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Reposted with permission from the author,
James Freeman
Date: Fri, 21 Jan 2000 08:31:14 0500


Lockout. A word to send a tingle up even the most gungho pilots spine. Before we go on to look at lockouts in detail a very brief description of the theory behind modern towing will be of great benefit.

Donnel Hewett was an engineer who in the 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. To complete the technical definitions we also need to understand the engineering concept of stability. There are three types of stability: stable, neutral and unstable.

A stable system can be represented by a ball in a round bowl. No matter what we do to the ball it will eventually return to a position of rest at the bottom of the bowl. Engineers use the term negative feedback to describe the fact that when disturbed the ball wants to return to its stable position at the bottom of the bowl. This is because the ball experiences forces trying to return it to this stable position.

A neutrally stable system can be represented by a ball on a flat surface. The ball will stay put or move a constant velocity unless disturbed.

An unstable system can be represented by a ball on the top of a hill. While the ball may balance in its precarious position the slightest disturbance will cause it to roll down one side of the hill or the other. Engineers use the term positive feedback to describe the fact that when disturbed the ball does not want to return to its position at the top of the hill. In fact it experiences forces trying to send it rolling down the hill at an ever increasing rate.

OK, OK you say. Enough of the technobabble. What Hewett did was analyse the problem of towing a hang glider and devise the familiar Vbridle 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 pilots body was pulled across the A frame by the tow line. This caused a weight shift towards the towline and a resultant turn back towards the on line condition. This was a brilliant innovation as it introduced negative feedback to the system making it stable. There is however one small flaw. If the glider gets too far off line the Vbridle comes into contact with the upright or front wire. At this point a problem occurs because the towline now ceases to move the pilots weight across and now 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 the weight shift of the pilots body may be sufficient to cancel out this physical leverage effect this is what I call an incipient lockout (neutral stability) and occurs when the glider gets more than about 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 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.

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 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:

  • 1 stalling a wingtip
  • 2 secondary to severe turbulence, probably causing 1
  • 3 the development of yaw roll oscillations
  • 4 when launching in strong crosswinds which prevent the nose being pointed on line
  • 5 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 tow line will bow around the upright/front 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 online. The forces applied by the towline may quickly exceed the pilots weight shift roll 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. 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 weightshift MAY allow the situation to be salvaged.

So here is the bottom line. When the bridle contacts the upright or front 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 weight shift 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 23 times the force required to sustain a lockout. 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=92s 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=3DRelease. 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 thermalling off into the sunset.

Lockouts Continued

Apr 9 2004   Top Top