Aero Mythbusting: Tackling Bad Questions, Bad Info

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 …

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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.

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.

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?

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.

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

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.

In reply to Shaun :

Chaparral 2J.

[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]

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

I always thought the chaparral used solid skirts.

buzzboy said:

I always thought the chaparral used solid skirts.

Plexiglas

And the $2000 Challenge version: the  Cheaperal. 

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.

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.

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.

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.

In reply to TurboTodd :

Any idea what the cfm of your fan was?

Edit, nevermind reading fail.

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.

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.

Great pic, thanks! What happened to the car?

One of the reasons I like Vintage racing. Cars are left as originally. Designed. 

Found this pic of the Chaparral 2J's leading-edge skirts, not quite broom bristles but a set of flexible phalanges with wearable tips:

I don't want to be "that guy" but I've always been curious about this.  How is it that articles a few years old are sometimes bumped to the top and become grm+ only again?  Is it when they are updated, or are they brought back so that newer users who haven't seen them before can be aware of them or something?  I don't have a problem with it at all, just wondering why it happens.  

In reply to GameboyRMH :

the August issue of Racecar Engineering covered the 2J. The front skirt was 3 layers of Lexan in 12 sections, replaced as needed between races.