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Aerodynamics help -- small scale car powered by a CO2 cartridge

  1. Jun 13, 2017 #1
    I am currently engineering my v2 version of my f1 in schools car, it is a small scale car powered by a co2 cartridge designed to go down a 25m track, my current version does 25m in under 1.4s, however, I would like to shave off another 0.3-0.2s I believe my main issues are the bearings (only ceramic hybrids) and the weight. It weights about 70g I believe. I have attached pictures of the model and the one I am currently designing to be CNC'd. Will taking material off the side of the body underneath improve speed, any tips?

    https://i.gyazo.com/d10bb98329a639e96fbd9d942d0da553.png
    https://i.gyazo.com/dd0c44a7416831d6905dab4849031632.png
    https://i.gyazo.com/b4f70659cfef30fe55607dee724c5fd3.png

    As you can see on the second model the front wing has a little funnel to direct the air into the under slot, should I place the wheel as high or low as possible?
     
  2. jcsd
  3. Jun 13, 2017 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. :smile: Looks like a fun project.

    Do you need downforce on the front for some reason? If not, I'd use a triangle shaped piece instead of the wing shape to smoothly deflect the air over the front tire. That wing will have more drag than a shaped triangular piece, IMO.

    How is the car guided? Are there side walls on the track to keep it going straight? Or is there some runner of some kind underneath that fits into a slot down the length of the track? Whatever mechanism is keeping the car going straight, you will want the lowest friction interface possible (obviously).

    I would also hollow out those wheels, and/or make them skinnier to lower the MOI, which resists the acceleration of the car. What are the rules for the cars that you are designing within?

    Also, I didn't understand your question about placing a wheel higher or lower...
     
  4. Jun 13, 2017 #3

    JBA

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    I agree with the above on the removal of the wings and wedges shading the front of the front wheels; also, as stated above the wheels should be absolutely as narrow as possible (wheel bearings as well). Down force wings and features and wide tires on F1 cars are strictly for cornering adhesion and are both very detrimental to maximum speed on the straightaways.

    For some good aerodynamic low drag and designs take a look at world speed record vehicles, soap box derby cars and world mileage challenge electric powered cars.
     
  5. Jun 13, 2017 #4

    jack action

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    I would say that the aerodynamics is somewhat wrong on your basic design.

    The aerodynamic resistance is based on 2 vehicle characteristics: The drag coefficient ##C_d## and the frontal area ##A##. A lot of people tend to focus on the ##C_d## and forget about ##A## which is often a lot easier to decrease. In your case, the minimal frontal area is the frontal area of the wheels (the rear ones should be aligned with the front ones to share the same frontal area) PLUS the cartridge frontal area. Anything else is superfluous and should be kept minimal to keep the structure integrity. I like this kind of design:


    See how it is basically 3 «cylinders»: One for the left wheels, one for the right wheels and one for the cartridge. They are held together by thin plates.

    As for the ##C_d##, the best shape is one that looks closer to a teardrop. What is important to note about this design is the following:
    • The front end can be blunt. Often, a half-sphere is OK. On a general principal, shapes with an angle less than 60° from a longitudinal plane offers no real advantages for reducing drag.
    • The rear end is where the shape is the most important part. The best slope angle is around 7°-10°. The small angle prevents flow separation which creates a vacuum that pulls back the vehicle. Your vehicle design behind the rear wheels doesn't conform to this principal.
    • Creating lift or downforce usually increase drag. The design of your front wing creates downforce which will necessarily creates drag. Worst of all, this downforce creates a higher normal load on your wheels, which in turn increases the rolling resistance. I don't think wings are useful on this type of vehicle as traction is pretty much irrelevant. If there are wings, I think producing lift would be best, just to reduce the rolling resistance (like your car is almost flying). If one was needed to increase stability (putting the center of pressure further behind the center of gravity), I would prefer a vertical rear wing.
    • Rotating wheel drag is awful. You should generally enclose the wheels as much as you can.
     
    Last edited: Jun 13, 2017
  6. Jun 14, 2017 #5
    At the relevant speeds, drag isn't the biggest factor. Focus on weight reduction for max acceleration. Super skinny wheels (saves drag and weight), lower the thrust line to minimize cross section and needed structure, then add by experiment the least amount of down thrusting wings that will hold it on the track.
    For stability, keep the centre of mass in front of the centre of drag - using small front wheels (maybe even just one, fully faired inside the nose cone) and larger rear wheels will help.
     
  7. Jun 14, 2017 #6

    JBA

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    @julianfer5, we have made several suggestions; but, unless you reveal the regulations or restrictions that you are working under we cannot know the relevant or better ones that might apply.
     
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