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Old 12-16-2010, 05:49 PM   #311 (permalink)
aerohead
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COP and handling

Quote:
Originally Posted by euromodder View Post
Why would Jim consider adding area to the rear, while the centre of pressure is obviously still forward enough not to cause handling issues in sidewind conditions ?
A fin wouldn't cure an aft shift in CoP, it'd make it worse.

If not perfectly symmetrical to eliminate "lift", a vertical tail surface would add a permanent resultant sideways force working on the vehicle.

A fin also adds to the frontal area, and thus to the aerodynamic drag, the reduction of which is why the tail extension was built in the first place.
Been looking at this and have ended up on same side of fence as lunarhighway.
Low speed understeer is preferred by carmakers selling to drivers that might freak out in an oversteer condition. And rather than require opposite-lock steering and throttle modulation to control the drift,the driver simply 'steers' harder in the direction they want to go.
In Jim's situation,if the car IS breaking loose first at the front,then as the car yaws,it grabs more air,increasing the yawing moment,and conceivably,the car could go to an out of control situation as lunar has mentioned.
Kamm was obsessed with this concern and put fins on all his personal cars.His last project,a DKW station wagon was festooned with roof-mounted slotted fins.
Ditto, Jaray's fast cars.
Ditto ,Walter Korff.
Dr.Alberto Morelli put fins behind the rear wheels underneath the CNR car to guarantee a rearward COP bias behind the CG.That car generates zero lift,so un-weighting the front axle was not an issue.
With Jim's tail,I'm pretty sure there's separation under there and wake pressure ( vacuum ) is no doubt communicating forward under the tail,but I suspect that there is so much pressure bleeding between the flanks and underside,that a pressure moment arm,as in race cars, cannot take form.Tufting in that crucial area would tell us a lot.
Thinking out loud,I would be tempted to construct as deep a rear wheel fairings as approach/ramp/and break-over would permit,which in addition to streamlining the wheels,would help with the COP as well.
P.S. GM's Epcot car movable rear valance would cancel the separation and move the wake completely behind the car canceling any lift should it exist.

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