Reposted with permission from the author,
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
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
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
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
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