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In Reply to: I'm sold, do you have one F/S???? posted by Aaron Wyatt on March 31, 2002 at 18:02:08:
Gents...
With this posting I am at the limit of my knowledge, so I will tread lightly...
It is my understanding that the reason the COD is reduced is that the rear spoiler helps the air break cleanly from the rear of the car reducing the burble and non-coherant airflow behind the car...
To put it into experience that I am familiar with....
In the C-130, the company did tests on what they called "strakes" for an undisclosed user. The strakes were basically 18 inch high 20 foot long "spoilers" attached to the rear of the aircraft, where the side skin wraps around to join the rear skin. They were placed to keep the air moving along the side of the aircraft from whipping around and filling the negative pressure area behind the aircraft. The strakes (spoilers) caused the air to seperate from the rear of the aircraft and flow off cleanly to the rear, rather than allowing the air to fold around the rear and cause turbulence...
With the strakes, the result was a noticable reduction in drag. The strakes were never adopted for "other" reason, but were proven effective in reducing drag.
This "strake / baffel / spoiler" (you pick the term, they all do the same thing...) method was also employed to build baffels which would unfold as the ramp and door was opened in flight during paratroop airdrops. The baffels kept the air from whipping in over the ramp and allowed us to increase the airspeed (for other missions) from 110 to 250 kias.... a significant improvment in survivability....
The point of all this is to help explain how the spoiler helps the air seperate cleanly from the rear of the car, there y reducing drag...
Like I said above, the rear spoiler is a modification of the Kamm theory used by the Cobra Datonas and birdcage Masseratis (sorry for the mis-spellings...)
And with that, I have reached the limits of my knowledge and step back from the discussion, perhaps somebody else has some additional theoretical knowledge they would like to share...
Regards all
John Stein
PS... After years of success on the NASCAR circuit with using spoilers to create down force.. The current NASCAR regulations now require the rear spoilers be set at higher "angles of attack" to actually REDUCE the top speed of the cars on the track...
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