Ghia Wool Tuft Testing?

[spectre6000]

Has anyone ever done any reasonably scientific testing on the aerodynamics of the Karmann Ghia in a similar vane to the wool tuft testing that was done on a Beetle as in http://www.shoptalkforums.com/viewtopic ... 5&t=136778? I know FJCamper has done a lot of good work, but I'd really like to see something a bit on the objective side. For instance, the duck tail (ugly as it is) makes sense, but FJCamper has his mounted right behind the decklid vents, and the STF salt Ghia had better results with it further back. Is there anything more than gut feeling and butt testing behind any of the aero work that's been done? I'd gladly lend a hand and do a good bit myself, but my restored '57 is in storage a good 1,000 miles away at the moment (and there's no way I'm drilling holes in it to mount things)...

[FJCamper]

Hi Spectre6000,

The right mounting location for a duck tail is right behind the Ghia's air vents. This is based off Porsche's location for the duck tail on the RSR.

We supplied the Bonneville Ghia's duck tail for real world testing, but failed to specify the mounting location, and the device got mounted farther back than it should have been.

The Bonneville Ghia (Britt's record-breaking car) even made a run or two without the duck tail, for comparative testing.

The Bonneville experience was inconclusive. The Ghia performed about as well with it as without it. Another way of stating this is the device did no harm. We're still experimenting. I think we can say with some agreement that, just as Porsche intended, the device added no drag. What we want to know, and will discover, is can it reduce drag and increase stability?

FJC

[spectre6000]

FJCamper, thanks for the reply! You're probably the most qualified person I can think of to field a response!

The right mounting location for a duck tail is right behind the Ghia's air vents. This is based off Porsche's location for the duck tail on the RSR.

If the Ghia's body were substantially identical to the Porsche's that might be sufficient reasoning, but they're simply not the same shape and the notion lacks any sort of academic rigor. The lack of any change between misplacement and non-placement of the duck tail in the salt flat car could very easily mean there is no laminar flow over the back 1/3 of the car, and it's not hurting anything because it's just not doing anything in the first place. In fact, if it's not doing anything appreciable having been misplaced just a few inches aft as large as it is, that indicates the buffer zone at that point is greater than the height of the tail, and it likely isn't doing much where it is "supposed to be" either. In fact, it would make sense that this would be almost intentional to avoid creating a low pressure zone in the engine bay as the high speed flow passed over the air vents... I have read quite a few of your posts on the subject of making Ghias great (and I have no small amount of respect for what you've written and intend to implement a number of your tips/findings/suggestions/what have you to a to be acquired dedicated race car) and I'm not trying to be argumentative, it's simply that I can very easily imagine how the laminar flow over the body would break at the rear window rendering the duck tail fairly moot (though the duck tail was not the primary motivation for the original post). If there were some objective test demonstrating the effect, my curiosity would be sated and I'd have one on my next Ghia, which is why I asked in the first place.

A baseline assessment of the aerodynamic situation would go a long way to assess what needs what type of aerodynamic aides where. What's the flow over the nose? Wheels/wheel wells? Break of the roof at the rear window? Decklid? If the airflow breaks at the rear window (an almost ubiquitous scenario), as smooth as the transition in the car in question something could very likely be done to fix the problem. If the air never lets go all the way down the back, something needs to be done there. Without some baseline though, it's difficult to tell.

If it hasn't been done, or at least hasn't been documented in a manner that's been published in any reasonably accessible way, I'll gladly help make that happen. Either I'll do it with my own Ghia (lowlight) when I finally get it up here some time this summer, or I'll assist anyone in the Denver/Boulder area with a Ghia that would be willing to have masking tape and yarn stuck all over half their car in the name of science.

[FJCamper]

Hi Spectre6000,

I follow your questions and reasoning.

Of course the Ghia and 911 body are different, but the constant between them that places the ducktail over the engine vents is the strong drag created by the slipstream breaking up just after the rear window.

Placement of the engine vents on both cars is based on the reversal of air flow that occurs just ahead of the vents. The air is actually "blowing backwards" there, and the vents are placed to take advantage of that.

The RSR's were fast enough to make turbulance so great the engine vents were no longer working as intended, and the ducktail fixed that, as well as controlling the airflow so that the rear end was not being buffeted and made unstable at top speed.

I know of which I speak. The first RSR victory in the world was with us at Brumos (Feb 1973) winning the Daytona 24 Hour. A test run without the ducktail showed the car to be very twitchy at 150-160mph. We could afford no downforce drag. Greenwood's Corvette had us by 20mph on the top end then. Thank God he broke down a lot.

Getting more recent, and to illustrate the force of the rear "drag" on the Beetle body, we had to run without rear windows at Sebring last year in the 1973 Super Beetle Blitzwagen, and oil drops from the case breather were propelled forward in the cabin to spatter on the inside of the windshield.

VW knew what it was doing by placing the engine vents under the rear window on the Bug. We think of that area as subject to drag, and it is, but to feel it as the teriffic reverse airflow it is, is impressive.

Aerodynamically, the 911 body is just marginally better than the Ghia body. We know the Ghia can go 145-150mph on the salt and stay on the ground.

Other than a front airdam and a small ducktail-type device, the Ghia needs nothing to be stable at 140mph + ... the reason we can get by with a shorter ducktail on the Ghia than the RSR is the RSR's engine lid slope falls off more quickly than the Ghia.

FJC

[spectre6000]

What is the claimed constant between the 911 and the Ghia? The notch of the rear window is admittedly smooth and fairly gradual, but it's present all the same. Additionally, they couldn't be much different up front, and the curvature of the rear is very different. They have the same mechanical layout and occupy approximately the same area within a silhouette, but to my mind that's all that can be stated without material evidence.

The 993 (the car I've seen tests on) has a small spoiler at the top of the rear window (to break laminar flow and reduce lift), and the air flow over the rear window is pretty bad. It quickly reattaches though, and by the time you get to the vents, it's laminar again until a spoiler/duck tail or the level of the top of the tail lights. In the case of the 993 Turbo, the air flow goes straight into the duck tail versus being turbulent over it. I haven't seen any tests done on a 911, but the 993 and the 911 are far more similar than a 911 and a Karmann Ghia in terms of apparent airflow characteristics.



The Ghia lacks the rear window spoiler. Is it in any way needed, or does the laminar flow completely break from there leaving a much larger than necessary hole in the air and a lot of reverse lift? If it doesn't break at the top of the rear window, where does it break? Can it be moved further aft (without affecting cooling) to reduce the cross sectional area A? This would create lift, but could be rectified by use of a properly designed spoiler. What about the transition from the headlights to the fenders (lowlights would seem to have the edge here)?



These guys took a completely different approach to yours, and I'd be willing to wager they feel just as confident that they know what they're doing. Looking at the photo (and with no other evidence to back the claim), I'll bet the spoiler is probably helping, but the wheel arches are displacing some of their drag reduction efforts. Additionally, the deck lid standoffs are an interesting choice... Maybe the buffer zone is sufficiently thin there as to actually be actively directing some air into the engine compartment (though this would be counter to what the Bonneville crew seems to have demonstrated as previously mentioned)...

The goal on the face of it is to keep the air flow stuck to the body as long as possible (including over the vents) and break it at the trailing edge in order to minimize cross sectional area. Add a well designed spoiler to counteract the lift this generates, and provide alternate means of getting cooling air to the engine (if necessary) by either strategically placed vents (something you have dabbled with), or moving the deck lid vents back further on the deck lid (the rear of the car even to take advantage of the high pressure zone created by the rest of the strategy).

For my satisfaction, there are too many unknowns without a baseline assessment to make any educated guesses as to what is helping and what just looks cool. As cheap and easy as the testing is to suss it out, it seems a given that it would have been done or can be done in short order.

[FJCamper]

Hi Spectre6000,

The "constant" between the 911 and the Ghia is the location of the duck tail, not the body shape. Porsche put the duck tail just behind the engine lid vent to dam up air so it would be forced down the vent, and as a side effect, make the laminar air break higher than the engine lid itself.

The Ghia actually has a better body contour than the 911 aft of the rear window, meaning higher, so the laminar air flow is more controlled. The Ghia with the big wing might be getting both downforce and taking advantage of the wing catching what air that flows over the rooftop (similiar roofline and wing height) and reducing overall rear drag.

Look back at the Porsche double wings which were trying to do just that.

One of the cheap aero test tricks is to apply a few drops of oil at key places on the body, and after the run, check the direction of flow.

FJC

[spectre6000]

So you're saying the duck tail's purpose in that position is to force air down the vents, and breaking laminar flow is a convenient byproduct? What if we assume the air vents can be relocated or eliminated? Where, in this scenario, would it best be located? Would it still be a duck tail, or more of a lip spoiler to simply break flow off the body at the back of the deck lid (combined likely with a pedestal spoiler to generate downforce)?

The oil method sounds interesting/effective, but a bit more difficult to document/clean up after.

[FJCamper]

ABOVE: Flow Illustrator red area is high-pressure air, green is lower pressure tending toward turbulance. A street-high front airdam and duck tail show their effect.
Note (at a software 100mph) how green air is trying to disrupt air flow into the engine vent.

Hi Spectre6000,

Yes, the ducktail is a very clever double-duty device. Part air dam, part spoiler.

The 911's original body shape was styled with some aerodynamics in mind, but not with 150mph and more for sustained lengths of time. To really cheat the wind it takes a long-tail car, the most famous of those Porsches being the Moby Dick.

When the RSR was introduced, it had more top end potential than any preceeding 911, and Porsche made it the first 911 to wear aero mods. And remember, the RSR had to compete in the pre-turbo days. Horsepower was limited compared to the competition. The RSR's aero had to work without adding drag.

The whale tail was a turbo add-on. The engine made the power to allow some drag. Note that the whale tail effectively raised the engine air intake so the airflow off the roof and down the rear window did not break off so quickly.

I had the good fortune to be right in the middle of the factory development, as crew and mechanic, beginning with the RSR and ending with the last of the twin-turbo 935's, at Brumos. We went through every aero mod the factory provided, and knew what worked -- or didn't work. Sometimes we'd swap out tails or air dams in testing, or because we broke them in racing.

All the Ghia needs is a few aero tweaks already proven by the RSR. Front airdam to kill the lift, ducktail to keep the engine airflow coming in, and breaking the airflow off a little cleaner.

FJC

[spectre6000]

How does that software work? Do you give it a silhouette/profile and it generates a prediction of air flow? I found what you used, but it's Windows only... :-/ What does it show with the stock profile?

[FJCamper]

Above: Stock Ghia, 120mph. The Ghia has an aero-friendly upper body contour. The software air density is at a syrup-like 45 within the program here, a perfect amount to display clear laminar flow, classroom style. In the earlier ducktail example, I'm using the more realistic zero (normal) setting, which shows the "dirty" air in turbulance.

Hi Spectre6000,

Yes, you supply the bitmap image and the software streams the air over it, with all kinds of variables. It is a dynamic process, a movie, with much swirling of the vortexes.

The program comes with its own set of shapes for aerodynamic testing.

https://www.imperial.ac.uk/aeronautics/ ... trator.htm

FJC

[noslzzp]

FJC, thanks for the link and information as usual. Did you ever experiment with canards on the front end of your Ghias? Also, will the software allow a top down view - so one could see the side air flows? I'm guessing this would simply be another bitmap..

[spectre6000]

It sucks that I don't have a PC, or I'd be playing with that all evening...

[FJCamper]

Hi Spectre6000,

The program will allow an overhead view but it thinks all images are profiles. You can fool it by only paying attention to the upper side!

Above is our 1973 Super at Sebring last year, no rear window, and the software shows us why we got oil spatter all the way up to the inside of the windshield.

Also, note how much air is coming in through the stock-location engine air vents! And that our low-profile roof spoiler was working, but not well enough.

Oh -- no canard test on our Ghias, just front air dams and duck tails.

FJC

[silkvw]

Great post! I've been thinking about what will really work to improve the aero of my Ghia too. So far, I have covered the air openings of the deck lid with a piece of aluminum, cooling air is drawn from under the car with a 4" dia. duct (dryer vent) and intake air to the turbo is taken from the passenger side 1/4 panel air box that draws air from under the car. This set up has worked at 120, 130 and 140 mph. I have a gauge to monitor the pressure in the engine bay, it's always positive when the car is moving, but I don't know if there is enough air flow through the engine bay. During a 141mph, 1 mile run the oil temp was below 200 deg. and CHT stayed below 300 deg.
This article may be of interest: http://www.hotrod.com/techarticles/body ... odynamics/ Perhaps we can at least learn from other cars.

[Fiatdude]

Hey FJC --- Have you thought to run the side of one of the promod Ghia's or possibly the side of Roger Crawfords SS Ghia???