Variometers for Winches

Thread on varios for tugs got me thinking. Why not extend the idea to winch launches as well? I know a winch can't tow you to the right piece of sky, but they can certainly extend the launch time and therefore the height gain by making use of transient thermal activity. Now I'm not suggesting that the winch actually soars, but it must 'rise on its suspension' when the glider on the wire goes through a thermal gust. Maybe a sensitive guage on the wheel assembly could detect the change in loading on the tyre as the winch rises in the thermal and transmit it to a vario in the cab. The winch driver would then be able to ease off the power for the duration of the gust. This would serve several purposes:

1) enhance the launch height achieved 2) save fuel, being more environmentally aware 3) make winch drivers feel part of the soaring day

Hello Howard

Not sure how sophisticated your winch is, but ours is certainly not going to allow you to measure individual thermals. Generally our winch is moved around by the glider on the launch, rocking and pushing against the chocks. We can certainly feel something causing variations in the load almost all the time. The problem would be in identifying the cause:

1] Wind gust. 2] Wind gradient. 3] Thermal. 4] Poor airmanship. (Frequent culprit)

In addition we try slow launches down to a minimum safe speed anyway to maximise the climb for the glider. There is little safe leeway for throttling back when you have an aircraft at 45 degrees or more and usually only around 50kt on the ASI.

As a fairly inexperienced winch driver, I am also wondering about the aerodynamics, a strong thermal would tend to increase the angle of attack. Reducing power always results in a need to lower the nose to maintain airspeed (Unless you are over your targer launch speed) Would you not be increasing the risk of an inadvertent stall and / or abandoned launch by decreasing power when the wire strain increases.

We generally set the winch at a known safe speed and let the pilot fly through gusts etc. We migh notify him/her of any strong variations, although the pilot should be able to feel the gusts too.

On 30 Apr 2002 08:33:13 GMT, Howard Franks

Not if it's properly set up. Our Supacat's instructions say that it must be jacked off its suspension before use. Strain gauges on the legs might show something, but I dunno - those legs are pretty stout and the Supacat is built like a brick outhouse.

Howe Hi you are still right out there in the weeds reg,s tracey

What you are getting at is a wire tensiometer - usually of the 3-roller type. They are often used with good results. In theory, if the winch driver keeps the wire tension roughly equal to the weight of the glider, this will result in a very efficient launch.

There are a few problems with tensiometers. They increase the complexity of the winch and create an additional point of failure since they are in the wire path. Also the tension reading is influenced by factors other than the airspeed of the glider - pilot technique, wire friction with the ground early in the launch and the weight of the wire late in the launch just to mention few.

What you really want to see at the winch is the glider airspeed. This can only be done with radio telemetry. Fortunately, there are off-the-shelf solutions for transmitting the glider airspeed to the winch driver. One of our young pilots, a frequent science fair winner, is putting together a spread spectrum digital system.

Armed with knowledge of the glider's airspeed, the winch driver should be able to deliver a near-perfect launch every time regardless of the pilot's skill or technique, thermals, gusts or anything else.

Bill Daniels

A simple way of doing this is commonly used: the winch operator uses the sag in the cable as an indication of how hard he's pulling on the glider. No need for a separate instrument.