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 Donnel Hewitt, 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 Hewitt did was analyse 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 Hewitt’s system as soon at the glider got off line the pilots 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 Hewitt 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 gliders handling will be under tow. The mechanism whereby a glider is turned back on line by the Hewitt bridle IS NOT WEIGHT SHIFT. The movement of the pilots 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 centre 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 recognise 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:
- Glider handling on tow is different to free flight, the higher the tow forces the more different.
- Available weight shift control authority decreases when you need it most.
- You have a new control element to deal with – yaw input.
We can now begin to appreciate some of the potential problems with the Hewitt 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:
- Stalling a wingtip
- Secondary to severe turbulence, probably causing 1
- Inappropriate pilot inputs in terms of type, timing and magnitude
- The development of yaw roll oscillations, usually due to 3
- When launching in strong crosswinds which prevent the nose being pointed on line (towards the tow vehicle)
- Crabbing on tow trying to lay off the drift and keep the rope over the tow strip in strong crosswinds.
Secondly once the glider is turned sufficiently from the towline direction the bridle or pilots 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.
Causes/Precipitants of Lockouts. 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. 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 moon-walking 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 (i.e. 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 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 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 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 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 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 (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.
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. Utilising the available yaw force comes with practice. For aerotowing both Quest Air and Wallaby Ranch emphasise 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.
Cross winds 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 (i.e. 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.
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 cross winds
- Stay pointed on line
- Incipient lockouts may be corrected but there is a point of no return
- When in doubt – RELEASE