controller advice for fastest car event - Page 3 - RC-Monster Forums

Mod Man · 01.29.2008, 08:46 PM

CD (coefficient of drag) is a hugely complex thing to calculate. There is alot more than merely frontal area to consider. There are factors like screen friction (air dragging on the sides of the car), the cowanda affect (spelling?). That is the tendency of the air to hug the body as it rounds a corner rather than skipping off the body creating turbulence and, therefore, drag.

When I designed my CF body, I stood it up (nose in the air) and ran water over the body (steady stream of water running over the body) looking for turbulence in the water and the tendency of the water to hug the body rather than skip off at various curved areas. Water is 800 time more dense than air, yet the same fluid dynamics apply with air as water. So, you can get a good idea of how the air will behave with your body by watching water run over it.

Am I making any sence?

Matt

zeropointbug · 01.30.2008, 04:45 PM

The Quark will handle 7s A123 no problem, not quite 6s lipo under acceleration.... but for top speed, it will hold near 23volts, which is all that matters for you, right?

Although, you might want to solder two extra low-esr caps onto the input power leads. I did.

zeropointbug · 01.30.2008, 04:48 PM

Quote:

Originally Posted by Mod Man

CD (coefficient of drag) is a hugely complex thing to calculate. There is alot more than merely frontal area to consider. There are factors like screen friction (air dragging on the sides of the car), the cowanda affect (spelling?). That is the tendency of the air to hug the body as it rounds a corner rather than skipping off the body creating turbulence and, therefore, drag.

When I designed my CF body, I stood it up (nose in the air) and ran water over the body (steady stream of water running over the body) looking for turbulence in the water and the tendency of the water to hug the body rather than skip off at various curved areas. Water is 800 time more dense than air, yet the same fluid dynamics apply with air as water. So, you can get a good idea of how the air will behave with your body by watching water run over it.

Am I making any sence?

Matt

Water and air are both fluids yes, but water cannot compress like air... the only way to test a shape for air turbulence is, well air.

rschoi_75 · 01.30.2008, 05:33 PM

How about a Pheonix HV 125amp? Isn't that what Nic Case was using for his serpent?

btw, where will the speed challenge take place? If its in CA again, I may attend. I don't have a crazy fast car since all of my rides are purpose built for tracks and bashing, but I'd love to go and watch. I may even enter one of my slow rides just to say I took part in it... lol.

Seriously though, I was thinking about entering my zip zap in the event. I think I can get that thing to about 10mph... plenty fast enough to take home a trophy... don't you think?

BrianG · 01.30.2008, 05:41 PM

Quote:

Originally Posted by Mod Man

CD (coefficient of drag) is a hugely complex thing to calculate. There is alot more than merely frontal area to consider. There are factors like screen friction (air dragging on the sides of the car), the cowanda affect (spelling?). That is the tendency of the air to hug the body as it rounds a corner rather than skipping off the body creating turbulence and, therefore, drag.

When I designed my CF body, I stood it up (nose in the air) and ran water over the body (steady stream of water running over the body) looking for turbulence in the water and the tendency of the water to hug the body rather than skip off at various curved areas. Water is 800 time more dense than air, yet the same fluid dynamics apply with air as water. So, you can get a good idea of how the air will behave with your body by watching water run over it.

Am I making any sence?

Matt

I kinda figured it would be extremely difficult to calculate, but was hoping for some miracle constant.

Like ZPB said, using water is not exact, but it sure is a lot better than nothing. At least it gets you in the ballpark. ZPB: you have a wind-tunnel I can borrow?

BrianG · 01.30.2008, 05:42 PM

Quote:

Originally Posted by rschoi_75

How about a Pheonix HV 125amp?

You have a link to that? The only thing I can find in the Phoenix line is the HV-110...

Dagger Thrasher · 01.30.2008, 06:29 PM

There's always the Hydra ESCs; a 120, 180 or even a 240 would be plenty enough! Or, you could wait for the MMM; that too should be plenty, and will of course have brakes you can use.

BrianG · 01.30.2008, 06:31 PM

The think about the Hydras is the price (compared to the Phoenix) and need to replace the water-cooling tubes with a finned heatsink...

Dagger Thrasher · 01.30.2008, 06:37 PM

D'oh...good point! I didnt think of that lol... :)

zeropointbug · 01.30.2008, 06:41 PM

One thing I know though, is if you use the general shape of fish... if you have some gold fish, these have a nice curve ratio that is appealing to a car body.

I have heard Mercedes made a van that has 80mpg and doesn't even look aerodynamic really, they used a 'box fish' design.

Another thing I know is to keep the front profile shorter and more rain drop looking, and the tailing end more prolonged (fish like).

I agree, someone else said a while back that frontal area has little to do with aero's.... it's all to do with the profile. Anyways, I better stop, I am yabbering again. ;)

Mod Man · 06:45 PM

Yes, air compresses and water does not. However, without a wind tunnel, water is one of the best methods to use. Many auto manufacturers use water turbulence testing as well. I was not nearly as concerned with localized compression as much as flow characteristics over the body. My main point of interest was trying to get 40% of the air flowing over the body and 60% flowing around the body for downforce reasons. I got it really close with the water testing. I was only off a tiny bit in my nose downforce calculations using the water flow method. So, I merely added a chin spoiler to increase the nose downforce just a touch and bingo, it ran very stable.

Anyway, I also have a smoke pen and an annemometer. Putting the car in a gentle breeze and following the smoke line shows the same flow characteristics as my water test showed.

There is still so much guesswork in this, though. Sometimes it is best to just go out and run it to see how it works!

Matt