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Old 05-13-2010, 04:16 PM   #1 (permalink)
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Experiment: tuft testing the underside of a variable-angle tail extension

The steeper the angle of the underside of a boat tail, the easier it is to live with. I want to know how steep I can make it without creating too much drag.



Here's what the test rig looks like. It's a tufted sheet of coroplast, with an L-shaped aluminium spine running perpendicular to the flutes to keep it straight. I attached the camera to the spine with shipping tape and bubble wrap, which is why the video isn't very stable.

A piece of mechanic's wire to adjust the angle is held in place by vise grips just inside the hatch. I made markings on the wire to indicate various angles, and calibrated with a protractor.

The good news is, even with a camera hanging off the back, it didn't come close to scraping at the end of my driveway. The bad news is, the airflow doesn't look great in ANY of the runs. I wonder what this test would look like on 3-wheeler's Insight with the smooth fiberglass underbody panels.

I was hoping to see a sudden flow regime change at a specific angle, but that didn't happen. I'm not sure what to make of the results. Thoughts?





I did four runs, at four different angles, which you can see at the following times in the video:

1. 10.5 1:50
2. 13 3:50
3. 7 6:05
4. 4 8:20

Vehicle speed: 25-35mph.



And yes, I tried to run some video editing software, but it's probably easiest if you just turn your monitor on its side and skip to the time marks indicated.

For comparison, there's some footage of MetroMPG's boattail tuft testing at 5:40 in the video here.

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Old 05-13-2010, 06:55 PM   #2 (permalink)
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May I suggest:




Also the leading edge of the coroplast introduces some turbulence.
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Old 05-13-2010, 07:05 PM   #3 (permalink)
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guessing

Robert,I couldn't get the video to play,but I'll try again.
Having not seen the footage,but reading your remarks ( and being well into my ignorance zone ) my guess is that without the side and top flow adjacent to the extended 'bottom',that all this is influencing your results.
From Hucho's book,the diffusers are effective for 2.5-degrees,and 4-degrees,depending on where the up-sweep originates.
The underside of the Insight should dictate how far you can 'cheat',after which you'll just have to live with the difference unless you can make the diffuser to move up and down.
As to the difference in drag,I believe that that will be a function of how much turbulence there is compared to the lack of turbulence with the 'ideal' form of same length.
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Old 05-13-2010, 10:38 PM   #4 (permalink)
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Aerohead,

I'm happy to say that the up-sweep starts around the middle of the gas tank, right behind the front seats. It's a gradual up-sweep, gradually working its way from 0 to 7, from 6" ground clearance to 10". I think this is an unconventionally good feature, and I hope it will allow me to have an unconventionally short boat tail.

I've added coroplast paneling to cover the rear axle, making a smooth transition from gas tank to spare tire box. There's still a lot of room for improvement under there, like in front of the gas tank, and around the exhaust. All four wheel wells are also wide open.

I'd offer my interpretation of the video, but I really want to hear from experienced tuft testers. I'm not sure that the 4 run was good, nor that the 13 run was bad. Tufts were lively in all runs, but not consistently attached or reversed in any of the runs.

Quote:
Originally Posted by silverinsight2 View Post
May I suggest:
You mean to suggest strakes in my diffuser? Hmm. That led me to read an eng-tips forum post on diffusers. Diffusers reduce drag and lift relative to a flat underbody. 7-10 is recommended, at least in the context of race cars with low ground clearance, looking to maximize downforce.

Strakes? They're supposed to make the diffuser less sensitive to body pitch and probably crosswinds. Okay, they're easy enough to add on. I will run at least two strakes, in the form of deflectors behind my rear tires. Speaking of which, I couldn't tell from the video at what angle to install those deflectors. I guess I'll have to build a prototype and tuft-test it.
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Old 05-15-2010, 01:57 PM   #5 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
....I think this is an unconventionally good feature, and I hope it will allow me to have an unconventionally short boat tail.
Robert, thanks for trying anyways

My car would not give you that much better results than yours at point.

Why?

Because as much as the under-body panels cover the underside, there are still some gaping holes next to the rear tires. This occurred from running out of warm weather late last fall.

I'm still waiting for the weather in Wisconsin to get consistently warm enough in the mornings to start riding the bike to work and then start working on the Insight again. First finishing the rear wheel holes, then working on the rear extension.

If it's any help, my extension length works out to 42 inches from the rear edge of the rear tire skirts to the end of the tail lights/license plate transom. That's the length I'm building to and then we will see what type of turbulence that remains on the underside of the car.

As for rear transom height?

It will be set based on a straight line drawn right down the middle of the back window, all the way to the tail end. The underside angle will be curved gracefully to meet up the underside of the license plate.

That way the clearance can be as high as possible and the rear window curvature will simply be straight to the end. It will be slightly lower than the nice rendering by Botsapper.

__________________________________________________ ______________

Robert, on a different note, you might want to retry that run again with side strakes as SilverInsight2 mentioned.

That will hold the air from slipping off the sides of the coroplast, and maybe giving better results.

Of course it might be that the turbulence under the car may have contributed to it as well. Mine has some large holes down the center by the cat-con and muffler, which would certainly stir things up. I might cover mine up some time, but it certainly won't be this season.

EDIT:
I just watched your video. It did work for me.

A fine tweak to smooth the underside can be made by adjusting the rear brake cable up out of the air stream. Mine was adjusted up last fall after spending so much time on my back looking at it during the panel construction. I think it's held by a 10 mm bolt and associated bracket. Loosen the bolt then adjust up out of the air flow. Mine allowed just enough play, that it's just about flush with the bottom of the panels, and that's with the rear suspension fully extended.

Jim.

Last edited by 3-Wheeler; 05-15-2010 at 02:11 PM..
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Old 05-15-2010, 02:22 PM   #6 (permalink)
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strakes 'n such

Quote:
Originally Posted by RobertSmalls View Post
Aerohead,

I'm happy to say that the up-sweep starts around the middle of the gas tank, right behind the front seats. It's a gradual up-sweep, gradually working its way from 0 to 7, from 6" ground clearance to 10". I think this is an unconventionally good feature, and I hope it will allow me to have an unconventionally short boat tail.

I've added coroplast paneling to cover the rear axle, making a smooth transition from gas tank to spare tire box. There's still a lot of room for improvement under there, like in front of the gas tank, and around the exhaust. All four wheel wells are also wide open.

I'd offer my interpretation of the video, but I really want to hear from experienced tuft testers. I'm not sure that the 4 run was good, nor that the 13 run was bad. Tufts were lively in all runs, but not consistently attached or reversed in any of the runs.

You mean to suggest strakes in my diffuser? Hmm. That led me to read an eng-tips forum post on diffusers. Diffusers reduce drag and lift relative to a flat underbody. 7-10 is recommended, at least in the context of race cars with low ground clearance, looking to maximize downforce.

Strakes? They're supposed to make the diffuser less sensitive to body pitch and probably crosswinds. Okay, they're easy enough to add on. I will run at least two strakes, in the form of deflectors behind my rear tires. Speaking of which, I couldn't tell from the video at what angle to install those deflectors. I guess I'll have to build a prototype and tuft-test it.
Robert,I don't have my book with me,but my failing memory is emphasizing that nobody would see attached flow with more than 4-degrees up-sweep on a diffuser.
The other thing mentioned,is that the diffuser is worthless without a complete and well-executed bellypan ahead of it.
I like the strakes.This is something Dr. Morelli used on his Cd 0.23 CNR car for directional stability in crosswinds.
I don't know about the racer stuff.I 'think' that in racing,that allowing the low pressure of the wake to be communicated forward under the car could help with downforce on the track,especially if spoiler dimensions were limited by the sanctioning body,and this would be an advantage during high-speed cornering.
So my intuition tells me that going over 4-degrees is a downforce issue.
And certainly,'departure' angles for the real world ( 10-degrees ) severely limit diffuser angles unless 'active.'
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Old 05-15-2010, 02:40 PM   #7 (permalink)
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4-degrees looks pretty good!

Quote:
Originally Posted by RobertSmalls View Post
The steeper the angle of the underside of a boat tail, the easier it is to live with. I want to know how steep I can make it without creating too much drag.



Here's what the test rig looks like. It's a tufted sheet of coroplast, with an L-shaped aluminium spine running perpendicular to the flutes to keep it straight. I attached the camera to the spine with shipping tape and bubble wrap, which is why the video isn't very stable.

A piece of mechanic's wire to adjust the angle is held in place by vise grips just inside the hatch. I made markings on the wire to indicate various angles, and calibrated with a protractor.

The good news is, even with a camera hanging off the back, it didn't come close to scraping at the end of my driveway. The bad news is, the airflow doesn't look great in ANY of the runs. I wonder what this test would look like on 3-wheeler's Insight with the smooth fiberglass underbody panels.

I was hoping to see a sudden flow regime change at a specific angle, but that didn't happen. I'm not sure what to make of the results. Thoughts?





I did four runs, at four different angles, which you can see at the following times in the video:

1. 10.5 1:50
2. 13 3:50
3. 7 6:05
4. 4 8:20

Vehicle speed: 25-35mph.



And yes, I tried to run some video editing software, but it's probably easiest if you just turn your monitor on its side and skip to the time marks indicated.

For comparison, there's some footage of MetroMPG's boattail tuft testing at 5:40 in the video here.
Robert,just now got to see video.Great investigation,thanks!
Looks to me that 4-degree run was best.The tufts appeared to be forced onto the surface the best,rather than dangling out into space.The spanwise wiggling is indicative of turbulence,but it appears to be the least.
I'd be curious to see what less than 4-degrees looked like,but I realize the practicality of the shallow angle.
Thanks very much for the grunt work! These kinds of efforts are invaluable for all who will benefit.
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Old 05-15-2010, 11:04 PM   #8 (permalink)
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I agree with Phil: the 4 degree run was best, and comparing it to the 10 & 13 degree runs, I think there's a marked difference - tufts swirling "in space" at the higher numbers vs. staying close to the surface in the 4 degree run. I'd go out on a limb and call that the difference between attached/turbulent vs. detachment.

Also, my videos show a clear improvement in flow with the full belly pan. I think it's pretty important for the best performance of a boat tail.
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Old 05-16-2010, 12:13 AM   #9 (permalink)
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I was under some mistaken impressions about what the tufts would look like, hoping for a sudden and easily discernable transition from attached to separated.



I studied this video of an airfoil for a while. It's a more intuitive flow visualization than tufts. It looks to me like a very thin seperation bubble begins to form around 4s into the video. It becomes quite thick, with corresponding increase in drag until 6-7s, but the airfoil hasn't stalled until 8s.

Comparing this airfoil to my tufts, I have to agree, the higher angles I tested have higher drag. The seperation bubble moves farther forward the steeper I make the tail.

I guess the tuft test confirmed that the underside of the tail will be a compromise between drivability and drag (neither of which I have good data on). I'm leaning towards 7. However, another idea is to just go with 12 in fiberglass, and have a clip-on tail extension extension for road trips and FE runs, to bring the underside angle to 7 or even 3. Although, would the reduction in drag underneath the car make up for the increase in transom area?



Btw, Jim, thanks for the tip on the brake cables. Those things annoy me every time I see them dangling from my car.
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Old 05-16-2010, 08:20 AM   #10 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
Aerohead,

I'm happy to say that the up-sweep starts around the middle of the gas tank, right behind the front seats. It's a gradual up-sweep, gradually working its way from 0 to 7, from 6" ground clearance to 10". I think this is an unconventionally good feature, and I hope it will allow me to have an unconventionally short boat tail.

I've added coroplast paneling to cover the rear axle, making a smooth transition from gas tank to spare tire box. There's still a lot of room for improvement under there, like in front of the gas tank, and around the exhaust. All four wheel wells are also wide open.

I'd offer my interpretation of the video, but I really want to hear from experienced tuft testers. I'm not sure that the 4 run was good, nor that the 13 run was bad. Tufts were lively in all runs, but not consistently attached or reversed in any of the runs.

You mean to suggest strakes in my diffuser? Hmm. That led me to read an eng-tips forum post on diffusers. Diffusers reduce drag and lift relative to a flat underbody. 7-10 is recommended, at least in the context of race cars with low ground clearance, looking to maximize downforce.

Strakes? They're supposed to make the diffuser less sensitive to body pitch and probably crosswinds. Okay, they're easy enough to add on. I will run at least two strakes, in the form of deflectors behind my rear tires. Speaking of which, I couldn't tell from the video at what angle to install those deflectors. I guess I'll have to build a prototype and tuft-test it.

1.Was the added coroplast covering the dog leg in the muffler pipe? Were the other hot bits covered or exposed? Turbulent flow at the start of the rise should be minimized. You know that, of course.
2. Outboard strakes are really wheel fairings IMO. My suggestion is to use two or three more centrally located.
3. This may be contentious (stupid), but consider making the bottom diffuser with small slots, pin holes, or screen on one side of the coroplast. Seal one open edge with RTV silicon, create a manifold out of split soft tubing on the other. Use boundary layer control with vacuum created by flow thru the boat tail, or engine vacuum.

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