Quote:
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.
--------------------------------------------------------------------------
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.