Poor Man's Aero: Building Your Own Wing (many pix)
 

For those of you who haven't been following along, this was my dilemma: I decided my 3.8RS tail and 58-inch wing was too ugly for long drives to the track, and that I'd rather have something that did as much for my car on the track (where I don't care so much about looks) but would allow me to drive to the track itself looking like a normal widebody early 911.

Yes, I know. A widebody early 911 ain't exactly normal. But I prefer the look of a ducktail on it to the huge apparatus that I would previously put on for track weekends:

http://forums.pelicanparts.com/uploa...1134533115.jpg

Plus, the 3.8RS deckild is heavy (45 pounds, with the wing and extensions), and it's not the right decklid for my era of 911.

Now, I know what you're thinking. Who really cares about what their car looks like on the drive up to a track? Well, I don't care that much, honestly. But once I get an idea in my head, I often have a tough time stopping things. Plus, it appealed to me to run the same decklid on the street as I do on the track.

Everything got its start in this thread. And this was my rough sketch:


http://forums.pelicanparts.com/uploa...1157211167.jpg

I got lots of feedback, and I did some wool tuft testing, and things evolved into this thread, where we worked out an idea for an airfoil to use.

So the goal was this: build a full-size wing that I could take apart and put inside the car pretty easily. I don't care how ugly it is at the track, although I have to admit I thought the previous test wing, at 42-inches wide, looked a little goofy and maybe a little too 'girly' for my tastes:

http://forums.pelicanparts.com/uploa...1149366639.jpg

This was the profile that Pelicanite Peter Bull worked out for me:

http://forums.pelicanparts.com/uploa...1157211148.jpg

I ordered up some aluminum and did some tests with different adhesives. I'd never built anything like a wing, previously, so I kind of made it up as I went along. I'm going to post a whole bunch of pictures showing the steps I used. I'd appreciate it if the guys who know about this sort of thing let me know where I screwed up (or, if it happened, what I did right).

I'm reasonably happy with the end result. It's big -- since my thinking is to find the point of diminishing returns with this and then back off to a smaller wing down the line. But it's surprisingly strong. I don't think it will have any problems with the forces involved once it's up to speed.

Final size: 70 inches wide, 10.25 inch cord length
Final weight: 4 pounds 12 ounces for the wing itself.
Final cost: About $75 for the materials, and $50 of that was for the Scotch-Weld DP460 structural adhesive. It's expensive stuff.

For most people, a pre-made wing from a fiberglass/carbon fiber place is probably a smarter way to go. I was limited to a straight wing, and it will no doubt get more damage over time than my old carbon fiber one did.

Comments? Questions?

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nice job jack- about how many hours for the actual fab and assy?

Nice work Jack, it looks really good.
I appreciate the design, fabrication and effort that you put into it.

Regards,
Ed

Wow! Not too shabby! Impressive fab work, and lots of it with home tools. Nice!

Does the FAA and NASA know about this;)

http://forums.pelicanparts.com/uploa...1157157598.jpg

I see that I'm not the only one that built model airplanes when I was young.

Nice fab.

Very cool, I'm impressed again. Between Your and Thom's fabrication skills you guys should start making cars. ;)

At this point, the only thing that I wonder about isn't the strength of the airfoil, but the strength of the duck tail and uprights that attach it to the car.

Have you done any high speed tests?

Jack,
It looks like you put a lot of thought into this. Nice job, especially the nice smooth bend at the leading edge. Not sure where I could improve upon it. All you need to do is fix the camera on it at speed and a stopwatch to tell you all is well.

Sherwood

Neat - I look forward to seeing her run at speed at WSIR.

Minor fabrication suggestion for v2.0: Bend ever other tab on the ribs in the opposite direction (except the ends). But given the size of the box section probably unecessary.

Very cool. I would be curious to see how the uprights mount to the body.

Fantastic!

Nice work jack. Makes me think of the modern GT3 wing mounting style.

That's is pretty damn clever man. Great DIY fab work.

Thanks, guys. As a non-fabricator by nature, this was a learn-as-you-go type of deal. But I think it will hold together just fine on the track, and it will allow me to fine tune a wing solution for my car without breaking the bank.

Answers to questions:

Time involved: It's hard to say, since I did it in little pieces, allowing the glue to cure in between. Putting on the skin was done in four steps, each with at least four hours of curing time. I think it's safe to say that if my fabrication time is worth $10/hr, then this wing is a great deal. If my time is worth five times that amount, then it probably would have made more financial sense to buy somebody else's wing.

Most of the time for the job was spent learning about how it's been done in the past -- mostly from RC aircraft sites. I've never done any modeling with airplanes or anything else, so this was all new to me. But the problems faced by guys building their own aircraft are similar to the challenges for this project -- and RC guys usually are on a strict budget.

The trickiest part was thinking up a way to have the two halves join together, and lock and unlock easily, in a way that didn't add too much weight. There are two 18-inch aluminum tubes set into each center section, with end stops, and a 35-inch length of smaller tube goes inside them (half on each side), bridging the two when they're together and also making the wing more rigid. There's a shorter tube aft of the longer one. It also has an insert whic is permanently fixed inside it. There's a pre-drilled hole for the pin that holds the two halves of the shorter tube (and the wing) together.

Don, that's a good idea about alternating the tabs. It would mean less snipping to create enough of a gap between the tabs so they don't overlap.

I've already tested the uprights and the base for three full track days. The base mounts are really simple, but I think they'll hold up fine, even over time. All of the force from both drag and the (reverse) lift of the wing go right to the frame, with fasteners in the fiberglass only having to hold the mounts in position. I'll try to get some pictures of it all tomorrow.

Of course, I shouldn't have questioned, you've thought of everything. If you wore glasses, I'd say you had 2 personas. Glasses on you'd be a mild mannered writer, and then glasses off, automotive engineer extraordinaire.

Outstanding - the wt. & cost comparison at top is fabulous.

IT will be easier & faster to do the next one...

YOu might also make a wooden buck to form strips into various curves.

How thick are the tubes?

Hi Jack,
Next thing you know you'll put a difusser under the car or even on the under side of the wing.
Good luck
Richard

You'd be correct. I did the world's least-scientific diffuser a while back.

http://forums.pelicanparts.com/uploa...1112209185.jpg

http://forums.pelicanparts.com/uploa...1112290827.jpg

I'm going to do another one in the future, this time with some input from a friend who's an actual aerodynamicist.

I've got one other crazy idea in mind, as well, although it's one the aerodynamics guy has told me is nutty.

But I may try it anyway.

Awesome and quite inspiring! Thanks for sharing... Love your 911 by the way :-)

Eric

They're .5 inch outer diameter (6063 alloy). The plastic pieces snapped over for reinforcement are PVC pipe with a slightly-less-than .5 inch inner diameter. The sliding insert tube that goes inside the fixed tubes is .345 inch outer diameter.

But the lateral rigidity comes mostly from the spar pieces, which have a T-shaped profile (5/8 inch by 1/2), with glued-in bridge sections to create an I-shaped beam that runs the length of the wing. Even on their own, the T-shaped pieces (6061 alloy) are surprisingly rigid.

Very nice, looks like you really did your homework on this project. It sure is nice that NACA makes their data readily available to us.

Here's how it breaks down for travel:

http://forums.pelicanparts.com/uploa...1157221850.jpg

The black piece is the diagonal support. It's a threaded bar with an airfoil-shaped cover over it, to reduce drag.

The aluminum tube next to it is the one that fits inside the tube that runs between the braces in the center of the wing. If you look at the ends of the wing pieces, you can also see the rear tube insert (it's a little washed out in this photo), which has the hole for the pin that holds the two halves in position when they're assembled.

And here's a picture of the base pieces (they also come off for normal driving) from the side:

http://forums.pelicanparts.com/uploa...1157221919.jpg

And from below:

http://forums.pelicanparts.com/uploa...1157221901.jpg

The key part about the base is that there really isn't much stress placed on the fiberglass decklid. The steel braces are sandwiched down on top of the body of the car (along the edges of the engine compartment) so that the force goes down, right to the chassis steel. The T-shaped front piece is very sturdy, which controls fore-aft movement. The rear mount is just there to distribute force down to the chassis. The base pieces abrade the paint somewhat, but it's the paint deep down in the gap between the decklid and the chassis, which isn't visible when the decklid is down.

"The base pieces abrade the paint somewhat, but it's the paint deep down in the gap between the decklid and the chassis, which isn't visible when the decklid is down."

Aha. A place to improve!
A piece of inner tube or any convenient squishy material glued here should prevent further erosion of paint and metal. Punch appropriate cutouts to access the bracket fasteners.

Happy to contribute to the engineering of this. :)

Sherwood

That's a good idea, Sherwood. I use adhesive-backed felt pads on the parts that could conceivably touch the exterior paint. But something dense and slightly elastic (like inner-tube material) underneath would eliminate damage down there.

Well done Jack. My understanding of the diffuser is that it doesn't need to be as long as the one you have already tried. Look at a surfboard with 3 fins that is enough to do the job and I believe that is all that is required on a car.Perhaps your aerodynamisist could correct me if I'm wrong.
Richard.

A weight update: Total for the wing, uprights, caps, hardware and ducktail itself: 22 pounds.

That's less than half the weight of my old setup (45 pounds). And it's weight that was far back and high up.

The old carbon fiber wing was 9 pounds all by itself, and it was 12 inches shorter than the new 4.75 pound one.

Nice work.

Weight testing report: I put 250 pounds (five 50-lb bags of sand) on the wing today, with some cushioning to distribute the weight more evenly over its surface. It turns out that I have to make my connecting tube more rigid (maybe steel instead of aluminum?), and distribute the load to the fiberglass decklid a little more (I started to hear the fiberglass complaining).

But nothing got damaged, and the wing itself held up fine, which isn't bad for a 4 3/4 pound collection of thin aluminum pieces.

"...and distribute the load to the fiberglass decklid a little more (I started to hear the fiberglass complaining). "

Maybe keep the basic attachment location under the FG lid, but make the base longer/shape it so when the wing is compressed by air pressure, the load transfers to the chassis rain gutter directly under the mount instead of straining the limits of FG strands. You could also shape one base edge so it sits over (U-shape) the lip of the engine compartment edge. Might add some rigidity to reduce any twisting tendencies of the upright brackets as car direction goes off-axis @ speed. Or just mount the uprights in the side gutters of the more-rigid eng. compartment opening. The height of the wing should allow the engine lid to fully open. Need a lightweight CF engine lid support? :-)

FWIW, F1 pays a lot of attention to vehicle dynamics during high speed cornering, not only straight line aerodynamics. Can you measure change in attack angle or other wing/bracket deflection during high-speed cornering with the equipment you have (e.g. strain gauges)?

Sherwood

Yeah, that's my plan. The downward force actually does what you're describing -- it pushes down onto the chassis underneath the decklid. But the way the wing extends back means that downward pressure on the wing also pulls up on the lid -- imagine pulling straight back on the wing it wants to lift the forward edge of the decklid. It's like a lever effect.

I'm going to keep the existing mounts, but also integrate in a triangle-shape on the side that folds down below. It will run up a little higher on the rain gutter part of the chassis, but it will also distribute the force more evenly to the decklid itself.

I've been making home brewed things all my life from the first skate boards to be seen, to 20" bikes with kart motors welded above the removed pedals. Then, of course, the car thing, now for some 45 years and probably 45 cars. I've never been more impressed. With the amount of time that you've spent with your tools compared to me and the fact that I'm a journeyman carpenter with $1000's in tools, it's a real accomplishment to conceive, design and create a product like that.

You have my due respect.

Now, a question. How much drag force will be applied in your guess? Will the wing want to roll back putting a lot of stress on the deck? Seems like the forward mounts might not be pushing down all that much. I'm a real novice at this kind of aero engineering.

Edit: I was tpoing this as the above posts were posted. I see the question has already been addressed.

Jack,
The really rigid structure is but 8" below where you are now - the fore-aft longitudinal rails below the rain gutters. The rear engine mounts are close by as well. However, it's a convoluted path from here to there so maybe save this in case you need a plan C.

Sherwood

Thanks, Milt. Coming from you, that means a lot. But I have to say, when you see it up close, you'll smile, pat me on the back, and be glad that I'm keeping my day job. I love doing this stuff, but I'm not especially skilled at it, and I don't have the patience or the persistence that a real craftsman has.

Then again, maybe my lack of engineering and fabrication experience is why I'm willing to jump into projects like this with both feet.

Or maybe I just have too much free time.

As I understand it, though, the drag issue is minor (when it comes to my decklid and mounts) compared to the way the design of the uprights makes downforce mimic a rearward tug (that lever effect). Smdubovsky illustrated it in a different thread:

http://forums.pelicanparts.com/uploa...1154116506.jpg

I've gotten input on this project from a bunch of guys on Pelican, but also from a couple of other guys. One of them does this stuff for a living, but doesn't want me to share data he's produced based on actual simulations he's run. But the other guy (who I met through the S2000 BBS) did simple tests on the airfoil for lift and drag. The drag numbers would be kind of lousy if I was designing an airplane, but aren't going to pose a structural problem for my uprights or mounts. (Peter Bull, the Pelicanite who worked out this airfoil for me, also looked at drag coefficients when we were comparing airfoils. I don't think we talked about actual numbers, though.) The S2000 guy's charts put the maximum drag at about 29 pounds at 120 mph.

http://forums.pelicanparts.com/uploa...1157221901.jpg

Agreed. I think the stress on the fiberglass was coming from the single bolt in the mount of the rear base piece. I didn't anticipate the way the rear mount would twist from pressure from where it connects to the aft side of the upright. Adding a second bolt might be enough to fix it. And simply tying to the two base pieces together would probably eliminate the problem -- but I'm going to take it a step further for my Plan B.

Jack, nice job. FWIW, epoxy will bond aluminum when prepped properly (Grumman Yankees, Tigers and Cheetahs are all simply glued together with epoxy as opposed to more traditional rivets).

I see you installed a diagonal round tube to keep the wing from flexing side to side. Round tubes produce a lot of drag and your single small tube will only be effective in preventing flex when in tension. When in compression, it will tend to buckle. This may not be an issue, however I am a bit concerned that you may get a flutter at high speed without a little better bracing to keep the thin side plates from going into a destructive flutter mode.

The drag from your "round" diagonal tube is probably not worth worrying about, but streamlined tubing would be better. If it were me, I might think about adding an additional "jury" strut to give this lonely tube some help in the buckling dept or make your side plates shorter or make them from 4130 streamlined tubing which would make them stiffer yet still relatively drag free. I realize that theoretically you are not introducing side loads, but flutter once initiated is wicked.

These are just friendly observations, I think you did a great job.


OOPS EDIT: After re-reading, It looks like I missed the part where you stated the diagonal had a streamlined cover.

Wow, no technical comments to add, but I am seriously impressed by your inginuity and workmanship. Congrats, I'm subscribed!

Wow... very impressive!

I swear, you're making it harder and harder for me to tell you and Jack Ridley apart!

(FYI, Jack Ridley was, among other things, the Project Engineer on the X-1 tests with Chuck Yeager, as well as one of my all-time heroes)

this will make a nice new product for Olsen Enterprises, Inc.


- eventually you may want to move the ducktail to a different location