(Click on an underlined title to go to that story)
Bill Walsh Lockouts (first story below)
Bill Walsh, My Lockout Experience
In Pueblo, Co. we tow most often - the mountains are available, but the towing allows us to get directly into the wind. Just after getting my rating I had a wild tow. The glider had gotten out of alignment with the tow rig, I overcompensated and oscillated quite a bit. Luckily I was able to get back in line.
After landing some of the pilots came over and showed me how to "short tie" the release line. They shortened the release line and tied it to my shoulder pad (or to the aero tow loops). Then if the glider pitches up too much on tow, or you get too out of alignment the bridle releases automatically. It works!!! The very next day I was once again on tow and got into a REAL lockout. I was 75' off the ground and going almost straight into the ground when the release engaged. I was free of the line, the glider stalled with the release of all that weight from the tow tension, and I was able to land perfectly after diving and flaring. Now I fly "short tied" always and we demand that all pilots are "short tied".
As an additional comfort I fly with a 2nd line tied to the release pin. This line goes under the control bar (of course) and is looped around my thumb. This way I can "punch into the air" if needed while keeping my arm on the control bar. All the books say you can just let go of the control bar and sweep your hand down to engage the release line. This is not true. When I was in the lock out just before the "short tie" engage I looked down and my release line was buried into my harness from the pressure. There was no way I was going to be able to let go and grab it. Heck, I couldn't even get a finger nail in between the release line and my harness. This method has worked on four people that I know and several others have seen it required in other clubs.
Bill Walsh Pueblo, CO
FLY THE GLIDER!
Towing is or should be a team effort. Talk often with your teammates about all the minutia of towing. Things you wouldn't normally chew the rag about over a beer. Get into detail, go off the deep end. Discuss the smallest point of the least part ad nauseum. This does several things. It helps bring everyone of the team members up to a certain speed; it helps educate the team on all the parts of the system; it brings everyone's level of awareness higher, including yours. And that's never a bad thing.
Question: OK, I'm locked out and quickly heading into the ground, but I have my wits about me and so I release; now what?
Answer (by Peter Birren): There's an old line that you're probably more than familiar with: Plan your flight and fly your plan. That goes for when things go wrong just as with normal flight.
Let's say you're at 1000' and got trashed so bad you head into a nose down attitude. By the time you're actually nose down you'll have a lot of energy and speed built up. If you wait until then to release, you'll be just about ripe for a loop... or you might be able to gradually pull out of it.
Let's say you're at 50', slightly pulled in to maintain control speed, when you get 45 degrees off kilter. Releasing at that point will put you into a turn that's easily recoverable IF you think and act fast. If you are at that 50 feet and find yourself in the above nose down attitude, better hope your life insurance is paid up. I think both the above scenarios are reason enough for (1) recognizing a developing out-of-control situation BEFORE it gets that far, (2) releasing early enough in the situation so that after releasing you'll be able to control the glider.
Question: I'm in the panic mode and can only reflexively think about control bar inputs?
Answer: Pull in quick and hard to (re)gain airspeed, ergo control. Then FLY THE GLIDER! In static line towing, when you're close to the ground, soon after launching you should be pulling in slightly, but not too much that you don't climb. Once away from the ground, say 200+ feet, you can ease out the control bar for a better climb rate. ALWAYS stay aware of your climb/flight attitude and stay a step ahead of the glider's movements.
With aerotowing, right off the dolly you'll be pulling in so as not to outclimb the tug. And of course, if you get too far off-line (away from the direction of the tow) the tug pilot will likely give you the line if you don't release soon enough. Towing is not something to fear. It is different than foot-launched mountain flying with different considerations... each type of HG launch style is different enough from each other to require its own separate style and discipline; one style does not necessarily transfer directly to another. And the dreaded lockout IS something to be feared... so that you don't let yourself get so far off the towline that a simple turn develops into a lockout.
Everyone, repeat after me: Release! Release! And fly away, So you can fly Another day. Chant it like a mantra. Say it to yourself while shaving. Repeat it every time you tear off a piece of toilet paper. Type it on all memos. Whenever you tow, become intimately familiar with your release line it's your key to freedom. It's more important than your wife. It's more functional than sex. It's better than money. And it's yours... all yours. No one else can do it for you, and you wouldn't want them to. And when you use it you'll be FLYING.
The years are just flying past and it's hard for me to believe my log book when it tells me I got my towing endorsement in November 1995. Only 4 tows in a Moyes XL training glider then 7 more in my Xtralite. The next week I went to Birchip and had 9 tows in two days in the flatlands with only one 70km XC. It only seemed like last year all that happened but in reality it was four years until I was Back at Birchip on the 5th December this year. Only 11 tows this time, the first on my SX4, so you could say that I'm still a beginner. Most of these were foot-launched static car tows and apart from one weaklink break, and a bit of yawing in gusts, they have gone perfectly.
The tricks learned are simple and basic. Strictly speaking they apply to gliders that will handle like my SX4 (or worse!) Floaters and trainer gliders will tow with almost no pilot input required so often the best response is to let loose your grip and let the glider fly itself if your glider is this type.
I plant my feet firmly and hold the wings level and try to hold back as hard as I can on the tow rope until it pulls me forward. I've had no problem with the loose rope that can follow this slingshot type of launch (it doesn't go slack for me), but I will pick the most powerful tow vehicle if there's a choice.
Don't attempt to turn the glider, just a short punch and re-center and wait for the glider to come back on line. A bit of push-out helps to keep the glider responsive, rather than pulling in as you would for a turn off the tow. Trying to turn the glider (back to the straight position) always causes overcorrection, just short punches of 6" or so to the side and then wait for the response.
Nice relaxed grip so you can pay attention to the air, gusts, and initial movement of the glider off line. It's important to be able to recognize the difference between a yaw and a turn. With gusts coming from different directions, my glider yaws a lot.
I remain upright and fly holding the down tubes. Even after taking off in prone on the dolly I'll lean back and fly hanging semi-upright, this stops the upper bridle from hitting my helmet, and it is a familiar position used in final landing approaches when airspeed and reaction to gusts are critical. Pitch or bar pressure controls line tension and makes it easy for the driver, it also maximize height gain on tow. At the top of the tow I am pushing out strongly with the upper bridle trying to pull the control bar back.
I've got into the habit of setting my radio time-out to two minutes and talking to the driver while on tow. If it times out, it has been easy to reset, or I tell the driver I'm going to leave it off and just give a call at the end of the tow. (The Icom beeps a couple of times a few seconds before it times out).
These observations come from personal experience and from talking with other pilots flying both similar and different types of gliders, as well as lectures from, and talking with some of the best towing instructors in the business. They've changed my attitude from being wary (or even terrified) of towing for 20 years of hang gliding, to as comfortable with towing as I am with mountain launches. Now if I can just learn to LAND!! Chainsaw.
Taming the Beast
This is the full article from Dr. James Freeman. It was presented on the HG Digest in Jan, 2000. I (PeterB) disagree with only a couple of minor details he presents, but in the whole it's a grand piece from James, who graciously granted permission to publish.
Lockout. A word to send a tingle up even the most gung-ho pilots spine. Before we go on to look at lockouts in detail a brief description of the theory behind modern towing will be of great benefit.
We all owe a lot to Donnell Hewett, a physics professor and pilot, who in the late seventies and early eighties applied his mind to the physics of towing a hang glider. I will define the term "on line" as meaning having the nose of the glider pointing towards the end of the tow rope furthest from the glider being towed. "Off line" therefore means having the gliders nose pointed anywhere else.
What Hewett did was analyze the problem of towing a hang glider and devise the familiar V-bridle system (amongst other things). Up until this point hang gliders were mainly being towed with a rope attached to the base tube (or some other part of the glider) which in engineering terms formed an unstable positive feedback system. Sure it worked but it required constant pilot input to keep the glider on line. In Hewett's system as soon at the glider got off line the pilot's body was pulled across the A frame by the tow line resulting in a turn back towards the on line condition. This was a brilliant innovation as it introduced negative feedback to the system making it stable, or at least less unstable depending on the characteristics of the glider being towed.
The exact mechanics of how the Hewett bridle actually works are surprisingly complex. Under tow, the forces acting on a glider ARE NOT the same as in free flight. It follows that the higher the tow tension the more different the glider's handling will be under tow. The mechanism whereby a glider is turned back on line by the Hewett bridle IS NOT WEIGHT SHIFT. The movement of the pilot's body across the A frame caused by the tow tension (although it might seem like a weight shift) does not cause the same forces as a weight shift in free flight, although its ultimate effects are similar, i.e.: the glider turns. The dynamics involved are complex and include keel movement/billow shift, side slipping, yaw roll coupling and yaw stability.
Under tow, weight shift as we understand it in free flight, only occurs from a neutral position defined by where the pilots body is pulled by the towline. An unfortunate result of this little understood fact is that the further your body is pulled off center the less the available weight shift authority in the desired direction. Moving your weight from this neutral position does cause a weight shift control response exactly the same as occurs in free flight except that the increase in your apparent weight caused by the towline tension will amplify the response.
By way of example say you are in a right turn on tow. To correct this you need to weight shift left. Unfortunately the tow line already has you pulled over to the left so your available weight shift to the left is actually reduced - the more off line you get the less ability you have to correct this condition as your available weight shift authority in the desired direction steadily decreases. To further complicate matters under tow there is a completely new element introduced - this is yaw. If free flight yaw plays a minor albeit important role. The yaw force that a pilot can apply is quite limited. Even if you might not recognize it, the act of leading with your hips when applying a weight shift input in prone also applies a yaw force to the glider - to shift your hips right you must push with your left hand and pull with your right. But now consider this. What would happen if you attached the tow rope to the bottom left A frame corner? The glider would go spearing off to the right of course. Now consider what happens under tow if you put both hands on this same point and braced your arms. You are now redirecting a significant part of the tow force to the left hand side of the glider and producing a big yaw. The magnitude of this yaw is potentially far more than can be applied in free flight.
For a glider under
tow there are several points you need to note:
We can now begin to appreciate some of the potential problems with the Hewett bridle tow system. If the glider gets too far off line the V-bridle or pilots body may come into contact with an upright or wire. At this point a problem occurs because the towline tension force starts physically levering the glider into a turn. The direction of this leverage force is the exact opposite to that which is desired to correct the off line condition. Initially, sufficient weight shift/yaw authority may be available to cancel out this physical leverage effect - this is what I call an incipient lockout and generally occurs when the glider gets more than about 30-40 degrees off line. If the glider continues to become more off line at some point the system passes through being neutrally stable to become an unstable positive feedback system. This is the point where the shit hits the fan and a true lockout occurs. We can now define what we mean by a lockout.
A lockout occurs when a glider becomes turned away from the towline direction and reaches a point where the pilot cannot recover because he/she is unable to exert sufficient force via weight shift/yaw to counter the effect of the tow tension.
A lockout may also occur if a wing tip (or the whole glider) remains in a stalled condition although this is perhaps more correctly a spin on the towline with the towline forces simply exacerbating the situation.
A third and somewhat unusual form of lockout can occur it the glider overflies the towline, this will result in a steadily increasing dive as the tow tension pulls the bar in.
You should also now be able to understand that the towline forces in a lockout need not be very high. They only need to be sufficiently high to cancel out the effect of a maximum pilot weight shift/yaw in order to cause a continuation and worsening of the situation. Lockouts can and do both occur and continue without ever exceeding normal tow tensions. As a result A WEAK LINK OFFERS LITTLE PROTECTION FROM A LOCKOUT.
There are two distinct and different processes involved in the development of a lockout.
Firstly, to initiate a lockout the glider must be turned away from the towline direction. The reasons why this may occur include:
Secondly, once the glider is turned sufficiently from the towline direction the bridle or pilot's body will come into contact with an upright or wire. As detailed above this bowing will cause a roll force in the opposite direction to that which is required to correct the incipient lockout and turn the glider back on line.
The forces applied by the towline may quickly exceed the pilots yaw/roll control authority and the lockout will rapidly worsen. Experience leaves no doubt that there is a point of no return. Once this point is reached the only solution is to release. Prior to this point the pilot can often salvage the situation by pulling in to simultaneously reduce both the angle of attack (correcting any tip stall) and the tension on the tow line and applying a full weight shift/yaw.
The pilot may also be able to get the tow operator to reduce tow tension - this is easiest for winch and static tow, may be possible with monitored platform tow, but not really applicable to aero tow. The combination of reduced towline tension, lower angle of attack and strong weight shift/yaw MAY allow the situation to be salvaged.
So here is the bottom line. When the bridle or your body contacts an upright or wire you are approaching the point of no return (incipient lockout). At some point the forces exerted by the tow line will exceed your available control authority. If this situation is not corrected a full blown lockout will ensue. The ONLY solution at this point is to release.
The biggest fallacy in towing is that a weak link will protect you from a lockout. For ground towing this is wrong. The tow line force required to break the weak link is roughly 2-3 times the force required to sustain a lockout - I have seen this demonstrated on numerous occasions. As a result you could potentially continue a lockout all the way to the ground without ever breaking the weak link. If you have ever seen a child's kite lock out and arc into the ground you should intuitively understand this. Yes the weak link MAY break but remember all sound ground tow systems are designed to control the tow tension below weak link breaking point. In a lockout your winch and/or driver will actively be working to maintain a normal tow tension below the weak link breaking point. You CAN NOT rely on your weak link to break. In a lockout your only option is to release.
I have heard it suggested that you get the driver to floor it to break the weak link when locked out and using static tow. In my experience a weak link break in a locked out vertical dive usually results in a loop, followed by a wingover and then a massive stall. I'd prefer to release personally. On aerotow a weak link will limit the duration of a lockout because the short rope and lack of direct tension control gives less scope for the glider to diverge from the appropriate flight path - of course you could still hit the ground before the weak link breaks.
Moral. Lockout=Release. Now!
OK so now we understand the beast how do we tame it and make sure all our tows have a happy ending with us thermaling off into the sunset.
Addendum: after seeing a couple of static line launch incidents, and having been in a few myself, it finally dawned on me that a weaklink's real-world use is on the ground to prevent the tow car from dragging the pilot in case of a launch problem. The glider may suffer some road rash and bent tubing, but with a weaklink the pilot just might walk away without a scratch. Been there, done that, seen others do that too.
High tow tension
High tow tensions increase the undesired opposite roll effect as we approach the point of no return. They also introduce the element of a pilot induced yaw force as discussed above making the handling characteristics of the glider different to those found in free flight. 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 0.4-0.8G. 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 0.3-0.4G 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 weight shift authority to tow force in the event of an incipient lockout. Under low tensions the glider handling is more like that in free flight so inappropriate input and over control problems are reduced.
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 take off 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 take off is completely different to a hill launch. On a hill you are rewarded for a strong take off 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 direction and 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 of the pressure you have been using to hold the bar in 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 and the elasticity of the tow rope. This drop in tension is usually followed by a period of excessive tension as the driver floors it in response to your desperate go-go-go-gooooo as you sink back towards the ground, usually still in prone. If this regularly happens to you, you 're exiting the dolly too early.
Towing = Flatlands = Thermals = Turbulence
OK so its hard to avoid turbulence completely but you can minimize 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 canceling each other out or enhancing each other. What this means is that if there is a light prevailing 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 maximize 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 weak link 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 directly 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 1-200feet. 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 dusty ever arrives.
Under tow the towline tension increases your effective weight and hence enhances your gliders response to a given input. The increased effectiveness of weight shift under tow necessitates making smaller corrections than you might expect. You also have the addition of a new ability to yaw the glider. Experience has show that the original 2:1 Hewett 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 (weight shift and yaw) 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.
(Editor note: This is where Jim and I disagree. I believe the 2:1 is more "stable" because it places more of the tow force on the pilot, making him seem heavier to the glider, while still attaching some tow force on the glider.)
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 minimize PIOs. Utilizing the available yaw force comes with practice. For aerotowing both Quest Air and Wallaby Ranch emphasize a lead with your hips approach for control inputs under tow - this is simply a practical explanation of: Use the (yaw) force Luke.
I have found pulling some VG on (1/4-1/3) 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 easier to tow than my old CSX anyway.
are the most underrated risk in towing. Consider a high performance glider launching in a strong cross wind. 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? Cross wind take offs are dangerous.
My rule of thumb is that if I can't get the gliders nose to within 10-20degrees of on line (ie pointing down the strip) the cross wind 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 all right. Hey hang gliding is pretty forgiving really. To drop the rope on the strip in a cross wind 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.
or other obstructions
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.
I'll address a few points in Doc's comments and answer some previous questions. 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 weathervaning 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 weathervane 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 weathervanes, 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 overcontrolling 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 downtubes 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 towline never touch the glider.
The second situation deals with non-turned 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 20-30 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 glider's nose to within 10-20 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. It's there to get you thinking about crosswinds the correct way and present you with something the start with.
It 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.
and produced by Peter Birren