Air Deflector - Fiberglass RV


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Old 05-07-2006, 02:07 AM   #1
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Has any one used a air delfector on the towing vehicle? I pull my Ventura 12' with a 1989 Chyrsler Fifth Avenue and there seems to be a lot wind drag. The Ventura is quite tall comapred to other small Fiberglass RV's, as it has 6' 1" headroom clearance. I was thinking of making my own wind deflector...any thoughts??
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Old 05-07-2006, 06:09 AM   #2
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I will repost this diagram from before the crash. It is from tests done in the VW wind tunnel - a good few years ago, judging by the model of VW Passat used.

The first case has a Cd of 0.53 but at 50mph produces 76lb less hitch weight than when it was stationary - this might be 1/3rd of the typically small European hitch weight.

In case 2, with a roof spoiler, the Cd drops to 0.45 (15% less drag), but still produces a 71lb reduction in hitch weight at 50mph.

Case 3, with a sloping trailer front and no spoiler, has the same 0.53 drag coefficient as the first case, but has no loss of hitch weight at speed.

So VW reckon spoilers work.

Andrew
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Old 05-07-2006, 11:25 AM   #3
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Cd = Coeffecint of Drag. the lower the better.

Hitch weight at speed.= What do you want for towing?
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Old 05-07-2006, 02:48 PM   #4
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Cd = Coeffecint of Drag. the lower the better.
I'm an engineer - so I sometimes forget some people don't know this!

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Hitch weight at speed.= What do you want for towing?
Most US sources say you want a hitch weight of 10-15% of the total trailer weight. What you don't want is for this to reduce the faster you go - this reduces the damping on sway motions, so less hitch weight can allow sway to start. This is perhaps more important in Europe where hitch weights tend to be lower (7% of trailer weight is a good traget, if possible) - a 70lb reduction may be 30+% of the hitch weight. The results illustrate why the sloping front trailer is now the European standard, as it does not suffer a loss of hitch weight with speed.

Andrew
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Old 05-07-2006, 06:48 PM   #5
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Interesting. Maybe that's why my stick-built Scotty tows so nicely - the front is somewhat wedge-shaped. The typical egg must have better overall aerodynamics, though.

I have noticed a difference in how easily it pulls at speed since I changed from a cab-height cap over the bed of my S-10 pickup to a high-rise cap that steps up at the front, thus acting like a wind deflector. That was one of the reasons I switched, and it seems to have made a difference. I believe the wind now strikes the sloping front section and not the flatter part below it. Someone towing with a full-sized truck or SUV probably wouldn't notice the difference.
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Old 05-07-2006, 10:17 PM   #6
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Andrew, if you are towing a trailer with a sedan, wouldn`t the spoiler have to extremely high to get the air off it to clear the trailer?....is it not true, and this would be the same principle, that if you have trees as windbreaks, once the air blowing over the trees gets twice the distance past the trees as the height of the trees, it`s back on ground level again.....I realize that the semi trailers use spoilers but they are mounted on the rear of the sleeper cabs and relatively close to the front of the trailers......wouldn`t mounting a spoiler on the roof of a sedan cause a turbulence between the trailer and car to have a negative effect on the use of a spoiler? ...Benny
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Old 05-08-2006, 06:02 AM   #7
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wouldn`t mounting a spoiler on the roof of a sedan cause a turbulence between the trailer and car to have a negative effect on the use of a spoiler?
I expect it does, but the benefit of the spoiler certainly outweighed any losses, since the drag (and hence fuel used) went down 15%.

Because of the size of the gap, there may be a good argument for using vortex generators such as Airtabs or the home-made ones below to minimise the amount of this turbulence.

Andrew
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