There's been some debate over the aerodynamics of the Fiero rear spoiler. Some people think the Fiero spoiler is a functional airfoil, and some think it's a drag spoiler. Some people think the air underneath the spoiler flows forward (toward the rear window), and some do not. I decided to take some video footage to see exactly what's going on back there.

I taped some paper streamers to the front and rear edges of the spoiler, and stuck a video camera in the back window. I then drove the car on a fairly straight and level road, accelerating to approximately 70 MPH. There was a slight tailwind during the first run (the first half of the video), so I turned around and made a second run in the opposite direction.

Here's the video footage:



The results are very interesting. First of all, it seems that the air underneath the spoiler does in fact flow forward. However, it only does so in the center section of the spoiler. The outer edges of the spoiler appear to be acting as airfoils. At the outer end of the spoiler, you can see the streamers wrapping around the bottom, pulled by the front-to-back airflow.

In the second test, I used a long piece of PVC L-bracket to block off the airflow under the center section of the spoiler, to stop that back-to-front airflow pattern. Unfortunately, there was more traffic during this test. The traffic only let up long enough for one quick run.



The outer sections of the spoiler still appear to be acting as airfoils. I was concerned that the "air dam" I installed under the spoiler would cause pressure to build up and spill around the sides onto the decklid. But it seems the airflow at the ends of the spoiler is strong enough to prevent that.

In my third test, I wanted to see the airflow characteristics of the decklid vent gratings. I decided to compare a stock vent to a modified, free-flowing one to see if there would be a noticeable difference. This will help me determine if it's worth the effort to modify the decklid vents.

In the photo below, you see the two vents that I used. On the left is a stock vent, complete with the stamped sheet metal "leaf catcher" underneath. On the right is an '84 Fiero engine cover, with the center cut out and some steel mesh installed. The only purpose of the mesh is to provide a mounting point for the string. Speaking of which, I've switched to using 3" long pieces of yarn, instead of paper streamers.



And last but not least, here is the video. The first half of the video shows the airflow (or lack thereof) through the stock vent grating. The second half shows the airflow through the modified one. In both cases, I accelerated to about 70 MPH. I was going the same direction, on the same stretch of road, in the same weather conditions.



What a difference! The modified vent grating allows much more airflow than the stock one. That should help with engine compartment cooling, as well as helping to fill the void behind the rear window (i.e. less drag).

I also wanted to see what's going on at the front of the car. So I built a roof-top camera enclosure to get a good viewing angle of the hood.

The first test was to see how the pop-up headlights affected airflow over the hood. My headlights are the slim pop-ups using the brackets from Fiero1Fan and Hella 90mm lamps. So they don't pop up as far as the stock ones. The effect on airflow was still pretty crazy. This is what it looks like at 60 MPH:



Notice how the tufts behind the headlights flare out when the headlights are popped up. That appears to be the twin-vortex affect... the same thing that causes the big swirls in the tire smoke when airplanes land. The turbulence from the pop-up headlights, combined with the turbulence from the hood vent, turns the airflow over the hood into a swirling mess. NOT GOOD.

[This message has been edited by Blacktree (edited 02-15-2009).]

There is an archived thread where this was done before. I haven't looked for it, but it was about 2004-maybe 2005. Some pics if I remember correctly

Keep in mind that just because there is air coming forwards below the spoiler that does not necessaryly mean that there is not downforce created from the spoiler. There is still air flowing over the top and then dropping over the rear of the car into the low preasure area. Some of that air then comes back forward under the spoiler. The spoiler effectively raises the rear of the car up higher into the airstream, ceaating some down force. If air was rushing under the spoiler, it could create lift.

Either way, I feel the car looks far better with the spoiler and I drive at rational speeds, so there is no control issues either way.

I think the fact that those on the leading edge of the spoiler are dancing around and not folding under the wing is proof that the wing doesn't have any airflow.

On a stock fiero GT, there is 160# of up force on the front end at 90mph. (measured)

Here is the passenger side:



It seems to be about 0 until 35mph. Ignore the outlier and double for total on the front.

I was working on an underbody pan and diffuser to correct this, but had other things to work on.

[This message has been edited by ryan.hess (edited 05-30-2008).]

quote Originally posted by Electrathon: Keep in mind that just because there is air coming forwards below the spoiler that does not necessaryly mean that there is not downforce created from the spoiler.

That's true. You can see from the streamers on the back edge of the spoiler that there is a lot of air flowing over the spoiler. This would suggest that there's some downforce being generated.

However, we must remember Newton's third law of motion: for every action, there is an equal and opposite reaction. If the air under the spoiler is being accelerated forward, then likewise that air is accelerating the car backward. In other words, that airflow pattern generates drag.

How about someone tapes a wing to a scale to the decklid and puts this discussion to rest right now?

quote Originally posted by Blacktree: In other words, that airflow pattern generates drag.

Downforce is always going to create drag.

quote Originally posted by Electrathon: Downforce is always going to create drag.

That's true. However, not all drag is generated by downforce. In this case, I think the drag is being generated by turbulence.

I figure there can't be a great deal of force in any direction, consider what's holding it to the deck lid. 4 6MM screws into plastic.

I think we need to rent a wind tunnel for a few hours. lol If only it wasn't so expensive

quote Originally posted by Silentassassin185: I think we need to rent a wind tunnel for a few hours. lol If only it wasn't so expensive

GM has a lot of money and they did test it. They have the specs on the force it creates. Had something to do with keeping the Indy cars stable at speed. I am sure someone can tell the story better than me.

[This message has been edited by Electrathon (edited 05-30-2008).]

quote Originally posted by Electrathon: That would not work at all. The downforce is not the preasure down on the spoiler, it is the preasure down on the rear of the total car. If you alter the flow over the top of the car slightly you can easily alter the "weight" of the car on the road. Adding 200 pounds of weight to the top of the decklid does not necessaryly add 200 lbs of downforce.

Okay, but there's only a limited amount of places to "push down" on a car...

So pop the decklid and stick a spring in the trunk to hold it open. Find a way to record the height (passenger?). If the spoiler is doing anything, it will either cause a high pressure behind the rear window, or create a force down upon the wing, forcing the decklid down. Recreate the deck lid movement when stationary with weights.

But the real story is the huge void behind the window will create a LOW pressure center and give you plenty of LIFT at high speed. Even if the wing or spoiler were in a decent airflow zone (which it isn't, as evidenced by the backwards flow underneath it), it would be limited to around 20# of downforce. This is using 1.5 as a lift coefficient (very liberal).

If you want to do it right, put a potentiometer on the suspension and measure suspension travel at speed like I did. Measure down force directly.

[This message has been edited by ryan.hess (edited 05-30-2008).]

Very nice. However, take into consideration that there is an air current similar to that in a pick up bed. This would cause an eddie, and it could "pull" those streamers, its possible the air is not going in a circle around the wing and "pushing" them. I'm sure the wind does what its supposed to, but maybe not as effeciently as a more aerodynamic car...

Edit: I'm sure the winG does what its supposed to,

[This message has been edited by Jeckel (edited 05-30-2008).]

quote Originally posted by ryan.hess: Okay, but there's only a limited amount of places to "push down" on a car... So pop the decklid and stick a spring in the trunk to hold it open. Find a way to record the height (passenger?). If the spoiler is doing anything, it will either cause a high pressure behind the rear window, or create a force down upon the wing, forcing the decklid down. Recreate the deck lid movement when stationary with weights. But the real story is the huge void behind the window will create a LOW pressure center and give you plenty of LIFT at high speed. Even if the wing or spoiler were in a decent airflow zone (which it isn't, as evidenced by the backwards flow underneath it), it would be limited to around 20# of downforce. This is using 1.5 as a lift coefficient (very liberal). If you want to do it right, put a potentiometer on the suspension and measure suspension travel at speed like I did. Measure down force directly.

I definatly agree that the suspension would be the place to actually measure it.

The low preasure are created behind the rear window does not necessaryly create "lift". The airstream being slightly altered does alter the "preasure" down over the total top of the car. Not just over the trunk. The readings would have to include the alteration on the trunk lid and the foof area.

quote Originally posted by Jeckel: Very nice. However, take into consideration that there is an air current similar to that in a pick up bed. This would cause an eddie, and it could "pull" those streamers, its possible the air is not going in a circle around the wing and "pushing" them. I'm sure the wind does what its supposed to, but maybe not as effeciently as a more aerodynamic car...

I think this statement ls likely dead on. A common misconseption about PU trucks is that they are more aerodynamic with the tailgate down, they are not. The tailgate being up creates a preasure "pillow" in the bed, making the PU more aerodynamic. I think this is likely the case as to why the aditional downforce is created on the Fiero. The air coming forward under the wing could easily be adding to the proper aerodynmics of the car.

Since the lower ones pull forward that would also indicate a possible negetive pressure area in front of the spoiler which Might also help to pull air through the hood vents.

I sort of had an interesting test without meaning to.

It was going to rain and I usually don't drive my 85GT if it is going to rain much. So I had a piece of cardboard that i put over the battery vent and put a U shaped wire from a coat hanger thru the cardboard into the holes in the screen so it would not blow off. (made sure it would not touch the + of the battery)
I forgot to take it off and started home. Everytime I started driving over 20 MPH the carboard would start to float up to the top of the wire. (about 4 inches) Everytime I stopped it would slowly lower as I slowed down. Watch it a couple time before I stopped and removed it.
There was no wind this day so it showed it could only be the air flow out the vents.

[This message has been edited by Dodgerunner (edited 05-30-2008).]

The fluttering simply means the air flow is not clean and that there is a pressure differential between the bottom air flow and the top.

My congrats on a good test. Now you need to do the same for the vents. You'll be amazed how inefficient they are.

Here is a pic of my modified deck lid after wetting it down and driving down a dusty road. It shows the air movement well.



Again, thanks for a good test.

Arn

I've been working a dif spoiler for one of my fastabkcs and have t admit It's really just for looks. There's way too much dirty air from that roofline (fastback is nothing is a notchie with side skirts) to make most spoilers work, unless you raise it up lot and then it will look like just another ricer..

Louvers would probably fix the problem

quote Originally posted by Electrathon: GM has a lot of money and they did test it. They have the specs on the force it creates. Had something to do with keeping the Indy cars stable at speed. I am sure someone can tell the story better than me.

Yes but with said tunnel one would be able to test any number of things. I would like to see how air flows on a few different cars on the forum. Like how does a choptop affect things, or how about the decklid vents with fans in them, raised wings or fiero turbo scoops and the list goes on and on. Would love to know how some of these things affect fiero aerodynamics.

[This message has been edited by Silentassassin185 (edited 05-30-2008).]

Anybody have a good 3D model of the Notchie? Simulators can tell you alot about the airflow and show where the pressure zones are without the expense of a wind tunnel.

Kendall

try this test again .only put the streamers in the middle of the wing on the bottom and the top .i think with the streamers on the leading edge they are exposed to the air where it is being ' split 'and are being pulled in both directions .i dont know if the wing does anything other than block the view in the mirror , but experiments like this could help us find out .thanks for doing this .

quote Originally posted by Silentassassin185: Yes but with said tunnel one would be able to test any number of things. I would like to see how air flows on a few different cars on the forum. Like how does a choptop affect things, or how about the decklid vents with fans in them, raised wings or fiero turbo scoops and the list goes on and on. Would love to know how some of these things affect fiero aerodynamics.

I agree some of this would be fun to see. My guesses:

how does a choptop affect things: The roof is not up high enough to create a proper lift and drop of the airstreem. Sort of like a teardrop shape is more aerodynamic than a flat top and bottom wing would be.

decklid vents with fans in them: There woudl be some upward airflow, but the rushing force of 60 mph air and the massive volume of air would dwarf the airfluw upwards through the vents.

raised wings: definatly more downforce (that is generally not needed or desired) and a real increase in turbulance and drag

fiero turbo scoops: The ugly scoops that stick over the roof? Too ugly and dorky looking to use no matter if they work or not.

Going back to a method used in aerospace engineering some 50+ years ago:

Instead of paper streamers, use brightly colored tufts of yarn approx. 2 inches long. Tape them to the surface at approx. 2-3 inch intervals and in rows 2.5 inches apart. Stagger the tufts in each row. This will give a little better idea of whats happening to airflow close to the surfaces and allows you to place a lot more of them on the test subject to better see any patterns in the airflow.

http://www.airflowsciences....ortsdirectindex.html

[This message has been edited by randye (edited 05-31-2008).]

This is a fun and interesting conversation. I am surprised that more people are not getting involved.

I wonder if your spoiler was moved in on the trunk lid would you loose that reverse are flow?

quote Originally posted by Blacktree: I decided to take some video footage to see exactly what's going on back there.

Congratulations on actually doing the experiment. One good experiment is worth a thousand pages of Fiero "experts" theorizing.

Randye's suggestion of using yarn tufts is a good one. This is still a standard aircraft practice for low speed wind tunnel or field testing. An even better, cheaper, but much messier technique is to place large "dots" of a viscous material (usually in a contrasting color) all over the surface of the vehicle. The dots will flow out and leave a lasting record of the airflow. At the most basic level, used motor oil can be used for this purpose, but cleanup is usually a real mess. However, this technique is cheap, simple, and very effective.

quote Originally posted by Jeckel: I'm sure the wing does what its supposed to,

Have you considered that maybe GM didn't intend it to do anything (on the production cars) but look cool? Car manufacturers often add aerodynamic "features" that actually do little or nothing but add marketing value to the car.

The same thing is also sometimes seen in automobile racing. Put something new and bizarre-looking on a car, then test it. Even if it doesn't provide any performance gain, you may leave it in place anyway ... because it doesn't hurt anything and it will have the competition scratching their heads, and thus it may provide a short-term psychological advantage. Back in the '60s and early '70s, Jim Hall and the Chaparral team were masters at this. One example was when they replaced the normal brake light switch on a car with a mercury switch, so the brake lights lit up anytime the car was decelerating.

My opinion (and without data it is only that) is that the real reason for the Fiero wing was to reduce C_d slightly using something that looked "cooler" than a simple rear spoiler. It is also possible that the rear wing didn't obstruct rear vision as much as a properly-sized plain spoiler would have. GM certainly wouldn't have spent the money on a rear wing unless they thought it would help their bottom line through increased Fiero sales, whatever the reason.

[This message has been edited by Marvin McInnis (edited 05-31-2008).]

quote Originally posted by randye: Going back to a method used in aerospace engineering some 50+ years ago: Instead of paper streamers, use brightly colored tufts of yarn approx. 2 inches long. Tape them to the surface at approx. 2-3 inch intervals and in rows 2.5 inches apart. Stagger the tufts in each row. This will give a little better idea of whats happening to airflow close to the surfaces and allows you to place a lot more of them on the test subject to better see any patterns in the airflow. http://www.airflowsciences....ortsdirectindex.html

I would prefer to use yarn, but I'm worried about visibility. The streamers in that video are about 6 inches long, and about 1/4" wide. Pieces of yarn would be much smaller, and might not be clearly visible to the camera... especially after being compressed into the crappy YouTube video format.

I can add some streamers to the middle area of the spoiler, though.

quote Originally posted by Blacktree: I would prefer to use yarn, but I'm worried about visibility. The streamers in that video are about 6 inches long, and about 1/4" wide. Pieces of yarn would be much smaller, and might not be clearly visible to the camera... especially after being compressed into the crappy YouTube video format. I can add some streamers to the middle area of the spoiler, though.

You'll see the yarn if you get thick stuff. BTW: while youre at it' put rows of yarn on the rear decklid, be interesting to see turbulence the that drop off roof creates.

quote Originally posted by Marvin McInnis: Have you considered that maybe GM didn't intend it to do anything (on the production cars) but look cool?

If thats what GM intended to do, then they IMO succeeded...

quote Originally posted by Blacktree: I would prefer to use yarn, but I'm worried about visibility. The streamers in that video are about 6 inches long, and about 1/4" wide. Pieces of yarn would be much smaller, and might not be clearly visible to the camera... especially after being compressed into the crappy YouTube video format. I can add some streamers to the middle area of the spoiler, though.

Just use a color with high contrast to the car color and it should be very visible.
When you cover large areas you can frequently see flow patterns that a single row will never show.
"Test materials" shouldn't cost more than $10 at Michaels crafts...a skein of fat, colored, yarn and roll of Scotch tape...

You might also try "tufting" the taillights and rear bumper and having a 2nd car video from behind or at an angle off to the side from behind.

We have a couple of overused saying in my lab:

"Best by Test"

and

"Evidence not Argument"

By covering the wing, rear decklid, and heck, maybe even the hood of the car, you should be able to gather some nice visual data.

[This message has been edited by randye (edited 05-31-2008).]

I use snow! I know from experience that the air flow forcefully up through the grill, On my coupe the trunk will stay snowy at 75mph+ with a tapered rear and two hollows over the dry grills. I frequently start out with 8-12"+ of fresh snow. With smaller depths the snow circles forward and down scrubbing the rear window free of flakes, not ice though...

quote Originally posted by Electrathon: fiero turbo scoops: The ugly scoops that stick over the roof? Too ugly and dorky looking to use no matter if they work or not.

No the fiero store fast back quarter panels

[This message has been edited by Silentassassin185 (edited 06-01-2008).]

I know for a fact the spoiler doesnt even add #20 of downforce. Wanna know how I know? My rear deck ajar switch got out of adjustment one time. While it was out of adjustment the light would stay on even when the deck was closed. Driving down the highway at 80+ the light would stay on unless I hit a bump. When I went to adjust it I went ahead and added weights to see how much it took to turn the light off - only 20#s.

The spoiler is for looks, just like 90% of spoilers on production cars.

------------------
Chris

quote Originally posted by FieroReinke: I know for a fact the spoiler doesnt even add #20 of downforce. Wanna know how I know? My rear deck ajar switch got out of adjustment one time. While it was out of adjustment the light would stay on even when the deck was closed. Driving down the highway at 80+ the light would stay on unless I hit a bump. When I went to adjust it I went ahead and added weights to see how much it took to turn the light off - only 20#s. The spoiler is for looks, just like 90% of spoilers on production cars.

I don't think anyone that understands aerodynamics believes that it is supposed to add downforce to the trunk lid. Spoilers and wings are supposed to add downforce to the car. They do that by altering the shape of the airstreem. I do not know how much and at what speed it does alter it, but the trunk lid is not where it changes anything. Eevn adding 20 lbs to the trunklid likely does not add 20 lbs of downforce at speed.

Compare them to this: Nascar has little pop-up spoilers on their cars that come up when their cars go out of control. Without them the car goes airborn. The litle pop-up alters the airstreem and the very heavy car slams down into the track. They do this by altering the airstreem, not by adding weight to the car.

I have a second video up on YouTube. This time, I blocked off the center section underneath the spoiler. It made a noticeable difference in spoiler airflow.

Now I can see why the IMSA racers went with whale-tail spoilers for their race cars.

Scroll up to my original post to see the test results. I decided to consolidate all the test results into my original post, so people wouldn't need to scroll through the entire thread just to see it all.

[This message has been edited by Blacktree (edited 06-04-2008).]

If anyone has an ACCURATE solid model file of the Fiero with wing, I can try to run some CFD on it.
I can't promise that I can get particle density high enough, considering the scale of the full size car, but I can see what we can do.

Just keep in mind that an accurate model will need to include the decklid vents and engine compartment. The vents play an important role in the Fiero's aerodynamics. They help to fill the void behind the rear window, which in turn sucks air out from under the car, which creates downforce under the floorpan.

Going back to the experiment, I think I need to put together a roof mount for the camera, so I can get an overall view of the decklid. I have a hunch as to how the eddy currents are moving, but need solid proof. Once I get the air currents figured out, I can begin to address them.

I'm already getting quizzical looks from people who see the streamers on my spoiler. I can just imagine the looks I'll get with a tripod sitting on my roof.