Trying to create more aerodynamic design

[Billbagley1]

Hi all. I am new to this forum but have spent many nights lurking and reading old posts. I am building a teardrop trailer and am in the design / materials purchase mode. I will note several of the choices I have made so far and materials I have acquired, and then pose some design questions.

* I have read in the Ecomodder Forum that the traditional teardrop design is not really as aerodynamic as it could be, so I am trying to design in aerodynamic features that are not associated with a traditional teardrop as seen in the TNTTT forum.

* I will pull this trailer with a 2004 Jetta TDI Sportwagen.

* I have acquired sheets of 5052 aluminum sheet with a clear anodized gloss finish that are 4 x 12 feet and 5 x 12 feet that I will use for the skin. I hope to attach this to the wood and foam frame using 3M double side ultra tape to minimize rivets.

* I have acquired a Harbor Freight trailer with 1720# capacity that I will stretch to be a 4x10 foot frame. I plan to skin the underside of the HF trailer with a sheet of continuous aluminum hiding the axle within the skin and place the axle on top the springs to lower the profile and reduce area under the trailer.

* I plan a Kamm back design using the Ecomodder AST II template for the top surface, running out to the 50% to 60% bulkhead. (In the attached picture count 4 to 5 sections from the point in the back. Picture a Toyota Prius with out the hood. Split the photo in half at the center line.

* The front of the trailer will lift up, hinged at the rear to increase headroom and interior space, while minimizing height while traveling. (Picture a car hood or hatchback.)

Now for the questions...

1. ? For the front of the trailer profile, I am thinking of several options.

a. try to recreate the shape of the rear of the Jetta, making the front of the trailer concave to try to match the convex profile of the rear door of the Jetta and matching the shape of the Jetta as seen from the rear. (although I wouldn't be able to run the trailer THAT close to the rear of the Jetta, this might allow the air stream of the Jetta to continue as if the Jetta and trailer were one aerodynamic unit)

b. make the front traditional with a rounded front like the leading edge of a wing and the sides of the trailer flat (very easy to construct with flat panels.)

c. use the superellipse or square half body plans documented by freebeard in the Ecomodder forum... (see attached... super ellipse in the center is more aerodynamic but the the square would be much easier to construct.)

3D representation of Aerodynamic Template by Freebeard Ecomodder Forum Teardrop Superellipse Square.png (236.92 KiB) Viewed 1637 times

d. try to reproduce schlorwagen shape and raise the top half of the trailer with pop up trailer jacks.

Schlr-LoftingLines-fs8-fs8.png (234.24 KiB) Viewed 1637 times

2. It there that great of a benefit to round / slope the trailer sides from bottom to top or are straight sides acceptable?

3. How great a benefit are rounded edges at the corners.

I thank you in advance for some discussion and apologize for my newbie questions :-)

Once I am further in the design process and make some decisions I will post some drawings and pictures...

[PaulC]

Unless you are going to push it, rather than tow it, aerodynamics are almost a waste. This subject has been covered ad infinitum on the forum and our resident Engineer, Andrew to some, has given up. Your trailer will be traveling in dirty air all the time it is behind your vehicle. The best way to get better fuel efficiency is to have as small a gap as possible between both. If you can design a sleeve arrangement that fits onto the TV and the Trailer you will achieve the best efficiency.
With that said, let the debate begin
Cheers
Paul

[grantstew8]

It's great you are starting a build and the design stage is always interesting.

Paul's right, if the td is huge , wider and taller than your jetta then worry about aerodynamics otherwise it's not an issue. Myth busters did a fuel economy test sitting behind a truck.

The other issue is build-ability, curves and bullet shapes can be difficult to create. (Certainly with my skills and tools)

Take a look at the polls on this forum and read the one where they ask about " what would you change if you build again". Biggest hitter width is 4' to 5'.

Good luck!

[Kody]

Aerodynamic design is an enormous box of worms. The only way to achieve anything is to make a prototype as big as possible and place it in a wind tunnel. The velocity of the air plays a huge part in the design of the part/s being tested. Both the tow vehicle and the TD need to faithfully copied and tested together as they would be on the road. At 100 kph (60 mph) there is little to be gained by aerodynamic design but the greatest benefit comes from eliminating sharp corners at the end of the trailer/van and don't have a flat square end. A square box shape seen so often on the highway is the worst shape to be towing. The efficient airflow around the tow car is the first vital part of the equation and small things can have a huge effect. A very notable "small thing" that has a devastating effect is the tiny winglet on the F16 fighter plane. Ok, this is traveling at a velocity just a little greater than our tow vehicles (well I sure hope so) but it serves to show the effect. Without this small winglet to straighten the airflow, the turbulence from the fuselage will totally destroy the tailplanes ( or rear stabilizer) and that's end of the aircraft. The velocity of our tow vehicles is a bit less but the turbulence is still there and its affect and shape of the air flow changes as the velocity changes. Without a wind tunnel, you have no idea of the smoothness or otherwise of the air stream. All this to consider and there is still the problem of parasite drag. This takes the form of anything that disrupts the smoothness of the airflow. The biggest parasite drag comes from those side mirrors fitted to the doors. Then there's door handles, door shapes, tow bars, bumper bars, window wipers, dogs heads and the list goes on.
To build a shape that is super efficient for towing you will need to build a TD the shape of a dolphin. There is an enormous cost to building an efficient shape and this is the ability to put anything inside it. The best shape is the typical teardrop shape, large rounded front curving down to the fully curved and tucked under galley that gives these trailers their name, "Teardrop". Keep away from square ended forms with high panels and you will have the best compromise.

Kody

[Bogo]

I'm taking the attitude that most times I'm pulling the TTT, there is going to be some degree of side wind. With that in mind, I'm planning some streamlining at the front and the rear. Also I'll be making the top, bottom, and sides of my TTT as smooth as reasonable. Even though my current tow vehicle is not that aerodynamic, I expect I'll be getting a more aerodynamic SUV in the future. Look for the Foamie Aerodynamics thread. http://www.tnttt.com/viewtopic.php?f=55&t=49036

[Roo Dog]

Wot Paul sed !

RD

[rmclarke]

Sorry for the rabbit trail but that one picture looks like Fuller's Dymaxion car circa 1933 http://en.wikipedia.org/wiki/Dymaxion_car
There was such nice design going on in the 30's (my 2 cents)...whether or not it was aerodynamic or not is another matter....but it looks cool!

Richard

[mezmo]

Hi wrbagley,

Welcome to the forum.

As someone stated, to paraphrase, this is an often debated/discussed area.

This Foamie section thread has the most, gathered in one place, discussion on Aero:

http://tnttt.com/viewtopic.php?f=55&t=49036

The center, more oblong shape, would probably be the more "ideal"shape.

From what I've read on the subject, the three most practical practices a DIYer
can do are:
1) Have the trailer frontal cross-sectional area/shape match, or be slightly less
than, the cross-sectional area/shape of either the end of the tow vehicle
or its maximum cross-sectional area/shape. [Think blunt front with all
the edges rounded/curved/radius-ed.]
2) All edges/joins need to be [must be] radiused/rounded over. This is the
simplest item with the most practical benefit.
3) Taper the end/rear of the trailer - i.e., end in a Kammback at the point
where the cross-sectional rear area would equal ~ 50 percent of the maximum
trailer body cross-sectional area/shape.

I've always thought that the more square cross-section aircraft fuselage would be a design
that could work for a Travel Trailer. Here are some examples:
[Most pics will enlarge if you click on them.]

Its nose shape could be used for a combined enclosed hitch-tongue box
at the front: 1930s era DeHavilland Dragon Rapide
http://en.wikipedia.org/wiki/De_Havilland_Dragon_Rapide
http://www.zweiterweltkrieg.org/phpBB2/ ... =33&t=7929

Unusual looking: The Boeing Pelican Cargo Plane Concept
http://www.aviationexplorer.com/boeing_ ... _facts.htm

Ugly but it flys: C-23 Sherpa
http://www.fas.org/man/dod-101/sys/ac/c-23.htm

They all have flat or minimally curved sides, but all edges/joins
[or cross-sectional corners] are curved-over/smoothed/radius-ed.

Cheers,
Norm/mezmo

[Bogo]

Your trailer will be traveling in dirty air all the time it is behind your vehicle.

Modern aerodynamic cars like the Prius have very smooth well controlled tail wakes. Yes there is some turbulence and a vacuum zone, but it surprisingly small, and shrinks fast behind the vehicle. Any TD trailer towed by a Prius, or similar, will be parting allot of wind. A vehicle like a Jeep will have a large tail wake zone and it will be very turbulent in it's wake. For a TD towed by one of them in still air or a perfectly aligned head or tail wind, there is no need. Unfortunately rarely is a vehicle operating in those conditions. Often there is some degree of cross wind. That means the outer edge of the front of the upwind side, and the whole up wind side, of the trailer will be in clean or minimally to moderately disturbed air. In my opinion it is well worth it to do some streamlining, even if pulled by a brick. When you finally trade that brick in for a more streamlined vehicle, it will pay off even more.

The best way to get better fuel efficiency is to have as small a gap as possible between both. If you can design a sleeve arrangement that fits onto the TV and the Trailer you will achieve the best efficiency.

Vortex generators is another option, but there is still a limit to the length of gap they can help with. http://www.airtab.com/main.html NASA even likes and uses them on it's trucks. In the transition zone from TV to trailer they work by creating a small amount of drag from the vortexes which creates an air curtail wall which prevents a large amount of drag from air sneaking it's way between the TV and trailer.

My streamlining efforts include making all surfaces as smooth as possible, including the bottom. Even my axle is located in the basement area rather than under the trailer. Where transitions happen, like from the front streamlining cover to the side wall, the transition is smooth with no abrupt changes.

The front streamlined cover has as much of it's curve radii over 24" as reasonable. NASA found out in truck streamlining tests that 24" radius curves that transition from a flat front to the sides were very nearly as efficient as making the whole front as close to spherical as possible, and solved the added length issue. On a TTT or TD there is little penalty, other than construction complexity, for making the front as close to a sphere as possible because of the trailer tongue. Much of the additional length can be over the tongue. I'm using it as a cover over the propane tanks, spare tire, and air conditioner.

The tail cover has a little more complex requirements. It can't have more than a 15 degree slope for the airflow to follow as any greater is more likely to cause separation at highway speeds. Uncontrolled separation is bad for aerodynamics. The other feature I'm using is a set of short walls around the perimeter of the back that extend the side walls a few inches further back than the back wall. They have been shown to smooth the shape of the tail wake zone so it isn't whipping all over the place. As such they add a few more percent in efficiency. I'm using the tail cover area as inside accessible storage.

The faces of the doors and windows will be coplanar to the side walls, and the gaps around the edges will be kept as small as reasonable. For the door, I'm planning on having a bulb seal fill in the gap. It is also there to help keep the door from getting iced shut in the winter.

TTT I'm working on designing.

[SaabRally99]

I think a benefit could be had from an elongated cone on the rear end. The TT has the most control over how the slipstream "finishes," and most cars are terrible at this. Brings to mind a certain Ecomodder member.

[citylights]

Aerodynamic shape is a trade off between difficult construction, additional materials, and functional use. A standard tear shape is a nice compromise between those. Your sketches go more extreme shape and have great "cool factor" and likely better aerodynamic, but are not nearly as practical. When you layout, bed, galley, cabinets, storage, electrical, you will have a difficult time fitting it all.

[working on it]

Aerodynamic shape is a trade off between difficult construction, additional materials, and functional use. A standard tear shape is a nice compromise between those. Your sketches go more extreme shape and have great "cool factor" and likely better aerodynamic, but are not nearly as practical. When you layout, bed, galley, cabinets, storage, electrical, you will have a difficult time fitting it all.

The best laid plans often go astray (paraphrased quotation)- In my first post, on 10/4/11, I outlined a few of my trailer concepts, and stated that I intended to build a Kamm-backed trailer (for aerodynamics), with a profile matching my HHR (i.e. sloped front, intended to ride in the wind shadow of the TV). Best intentions. Couldn't implement the Kamm rear profile due to the weight of the trailer going beyond the goal (couldn't build a Kammed rear fairing of plywood, too much extra weight for too little aero advantage). Did build the sloped front lower than the rear wing of the wife's Cobalt, presumably the smallest, shortest wind shadow of our "fleet". Couldn't do any other mods due to weight constraints (and lack of woodworking skills). I did put a crest, just aft of the front slope, to try to disrupt the flow over the top of the trailer, and made my hatch to body rain shield sit 2" above the plane of the roof, also to disrupt the airflow (hopefully enough to lessen the vacuum affecting the flat, vertical, rear hatch). Other than that, I could've, but didn't, rounded off the square edged intersections of the flat sections of ply that formed my structure. I could not risk exposing ply end surface to weather; paint and sealants are not perfect. As citylights said, trade-offs happen. But, I think that I my not have a "brick on wheels", aerodynamically, because in the following quotation, I may have one good thing going for me: As angib said in an earlier discussion [quote]
Re: Foamie aerodynamics.

Post by angib » Mon Dec 17, 2012 8:59 am

It might be worth giving the drag coefficients that NASA measured for that test truck as it may show some effects that folk don't expect:

1) All edges square, front and rear - 0.89
2) Horizontal edges rounded/vertical edges square, front and rear - 0.54
3) All edges rounded, front and rear - 0.41
4) All front edges rounded, all rear edges square - 0.365
5) Rounded front, partial boat-tail rear (as my photo above) - 0.242
6) Rounded front, full boat-tail rear to point (as GPW's photo above) - 0.238

Remember these are data for an independent vehicle, not a trailer, so don't read too much into them. But one thing to look at is the comparison between 3) and 4) - the one with the square back end has less drag than the rounded back end [quote] Maybe the square edged rear hatch is ok, after all? I'm still considering a sheet metal fairing in front of the tongue box / spare tire, but don't think it would be much help.

[Roo Dog]

If the quest for better aerodynamics is to achieve optimum fuel consumption the most cost effective and practical solution is usually found in a sensitive and thoughtful right foot.

Take it easy.

RD

[Billbagley1]

Created the following attachment today...

1. I decided to do out and measure actual values on my Jetta today rather than relying on computer pictures and estimated curves. I measured the peak of the roof line to be back 48 inches from the back of the Jetta. Maximum height at that point was 56.75 inches. Drop to the back was 1 inch. Height at back was 55.75 inches.

2. I used these values along with the xy coordinates of the AST II template to scale a spreadsheet graph and then fit an image of the Sportwagen under the curve.

3. I generated a scale factor to create the rest of the xy coordinates.

4. The top of the trailer frame is 17". The frame is 3.5" channel, so the underside is 13.5". The A frame tongue is 42" from the front of the frame to the hitch.

5. Using these constraints, I generated a trailer body at 8', 9' and 10' lengths of the frame, fitting close to the back of the vehicle

6. The top will be a simple lift of back first... block, then front... block. I'm thinking a simple pull out block (shelf) once the trailer body is lifted.

7. The reverse concave front is perhaps counter intuitive, but is intended to follow the curve of the back hatch of the Jetta to create a single aerodynamic unit instead of allowing flow separation...

Thoughts so far?

BB

[rowerwet]

for all those swoopy curves you would need to go with fiberglass or foam, metal or wood would be a killer to form lke that.
As for actual towing, I found my 5" wide TD with the boats on the roof got 28 MPG at 50 MPH, 25 mpg if I drove at 60 MPH, 22 MPG at 70 MPH, and 21 MPG at 75 MPH. Speed is the killer.
I don't know what the TD without the boats can do, I wouldn't go camping without them.
I don't have a number for just the boats either.
On the highway the car gets 32-33 MPG at 75 MPH,
sticking to the backroads, max 45 MPH, I get an average MPG of 35-38 MPG.
with just the one boat on the roof (flat) the car computer is showing 35 MPG, as I've only driven back roads to work all week.
Aerodynamics matters, but the TV matters more, and Speed matters most.