Posted on 10/22/2015 7:00:44 PM PDT by Paul R.
I wonder if we have on FR someone familiar enough with aerodynamic design to roughly estimate the upward force ("lift"*) on a pop-up camper being towed at 60 mph if the camper is tilted, front up...
The camper shell is approx. 7.3' wide by 9.8' long x 3' high. The shape overall approximates a VERY crude upside down airfoil -- I assume negative lift (with the camper level) at speed was considered desirable, in the design, but I also assume the effect is not large.
So, what I'm really wondering is if the upward "lift" of the "tilted panel" (or "tilted block" if you like) is enough to be a problem. I really don't want a "skipping trailer" behind me!
I looked around on the web for equations to do the calculation, but quickly discovered this is tricky business, and I don't know what secondary variables can be estimated, or just thrown out. (It IS interesting stuff: If I was young, an elective course might appeal to me, but at my age I no longer have that sort of time...)
*A tilt of 0.6 degrees (or perhaps 1.0 degrees in the "wrong" spot in the road) does not sound like much, but when one views the trailer, the "tilt" is very obvious. The tilt exists because our SUV has a factory welded in Class II receiver that puts the bottom of the trailer coupler over 14" off of level ground even using a 6" drop ball mount. This is with the SUV and camper typically loaded (which helps a little.) If the camper is level, the bottom of the trailer coupler is 11" off the ground.
Meanwhile, the camper coupler is approx. 11" off the ground when the camper is level: Then subtract about 1/2" because when the coupler is on the ball, there is about 1/2" gap to the bottom of the ball base ( = top surface of the ball mount bar).
So, for the camper to be level, the coupler would have to come down 3+ inches further.
Class III ball mounts are available in up to 10" drop, but, unfortunately, there is no easy way to use a Class III receiver. One could for around $200 plus labor ADD a Class III hitch UNDER the existing Class II hitch, but, that drastically reduces the SUV's angle of departure. So, if one wants to go exploring after setting up camp, the vehicle to do so is seriously compromised. The SUV mfgr. obviously did not consider what height most Class II trailers are at. (Our popup, jonboat trailer, and utility trailer, are all 11-12" coupler height when level.)
Thanks in advance to anyone who can help...
Oops, I should have posted “90 mph” including a headwind gust.
And I typo’d the gust! Assume 30 mph (headwind gust) + 60 mph towing speed.
Heh-heh...
This extremely complicated as you also need to consider the bounday layer turbulance created by the tow vehicle.
Rather than get all scientific and running some sort of Computational Fluid Dynamics simulation... how about you hitch it up and take the rig out on the Interstate and do 90... if it rides ok, problem solved, question answered.
I half-seriously considered posting this with the title “Air Force hits camper”. It’d probably get a lot more views that way...
I once pulled a Jayco all the way from Florida to Garden City, Kansas. Over a thousand miles and the hitch broke a hundred yards from home.
They had these fairly severe dips in the pavement to allow water to flow and that is what caught it. Fortunately a guy I worked with cut out the right size piece of metal and welded it back. He had partially welded it and was doing an acceptable if rough job when another friend took over and made it look easy and really smooth weld.
I think the wake left by the towing vehicle would tend to break up the wind flow and lessen the chance of enough wind getting under the trailer to make it lift off the ground tho I guess it has probably happened some time some where.
spoiler alert?
When you are king you must know these things
My best guess is that it is highly unlikely. While your trailer could possibly get enough lift if total wind force underneath it was 90MPH, the draft from your vehicle will counteract the lift imparted on the trailer and the tow vehicle will push most of the air away from the front of the trailer (even in a 30mph wind).
Yes, and I assume there’s an effect from there being “road” under the camper and not empty airspace. (Slight compression of air under the tilted camper.)
Still, an estimate within, say, +/- 30%, would be better than nothing.
The Interstate test might get hairy indeed, if the trailer lost tire contact with the road... That’s what I’m trying to avoid!
Ooh, I like that. I could sit up front and fly and tell the wifey “get in the back and make me a sammich”. Right?
Hahaha! Great image!
I would be more concerned with braking. Under a hard stop, the trailer is going to try to lift the rear wheels of the tow vehicle.
Electric trailer brakes will help or eliminate this.
I would go with a longer drop mount on a class 3 hitch.
Alternately look at putting blocks under the springs on the trailer.
I have experienced the lift problem running heavily loaded and it was bad.
Yeah: give it a try!
Can’t you find someone to hold your beer?
Someone who knows trailers would be more helpful than an aerodynamicist IMO.
I doubt it would be a problem unless the trailer is empty.
It has more to do with the surface area of the top and bottom of the camper. If the top of the camper has a higher surface area than the pressure over the camper will be lower.
Look up Bernoulli’s Principle.
Ideally, one could rig some type of tension gauge to record axle spring travel when towed at speed for a accurate determination. :-)
You forgot to state how much beer is involved...
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