Cutting hole/shape in Back Window

[Mike]

I have hand-held jig saws, dremel, roto-zip tools etc. I'm thinking of tackling the installation, over this weekend, of my naca ducts into the rear lexan "curved" window. Has anybody had success with any of the above tools or others. Don't get many 2nd chances on this type of project.

Thx, Mike

 

[Lee Patterson]

I have tried jig saws on lexan - do NOT use. They are way to violent and will crack the material. The other problem is as you cut you heat and then MELT the material. I don't know what the pro's use, but a jig saw is bad. Someone else here probably has better ideas!

 

[Jack]

Tape off the area to be cut as well as the surrounding places where the base of the saw will contact. to prevent scratching. You can use a fine tooth metal blade in a jigsaw at very low speed to prevent cracking and melting. Go slowly, let the tool do the work. Don't push the saw, let it do it. Don't stop until the cut is finished. Did I say go slowly?

It will be difficult to keep the Lexan from chattering, so go slowly. You only get one chance at this.

 

[Dave]

As jack mentioned, a slow cutting speed is the key along with a fine toothed blade.

 

[Rebeca Lynn]

I would also suggest using painters tape or such. One to protect against scratches, it also helps to hold the put together as you cut it out. The other suggestion is to support the rear window as best as you can to reduce the amount of movement. The last suggestion is to use a drill in the corners so you do not have to make a sharp corner with the saw.

 

[Howard Jones]

I cut Lexan by hand with a very fine pitch hacksaw blade with all but about two inch's wrapped with tape until the tape is about a inch in diameter. Then I use a metal grinder to shape the end of the hacksaw blade at a angle to remove the rounded end with the hole in it. You can start the hole this way with the sharp end if you take your time. This way the cut will be the same all the way around and you don't have to start with a hole.

The little bit of blade will force you to go slow and pay attention. This will take a bit of time but there is a very low risk of FIU. All power equipment will FIU, at least in my hands.

Also the less the Lexan moves around the better. If you can, have a friend (wife) help. Two people, daughter and wife, is better. Out to dinner after you are done.

Where did you get the NACA ducts?

Cool found this. Now you can make more!

[ame]http://www.youtube.com/watch?v=bd8yQSbcS_E[/ame]

 

[Will Campbell]

I thought the purpose of NACA duct was to capture laminar at flow and direct it to the output with minimum drag. But what you are trying to do is vent hot air out.

I'm thinking the best solution is just simple openings like the 01 car, or some sort of vent or mesh like the current R8s and Ferraris.

 

[Jack]

NACA ducts can work in both directions. When installed on the SL-C, the wide end is toward the front. As the duct narrows, it creates a small low pressure area aiding the outflow of air. The issue is that you must be moving to create the laminar flow (for either direction). The major reason the open vents or mesh (R8's, etc.) is to allow heat to escape by "stovepiping". Especially when in slow traffic or stopped at a light.

 

[Mike]

I made the cut-outs today - no issues. The templates I made, and all the mock-up taping and arranging took the longer than the actual cutting. Bought some 32tpi jig saw blades, made round corner holes with a Forstner bit and blue taped the area. The left and right naca vents were not exactly symmetrical. Ran out of connecting bolts so will put off completing till tomorrow. More air-flow into the engine bay cannot hurt and when idling or parked at least the rising heat has somewhere to go - although the openings in the naca vents are quite small and undersized for "venting" purposes. I spent days looking for a screened oblong acceptable vent(s)... I didn't find anything I liked at that time. Pictures to follow in the next day or two.

 

[David]

Those are poor NACA ducts. Depressing how many people just throw something together that looks vaguely like a NACA duct.

It's not fun reading but this goes into how they should be constructed.
http://naca.central.cranfield.ac.uk/reports/1945/naca-acr-5i20.pdf

They'll work as vents but don't expect them to work as proper NACA ducts.

 

[mark b.]

Don't leave the tape on the lexan for extended period of time, the adhesive can embed into the lexan and you will not get it out, even the painters tape. I know someone who had to have a complete windshield replaced because of this!

 

[Jack]

Happy it turned out okay Mike. I have a program to plot proper NACA duct shapes using inputs from throat width and depth. They usually come out a little longer (length) than one may expect. Have it on an Excel spreadsheet if anyone wants it send me an e-mail.

 

[Howard Jones]

Here's a picture of the Audi R18. You can see that they are using a small NACA duct to intake air below a larger exhaust duct to remove engine room heat. The differences are clear. The NACA duct continues the attached airflow downward into the expanding low pressure area and into the ductwork inside the car.

The other bigger duct does a completely different thing. It allows the internal heated air to leave the interior of the engine room into a low pressure area on the outside of the bodywork. I would think that a small gurney along it's leading edge would improve efficiency at some small cost of drag.

The larger exhaust duct with a small gurney would be my choice to remove hot engine room air on a moving car. The other simple open holes idea would work just as well on a stationary street car in traffic and depending on area might even work better.

Turning around a NACA duct isn't the way they are intended to function. The airflow across the ramp area is going the wrong way. It's flowing into a DECREASING area and thus a higher pressure. If it works at all, it would be very inefficient.

 

[Mark Hamstra]

If it works at all, it would be very inefficient.

Which is true for most automotive applications of NACA ducts, where even using them the right way around is using them the wrong way -- or misusing them to do a job for which their designers specifically noted they were not well-suited.

The Scoop on the NACA Scoop | Flying Magazine

 

[Mike]

Installed and done... till Fran comes out with his vented rear panel design.

If they help, then good. If they don't - I'll just consider it additional car swag/bling.

 

[Dave Lindemann]

Mike -

I think they look good! Even if all they do is vent engine bay heat in traffic or while idling you're ahead of the game. Just be careful driving in rain (it could happen) or washing the car depending on what's in the engine bay below those openings - better than the race car in that regard.

Nice work!
Dave L