Aero trunk lid project

[COcyclist]

Originally Posted by California98Civic

Wouldn't a downward spoiler reduce the wake by taking the attached flow I know is coming down the window and sending it into the wake area? Wouldn't that reduce the wake area? Does that make sense?

If you have time to test this idea I will be following with interest. My armchair QB 2¢ worth is that you won't see much difference because what you have illustrated is well below the "template". When I look at your tuft tests I see evidence of counter-rotating vortexes crashing onto your back glass, not necessarily "attached flow". The tufts at the bottom of the C pillar are flowing out, not inward. There was a thread on here a while back about the New Beetle having attached flow down the back (shown in tuft tests) and a horrible Cd. The angle of the C pillars created huge, spinning vortexes and drag. Aerohead designed the template as the minimum necessary to allow smooth airflow from the top and sides to rejoin and fill in the trailing wake.

[California98Civic]

Originally Posted by COcyclist

If you have time to test this idea I will be following with interest. My armchair QB 2¢ worth is that you won't see much difference because what you have illustrated is well below the "template". When I look at your tuft tests I see evidence of counter-rotating vortexes crashing onto your back glass, not necessarily "attached flow". The tufts at the bottom of the C pillar are flowing out, not inward. There was a thread on here a while back about the New Beetle having attached flow down the back (shown in tuft tests) and a horrible Cd. The angle of the C pillars created huge, spinning vortexes and drag. Aerohead designed the template as the minimum necessary to allow smooth airflow from the top and sides to rejoin and fill in the trailing wake.

Thanks for this read on my tuft test. I have been wondering more about the side flow off the trunk lid and did not know what to make of it. This is a real possibility. I can see it in my mind. I might have made it worse with the latest spoiler design (no photo yet). My latest version tries to approximate the template lines I projected from overhead in post #29: http://ecomodder.com/forum/showthrea...tml#post245629 EDIT (image added):



I wonder about an end plate now that might start from the side of the car, just aft of the rear side window and "grow" into a plate that would capture that sideways moving downdraft from the roof, redirecting it to the tail. Just wondering. My fabrication skills are weak, so I am becoming reluctant to keep trying these until I have built more of the simpler forms, like a new grill block, some good looking wheel well skirts, airdam, undertray... I'd have somewhat better skills for this ...

[PaleMelanesian]

It was raining yesterday so I got to visualize my sedan's rear window airflow. The sides dried out from the airflow while the middle stayed wet. All the way to the top of the window. I sketched the part that stayed wet.

I think this supports the idea of air vortices swirling in from the sides. With the sedan's less-attached flow coming down from the roof, the side vortices have more opportunity to swirl. (I think)

I think this means you're on the right track to consider the horizontal taper as well as vertical. I'm visualizing something more like Phil's actual boat on the truck - tapering in from the sides and down from the top, so it comes to (or toward) a point at the rear. The sides of the trunklid would remain exposed.

[brucepick]

Including tapering sides for the false trunk lid is probably a good idea. aerohead suggested this somewhere- I believe it was in a pm.

I'd use panels attached to the C-pillars, hopefully tapering inward along the template line. The coupe's C-pillars do taper in so one might be able to just continue that line.

I do believe you'd end up with something very close to the Gen 2 Insight back end, by that method. Insight 2's dark glass rear vertical panel is noticeably narrower than Prius's, if you get a chance to see one.

[Cd]

Originally Posted by California98Civic

Interesting. Thanks. How might the vacuum help? Wouldn't a downward spoiler reduce the wake by taking the attached flow I know is coming down the window and sending it into the wake area? Wouldn't that reduce the wake area? Does that make sense?
.

Ill let the experts take the floor on the question about the vacuum pressure being good or bad.

I was guessing based on a comment from an aero engineer that worked on the UCLA Future Car over a decade ago. The link is dead now, but there was a PDF write up about a partial boat-tail that was created for the car. At the tail end a section was hollowed out - the reason being that it created a bit of vacuum pressure at the tail end and pulled the wake in farther.

As far as having a downward slope to the extention, it might not do a thing, but i can't help but think of the back end of the dodge ESX-3 ( which had a fantastic Cd in the twenties. ( Note the trunk section )

[California98Civic]

Originally Posted by Cd

Ill let the experts take the floor on the question about the vacuum pressure being good or bad.

I was guessing based on a comment from an aero engineer that worked on the UCLA Future Car over a decade ago. The link is dead now, but there was a PDF write up about a partial boat-tail that was created for the car. At the tail end a section was hollowed out - the reason being that it created a bit of vacuum pressure at the tail end and pulled the wake in farther.

As far as having a downward slope to the extention, it might not do a thing, but i can't help but think of the back end of the dodge ESX-3 ( which had a fantastic Cd in the twenties. ( Note the trunk section )

Odd looking car, eh? Wonder how well it works. Of course, my car looks bizarre these days. Here is a rough diagram of the build, which is better but not great. It is still three inches above the stock lid, but is tapered better for the tumblehome and at the trailing edge. The bottom row of my widow tufts is under it. Attached with magnets under the lid, its gaps still need to be sealed with ugly tape! My fabrication skills are just not yet up to this challenge. It seems like it might be making an improvement, but if it is, the difference is marginal and hard to measure, maybe 2%. I would post a photo but I took it off to use the trunk for lumber. With my seats and moldings out of the back. Ten foot boards slide in easily. Yay! Car didn't feel very heavy either, since I was only replacing the deleted stock weight. Yay.

[wyatt]

Hucho says a concavity that is 13% as deep as it is wide is the optimum for drag reduction. So for a boat tail end that is 8 inches tall and 30 inches wide, you would use the 8 inches since it's the minimum, and you would want the tail end to have a concavity of approximately 1 inch.

[aerohead]

Originally Posted by California98Civic

Thanks for this read on my tuft test. I have been wondering more about the side flow off the trunk lid and did not know what to make of it. This is a real possibility. I can see it in my mind. I might have made it worse with the latest spoiler design (no photo yet). My latest version tries to approximate the template lines I projected from overhead in post #29: http://ecomodder.com/forum/showthrea...tml#post245629 EDIT (image added):



I wonder about an end plate now that might start from the side of the car, just aft of the rear side window and "grow" into a plate that would capture that sideways moving downdraft from the roof, redirecting it to the tail. Just wondering. My fabrication skills are weak, so I am becoming reluctant to keep trying these until I have built more of the simpler forms, like a new grill block, some good looking wheel well skirts, airdam, undertray... I'd have somewhat better skills for this ...

California98Civic,with respect to the plan-taper of your car,I would establish the rate of curvature from your elevational view 'Template',using the height of the car,then transpose that curvature onto the plan-view of the Civic.
It looks like the body taper begins around where the center of the rear tires are.Try to dovetail onto that spot.
If you use too 'fast' a curvature on the sides,you'll lose your boundary layer and trash the airflow.
Also,you might set up attached longitudinal vortexes ( really high drag ) as the slow side flow intersects with the faster downwash coming off the roof.

[winkosmosis]

Originally Posted by Cd

Ill let the experts take the floor on the question about the vacuum pressure being good or bad.

I was guessing based on a comment from an aero engineer that worked on the UCLA Future Car over a decade ago. The link is dead now, but there was a PDF write up about a partial boat-tail that was created for the car. At the tail end a section was hollowed out - the reason being that it created a bit of vacuum pressure at the tail end and pulled the wake in farther.

As far as having a downward slope to the extention, it might not do a thing, but i can't help but think of the back end of the dodge ESX-3 ( which had a fantastic Cd in the twenties. ( Note the trunk section )

I think the reason they made it like that is because Americans hate hatchbacks.

[California98Civic]

The tape that I still have to use to seal the gaps and edges has damaged the paint job with sticky glue that I can't get off (yet). Bummer. Magnets hold it down via straps to the lid's underside. And I have tested the straps hold, but the tape seals gaps and smooths edges. I'm pausing this project until I'm better skilled in fabrication. I'll test the current build once I get the paint clean and devise a better way to seal gaps. Good luck with your efforts Brucepick. I'll be following. On to my airdam today after work and yard projects...