Quote:
Originally Posted by RobertSmalls
Jim,
I thought of an experiment I wish I had time to run before building my boat-tail. I'd like to know the maximum angle for mantaining attached flow across the underside of the tail. If I could get away with 9° instead of 7°, it would save a few inches of overall length, which would be great. Or if I can only get away with 4°, but I need 7° to meet my departure angle requirements, I'll make the first half 4° and the second half 11°.
Tuft testing at 30mph would determine that angle. Aerohead assures us Re* is achieved by that speed, so if flow is attached at 30mph, it will be attached at 80mph.
Attach a sheet of cardboard (with camera underneath) or plexiglas (with camera peering out through the hatch) to the stock rear diffuser, on a duct tape hinge. Attach 20 strands of yarn to its underside. Support the rear of the cardboard or plexi with a string, tied to the hatch. Adjust the length of the string with a clamp or something, to modulate the angle of the board (measured with protactor and plumb bob). Start at 0°, and test at various angles.
For extra credit, use a smoothly curved piece of cardboard or plexi.
Accuracy would improve if a cardboard boattail was already in place.
Did I just spend more time describing an experiment than it would take to execute it?
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Robert,I saw a trick General Motors used on their Aero 2000 concept car at Epcot Center.
Their diffuser was hinged at the attach point to the car,and once they were at cruise speed away from urban hazards,the panel lowered to the desired orientation for best airflow.
Piano hinge would do the job.A cable and pulley system could operate it,with no more than something like an emergency brake handle,with release for manual control,locking,and release.
Best of both worlds!