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Old 02-05-2015, 03:24 AM   #1 (permalink)
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Aero flaps continued, advice needed

for those like myself that is not too keen on doing a full cam back or boat tail these are some aero flaps I noticed on newer cars. I am still experimenting with materials and shapes and need advice on some positions.
here are some that ive seen out ther.

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this is my idea. im just not too certain if I should use option1 or option2. im worried that with option1 air would be deflected but then sucked back towards the light fixture? option 2 will be a bit more tricky to fit and it wont close the slight panel gap


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Old 02-05-2015, 09:45 AM   #2 (permalink)
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The way I read your descriptions it seems to conflict with your drawing.

At any rate, here are the facts as I see them, Option 1 on the EEE Dwg with the device on the rearward part of the light is 10 times more effective than Option 2 of the same drawing.

Sealing the gap would gain you such an insignificant result, you'd need a super computer, advanced wind tunnel, and a team of Formula 1 aerodynamics engineers to figure out the difference in Cd.

The whole idea is to get the air to make a clean break there at the back of the car so the air just flows smoothly instead of flapping back and forth in a wavy oscillation. So putting it forward would allow the air to try and reattach gaining you nothing or possibly hurting the Cd.

That said, the gain here would be very small even if you execute the shape perfectly due to it being such a small inefficiency that you're trying to correct.

The air in the "Boundary layer" within a few centimeters of the skin of the car is not the source of drag on your vehicle. It is the air out to 10 feet to the front, top and sides and 100's of feet behind being displaced by your car as you drive through it that is creating the drag. The drag comes from the energy required to move that huge volume of air, the more it moves, and the more of it that moves, the more energy it takes, and the less efficient it is. The most efficient shapes make the air move the least amount and it goes back to being calm within the shortest period of time.

There are papers out there that show how the rear edge radius of the vehicle affects the Cd. Very large and very small radius corners do well, there is a "Death Zone" radius of like 5 to 10 centimeters as I recall that really create a large drag. I'll try to find one the papers and post it later.

Hope that makes sense.
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Old 02-05-2015, 01:42 PM   #3 (permalink)
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Make kerf cuts in the wood piece to help it conform to the body contour. Or, make a female mold of the wood piece, and then make flexible foam duplicates of the wood piece's shape by using Gorilla Glue, which expands and foams while curing.


BTW, what sort of aero improvement in overall CD do you reasonably expect?
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Old 02-05-2015, 07:30 PM   #4 (permalink)
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https://en.wikipedia.org/wiki/Gurney_flap

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Old 02-06-2015, 01:43 AM   #5 (permalink)
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apologies I see that I have the descriptions wrong way around. im not too sure exactly how much this will save but if those automakers are adding such small ones im sure it makes a difference. as for the construction of the flaps the wood you can see in the images are balsa wood used in rc plane construction. its easy to shape and mould. once I have a mould the items are going to be made from carbon fibre. I happen to have a younger brother working with this stuff daily. these are some aero pieces he made for his car to cover a channel running along his pillars. he also has a full carbon fibre engine under tray. he will be making my parts too

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undertray a bit dirty but it is made of high density foam sandwiched between 2 layers of carbon fibre

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Old 02-06-2015, 01:14 PM   #6 (permalink)
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The A-pillar fillet is something I should probably make for my car, for wind noise.

What I am suggesting is a 'trip strip' located at the short leg of your Option A. Since some of it would be on the taillight clear plastic might be a better material. You could use your suggested method below the taillight. At the bumper it would be a 'skeg' that could integrate into a difusser.
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Old 02-06-2015, 01:25 PM   #7 (permalink)
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Quote:
Originally Posted by ChazInMT View Post
there is a "Death Zone" radius of like 5 to 10 centimeters as I recall that really create a large drag. I'll try to find one the papers and post it later.
Id be very intrested in that paper too, as I think every "edge" ... or lack of an edge on the rear of my car is in the death zone
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Old 02-06-2015, 10:57 PM   #8 (permalink)
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Old 02-07-2015, 03:46 AM   #9 (permalink)
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I vote Option 1.
I'm an armchair aero geek, and know nothing of aerodynamics, but I would think that Option 1 might be a fraction of a percent better.
I base this guess on the fact that every trip strip / air ramp that I have seen, is located at least two inches within the area of the car that has a radiused edge.

Some good examples are the pictures you posted. If you notice, the body panel begins to curve a couple of inches before the trip strip.

However, there are some exceptions, such as the air ramp on the Ford Focus tailights :



A curious thing abou the Focus is the fact that the air ramp is there on a section of the light, but directly below it is a massive round edge to the bumper and area above the air ramp.
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Old 02-07-2015, 04:03 AM   #10 (permalink)
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Quote:
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I I base this guess on the fact that every trip strip / air ramp that I have seen, is located at least two inches within the area of the car that has a radiused edge.
Cruze hatch has them located a long way inboard (on the window):



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