On Wednesday, December 25, 2019 at 9:46:40 AM UTC-8, Eric Greenwell wrote:
wrote on 12/24/2019 9:07 PM:
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.

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.

I did test my Cobra trailer with and without spoilers on the fin. I could not
detect a difference in stability, suggesting to me the fin is producing
insignificant lift when yawed.


--
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

Here is a couple of compilations of trailer towing accidents:
https://www.youtube.com/watch?v=CJtmOPdWrlE
https://www.youtube.com/watch?v=Z7Kfl97b57s
These accidents involve trailers of all types: travel trailers, utility trailers, and car hauling trailers. The number one common denominator is speed: most were passing the truck recording the video. Another factor is small vehicles towing larger travel trailers. A third factor is improper loading of the trailer.

A car-trailer combination is a very complex spring-mass-damper control system. Like any control system, there exists what is called a stability margin.. This is the margin you have to handle disturbances and still keep the system stable. Here is a paper I found that discusses this:
https://tinyurl.com/sux9ph9
The take-home message is Fig. 3; as the speed increases the stability margin decreases until it becomes zero (you are inherently unstable). So, the faster you drive, the smaller your stability (safety) margin. This paper confirms those findings by experimentation with an actual instrumented car-trailer combination:
https://pdfs.semanticscholar.org/e8e...b8065c44c2.pdf
Improper loading dramatically reduces the stability (safety) margin. We can control this by the tongue weight: higher tongue weights are more stable, lower weights are less stable. The most direct way to change this is moving the axle. Other significant factors are vehicles purpose-built for towing will have suspensions that are stiffer than passenger cars. Effects due to aerodynamics will be third-order and not significant compared to these primary causes of instability. So, don't think that you can put vortex generators on your trailer and drive 80 mph safely.

Tom