trailer sway mitigation TSM

On Monday, December 16, 2019 at 2:03:02 PM UTC-6, princiar wrote:
Anyone has experience towing a trailer with a car with the trailer sway mitigation installed?

Check out this technical paper (https://purehost.bath.ac.uk/ws/porta...JAUTO981.pdf); it is really pretty easy to digest.

Figures 9-14 provide the important results. Figure 10 shows that minimizing the yaw inertia of the trailer provides the largest impact on damping ratio, and thus the safe towing speed. Figure 9 shows that adding tongue weight above about 6-7% does not really improve the speed very much. Figure 11 shows that changes in the overall trailer mass do not greatly impact the safe towing speed. Figure 13 shows that making a large change in tire pressure (from 30 to 40 psi) only changes the speed for the same damping ratio by a couple of mph. Figure 14 shows that adding a friction stabilizer likewise only changes the speed a couple of mph.

Most composite glider since the 1970s load the wing roots to the front of the trailer, and this directly helps lower the yaw inertia of the trailer. However, many older wood gliders with large root chords load the wing roots aft. For these trailers, the axle must be mounted further aft to help control the yaw inertia so that the trailer can be pulled at highway speeds. Trailers that load over the tongue should have the wing roots forward, and these should be similar to more modern trailers. I have one of these for a Ka-6, and it is a real detriment for drag with the big flat door directly into the wind, even behind a full-size van.

As for tires, if one wants to travel above 65 mph, trailer tires are probably not what you want, since it is above their speed rating. If you drive on Interstate highways, this could be an issue, unless you are comfortable backing up traffic. I myself prefer the radial tires that are not speed limited (at least to normal road speed limits) and are likely to give a smoother ride for my glider as well.

As long as I load the trailer to have a safe margin with damping ratio and I have reasonable pressure in the tires, I have towed single-seat trailers with my Ford E-150 passenger van, my wife's 1999 Volvo V70, my 2003 Ford Focus, or even years ago, my old 1981 Volvo 245. Towing with the 245 (108 hp max at SL) limited my speed significantly when climbing over mountain passes in Oregon, but it did okay on the level, albeit normally not using overdrive. One should also respect the max speed when towing with a smaller car though, because of the time, distance, and stability to do a panic stop with the increase in total weight.

Read the paper and try not to overthink this topic. Load your trailer properly, air up whatever tires you choose, and drive safely.

... Neal P.

At 05:42 23 December 2019, wrote:
On Monday, December 16, 2019 at 2:03:02 PM UTC-6, princiar
wrote:
Anyone has experience towing a trailer with a car with the trailer
sway
m=
itigation installed?

Check out this technical paper
(https://purehost.bath.ac.uk/ws/portalfiles/=
portal/147993632/JAUTO981.pdf); it is really pretty easy to digest.

Figures 9-14 provide the important results. Figure 10 shows that
minimizin=
g the yaw inertia of the trailer provides the largest impact on
damping
rat=
io, and thus the safe towing speed. Figure 9 shows that adding
tongue
weig=
ht above about 6-7% does not really improve the speed very
much. Figure
11=
shows that changes in the overall trailer mass do not greatly
impact the
s=
afe towing speed. Figure 13 shows that making a large change in
tire
press=
ure (from 30 to 40 psi) only changes the speed for the same
damping ratio
b=
y a couple of mph. Figure 14 shows that adding a friction stabilizer
likew=
ise only changes the speed a couple of mph.

Most composite glider since the 1970s load the wing roots to the
front of
t=
he trailer, and this directly helps lower the yaw inertia of the trailer.

=
However, many older wood gliders with large root chords load the
wing
roots=
aft. For these trailers, the axle must be mounted further aft to
help
con=
trol the yaw inertia so that the trailer can be pulled at highway
speeds.
=
Trailers that load over the tongue should have the wing roots
forward, and
=
these should be similar to more modern trailers. I have one of
these for
a=
Ka-6, and it is a real detriment for drag with the big flat door
directly
=
into the wind, even behind a full-size van.

As for tires, if one wants to travel above 65 mph, trailer tires are
probab=
ly not what you want, since it is above their speed rating. If you
drive
o=
n Interstate highways, this could be an issue, unless you are
comfortable
b=
acking up traffic. I myself prefer the radial tires that are not speed
lim=
ited (at least to normal road speed limits) and are likely to give a
smooth=
er ride for my glider as well. =20

As long as I load the trailer to have a safe margin with damping
ratio and
=
I have reasonable pressure in the tires, I have towed single-seat
trailers
=
with my Ford E-150 passenger van, my wife's 1999 Volvo V70, my
2003 Ford
Fo=
cus, or even years ago, my old 1981 Volvo 245. Towing with the
245 (108
hp=
max at SL) limited my speed significantly when climbing over
mountain
pass=
es in Oregon, but it did okay on the level, albeit normally not using
overd=
rive. One should also respect the max speed when towing with a
smaller
car=
though, because of the time, distance, and stability to do a panic
stop
wi=
th the increase in total weight.

Read the paper and try not to overthink this topic. Load your
trailer
prop=
erly, air up whatever tires you choose, and drive safely.

... Neal P.

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.

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

--
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

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

--
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...ions/download-
the-guide-1

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

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

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.

So should we put zigzag tape on the fin?

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

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

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.

Tow safe so you can fly safe,

...... Neal