Realize that it is after 5:00 pm on a sunday afternoon down here in Panacea. Eat your ever lovin' hearts out Yankees.
Sun shining, no chop on the bay. Captain Morgan just arrived.
Please understand I'm on my way to a double national where i will be parked next to Carnut169. So, in anticipation of being blinded by the shine on all his equipment....
I spent the whole afternoon using rubbing compound on the whole race trailer, then following up with a coat of wax. It is something that should be done every 7 years i guess.
I understand nothin' about boundry layers, or such.
so my stupid question is:
Does all this newly developed slickness improve fuel economy?
I'm thinking maybe a tablespoon every 100 miles. Captain Morgan is saying it will deliver gallons of improvement (at least that is what he has been telling Mrs Frog).
It really doesn't matter, I had to wax it anyway because of the team motto:
It is not how fast you go,
it's how good you look...
and... you look marvelous!
The real question is whether or not the weight of the wax is less than the weight of the bugs that won't stick to the trailer.
I say yes; I've waxed both the truck and the trailer prior to the 1600 mile trip down to that double national.
(then again, at least until I've done the first 100 miles or so I'm just trying to keep the road salt up here from sticking to the trailer!)
__________________
Rob Zatz
FVs in the Past: D13, D13, BRD, Vector GB4
Cars in the Middle: SRF #496, Fiesta GTL, Vector FC, Van Diemen RF96 FC, MR2 ITB, SRF#750
Now: Another Vector GB4 FV and another ITB MR2
We actually tested a super slick low drag paint in the wind tunnel at Williams F1 (on some rear wing elements) and it made no difference what so ever much to the consternation of the young aerodynamicist tasked with testing it!
Wax the race car? Sacrilege. We are running with the FVs at Road Atlanta. Every time we lap them we pick up a bit more 'shine'.
Janice did walk out at sunset and admire the trailer... then she said, "when are you going to do the Motorhome?" My arms are so beat, i'm struggling to hold up the evening cocktail. May have to get a straw.
This reminds me of a question that a female caller made to to the Car Talk radio show several years ago. Her engineer boyfriend insisted that she wash and wax her car before thay took any long trips. He was convinced that the fuel mileage would increase, & hence their trip would be more economical. Their response was yes, it would improve the mileage, but they challenged him to try an measure the savings. I agree with their take on it.
The majority of drag on road vehicles is pressure drag, not viscous or skin friction drag. So no, there's not going to be any real gain in fuel economy.
Tried airtabs and couldn't really tell a difference. The only thing I noticed is that the back of the trailer is cleaner but there was no improvement in stability or mileage. Does anyone really see an improvement with these?
I drove north on I-35 with a nice direct tailwind on Friday. I got over 25% better gas mileage and the trailer was noticeably more stable.
Is there any type of wind deflector I can put on the Tahoe to keep the wind off the front of the trailer?
I was considering the airtabs on my motorhome to help with wander in high winds. It's a bit light on the front axl because it's a short diesel pusher with the genset in the back as well, so cross-winds tend to make it weathervane. A lot of people report Airtabs help, but then again, they're the same people that just shelled out about $700...
I figured if they helped the shipping industry would festoon every trailer with them. Have y'all noticed that in less than a year, the majority of trucks now sport two panels between the back wheels of the tractor and the fronts on the trailer? No real structure, just a shell, so I figure it must be doing good things for them to clean up the airflow underneath. Maybe Tovo will chime in here.
After a careful alignment and a steering stabilizer the size of a locomotive shock absorber it does better, but come summer it will have a 3x8 tube bumper stuffed with rebar and two more batteries up front.
I put them on the back of a 28 ft vee nose car trailer, and got around 1 mpg improvement plus a decent reduction in sway (which was why I did it). 1 MPG doesn't sound like much but when its 8.5 to 9.5 (or even a little better) it's over 10% improvement.
The majority of drag on road vehicles is pressure drag, not viscous or skin friction drag. So no, there's not going to be any real gain in fuel economy.
Unless it is a smooth shape such as a land speed record car where pressure and viscous drag are 50/50. F1 cars, about 10% viscous drag, 90% pressure drag.
Realize that it is after 5:00 pm on a sunday afternoon down here in Panacea. Eat your ever lovin' hearts out Yankees.
Sun shining, no chop on the bay. Captain Morgan just arrived.
Please understand I'm on my way to a double national where i will be parked next to Carnut169. So, in anticipation of being blinded by the shine on all his equipment....
I spent the whole afternoon using rubbing compound on the whole race trailer, then following up with a coat of wax. It is something that should be done every 7 years i guess.
I understand nothin' about boundry layers, or such.
so my stupid question is:
Does all this newly developed slickness improve fuel economy?
I'm thinking maybe a tablespoon every 100 miles. Captain Morgan is saying it will deliver gallons of improvement (at least that is what he has been telling Mrs Frog).
It really doesn't matter, I had to wax it anyway because of the team motto:
It is not how fast you go,
it's how good you look...
and... you look marvelous!
IIRC, and I'm not an aerodynamicist, unless the speed is great enough so that the boundary layer is thinner than the surface roughness height over a significant area, you will see no change. OTOH, designed surface features, such as the dimples on a golf ball or the air tabs, can delay flow separation and thus reduce drag by reducing the thickness of the boundary layer due to inducing controlled turbulence.
I've wondered how much these air-tabs would help, and did some research, but never found anything that gave a scientific analysis of any back to back experiments with these. I would imagine that their effectiveness is highly dependent on the vehicle shape, and what the air flow was doing (how it was coming off a tow vehicle in front, etc.) before it got to the tabs.
Dave is right that the dimples on golf balls reduce drag by causing the boundary layer to be turbulent, delaying separation and thereby reducing pressure drag. However, once an attached boundary layer is turbulent, relatively small roughness heights can start to increase skin friction drag. The classic view is that once the roughness height is greater than the thickness of the very thin viscous sublayer, skin friction begins to increase. So long as the roughness is buried in the sublayer, it has little effect on skin friction drag. To know the sublayer thickness, you have to know the wall shear stress, tau-wall. The sublayer thickness corresponds to a height from the wall, y, that satisfies the following equation.
y+ = 5 = yu*/nu
where u* = sqrt(tau-wall/rho) and nu = kinematic viscosity
By the way, to win the respect of anyone serious in fluid mechanics, you can call the sublayer the "viscous sublayer." Long ago, it was misleadingly called the "laminar sublayer." The current use of that terminology drives anyone who specializes in turbulence up the wall!
There are a number of ways to reduce turbulent skin friction drag a little (~10%). S[FONT=Arial]peedo swim suits used in the China Olympics received a lot of publicity for their use of drag-reducing “riblets” – very fine triangular grooves aligned with the flow direction. One theory holds that sharks are fast fish because sharkskin has similar grooves. There is some debate as to whether or not sharks really inspired the first work on riblets. NASA Langley was central to much of this work in the 1980’s, long before Speedo's application, and riblets were used on Dennis Connor’s “Stars and Stripes” America’s Cup winner in Australia in 1987. NASA indicates they developed the idea before looking at sharks. The optimal riblets were streamwise triangular grooves with a height and span of about y+ = 15. Speedo’s P.R. on the subject in the last few years before the suits were banned seemed to have a lot of hype![/FONT]
[FONT=Arial]http://www.nasa.gov/centers/langley/news/factsheets/Riblets_prt.htm[/FONT]
[FONT=Arial]Riblets have been tested successfully on aircraft as well. The grooves have to be very small. [/FONT]
I've heard that a good wax job on a typical single engine aircraft with a cruise speed of around 100 mph will add about 5-10 mph. Of course that would depend on how dirty or rough it was before hand.
__________________
Scott Woodruff
83 RT5 Ralt/Scooteria Suzuki Formula S
65 FFR Cobra Roadster 4.6 DOHC
Scott:
When Lockheed & McDonnell Douglass did tests of laminar airflow "gloves" on full size transport aircraft with NASA in the 80's, they used windshield washer fluid to remove the bugs collected during take-off and low altitude flight. A smashed bug acts as a trip producing about a 15 degree wedge of turbulence in laminar flow.
- Frank C
Air taveling over a waxed dry trailer will not show any MPG differance but when it rains the non waxed trailer will weight a lot more and have more drag.A spoiler at the back (top)of a trailer will also help about 10% to 15 % MPG with a 35deg. angle.I have tested this on 3 differant trailers and it works just like on your car!!
Gas prices got me started in racing !! A Car and Driver artical (in the early 70s) on how to make a PINTO more aerodynamic (and get more MPGs) got me interested in aerodynamics witch lead to racing.
They made and air dam,closed off some of the radiator opening and added a rear spoiler. They test ran each addition and how much mpg per change and then did all together.It was a very interesting artical.
Okay so for us aero noobs (okay probbly just me), spoiler extended past rear of trailer or just on top of it? (ending at rear edge of trailer) Im guessing this was a custom fab job or did/does someone actully make one?
I just did a Google search for "highway trailer aerodynamics," and in the 15 or so independent-testing articles I read, the only trailer additions that might apply to us that were shown to work were:
1. trailer side fairings under the box between the front end and the trailer tires (you see these on 18-wheelers now), and
2. "boat-tails," which are ~15-degree tapered inward 1-2' long rear trailer extensions on top, sides, and bottom of "box," somewhat reducing the effective rear area of the trailer. Unfortunately, these make access to the rear of the trailer more complex.
No other independently tested devices that I saw in these articles made any improvements in MPG.
To be honest, Lee's trailer "spoiler" was not tested, but, intuitively, I do not understand how this will improve MPG, since it would accelerate the air upwards, expending energy and exacerbating the low-pressure already there behind the trailer.
Lee, please tell me why I am wrong.
__________________
Dave Weitzenhof
Last edited by DaveW; February 25th, 2012 at 10:01 AM.
I would think vanes on the top and sides bending air into the area behind the door to fill the low pressure area.
As a matter of fact, I do think I saw that somewhere, but forgot to mention it. If done correctly, it could work similar to the "boat-tail."
However, a disadvantage might be the turning vanes sticking out the sides increasing frontal area (drag) and being vulnerable to getting caught on things.
The turning vanes on the front sides of a boxy tractor work by keeping the air coming off a box-like front attached to the sides instead of splaying out laterally. Keeping the flow attached decreases turbulence and drag.
__________________
Dave Weitzenhof
Last edited by DaveW; February 25th, 2012 at 11:51 AM.
I am sorry it was 20 degrees angle on the spoiler and 4" long.It has been a long time since I have had to really think about it to this extent but I will give it a shot. Adding a small low angled rear spoiler increases the static pressure (a low static pressure will try to suck the car backwards)on the rear.This increase in static pressure on the rear more than offsets the slight frontal area increase of the spoiler.
I might add that a perfect example of this is the currant ProStock Drag cars that have almost flat rear spoilers to reduce drag (also reduces lift)and add some down force at the same time.I hope this explains things a little clearer.
A review of the article I talked about: http://ecomodder.com/blog/11-on-mods...ves-mpg-by-25/
Anyone can peruse through the myriad of papers on the SAE site and find a LOT of trailer aero information. If you aren't a member, you can get a member friend to download it for you for free, or you can pay the few $$ as a non-member.
Tapered fronts on tags, skirts along the sides, smoothing panel on the undersides, and air defectors to help fill in the low pressure area behind the rear all work when done properly.
Anyone can peruse through the myriad of papers on the SAE site and find a LOT of trailer aero information. If you aren't a member, you can get a member friend to download it for you for free, or you can pay the few $$ as a non-member.
Tapered fronts on tags, skirts along the sides, smoothing panel on the undersides, and air defectors to help fill in the low pressure area behind the rear all work when done properly.
I am an SAE member, and the papers are NOT free. Downloads are $14 each and up for members, and there is no apparent mechanism to view a paper w/o downloading or otherwise buying it.
__________________
Dave Weitzenhof
Last edited by DaveW; February 26th, 2012 at 11:03 AM.
I am sorry it was 20 degrees angle on the spoiler and 4" long.It has been a long time since I have had to really think about it to this extent but I will give it a shot. Adding a small low angled rear spoiler increases the static pressure (a low static pressure will try to suck the car backwards)on the rear.This increase in static pressure on the rear more than offsets the slight frontal area increase of the spoiler.
I might add that a perfect example of this is the currant ProStock Drag cars that have almost flat rear spoilers to reduce drag (also reduces lift)and add some down force at the same time.I hope this explains things a little clearer.
A review of the article I talked about: http://ecomodder.com/blog/11-on-mods...ves-mpg-by-25/
The Pinto rear spoiler (linked above) reduced drag by partially filling in the turbulent area behind the too-steeply-sloped rear window. That spoiler was very low, well below the roof height.
I did some more searching, and given everything I saw, I am still of the opinion that almost anything that adds downforce by accelerating air upwards or sideways will add at least a small amount of drag, especially if it also increases frontal area.
However, a flat, or almost flat, rear extension (a smaller version of the "boat-tail" I referenced previously) that makes the rear air-departure surfaces relatively sharp, so that the air breaks cleanly with the rear end of the trailer, should reduce drag. That was the principle behind the "Kamm-back" automotive body designs popular 30+ years ago.
So, I am surmising that your rear spoiler helped in that regard, and the small amount of air acceleration upward caused little drag increase, resulting in a net drag reduction. If I were to add something in this vein to my trailer, it would be flat straight 4" extensions on the top and sides.
Comments?
__________________
Dave Weitzenhof
Last edited by DaveW; February 26th, 2012 at 11:06 AM.
Yes on the spoiler sides would help. I towed with a van and basically riveted it (spoiler)to the rear rain gutter on the van too. When air drops off the back of your normal car moving it speeds up and creates lift and drag to fill the gap at the rear because it has had to speed up going up and over the car or trailer. The air under the car or trailer has not and thus the pressure difference between the front to back and top to bottom.The spoiler keeps the air moving in a slightly upward angle(little drag) but at the same time speeds up the air from under and sides said trailer to fill the gap reducing drag and lift at the same time.You are trying to make the air going under the car or trailer travel as far and fast as the air going up and over to equalize the pressures as much as possible.The very little drag added by the spoiler(frontal area 1"x width) is more than offset by the drag reduction it induces.I would go with a 6" spoiler I think now.About 1 1/2" x width frontal area.
Lee A. Jordan
I need a Killians NOW!!
Last edited by LAJ; February 26th, 2012 at 12:06 PM.
Yes on the spoiler sides would help. I towed with a van and basically riveted it (spoiler)to the rear rain gutter on the van too. ...
Did you do a back-to-back with the van spoiler or the trailer spoiler ONLY vs the van and trailer w/o spoilers? If you did not, and only did them together, I would guess that most of the drag reduction you saw was due to the spoiler on the van, and that the trailer spoiler had much less effect.
If the trailer rear spoiler was totally horizontal or angled down slightly, similar to the one on the car in the last post, I would expect it to be more effective in drag reduction than one angled upward.
General Formula Car Discussion
Stupid Aero Question
It is something that should be done every 7 years i guess. 
My arms are so beat, i'm struggling to hold up the evening cocktail. May have to get a straw.
