Monday, February 07, 2011

Winch launch in the PW5

This weekend we again conducted winch launches at Crystalaire gliderport. The weather was nice - low 70's all day, enough to create a little "zero sink" lift, but not enough wind to help with the launches. (Sorry to any eastern readers... not trying to rub it in!)

You might recall that the other times I tried winch launching in the PW5 (almost a year ago) I had an aborted takeoff and then a winch failure, so I've never really had a good launch. Since I had four good launches in the Grob 103 two weeks ago, I was eager to try the PW5 again.

The CG hook on the PW5 is really low to the ground, well below the actual center of gravity, so there's a significant rotational moment which raises the nose. On a glider with a tailwheel on the ground, that's not a problem. But the PW5 rests on its nosewheel, with its tail up in the air, and that rotation caused the tail to strike the ground as soon as the winch started pulling. Our winch has been improved over the last few months, so it's even stronger than before and could really slam the tailboom, so we decide to start with the tail on the ground. The way to do that is to have a person hold down the stabilizer until the glider starts its ground roll. That means that the nose is pitched up at a rather unusual attitude. What does that mean for the elevator position - will I need to hold the nose up, or push it down to fly level? What about the trim setting, which for my weight is supposed to be fairly far forward? These are all things I'll have to work out in the first... um... 0.7 seconds of the takeoff roll.

The first launch starts out smoothly enough. The nose angle seems good, and there's no tendency to PIO like my first time. But I'm keeping an eye on the line and the parachute, and I'm flying faster than it again. Before I am high enough to rotate into the climb, the 'chute disappears below me on the left side, so I have to release and land straight ahead (which goes perfectly well).

The ground crew agrees it was straight and smooth and good, but they say there was a gust of tailwind just as I took off. That probably helped the glider accelerate and helped the parachute inflate, which contributed to outrunning the line. And our experience has shown that once the 'chute inflates, the glider always passes it by. What to do? We decide that the winch driver needs to not use full speed with the PW5, so it won't accelerate so fast, and will keep some load on the line in the early part of the flight. The PW5 empty is 400 pounds lighter than the Grob 103, and it only carries one person, so it's nearly 600 pounds lighter all told. It just doesn't need full power like the big Grob 103 does. We also think that keeping the nose up slightly will present some drag, which will help keep the line taut.

Launch #2 goes just fine. The line stays tight, I can gain a little altitude, and then rotate into the climb. The speed in the climb stabilizes at 60 knots, right where I want it to be (the maximum allowed is 65). It climbs just fine. Finally the glider starts to level out, and I can look down and see that I'm nearly over the western airport fence. So I nose over to relieve the line tension, and release at about 1300 feet AGL. If there had been a headwind it would have been higher.

We've designated an area to the left of the runway where we can look for lift without interfering with either the right (glider) landing pattern or the left (towplane) pattern. I look around there and find a little zero sink, but nothing strong enough to lift me up more than 150 feet or so. The thermal forecast for today did look like there could be lift - no inversion to stop it - but it was not to be. With only 450 feet above pattern altitude to play with, I couldn't go very far, and soon enough I was calling my pattern entry. I had a good landing and rolled off into the parking area.

Later three other pilots launched in the PW5. Two of them had done it before, and one had not. One of them thought that his weight would be enough to counteract the rotational moment, so he opted to start with the tail up. Wrong! Slam! After that one, we all agreed that we need to always hold the tail down. I think everyone's release altitude was about 1300 to 1500 feet.

Here's a video of one of the other pilots launching. From the time you hear the line boss say "Launch, launch, launch" until the glider is in the air, it's about 2.5 to three seconds! This one is noisy, but that's just normal bouncing noise since we're launching on dirt with some rocks. The tail-slam was even louder!


1 comment:

smith said...

Hi Roger - great news about the FAA's determination about Hemet! Hopefully you folks will be going back there.