disasterman

My research led me down the "flat bottom" path. Two things that we will be adding will be canards to balance the wing and a air relief hole behind the front wheel in the fender.

JustinL

I used this one on my car. I think it looks cool and fits with the Porsche theme as it is a 968 part. The IFC piece is nice, but not perfect. Mine was a little twisted and after many attempts at gluing it to my fiberglass hood, I ended up just riveting it down. It still looks ok and it's not going to fly out now for sure. Mike is great do do business with.

Sterling Doc

Got it started with the new cam and side exhaust.
It needs to be tuned with the new cam in, but sounds mean
Video :<p><a href="https://vimeo.com/152722070">Startup</a> from <a href="https://vimeo.com/user842850">Eric Kuhns</a> on <a href="https://vimeo.com">Vimeo</a>.</p>



Now on to fabricating the bottom.

67King

I'm not an aero expert, so take that into consideration. I think that the spoiler on the hatch is your enemy. Two things, First, it is designed to create a high pressure region. Well.......given that you have a wing above it, you want a low pressure region there. Now it may be that it is far enough below that it is helping, I'm not sure. But that is what I suspect. Removing it will also help the air expand as it rolls over the rear of the decklid, which will do two things for you. It will create a low pressure region (again, below the wing), and it will also help reduce turbulence at the rear of the car.

I want to think that Mark is an aero guy, maybe he has some info to share?

Longer wing will help. So will a deeper chord. I may end up making my own. I'm going to be running a 944S in SP3. So I'll be lighter than msot folks out there (2500 lbs race weight, compared to 2900 for the 968's). But Ill also be a lot more affected by aerodynamic drag.

Sterling Doc

I agree. The spoiler is gone, for all of those reasons, as well as fitting the upright. As an interim solution, I thought about a slightly larger Gurney for stiffness, and less AOA. Maybe Kibort has something to say about this

As for the hood vent, I like Patrick's option, and I thin I can fabricate something similar to rivet on.

mark kibort

I think if if the wing is high enough, it might help do what it is designed to do, which is , non intuitively, reduce the drag off the rear of the car. However, you are right, if by doing that, it creates a little high pressure zone and flow, it might hurt the wing effectiveness. I think the point is moot as he is removing it anyway. certainly at first glance dont think you need both

the hood vent is a great idea.... since the 944 and 928 share some frontal flow similarities, if you can find that one spot that is the low pressure zone (right about where you have it usually) , you can have the pressure of the nose inlet flow out the hood, vs under the car. (the main purpose of the vent). it then adds to the low pressure, makes more downforce, and the extra flow goes over the rear wing as well. (all good things)

as far as gurney flap.... definitely use only the 1/4" height, otherwise , you get into higher drag levels. generally, if the wing is near level (which really is near 9-10 deg AOA) the gurney flap gives the effect of downforce of what it would be if it was near 8 degrees. (pretty steep angle). the gureny flap just allows you go get more effective angle results, with out going into stall regions of the wing. (super high downforce with less drag than without the gurney flap) but, more drag if you are in the moderate angle ranges.
bottomline.. ..... little or no angle (AOL).... use the gurney flap.... lots of angle .... use the flap.....

there are some great graphs posted about the effects of wings like ours and the use of different height gurney flaps in our gurney flap discussion a while back.

Lemming

I have the 70" predator (carbon fiber) on my car with a gurney flap (1/4") and it still flexes in the middle.

67King

FWIW, I read through most of that thread, and there were two things I'd point out. First, I completely understood what you were saying about AoA versus a Gurney. There was one chart n there that showed Cd for a constant Cl with and without, and it clearly showed your point. I would expect that it would vary by wing. One of the things that I think may make the experiemental results differ is that the uprights are mounted on the low pressure side of the wing. That's a big no-no, although as a matter of practicality, most people do it. But it drastically cuts down the effectiveness of the airfoil. I haven't seen too many of hte swan neck style mounts, but I am wondering if those end up giving real world results that match the theory you were talking about.

The other point is that someone in there stated that a Re of 1.95E6 was about 22MPH. He was off by an order of magnitude. It is clsoer to 150MPH. 100MPG gives an Re of just under 900,000 (0.9E6).


ON the flexing of the wings........one thing y'all might try is to get some unidirectional carbon fiber, and lay it on the bottom wurface of the wing. You'll need sand it pretty well and make sure the epoxy/resin will stick to what is there. But C fiber will not stretch, so if you go wiht a unidirectional cloth, and lay it so it will be in tension, it should help. Try to keep the surface as smooth as possible when you do, though, as the bottom surface is much more affected by disription as is the top surface.

thingo

I think if anything you should extend the stock spoiler,it really helps the airflow on the underside of the wing,despite there being a drag penalty. The rear of the 944 is very turbulent.

stownsen914

I'm no aero expert, but have done lots of reading, including Joseph Katz's book on racing aero. He discussed gurneys in the book. He wrote that gurney flaps increase downforce but not efficiency. In other words, generally speaking, for a given downforce level, you will have less drag by increasing the wing's angle of attack or using a larger wing, rather than adding a gurney flap.

Gurneys are used extensively in pro racing for tuning a car's handling and to increase downforce. Keep in mind that in pro racing, wings are almost always limited in size and shape by the rules, so teams are looking for more downforce even at the cost of increased drag.

mark kibort

Yes, and as we discussed in the thread about gurney flaps, what you say is true , just until you get in the max lift range of the airfoil. meaning, if you go over the max Cl of the wing, then you dont get anymore lift, but drag continues to increase. the gurney flap allows to go into a higher Lift range, with only proportioal drag assocaited with it. BUT, at the lower lift ranges, its better to get the lift (or downforce) by use of angle alone. however, i must add, because of AoAs of the wing, most are in the max range anyway, so its good to use the flaps if you need the extra downforce that the wing doesnt provide at more level ranges. (level visually, but not level AoA.... more like, most are in the 10degree range at level anyway, and max is in the 15 degree AoA range)

yep, you are right about the uprights, but, a thin one has very little effect on the wing due to it being "thin". in fact, there are probably very little downside. the reason you can use "thin" is because the wing is supported in two spots , and the aspect ratio is so very very small compare to an airplane. so, subsequently, you can get 300-400lbs of downforce with very little support on the wing at the ends. sure, thats 1/8" x 2 of an area on the wing that is not activly adding to lowering the pressure , but on a 55" wing, 1/4" is not a big deal and there is very little turbulent flow around it, so call it double that... 1/2" less effective wing area overall.

chartersb

Interesting thread. Here's some stuff we learned in the UOIT tunnel.

It was easy to add downforce at the rear wheels. It was very difficult to add anything even close to matching downforce at the front wheels. A few hundred pounds of DF on the rear wheels can add .1 to .2 g at the rear wheels in a 100+- mph corner; but without a similar amount on the front wheels, you have only a marginal increase in the max g load at the front. The result is a a stable, planted feeling in the car but not a significantly increased cornering capability since you can only corner as fast as the slowest end of the car will let you.

With the wing at it's most efficient, designed AOA, it will deliver the most lift (downforce) with the least drag. To increase the DF with minimal drag cost, a wicker was far better than changing the AOA.

JustinL

Discuss this point more please. I looked at pictures of daytona prototypes and it seems some kind of spoiler is better than none even with a wing not far above... but how big and what angle? It is adjustable on the DP and it's got to be better than running the rounded *** end of the stock 944 right?

stownsen914

Katz covers spoilers in his book as well. Really good read if you're interested in racecar aero, and lots of good, relevant stuff for us home-brew racers playing with aero. Wing and aero interactions are complex, and highly dependent on the individual car design and setup. The guys with the big bucks spend many hours in wind tunnels and/or doing CFD to get it right. Wing height, angle of attack, spoilers and other add-ons, and it gets even more complex if the car has tunnels or a diffuser underneath.

Generally, stand-alone spoilers are much less efficient than wings, but they do still give downforce. For the DP in the pic above, the spoiler could be designed to interact with the wing, so it may add a lot more downforce than a stand-alone spoiler would. Or maybe the builder just wanted more downforce and decided to take some aero penalty with a good sized spoiler in addition to the rules-limited wing. Hard to say for sure, unless there are some aero details out there about the car in question.

If you want to get fancy, you could have a twin tier wing with the lower one just behind the hatch and an upper one above it. If rules allow, it is supposed to be a great setup, especially if you have a diffuser ... I got ambitious a few years ago and built a diffuser and a twin tier wing setup for my 914. I have done limited testing and definitely nothing fancy like CFD or wind tunnel testing, but the car sure does stick well. Here's a pic. It's not easy to see in this picture, but the sliver-ish thing below the large black wing is as second wing, and the diffuser is visible below that. (And ignore the engine cover sitting on top of the car and me fiddling with something just in front of it.)

thingo

The lower "spoiler" provides more total pressure below the rear wing, without it the air follows the curve of the rear end further down which lessens the efficiency of the wing, it's the flow underneath that produces the downforce. As for the angle a gentle curve should be fine, we put a gurney on ours at the track, this is before we mounted the wing, but you should get the idea.