Some Towing Thoughts

[TomK]

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

[honda03842]

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.

[TomK]

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

[honda03842]

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.

[TomK]

Never too old to learn are we? I hope you get your chance some day, Norm.

Speaking of getting old; my member status just changed to "Senior Member" and is now suited to the color of my hair.

Tom

[Andrew Gibbens]

I think the most aerodynamic trailer I've ever seen (and I'm going from a lifelong interest in vehicle aerodynamics, not actual drag testing) is this one-off. The front is rounded enough to smooth out some of the turbulent airflow coming off the tow vehicle, all important corners are well-rounded, and the body tapers to the rear as much as practical. And before you ask who moulded it, it's actually plywood over an aluminium frame, faired by boat builders (who know something about fairing). And it's all spoilt by having the fenders and roof-top AC sticking out! I don't think the 'trolley top' has any benefit, except providing headroom without adding much drag.







And this proper aerodynamic testing suggest that the amount of curvature on the corners of a typical egg (or an Airstream) is just too little to have the dramatic effect that, say, double the corner radius would have:

[honda03842]

Andrew,
Reminds me of the Oxygen Trailer which has always looked aerodynamic to me. All these posts and PMs has caused me to learn how little I know about the subject of aerodynamics except to know it's complex and important.

Does that trailer have a name?

[Craig D. Thompson]

About 30 years ago my parents drove coast to coast with a 19’ stick built, quite squared, towed with a Ford Fairlane station wagon powered by a small V8. When they started out the rig would not go faster than 55 mph flat out on a level road with no wind. Somewhere mid country they stopped at a home center and purchased some plywood and made a simple air deflector for the roof of the station wagon. Now they could easily cruise at 65 or better. I do not have any photos but it was a simple flat ramp on the car roof that helped dramatically.

About 25 years ago I drove from SE Pennsylvania to Montana and back with 4 adults and one 18 month old in a Ford Taurus station wagon towing a small pop-up in the summertime. The fuel economy averaged about 32 mpg if I am remembering correctly. This is better than I ever experienced with the car by itself. Similar but not as dramatic an experience with the same pop-up towed with a full-size van.

(Disclaimer, I am a mechanical engineer working the the aeronautical field, not an aerodynamicist so anything I write is subject to review by more knowledgable experts.)

As I understand it...

When determining overall air drag (as previously mentioned it is a very involved set of calculations that analyzes the tow vehicle/trailer combination) there are two primary components to consider:
- Profile drag, a function of the overall shape of both tow vehicle and trailer.
- Parasite drag, caused by all the little things like door handles,
vent hatches, window frames and such.

The rounded corners on many of our molded fiberglass trailers help, especially in the front but as previously mentioned, the back is also important. It takes energy to push the air out of the way in the front. When the air flows to fill in the void behind the trailer it creates drag.

The roof deflector made a slightly but significant improvement in the overall profile of the station wagon / square trailer combination.

With the Taurus / pop-up combination, I believe the pop-up created an overall (profile) aerodynamic shape with slightly less drag by providing a smoother transition for the air to flow into the "hole" behind the trailer even though it was squarish and had a square back end.

I believe a large source of air drag for our tow vehicle / trailer combos is the air closing in behind the tow vehicle than hitting the front of the trailer. When I finish the refurbishment of my UHaul I want to experiment with a roof deflector.

Sorry for the long post. Hope it is of value.

Craig T.

[TomK]

Excellent reply Craig. It always carries more weight when you have anecdotal evidence. I have to give your Dad some kudos too. Pretty clever guy. I wonder how many people would think that through, and take the time to fix it.

Your Dad's first name wasn't Mickey, was it?

Tom

[MCDenny]

Most aero (and most beautiful) trailer I have ever seen: Bolus Road Chief.

[Craig D. Thompson]

Oh, I am sure it would have been great if my dad was Mickey Thompson! As it was, my dad is an aeronautical engineer who among other things, designed rotor blades for several experimental helicopters. Then we built them in our basement. And we went to watch them fly. How many kids get to do that?

I realized that I didn't quite finish my thoughts on drag. A point I meant to make is that parasite drag for our trailers is probably a small portion of the total drag. We could spend a lot of time cleaning up the little things, bumpers, propane bottles, vent hatches and such, and not make much change.

If we could lower the profile drag like the Bolus Road Chief, then the parasite drag would be worth addressing.

Craig T.

[TomK]

Craig, couldn't help but notice the name. Especially since we're discussing this topic. I'm an A&P mech from way back. Most of my background was working on rotary wing aircraft. I never designed them but we did have to fly what we fixed. Nothing lasts forever, but that was one fun job! I did that for 25 years.

Denny, that trailer is beautiful. Very Buck Rogers. Do you have any idea what year it was made?

Tom

[rdickens]

I've considered putting an air deflector on either the top of the tow vehicle or on the tongue of the trailer to kick the air up&over or around the trailer. Most of the cabs of 18-wheelers now seem to have some sort of a deflector to send air over or around the trailers. I just haven't located a good one for sale or yet had the time to create one myself.

I've also wondered if the reason why Airstream trailers and Scamps seem to be a little more efficient than more square cornered trailers/motorhomes is due to the Coanda Effect. (See wikipedia.org/wiki/Coandă_effect)

The Coanda Effect describes the observation that fluids (air or water) tend to follow a curved surface (like the top of an airplane wing or maybe the curved top/corner edges of some trailers).

For an airplane wing, as the air follows the curved upper surface, it creates an area of lower pressure above the bend radiius (i.e. some have said that, perhaps even better than the Bernoulli explanation, Coanda low pressure above an airplane wing might be the (greatest?) contributor to lift). I think that Coanda may also be the principle used in the Dyson "Air Multiplier" fan.
(Dyson "Air Multiplier" fan => YouTube /watch?v=8he8afjQyd8 )

The Coanda Effect may also explain why the radiius on the top of the of some gutter covers bends water around the nose on the top cover and redirects it into the gutter while the leaves just blow off the top. (Gutter Cover of KC => YouTube /watch?v=K0JwbDiap5o )

At the rear of the trailer, I've wondered if the Coanda Effect at the the rounded top and side corners brings air down and/or around from the sides and thereby reduces some of the apparent/effective "flat plate" area at the back of the trailer.

At the front of the trailer, perhaps air hitting at or near the top and the rounded side corner edges pulls some air from the trailer's front "flat plate" area to the top and to the sides of the trailer.

(Just FYI - Many years ago, I was schooled as an engineer .. just not an aeronautical engineer, so any of the previous could be wrong .. ..
As always, YMMV)

Just some random thoughts for your amusement .. .. they might possibly be worth just about what you paid for them .....

Wishing you safe travels and wonderful memories,

Ray

[MCDenny]

Originally Posted by TomK

Denny, that trailer is beautiful. Very Buck Rogers. Do you have any idea what year it was made?



Tom

There is a new company making reproductions of the Bolus Road Chief.

From their website: "The original Bowlus Road Chief was produced in 1934. It was designed by Hawley Bowlus, a pilot, sailplane enthusiast, aircraft designer and builder of the "Spirit of St. Louis” – the airplane made famous by Charles Lindbergh’s first solo flight over the Atlantic in 1927. Being the brainchild of an aircraft designer, the Road Chief was, not surprisingly, based on aircraft principles with a streamlined aluminum body. Unfortunately, it wasn't very successful. Only 80 Road Chiefs were built, but a similar design was adopted by Wally Byam in 1936 for the much more successful Airstream line."

It's really cool but big bucks, I believe over $100,000.


Denny Wolfe
Wanderingourway.wordpress.com

[Andrew Gibbens]

Sadly, the Bowlus has the same 'aerodynamic styling' as the Chrysler Airflow or the (original) Volkswagen Bug/Beetle, which is not at all the same thing as low aerodynamic drag. If a body tapers too fast, the airflow cannot remain attached to it and it breaks away, making the remainder of the body pointless. Even the trailer shown higher up probably has too steep a slope on its rear roof, though it's rather unimportant since the airflow has been destroyed by the roof top air conditioner upstream of it.

The Bowlus however is gob-smackingly beautiful. But, sadly, in vehicle aerodynamics, pretty does not mean low drag - though there is no shortage of customers who think they are the same.

[honda03842]

Andrew,

Small point but the Bolus does not have a roof top air conditioner.

As to it's shape, sometimes 'pretty in pink' can make up for failure to make the bed in the morning.

What is the name of the plywood trailer you posted about earlier?

[honda03842]

Andrew, Small point but the Bolus does not have a roof top air conditioner.

It may not be areodynamic but if you're 'pretty in pink' you don't have to be able to make the bed in the morning.

By the way could you explain the phrase 'double radius' and did your plywood trailer shown earlier in the thread have a name?

[Andrew Gibbens]

Sorry, the roof top air conditioner comment referred to the one-off trailer photos I posted a bit earlier. I don't remember that trailer having a name and I can't find the postings made on the Teardrops Forum (Teardrops n Tiny Travel Trailers • Index page) by the builder. He was definitely a boat-builder so maybe we should call it the 'Boatbuilder's Trailer' for short.

I didn't use the phrase 'double radius' and, though I am fond of using 'double curvature', I don't think I have done so here.

And for Craig Thompson, below is a report on the wind tunnel tests done when roof-top spoilers were all the fashion in Europe, showing a drag reduction of 15% by using a spoiler on a vertical-fronted trailer. Apologies to those who notice that I've posted this on this forum before.

[TomK]

Andrew, this goes back to my original point. Everything is a compromise. The Bowlus may not be perfectly aerodynamic, but it probably comes closer than most RV's on the market today. It APPEARS to be about as streamlined as anything that is actually inhabitable and practical. To taper any slower, there would be a long unusable space that would add more length, but not much more capacity.

What trailer would you point to as a good example of truly aerodynamic shape?

I'm a noob to TT's and campers. Just winging' it in this thread, but having fun throwing theories at a wall, and seeing what sticks. I learn something every day I look at this forum. There are Subject Matter Experts (SME's) willing to help answer almost any question that crops up. That's pretty amazing!

Tom

[Andrew Gibbens]

Originally Posted by TomK

The Bowlus....APPEARS to be about as streamlined as anything that is actually inhabitable and practical. To taper any slower, there would be a long unusable space that would add more length, but not much more capacity.

The problem is that if was made more aerodynamic, many people would think it looked less aerodynamic - all that 'eye training' on supposedly aerodynamic shapes like the VW.

The Bowlus would be better if the back end tapered more slowly and just stopped at the same length in a flat bulkhead - a Kamm tail, as suggested by Herr (Professor?) Kamm back in the 1930s and still in use today on low drag shapes like the Toyota Prius or the GM EV-1 (the lowest drag production vehicle).

Incidentally my personal rule of thumb is that airflow can stay attached to a body that tapers up to about 15 degrees from the horizontal or straight-ahead. More than about 15 degrees or almost any sharp change in taper and the flow won't stay attached and any body behind that will be doing no good.

If you have your own wind tunnel you can do better than this, but then if you have your own wind tunnel, nothing I write would tell you anything new.......