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#61
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trailer sway mitigation TSM
So should we put zigzag tape on the fin?
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#63
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trailer sway mitigation TSM
On Wednesday, December 25, 2019 at 11:40:03 AM UTC-6, George Haeh wrote:
So should we put zigzag tape on the fin? Too thin. Zigzag tape is used to force transition and avoid a long-bubble separation as the flow goes from laminar to turbulent. This paper describes some different vortex generators (figure 2): https://pdfs.semanticscholar.org/517...170.1577303063 These types are less likely to cut someone than simple swept sheet metal ones. If you google 'wheeler vortex generators' you can find several of them for sale. Large trucks use them as well. |
#64
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trailer sway mitigation TSM
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 |
#65
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trailer sway mitigation TSM
Wow, great examples of the tail wagging the dog.
On a less scientific note, and maybe more to the point of simple common sense, I have trailered my glider in a tube style trailer using a V6 Toyota Rav 4 from California to Utah going over the Sierra Nevada's and it was an easy comfortable trip. To mitigate trailer sway issues I simply noted the speed at which trailer sway started when doing simple lane changes or getting passed by a large vehicle, and set myself a top speed limit of roughly 7 to 10 mph below that. Problem solved. |
#66
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trailer sway mitigation TSM
On Thursday, December 26, 2019 at 9:19:35 AM UTC-6, Mike N. wrote:
Wow, great examples of the tail wagging the dog. On a less scientific note, and maybe more to the point of simple common sense, I have trailered my glider in a tube style trailer using a V6 Toyota Rav 4 from California to Utah going over the Sierra Nevada's and it was an easy comfortable trip. To mitigate trailer sway issues I simply noted the speed at which trailer sway started when doing simple lane changes or getting passed by a large vehicle, and set myself a top speed limit of roughly 7 to 10 mph below that. Problem solved. Thanks! A good practical solution! |
#67
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trailer sway mitigation TSM
wrote on 12/26/2019 7:42 AM:
On Thursday, December 26, 2019 at 9:19:35 AM UTC-6, Mike N. wrote: Wow, great examples of the tail wagging the dog. On a less scientific note, and maybe more to the point of simple common sense, I have trailered my glider in a tube style trailer using a V6 Toyota Rav 4 from California to Utah going over the Sierra Nevada's and it was an easy comfortable trip. To mitigate trailer sway issues I simply noted the speed at which trailer sway started when doing simple lane changes or getting passed by a large vehicle, and set myself a top speed limit of roughly 7 to 10 mph below that. Problem solved. Thanks! A good practical solution! A lower speed provides another advantage: tire longevity. I'm reminded of a motorglider pilot towing his trailer from Florida to Parowan years ago, suffering four blowouts along the way. That meant four separate tire changes, and purchasing four tires, one at a time. His wife said she couldn't get him to slow down, even though it was obvious (at least to her) they'd arrive sooner if he did so! At least it wasn't unstable at the 75+ tow speeds. -- 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 |
#68
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trailer sway mitigation TSM
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