Aero Mythbusting: Tackling Bad Questions, Bad Info

Image Courtesy Richard Prince/GM Racing

Story by Steve Stafford • Photography as Credited

The area of aerodynamics poses problems for amateurs and professionals alike. You can’t see the air you are trying to work with, and that air doesn’t react linearly. As a full-time aero engineer–you’ve likely seen my work at places like Daytona, Sebring and Talladega–I have five items that get under my skin when discussing operation aerodynamics.

Bad Question: How Much Downforce Does It Make?

This question is probably the first one that I get asked about an aero device. The real answer is “it depends.” How the device is incorporated into the whole car and the speeds involved are needed to accurately answer the question.

A better question, which should make the questioner think through the entire process, is this: How does this device work with the rest of the car? Adding devices at the back of a body can and often will change what happens at the front. It is a system to be considered as a whole. As a side note, mostly we don’t use forces for comparing aero, we use coefficients that are almost speed-neutral.

Bad Advice: Do It Like the Pros

While imitation is a high compliment, most professional series employ rules and limitations that prevent the engineers from getting too much performance. It might be size, location, adjustability or an outright ban on a device, but the limit is there. Use the pro series configurations as a starting point, not a final destination.

Bad Info: Spoilers and Vortex Generators Reduce Drag

Every time I hear this, I gnash my teeth. While there are a very few instances where this works, it is much more likely that spoilers and vortex generators increase drag. Forcing the air around stuff–like a spoiler–takes energy and therefore creates drag. This is a guarantee locally, but it may reduce drag effects elsewhere, providing a net drag reduction on the car. Remember, the entire car operates as a system.

Bad Plan: Bigger and More Is Better

Even at the professional levels, this crops up time after time. Usually it occurs with things like the angles of splitters and dive planes as well as the angle of spoilers and wings.

Once wings and spoilers achieve their peak downforce, more angle reduces the downforce. It does not disappear; it is just reduced, usually with increased drag.

Splitters and dive planes suffer by typically being made of materials that can bend under load. If you make those items too big, they will deform at speed and start causing issues like ground contact, flow separation that reduces downforce, or even ultimate failure and departure from the car. If the car feels unstable at the limit, back off angles and/or stiffen the aero devices to get a more consistent feel that will also make the car easier to drive at the limits.

Bad Logic: Aero Does not Work at Autocross Speeds

The very nature of aerodynamics says that something is happening if there is any airspeed. At autocross speeds, the effects are likely small compared to the end of the Mulsanne Straight, but aero is doing something.

Since the forces are a function of square of speed, aero forces at 60 mph are four times greater than at 30 mph–your typical range of speeds between the cones.

With that idea in mind, a dedicated autocross car will have much bigger aero devices than a track car just to get useful downforce. Also, autocross can often (legally) make some big power, so the drag impact can be ignored for all but the lowest-powered cars. This is a case for bigger is better, since the aero forces also increase directly with size.

Good Aero Logic

Remember, treat the car as a system with all of the aero parts working together. Determine the operating range for the car and components, and test to make sure you have not exceeded the peak operating conditions for the devices. Finally, think beyond professional series cars: They are working to a specific set of rules that may not apply to you.

Steve Stafford is a professional aerodynamicist working for factory efforts in NASCAR, IndyCar and IMSA. He’s also a frequent question-answerer on our message board.

 

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Comments
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sleepyhead the buffalo
sleepyhead the buffalo Mod Squad
2/4/20 9:52 a.m.

I reckon this ended up in 'GRM' thanks to the 'article posting system'.  So, if no one has an objection, I'm going to move it to the Aero sub-forum.

Colin Wood
Colin Wood Associate Editor
2/4/20 10:34 a.m.

No need, this was entirely intentional. We thought this was an interesting bit from our December issue, and we wanted to make sure it got shared with the rest of the class.

With that being said, everyone who is reading this should check out the Aero sub-forum. It has some great information there.

buzzboy
buzzboy HalfDork
2/4/20 11:36 a.m.

So what I'm understanding is that I need a big wing mounted high up in clean air and midship as well as big, balanced, front and rear aero?

wspohn
wspohn Dork
2/5/20 1:11 p.m.

It always made me laugh to see other cars in my production class at the track sticking on angled spoilers that did nothing except to slightly reduce top speed, given the modest power output of the cars.

My approach (subsequently outlawed by the organizers) was to add a vertical air dam that excluded air from going under the car. They made the mistake of not specifying a minimum ground clearance, no doubt figuring that it would be self governing as people went too low and had their air dam ground off under braking.  But I made the last few inches of dam out of flexible conveyor belt material, allowing me a basically zero clearance at rest height and the flexible lip only needed to be replaced about once a season. And it worked - an additional 4-500 rpm at the end of a long straight. Also helped keep the gravel off the track. But it wasn't legal the next season.

buzzboy
buzzboy HalfDork
2/5/20 3:29 p.m.

I've wondered, would using broom head material work? It could just touch the ground but bend away from damage.

Shaun
Shaun Dork
3/15/20 8:53 p.m.

In reply to buzzboy :

IIRC There was a can am car I saw run at Riverside when I was a kid that had bristle broom skirts and an extractor fan.   A white car #44 something to do with the name Jim Hall.  Thats what I remember.  I'll go search the interwebs and see how my brain is holding up.

David S. Wallens
David S. Wallens Editorial Director
3/15/20 9:12 p.m.

In reply to Shaun :

Chaparral 2J.

NOT A TA
NOT A TA SuperDork
3/15/20 9:45 p.m.

[URL=https://s240.photobucket.com/user/NOTATA/media/AERO/gallery-1484852151-dsc-4029_zpspp0hgw7v.jpg.html][/URL]

[URL=https://s240.photobucket.com/user/NOTATA/media/AERO/45192617_10156282462072946_4777454722212691968_n_zpsldck7t5p.jpg.html][/URL]

Cactus
Cactus Reader
3/15/20 11:00 p.m.

Were they broom skirts? I know F1 side skirts were sacrificial elements, but they were solid, and not brooms.

buzzboy
buzzboy Dork
3/15/20 11:20 p.m.

I always thought the chaparral used solid skirts.

slantvaliant
slantvaliant UltraDork
3/16/20 8:14 a.m.
buzzboy said:

I always thought the chaparral used solid skirts.

Plexiglas

David S. Wallens
David S. Wallens Editorial Director
3/16/20 8:34 a.m.

And the $2000 Challenge version: the  Cheaperal

03Panther
03Panther Reader
3/16/20 7:27 p.m.

Great article. Aero is one discipline where most engineers I've met DO know more than me... but that's more an example of how little I know 'bout Aerodynamics!

I've had "degreed" mechanical engineers tell mt their new V nose trailer gets better mileage and is easier to tow... They tend to get mad at me whin I point out that NASA's wind tunnel tests don't agree.

NOT A TA
NOT A TA SuperDork
3/16/20 8:29 p.m.
Cactus said:

Were they broom skirts? I know F1 side skirts were sacrificial elements, but they were solid, and not brooms.

IIRC there were two rows of broom skirts in the front.

TurboTodd
TurboTodd New Reader
12/6/20 1:23 p.m.

In reply to David S. Wallens :

Thanks for remembering us.  Jim Hall used lexan plates on linkages , articulated via cables attached to the suspension. F1 tried brushes, then inward and outward bending aluminum, before settling on a floating system on linkages, with ceramic wear rods to scrape the asphalt at the bottom.  Cliff Papsdorf, Cheaparral chief engineer, welded a free floating rectangular steel frame with 4 locating linkages under the vette, and used sacrificial casters to set the ground clearance.  The flexible (blue and white tarp, spray painted black) skirt was pulled down by vacuum to keep it on the ground, although some dampers would have helped at bumps. Drove pretty normal, except felt like you had switched from street tires to race tires.  Addictive.

NOT A TA
NOT A TA SuperDork
12/7/20 10:28 a.m.
TurboTodd said:

In reply to David S. Wallens :

Thanks for remembering us.  Jim Hall used lexan plates on linkages , articulated via cables attached to the suspension. F1 tried brushes, then inward and outward bending aluminum, before settling on a floating system on linkages, with ceramic wear rods to scrape the asphalt at the bottom.  Cliff Papsdorf, Cheaparral chief engineer, welded a free floating rectangular steel frame with 4 locating linkages under the vette, and used sacrificial casters to set the ground clearance.  The flexible (blue and white tarp, spray painted black) skirt was pulled down by vacuum to keep it on the ground, although some dampers would have helped at bumps. Drove pretty normal, except felt like you had switched from street tires to race tires.  Addictive.

Todd do you have any more pics from that build? And just so you know, the Cheaparral has been mentioned a couple times over on the F-1 technical forums when the subject of sucker cars comes up.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
12/7/20 2:53 p.m.

In reply to TurboTodd :

Any idea what the cfm of your fan was?

Edit, nevermind reading fail.

TurboTodd
TurboTodd New Reader
12/8/20 6:26 p.m.

In reply to Robbie (Forum Supporter) :

I saw 8 in H20 peak on a dwyer gauge plumbed underneath.  That's about 9,500 cfm at rated speed.  We were running about a hundred rpm higher. I calculated 1.4g's at the time, which seems low by today's standards. ~4ft × 4ft plan area.  Actual was probably 4-6in H2O most of the time, 8 in wg in the straights.  Rated hp draw of 25hp. We'd choke the fan and go into the compressor stall part of the curve once the casters wore down. The fan blades had about a 45deg pitch and remember it was a dual stage fan with stator vanes after the rotating vanes. If we hit a bump, we lost pressure, but it wasn't crazy like f1 cars.  The volumetric flow was large enough that you just moved lower on the fan curve rather than losing all downforce. Very easy to drive.

TurboTodd
TurboTodd New Reader
12/8/20 6:48 p.m.

In reply to NOT A TA :

Yeah, I found my old folder and moved it to OneDrive.  Probably have more on the PC.  Any particular shots?  Too bad we didn't have 10hz gps and trackaddict to measure back then. I could have recorded Danny's line and mine to see where I messed up.

Below is the carriage. 4 locating rods between a fixed rectangle and a lower floating one.  Tarp between them. Leaxan to fine tune the clearance.

NOT A TA
NOT A TA SuperDork
12/8/20 7:58 p.m.

Great pic, thanks! What happened to the car?

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