Erik N

Jerry, you may encounter an issue at the joint where the plywood meets the blocking. The solid material will want to expand and contract, where the ply will not.

Have you worked with rigid polyurethane foam? There is lots of kinds, thicknesses, and densities available for use as core material in the boat construction and surfboard industry. It is easy to work with, then can be glassed over. Polyester resins won't melt it.

Jerry Feather

Erik, that is probably a good suggestion, but you need to realize that this thing I am making is just a buck for the fiberglass mold that I plan to make off of it. Its usefullness is going to be pretty limited in time, so the potential changes in shape resulting from the differences in the wood will be of little concern, I think.

Thanks for your input.

The Fixer

Just got back from borrowing the use of a new friend's beautiful spray booth, what a difference they make. His name is Enzo Tecci and he does exotics, was fixing a GT3 when i was in there today. He will allow me to use it from now on as I promised to help him when he needs it. Awesome.

Anyway, Jerry the wooden mold you've made is really beautiful.

You could make furniture for a living, maybe a 928 coffee table?

Very impressive, look forward to seeing it completed.-Matt

Jerry Feather

Thanks, Matt. You are just too kind. If you watched me in the process you would think differently, I think. I have made some furniture in the past and it turned out ok, but that is not my area of interest. Nevertheless I have taught myself to work in many mediums and with many processes. Woodworking is just one of them, at least up to the point needed in a given project.

This morning I reshaped the foreward extensions so that they are a little more pointy and then I turned the buck upside down and began filling in some of the gaps there. I need to have a pretty good surface around the foreward edge of this spoiler and along the back edge so I can include them in the mold. Those surfaces will need to be very true to the hatch so the thing will lay there snugly.

I also decided to round the back two corners in the lower part so they flow a little better. Here are a couple of pictures.

I'll show the top view of the extensions after this glue sets and I can lay it up with glue on the hatch.

Nicole

I don't.

The side extensions change the airflow dramatically. Neither you nor I have any way of knowing the difference without a wind tunnel test.

The Fixer

Awesome info, thanks.

Jerry, Just keep going and build the first 928 Woody!

Imo000

I don't like that spoiler either.

The Fixer

Many especially 928 Folks are simply not used to looking at the Duck tail spoiler and how 'abrupt' an angle the Ducktail spoiler has.

When i was new to the Porsche 911 years ago the RS Ducktail looked really wrong to me also, but it's not.

It just looks wrong. The air does not flow over the car as you believe Jerry, otherwise Porsche would not have designed a spoiler with such an angle. (your logic regarding the S Spolier, which i believe was also about styling the rear to look more up to date at the time)

With Porsche function is usually equal to, if not greater than form.

When you really get the car going, think about it, a little air gets trapped and swirls down there in that abrupt angle but that air pushes up the rest.

The air does not just trap down in that "90 degree angle" IT Serves A Purpose. The bit that does, pushes the rest up and it begins to fluidly flow over the rear end and spoiler creating down force, and it also redirects the airflow up and away from the rear, cutting it off from the car to lesson drag.

The air speeding over and around the Porsche doesn't walk over the shape of the car like a bug. Check this out:

Jerry Feather

Thanks, Martin, for this lead. I have checked those out and put one of them in my eBay watch list. When I get some extra money I will buy one of those. They look really agressive.

In the meantime I did find another item on eBay that is more like what I had in mind and that is a curved face sanding block that uses round velcro-like sanding discs. That is more like what I need for this project--something I can sand the concave face of the Spoiler, particularly after I start to true it up with some bondo.

Jerry Feather

If we can't tell "the difference" without a wind tunnel test, what is your basis to suggest that there is a difference, particularly one that is "dramatic?"

Given the two distinct "dorsel fin-like" foreward extensions the factory put onto the center section of the subject spoiler, the ones running along the sides of the hatch window nearly its full length, I suspect that whatever change in the airflow the two outward extensions they added to the spoiler has is limited just to those outward extensions and that they do not change flow over the subject center section, which section I think is the result of the original testing of the prototype "bikini" spoiler. I appears to me that the result of the testing shown in the book and pictures was something like "if a little is good, more will be better."

Jerry Feather

Matt, I checked the video out, but didn't see much in it about the spoilers or wings we are discussing. I could tell that the factory apparently began doing something like the "duck tail" finish to the car body starting with perhaps the 904. There they have incorporated a little upward flare to the top of the trailing edge, but only about 10 degrees or so, and not for very long, front to back.

The other picture in the video, the one shown still in your post has the whale tail at about level to the world or about 35 degrees to the back of the car.

The exagerated S spoiler in Marks' post measures off the picture about 40 degrees to the flow of air over the top and back of the car.

The S spoiler by itself measures only 20 degrees to the rear window; and I can't tell what the little spoiler on the prototype car has for its leading surface.

Yours however, measures about 70 degrees to the approaching air. That is very much more abrupt than any of the rest of them.

Mine measures about 40 degrees to the slope of the hatch at the top and gets there only with a short but gentle curve starting at about 20 degrees to the flow of air.

This all assumes that the air is flowing somewhat like a bug crawling front to back along the contour of the top of the car, and that may not be entirely true, as you suggest, but I don't think it is too far off. When we do some tuft testing I guess we might be able to see.

I have been thinking of this Bikini Spoiler sort of in the nature of the elevator on my airplane. What I have to wonder though is just how far the elevator might be deflected upward in generating down force when it stops doing that and just converts to pure drag and perhaps with no down force. I suspect that is what would happen at 90 degrees to the flow of air, but I don't know about 70 degrees or anything in that neighborhood.

Jerry Feather

I have the under side of the buck pretty well filled with epoxy and now have to grind some of it out to get the thing to fit closely to the hatch. Then I'll bed it with a coat of epoxy so that it is exact.

Here are some pictures of how it looks on the tub I am showing it with. You can see how I have trimmed the foreward extensions to slim them down some. I might be able to do more of that, at least from the horizontal point of view, since I think I will be utilizing only the rear of the S4 wing mounting points, so I only need the spoiler to cover the front holes and not need to bury a nut or stud in that part of it. Therefore it can still be somewhat thinner.

The Fixer

I think it looks awesome Jerry, very Porsche indeed.

It has elements of the whale tail to my eye. Great Work!!

Just one thing I noticed, you need to take a belt sander to the sides along the hatch to quarter seam,
because when the glas spoiler is made, the size will increase by a nom. 1/8" in every direction.
It will be too tight to the edges of the hatch IMO. You probably know this.

Again, awesome job.

-Matt

Jerry Feather

No, Matt, I didn't know that. I usually work with things that will give me a small amount of shrinkage, but not expansion. I do have about 3/32 of an inch allowance on each end of the spoiler buck, but your advise will give me motivation to add a little more. Thanks for your input.

hans14914

I suppose it depends on how you are making the mold. Perhaps the better way is to get your plug exactly the way you want it, and make female molds from that. That is generally how most production shops make things. Then you can store the plug away in-case you need to make a new mold in the future.