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.......

Or:

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.

Or, I could add a fifth wheel to the trailer that would hold the trailer body (and the rear wheels) off the road.

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)
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

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.

Scanning the first page of DDG "minimum departure angle' one finds:
The SAE paper with the answer is available for $78.00.

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.

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.

Hinge it at the top, bottom or mid-point (for minimum gaps).
http://i.ebayimg.com/t/RC-Model-Airc...uLQ~~60_35.JPG
http://i.ebayimg.com/t/RC-Model-Airc...uLQ~~60_35.JPG

This is for an R/C model airplane, but the construction is very simple. You could use four of them, or make you own.
https://www.youtube.com/watch?v=jAawhg6JtyY
Also — Retractable skegs.

10.5% grade = 6° angle. (Had to look it up)

Bungie's would be great for the experimental stage of the project. For the long-term I would prefer something a little more resilient than rubber filaments.

I like option B better with a top hinge. A bottom hinge requires armadillo slipping plate armor crossbred with a slinky in order to allowed to articulate and accordion
The bird cage/ skin supporting structure would have to be extremely rigid(read heavy) to survive the torsional load applied to cause it to slide up an example of .A
https://www.kurveygirl.com/woodcraft...01.444x225.jpg
I was thinking more in terms of MotoGP for the skid points extremely abrasion-resistant bolt on out of the box at this time 2 pucks are $59
Found hear: http://kurveygirl.com/shop/product_i...hoC3MoQAvD_BwE
With the air dam I experimented with I went beyond what I saw as an acceptable lower limit just to determine how far I could go before I would start hitting the ground 2.5' forward of the contact point of the tires at 10 in of height is where I ran into the ground on the aggressive driveways the normal driveways that don't have chunks of pavement missing from their impact points, 10" was just fine. @8" parking blocks for Troublesome and driveways folded up like a taco shell. Fortunately this prototype was set up for rapid prototyping and it was not difficult to set right after an impact.

Go to a track where they race motorbikes, the guys throw away the pads on their suits.

They should be adequate for the job especially if you have a free source of them.

I like that idea!

https://images.duckduckgo.com/iu/?u=..._0342L.jpg&f=1
https://duckduckgo.com/?t=palemoon&q...e+landing+gear

Not talking about a strap with hooks on the end.

Why would the bottom of the boattail be flat? We don't want the top or sides to be flat. Why not like this?

http://ecomodder.com/forum/attachmen...1&d=1518472998

Also, I believe the angle freebeard shared refers to the angle from the ground to the back bumper, boat tail, or what have you.

The thing I am fascinated with, today, is the M-80 Stiletto. Hydrodynamics is related to aerodynamics courtesy of Reynold's number.

The M-80 trades fineness ratio, as you see in corvettes, for modulating the interface with the medium it rides on. The 'underbody' is highly refined, a 'pentamaran'. The intermediate hulls are thickened for the surface-piercing propellors. (Their fluted blades are interesting in themselves. The forward aspect of the hull is viewable at:

https://img.newatlas.com/5151_40206114026.jpg
https://newatlas.com/stiletto-experi.../5151/#gallery

Long story short — the underbody shouldn't be flat, it should be two pontoons to enclose the wheels and a Bucky Fuller 'air keel' in between. I'll leave how to apply that to a Prius as an exercise.

That sounds like one of those car companies that died in embryo.

You mean, more like this?

I would say more like this:

http://ecomodder.com/forum/attachmen...1&d=1518484033

I do not believe air would stay attached if you simply flipped the template because you are skipping the first half of the curve. This one has the curve from the peak of the roof until the termination you chose.

More like this:
http://ecomodder.com/forum/member-fr...80-silver4.jpg

A single loft line on a side view is a long, long way from three-dimensional reality.

Compare the Lotus to the pentamaran boat. If the outer fence were thickened into a wheel spat, that would correspond to the shrouded propellers. F1 cars have similar refined underbodies with tunnels and vortex generators. [citation needed]
An highly evolved underbody could give as much benefit without adding [much] length.

It does, but drastically shrunken in length. The lower curve is twice as abrupt in it's angle change, and should work as well at half the speed, but not at full speed.

Yes, Beard, but F1 cars have high drag, and are about downforce at the expense of mpg.

They're fat-tired open-wheel cars. The high drag comes from the tacked on wings. The underbody does contribute downforce. It does it by expediting the airflow through a closed plenum with tunnels and induced vortexes. I don't think that adds drag, but I've been wrong before, once.

I talked above about a 'pontoons enclosing the wheels'. More specifically look at the spats on the Template as Xist showed at Permalink #16 (that you keep cutting off). And look at basjoos' Aerocivic:

http://ecomodder.com/forum/showthrea...1-0-a-290.html
https://www.aerocivic.com/imgs/aerocivic-top-2.jpg
https://www.aerocivic.com/

https://www.aerocivic.com/imgs/rear-3-4-s.jpg

It has double-walled side skirts flushed to the inner and outer walls of the tires, 'catamaran-style'. It has zero break-over angle but I think the skirts are conveyor belt material. Notice also the wedged out nose and high boat tail. They work together.

Here's my own theory on how to terminate a Template shape (this one is squircular):

http://ecomodder.com/forum/member-fr...14-1-42-00.png

It is an half-bellhorn diffuser that couples a flat bottom to the truncated tail.

It's all the same from approximately 25mph to approximately 250mph. More benefit going faster.

I notice the Aerocivic has the same high tail that I originally started with. Someone told me that I needed to keep the bottom at 2.8 degrees, or did I misunderstand? Very possible.

I don't know what a breakover angle is.(?)

Interesting. Don't the lower sides of the bellhorn drag in dips?

So, do you favor the flat 2.8 degree, or the profile curve for the bottom plate of the boatail?

Mr. freebeard uses the English language differently than the rest of us. I am pretty sure he makes up half of the terms I do not know! :)

I agree but he declined to entertain ideas dealing with diffusers early on in the thread. In my opinion nothing should be left unexplored or predetermined to be a waste of time as design is circular not linear.

I also think it's a mistake to go this far without top view, bottom and rear views.

I have a 1/8 scale die-cast model of a Porsche 911 that has proven useful in 3D thinking about aerodynamic modifications on my car.

I'm suggesting here and now that a Prius model be purchased and a scale mock-up be fabricated.

Design is a process of making decisions, informed decisions. Design is not an end product, the product is the fruit of the design tree with lots of dead-end branches.

Working in model form may be time consuming but many things just become apparent that otherwise are surprises once you start on the real thing.

Surprises are even more time consuming - and expensive.

EDIT:
One of my favorite solutions to the given problem, pause the video at 2:20.

http://www.polyjoule.org/urbanconcept

I would think making the rear wheel spats/diffusers hinged or out of a flexible material would be easy enough to do.

EDIT-2:
A road going car with similar design approach.

https://www.riversimple.com/the-tech...-hydrogen-car/
https://www.riversimple.com/wp-conte...st-650x415.jpg

If one were to look at the diffuser articles I posted links to and understood the vortex entrapment principals, many other options to the 2.8 degree flat bottom could become apparent. Fixated on a single concept is just one way to go, opening up your mind to other possibilities is invaluable. Just my opinion.

I was a time-consuming and expensive surprise. At least my parents were already married.

I remember talking to a new father once. He was explaining to me that he was going to have to sell off his prized possession a blue late 1970's Trans-Am. He said the child was going to cost him 2 or 3 times that before his 5th birthday.

That was like 20 years ago, I bet the kid is in college now. That Trans-Am would have been a great investment in retrospect - if he only knew.

https://www.focus.de/auto/elektroaut...d_5678434.html
https://p5.focus.de/img/fotos/origs5...962-bild03.jpg

EDIT:
So far I have not heard plans that include a belly pan. I don't even know what year the car is.


https://priuschat.com/threads/skid-p...0-prius.84539/
http://mannphoto.com/prius/bottom/smbottomfront.jpg
I think I'm out until more is known.

Kawaii!

Here's the design I made for a conveyor belt wheel spat. Pinned on the sides but open at the front and back, so it will deform easily:
http://ecomodder.com/forum/member-fr...42-mudflap.jpg

It's drawn flat, but conveyor belt would give curved spats.

At least this OP reports back, unlike the guy who started that argument about scientific consensus or whatever.

This from schumiGO on F1 CD

"F = 1/2 * rho * Cd * A * V^2
I don't know exectly situation now but in 2000 and 2001 in soe track not so fast and slow maclaren has Cd that used in F = 1/2 * rho * Cd * A * V^2 formula near 0.67 and 0.65
So now it must be a little mort 'cause aero rules had been changed..
So now range [0.75:1.25] is realy possible."

https://www.f1technical.net/forum/viewtopic.php?t=1861

Soe track...mort...realy?

You're making me think about F1 more than I wanted to (it's sunny outside), but let's ask http://formula1-dictionary.net

http://formula1-dictionary.net/Image...0side_merc.jpg

Now I'll need to read up on stepped planes and barge boards.

]

Guess I should have used *sic* more. That was all quote from the URL I included.

No slight intended. I was using it as a counterpoint to Xist.

What's your current thinking on adding a bustle to the Prius?

A WHAT? I'm not familiar with that term.

Once again, baiting Xist. Sorry [to both]. Multiple layers of meaning

OK, I went ahead and checked, and the profile averages 15 degrees where it would apply to a boat tail:

I'm looking into a way to direct a 48" wide x 4"tall under bumper air scoop to a 12" wide by 16" tall exit under the Boat tail, with the idea that throughout the passage under the boat tail, any cross section will be 192 square inches, thus maintaining an equal pressure, and hopefully preventing the low pressure area that would draw the tail down Drawing to follow

I gave that my best shot at Permalink #24 with the half-bell. Think about a NACA duct. Vortexes generated by the curvature of the duct sides pulls air down into the throat of the duct. This acts like a reverse NACA duct, pulling underbody air in toward the center and up into the bell.

For balance, here's a different approach, from Bonneville:

http://ecomodder.com/forum/member-fr...ture5476-9.jpg

This is also a bell — the roof has some curve to it. But it progresses from the full width, narrow inlet to an outlet that is ~4-5x the opening. I think this slows the air down and decreases it's static pressure to match the wake. But... I'd want to get aerohead's opinion on whether that's even wrong.

and again:
http://ecomodder.com/forum/member-fr...dynamics-c.jpg
2nd from the top left is the High Momentum Mudflap. It's boat tail shaped.