Glider winch launch question
 

I saw my first glider winch launch the other day. I understood the principle but had never seen it done before. The first thing that struck me though was what would happen if the the cable release failed?

With an aircraft tow you clearly have time to work the problem. But with a ground winch, which seems to involve release when the cable approaches vertical, then a failure to release looks like it would very rapidly get very serious.

The glider winch-hook has a back-release mechanism, which should operate as the the glider overflies the winch. If that doesn't work,then the winch driver uses a guillotine to cut his end of the cable.

The subsequent landing with >1000' of cable attached should be "interesting" :eek:

From a design point of view, the winch launch is a very important case. At the top of the launch, the wing has to generate lift equal to the weight of the glider PLUS the tension on the cable.

Overlap with Above The Clouds, who points out the weak link which I forgot about!

You would treat it in a similar manner to an engine failure after take-off, depending on the altitude obtained would determine your choice of landing area/field.
If by the top of the winch launch and cable could not be released from the cockpit then there are two fail safes, once the glider has flown past the vertical point of the cable then the tow cable will release aft, if that doesn't work then there is a weak link in the cable system that would break once the downward pull force is exceeded.

Thanks for that. Very interesting.

The other things that struck me...

1. You need a pretty long runway because the glider can only get as high as the runway is long - less the length of cable that the winch has drawn in to achieve the launch.

2. At the end of the launch most of the cable tension must be in the downward direction. The glider must need to be generating a phenomenal amount of lift.

3. The hook (nearly). It is scary as a spectator when you think it is about to hit you from a great height - even if it is on a drag chute.:O

EFATO I know about. Stuff the nose down and aim for something flat. But this I envisage more like being pulled rapidly into a vertical dive with very few manouevering options.!

However I am now clearer what the procedure is. Thanks for that.

Yikes! If you're close enough to be at all worried about the strop hitting you as it comes down, you are far, far too close to the winch for safety.

The only safe place is inside the winch cabin. If the cable breaks, it can violently whip backwards or sideways. I have seen a car written off by a snapped winch cable. I also had the memorable experience of driving a winch when the cable snapped, wrapped itself in a knot and then disintegrated as it hit the rollers, spraying fragments of cable thread all over the place with such force that some of them wrapped themselves around the chicken wire that protected the windscreen.
The spectator who I had insisted joined me in the cabin before the launch (thye had wanted to take photographs) went very quiet.

As with all things to do with aviation there are several different options depending on when the cable breaks during the launch - including landing straight ahead, carrying out 'S' turns or completing a circuit to land (gliders 'glide' therefore have more options than powered aircraft in this situation).

Naturally, preventing a stall, by lowering the nose, and getting rid of the cable are important aspects of the process!

All practised during training and 'par for the course' (in my experience at least).

We were at least 100 metres off to the side of the centreline. It was just that as the drag chute started spiralling down it seemed to be heading our way, possibly due to crosswind. It never really got near us - just a few moments of alarm.

The first thing to note is that a cable break is NOT considered an emergency. I was always told a 1% launch failure (usually cable breaks) was par for the course.

Safety-wise, in many ways, winch launching is much safer than aero-towing anyway. A winch launch can be split into 3 phases:
- Enough runway left to land straight ahead
- Overlap: Enough runway to land straight ahead AND enough height to complete a safe (if abbreviated) circuit
- Enough height to complete a fairly normal circuit
If conditions (wind strength/direction, winch power, runway length) do not provide a sensible overlap then the duty instructor will not allow winching.

Compare that to an aero-tow - in the early stages (once you can't land straight ahead) you often don't have enough height to pick a perfect field/return to the airfield.

OC619

Cable breaks teach you a very good 'get the nose down' reflex That can be life saving if you are flying low inertia, light aircraft when the engine stops!

We used to use solid piano wire, now that went with a bang and came towards the winch like a coiled spring.

There are several ways of severing the connection between the glider and the winch.

Firstly, at the glider, there is the connector that can be either opened manually by the pilot (by pulling the yellow lever/ball/whatever the aircraft has) or will release the ring by itself when pulled back under a certain angle. After the disconnect, the pilot will actuate the release lever a few times by himself to ensure no cable left connected. These are the normal ways to end a winch launch.

Secondly, the cable might break either at the weak link, the parachute or some point in the cable itself. This requires the pilot to fly the above mentioned procedure, not forgetting to drop whatever rope may be still attached to the aircraft again by pulling the release, the ground crew to search for the loose cable and the winch driver to repair the rope using one of the many ways available (occasionally a bloody business).

And thirdly, if the winch driver sees the glider pass behind the winch without dropping the rope or if he is otherwise unhappy with the gliders behaviour (e. g. if it underflies the cables parachute or makes weird movements while under tow), he can use the "guillotine" to cut the cable at the winch. This one typically consists of a blade mounted on a pre-tensioned strong spring that when released will cut through the cable and is required to show its ability to cut several strands of tow line (IIRC, 3 or 5 of them - it has been a while since I last operated a glider winch) at once during the annual check.

This of course leaves the glider with several 100m of cable attached. The options are generally the same as with a breakage plus the silk way down in case of sufficient height: having the line entangled somewhere on the ground during approach will easily lead to nasty consequences, so this option may well be considered as well.

I heartily agree with the words on safety around a winch. A parachute with the weak links to the head can ruin ones afternoon quite well, as can receiving a hit from a broken line. Once after a rather late disconnect of the glider (IIRC, a slight tailwind was involved), I had the parachute assembly come down right beside the winch and hit the roof of the old vehicle used to return the lines to the launch side. The driver was rather white about the gills when he got out, but fortunately received no injuries. He made sure to park a bit further behind the winch after that.

I shall take advantage of having some clearly well informed people taking an interest. So here goes with a couple more observations.

To overcome the downforce of the near vertical cable at the end of the tow, the glider must be producing (I estimate) 3 or 4 G of lift. Is that not a pretty teeth jarring kick in the backside when the cable is released?

The winch must start off fast with low torque while the glider is on the ground, but must rapidly slow down with increasing torque as the glider climbs.That sounds like a job for an electric motor, but the winch is clearly a combustion engine. How is that geared as the torque and speed requirements of this particular usage sounds like a perfect recipe for it to stall?

There are several winch designs around. The one I operated most often had a Magirus-Deutz air-cooled V8 Diesel working on a hydraulic clutch to convert rpm to torque as required. This worked rather nicely and in windy weather with a light glider even allowed the cable drums to run backwards (release heights of above 800m have been achieved that way on a 1100m field), although the clutch had a certain tendency to overheat in hot weather. Rapid-fire launches of heavy gliders and insufficient cooling-down periods made it tend to shed all its oil via the melting fuses. Refilling all the IIRC 15 litres (= nearly 4 gallons) of oil and replacing the fuses was not the most popular job and of course the privilege of the unlucky winch driver. An hour or so under the shower usually followed.

Older designs use a petrol V8 engine (often from an Opel Admiral) and may even have a stickshift gearbox in the drivetrain - I have never operated such a device but have heard that stalling the engine is not impossible on those.

I am not familiar with the newest developments in this field; the last time I operated a glider winch is more than 10 years ago already. An electric motor would sound interesting and like a plausible choice though.

The rope disconnect can be everything from a jolt to hardly feelable. A break somewhere mid-tow is usually on the rougher side, but the earlier or later in the tow it gets, the lighter it is; I had a rope break shortly before disconnect that I did not even notice in the glider and learned about only after landing.

The reason for this is the variable power requirement for a launch: the winch will not always pull with the same force. In order to accelerate the glider, firstly the throttle is rather rapidly opened (how far depends on the individual glider type, its mass, weather etc.). When it is airborne for the first 50 meters of height, a fair bit less power is required in order not to overspeed it: the first 50 meters are climbed rather gently as otherwise altitude would be insufficient to lower the nose in case of a break. Then, when the winch operator sees the glider pull up, he is required to increase power again to keep it at speed during the now more rapid climb. The glider will follow an approximate quarter of a circle then, so in the beginning, more power will be required to "lift" the glider, while in the end, the gliders climb rate will decrease and it will tend to overspeed if the power is not appropriately reduced. Finally, the winch driver will cut the power to idle and the pilot will return the elevator to a more neutral position. Both serves to reduce the tension on the cable and allow it to disconnect without a major jolt.

Operating a glider winch is a bit of a black art involving observing the climb rate of the glider, the slackness of the rope, the engine and drive train sound and many other things. If a guy is proficient on one winch, this does not at all mean he can just switch to another winch and operate it with the same expertise in a second.

Thanks Tu.114.

Clearly I was thinking in terms of kite flying when I was considering the tension on the cable at release! Obviously the glider pilot is not a helpless participant in the process and can take the tension off the cable with some forward stick before releasing. Obvious after it is pointed out.

Yes a hydraulic clutch makes sense too. Perhaps a simple temperature gauge could save some heartache with melted fuses?

My guess is that the poor winch driver is a glider pilot himself/herself most of the time? Just taking a turn on the rota.

Yep - some of my 'longest days' were left and forgotten at the far end of the airfield for hours on end manning and operating the winch.

Yep!

In my case the winch driver was me an impoverished student and lowly member of the gliding club who spent an entire day winching (and mending cable breaks) in return for a quick circuit in the K7 at dusk.

Often got : nah it's too dark now.

Gave up gliding and G Club membership soon afterwards.

Cusco

When the glider is approaching the top of a normal launch the cable is at a significant angle to the glider and therefore the glider forward speed will be a lot higher than the cable speed. (a comparison is a water skier doing tight turns, when his forward speed is much more than the boat speed).

To compensate for this, the winch driver steadily backs off on the throttle as the glider approaches the top of the launch.

The winch driver can further cause the cable to back release from the glider under minimal cable tension by closing the throttle completely. The cable disconnection is then barely noticeable other than as a minor click. This minimises stresses on the hook and cable and is also great for early pupils who can be discommoded by an almighty bang if the cable is released by the instructor under full tension.

Full tension cable releases are normally caused by the instructor at about mid point in the launch in order to give the pupil practice in dealing with mid-height cable breaks (i.e. he/she has to decide whether there is room to land straight ahead or turn a turn is necessary after getting the nose down and adequate airspeed.

The sudden release of tension and the "elasticity" of the cable can then cause loose loops of cable to form on the winch drum and/or the cable to fall off the side of the drum, creating a mega birds nest and the winch driver to swear about f****** instructors doing simulated cable breaks!

Astir8,

ouch, had totally forgotten about these ones. Beside a broken cable, the pull-out car driver not keeping a steady speed and pulling out the cable at about the same speed the drums deliver them was a frequent cause of such a mess. Much cursing ensued; more than once I have had to take the chisel, simply cut the knot up in order to untie it and then resplice the cable - this was sometimes the fastest method to unjam the cable. It was most fun during thermal season with all the cross-country guys waiting over at the launch pad, the phone keeping ringing every 4 seconds ("Are you done yet?" - "NO!" (SLAM!)) and the flight instructor coming over to berate You for taking so long, but then at least lending a hand.

At my club, everyone wanting to fly plastic gliders had to get the winch license and got half a day on the winch about once every 4 to 6 weeks. Procedures at other fields were different.

A nice thread btw - the winch driver is an oft forgotten hermit at the far end of the field.

Me too. I often wonder how many folk were lost to gliding through that sort of inexcusable experience.

My first experience of driving the winch went something like:
Instructor: "You're a solo pilot, aren't you?"
Me: "Yes"
Instructor: "Go and drive the winch then"
Me: "I've never done it"
Instructor: "Tell whoever is in it to show you 2 cables then"
Me: "I'm worried I'll get it wrong / give people cr*p launches"
Instructor: "Good practice for them"

Another time I heard:
Instructor: "Go and drive the winch"
Volunteer: "I'm scared"
Instructor: "Never been anyone killed that end of the cable"

And the most important bit of advice when converting from winch to aerotow...
Instructor: "What's the most important difference between winch and aerotow launching"
Student: Runs through the expected list...
Instructor: "As a glider pilot you can't kill the winch driver ... always remember that"

OC619

P.S. In both cases (mine and the "volunteer") we had both been gliding for a some time, whilst skillfully avoiding winch duty - so it wasn't nearly as bad as the snippets above sound. Also it was many many years ago.