Thursday, May 31, 2007

Winch Launch

Our club is joining with two others for two weekends of winch launching, one of which was last weekend. Our club just finished rebuilding a winch and loading it with Spectra cable. We have instructors who have extensive winch experience, but none of the members did, so we thought it would be good to learn from other clubs who do. Some members worked on learning to "drive" the winch, and most others worked on learning to fly the gliders. We have done autotowing before, and some of us already have our "ground launch" endorsement, which you can get via either auto or winch. Technically you don't need another endorsement to do the other kind of ground launch, but you'd be crazy not to get instruction in it before doing it.

Saturday I did one launch, and Sunday I did two more. All three were very straight. It's the speed control, the angle of attack, that is tricky. My first one was not very controlled, my second one was better although I let off early when my speed got low. My third was quite good, achieving an altitude of 1400' AGL.

Ground launching involves redirecting the forward-downward force of the cable motion, using the resistance of the bottom surface of the wing against the air, into a forward-upward force. The amount of force converted depends on the angle, so increasing the angle of attack (with the elevator) directs more of the force, increasing airspeed. This is the opposite of the action of the elevator when in free flight. So you have to reverse your thinking: if the speed gets low, pull back to increase the force... up to a point, of course.

Now, there are two schools of thought on this, so I inv1te comments. A few months before doing this for real, I studied it in books and on the Internet and RAS. Some highly respected instructors insist it does not work this way, that lowering the nose is the way to increase airspeed during winch launch, just as in free flight, that it is NOT reversed. In my limited experience, six autotows and three winch launches, it IS reversed. The HRI's say that if the winch is underpowered, the reverse method is true, and if the winch has excess power (big European diesel or diesel-electrics), it is false. All three winches in use at our event were traditional gas V8's... powerful enough to launch pretty darn high, but maybe not in the excess-power range.

The other tricky part is that since you're going up at a 40-degree angle, directional control is harder. Until you know the surrounding landmarks, you can't judge direction very well.

Things happen REALLY fast. Zero to 55kt in about 3 seconds. Off the ground in as little as 50 feet. Up to 200' AGL in about 5 seconds. Then up at 40-45 degrees for about a minute or less. Wow!

Our club did some launches with our winch and steel cable some months ago, and did not get nearly as high. With about 3500' of lightweight Spectra, launches of 1400 to 1600' were common. That's enough to snoop around for thermals until you're down to about 800' and need to enter a pattern. Some folks found thermals and went off for hour-long flights of up to 10,000'.

Usually where we fly no one is on the ground near the approach end of the runway. At this site, we took off from a taxiway and landed on the adjacent runway, so we were quite close to the round-out point. In this shot you can see the flaps and airbrakes open.

I'll have more to say about some critical things that I learned about safe launches... after our second winch event weekend is over.

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