enclosed trailer aero kammback

[aerohead]

Originally Posted by hotrodf1

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?

You're rig will basically respond to the tail as any other vehicle,save for the exception of losses between the tow vehicle and trailer face.
In a perfect world,where there was no gap in your rig,all your wheels were flush and had covers,and everything was bellypanned,you'd essentially have a 'bus' or 'motorhome.'
At the following lengths of tail,you'd have the potential for:
50% = Cd 0.177
60% = Cd 0.153
70% = Cd 0.14
80% = Cd 0.133
90% = Cd 0.13
100% = Cd 0.13
So you can see that the last 20% is statistically insignificant.This region is 'phantom' tail,where the turbulence itself is behaving as a solid structure,with the outer flow ricochetting off of it.
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The whole thing about the tail is that it is converting dynamic pressure back into static pressure.With 80% of it,you end up with the forward stagnation pressure very close to that of the rear base pressure,with only the unavoidable skin-friction losses in between.Which according to Hucho and others,THE fundamental premise of all road vehicle streamlining.
PS,if you'll go to the 'full-boat-tail-trailer' thread,at permalink 245 or so,there are some pictorial drag tables for different boat-tailed vehicles.Look around,you'll find 'em.

[drmiller100]

aero,
Thank you for your comments and thoughts.

Just to bring it back for perspective for me, do you have any estimates for
0
10
20
30
40 percent?????

[hotrodf1]

Originally Posted by aerohead

You're rig will basically respond to the tail as any other vehicle,save for the exception of losses between the tow vehicle and trailer face.
In a perfect world,where there was no gap in your rig,all your wheels were flush and had covers,and everything was bellypanned,you'd essentially have a 'bus' or 'motorhome.'
At the following lengths of tail,you'd have the potential for:
50% = Cd 0.177
60% = Cd 0.153
70% = Cd 0.14
80% = Cd 0.133
90% = Cd 0.13
100% = Cd 0.13
So you can see that the last 20% is statistically insignificant.This region is 'phantom' tail,where the turbulence itself is behaving as a solid structure,with the outer flow ricochetting off of it.
--------------------------------------------------------------------------
The whole thing about the tail is that it is converting dynamic pressure back into static pressure.With 80% of it,you end up with the forward stagnation pressure very close to that of the rear base pressure,with only the unavoidable skin-friction losses in between.Which according to Hucho and others,THE fundamental premise of all road vehicle streamlining.
PS,if you'll go to the 'full-boat-tail-trailer' thread,at permalink 245 or so,there are some pictorial drag tables for different boat-tailed vehicles.Look around,you'll find 'em.

Sorry, just to confirm here.
The % all relate to "full" length tail, which is the 1.78X relationship right? And the X is the height of the trailer in this case? If trailer is 85" high or so, than a full tail is 151.3", a fairly "healthy" length. So the 50% is then 75.5 or so long. Skinning a tail that long will be more difficult with the material I have, but I will work around it.

And speaking of other aero treatments, I was thinking of side skirts that go forward to a point behind the hitch, and also perhaps some fender
skirts on the trailer as well. Hoping to avoid a pan on the trailer that way. In the grand scheme of things are the skirts worth doing on a trailer?

[ChazInMT]

Check out this post Doc Miller.

Aero Drag Reduction Potential Thread

Summary is this.
zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115

[ChazInMT]

Here is a chart I made for myself to help quantify the difference.

I use it by looking at where I start, ie. at 30% length (3 in the left column), and I figure out how much I'm going to add on to the shape to bring it closer to the template, so if I go to 60% (6 in the left column) a change of 30%, I get a 44% improvement on the Cd (44% in the far right column).

I hope it's not too complicated. The tool helps me figure it out better.

[drmiller100]

Chaz, Thank You.

I assumed the corners are rounded at the back. And I assumed 7 feet by 8 feet, for 56 square feet.
Then I went to the calculator, and typed in 15,000 pounds as vehicle weight just as a place to start.

So, I WAGged .40 for rounded corners and Cd.

I got 10.8 mpg at 60 mph, which passes a sanity test but maybe a bit high based on personal experience.

Then I typed in .177 as the 50 percent mark.

I got 16.7 mpg at 60 mph. HOLY CRAP.

So. Assume a 1000 mile trip, and assume 4 dollars per gallon.
(1000/10.8)*4 = 370 dollars without.

(1000/16.7 ) *4 = 240 dollars with.

Wow. HUGE difference!!!!

[ChazInMT]

Originally Posted by hotrodf1

Sorry, just to confirm here.
The % all relate to "full" length tail, which is the 1.78X relationship right? And the X is the height of the trailer in this case? If trailer is 85" high or so, than a full tail is 151.3", a fairly "healthy" length. So the 50% is then 75.5 or so long.

So if you look at my chart, and you went 30% longer or 45 inches, you'd gain a 51% decrease in your Cd. Not too shabby Scoobie Doo. The biggest gains happen as soon as you start adding anything.

Of course all these numbers assume you're dealing with an ideal type shape to begin with, the variables are daunting to try and accommodate, but still, even if you only got half of 50%, you're still looking great. Well worth the effort because the gains are very significant.

[drmiller100]

Chas, There is almost NOTHING more ideal then a LOOOOOONG straight trailer.

I believe we will see the benefits listed, perhaps a bit more.

As I interpret Aero's comments, he is saying the same thing. We might be off 5 or 10 percent, but the savings are HUGE.

[ChazInMT]

Originally Posted by drmiller100

Chas, There is almost NOTHING more ideal then a LOOOOOONG straight trailer.

I believe we will see the benefits listed, perhaps a bit more.

As I interpret Aero's comments, he is saying the same thing. We might be off 5 or 10 percent, but the savings are HUGE.

Keep in mind that the "aero drag" part of the fuel economy equation here is still only a one part of the overall picture. Sheer engine size, large frontal areas, unsavory aerodynamic interactions between the truck and trailer, and even skin drag on such a large vehicle comes into play more. None of these items are being affected by the addition of the tail cone, so a 50% improvement at the rear, is still only 50% of maybe 20% of the equation (WAGing myself at this point) The improvements at the rear are only going to get you 10-15% better mileage tops if done properly. Driving habits are still the biggest area for improvement.

I highly doubt you'll go from 11MPG to 16MPG by slapping a tailpiece on your rig. If your definition of HUGE is 11MPG to 13MPG, and good driving habits end up getting you to 14MPG, then I guess we're talking the same language.

[euromodder]

Originally Posted by drmiller100

Chas, There is almost NOTHING more ideal then a LOOOOOONG straight trailer.

Donbur's experience shows that a curved trailer is better than a straight one - even at the expense of some added frontal area.
If they didn't add that extra area, the results would be better still.

Curving the sides as well as the top, would further increase the gains.

Length is limited by legal and practical issues, so loooooooooooong really can't be that loooooooooooong.