Podmods, Maintenance, Tips and Tricks: air tabs

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...

http://www.airtab.com/gallery/gallery.html - http://www.airtab.com/gallery/gallery.html and http://www.buyairtab.com/ - http://www.buyairtab.com/

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.

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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.

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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.