Which Trailer shape is better??

[aerohead]

Originally Posted by Joggernot

Would a 2-4 inch spoiler work on the top of this trailer?

Typically,trailers don't produce induced lift from separation so the spoiler wouldn't really have a job to do.On vans they only increase drag.

[Hersbird]

I'm no photoshop expert but this is what I was thinking. It would be smaller then this actual trailer which is a 20" and have a single axle not sticking out. I would also taper the rear narrower as it goes back which I can't figure out how to photoshop. Also possibly pull the floor up at about 10 degrees after the axle.

[freebeard]

Here's a teardrop based on the Template. If it was done in five sections instead of three, it would be a closer approximation.

Joggernot -- Here're things that work on a square box van:

[ChazInMT]

Here's what I'd do to the design. Everyone gets all geeked up over 20° rear angle, but I think that is too aggressive. Based on a paper I read by a Cal State Masters Thesis candidate exploring pickup truck aerodynamics, you get 97% of your aero gain by going only 10°, but you maximize lift reduction. The usable space is way better with the design I illustrate here too. So in a sense, you get the best of everything, huge drag reduction, big lift reduction, and a usable design. Win Win Winny Win.

Reason you don't want to get too aggressive on the top taper, is that you start to create a large area of low pressure that wants to get filled in from the sides, this sets you up for a vortex generation situation which produces a lot of drag.

The paper I reference is here. Paper The table on pg. 85 sums it up.

Why this paper isn't the gold standard for truck aerodynamics is beyond me. The author Feysal Adem clearly put a huge amount of VERY knowledgeable effort into its creation, yet somehow everyone bases their opinions on what looks right to them instead of the real science Feysal did.

I can only take credit for putting 2 & 2 together here on this concept. All due credit rests with Feysal.

[aerohead]

Originally Posted by Hersbird

I'm no photoshop expert but this is what I was thinking. It would be smaller then this actual trailer which is a 20" and have a single axle not sticking out. I would also taper the rear narrower as it goes back which I can't figure out how to photoshop. Also possibly pull the floor up at about 10 degrees after the axle.

*the inboard wheels would be great,they could have skirts
*a little roof curvature leading into the down-slope would really help.Lay used 18-degrees max. for this simple roofline.
*plan taper behind the axle is great.Again,some curvature leading into the angle is preferred.Lay used about 11-degrees max.convergence angle with this type of boat-tailing.
*the SAE 10-degree departure angle clearance criteria would help with optimizing the diffuser angle.(Hucho favors 2.5-degrees upsweep)
*a gap-filler made like an 'Inflate-A-Bed' along with it's 12-VDC inflation unit could cut drag over 10% according to NASA testing.SPANDEX stretched across the void has demonstrated savings with the VW Jetta aero trailer project.
*she'd be 'golden' "Oro Azul" blue gold,from the sky!

[Hersbird]

The upslope on the rear would also help with departure angles on driveways and such. I like Chaz's 10 degree but the 18 is also ok for me as that area is just where your feet would go on a rear bed. So the side taper would also help the vortexs right?
I was just going over TV to trailer gap ideas today also. Something flexible would be good that could absorb the turns, or I was contemplating a movable tongue or hitch. Then I thought a 5th wheel design instead might be better but then I'd have to get a pickup that I would otherwise not use where the minivan is a daily driver.
I like freebeards design but the rear door makes for what seems like a wasted hallway where you can't use that rear floor well. A side door would have to be complicated not an off the shelf $200 ebay unit.
My goal here is to be able to tow it at about 65 getting 15mpg or better. A normal box trailer seems to drop mileage to 10 or worse.

[ChazInMT]

Originally Posted by Hersbird

I like Chaz's 10 degree but the 18 is also ok for me as that area is just where your feet would go on a rear bed.

Did you look at the paper I referenced? If so did you just decide that you'd rather maybe do an 18° slope and only gain a 10% reduction in your Cd and INCREASE the lift coefficient over base by 65% instead of a 16.2% reduction in Cd and a 50% decrease (Down force added) that you'd get with 10°? That way with the 18° you'd have less usable space too?

[aerohead]

Thanks Chaz,I printed out 119-pages of it.
The plan taper would change the equation a bit,as well as any diffuser.
Ahmed et al. got into this.I'll do a search for it.
Thanks again in advance.

[aerohead]

Here is some data from Ahmed and associates which includes plan taper at the rear as well as some diffuser.
Every 'best' angle seems to be contextual and we really have to stay on top of the conditions under which results were obtained.
*with only roof angle we get 9-degrees
*with 10-degrees simple plan-taper,and 10-degrees of diffuser our drag is lower with 25-degrees of simple roof angle.
And Ahmed et als. research gives only 9-configurations out of hundreds we might consider

With Hucho,we see some numbers with frontal softening,edge softening,and 5-degrees of simple contraction for both sides and top,with zero diffuser.

[Hersbird]

Originally Posted by ChazInMT

Did you look at the paper I referenced? If so did you just decide that you'd rather maybe do an 18° slope and only gain a 10% reduction in your Cd and INCREASE the lift coefficient over base by 65% instead of a 16.2% reduction in Cd and a 50% decrease (Down force added) that you'd get with 10°? That way with the 18° you'd have less usable space too?

I will Chaz, I'm working and posting from my phone today. I do get what you are saying but now have a new question. Why do I want any downforce? Does downforce come from drag? Forgive me I'm new to all of this.