Hi Joe:
I'm not an aerodynamicist, so all I really know about airflow over control
surfaces comes from the data the aerodynamicists gave me when I was
spending
40+ hours per week making decisions about spoiler vs. aileron responses to
wheel inputs vs. trim inputs vs. speedbrake inputs vs. autopilot inputs.
(And yes, many spoilers are subjected to positive pressure from a vented
area underneath them, but I was lead to believe that even spoilers over
unvented compartments could float up with a disconnected actuator,
depending
upon the airfoil design.)
Take a look at paper number AIAA-2004-1243 submitted at the 2004 AIAA
Aerospace Sciences Meeting. Here's a link to it online:
http://www.dlr.de/at/PortalData/2/Resources/dokumente/at/aiaa-2004-1243.pdf
These folks analyzed the use of freely floating flaps mounted to the upper
surfaces of aircraft wings. The reason they pursued it was to see if they
could improve wing performance at high angles of attack. But they're
examining exactly what we're talking about. They found that the flaps
mounted on the aft section of the wing *can* lift slightly into the airflow,
due to the adverse pressure gradient on that region of the wing. If they're
mounted farther forward, where the pressure gradient is positive or zero,
they don't lift at all. They also found that, if they do lift into attached
flow, they cause separation, decreasing lift and increasing drag.
The drag coefficients for the Porsche 928 with the different factory
spoiler
configurations are well documented (I didn't measure them myself).
There are things to consider here. Is the bodywork on all three models
exactly the same, except for the spoiler? Do all three models sit exactly
the same height from the ground? Do all three models have the same width
and diameter wheel and tire combination? These things--especially height or
the presence or absence of a chin spoiler--all affect the lift and drag on
the cars. If they're identical, except for the spoiler, then we've got
something interesting to look into.
Regards,
Rich Wagner
Montrose, CO, USA
'82 GTV6
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