Airtabs

[aggie79]

I think the large truck pulling you over and pushing you out aren't as much about the airflow around and over the trailer as much as they are about the low pressure area behind the trailer (which the vortex generators affect).. At least that is the way I understand it. I am not thinking about efficiency in this case.

Maybe someone can explain it better, but as the truck approaches from behind it is pushing air causing a higher air pressure and the low behind the trailer is attracted, the opposite when they pass by with the low pressure behind them and high in front of me..

Certainly crosswinds stability is affected by the surface area and profile, but I am think specifically of that sucking in and pushing out semi trailer and large buses create when they pass by me in the same direction of travel when I am towing the CTC

Anybody?

Don,

I think you bring up a good discussion point. It makes sense that if you reduce some of the drag/suction at the rear of CT, then the push and pull of large passing vehicles should also diminish.

This website has a brief description of ideas to reduce trailer drag: [urlhttp://www.part20.eu/en/applications/trailer/[/url]. They're not too aesthetic but the vanes applied to the sides and top of the trailer seem to be pretty effective in reducing the negative pressure at the rear of the trailer. I think this is the concept of air tabs.

Another random thought came to mind would be trying out "Gurney flap" or "wickerbill" on the sides or top of the trailer. A cheap way of trying this out would be to use metal roof drip edge from HD or Lowes. They're about $3 for a 10' stick. I'm an open wheel racing fan - primarily F1 - and have seen a sawtooth pattern cut into the wickerbill to increase its effectiveness. Perhaps triangular notches could be cut into the drip edge to generate additional vorteces. To me, this might be a way to test out the concept before spending big bucks on air tabs.

Take care,
Tom

[JaggedEdges]

Specifically to improve crosswind stability, the top side edges of the box should be rounded. If they are, good, if they're square edged, well there's twofer bandaid...

Get some jumbo pool noodles, cut them into quarter rounds, now glue them all along the roof each side...

What this does... makes the side on coefficient of drag a bit lower so crosswinds have less grab on the trailer... (If you've got leftover, put a halfround on the top of the front too while you're at it.)


The twofer...

This doesn't show terribly well on the pics on the link in the last post, possibly because the airflow was not done in ground effect, but to a box floating in mid air. However, driving along on the ground, against the ground, your vehicle is kind of like a lift producing wing, and as such, there's a pressure differential on top of it, this causes an outward and downward flow off the top of the vehicle, which is half what drives those large vortices and helps them form in the wake. Now, much like winglets on Jet, or a "wing fence" , the inner vertical edges of the pool noodles will help to resist this flow sideways and downwards off the roof... This does two things, stops some of the pumping force on those vortexes, also, if you picture the airflow... it's hitting the front of the vehicle straight and level, but washing out and down off the back and sides of the vehicle. This is like an airfoil at high angle of attack, and is causing what is called shape drag or induced drag. Aircraft aerodynamicists like to lock the force of lift at 90 degrees to the ground, but if we take the frame of reference of the airfoil, what's happening is that it's "lifting you backwards". ... Okay so when we resist the flow of air off the sides of our very low aspect ratio wing, preventing the downwash, we're effectively pitching the wing as a whole at a lower angle of incidence and bringing that lift vector back up to 90* so it's not pulling us backwards...... all in all this should show a slight mpg increase also.

That sounds so good you wanna do it on your round cornered trailer right? Well when you've got a square as square can be one, you can't do worse... when you've got a rounded one already, you can screw it up. Don't do pool noodles, because side profile won't blend. Don't do vertical vanes, because you've just added the crosswind problem the square trailers have. You'd have to blend in a ramp profile to curve, about 20 degree angle, then give it that sharp step on the inside.... annnd I don't know a cheap effective way to do that, unless you just wanna carve wedge sections out of styrofoam or something.

[JaggedEdges]

Another random thought came to mind would be trying out "Gurney flap" or "wickerbill" on the sides or top of the trailer. A cheap way of trying this out would be to use metal roof drip edge from HD or Lowes. They're about $3 for a 10' stick. I'm an open wheel racing fan - primarily F1 - and have seen a sawtooth pattern cut into the wickerbill to increase its effectiveness. Perhaps triangular notches could be cut into the drip edge to generate additional vortices. To me, this might be a way to test out the concept before spending big bucks on air tabs.

Ah no, don't do it quite like that, remember it was trying to slam the car into the ground, not save fuel. It's going to kick air up and give you a bigger effective wake.

However, using the same drip strip in a different manner may have benefit. as a trip strip or turbulator. The theory on those goes, that you separate attached air from a surface, creating small scale turbulent flow, that reattaches to surface behind the strip, and sheds more easily from it at the rear. Think spraying off marbles instead of towing bowling balls. But in order for this to work, they have to sit some distance in from the rear, probably a foot or so. That's for a relatively short one. However, another mode of operation, which is kind of tricky to arrange right, needing some experimentation on the spacing to rear edge is to use them to throw up a single vortex "bubble" (Okay it's gonna be more like rolling a snake out of playdoh but just trying to use easy visualisation here.) Such that the air further away goiing round it, comes back in behind the vehicle at an angle, thus creating a virtual boat tail effect, and calming the tits of those nasty big vortices again. So that's the trick, getting the distance just right so it's coming in an an angle still when the end of the vehicle isn't there any more, instead of still being there and reattaching to it. or, in too short a distance, the smaller vortex not forming on it's own and the vacuum behind the strip just making the wake vortex a few inches bigger and badder all round.


However, I would not recommend the metal drip strip be used to create the 15 degree angle pattern I recommended for attaching weather strip, because then you've just created a child slicer and dicer, which are incredibly unpopular for some reason.

[flboy]

All way to complex for a CTC. I am not building a stealth bomber. LOL

Sent from my SM-G935P using Tapatalk

[hankaye]

flboy, Howdy;

Best thing that I can recommend is one of these ( http://www.equalizerhitch.com/index.php ).
Yea, a bit pricey but IMO very much worth every penny, and for the Canadians ... each nickle.

Had one for my old Travel Trailer. used it when I towed from Central Utah to SE Ohio and back.
Never new when a big truck or buss went by on my Left from trailer movement. Even kept it square
behind me when I had a flat on I-70 at highway speed. I'm planning on getting one for mine as I
do feel it wanting to wiggle a bit from cross winds or the usual big truck.

hank

Disclaimer; I have zero vested interest in the company liked above. Just a very satisfied customer.

[flboy]

flboy, Howdy;

Best thing that I can recommend is one of these ( http://www.equalizerhitch.com/index.php ).
Yea, a bit pricey but IMO very much worth every penny, and for the Canadians ... each nickle.

Had one for my old Travel Trailer. used it when I towed from Central Utah to SE Ohio and back.
Never new when a big truck or buss went by on my Left from trailer movement. Even kept it square
behind me when I had a flat on I-70 at highway speed. I'm planning on getting one for mine as I
do feel it wanting to wiggle a bit from cross winds or the usual big truck.

hank

Disclaimer; I have zero vested interest in the company liked above. Just a very satisfied customer.

Thanks Hank. I am running with one just like that. I am adding the anti-sway bar this week before I head out. That will make a difference also...

[DrewsBrews]

I have heard of air tab like devices used on plane wings. This effectively increases it's weight capacity by reducing the wing's tendency to stall. However the air tab actually increases drag so it limits the plane's max air speed. It's a tradeoff

You also see vortex generators on some "performance" cars at the top just before the rear window. This is in an effort to draw more air downwards to the rear wing to increase downforce.

[RandyH]

Anyone got pictures of these homemade air flow smoother outers?

[JaggedEdges]

I have heard of air tab like devices used on plane wings. This effectively increases it's weight capacity by reducing the wing's tendency to stall. However the air tab actually increases drag so it limits the plane's max air speed. It's a tradeoff

Don't forget though, in this situation only the first is ever happening because it's like a wing at high angle of incidence all the time, the only time you'd get the other situation is if you're skidding down the interstate at 30 degrees to the direction of travel at 200mph...

[hankaye]

Howdy All;

Thought I'd post a link to the Air-Tab folks so y'all can read their sales pitch.
I have absolutely zero connection with the "tab" folks just thought I'd provide
ya with some reading material ...
Don't forget, their goal is to sell you their product.

hank