Rear aero project - Civic notchback coupe

[aerohead]

Originally Posted by brucepick

Thanks. Yes I will beef it up.

I think the air impact is mostly around the middle. Based on: Before I added the stiffeners, it was bowing down in the mid section and up at the rear - but I suspect the up-at-rear might have been a kind of lever action, with the panel pivoting half way between the low and high points.

Does the bowing tell you anything about whether I put the horizontal trailing edge at the template line as I hoped to?? Or merely proves your point that it's subject to a lot of pressure?

If you've ever seen a soft tonneau cover on a pickup going down the highway,you may have seen how the back is 'bird-bathed' with a distinct depression,and the front is bulging upwards as if Yoda was under there trying to get out.
The depression is created by air slamming straight down on the fabric ( this is what air-cooled VWs and Porsches use for induction and cooling.
The bulge is created by the partial vacuum of the locked-vortex directly behind the cab,literally attempting to suck the fabric into it.
These forces will be present on top of your wing.If the wing is not properly 'tuned',it may reach the resonant frequency,or any harmonic of the vortex and undergo destructive flutter as can happen with aircraft.They just explode!
The reinforcement alters the resonant frequency,moving it to where it is dampened against the destructive forces.
Also,adding a short distance of roof extension will help direct the momentum of the air towards the trailing edge of the wing and not down into it.This worked very well on the CRX.

[brucepick]

Originally Posted by aerohead

If you've ever seen a soft tonneau cover on a pickup going down the highway,you may have seen how the back is 'bird-bathed' with a distinct depression,and the front is bulging upwards as if Yoda was under there trying to get out.

The depression is created by air slamming straight down on the fabric ( this is what air-cooled VWs and Porsches use for induction and cooling.
The bulge is created by the partial vacuum of the locked-vortex directly behind the cab,literally attempting to suck the fabric into it....

Also,adding a short distance of roof extension will help direct the momentum of the air towards the trailing edge of the wing and not down into it.This worked very well on the CRX.

Aha. Thanks, aerohead.

That pretty much explains the center panel flexing. If I perceived it correctly (via rear view mirror only of course), mine was "bowing down in the mid section and up at the rear", which was before adding the stiffeners. The forward edge was laying on the glass (not taped down at that point) so couldn't go down. If it had gone up I would have seen that, with the edge not lining up on the defroster wires as it was cut to fit there.

So it seems the air is slamming down around the mid point of the panel. And possibly there's a suction pulling up on it at the rear.

Roof extension should be helpful if/when I can make one. I'm sure it will be one of those conical section things, a lot like a front windshield. Patience is needed to make one of those panels. I can do it but it's not something to attempt to do quickly between other responsibilities.

Anyway, it's back to the drawing board, one way or another, next time I get to work on it.

[brucepick]

Either of the red/orange line roof extensions could be added to the project. The blue line, with side panels, is not yet giving any aero improvement.

The upper line is closer to the template. The lower line would be far easier to build.

The upper, longer roof extension option would likely require several supports as it would be over a foot, front to rear. I have a piece of coroplast that could be made into the lower red line. Building the upper line would mean buying more coroplast. Would also require more support structure as the panel would be more than a foot, front to rear. Or I might make the upper line of 2.7 mm. lauan plywood, which is much stiffer than coroplast.

Re. the upper line option - which I think holds more potential since it's closer to the template - its leading edge would lay on the roof, would be taped down during testing. There's a limit to how much I can shave or bevel the material's leading edge to reduce the height of the joint. Will air flow separation be a problem there? On the lower line option, the leading edge would be butted up against the window molding so that should not be an issue.

aerohead, any thoughts?

[brucepick]

Received PM from aerohead, pasted below:
[EDIT] This is ref. post #63 above. [/EDIT]

Originally Posted by aerohead

Bruce, this is exactly what I would do. The air will be directed along a path which mimics the start of the Kamm roofline, and when it separates it will have some momentum to help it stay on that path, capturing the vortex underneath, then re-attaching before it leaves the new trailing edge. This worked very well on the CRX.

The upper line would be preferred, as there'd be no additional vorticity, but a straight shelf would still 'force' the air to follow a more favorable path, with a small penalty paid to the vortex which would be present above it.

Now I just need the time to build it (see signature, Oct. 2011) - - - the latest version of the rear spoiler design is currently off the car, stored safely where I can get it immediately.

[California98Civic]

I'm sorry there will be delays--not for the sake of the project so much as why. I hope everything works out well for your family.

Originally Posted by brucepick

Received PM from aerohead, pasted below:



Now I just need the time to build it (see signature, Oct. 2011) - - - the latest version of the rear spoiler design is currently off the car, stored safely where I can get it immediately.

[JasonG]

Digging up an old thread here.

Notes from me:
Aero. As well as the slamming Aerohead explained, the tapering fins may be working like a funnel. This tries to force the air from top of the window width down to a narrower gap creating drag or forcing the air up over the sides. Neither is good.

Reinforcing. It seems you terminate the rods/flat bar/plywood at the inside of the fins. This creates a moment connection that will flex considerably due to the forces on it. Instead try an aluminum flat bar, but bend the ends and attach there to the fins. This will transfer the flex from a moment connection to pure tension force on the sides.

[Blacktree]

I'm not an aerodynamics expert. But my hunch is that a differently shaped "greenhouse extension" would get better results. For example, a fastback might be more apropos than a kammback. But a fastback would need to be made of transparent material (assuming you want to retain rear window visibility).

I hate to say this, but your time / effort / money may be better spent on aero mods that are easier to execute. After you've done all the easy stuff, if you're still itching for that last bit of aerodynamic efficiency, then you can revisit this project.

[brucepick]

With the return of warmer weather I'm readdressing this project.



I plan to remove the low rear center panel and instead use a Lexan panel going all the way from the rear of the most forward panel that comes off the roof line, extending back to where the current spoiler ends. That's several inches rear of the trunk lid and about 5.5" above it. The step-staggered idea didn't test out as well as I hoped.

Because I can't build a compound curve, I must choose between:

• A straight line front to rear and a raised center
• Or with a straight line left to right, with a gradual downward curve as it goes front to rear.

My guess is the raised center is more helpful, but what do you say?

If you're wondering why not Lexan all the way from the roof to the rear, that's just a cost concern. I'll be saving noticeable $$ by using thin plywood over the roof and forward part of the rear glass.

[ChazInMT]

Sorry, but I can't help thinking that the air needs to follow down the back end on something to get the best return. That pressure building back in my mind is helping to push the car along. When you do it in steps, it just is not the same.

On my 04 Civic, it worked out to where the back end of the "Kamm Over Trunk" was tall enough for me to put in a 8" window....were I to get off my lazy butt and build it. This would have allowed me to put a small rear window on that would have allowed me a view straight back.

[jakobnev]

I'm curious of what would happen if the "rear deck"-part was replaced with a vertical bit along the rear edge of the trunk.