Solution to boat tail dragging On Ramps
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I've been told the optimal lower angle on a boat tail is 2.8 degrees. to prevent separation of the air from under the car as the car/tail passes through it. The problem is that on extended boat tails this leads to the dragging of the tail when crossing from one grade to another, such as entering a driveway from the street. If only one could raise the tail out of the way temporarily. If one could mount the tail to the rear of the car on two rack slides with fairly vertical electrically driven screws, and flip a switch on the dash.... Well.......
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Your second option is preferable but I'd replace the servo with a bungee cord wrapped like the landing gear of a light plane. IOW it would deflect and return without powered control.
And add one or two wheelie bars or tail skids, so the boat tail itself doesn't impact the ground. |
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However rear diffusers may hold the answer as an alternate solution. https://en.wikipedia.org/wiki/Diffuser_(automotive) I'm not saying that a rear diffuser is preferred or works better, only saying it may offer a real world solution or compromise that is simpler and more cost efficient than the illustrations posted so far. The winged or multi-deck diffuser is the option I drew up on one of my designs reviewed by an aerodynamists (see similar image below). I was told it does not work as well as simply holding the angle to the attachment threshold (2.8 degrees?) but will do in a jamb. https://en.wikipedia.org/wiki/Diffuser_(automotive) https://upload.wikimedia.org/wikiped...29_%282%29.jpg EDIT-1: http://www.racecar-engineering.com/t...-aerodynamics/ http://www.racecar-engineering.com/w...4/Diffuser.jpg EDIT-2: Burst vortexes? Yep, it's an interesting read. https://www.linkedin.com/pulse/how-d...rk-willem-toet https://media.licdn.com/mpr/mpr/shri...OWVhODI1ZA.jpg Quote:
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Kach22i, thanks for the links, but the problem I'm looking at is driveway scraping, and how to modify a tail to move it out of the way on more drastic grade changes. highway driving is OK.
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I'm with Freebeard on this. Hinged at the top, bungied at the bottom. Totally passive system, no user interference required to make it work.
HDPE wear pads at the extremes. Or what the heck, pine. If it only has to resist the downforce of the bungie and not the entire weight of the car, wooden rubbing blocks will do. |
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Sounds like you are married to a single concept, and exploring several solutions - which is cool.
I think you covered all the really simple solutions, anything else may be more complex. However the one solution left on the design table moves less mass/weight than any yet shown. Hard to explain in words, but imagine a piano hinge at the lower bumper edge. The license plate taillight lower edge moves up. The bottom panel is a large "U" in section. You have an unsealed with a gap/crack where the lower hinged moving "U" slides up into the upper shell (also a "U" for strength), an open overlap joint. Think of it as a hinged dustpan. The plate and lights would also have to be hinged, I've seen this done somewhere, at least the plate when the gas cap is behind it. An upside down (and backwards?) version of the Russian STOL jet below. Tails Through Time: The Experimental STOL Demonstrator That Fooled NATO http://3.bp.blogspot.com/_8DAL7gPYBi.../mig23pd-4.jpg In all of the designs I would allow air to escape out the tail, just in case the joint between the car and tail assembly is not 100% air-tight. EDIT-1: Like a dryer vent with triangular return sides. https://www.pinterest.com/pin/380906080958536898/ https://i.pinimg.com/236x/9d/94/f5/9...ers-dryers.jpg EDIT-2: I sketched it up, looks to be a drag penalty for being operable, open air-gaps not helping much. A cable and pulley system could operate it. Or just add some springs so it moves upon impact - that is if you don't mind a few scratches on your lower panel. |
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