Bill, in the case of the trailer you were pulling, would it have been beneficial to affix some type of "deflector" to the back of the FEH to push air up and prevent the downdraft from hitting the front of the trailer?

 

I believe so and have thought about it. If you could set up a deflector to cause the airflow to skip over the gap and descend back on top of the trailer it would have some benefit.

There was a time when some of these trailers had a big plastic bubble from about halfway up to the top of the trailer that was intended to intercept the descending flow and smooth it around the trailer instead of hitting the flat front of it. When gas got cheap again these disappeared. I suspect that U-Haul decided they just weren't worth the effort in the 90s.

There are such deflectors available for those who do travel trailers but I have not heard whether they really deliver savings. I'm sure they're designed for bigger tow vehicles like 3/4 ton Suburbans and such.

 

Bill, I don't think it is as great of a drag on your trailer as you suspect.
Yes, the air displaced by the vehicle wants to reoccupy the same space.
This takes time.
The time your FEH moves 5 feet at 60 MPH is... 0.055 seconds.
Moving faster, this time is even shorter.
Just how much air can move into that space in 0.055 second?

Is the whole trailer free from drag? No, of course not.
But I'll put money on the line that most of the frontal area is.

I've recently towed for 2000 miles.
There is a clean "shadow" on the front of my trailer, painted white, very nearly the shape and size of my FEH that is free from splattered bugs, dirt, and road grime. I'll take a picture if you wish, but this is a practical example of a "dead air space" if you ask me.

Did the FEH windshield catch ALL the bugs? Don't think so. Did the FEH block all the water spray and road grime from the truck tires in front of me? Don't think so. But there's a space on my white trailer with literally no bugs or dirt after 2000 miles of passing through the mid-west.... in spring/summer.


Caution- real data follows:

My trailer is 5 feet behind my FEH
My FEH is approx. 69" tall x 60" wide ( don't have the spec book in front of me, but that was from a tape measure )
The "clean shadow" on the white trailer is 68" tall x 44" wide and is nearly a perfect rectangle. The shape of the edge is amazingly clear.

So to me, at a glance, it looks like more air drag is occuring from the sides, than from the top. -John

 

With regard to the air from sides as well as the top filling the "hole" between the FEH & the trailer: yeah, I didn't go there to simplify the discussion. But the effect is precisely the same.

With regard to your calculation: I disagree. Gasses make turns around things quickly. Although there is a vacuum behind any moving vehicle it fills up quickly.

Car designers get better mileage with cars that curve the air up, over and then back down as the car moves. The back down is important as it reduces the drag from filling up that "hole." Even the FEHs have this effect in their design. When you then interrupt that up, over, & down flow with a trailer you lose.

The real thing to keep in mind is that the increase in aerodynamic drag is a cubic function. So when you go from 20 to 40 it goes up by 8 if you then go to 80 it goes up another 8.

Couldn't disagree more and I don't agree that the "clean" footprint on your trailer tells the story.

And then don't forget the same effect at the back of the trailer.

 

I think firmly, that with only a 5 foot gap, you can consider, for practical purposes, this to be one vehicle, one that is just longer in your case, and much taller in the rear in my case.

Don't get me wrong, the aero drag is high.
I think we're just discussing the "gap".

If 5' is not close enough in your book to treat the combo as one body, what would be close enough? 5" ?? 2ft ??

What was the original point again????

 

That's simply not true.

Consider the way a pure gas flows versus one with entrained liquid or particulates. Your analysis of the clean area on the front of your trailer only tells you what the entrained "stuff" is doing, not what the gaseous air is doing.

What we're talking about here is what the air is doing. A gas turns corners much more quickly than any entrained "stuff." That's how industrial systems that purify gases with entrained contaminants do it. They cause the gas to make sharp turns and the "stuff," since it can't make these turns, gets collected and drained off.

Go do a wiki search for gas flow and such, I think you'll see what I mean.

 

The low tow rating compared to the regular Escape (5000 lbs.) is entirely due to issues related to backing the vehicle up, as it only backs up using the electric motor (even if the gas motor is running, it's provided nothing but electrical generation)

 

The V6 FE is rated at 3500 lbs. not 5000 lbs.

 

My bad, or maybe they changed it

 

You can't go by what people install on their vehicles either or even at U-Haul's site which doesn't even ask what engine you have. The I4's capacity is less than a V6.