Some Towing Thoughts - Page 6 - Fiberglass RV


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Old 07-10-2014, 10:17 AM   #71
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Name: Ray
Trailer: 2017 Scamp 16 Deluxe
Missouri
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I too really liked MC1's posts!
I often refer to our Scamp13 as a "miniature white Airstream".

Less aerodynamic shapes are far more affected as speed increases than more aerodynamic shapes. (For a while I didn't realize the importance of the shape of a trailer - including both the front and back sides of a trailer as well as the rounded corners).

Barnaby Wainfan writes a regular "Wind Tunnel" column in issues of Kitplanes magazine. In the June 2013 issue of KitPlanes, Barnaby's column was entitled "The Zen of Drag Reduction".
(Barnaby Wainfan is a principal aerodynamics engineer for Northrop Grumman's Advanced Design organization.)

Barnaby suggests not mounting anything on the outside of an airplane (in our case, our trailers) that doesn't really need to be there ... especially in the slipstream. He also pointed out the drag caused by cylindrical shapes (propane bottles, stinky-slinky tubes, gray water dump tubes, etc.) on drag and suggests putting more aerodynamic fairings around some of those things.

Also, the flat "plates" on parts of our trailer frames and the flatter front and back areas of some of our trailers can be a significant drag source. (Some trailers on 18-wheelers are starting to use something called a TrailerTail, by ATDynamics - atdynamics.com/trailertail_benefits.htm , to reduce the drag from the flat back sides of the trailers.)

I suppose this is probably more information than anybody really wanted to know and, as always - YMMV )

I guess the moral of the story might be ... either keep the speed down, clean up the drag, or use more horsepower and burn more fuel. The choice is ours.

Safe and happy travels to all,

Ray
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drag_reduction.jpg   Shape_n_drag.jpg  

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Old 07-10-2014, 10:35 AM   #72
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Name: Norm and Ginny
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Ray, I also like MC1's posts. He often looks beyond the often repeated, rote answer.

As to Airsteams I find them interesting though I take an opposite view. To me they look like rough surfaced, aluminum fiberglass trailers. Just kidding.... they are nice trailers, just too expensive for me.

I like people who provide a technical view and I love information on just about any topic. For the reasons you described we do not have an roof top air conditioner or a max vent. It would be interesting to know the Cd of various trailers and how much they could be improved. Improvements have certainly been made in automobiles.
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Old 07-10-2014, 01:04 PM   #73
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Yeah, we don't want trailer towing to be a drag!
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Old 07-10-2014, 02:36 PM   #74
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Mike, You do have a sense of humor but I seriously wonder what the coefficient of drag of trailers are and how much it can be improved. I'll have to search and see if there's any information.

I believe the Prius is about 0.26 and the Odyssey we hope to buy is around 0.39. Discounting frontal area, that's about a 50% difference.
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Old 07-11-2014, 12:38 PM   #75
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Drag reduction is something to consider when purchasing or doing modifications but I'm not sure I would eliminate something useful such as fantastic fan because of drag. I would think about it if mounting solar on roof. Also worth considering for storage on tongue or rear bumper.

I would for example favor Norms storage containers under floor before a box on the rear bumper of the trailer because I would think the skirt of the trailer would make under floor storage have less drag.

Or want to provide a rounded cover if I was going to haul a generator on the outside of the camper. Give some weight to thin stuck on solar panels over thick square edged panels on brackets if mounted to roof.
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Old 07-11-2014, 01:00 PM   #76
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Roger,

I have both a rear bumper box and underneath storage. The rear bumper box is in the frontal area shadow of the trailer. I don't think it has much effect on the aerodynamics, but I'm no expert just going on how little the car effected the motorhome.

My major rear bumper box consideration is keeping the weight down so I don'rt effect the trailer.

Fantastic fans, though we don't have one are not too bad while the big max air vent covers are another matter as are roof top air conditioners.
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Old 07-11-2014, 01:08 PM   #77
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Name: Carl
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Roger,

I have both a rear bumper box and underneath storage. The rear bumper box is in the frontal area shadow of the trailer. I don't think it has much effect on the aerodynamics, but I'm no expert just going on how little the car effected the motorhome.

My major rear bumper box consideration is keeping the weight down so I don'rt effect the trailer.

Fantastic fans, though we don't have one are not too bad while the big max air vent covers are another matter as are roof top air conditioners.
Norm,
What do you mean about the big Max air vent covers and roof top air conditioners or another matter?
Do you think these reduce your gas mileage my very much?
Just wondering?
Carl
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Old 07-11-2014, 01:27 PM   #78
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The vent covers stick up higher so they have to create some drag, but they are also low and narrow (relative to the entire trailer's size), so their effect is minor IMO. Probably on the order of 1/10 of a mpg or something like that. The difference between, say, 48 square feet and 49 square feet of frontal area is not huge, about 2%.

In comparison, my KZ Escape had (not "has", because I sold it this morning) about 12%-13% more frontal area than a 16' Scamp, plus the KZ's shape would cause more airflow disturbance than the Scamp's. I believe I would get 15 mpg with a Scamp (the wider Burro 17' was yielding 14 mpg), but I was only getting 11 or so with the KZ.
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Old 07-11-2014, 01:54 PM   #79
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Roger,

I have both a rear bumper box and underneath storage. The rear bumper box is in the frontal area shadow of the trailer. I don't think it has much effect on the aerodynamics, but I'm no expert just going on how little the car effected the motorhome.

My major rear bumper box consideration is keeping the weight down so I don'rt effect the trailer.

Fantastic fans, though we don't have one are not too bad while the big max air vent covers are another matter as are roof top air conditioners.
I'm not sure how much any of these modifications would impact drag or fuel economy (the reason we care). I was just suggesting that it might be a factor to consider and weigh in deciding how to meet ones camping needs.

You full timing folks I'm guessing have more use and need of rear bumper storage than us weekend warrior types making short trips for a few days camping. Folks in the south doing those same short trips would be more willing to trade off wind drag to have larger roof top AC than myself living in a cooler climate.

Wind drag is just one more factor to consider, along with as you mentioned weight distribution.
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Old 07-11-2014, 02:14 PM   #80
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Roger,

I agree that there's a difference in the views of weekend campers and 'fulltime' campers. Weekend campers only stop once a weekend where as we travel we may stops every few days, and sometimes every day.

There are always choices to be made.
Also there's a big difference between gassing up for the weekend or gassing up for 300 days of travel. Even with the 20+ mpg that we get, 20,000 miles of towing represents about $4,000, or $400 a month, one of our biggest monthly expenses.

If we got 11 mpg as Mike described that would quickly jump to near $700-800 a month, approaching motor home range for fuel. Additionally, Mike, the addition of adornments is not simply a frontal area situation. Additions also generally increase the Cd, coefficient of drag, of the vehicle and air resistance for a constant speed increases by coefficient of drag and the frontal area.

I really wish there was some data on Cd for trailers. It would be interesting.
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Old 07-12-2014, 05:40 PM   #81
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I believe in every choice, there is a trade-off. When we ordered our Scamp last Jan, we didn't opt for A/C or an awning. One of the reasons was the extra drag, and the other was the requirement for taller storage requirements. That all went out the window when our daughter announced they were moving to FL. We plan to go there frequently, and don't want to wait for the cooler months. In fact the A/C will open up many more opportunities to visit friends and family down south in warmer weather than would be comfortable without air.

We'll spend the extra for gas to gain the all-weather flexibility when traveling. As Donna would say, "YMMV". If aerodynamics was that important to us, we'd have ordered a Camp-Inn Teardrop. A Scamp, even with the the extra lumps is a good compromise in my opinion. Everyone makes their choice based on needs.

One more observation. When I first saw the shape of the Lil Snoozy, I wondered if it wouldn't be more aerodynamic if the shell was turned around 180 degrees. Flat end in front, and pointy end in the rear. When I was in college years ago, we learned that the trailing edge (rear) was more influential in reducing drag. Look at an aircraft. The leading edges are blunt and they taper to a finer trailing edge. The wings and all the control surfaces, as well as the fuselage are made that way in an effort to make them more "slippery". A square trailing edge sets up a vacuum and drag along with turbulence, whereas a sharper edge lets the air close up more cleanly. In the Lil' Snoozy's case, the blunt edge (if reversed) would be in the disturbed or dirty air behind the tow vehicle anyway. A previous post points out the 18 wheelers are now trying to achieve the same results with fairings on the rear.

I've never heard anyone else mention this. I'm sure they had their reasons for the design. Maybe rear entry has something to do with it?

Just my .02 cents. I feel better now. Thx. I've been quiet on this thread up to now, but find it fascinating, like so many of Norms posts.

Tom
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Old 07-12-2014, 06:32 PM   #82
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TomK,

Certainly we all have different priorities. For us AC is not too critical because we travel in the south mostly between October and April. During those months AC is hardly needed. Actually this week's trip to Florida will be Ginny's first summer trip to Florida. I'm interested in seeing how she responds.

If you look at the free form of a water droplet falling thru air it is big in the front and tapers towards the rear and parachutes take the opposite form.
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Old 07-12-2014, 07:14 PM   #83
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Correct Norm. The drop of falling water assumes the shape it needs to efficiently speed through the air. Parachutes are a little different due to the way they are designed. Air on the flat bottom is dense. It rushes around the parachute and has to speed up on top of the parachute to join up with itself. Doing so creates a low pressure area on top, which causes lift, much the same as an airplanes wing.

Bernoulli's Principle.
Bernoulli's principle - Wikipedia, the free encyclopedia

If aerodynamics were the only thing considered when designing the Lil Snoozy, the square end would be up front and taper down to the pointy end in the rear. Obviously that wasn't the only factor when they came up with that shape. That said it is still a very slippery shape.

Not sure if you are familiar with ballistics; but the highest performing bullets have what is called a "boat tail" shape. The trailing end is tapered which lends itself to faster velocities, flatter trajectory, and less prone to be affected by side winds.

Any aeronautical engineers out there? My theories and beliefs are based on education I received 40 years ago.

Tom
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Old 07-12-2014, 07:40 PM   #84
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I know the water drop and the parachute comparison was not ideal other than the generalization of shape. I love to have access to a wind tunnel and do some testing or if younger design what I consider a more appropriate trailer.
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