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GlueGuy View Drop Down
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Posts: 2702 		Direct Link To This Post Topic: air tabs
    Posted: 26 Sep 2017 at 7:38pm
Originally posted by Keith-N-Dar

His after mileage isn't great either. 
Good point, but I figured everyone would kinda notice that. Embarrassed
bp
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Posts: 1447 		Direct Link To This Post Posted: 26 Sep 2017 at 6:28pm
Originally posted by GlueGuy

There is now an example of the use of the airtab vortex generators on a Winnebago Winnie Drop 1780 here: http://www.airtab.com/application-rv.htm. The example is about the 3rd from the top. The guy claims about a 20% improvement in fuel economy. Admittedly, his previous MPG was particularly awful.


His after mileage isn't great either. 
Keith-N-Dar
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DavMar View Drop Down
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Posts: 592 		Direct Link To This Post Posted: 26 Sep 2017 at 4:39pm
Air Tabs .... sounds like Voodoo medicine to me! Tongue 
Dave & Marlene J with Zoey the
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GlueGuy View Drop Down
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Posts: 2702 		Direct Link To This Post Posted: 26 Sep 2017 at 9:21am
There is now an example of the use of the airtab vortex generators on a Winnebago Winnie Drop 1780 here: http://www.airtab.com/application-rv.htm. The example is about the 3rd from the top. The guy claims about a 20% improvement in fuel economy. Admittedly, his previous MPG was particularly awful.
bp
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techntrek View Drop Down
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Posts: 9062 		Direct Link To This Post Posted: 18 Sep 2013 at 7:43am
Rex asked me to post the following email received from the Airtab company last October:
----------------------------------------------------------------------------------------------------------------------
 
Rex here is how we answered a question from a RPod customer back in April 2012.
 
Here are two sites you might want to look at...
 
 
 
Jack Latimer
Airtab, LLC
 
From April 2012 with a customer in Texas
I have ordered a “Forest River R-POD” tear-drop shaped travel trailer(see attachment). Approximate dry weight is 2,775 lbs, pod width is 6’5’’ (8.0’ to outside of tires), 8’8’’ high (9’7’’ to top of AC). My questions and comments are as follows:
1.       There is considerable distance from the end of the roof of my tow vehicle to the beginning of the trailer front (distance of the covered PU bed). Where should I apply air tabs on the tow vehicle?
2.    For the trailer, due to its “tear drop shape”, where should air tabs be mounted? On the front, the sides, etc. , as there is not the “squared off flat back to the trailer:? Should air tabs be placed on the ac unit(top? Sides? Both?)
5.       Any other aero suggestions for the trailer?
Any help and advice will be greatly appreciated.
JC
=================================
JC, 
I remembered we answered a question similar to yours about 18 months ago and here is what our aero guy said then. 
Sorry I didn't think of this before.
The Rpod trailer has a particularly evil fastback design (in side view) that much like the early Porsche 911 fastback body style, suffers from terribly high aero drag.  That's because the location of flow separation on the curved roof top surface wanders forward & backward over the tail.  As the rooftop's separation line wanders aft, the adverse pressure gradient builds to unsustainable levels and the flow  separates, then the flow attachment line snaps forward and gets re-established.  This unstable flow oscillation builds and collapses much like the current in an old ignition coil.  Every time the flow separation location snaps forward it creates a huge unwanted burble of large scale turbulence that migrates back into the near wake.  This unsteady flow has a periodicy that varies with vehicle speed.
 Porsche stopped it by inventing the Whale Tail which also gave some downforce.  Today's modern VW "Bug" has a smaller lip above the rear window.  The purpose it is to fix the separation point at one location  (in side view) so it doesn't migrate fore-and-aft over the rear window (which creates very high drag).  A side benefit is that the vehicle then becomes more stable.
 The little teardrop-shaped Rpod trailer badly needs a similar "lip" installed transversely across the entire rooftop - to fix the point of separation at one point.  I would suggest  it be located about one-foot aft of the rear of the rooftop air conditioning housing.
 You could place a row of  Airtabs across that station on the rooftop, but the vorticity would simply help the flow remain attached further aft.  Unfortunately, eventually the flow would separate farther downstream, and then the unsteady flow mechanism would once again occur.  Only this time it will be worse, because the unsteady aero forces are occurring farther aft on the trailer - which might make the tail wag the dog so to speak.
So don't let him use Airtabs transversely across the roof.  Instead, have him mount them only on the sides - not in a straight vertical line but following the side contour.  In other words, the trailing edge apex of each Airtab should be located just a couple of inches forward of the trailing edge of the side of the trailer.  The line of Airtabs will be curved to mimic the side contour as viewed from the side.  Much like the red painted "swoop" graphic except positioned far enough aft that the line of Airtabs will fit aft of the side window.
========================
The customer, JC, writes back in May 2012 saying this....
Mr. Latimer- again a very helpful reply, which I greatly appreciate. Forest River must have been talking to your aero guy, because they began installing a “lip” or spoiler, that goes transversely across the roof in 2012 (my unit has it) and all the units going forward. It looks exactly like what your aero guy is recommending. I’ll sketch out, my understanding of your air-tab scheme for the side of the trailer, scan and e-mail it, to make sure that I am understanding the advice. Then I can get a count together and place the order. Again thanks for your follow-up, which is both rare and appreciated.
 
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Posts: 9062 		Direct Link To This Post Posted: 05 Sep 2013 at 9:24pm
Wouldn't hurt to play around with some of the shapes suggested, none would be permanently mounted.  Don't like the results or the look, remove two straps and pull it out of the rear receiver.  For the cost of some cardboard it would be worth some test rigs.  I might just try one out on our next 3-week trip next July.  Approve 
Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual
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Sleepless View Drop Down
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Posts: 556 		Direct Link To This Post Posted: 05 Sep 2013 at 4:05pm
Originally posted by Keith-N-Dar

Originally posted by Sleepless

To me, much of the R-Pod's design problem is the backward slope of the lower part of the front.  Bring the front down perpendicular to the road and I bet mpg will improve.  Again, this is just me thinking out loud.Bob



The interesting thing about this comment is the "problem" is one of the things that made Pods attractive to many people. It is the thing that sets them apart from the rest of the little trailers. Making the changes you suggest would eliminate the attraction.


I would never think of modifying the design of my pod.  I was merely commenting, as were several others, not suggesting.  Again, I was just thinking out loud.

Bob
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Keith-N-Dar View Drop Down
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Posts: 1447 		Direct Link To This Post Posted: 05 Sep 2013 at 3:23pm
Originally posted by Sleepless

To me, much of the R-Pod's design problem is the backward slope of the lower part of the front.  Bring the front down perpendicular to the road and I bet mpg will improve.  Again, this is just me thinking out loud.Bob



The interesting thing about this comment is the "problem" is one of the things that made Pods attractive to many people. It is the thing that sets them apart from the rest of the little trailers. Making the changes you suggest would eliminate the attraction.
Keith-N-Dar
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Posts: 767 		Direct Link To This Post Posted: 05 Sep 2013 at 1:26pm
I did road tests and computer simulations on airflow over my vehicle and r-pod several years ago.  The gap between the vehicle and the front of the r-pod really didn't create significant drag.  The problem area was created directly behind the r-pod: fast moving air travelled along the roof slammed down to the ground immediately behind the trailer and created a large swirling vortex of low pressure about 5' high behind the trailer.

Next time you're pulling the trailer in the rain/drizzle, watch around the rear of the trailer in your mirrors - you can see the maelstrom that's created back there. 

In reality, the r-pod's shape creates far from a sleek slipstream.  I think that Doug's approach for a squared-off rear (or maybe something more rounded-off and airstream-ish) would likely perform much better.

Question for rex: Have you tried the air tabs across the top of your trailer?  I suspect that installing them along the rear edge of the air conditioner (and extending that line across the top of the trailer) would yield the best results.
Craig :: 2009 RP171 towed by a 2017 F150
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dsmiths View Drop Down
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Posts: 866 		Direct Link To This Post Posted: 03 Sep 2013 at 10:36pm
The tabs look similar to vortex generators on some aircraft, these generate turbulence and break up the flow of air for specific purposes. I don't see how disturbing the air flow at the back of the pod could help induced drag. I agree with many that the problem is the shape of the front of the pod and the distance between the T.V. and the front of the pod, this area is subject to a vacuum and turbulence and increases induced drag. I don't care, its still fun and cool looking.
Dane and Donna Smith
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