently works at Pratt & Whitney. Hats off to him.

This is how he contributes to our Bonneville effort from San Diego when he cant be here nights and weekends to help prep and build the car like the rest of us.

Here is a sample of one of his iterations. With it in Flow Works now, we can try different air dams and rear spoiler combinations and select the most promising outputs. There is still a difference between a computer simulation and a wind tunnel test and real-world results; but it does lesson our guessing and direct us to spending our time on the configurations most likely to produce desired results.

eijun

Here is my question, how is the engine heat like when you the restricted airflow from putting the cardboard in the front with only a slit hole like that?

mark kibort

for no drag cost, you can vent through the hood and gain frontal downforce and still keep cooling up.