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  • Evan
    replied
    Yep. The above trick will not only provide some back pressure but it also will use some of the exhaust heat to provide additional thrust. That idea is what made the P51 the fastest aircraft around in WWII. They used the waste heat from the radiator to overcome nearly all the cooling drag of the system by expanding the cooling air and expelling it out the restricted exit of the cooling shroud.

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  • topct
    replied
    Do not attempt to free up the exhaust.

    You want to keep as much heat in the engine as possible. A more restictive exhaust is the way to go.

    As far as an aluminum chassis goes. Look at how long a welded boat can last. They flex a bunch and seem to hold together.

    Using the proper alloy, a 5xxx series, and proper welding techniques your chassis would be strong enough.

    What do the rules say for materials?

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  • Evan
    replied
    i don't think 2.4 hp will generate a very noticeable amount of thrust...
    I wouldn't ignore it. A ten cubic inch engine running at 2000 rpm will be pumping about one half pound of air through it per minute. It isn't much thrust but in your application every tiny bit helps and it costs basically nothing to implement. If you can double that to one pound per minute out the back at maybe 100 mph exhaust velocity it isn't negligible.

    Dimensions? Nope, you will have to experiment.

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  • Fasttrack
    replied
    Hey evan - i don't think 2.4 hp will generate a very noticeable amount of thrust...but do you have some dimensions for that? Looks like a fun little thing to whip up - i love finding tiny little projects to do on a lathe because the small projects are pretty much right at my skill level

    I like the idea of using some kind of composites except for the money involved and the difficulty of construction (that is difficulty in constructing a vehicle on the fly with composites - read w/o any plans )

    Also - i realize aluminumm will work harden but these frames won't be used more than two maybe three years anyway - thats only one to three races which means max of 75 miles on the frame. (thats with practice runs added in)

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  • abn
    replied
    As others have stated I would consider the whole Smith lineup immediately, I've seen some specials if you buy them all at once:

    http://www.CarrollSmith.com/main.html

    I'm lucky to have Cerritos Community College relatively nearby, it has the only Community College based composites curriculum in California, I still have a sample of kevlar/aluminum honeycomb I stand on once in a while to amaze myself at how light/strong/stiff it is.

    In some situations steel can actually be the best material, especially if you are limited in cross sectional area and you have the time to machine the part and remove material in un-needed areas.

    I have some AZ61 magnesium it's easy to work with, but I don't know a cheap source.

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  • Scishopguy
    replied
    Aluminum

    You may want to avoid aluminum in the frame and support members and opt for the carbon fiber material. The folks who made bicycle frames from aluminum tubing soon found that it hardens with age and after a few years, tend to fail at bad times.

    Jim (KB4IVH)

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  • Evan
    replied
    Here is a little trick to recover a bit more energy from the engine. Naturally you would point the exhaust out the back to gain the small amount of thrust it produces. That can be considerably augumented by using a simple arrangement to cause the exhaust to entrain additional air. The mass of the air entrained will more than double the thrust from the exhaust. It's the same principle that bypass jet engines use compared to pure jets.

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  • Duct Taper
    replied
    Fastrack, I just checked with my son who was in the Supermileage program in high school and the Formula SAE in college.

    The supermilage car that they built in high school had an all aluminum frame and only weighed about 75lbs. I do recall going to the high school competitions and seeing some of their aluminum welds break. They had to repair them on-site with that perforated strap that plumbers use to hold up pipe sometimes.

    In the Formula SAE car they used mild steel tubing, that way it was easy to repair with a torch.
    Last edited by Duct Taper; 05-02-2006, 11:13 AM.

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  • SJorgensen
    replied
    Machine it out of heliuminimum. This metal is very easy to work with except that you have to vacuum the chips off the cieling. Care must be taken. Don't let go of the part when you are outside. You'll never see it again.

    Spence

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  • john hobdeclipe
    replied
    "Carroll Smith's Nuts, Bolts, Fasteners and Plumbing Handbook"

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  • Fasttrack
    replied
    Evan - yeah i did a little research online about graphite epoxy rods and realized that they must have had fiberglass. Shoot it all looks the same to me

    Joel - Thanks for the recomendations; those will go on my growing wish list of tools and literature!

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  • Joel
    replied
    Engineer to Win by Carroll Smith
    Any other book by Carroll Smith, like Prepare to Win and Tune to Win

    How to Make Your Car Handle by Fred Puhn
    Chassis Engineering by Herb Adams

    Engines are a broad subject, but a good book with a fair amount of explanation as to why certain modifications help is Power Secrets by Smokey Yunick

    Leave a comment:


  • Evan
    replied
    If they were able to deflect graphite epoxy rods they must have been using tiny sections. Don't confuse it with the same looking fiberglass epoxy resin rods. Graphite epoxy hollow tubes are incredibly lightweight yet incredibly strong and stiff. I used .5" tubes with .030 wall on my telescope for some of the truss. On a length of 2 feet supported on the ends with a one kilo weight suspended in the center they deflect about .010" but only weigh about 30 grams.

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  • Fasttrack
    replied
    Great! Thank you for the suggestion - this kind of leads me off on a tangent which i may post as a thread later on, but anyone have any suggestions for good reading/reference material for designing cars, engines, etc? Any kind of Audel's type handbook for racing? That would be awsome; i've looked around at the local book stores but haven't turned up anything besides Monster Garage Weld Anything and etc type books.

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  • QSIMDO
    replied
    A good place to start is the "Racer's Encyclopedia of Metals, Fibers and Materials" by Forbes Aird and published by Motorbooks International.

    Border's, Barnes & Noble maybe even Amazon will have it.

    Leave a comment:

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