Anyone tried this?

[Bullet]

Thx all for the input. Will give it a try and probably find the same thing as your testing, but should be fun to watch anyway, or anticipate. Thinking of another version that may be better...

Trying to learn the standard stuff as well as test some fun ideas at the same time....maybe too much for just starting out with regards to time.

 

[Guest]

Thx. Was concerned that the path down the middle would be about the same drag as going over. The bottom is open but has mono over it now. If the rest of the car construction runs ok, may take off the bottom and try it again.

Would sure be nice to have a track, instead of only having one shot a month to try things out...seriously considering a track if I can talk my wife into parking outside for a week or two a month when its set up!

Good plan. Ease her into the idea.

Yeah sweetheart! It will only be for a few weeks.

 

[Bullet]

Well, I performed some air drag calculations on the car above. It looks like OPA is correct and my numbers follow his experience. depending on the coefficient calculated (or estimated) that is used, its a little worse than a standard thin design. It would require some more area removed to overcome the extra drag, but is still in the realm of possibility to be better with the right setup. Btw, the Reynolds number is higher than I thought it would be for these little cars at relatively low speeds. I will probably run it anyway since I can still put it in the novice class. Will do the calcs before the build next time, but had fun building it!

 

[Guest]

Amazing!

How did you calculate this?

Can you give an abridged version?

Thanks

 

[Bullet]

D = drag (force) = 0.5 x Cd x A x d x V^2

where Cd is the drag coeficient for the surface; A is the area of the surface the wind is blowing over; d is the density of the air; and V is the velocity that the wind is travelling relative to the surface.

Careful with the units...

The drag coefficient is the tricky part, mostly an estimation from other emperical data, not much out there for complex shapes...was going to look into finding a better one when I have the time. The drag coeff is actually a function of the reynolds number, but can probably be considered constant for the range of speed these cars are traveling.

 

[Bullet]

BTW, just to give you and idea... Not sure how accruate it is, but it can probably be used for comparison purposes if calculated the same way.

EDIT: I had the air density in lb/ft^3 and everything else in inches and ounces...yikes! OK, so the numbers I came up with now are actually in the .08 ounce range for the car body only, traveling at 12 mph.

Sounds more realistic than the 0.5 oz number I earlier listed....

Careful with the units...

 

[Obsessedderbydad]

I often wondered if this would work or not. Never knew how to do it myself so never tried. Also was worried about wind making the car shimmy more.

 

[DerbyDad4Hire]

In a previous life I pulled a flatbed trailer over the road. If I had a load of pipe, either a few large diameter pipes or many smaller diameter pipe the results were the same. Better fuel mileage if the front of the pipe was covered with a tarp instead of leaving it open for air to flow through. With the center open, you have a bunch more surface to build friction with air flowing over it. Just my thoughts../images/boards/smilies/wave.gif

Thank you B-4. You speak my language! You guys are way over my head with these formulas. Thank goodness for guys like you. If it were on me we would still be working on inventing the wheel.

 

[Guest]

Oh man!

I feel dizzy.

 

[B_Regal Racing]

Where is TXCHEMIST? I need validation...

 

[Guest]

Ok.

I have wondered about this.

If wind is forced thru a constriction then would this not set up a scenario for the Bernoulli principle?

Could this force be used to steer the car rather than toe in on the DFW?

 

[Bullet]

yes, Bernoulli just says that pressure is turned into velocity and vice versa through a constriction/expansion of cross-sectional area, conservation of energy...pressure equals drag for our purposes. I would think it would be hard to predict and plan to have a certain pressure in a certain area at a certain speed for steering, and the restriction would cause drag also. The idea for cars would be to avoid Bernoulli effect if possible. We only have one option since we cant induce energy to the equation, which is to add backpressure with restriction...

I was trying to keep the same or larger area in the back as compared to the front, but the geometry of the hole adds to the drag coefficient, which is hard to determine.

 

[Guest]

whoa , whoa whoa.

Bernoulli doesn't just say anything like that.

 

[Bullet]

/images/boards/smilies/smile.gif

I found a better way someone else said what I was trying to say...

From Wiki..."Fluid particles are subject only to pressure and their own weight. If a fluid is flowing horizontally and along a section of a streamline, where the speed increases it can only be because the fluid on that section has moved from a region of higher pressure to a region of lower pressure; and if its speed decreases, it can only be because it has moved from a region of lower pressure to a region of higher pressure. Consequently, within a fluid flowing horizontally, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest."

Like putting your thumb over the garden hose...or a nozzle.

Guess this one is out in the gigglesticks now! lol

 

[Papa V Racing]

I'm the guy with outside the Box thinking.... Here is a question as it comes from watching our first Scout race back in 2011. The used a car shaped as a Bullet.. No Pun intended... But IMO without testing and reading on this shape... Would it be better instead of a flat design to go round with a point that opens?

Again I have not read up on anything and I am not a whiz kid on drag... I look at it from a simple point... again No Pun... I can see where the wheels could be an issue but if the wheels were recessed into the body or with a different fender to help bush air away.. The point would force more air through the middle .. guessing this of course... allowing for air flow on the out side to do as air does flow evenly across a round or close to round body as possible...

LOL ---->>> Bullett you & a couple of the others may be creating an experimental class with more to follow soon... It would be cool to see more cars show up with crazy designs... I know one race I watched on You Tube had the slowest car design... The cars were prety quick off the line until the transition.. SO who is to say that the tunnel effect is outdated when curious minds continue to wonder and who knows maybe you find a different design that works... Think of a Catamaran or some of the other Hydrofoil boats instead of water it is air... Here is a crazy idea.. Nellie Faye fenders with super narrow and a very wafer thin like body until you get to the back where the weights are... Who is to say it can't work as you have completely changed the set up...

Ok sorry to have thrown this out there but Bullet you got me thinking now... I like the fact you are getting people to say Hmmmm... or I tried it and it didn't work for me BUT.. that is cool ... Now I will be working on something in the back of my head.. SHOOTS!!!! .... Wheels in my head go round and round...

 

[Papa V Racing]

Just a thought that is way beyond PWD but some ideas come from this video...

[video]http://www.youtube.com/watch?v=IWCgArIxU2o[/video]

 

[ChrisF]

Not long ago I had a nice chat with Lightning Boy regarding Pinewood Aerodynamics. We shared thoughts and experiences. He's done a few out of the box things. I'm just waiting for him to ring in.

 

[ChrisF]

The used a car shaped as a Bullet.. No Pun intended... But IMO without testing and reading on this shape... Would it be better instead of a flat design to go round with a point that opens?

PapaV, would you be thinking of a design like car number 15?


Funny thing is I didn't think of "bullet" initially...

Incidentally, you can just see part of car 17, and if you look closely,you can see the "aerodynamic tunnels" in the rear.

 

[JAG Racing]

I've thought about some of these things in the past and soon realized that form follows function as they say.

Most "vehicles" have different sets of problems they are solving when considering aerodynamics.

Pinewood derby cars have a very unique set of problems very unlike other vehicles.

 

[LightninBoy]

Not long ago I had a nice chat with Lightning Boy regarding Pinewood Aerodynamics. We shared thoughts and experiences. He's done a few out of the box things. I'm just waiting for him to ring in.

Hey thanks Chris. Yeah, I love the outside the box stuff like what's being discussed in this thread. I don't have any experience with the tunnel approach. I'd be worried about skin friction as others alluded to, but skin friction at PWD speeds is such a minor thing .... so who knows?

I'm no expert, but I will throw out that IMHO trying to draw PWD aerodynamic inspiration from a 450+ HP car designed to accelerate, drive and corner at high speeds is a mistake. PWD aero is all about conserving energy. Therefore, a better analogy can be found in high efficiency cars. Or something like this ...

http://en.wikipedia.org/wiki/Nuna