Potential mpg loss % with custom aero trailer.

[aerohead]

Originally Posted by Hersbird

If you had side skirts attached to the tow vehicle that went all the way to the sides of the trailer, then had them on spring hinges that allowed them to swing out for turns even to the point of jack knife, then just return to their neutral.

What you don't want to run 60 sheet metal screws into the side of your SRT so you can hang a 4'x4' aluminum panel on each side?

I also wonder what the turning limit on the old GMC aero truck was? This was the coolest thing ever.

This was NASA's solution for their semi-trailer.
Tesla has chosen same for their Semi.
I had trouble over-extending and over-stretching springs, so I defaulted to cables-and-pulley-to- window-drop-weights, inside elevator shafts on both sides. It's impossible for gravity to fail, the weights are incapable of sticking, and suspending the weights above the bottom of the elevator shaft allows a constant pre-load on the gap-filler panels, holding them snug against the trailer sides when going straight.
WARNING: the panels are in full cantilever, with a gravity moment ( torque ) acting at all times over the entire length of the panels. Striking frost-heaves and pot-holes creates an instantaneous shock-loading ( mechanical 'jerk' ) on the supporting hinges.
Panels need to be as 'short' as possible.
Panels need to be as 'light' as possible.
Hinges need to be as 'strong' as possible.
No 'spontaneous disassembly' is permissible!

[euromodder]

Originally Posted by Gschuld

The difference at 75 and 80 mph is striking. Aero matters…..

Sure does
Around 65-70 is where you'd usually really start to take a rather obvious hit on a "normal" (ie really non-streamlined) vehicle

Originally Posted by Gschuld

What gives me some pause is cross wind effects.
Sway control in particular has my attention as my as designed single axle distance to hitch is on the short side and I sure don’t want it to be dancing around back there at 75-78mph

What do you call on the short side ?
In the US, axles are way too far aft for European eyes
In EU, we have the axle more towards the middle, try to centre weight on the axle as much as possible, and balance weight on the axle with a slight forward bias - say under 100 lbs on the hitch.

I lift the hitch over the ball by hand as a weight-balance check on my single-axle trailer (1650 lbs MGW).

More side-area forward of the axle, say to cover the hitch-gap will help to even out the side-area fore & aft.


One of the members here once made an aero trailer with a circular aft end.
DON'T do that
It induces instability and drag, as air flowing over it will result in a force pulling outward & aft - Coanda effect.

The forum member pulled it off alongside the road IIRC ...

Weight, yes, no way around that.
This is also where I suspect the overpowered 6.2 475hp Hemi comes in handy.

Shouldn't have much trouble with it

The wildcard, for me, is the “ECO” 4cyl mode on this Hot Rod engine.
But I doubt the engineers would have gone through the trouble if it didn’t have some benefit.

Always ask yourself where the benefit is with these things
Auto stop-start was getting used because the European driving cycle (NEDC) had cars @ standstill for large parts of this unrealistic test ... so it dropped the fuel consumption significantly during the test.
In real life, nowhere near as much.


Even half that HEMI is still a 3L , 240 HP engine

[aerohead]

Just for giggles I ran your velocities through the aero power vs V-cubed function.
Compared to aero power requirements at 65-mph:
* 70-mph requires 24.9% more hp.
* 75-mph = 53.6% more
* 80-mph = 86.4% more
-----------------------------------------------------------------------------------------
When you compare 80-mph, to 40-mph, you get your 8-X increase.
1) you're hitting twice as much air
2) you're hitting it twice as often
3) and you're hitting it twice as hard
Triple-whammy! ( courtesy of Walter Korff, Lockheed Aircraft Company, 1963 )

[freebeard]

Originally Posted by euromodder

One of the members here once made an aero trailer with a circular aft end.
DON'T do that
It induces instability and drag, as air flowing over it will result in a force pulling outward & aft - Coanda effect.

The forum member pulled it off alongside the road IIRC ...

[citation needed]

This contradicts Morelli.

[euromodder]

Originally Posted by freebeard

[citation needed]

It has to be on the forum ... somewhere

This contradicts Morelli.

Did he even use (semi-)circular shapes aft ?


I know there have been experiments to try and get air to stick on circular and/or "faster than template" tapering sides - even by trying to suck it down - but was that Morelli ?


Anyways, (semi-)circles or spheres don't really work aft.
Undimpled golfball effect.

You can even create lift by spinning a horizontal cylinder, the Magnus effect. Or by making the air flow over it ,as Coanda did.


Anyways, you get a force perpendicular to the circular surface, and depending on air speed over it.
In real life you're never straight into the wind, so there's always a resultant sideways force.

[freebeard]

ecomodder.com: Morelli's 'Fluid Tail'

Originally Posted by aerohead

*Morelli's test vehicle is a FIAT Punto.You can see an oval Kamm/box-cavity with low Coanda jets which help compensate for the added wheel drag.

[freebeard]

I'm a fan of skegs, but ultimately a diverging horn difffuser AKA *ss trumpet. The longitudinal edges produce vortex jets at the rear corners.



Someone once did a CFD analysis of this. It's on the forum somewhere.

[Gschuld]

Aerohead: good reality check

Euromodder: yes 4cyl ECO mode from a 475hp 6.2 v8 is still a good bit of power. Just calling it “ECO mode “ makes me giggle a bit

And the most compact version of my designs call for a 9’3” (290 cm) distance between hitch ball and trailer axle. That’s reasonably short for a 2700lb (1225kg) all up trailer running at 75mph (120 kph)

Given the choice I would trade a little aerodynamic efficiency for a very stable high speed behavior. Cross winds, in my experience, are one situation where an otherwise well balanced trailer can start to push the TV around.

Granted this situation is for a 2700lb trailer being towed behind a 5200lb SUV with a tow mode that includes real time all 4 corner torque vectoring(or whatever they call it) and with the trailer lower (just slightly) than the rear of the TV. The trailer would have a pretty low ground clearance and center of gravity. All things that help stability.

The less ideal situation is the relatively short ball to axle length. It may be just fine. I’m used to towing 24ft and 28ft trailers around on the highway, so maybe I’m just used to longer trailers. I have a 26ft trailer for a super lightweight racing 28ft sailboat (melges E-scow), with a mere 70” width to the outside of the single axle tires. That thing is only 1800lbs all up. But I’ve never towed anything that followed so smoothly that you can hardly tell it was back there on the highway.

That one has a 16ft ball to trailer axle distance.