enclosed trailer aero kammback

[skyking]

1) To use the template properly, you start with the end of the trailer at the 0 degree point. Scale the template to match the height of the trailer.
2) you will have a pronounced separation with that 2" step. It will re-attach before the end of your tail, provided you keep the angle shallow.

[drmiller100]

I'm a minority of one.

I think you take your flat trailer, and put flat angles on the back of the trailer and get a much better result.

[aerohead]

Originally Posted by hotrodf1

Sorry for the delay on the pics. They aren't great but do show the trailer and the start of my framework.

Aerohead - I didn't know that 50% was a less than ideal number. Got it from widipedia. Perhaps not a great source, but when I searched for Kamm back that's what came up first, so . . . .

I kind of thought it seemed "too easy" to get there. Was planning on making it deeper, but then at 50% cross section and 15 deg. it was only 44" deep on the hypotenuse.

Anyway,
The piece was going to start out with straight corners since the corners on the trailer are straight anyway. Does this make sense or not? Could have easily used the bender to make them 6" radius curves.

The rear of the tail was going to just use the curves that the bender produces (seen in photo). I guess that's probably not enough curvature to make the most of it.

Thanks for the heads up on the template. that's what I needed yesterday but I didn't know what to look for. I'll look at that before I go any further.

Let's see if these pics will show up from photobucket.

First the trailer



Then the rear section of frame.

Wikipedia as a data source is way out ahead of nothing,but we've been able to go straight to the horses mouth through original publications.
The 50% 'thing' originates with a 'practical' length issue,when considering traffic and driveways,and parking.
Also,Koenig-Fachsenfeld's model (of which Kamm is given credit) would suffer a ground-strike exactly at the 50% point due to the 10-degree departure clearance angle.So he just lobbed off the tail there,and added an inflated boat tail to get it out to 80% when out of town.
If you look at the 'Template',you see that there's essentially very little curvature at all where the tail begins.This is essential for high performance.If you just jerk the body into an arbitrary angle, boundary layer separation will be triggered right there and you're basically screwed.
NASA did a relatively primitive boat tail behind a Ford Econoline but respected the subtle beginning curvature.There are images of it online and here at EcoModder somewhere.
If I were doing your trailer,I'd use the height as the layout parameter,since the trailer is wider than it is tall.the With the imaginary end-point 1.78X of the heigh behind the trailer.
Your 50% frontal area point would be at around 51% of that dimension. Kinda 'long.' This is why Don Burr started his curvature 'early' on his trailer so it wasn't all 'behind' the trailer.
The final length is something you'll have to determine.I would recommend a short but proper chopped-off tail section over an 'aggressive' but 'dirty' longer tail.
I use an EMT bender to make small incremental bends of large radius.
Harbor Freight has a nice entry-level tubing bender at reasonable price.If you have any awning companies in your area,they can do perfect curves and might take on your work at a reasonable fee.
If the lengths scare you off,let's talk about DR Miller's idea.

[aerohead]

Originally Posted by drmiller100

I'm a minority of one.

I think you take your flat trailer, and put flat angles on the back of the trailer and get a much better result.

The lengths for hotrod's trailer tail ARE 'something.'
I have only one alternative which was part of Texas Tech's Project Tailwind,involving General Motor's 'Optimum' boat tail.
It has more simple angles but with definite edge radii all around.I'd have to dig it out.
So there may be a minority of 'two.'

[ChazInMT]

Here is what you want to try and do. You'll want to radius the corners of the tail extension as much as possible to help you in a cross wind. My angle may be a bit too steep, you certainly would not want to go steeper, going at less of an angle would probably get nearly the same result.

Notice how the shape starts out "Blended" into your trailer? Then it slowly curves to an angle, this is what Aerohead meant, I believe, when he said you don't want flat sides creating an abrupt transition.

I have about a 13° overall change and the back of the shape is at about 21° before it cuts off there. FWIW

Here is a link to a bigger slightly more detailed version of the same drawing as below.
Bigger Pic Of Trailer Kamm Dwg

[drmiller100]

Originally Posted by aerohead

If I were doing your trailer,I'd use the height as the layout parameter,since the trailer is wider than it is tall..

but there is only one top, and two sides.

And the advantages on the bottom are TOUGH to not screw up.

Again, I'm not very sure on this, and am TOTALLY willing to discuss it.

I think you do 1:3 on the top and sides, and radius about a foot at the transition from the tail to the trailer.

Be CAREFUL during that foot - don't do anything to create separation, but don't worry about the 4 corners around your tail- if you screw up the corners, you create a vortex which helps fix any separation.

[hotrodf1]

Originally Posted by ChazInMT

Here is what you want to try and do. You'll want to radius the corners of the tail extension as much as possible to help you in a cross wind. My angle may be a bit too steep, you certainly would not want to go steeper, going at less of an angle would probably get nearly the same result.

Notice how the shape starts out "Blended" into your trailer? Then it slowly curves to an angle, this is what Aerohead meant, I believe, when he said you don't want flat sides creating an abrupt transition.

I have about a 13° overall change and the back of the shape is at about 21° before it cuts off there. FWIW

Here is a link to a bigger slightly more detailed version of the same drawing as below.
Bigger Pic Of Trailer Kamm Dwg

Thanks for the illustration. That looks like the ticket - it's about what I was thinking I guess. I understand the importance of the tail curve being tangent to the vertical of the rear of the trailer. Makes sense. Gradual transition so as not to upset the flow so to speak.

So, the 2" drop at the top due to the lights- it sounds like this is not a huge deal breaker?

Just looking at the illustration - I wonder what kind of drag reduction that will be. Appears to be such a small cross section change but if that's enough to make a significant difference then I'll roll with it.

I guess at the end of the day longer is better, but how much would the difference be if I were to jump from 60" deep on the tail to a 90" or something.

That trailer sits pretty low actually in the rear and very rarely scrapes, so the length would only be a pain getting fuel, etc. I think. Mostly highway miles, so maybe not a huge disadvantage, other than more weight and more fab time?

If I get most of the benefit from the pictured above, and only an additional few % by extending, I would then say it's not worth making it really long.

So that brings me to this question: is there a rule of thumb for amount of effective drag reduction along the length direction? As in, at the first line on the template you get 50% benefit, next line is 70%, next line is 80%, next is 85, next is 89, so on, so on?? A sort of curve that nears 100% reduction as you get closer to the 1.78x dimension that Aerohead referred to as the end point?

[hotrodf1]

Originally Posted by drmiller100

but there is only one top, and two sides.

And the advantages on the bottom are TOUGH to not screw up.

Again, I'm not very sure on this, and am TOTALLY willing to discuss it.

I think you do 1:3 on the top and sides, and radius about a foot at the transition from the tail to the trailer.

Be CAREFUL during that foot - don't do anything to create separation, but don't worry about the 4 corners around your tail- if you screw up the corners, you create a vortex which helps fix any separation.

DR - so this means a radiused area to go from the flat top and sides to get a nice transition to the flat sides of the tail section - and then not worrying about actual curved sides on the tail itself correct?

Fab would obviously be easier on this concept. Does it end up too abrupt and still create separation?

[drmiller100]

Originally Posted by hotrodf1

DR - so this means a radiused area to go from the flat top and sides to get a nice transition to the flat sides of the tail section - and then not worrying about actual curved sides on the tail itself correct?

Fab would obviously be easier on this concept. Does it end up too abrupt and still create separation?

You've got the gist of my idea.

Danged great questions.

I think it would work fine. I'm working on building one between all my other projects.

I'm a bit surprised Aero agrees it should work - let him noodle on the idea a bit, and lets see what he comes back with.

[aerohead]

Originally Posted by drmiller100

but there is only one top, and two sides.

And the advantages on the bottom are TOUGH to not screw up.

Again, I'm not very sure on this, and am TOTALLY willing to discuss it.

I think you do 1:3 on the top and sides, and radius about a foot at the transition from the tail to the trailer.

Be CAREFUL during that foot - don't do anything to create separation, but don't worry about the 4 corners around your tail- if you screw up the corners, you create a vortex which helps fix any separation.

*Looking through my materials I've noticed that others have apparently used the 'width' as the datum for streamlining when the vehicle was 'tall',and the height as the datum when wider than tall.
*When I did my VW Bus I used 20-degrees for top and sides incorporating some beginning curvature provided by VW's original design.There was no 'diffuser',as in 1980,there was no talk of such things yet.
*NASA did the Econoline slowly wrapping the rear corners into a progressive slope,ultimately acheiving 20-degrees.
*GM's 'OPTIMUM' boat tail which appeared on the cover of Texas Tech's Engineering magazine appears to brake directly into a constant 19-20 degree slope at top,maybe same at sides (no true-length images provided in article),10-degrees at bottom,with outer edges softened with radii.
GM's tail extends back 65% of trailer height and ends at about 40% of frontal projected area of the trailer box.
No figures are given,as this may have been a proprietary project (they attempted to camouflage the tail markings) and may not have been for public consumption.
*This tail would be a lot easier to construct.
*I couldn't vouch for any numbers,but GM's got some really sharp aerodynamicists and this may have been an 'optimized' tail when looking at the entire life-cycle-cost-analysis,versus an 'ideal' tail.I'd be compelled to give it my blessing!