On Monday, December 23, 2019 at 12:16:38 PM UTC-6, Eric Greenwell wrote:
wrote on 12/23/2019 8:13 AM:
On Monday, December 23, 2019 at 9:48:13 AM UTC-6, Dave Nadler wrote:
On Monday, December 23, 2019 at 12:42:14 AM UTC-5, wrote:
Check out this technical paper (https://purehost.bath.ac.uk/ws/porta...2/JAUTO981.pdf)

This paper uses a model trailer with negligible aerodynamic characteristics.
Caravans have enormous side areas, glider trailers usually less but with rear fins.
So this paper is missing a rather important contributor to sway, which is the
aero force accelerating the trailer back to center.

Some glider owners have had good results reducing sway by fitting spoilers
on the fin.

Be careful out there,
Best Regards, Dave

First of all, I'm an applied aerodynamicist who has designed wings for transonic business jets, so I do appreciate aerodynamics. However, when a trailer is swaying violently from side to side and the driver is trying to hold the steering wheel firm, aerodynamics is not the issue; it is the inertia of the system. An aerodynamic impulse from a gust or passing truck can start the oscillation, but once going, the aerodynamics are secondary.

For regular operation at a safe margin below the critical towing speed, adding fins or removing sharp corners from the front of the trailer box may make the rig less twitchy, for instance less susceptible to irritating behavior in a sustained crosswind. My large aluminum box for the two-place, wood gliders is an excellent example of what not to do. It has sharp, swept corners up front that I am sure shed very nice sets of vortices that tend to impart a nice cyclic wiggle to the back end of the trailer. I've thought about putting a fin or two aft to help keep it pointed straight. But this is a low amplitude effect and never becomes a problem.

What I am concerned about is behavior that could impact the safety of the rig. This is the lightly damped oscillation of a trailer near the critical towing speed. Once I get to that level, it is more like an undamped pendulum problem than an aerodynamic one.

So you meant that aerodynamics of the trailer can trigger the swaying motion, but
the motion itself depends on tires, tongue length, etc?


--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

Pretty much. For the case is of most interest to us for safety, violent oscillation, I would be most concerned with:
1) loading the trailer to get a relatively low yaw inertia (high damping ratio)
2) adjusting the axle placement and tongue length (within practical constraints)
3) keeping the tires inflated to a good level (measured, not by eye)

Since most people are not mechanics / trailer builders, items 1 and 3 are most important.