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Old 06-26-2015, 03:31 PM   #1
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AirTabs

Has anyone installed these? And if so do they help with stability as claimed? My Challenger is actually pretty stable when being passed by trucks, but has some issues in cross winds. I was thinking this might be an inexpensive option that I can install myself.

Airtab | Aerodynamic Fuel Savers | Application
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Old 06-26-2015, 04:48 PM   #2
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I studied aeronautical engineering at the school of big books.

While I commend the fine marketing and promotion of AirTabs, and truly promote capitalism, free enterprise, U.S.A. Manufacturing, and the free market economy, they only serve to reduce rear turbulence somewhat.

Air tabs do absolutely nothing to improve wind resistance. They will not improve the fact that you are pushing an approximately 110 sq.ft. wall directly into the wind every moment that you drive a motorhome.

If you can stand to look at them once they are installed, (and are capable of completely ignoring the laughter and stares that they cause to those that notice them and point when they pass or at the fuel stop), then they are great!
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Old 06-26-2015, 05:02 PM   #3
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You could also try the edge fairing:
Edge Fairings Gain SmartWay Elite Status - Trailer Talk - TruckingInfo.com

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Old 06-26-2015, 08:41 PM   #4
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Funny you mentioned Air Tabs...

Ironically, I have been installing a set of Air Tabs on my rig today. I have been thinking of buying them for awhile now, but finally decided to after the second time towing our car in the rain, which made it a filthy mess. Even with a skirt on the bottom, road dirt just gets thrown on the car in the rain.

The tabs are supposed to prevent this from happening. As well, they are supposed to improve gas mileage and handling when a semi passes you. But the main reason I bought them is to hopefully keep the toad cleaner.

Beacher; I think it might be more of a fluid dynamics issue. From that perspective, do you think they help in reducing drag?

As for the ugly factor, I don't think you can do anything to make a motorhome more ugly than it already is.

I did notice that I lost about 1/2 mile per gallon when I installed the vent covers, so I plan on putting a couple of tabs on those to see if I can recover that mileage.


Before installing the Air Tabs:



After:



I ordered 84 of them ($250 with shipping) and did both sides today. It looks like rain so I am going to hold off on the roof tabs until tomorrow.

I put 25 on each side, including 3 just forward of the rear wheel well, and I damaged one trying to paint it, so I have 33 left for the roof.

The problem I had with painting the tab is it apparently had mold release agent still on it as the paint did not stick evenly - but splotched. It will be hard to wash and clean them as the adhesive backing is already attached to the tabs and I am not sure if I would damage them by washing them or not.

But they look OK not painted in my view.

I did have a tough time in routing the tabs around the back side of the storage doors. The tabs are supposed to be 14" from the rear, and I could have put them on the doors themselves, but then the baggage catches would not work, so I could not keep the doors open.

So I ended up mounting the tabs behind the storage doors. Unfortunately, the surface is uneven due to the metal corner trim pieces for the door and rear wall.

To bridge the gap in the uneven area, I used some VHB tape with a 1/8" thick foam backing and used that to fill in the void between the metal strips. Then I used some white silicone to fill in an left over gaps to make the tab underside waterproof.

I will report back in a couple of weeks as to how effective they are.
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Old 06-27-2015, 03:26 AM   #5
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Originally Posted by JamieGeek View Post
NASA did a lot of the early work in this area. Makes for interesting reading.

It's been applied to motorhomes, although I've seen little hard data on just how effective it's been.

Compared to tractor trailers that are squared off at rear to accommodate doors, a motorhome without rear doors can be optimized on a permanent basis. And it's not really that new a concept in that buses from before I was born had rounded rears that reduced drag.
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Old 06-27-2015, 05:16 PM   #6
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...
Beacher; I think it might be more of a fluid dynamics issue. From that perspective, do you think they help in reducing drag?..
Their only actual function is to stabilize rear air turbulence. The only thing that can reduce drag is to minimize the front end and make it more aerodynamic, (like a Vegas/ACE perhaps).

That being said, if you look at the Airtab website they show a picture where the tabs are mounted on a the trailing edges of semi tractor pulling a trailer. The tabs would help in that one situation. And, they should help to direct the turbulence behind your motorhome rearward, past your toad. Instead of dust and dirt accumulating as much on the flat back surfaces of the coach, it should be swept to the sides and top and bottom of your toad.
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Old 06-29-2015, 06:00 PM   #7
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I believe the concept of the Air Tabs is that the shape of the trailing edge as well as the leading edge of an object going through air affects it's aerodynamic characteristics.

I will have an opportunity to test my Air Tabs in a week or so with a 500mi trip (if it stops raining long enough that I can finish the installation). So I will be in a better position to know what effect, if any, they provide.
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Old 06-29-2015, 07:22 PM   #8
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Originally Posted by FW28z View Post
I believe the concept of the Air Tabs is that the shape of the trailing edge as well as the leading edge of an object going through air affects it's aerodynamic characteristics.

I will have an opportunity to test my Air Tabs in a week or so with a 500mi trip (if it stops raining long enough that I can finish the installation). So I will be in a better position to know what effect, if any, they provide.
Do you expect so much difference in fuel economy or dirt on your toad that you can rule out normal variations in measured data? Assuming they work I would think it would be very difficult to measure effect accurately given all other background variables that change from one trip to the next.

And this could work both ways. If you get same MPG result as previous trip it doesn't confirm they didn't work at all.
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Old 06-29-2015, 11:16 PM   #9
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My main purpose is to keep gunk off the toad, but if they improve gas mileage, so much the better.

I have fuel mileage data from last year and so far this year using a scan gauge, and we're traveling some of the same roads, so I should be able to see if there is a gas mileage improvement.

True, any number of infinite variables could affect the reading such as temperature, wind and weather, etc. and even where I buy gas (whether or not they sneak in 5~10% alcohol), and even which of the current boutique of summer blends is used.

But an improvement should be detectible I would think, although it may not be easy to quantify that improvement.

If there is no improvement, as you indicated, that might not be truly indicative as perhaps one of those variables changed (say a gas station is putting in 5% alcohol where they were 100% gas before). But I should be able to tell if there is any improvement at all, (unless of course a gas station I used last year had been "cutting" their gas with 5% alcohol, but is now 100% gas).

It is all subjective to be true, but over time, I should be able to tell if there is an improvement or not.
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Old 06-30-2015, 11:43 AM   #10
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I look forward to reading your report!

As far as function..... I did a research paper in college about vortex generators, which at the time were being marketed in aviation applications. Those look different, but these air tabs perform a similar function.... adding energy to the boundary layer.

I found this little graphic with the help of google


On a wing at high angles of attack (pitched up steeply), the airflow separates from the surface of the wing causing turbulence and loss of lift. These little tabs create a little vortex, sort of a small tornado coming off each tip... so this added energy helps the airflow to conform or stick to the wing surface better.

So I suspect that these airtabs, each of which look to produce two vorticies, would in an RV application help the airflow to flow behind the RV rather than folding into the low pressure area behind the coach..... or between the cab and trailer of a semi, or a Toad or trailer, etc....

So, to Beacher's point.... they can actually change the shape of the coach, but aerodynamically, into a more clean shape. My best guess is that it would be sort of similar to adding a fairing to the rear of the coach, similar to the bike helmet
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Old 07-03-2015, 04:31 AM   #11
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I personally see the two examples above as very different from a typical RV with its truncated rear. So much so that I don't think the same theory is applicable.

The bike helmet reduces the rider's coefficient of drag by tapering gradually so that air flow doesn't separate as early as when using a standard helmet. This is same principle as the tail cone of an airplane, which tapers very gradually. Most people think it's the front of vehicles that offers the most potential to reduce drag, but in most airplanes the back end tapers very gradually. And most modern cars with low Cd also taper gradually to reduce creating a partial vacuum behind the vehicle at speed.

By comparison a typical RV has the side walls forming a 90 degree sharp angle with the rear cap. Because of this I expect separation at the rear edge of RV with or without tabs. Likewise I expect the partial vacuum at the rear to be essentially the same. If the pressure is similar, then the total force the engine has to work against is also very similar.

I hope I'm wrong because it'd be great for improvement to be that easy.
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Old 07-03-2015, 05:29 AM   #12
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The only thing that leads me to the conclusion there is some degree of benefit is that you see a lot of semis on the highway with aerodynamic fixtures on them. Not necessarily Air Tabs, but it is becoming very common to see this stuff:



I find it difficult to believe that trucking companies would go to all of that expense if there was no benefit.

While this is a somewhat different approach than the Air Tabs, I think the drag issue is valid, whether the Air Tab design is the best approach to reducing drag or not.
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Old 07-03-2015, 05:50 AM   #13
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According to this article published by NASA, they did the initial research on vortex generators. The company that makes Air Tabs took that research, available from the NASA Technology Spinoff Program, to develop the Air Tab product.

https://spinoff.nasa.gov/Spinoff2008/t_3.html

So there does seem to be some validity to the concept.

Most of the comments I have seen from those using them is that they are an improvement, but just keep your expectations realistic.
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Old 07-03-2015, 10:27 AM   #14
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I personally see the two examples above as very different from a typical RV with its truncated rear. So much so that I don't think the same theory is applicable.

The bike helmet reduces the rider's coefficient of drag by tapering gradually so that air flow doesn't separate as early as when using a standard helmet. This is same principle as the tail cone of an airplane, which tapers very gradually. Most people think it's the front of vehicles that offers the most potential to reduce drag, but in most airplanes the back end tapers very gradually. And most modern cars with low Cd also taper gradually to reduce creating a partial vacuum behind the vehicle at speed.

By comparison a typical RV has the side walls forming a 90 degree sharp angle with the rear cap. Because of this I expect separation at the rear edge of RV with or without tabs. Likewise I expect the partial vacuum at the rear to be essentially the same. If the pressure is similar, then the total force the engine has to work against is also very similar.

I hope I'm wrong because it'd be great for improvement to be that easy.
What I was trying to get at, was that I think the air tabs simulate the tapered bike helmet by adding energy to the airflow, so that it flows back smoothly similar to the bike helmet, rather than folding back in behind the blunt rear which causes the eddy currents, turbulence, and drag.
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Old 07-03-2015, 10:43 AM   #15
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What I was trying to get at, was that I think the air tabs simulate the tapered bike helmet by adding energy to the airflow, so that it flows back smoothly similar to the bike helmet, rather than folding back in behind the blunt rear which causes the eddy currents, turbulence, and drag.
That is kind of what the NASA article was saying... I think.

From the NASA article;

"At the back of a trailer, box van, or RV, Airtabs radically alter the airflow to reduce drag in two ways: Shifting the airflow pattern from vertical to horizontal to eliminate large eddies, and smoothing the airflow to artificially simulate a tapered rear of the vehicle."
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Old 07-03-2015, 12:26 PM   #16
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What I was trying to get at, was that I think the air tabs simulate the tapered bike helmet by adding energy to the airflow, so that it flows back smoothly similar to the bike helmet, rather than folding back in behind the blunt rear which causes the eddy currents, turbulence, and drag.
The technical papers I've read on the original research were objective and included hard data. By comparison this spinoff reads like marketing to me. It covers a lot of history of previous work but doesn't document, in a scientific manner, the results of testing on air tabs. Just because rounding corners or adding boat tails in previous research reduced drag by 50% or whatever doesn't mean air tabs can achieve the same results. I personally don't see the connection and would like to see specific research with results on air tabs.

A concern I have is the use of words like "could", "may", "seem", etc... included in claims. As a technical type I prefer objective data more than subjective theories.

"At the back of a trailer, box van, or RV, Airtabs radically alter the airflow to reduce drag in two ways: Shifting the airflow pattern from vertical to horizontal to eliminate large eddies, and smoothing the airflow to artificially simulate a tapered rear of the vehicle. In fact, Airtabs have been shown effective on any vehicle with more than a 30-degree slope to the rear; the potential benefits stretch across vehicular applications and could thus benefit a considerable number of vehicles."
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Old 07-03-2015, 12:40 PM   #17
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That paragraph reads like an executive summary. It documents that they have been "shown effective", which would imply that data does exist. I consider "could" in this context to be scientific extrapolation of data.... not smoke and mirrors marketing and not false claims.
How grounded the extrapolation is to reality, well.... that's another question.

As has been discussed already, there are likely too many variables for anyone outside of a wind tunnel test lab to really verify performance....
But I for one am interested in reading FW28Z's observations.
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Old 07-03-2015, 04:21 PM   #18
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I just drove 60 miles today with the RV on a route I have often travelled. This year, I have done this trip 6 times (without AirTabs until today).

The route is a combination of about 30mi of freeway driving (with the cruise control set to 60 mph), and 30 miles of secondary highways, a few hills, and a few towns (cross streets, stop signs, etc), with 25, 35, and 55mph speed limits.

Before the AirTabs, I was averaging 8.5mpg on the scangauge on this route. Today I averaged 9.2mpg.

Now this was the initial test so I don't know if it will hold up over time or not, so we'll see how it averages over time.

With semis passing in the opposite direction at highway speeds, sometimes I felt no push, but a few times I still felt a push, but it was greatly diminished. But this push has never really been bad anyway. All I can say is it seems to have been reduced.

I have not yet towed the car with the tabs on, so I don't know how much cleaner it will be (if at all) or what the gas mileage changes will be.

I guess at this point all I can say is that I am encouraged by the initial results. By nature, I am a bit skeptical, but I believe there is some sound principles behind the phenomenon.

So my initial conclusion is there is indeed something going on here... but how much of an improvement it makes is still to be determined.
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Old 07-03-2015, 04:51 PM   #19
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yes, interesting indeed. I'm skeptical though, that your avg MPG will improve that much. There are way too many variables... including subtle even sub conscience driving habits, such as accelerating just a wee bit easier, even if you think you're not.

I'm betting on some improvement though.....
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Old 07-03-2015, 05:09 PM   #20
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I expect the results to vary over time. And yes there can be subtile differences (which is why I am using cruse control when I can and so on).

I think though, that given enough testing, any idiosyncracies for any one characteristic will average out.

All I can say for sure is the initial results are encouraging.
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