Re: Help me build an aerodynamic car

Steven,

First of all, be very careful. Take safety seriously whenever you handle or
use energetic materials, or eventually they will get you. If you have
doubts about how to handle them or how to launch your projects safely, find
a local model rocket club and ask for safety tips. Okay, preaching over, on
to the suggestions.

One thing that could cause you problems is improperly aligning your rocket
motor. You want it to be as level as possible so that the thrust is not
directed up or down, as that could lift one or both of the axles off the
ground. Besides, you want all the thrust pushing the car forward so that it
goes as fast as possible.

Even if it's perfectly aligned, the shape of your car could easily generate
enough lift to turn your rocket car into an unguided missile, and
unfortunately it sounds like you've already had some experience with this.
Throughout their history, people who race automobiles have come up with
innovative ways to deal with this problem, from spoilers, to creating a
lower pressure under the car with vacuum pumps, to basically strapping
wings on the car (but to push it down, not lift it up). This is so critical
to the performance of the car that modern racing leagues have very strict
rules as to what exactly can be done to the cars to keep them on the road. 

So you already know that downforce is important, but there is a down side
to downforce. As an object generates lift, it also generates some
additional drag. This is because the pressure forces that cause the lifting
force (or downforce in your case) are not evenly balanced and they will
push at least a little bit backwards in addition to up (or down).
Aerodynamicists call this "induced drag." It's important enough that
aircraft are designed to generate as little lift as possible during their
rollout prior to takeoff so that they can accelerate faster, then rotate
upward to create the lift that will get them off the ground. 

Back to your rocket car: as I see it, there are several ways you can go
about it. You could dig in to the aerodynamics and design spoilers or wings
that will generate just enough downforce to cancel out the lift caused by
the rest of the car, or you could redesign the car shape to produce a
slight downforce instead of lift. Although the basic equation for lift is
simple, your car is probably a complex shape and it would be hard to
analytically determine the lift coefficient (you'd probably be better
served by finding a wind tunnel and using that to measure it).
Alternatively, you could just add wings that will produce a lot of
downforce and just accept the fact that the extra induced drag will keep
the car from going as fast as it could without the wings. You could even
put a set of wings over each axel so that you don't inadvertently tip the
car because of the moment (torque) created by the downforce. These wings
don't need to have a sophisticated shape, they could be as simple as a flat
plate angled so that the back end is slightly higher than the front (three
to five degrees from horizontal should be plenty). 

If you really want to play around with it, you could design the car so that
wing shapes/sizes/locations could be changed easily, then do a series of
runs starting with the most conservative (largest downforce) and gradually
finding ways to reduce it until the car performs just the way you want.
Even with all the modern technology available to today's racing teams,
including sophisticated wind tunnels and computational fluid dynamics, they
still rely on the driver's feel for the car to make final adjustments to
the aerodynamics, so my guess is this will ultimately give you the best
performance in your rocket cars. 

Good luck, and if you end up running into more specific questions along the
way, feel free to ask.

David Coit

References:
 http://www.nas.nasa.gov/About/Education/Racecar/aerodynamics.html
 http://www.grc.nasa.gov/WWW/K-12/airplane/lifteq.html
 http://www.desktopaero.com/appliedaero/wingdesign/wingdesign.html
Introduction to Flight, 3rd Ed., John D. Anderson, Jr.
Aerodynamics for Engineers, 3rd Ed., John J. Bertin and Michael L. Smith