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Old 02-11-2018, 09:55 AM   #1 (permalink)
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Solution to boat tail dragging On Ramps

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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Old 02-11-2018, 05:33 PM   #2 (permalink)
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Old 02-11-2018, 11:38 PM   #3 (permalink)
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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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Old 02-12-2018, 01:55 AM   #4 (permalink)
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Quote:
Originally Posted by freebeard View Post
And add one or two wheelie bars or tail skids, so the boat tail itself doesn't impact the ground.
Or, I could add a fifth wheel to the trailer that would hold the trailer body (and the rear wheels) off the road.

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Old 02-12-2018, 04:44 AM   #5 (permalink)
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Old 02-12-2018, 09:01 AM   #6 (permalink)
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Quote:
Originally Posted by Angel And The Wolf View Post
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. .......
I've been told similar and have read similar.

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)


EDIT-1:
http://www.racecar-engineering.com/t...-aerodynamics/




EDIT-2:

Burst vortexes?

Yep, it's an interesting read.

https://www.linkedin.com/pulse/how-d...rk-willem-toet

Quote:
Then at a certain point, as the body is lowered further, one of the two vortices bursts and a sudden reduction in downforce is the result. Which one bursts first is almost random but will depend on tiny differences (e.g. of dynamic roll)....................

This change of flow state is significant and results in aerodynamic “hysteresis”. The two vortices hang on fairly well on the way down but, once burst, you have to come back up to a higher ride height than the burst ride to get the flow back into a nearly “symmetrical” state. The fact that burst and re-creation happen at different heights is what we call the hysteresis. This is why the plot of downforce showed two lines – they are for lowering and for raising the diffuser. Clearly drivers will hate the feel of the car if you allow it to get to these areas of performance change.
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Last edited by kach22i; 02-12-2018 at 09:21 AM..
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Old 02-12-2018, 10:55 AM   #7 (permalink)
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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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Old 02-12-2018, 11:20 AM   #8 (permalink)
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Quote:
Originally Posted by Frank Lee
Is this a solution in search of a problem? It doesn't look too low to me; it's at or above axle height and it really doesn't need to be that long.
Scanning the first page of DDG "minimum departure angle' one finds:
Quote:
Approach and Departure Angle Design for Fire Trucks - Traffic ...
The local fire department has established a standard that sets the maximum approach/departure angle at 10.5%. I understand this to mean that I can not have an i....
The SAE paper with the answer is available for $78.00.
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Old 02-12-2018, 11:27 AM   #9 (permalink)
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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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Old 02-12-2018, 11:43 AM   #10 (permalink)
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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


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/


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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Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html

Last edited by kach22i; 02-12-2018 at 12:38 PM..
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