Hi, Dad. Interesting project.
Been daydreaming about such a thing myself, but it will be a while before I
can get to it.
I have a book-- Aerodynamics for Performance and Racing Cars-- that has a
little to say on that subject. One thing it says is that the test object
should not block over 5% (area) of the tunnel. In other words-- for a 6'
diameter tunnel, scientists wouldn't want a test subject higher than about
16"! Now I know "Lucille" is low and slinky-- but she ain't THAT damned low
and slinky (LOL).
But I'm not meaning to discourage you. The scientists are trying to take
really precise and reliable ("quantitative") measurements, and what we
racers are usually trying to get is "qualitative" data-- that is, to
visualize and study the airflow patterns and make improvements until we get
the flow over the vehicle as smoothly as possible.
Ya don't need no stinkin' 16' tunnel to go get that "qualitative" stuff for
a full-size bike and rider. Your 6' tunnel will probably work just fine for
that. Just don't try to use it to develop an accurate Cd number for the
full-sized rig.
(Shoot, one ol' boy on our list is reputed to have developed some of the
best streamlining on the salt using a leaf blower!!)
One way you could easily (and cheaply) see whether "...what i'm doing is on
the right track or making me slower" is just to measure the "pressure-drop"
across the bike at a repeatable air flowrate. Higher pressure-drop means
you've hurt the streamlining, lower pressure drop means you've helped it.
There could be several ways of instrumenting that. The easiest and cheapest
way would be to use an unvented air pressure gauge tapped to the tunnel in
front of the bike (3', or so), and set your flow so that it is the same (as
close as possible) every time. Tee-off that same tap to one leg of a U-tube
manometer. The other leg of the U-tube should be tapped into the tunnel a
good distance behind the bike (to avoid the bike's turbulence). 15' or so
would be nice (still needs to be a good distance upstream from the fan, if
you use a suction fan).
Run your tunnel, set the correct pressure on the unvented gauge, and measure
the delta-P (pressure difference) on the U-tube.
The unvented pressure gauge helps you avoid having to do atmospheric
pressure corrections. The one drawback is that you will still have air
density errors unless you run all your tests when it is about the same
temperature outside. (Air density errors could cause you to incorrectly
evaluate the effect of SMALL streamlining changes.)
You can buy U-tube manometers pretty cheap. You can also make one with a
ruler (2 rulers can make it easier to read)-- a length of clear fuel line
tubing-- and a piece of plywood or plastic to mount it on. Fill it with
gauge oil, distilled water, or mercury-- depending on the pressures your
tunnel generates.
When you get it working, I want to drag on over and check my little gixxer--
unless you're gonna make it "Harleys ONLY"!
Russ, #1226B
p.s.--You can get the aerodynamics book I mentioned from Motorbooks.com.
It's by Forbes Aird. Not everything we want to know, but some pretty good
stuff. Not engineer-y or boring. But not much specific to bikes.
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