Hi folks, start to be a very interested.
Jack got very good points to the aerodynamic issue. In my new article
"Bonneville 2002" for the Fast Facts - the Speed Record Club -
magazine, is a picture I done from Jack with #988 during the World
Final - the title is "Jack Costella Innovator".
Jack and I work the same way - what I call inspiration design, may be
you can call it also innovation design. Using from all known rules and
laws the best to reach the goal. This is one of the reason, why I
like to discuss with Jack new ideas. He got all the time new points of
view, from where you have to see the "problem".
First of all, the Cd alone is not important - it's the Cdw (so we call
it in Germany), this is the Cd x size of the cross section - this
number has to be so low as possible.
A big teardrop get a bigger Cdw number than a small brick, good comment Jack.
Jack, Tom Burkland, myself follows the length rule. smallest possible
and smooth length.
The length rule comes from boats. The longer the line is on the
waterline, the faster the boat can go - longer line + more speed. Due
to this, that water resistance is more critical than air resistance,
this rule will be used today also in the automotive industrie.
Important is, that the airflow get a chance to clean the turbulences,
which the nose creates, to a smooth airflow along the body of the
car. The longer, the better. But it needs a clean body - best is a
straight surface - so as the McBride&Moreau or the Saltwinder from
Jeff Shipley (#438). Now, someone will ask, what's up with the 6
degrees (taper) rule. Depends where your critical cross section is -
most the area where the engine is - you run in a trouble, when you
like to use the 6 degrees rule from the front end, due to this that
the engine is close to the rear axle. This means that the width of the
nose will be much wider than necessary - which increase your cross
section size and increase the Cdw.
Let us make a look on the different concepts.
Tom Burkland - he creates a cross section which looks like a square
standing on one corner, this square is only so big as it was necessary
to cover the engine. Left and right he cut the corner off, so as it
was possible. The wheels are covered with small spoilers on the bottom
of the wheelhouse to clean the airflow in front of the wheelhouse. He
used not the 6 degrees rule - his starting cross section would be the
end of his rear engine with the result, that if he used the 6 degrees,
his cross section behind his cockpit would be 3-4 inches wider without
any aerodynamic profite.
Jeff Shipley - he used his rear engine as the reference cross section
for a 1 1/2 - 2 degrees taper. He got the advantage, that his cockpit
is on the end of the streamliner and also, that he use unblown engine,
so the height of the engine package is much lower. Only behind the
cockpit he used the 6 degrees rule, to get a clean rear end. The
percentage of the increase of the front nose cross section to the base
cross section is very small and due to this length of the car
acceptable.
Jack Costella - his base was the critical - single engine - behind the
driver - cross section. His idea was, to get the smallest cross
section in front and from there on a smooth surface/body, which
allowed the airflow to go around the body without to much
turbulences. He done this more radical in the #788 than in his former
car #988. But also to get an airpressure area which produced him the
downforce to get the power on the salt. Jack got here the advantage,
that he is not overpowered.
McBride & Moreau - this streamliner is in reality a motorcycle
streamliner with four wheels. So as Jack, we tried to get the
smallest(lowest)possible cross section in front, with one
different. From the beginning there was the idea to use a straight
sideline. If we tried the 6 degrees rule, we would get a so much wider
front end that we would loose all the advantage of the straight
sideline. Behind the rear wheel we went a little bit more than 6
degrees due to necessary short rear end which we need for the trailer
length.
Now, you like to hear a number, when it's better to go teardrop/6
degrees/length rule.
For the rear end, try to get the 6 degree - most you can start behind
the rear wheels.
Teardrop - so more the engine is in the front, so more you got the
profite from the tear drop design.
Length rule - if your cross section width to the length get a 1 to 8
(or more) rate, you be on the advantage side. Behind the rear wheels
you can use than the 6 degree rule. The McBride&Moreau stramliner got
a 1 to 12+ rate.
All is a mathematical calculation. Depends where your most critical
cross section is, calculate how much will the 6 degrees rule (you can
use also less than 6 degrees)increase your front cross section. Up to
5 %, no problem - up to 8 %, if you can use this more space for better
function (steering butterfly), alright. More than 8 %, forget it, use
a straight line and get the 6 degrees on the rear end. These are
roughly numbers.
Important is, if you use a teardrop design, that the shape
(left/right) of the teardrop is not used for the top (roof) line,
otherwise you create a lifting body. A mathematical correct teardrop
shape is very similar to the top of a wing design.
Hope this clearify the question about the length rule a little bit.
See ya
Pork Pie
Ps. hope that this time my English was a little bit better than last
night - I was too tired, sorry.
/// unsubscribe/change address requests to majordomo@autox.team.net or try
/// http://www.team.net/mailman/listinfo
/// Archives at http://www.team.net/archive/land-speed
/// what is needed. It isn't that difficult, folks.
|