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Old 01-22-2023, 02:23 PM   #21 (permalink)
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Originally Posted by Cd View Post
If you wanted to have straight sides, but get the benefit of tapered sides, you could perhaps create ducts that channel air inwards to a second wall. You could then have the air exit out into your wake to make it even smaller .


Whose Ioniq is that? Or did you draw that? That actually kind of looks cool!

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Old 01-22-2023, 02:59 PM   #22 (permalink)
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The purpose of tapering all the sides is to reduce the total area of the back, low pressure zone area, that low pressure can "suck" the car to slow it down. If you can only get a 10% smaller back area by tapering the sides, then you are leaving a little drag reduction on the table, but saving a bunch of work. As you know, forming compound curves with cardboard or corrugated plastic sheets is very difficult.

There is a guy on youtube that built his boat tail out of foil back 2" styrofoam using different rear cross sections (large to small) and then skinning them with tapered pieces of the 2" foil back foam (shiny side out) then taping seams with the aluminum foil tape. Looked pretty decent and strong.

Since you don't want to permanently drill holes, look what you can use as a starting anchor point for your aero mods. Rear tow hooks and their mounting point?. Maybe some straps that start in the car, go under the hatch lid, then shut the lid on them. The rear license plate anchors. Bolt/ screw some structural members to these then screw the coroplast or cardboard to these so the whole thing won't blow off at once if the tape fails. Then gorilla tape the seams from the coroplast or cardboard to the body. Gorilla tape is very strong and won't blow off or come off from heat from the sun. You can use a heat gun to carefully remove the gorilla tape to minimize glue residue later. Duct tape sucks and will come off after a few weeks of sun beating on it and leave horrible glue smears.

Anyway, trial runs with your temporary aeromods should be on back roads with no one behind you. Then take it up to about 10 mph over your design traveling speed to make sure it won't blow off.

My cheesy aeromods are coroplast but are gorilla taped then truss head screwed to the sheet metal after the gorilla tape. I don't care about drilling holes in my car. I can always weld them shut, and the car needs painted anyway. My temp aero mods have become permanent. Some day, I will take all of the mods off to complete the A-B-A testing, then make them permanent with sheet metal or ABS sheet and welding.
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Old 01-22-2023, 03:31 PM   #23 (permalink)
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As you know, forming compound curves with cardboard or corrugated plastic sheets is very difficult.



Follow the geodesics.
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Old 01-22-2023, 03:39 PM   #24 (permalink)
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Old 01-22-2023, 04:49 PM   #25 (permalink)
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Quote:
Originally Posted by Phase View Post
Whose Ioniq is that? Or did you draw that? That actually kind of looks cool!
No idea. Just a image I found.
Just a doodle.
After looking at it now, and lining it all up on a side image, you would have a much larger area at the far end of the tail.
Also, not sure why the image is posted sideways.
The " 1, 2 and 3 " on that first image was 1) curved tail, 2) channel within the wall of 3) the flat section of the tail.

I notice quite a lot of 'tails here have detached flow - even without a lot of curve, and following a really gentle arc.
I planned to ventilate the walls of my tail so that it would create a slight suction for this very reason.

I chose to curve in my 'tail sides because the sides have twice as much surface area as the roofline.
As others have pointed out, it reduces your wake size considerably.
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Old 01-22-2023, 08:04 PM   #26 (permalink)
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The Ioniq sketch rear roofline looks kind of sharp and steep. Maybe detached. Also don’t get the hole behind the rear wheel area. Is probably be less steep with the tapering and have nice smooth attached flow. My wake area may be a little bigger though
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Old 01-22-2023, 11:53 PM   #27 (permalink)
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Also don’t get the hole behind the rear wheel area.
I hadn't said anything because I'd need to rotate the pic to be sure of what I'd be saying.

Is that an air scoop on the backside of the fender flare?
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Old 01-23-2023, 12:43 PM   #28 (permalink)
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'difference'

Quote:
Originally Posted by Phase View Post
Whatís the impact difference between tapering just the roofline versus tapering the sides too as a full boat tail? I noticed all the eco modders taper all directions for their boat tails. Makes sense. But what about if I just extended the roofline and didnít do too much to the sides? Think Mercedes EQXX or the light year zero. The sides of the rear only have a super small taper that isnít very noticible, the main thing is their rooflines going down and extending

How much CD drop would I see just be extending down my roofline while only lengthening out the sides like those cars?
1) Ph.D. Elliott G. Reid, of Stanford University, studied this very subject.
According to his research, you'd suffer a 16% drag penalty by not providing the plan-taper.
2) Lightyear and EQXX are both boat-tailed in plan-view. From rear axle centerline back.
The tail on the EQXX constitutes 28.5% of the car's total length, and ends with a departure angle of 13-degrees to body centerline.
15.5-degree departure angle up on top.
The diffuser looks like 8-degrees. I can't really tell.
The rear transom is 337mm narrower than the car's maximum width.
You'll discern it in the overhead photograph provided by Mercedes.
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Old 01-23-2023, 12:59 PM   #29 (permalink)
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plan-taper

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Originally Posted by Phase View Post
I would extend the sides. Iím saying the angle would be very minimal. Most of the low Cd cars out there just have a long roofline and the width in the rear isnít as narrow as how most ecomodders do their after market boat tails. I understand itís not as optimal, but in theory a proper boat tail would give me an average of 10 extra mpg, but Iím willing to add some styling personality and say, get only 8 mpg
I would respectfully submit that, all low drag cars use as much plan-taper boat-tailing as length will permit. And the taper continually increases along a continuous, non-uniform, streamline path.
The 'angling' of the sides is pressure recovery, along with injecting the higher pressure flow 'towards' the wake, 'multiplying' the effect. Ask a tuna.
Also, since we're talking about a 'fastback', their drag is the most sensitive to tumblehome.
As the sides extend rearwards, their 'tops' should also simultaneously 'fall' inwards, along with incrementally increasing in radii.
It's a very 'complex' build if you're going to get it right.
Any benefit accrued to the tail will be dependent upon the care you take in it's design and dimensions.
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Old 01-23-2023, 01:08 PM   #30 (permalink)
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'air-scoop'

Quote:
Originally Posted by freebeard View Post
I hadn't said anything because I'd need to rotate the pic to be sure of what I'd be saying.

Is that an air scoop on the backside of the fender flare?
Sure appears to be one. And to what purpose I'd have no clue. Creating a pressure 'kink' anywhere in the aft-body is strictly forbidden.

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