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Tips on Taming the Beast

High Tow Tension

High tow tensions increase the undesired opposite roll effect as we approach the point of no return. Lower tensions allow us to tolerate the glider being off line to a greater degree before the forces from the towline exceed our weight shift authority. So how much tension do we actually need to get airborne. The answer is not much. Typically we calibrate our static tow gauge to 1G by the highly sophisticated method of attaching the gauge to a convenient high point and then suspending 1 pilot + 1 glider + 1 harness below. Marking the hydraulic gauge give us the 1G point. We mark the gauge with a blue working range of 0.40.8G. Now for a typical glider with a L/D of 10:1 the amount of drag we need to overcome to create 1G worth of lift is only 0.1G. Add a bit extra for some climb and a towline tension of 0.30.4G is more than ample to get us airborne. The critical phase of tow flight is when we are low because a lockout down low can make recovery difficult before ground impact. Keeping tow tensions around 0.4G when low will give us a good climb whilst maintaining the best possible ratio or weightshift authority to tow force in the event of an incipient lockout.

In static line towing your driver can give you too much tension down low. They can potentially kill you if they wanted so teach them well, stress the importance of their role in keeping tensions at safe levels down low, and treat them with respect. They really do hold your life in their hands when you're below 100feet. Similarly with a payout winch you depend on its correct function to keep tensions at safe levels.

High angles of attack

Too many pilots take off on tow at low airspeed and a high angle of attack. I'm sure you've seen them three steps, shove the bar out, dive into the harness.... We all know the benefits of extra airspeed/low angles of attack on takeoff as it gives us better roll authority in the potentially turbulent air near the ground and helps prevent a tip stall. It is important to understand that a foot launch tow takeoff is completely different to a hill launch. On a hill you are rewarded for a strong takeoff run. On tow a strong run will remove the towline tension so a different (more lazy) approach is required. The concept we teach is "let yourself be towed". By let yourself be towed we mean let the towline control your acceleration. Initially shuffle along, then break into a trot. At this stage even in light winds the glider will be flying and taking its own weight. As the tow continues the key is to fly the glider level with the ground. Correctly executed this is great fun as you get to do a moonwalk as you take impossibly large steps as the glider accelerates. This moonwalking can be continued for as long as you need to build up a good reserve of airspeed. A gentle relaxation allows the glider smoothly climb away from the ground.

If you can't master this technique use a launch dolly in light winds. Once again do not come off the dolly until you have built a good reserve of airspeed which you can use to soar clear of the ground. The technique I use is to hold the bar at my chest until the glider starts to feel very light in the dolly (ie it is flying at bar to chest speed and lifting my weight). Building this reserve of airspeed before exiting the dolly also helps prevent the precipitous drop in tow tension which can occur as the glider accelerates due to the loss of drag from the dolly. This drop in tension is usually followed by a period of excessive tension as the driver floors it in response to your desperate gogogogooooo as you sink back towards the ground, usually still in prone. If this regularly happens to you, you 're exiting the dolly too early.

Turbulence

Towing=3DFlatlands=3DThermals=3DTurbulence. OK so its hard to avoid turbulence completely but you can minimise its degree and effects to suit your skill level. We get mechanical turbulence from wind, shear turbulence from shear layers and thermal turbulence from thermals. When learning to tow a light dawn breeze is perfect, whereas 3pm on a windy summers day is sub optimal.

Interestingly the best time to tow when you are trying to catch a thermal is when the winds are lightest and the mechanical and thermal turbulence are at their smallest. Why so? Well every year at the Flatlands competition some pilot will relate the same sad story to me while crying into his pretzels at the bar. It goes like this. "How did you go today?". "I can't f$%&^%g believe it, I had eight tows and couldn't get out of the paddock!". "Oh, I suppose you were waiting for a bit of wind to launch in?". "Ah, yeah, how did you know that?". Its simple really. When a thermal lifts off the surrounding air must rush in from all directions to replace the rising air lets call this the thermal filler wind. Wind is just moving air so what we experience at launch is the combined effects of the prevailing wind and the thermal filler wind. The wind we get depends on whether the prevailing wind and thermal filler wind are cancelling each other out or enhancing each other. What this means is that if there is a light wind and you stand in a tow paddock when the wind is light/tail there is a thermal out in front of launch. If the wind is very crossed then there is probably a thermal off to the downwind side of you. When the wind is blowing strongest it is because there is a thermal behind you, so if you tow at this time you tow in the sinking air between thermals and not only get a dud (low) tow but also don't find a thermal because the next one is probably still ~2000m upwind. Moral: tow when the winds are lightest to maximise your chances of jagging a good thermal out in front. Yes this does mean on light wind days the optimal time to tow is when it's tail. This is where a dolly comes in handy. One cautionary note don't take off in a stronger tailwind than you are willing to land in because you just may have to. Of course by using a moderate tow tension down low and a 1G weaklink this should rarely be an issue.

A very useful technique we use is the 200m windsock. This is a windsock placed directly upwind (which is not dirctly up the strip in a cross wind). In conjunction with a 50m windsock it allows you to "see" that critical parcel of air which you must fly through to get to a safe altitude. These windsocks show the character of the air you will meet on tow in the first critical 1200feet. It makes no sense to me to have a windsock just in front of you on a tow strip. You can feel this wind on your face and by the time you do it is gone and of little relevance to your tow. What you need to know is what that air out in front is like. Put out a 200m windsock and avoid any nasty surprises like "invisible" dust devils you will see your 200m windsock doing circles well before a dustdevil arrives.

Overcontrolling/Oscillation

Under tow the towline tension increase your effective weight and hence enhances your gliders response to a given input. The increased effectiveness of weightshift under tow necessitates making smaller corrections than you might expect. Experience has show that the original 2:1 Hewitt bridle makes overcontrol more of a problem that the current 1:1 V bridle. This is simply because the 1:1 system applies less of the towline tension to the pilot, hence the pilots control inputs are not as enhanced as with a 2:1 bridle which apples twice as much of the towline tension to the pilot compared to the glider. While distributing the tow force in a similar manner to gravity with a 2:1 bridle makes nice theoretical sense, in practice 1:1 just works better.

Some gliders are more prone to overcontrol/oscillations than others. Increasing oscillations will invariably lead to a lockout. As a rough guide from best to worst we would have: floaters/open cross tube gliders, sport/intermediate gliders, square tip high performance gliders, curved tip high performance gliders, latest generation topless gliders, early generation topless gliders.

It makes sense to learn your basic towing skills on a docile easy to tow glider and work your way up. You can pick conditions to make the task easier as discussed above. I would advocate flying your new HP glider off a hill and getting used to flying it fast without oscillations before towing it if the option is available.

Keeping tow tensions low, making moderate inputs and waiting for a response, and slowing the glider down can all help to minimise PIOs.

I have found pulling some VG on (1/41/3) works well to damp out oscillations on the Xtralite and CSX. Of course you are sacrificing a little roll rate when you do this and potentially making the glider more prone to a tip stall. Any VG seems to make my Lightspeed tow worse but fortunately it is far better than my old CSX anyway.

Cross winds

Cross winds are the most underrated risk in towing. Consider a high performance glider launching in a strong crosswind. The glider will want to yaw into the wind. If the pilot starts the tow without the nose of the glider pointing into the wind here is what must happen. Initially the tow bridle is probably touching the uprights/front wires (incipient lockout). As the glider accelerates down the strip the change in the relative wind causes it to yaw/roll around toward the towline. OK so this is good but this yaw/roll must be countered by a pilot input due to the inherent yaw/roll instability of modern designs. So to counter the yaw/roll the pilot high sides the glider. At the same time he/she may well be pushing the bar out to get the glider to take off because even though the wind is strong because it is crossed the useful headwind component is small and this is effectively a light wind launch. For those of you who don't know high siding a glider in a shallow bank and pushing the bar out is the exact technique required to make a HP glider spin. Add a bit of turbulence.... Get the picture? Crosswind takeoffs are dangerous. My rule of thumb is that if I can't get the gliders nose to within 1020degrees of on line (ie pointing down the strip) the crosswind is too strong. If the wind is so strong and crossed that the tow bridle is touching the glider you are asking for trouble.

OK so we get airborne alright. Hey hang gliding is pretty forgiving really. To drop the rope on the strip in a crosswind requires that we crab. Crabbing on tow puts us much closer to an incipient lockout than I care to be as the bridle is often already touching the upright/front wire. Keeping on line and allowing the glider to drift downwind is MUCH safer. If you must crab do it when high and know the risk. Down low keep the nose on line and accept the ensuing downwind drift.

Instruments or other obstructions on the base tube

Placing instruments on your base tube when ground towing is inviting a lockout. The reason is simply that the bridle no longer needs to contact the upright or front wires to exert leverage in the opposite direction to that which is desired your instrument mount will do just fine as a fulcrum. In effect you have wound back the clock by twenty years and are now effectively towing off your base tube. Similarly the rubber grip material on some base tubes has also been proven to cause problems. We discovered this at our school when a course of students experienced unexpectedly frequent lockouts, always right at the top of the tow. Examination of the base tubes of the brand new floater gliders in use showed that the manufacturers recent addition of rubber grip material to the base tube was causing the top bridle line to grip the base tube at the top of tow. Scuff marks were evident on the rubber. After taking these rubber grips off the top of tow lockout problem completely disappeared.

So the keys to avoiding lockouts on tow are simple:

  • 50m & 200m windsocks to "see" that vital parcel of air
  • Low angle of attack and adequate airspeed especially down low
  • Keep tow tension low until a safe altitude is reached
  • Train and respect your driver and maintain your tow gauge/payout winch

Avoid overcontrol and oscillations by picking suitable conditions and gliders for your skill level and making moderate inputs and waiting for a response.
Avoid gnarly conditions, pick the light wind bits to maximise both safety and thermal prospects
Beware crosswinds
Stay online
Incipient lockouts may be corrected but there is a point of no return =B7 When in doubt RELEASE


From: Peter Birren
Doc, you did it again! Wonderfully succinct and fact filled.

I disagree with only one thing, have a differing opinion on another.

The disagreement: "While distributing the tow force in a similar manner to gravity with a 2:1 bridle makes nice theoretical sense, in practice 1:1 just works better."

In our collective experience static towing here in the flat US Midwest, the 1:1 takes some feel and feedback away from the pilot, especially during the initial launch phase when it's needed. It seems to make a glider that's stable enough using a 2:1 feel squirrelly. Close to the ground this can certainly be a problem. (As with many things, it may be what the pilot is familiar and comfortable using that determines the feel and usage.)

The opinion: (For crosswinds) To rehash what was offered in The Oz Report, using a Launch Post can make a crosswing launch a snap. It takes away much of the trepidation of where to point the glider and when to do it. YMMV

If I may, I'd like permission to reprint it in the ReelNews, the newsletter of our club.

Thanks, Doc. You've really nailed it.



Thanks for forwarding this discussion. I'll address a few points in the author's (someone called Doc I presume) comments and answer some questions you asked me earlier about this stuff. Everyone might find this of interest, but its a bit technical. Don't worry if you don't understand all the nitty gritty details of the physics and all that jazz. The important thing is you understand what the situations look and feel like when a lockout can occur and you know how to prevent one from even getting close to developing.

Most of the information he presents is in the book "Towing Aloft." I concur with 95% of what he wrote. And the stuff I disagree with is not that significant. The few inaccurate bits won't hurt you. Most of my comments will be to correct those few errors so DO NOT get the impression I am panning his post. It's basically right.

A notion he presented, and that's been printed in "Hang Gliding" before, is that if a glider gets turned to the tow line, the line connected to the pilot will pull him to the side and effect a weight shift to that side and turn the glider back towards the tow line. That is wrong. That does not occur. If it did, then why doesn't the glider tend to dive when the tow line pulls him forward, isn't his weight being shifted forward? You'd especially see it when towing with the line only connected to the pilot as often done with platform launch, payout winch towing and aerotowing wouldn't you?

There is no weight shift because when the tow force pulls the pilot forward or sideways, the effect on the glider in NOT a lateral weight shift as accomplished by the pilot when controlling the glider. Rather the forces are ANGULARLY displaced; the pilot's weight is NOT laterally displaced. Just as the nose of the glider tends to raise to a higher attitude during a normal tow, a glider sideways to the tow line will want to roll away from the tow line such that the vector sum of the tow force and gravity are perpendicular to the plane (as defined by the wings) of the glider. In other words the inherent stability of the glider tries to orient the glider such that the vector sum of gravity and tow force are about in line with the king post. If the physics work this way to get a glider into the air when you view it from the side, they have to work the same way when you view it from the front with a glider turned away from the tow line.

I cognitively comprehended this aspect of the physics but it didn't really sink solidly into my gut until I took my first paraglider lessons. When kiting a paraglider wing (just standing there as the wind lifts it overhead) if it gets a bit to one side and you try to pull it back overhead by pulling to the opposite side, you instantly ground the wing. That's because the forces are now angularly displaced and the wing tries to fly normal to them and that drives it sideways into the ground. The concept is the same for a hang glider though not as pronounced.

This stuff is all in the book plus diagrams.

I suspect the confusion and why some people believe there is a weight shift is because they have gotten off line, nose pointed away from the line, with the wings relatively level (to the ground) and the glider seemed to self correct. Yep, it generally will do this. Thinking that a weight shift occurred sounds logical and its OK to think that. But technically the reason is NOT a weight shift. But it might seem that way.

Some pilots have argued that the glider is weather vaning back into the wind but technically that is not what's happening either. There is a simple tests you can do to prove to yourself that it doesn't weather vane if you doubt this. Mount a pole or stick off the nose and fasten a piece of yarn out in clean air in front of the glider. If it weather vanes, then you should see a slight cross wind on the yarn prior to it correcting. I've done this, you don't see any cross, the yarn points right at your nose the whole time, especially on a nice beginner glider.

Before beginning, lets be sure you understand what a vector sum is. If I am pulling a piano due north and Greg is pulling just as hard due east, the piano moves northeast. So I created a force vector due north and Greg created a force vector due east and the vector sum of Greg's and my forces on the piano would be a force, or vector sum, to the northeast.

To understand what is actually happening, you must view the glider relative to an alternate horizon formed by drawing a plane perpendicular to the vector sum from the forces of gravity and the tow line. If the tow force is zero, this plane would be the surface of the earth. But with the tow force, this plane is like a huge sheet of plywood that rests on one edge but the end towards the tow line is lifted up. A glider in line on tow will have his wings approximately parallel to this plane side to side and front to back, the same as he would with the ground when free flying. The glider will glide down towards this plane with about the same glide ratio as it flies towards the earth in free flight.

When the glider gets out of alignment with the tow line but his wings are still level with the ground, they are actually banked relative to this alternate plane. When loaded by both gravity and tow force, the glider doesn't relate to the ground any more and just thinks this alternate horizon is "level" and flies relative to that. So, if the winds are level to the ground and the glider is off line, technically, it is already "banked" back towards the line and will generally turn back if turbulence, the pilot or some other variables don't interfere. This is very handy when instructing new pilots to surface tow since you can teach then to focus predominately on keeping the wings level and don't worry about a little yaw (just bigger yaw that starts getting close to lockout potential because of the bridle, etc.). If you keep the wings level, students generally won't have to provide very much correction for yaw and that helps reduce over controlling and PIO. A lot of oscillation begins when the wings are level (to the earth) but the glider is yawed a bit to the tow line and the student then "banks" it even more!

As the author mentioned, when the tow bridle contacts the control frame, a lockout can occur. He is correct. And his comments surrounding that I agree with.

However, there are a couple additional scenarios that can generate forces on the control frame that will swamp a pilot's weight shift ability and can cause a lockout. The pilot can be pulled sideways enough that his body actually contacts the down tubes or rear flying wires and exerts a sideways force on the control frame aggressively turning the glider away from the tow line. This can occur with platform launch bridles and twin chest release systems often used for step towing that route only under the control bar. The pilot's body can effect the lockout and the bridle or tow line never touch the glider.

The second situation deals with nonturned lockouts. The author states that a glider must be turned away from the tow line for a lockout to occur. This is not correct. Well... if you limit the definition of a lockout to only occur when the glider is turned, ok, then its true by definition. However, the author defined lockout the same as I, as being a situation when the pilot can not produce enough weight shift to overcome the effect of the tow force on the glider and control it. And the glider does not HAVE to be turned for that to occur, though it usually is.

There have been at least 2 fatalities in the last 10 years from "lockouts" where the glider never turned or banked away from the tow line at all. In both of these the glider overflew the tow vehicle or line pulley and was flying straight AWAY from the tow line. A bridle connected to the glider's keel and this wrapped down, around and then back under the control bar. A few pounds of line tension pulled the bar in, diving the glider. Just 2030 pounds of line tension effectively stuffs the bar diving the glider straight into the ground. I discussed this in the incident report column in Hang Gliding magazine October 1999.

Additionally, a glider that starts in a more conventional banking turning type lockout can transition into this exact same situation (mentioned above) after the glider rolls completely over and the nose is pointed straight down. The tow force will no longer be pulling sideways on the control frame and unfortunately won't consequently turn the glider out of the dive. I recently witnessed this precise and tragic occurrence.

While researching for "Towing Aloft," I heard a couple rumors of "lockout" type situations platform/payout winch towing with the tow line straight ahead. Their tow bridles were presumably routed high under their armpits and then under the control bar of the glider. They launched and drifted well back of the truck so the line was mostly horizontal. The tow force pulled the pilot(s) far enough forward that the bridle contacted the back of the control bar and pulled it forward nosing the glider up. The rumor was the glider was stalling and mushing and the pilot couldn't control it but didn't crash. I was not able to confirm this story and didn't attempt to repeat/create it. Theoretically, I suppose it could happen and that would certainly be a lockout situation because the pilot would not be able to effect pitch control over the glider.

Physics professor, Donnell Hewett (the correct spelling) chronicled many of the early developments of surface towing in his newsletter "Skyting." If you find towing history interesting, there are 47 "Skyting" issues that were published from Oct 81 to Oct 86 and several articles he wrote for "Whole Air" and one or two for "Hang Gliding" in the same time frame. I don't know if he still has back copies of "Skyting" for sale or not. It's been a long time since I last talked to him but I presume he still lives in Kingsville, Texas.

The author's comments regarding the weak link not protecting you from lockouts are right on. For more discussion on this see the Incident Report column in "Hang Gliding" December, 1998.

The author addresses crosswinds and I won't argue against his suggestions about the amount of acceptable cross. However, I will add that there are two important aspects to crosswinds, direction AND velocity. A 90 degree, 2 MPH cross wind might violate his recommendation (depending upon what he exactly means by "get the gliders nose to within 1020 degrees of on line" and how much effort he finds acceptable to accomplish it) but once the glider is moving 10 MPH forward, the relative wind is now only an 11 degree cross and the glider can generally be yawed that much during the launch. As you accelerate, airspeed increases and your feet are still well on the ground, the glider will continue yawing and lining up more with the tow line. Page 155 in "Towing Aloft" presents a guideline for dealing with acceptable cross winds. That chart is not the final word and folks are free to disagree with it. Its there to get you thinking about crosswinds the correct way and present you with something the start with.

Its was especially nice to see another person preaching that weaklinks do not prevent lockouts (with surface based, tension controlled towing) and recommending against efforts to break weaklinks in emergency situations. These are two of the most dangerous myths circulating in the towing world.

Most everything else he suggested was good and pretty much consistent with the tow practices at our site and in my book.

Bill Bryden

May 1 2005   Top Top