Well, only secondarily – his isn’t until February. We did this morning for this month’s school project – they get to choose anything, and he chose aerodynamics this time. Actually, he wanted to build a wind tunnel to use later for his Pinewood Derby car – though we all realize it’s about mechanical friction, not aerodynamics . . .
Anyway – this can measure bodies of different shapes and orientations at three different “wind” speeds. He came up with the idea for the quadrant and pointer on the top – I wanted to set an old OHaus balance scale on top, but he said that was too complicated.
All scrap, an old box fan, cardboard and duct tape for the reducer, a liquor box with dividers for the air straightener, and a box from leftover stoop ceiling for the test section. Clear plastic sheet from somewhere. Brazing rods for the “trapeezes”. Melamine quadrant and dry-erase marker for a scale.
Basically, air blows from left to right, pushes test subjects back on the trapeez, and the pointer registers how much it moved.
This is just a tool – his project will be loading it up and learning stuff!!
Forrest – now I gotta’ get back to handrails!
Edited 11/16/2008 7:06 pm ET by McDesign
Replies
Very cool! I was soooo on my own when I made my pinewood derby cars. My dad was too busy being a lawyer, plus soooo disinclined to anything mechanical.
As you said, it's all about the wheels anyway...
Steve
I was lucky - dad's an engineer.
Mechanical parts detail - still some tuning to do here - angles and friction and stuff -View Image
Forrest
Edited 11/16/2008 7:10 pm ET by McDesign
Neat project! Aerodynamics has a lot to do with winning. Any little reduction in drag will help. Flying helicopters makes me very aware of the little things that reduce drag.
Putting a point on the front is not near as important as tapering the rear so as to reduce the suction drag. The air will dam up in front of a box shape and actually kind of streamline itself....but the killer is the vortices that corners make on the rear as the air tries to suck back into the low pressure just left by the void of the shape thats moving. Its all about transitioning this air over time. Ideally a 7 to 1 ratio taper is optimum but any taper at the rear will help. Maybe there are strict rules that must be adhered to in a soap box derby race??
Anyway...what a cool project to get involved with your son.
Stan
There is an envelope spec of maximum LxWxH, and a maximum weight of 5 oz. Must use the four supplied wheels, no bearings.
He'd read about how much the "suction" drag is on the back of semi-trailers, but that there's no way to deal with it because of loading docks.
Forrest
Very nice. My kids are starting with school projects and its REALLY hard to not get involved too much. I try to let them do as much as possible on their own even if it doesn't look perfect. I get involved only to teach them about things and to use the dangerous power tools.
<its REALLY hard to not get involved too much.>
You know it! The rest is up to him - he plans to write up a test procedure and make a chart of the things he's tested.
Forrest
fill those square holes with straws to get better laminar flow. Then he can use incense sticks to make smoke and observe the air flow.
Cool!
Forrest - trying to remember wher I left my incense back in 1975
Nice setup!
BTW, you can convert the same setup to rollong friction with the use of a beltsander instead of a fan for your forward force. The car rolls on the fine grit belt, you may have to add a little spring to keep it on the same scale you've already made.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Tu stultus es
LOL! The rolling road addition would REALLY take it over the top!
Forrest
Mc Design
We could never win the speed award. So we started to focus on winning the best design idea. We were happy if we could win a couple of races. Usually we were in the top 1/3 for speed.
We built a shark car, an ear of corn, a rolling barn, a minivan. Those are the ones I remember.
Rich
With the belt sander, do you put an ammeter in line and measure the increased amp draw?"Put your creed in your deed." Emerson
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Handn't thought of that.
I was thinking that you substitute a spring on your swing for the force of gravity. That way you can measure how much force the road is applying backward against the spring via friction in the wheels. From there you can measure changes with lube, sanding the wheels, etc.Rebuilding my home in Cypress, CAAlso a CRX fanatic!
Tu stultus es
paul
I think the most critical factor is to get the car to run straight so that the wheels don't rub against the center or outside edge rails.
Advice from a Dad whose kids never won the speed trophy. LOL
Rich
Inside scoop I have heard is to set the wheels so only three actually touch the track. Aerodynamics don't matter at that speed. It's all rolling resistance and taking one wheels friction out of the picture makes all the difference.
I've also heard that slightly knife edging the tires reduces contact and thus drag.. the other option is when installing them they are tilted out at the top but aligned perfectly thus they roll on one edge.
Another trick is to seek out many sets of tires and grab the largest diameter ones in order to increase the radius. Larger wheels roll easier than smaller wheels. It goes without saying that great care is spent making sure the wheels are perfectly free of flat spots or low spots..
Drag wise the least drag configuration is a tear drop plus that gets you the added benefit of the heaviest mass at the front, best for acceration.
The greatest speed gain is attention to the fit of the"axle" and tire. The NASCAR dads are making bushings that are set into the tire and become invisable to detection.
It's reported that one well known NASCAR dad spent well in excess of $10,000 on his son's pinewood derby car most of that on wheels and axles..
Well, if those dads are putting all that money and sneaky bushings into into the pinewood derby cars it's a cinch that their boys are learning that dad is dishonest. Being honest and fair are Cub Scout ideals. If winning means taking away the basics of being a Cub Scout, then the whole system is being corrupted by overeager dads. I am a cubmaster and sure hope I don't see that in my Cub Scout Pack.
I really like the test rig to look at air flow. tAnd the belt sander idea to measure friction is also excellent. I may make that a den project to work on the scientist acheivement for my Webelos Den. Thanks for the great ideas on how to make the boys think.
mapletrim
Careful about using the word dishonest.
Roger Penske is accussed of using an Unfair advantage to win as many races as he does..
It's only unfair because other haven't thought of it though not because it's illegal..
It's called the grey area of rules.. For example if the rules say you can't do something then it's clearly a violation of the rules to do it. However if the rules are silent on the issue the exact details involved then it's an area that needs exploration..
The examples I listed of negative chamber to ride on the outside edge of the tire is one of those areas not covered by the rules. Lubricants are another one of those grey areas.. if the rules say you can't use any lubricant then clearly you can't however if no post race inspection is going to be done and you don't use a lubricant that does not mean you are precluded from reducing rolling restance by other means such as microfine granulation of the bearing surface with the material the tires are made of..
Accuracy of alignment is also one of those issues. For example if a child is supposed to assemble the car himself then it's wrong for an adult to do so,, it's not wrong to have the equipment to enable him to do a excellant job on his own and to practice building several cars to see which is the fastest.
Treating the tires for reduced rolling resistance is one of those areas.. for example if freezing the tires in liquid nitrogen reduces their rolling resistance and the rule doesn't specifically forbid it then clearly it's a grey area.
None of that says cheating is allowed.. but winning has a cost and boys need to understand the cost of winning is dedication to results. Measurable results.. once you've built your duplicate race track to test cars and built several cars then you clearly have an idea of what is required to win..
All sports and competition is like that.. To win you must be willing to use whatever legal means is available..
I agreee that inovation is the key to success. I like the wheel idea and this is not "forbidden" by the rules. The part that set me off was the comment: "The NASCAR dads are making bushings that are set into the tire and become invisable to detection." Now that is a rule violation and doing it anyway is cheating the other kids if you ask me.
I looked into the shape issue and what is most aerodynamic and supposedly a sphere is the best shape for low speeds as it has the least surface area for any given mass. Apparently at higher speeds shape is more importance and a teardrop is best with the weight in front.
That said, I agree that the secret of pinewood derby speed is really in the wheels and axles as they do not go fast enough for shape to make much of a difference. But, making them out of wood is the real fun and use for the imagination of the boys. The cool shapes that arrive at the track are always amazing to me and my boys.
MT
Yup, thats me, low speed..LOLSpheramid Enterprises Architectural Woodworks
Repairs, Remodeling, Restorations
They kill Prophets, for Profits.
Since I don't have access to the rules I am in a poor place to assume that what was done was illegal in any respect.
Using your judgement as to what you assume a rule to be is exactly why some cars are faster than others all things being equal.. I can envision a way to do exactly that legally depending on the exact wording of the rules..
Having spent decades racing where rules books are heavy bound units and suposedly everything is restricted etc.. I am very familar with what is and what is not legal.. For example one of the Indy cars I worked on had a shrader value put into the chassis. the purpose of that shrader value was to pressurize the chassis with liquid nitrogen. the rules clearly stated what the cars had to weigh at the start.. there was no weight limit at the finish.. liquid nitrogen was pumped into the chambers of the chassis and the car went across the scales meeting the minimum weight by about 17 pounds.. However after the finish of the race with chassis flexing and the heat of competiton the car was hundreds of pounds lighter..
That allowed a low buck team to lead the Indy 500 for a few laps and they wound up finishing well ahead of all but a few other teams..
Back in the early 60's I carved mine with my Cub Scout knife and sand paper,looked like and old style Indy car. I took 3rd place class B in the regional finals.
dedhed6b
Back in the early 60's I carved mine with my Cub Scout knife and sand paper,looked like and old style Indy car
Sounds like we built the same car with the same tools in the 60's.
I took 1st place in the church basement. Beat my big brother. Only time I ever beat him at anything.
Rich
Cool wind tunnel!
Don't forget that the other elements of a winning car are perfect weight, and axles that are parallel and polished. And for good measure, I also did what I could to impregnate the wheel surfaces with graphite.
And of course, "winning" in this case, is having some fun with the boy, and helping him learn something (perhaps while learning yourself). Any trophies bestowed by outsiders are just icing on the cake.
Very cool Forest.
On a cub scout side note, last spring for the Webelos Craftsman activity badge I designed a Derby car display stand for the boys to make. Tweaked a design is more like it.
View Image
View Image
Have plans for one, two, three, four and five car displays. Therefore the boys that did not start as Tigers could still have a full display for their cars.
Supplies are 1/2 dowels and 1x4 material. If anyone wants the plan(s) I'd be happy to share it assuming it is for cub scout use and not commercial production..