On Tuesday, December 24, 2019 at 9:06:36 AM UTC-6, Eric Greenwell wrote:
Don Johnstone wrote on 12/24/2019 2:20 AM:
At 15:35 23 December 2019, Eric Greenwell wrote:
Don Johnstone wrote on 12/23/2019 1:36 AM:

What is worth remembering is that the cause of the sway is
aerodynamic. Loading, tyre pressure, fiction dampers can only
ensure that the sway is damped out and does not increase.
Trailers
with less vertical surface area aft of the wheels suffer less from
snaking than those with equal or greater vertical surface area
behind
the wheels. You may not be able to change the aerodynamics of
the
trailer but a good friction damper and proper loading will
normally
ensure that the snaking is damped.
I have been a passenger in a car towing a Cobra trailer at over
100mph, scared the crap out of me but it was completely stable,
but
only when loaded with the glider. Empty it was a different story.

How did you decide aerodynamics was the important factor, and
not other
factors
like tongue length, tires, etc? That's counterintuitive and, for
example,
it's at
significant variance with Nelson Funston's paper on towing glider
trailers,
which
does not include aerodynamic factors.

http://journals.sfu.ca/ts/index.php/...wnload/779/737


From observation of driving trailers over many years. Aerodynamic
forces are almost always the initial cause of the onset of snaking,
the other factors take over in damping out, or increasing the initial
disturbance. I can be towing at say 60mph in clear air. If I am
overtaken, especially by a large vehicle I fist notice that as the
vehicle is alongside the trailer is drawn towards the other vehicle,
when the vehicle has passed the trailer moves away and this is
where the snake starts. From that point on the other mentioned
factors take over, which, almost without exception, we all use
stabilisers, mostly ALKO type

As you state, swaying typically starts with a "trigger" event, and crosswinds or
large trucks are common triggers; however, it can also be a twitch on the steering
wheel from inattention, dodging road debris, moving away from another vehicle
drifting too close beside you, uneven road surfaces, higher speeds while passing
another vehicle, etc. The trigger is not important, but the trailer's inherent
stability is.

On the other hand, there are reports of trailer aerodynamics directly affecting
the trailer stability. The two I know about were both the older Komet style
trailers with "doghouses" or "tail fins" that were airfoil shaped like vertical
stabilizers without the rudder. Both owners reported significant reduction in
swaying by putting "spoilers" near the leading edge on each side of the fin. The
spoilers were about 1/2" high and a few inches back from the leading edge.. The now
common Cobra trailers have blunt leading edges and flat, truncated trailing edges
that apparently don't produce the lift the Komet fins did.

--
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

My main concern is with trailer dynamics is safety, and for that, an aerodynamic disturbance can initiate a major oscillation that can be dangerous. Once started, the dynamics of the system is primary and aerodynamics is secondary.

However, for comfort while towing, one would like to deal with small-amplitude oscillations that are not dangerous, just annoying. For this, aerodynamics is important. In general, fins near the back of the trailer could be useful, however, most lack the finesse of a good airfoil and will have some separation. They tend to have a larger than desired thickness to chord ratio, and the closure angle of airfoil to the trailing edge can also be high.. Both of these design 'features' can lead to separation on the sides of the fin.

When separation occurs, it causes a change in the side force and an accompanied small change in yaw angle of the trailer. For a thick fin, the separation point can migrate forward and back on each side of the fin and this can lead to a small yaw oscillation. By forcing separation at a fixed point forward of max thickness (with spoilers or other devices), the drag goes up a bit, but the oscillation in side force is eliminated. Vortex generators at max thickness or other aerodynamics widgets could provide a similar fix.