Lifting rear of roof, what are the benefits Options

[race914]

Got inspired watching Sebring and since the weather was finally cooperating I painted the airtabs today in between Speedchannel coverage.

Hope to mount them on the roof tomorrow. Test day is April 15th at Buttonwillow! See unpainted mockup on page 4 of this thread for the color scheme.

[retrotech]

Anxiously waiting to hear. I am trying to decide between Air Tabs, and the Carbon WRX units.

[race914]

Mounted the Airtabs today. All ready for testing! (IMG:http://www.914world.com/bbs2/html/emoticons/cool.gif)

D-Day April 15th @ Buttonwillow. More to come...

[SirAndy]

QUOTE (retrotech @ Mar 18 2006, 05:07 PM)

Anxiously waiting to hear.

(IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif)

[retrotech]

I have read the research papers from Japnese engineers that designed the one for the Mitsubishi WRX. Certainly is technical, and claims to achieve the same results, but they are triangular. The pointed end, into the wind, opposite of the air tabs? Air tabs will certainly create a bit more drag, but I can imagine that being an issue. I just wonder if they are over kill for 914 application, versus semi trailer. I thik they will work, but will something less also be adequate?
Inquiring minds want to know.

[alpha434]

QUOTE (retrotech @ Mar 19 2006, 04:18 PM)

I have read the research papers from Japnese engineers that designed the one for the Mitsubishi WRX. Certainly is technical, and claims to achieve the same results, but they are triangular. The pointed end, into the wind, opposite of the air tabs? Air tabs will certainly create a bit more drag, but I can imagine that being an issue. I just wonder if they are over kill for 914 application, versus semi trailer. I thik they will work, but will something less also be adequate? Inquiring minds want to know.

The WRX ones are just square, straight up and down. Sorta. They are triangular shaped, but they look square from the front.

The "airtabs" look to be MUCH more refined and developed. I wonder if Subaru just did the vortex research as an "afterthought" when they had a little instability at high speeds.

Here's another $.02

[turboman808]

instead of going with the vortex what about adding a gurney flap to the back of the lid? No doubt those will probably work really well but man they are ugly. (IMG:http://www.914world.com/bbs2/html/emoticons/barf.gif)

[Joe Ricard]

Not if you make it out of CF or Lexan. (IMG:http://www.914world.com/bbs2/html/emoticons/rolleyes.gif)
Either one will make the car faster just by making the ashtray out of it.

[retrotech]

Picture of Gurnet flap?

[Matt Meyer]

I am anxiously waiting your reports also.

My $.02, and you are being overcharged.....

I think vortex generators are mostly a gimmick on these modern cars, but might work on a 914 because the stock aerodynamics suck.

The vortex generator on the Lancer and Subaru would probably not be effective on the 914. But I think it is worth a look anyway.

The "shark tooth" type vortex generators try to "re-energize the boundary layer" to get more airflow and keep the airflow attached as it travels over the car. These type of vortex generators are well researched and proven in the aerospace industry.

What is allegedly happening on the Subaru and Lancer is that the angle of the rear window is too great and the airflow starts to separate at the rear window. This creates excess drag and makes the rear wing or “spoiler” ineffective. The vortex generator takes energy from the flow and moves it into the (not moving) boundary layer to get the boundary layer moving. The airflow stays attached along the rear window, over the rear deck where a spoiler detaches it. In theory the drag being reduced is greater than the drag to create the vortices so there is a net gain. I would expect these would work well on an older 911.

The airflow will separate over the top of the 914, the undercut rear window insures that. It appears to me that “AirTabs” allegedly create (I assume larger) vortices that separate the airflow in a controlled manner and reduce drag. Like a spoiler on the rear of a car. This type vortex is notoriously hard to maintain, and I could find almost no research on the effectiveness of a vortex generator used to separate airflow (very quick search).

A small spoiler (¼ to 1 inch) on the back of the targa top might be effective as it would separate the airflow.

I possess no unique knowledge about the 914s aerodynamics but I doubt anyone can do much worse than what was provided originally at the trailing edge of the targa on a 914.

One cheap (crude) way to test these would be to do coast down tests. Take a car with a stock top up to a significant speed (80-100 mph) and then let it coast to 0mph. Time or measure how long it takes. Repeat in the opposite direction to negate any wind. Do the same with the aerodynamic aid installed and determine the net difference. If it coast farther/longer with the aerodynamic aid the aid works.

[brant]

I don't want to sound like a pessimist...
so I'm anxiously awaiting the seat of the pants impression too....

but I seriously doubt that without a wind tunnel or computer calibrated equiptment that it will be possible to "feel" any difference.

If you need stability add a wing.
if rules don't allow a wing, then they don't really allow vortex generators either.

This thread has split into 2 seperate discussions...
#1 about creating downforce
and
#2nd about reducing drag.

Lifting the roof is really just letting cockpit air escape in the hope that it will reduce drag... (see item #1)

I still think that in regards to downforce a Wing is the most effective technique

just my 1.5cents.
brant

[retrotech]

I didn't know how to attach this file, but if anyone is interested. I hope this link will lead to the in depth report by engineers in Japan, about the funtion of Vortex Generators on the Mitsubishi.

www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf

Its 6 pages and very convincing, that it is not a useless add on.
I am optomistic the VG will really help attach air to rear spoiler on 914.

The Arpil 15th test drive on the track will tell us the most.

[Matt Meyer]

I couldn't get your link to work but I think I have been to that site.

I did not mean to seem too pessismistic. And all ideas need to be tried and tested. There have been huge advancements in the past 30 years and the 914 was not at the leading edge then. With enough time and money I have no doubt that small barely noticible changes could produce some significant aerodynamic improvements. I wish we had the money to invest where there is a benefit that Mitusubushi

The type of vortex generators on the Mitusubishi work. The Aerospace industry has proven that and they have extensive use there (where they usually increase lift BTW). I acutally have more confidence that these shark tooth generators will do something positive than the AirTabs. I just don't know if it will help the 914.

I think they are a gimmik on the modern cars, because on a car for the street, 70mph max legally, the rear wing is a gimmik. The generators produce drag, the wing produces drag and at below 100mph have no real benefit. Furthmore, if they are really worried about seperation they put a little more angle on the rear window and you have the same effect with less drag.

IIRC the early 911 just missed the proper angle on the rear window to keep airflow attached. I would recommend the shark toothed generators first in this application.

On a 914 the air will seperate at the rear of the targa. The trick is to get it to reattach or virtually stay attached without braking up. AirTabs claim they do this with semis, so I see more potential there than on the "shark tooth" type on inspection. But making desisions by inspection leads notoriously to failure (and gloriously to success by those who do not know better) in the speciality of aerodynamics.

I am just predicting in order of likely success AirTabs, small spoiler (which the lifting of the back might be effectivelly doing), and then the shark toothed generators. I hope someone steps up and tests them all. See my above post for the best meaningfull test that is not NASA funding type money that I can think of. Anyone want to apply for a grant?

[brant]

most of the time when I see manufacture's claims about aerodynamics.. the numbers are relatively small. Possibly because the package is already so refined..

but here is an example:

the new lotus exige..
They reportedly changed nearly every single body panel, added a roof, added a splitter, the list goes on.

Now granted the "before" car was undoubtably pretty well refined. But my (failing) memory about the Aero tests for the "after" body work resulted in something like 15lbs of increased down force....

was it more? I could be wrong.. maybe it was more. But as I read about this vehicle and Wind tunnel proven numbers.. it shocked me that the amount of down force gained was so small.

I doubt that I could "feel" by the seat of my pants a 15lb down force gain at 100mph. In fact, I doubt that I could feel a 150lb down force gain at 100mph.

maybe my buttocks isn't sensative enough...
I'm not saying that small things can't be a step forward, I just doubt that anyone is going to gain 1 second a lap.. or even .1 of a second a lap.

The driver might "feel" more comfortable, but its going to be tough to assign a value to feelings without a stop watch to verify them.

and I hope I'm wrong..
anxiously awaiting the trial by fire.

brant

[Joe Ricard]

I don't know But I think them air flappy things are cool. I could benefit from them just by gaining 1 mile per gallon. Having to travel 380 miles just to race is probably worth it.

And the look cool (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) Gear heads love stuff that looks cool.

[alpha434]

Vortex generators fall into the "advanced" category of aero devices. Even though they increase profile, they don't increase drag. Well.. They do, but then they make up for it so the end result is less. If you look at the promotional website for the airtabs, they create a vacuum directly behind them, so that air behind the car will try to rush in and fill the void. This helps to propel the car forward. Theoretically, based on the info from their website.

The mitsubishi one was always just ment to turbulate the air for the rear spoiler. Has nothing to do with holding air against the bodywork.

I don't think that the advantages will be that huge. Even at speed. You may pick up some fuel efficiency, but who here runs 300 lap races? I've already mentioned the strategic advantages. Which won't help you go faster, but will make it harder for the car behind you to go faster.

And Brant.... If it's not specifically illegal, then it's legal. Until they MAKE it specifically illegal.

[alpha434]

Also anxiously awaiting the trial by fire....

(IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[tdsmoonchild]

QUOTE (retrotech @ Mar 18 2006, 07:07 PM)

Anxiously waiting to hear. I am trying to decide between Air Tabs, and the Carbon WRX units.

anyone have pics of the WRX ones?

[brant]

Chris,

most of the amateur racing that I do have rule books that specifically say:

"if its not listed as something that you can legally do, then it is illegal"

PCA-CR for example has this rule in print.
kinda the opposite of the SCCA, or Pro type racing.
supposedly to keep things more affordable.

gives the scrutineers an open back door to say nearly anything is illegal after the fact.

Plus in Vintage they clearly state that "No aerodynamic devices of any kind are allowed"

my holes in my rear window are technically for driver comfort. By letting the air in the cockpit escape I am promoting a cooler temperature cockpit and driver comfort. The rule book says that I can modify things to do that.... I am clearly motivated to do it for the benefit in reduction of drag. That is my 2ndary gain, as I achieve my primary intent of promoting driver comfort....

if you don't believe me ask the tech.
I told him too

(IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[retrotech]

Some seats are more sensitive than others. With a passenger I can definately feel the difference in acceleration. I posted some time back a wierd custom spoiler I had on my 914. It was about 10". It was super stable, but I "could" feel the drag on the straights, no spoiler, higher straight speed, but not confidence building over 100 mph. Now I made a custom 4" duck tail/spoiler. No seat of pants drag, and solid as a rock 100+. I still believe there is cheap speed to be attained from reducing drag on rear as air foil wants to wrap under rear end, and more stabilty cornering at 60+ with atached air to spoiler. I agree with the Lotus info, but the 914 is starting from such a disadvantage, I think even the little things will translate to big improvements.