My project is about testing the aerodynamic properites of Soap Box Derby (SBD) cars.
PART 1. I want to test different "skins" of the cars. One, completely smooth, and the other smooth on the nose with increasing roughness until the back of the car's surface resembles 16 grit sandpaper. (I'll apply plaster of paris to create this effect.) I do have a low tech wind tunnel. How can I measure drag on this?
PART 2. I am also going to use two different kinds of wheels, one air filled and one soild, to see if the air filled wheels absorb "bumpiness" on a SBD track. (It's an aerodynamic property because the more the car bumps up in the air, the more drag is created.) Again, how could I measure drag on this? How can I create a conveyor belt with bumps in it to simulate a bumpy track?
Measuring Drag...
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dynama_girl
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Measuring Drag...
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bradleyshanrock-solberg
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Your first challenge is going to be controlling the variables.
Ideally you would make a single car and use it for all experiments - swap the wheels, for example, or replace the "skin".
The shape of the "skin" will likely have a larger impact on aerodynamic properties than the skin surface. To elminate that effect, you want the shape to be the same in all experiments.
A simple way to do this would be to have the "skin" be something you attach to the front of the car, rather than a coating on the car itself. This could be as simple as a piece of cardboard attached to the front bumper and the roof (covering the shape of the car) although to minimize air turbulence you'll likely want the sides as well...so think of it kind of like a "shell" you put on top of the car. Kind of like a dust-cover for the car, except rigid, not cloth (to make it easier to keep the shape the same).
If you have a track to run the car on, take several runs of each 'skin' and 'wheel' combination and measure distance traveled - you should be able to tell which "skin" and/or "wheel" combination provided the most resistance. (shorter distance = more energy dissapated by wind resistance)
There are also techniques for using a wind tunnel, involving attaching the vehicle to springs and attaching a measuring device to the springs, but I'm not familiar with the technique myself. My instinct would be to measure distance rather than a wind tunnel for this kind of trial, especially as people will care more about how a car performs on a track than in a wind tunnel. Perhaps others can chime in on the wind tunnel piece, or you can look it up online.
To make a "bumpy" surface, you would do best to look for some kind of manufactured surface you can just put on top of your track. I would look maybe at a carpet store (either the carpet itself or the padding), or the bathroom section of a hardware store (bathmats usually have ridged surfaces, and there are things you can stick to your bathtub to give more traction). Many of these places will give away odds and ends (eg, a carpet store installs into a house from big rolls, and they have bits and pieces left over). Very coarse sandpaper might work too, depending on the size of your wheels. Something like bubble-wrap is probably too variable. Some corrugated cardboard might also work.
Ideally you would make a single car and use it for all experiments - swap the wheels, for example, or replace the "skin".
The shape of the "skin" will likely have a larger impact on aerodynamic properties than the skin surface. To elminate that effect, you want the shape to be the same in all experiments.
A simple way to do this would be to have the "skin" be something you attach to the front of the car, rather than a coating on the car itself. This could be as simple as a piece of cardboard attached to the front bumper and the roof (covering the shape of the car) although to minimize air turbulence you'll likely want the sides as well...so think of it kind of like a "shell" you put on top of the car. Kind of like a dust-cover for the car, except rigid, not cloth (to make it easier to keep the shape the same).
If you have a track to run the car on, take several runs of each 'skin' and 'wheel' combination and measure distance traveled - you should be able to tell which "skin" and/or "wheel" combination provided the most resistance. (shorter distance = more energy dissapated by wind resistance)
There are also techniques for using a wind tunnel, involving attaching the vehicle to springs and attaching a measuring device to the springs, but I'm not familiar with the technique myself. My instinct would be to measure distance rather than a wind tunnel for this kind of trial, especially as people will care more about how a car performs on a track than in a wind tunnel. Perhaps others can chime in on the wind tunnel piece, or you can look it up online.
To make a "bumpy" surface, you would do best to look for some kind of manufactured surface you can just put on top of your track. I would look maybe at a carpet store (either the carpet itself or the padding), or the bathroom section of a hardware store (bathmats usually have ridged surfaces, and there are things you can stick to your bathtub to give more traction). Many of these places will give away odds and ends (eg, a carpet store installs into a house from big rolls, and they have bits and pieces left over). Very coarse sandpaper might work too, depending on the size of your wheels. Something like bubble-wrap is probably too variable. Some corrugated cardboard might also work.
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bradleyshanrock-solberg
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A conveyer belt won't simulate a track very well.
Soapbox racers have two modes of operation...going down a hill and going level. When going down a hill, their speed is increasing, when going level, their speed decreases. A lot of what makes a good racer win is putting the weight in the car properly to handle the transition between dowhill and flat, but that's something beyond the scope of what you are trying to measure.
A conveyor belt (or treadmill) has a constant speed. This may be easier for some purposes but it isn't a state that a soapbox racer ever experiences.
By using the same car (except for wheels or "skin" or track surface) and the same track, you eliminate most of that complexity and can use distance traveled as your indicator of whether a change in one of your variables resulted in more or less drag.
Soapbox racers have two modes of operation...going down a hill and going level. When going down a hill, their speed is increasing, when going level, their speed decreases. A lot of what makes a good racer win is putting the weight in the car properly to handle the transition between dowhill and flat, but that's something beyond the scope of what you are trying to measure.
A conveyor belt (or treadmill) has a constant speed. This may be easier for some purposes but it isn't a state that a soapbox racer ever experiences.
By using the same car (except for wheels or "skin" or track surface) and the same track, you eliminate most of that complexity and can use distance traveled as your indicator of whether a change in one of your variables resulted in more or less drag.
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ChrisG
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Just wanted to add that another response was posted here:
http://www.sciencebuddies.com/mentoring ... php?t=2990
http://www.sciencebuddies.com/mentoring ... php?t=2990