Monday, April 26, 2010

Winch weekend - Day 3 - PW5 PTT x2

Today several of us plan to fly the PW5. After preflighting it, I spend some time with private pilots M and S explaining the features of the Borgelt B50 computer and the MicroAir radio. We also spend some time with the winch launching section of the PW5 manual. Important points include adjusting the seat position and the trim according to the pilot's weight. S is heavier than M or me... one concern we have is that with lighter pilots, the way the wheels and the CG hook are arranged, it's quite possible that the moment of acceleration could cause the cockpit to rotate up and the tail boom to slam down. The manual has some advice on how to hold the stick.

And then there's this interesting comment: "During the steep climb the stick forces are of small value." Some winching experts have pointed out that stable gliders tend to climb at the angle that is the most efficient, i.e. the horizontal stabilizer will tend to orient the wings directly into the relative wind to achieve stability, and so the angle of attack and speed will tend to become optimized. They claim you can unhand the stick and a stable glider will climb well on its own. (I won't try that for quite a while!) The PW5's elevator is really small compared to the wings and the horizontal stabilizer, so it can't override the orienting effect. More on that later.

We set up today on the main runway, launching to the west due to the wind direction. We get an earlier start, which is good for practicing, but it also means the lift isn't working for the first few flights. A visiting glider pilot (not a member) wants to go for a ride, so I go up with him in the Blanik (me in front, him in back). He's been a winch launch passenger once before, so it's not all new to him. We get a good launch, but with no headwind and since I'm not terribly aggressive in the climb yet, we only get up to 900' AGL. We turn left to look for lift, but we get into 5 knot sink right away, so essentially we have to turn back and never get out of the pattern. We come in for a good landing and roll right up to the launching point. Fun, but short. No one else was finding more than 1-2 knots of lift this early in the day, so I don't think we had much of a chance for a longer flight anyway.

Later in the day I set up for a flight in the PW5. As expected, the tail boom comes down, but not very hard. If this winch was as aggressive in the first two seconds as the OCSA winch is, this rotation could be an issue. Once I get off the ground, I get into a slight PIO (which means "pilot-induced oscillation", pitching up and down due to not controlling perfectly). It's not much as PIO's go, and only lasts for about two cycles. But as I mentioned yesterday, if you don't pitch up at least a little bit right away to present a load on the cable, you can overrun it. My two seconds of horizontal flight getting out of the PIO is too long, and I see the parachute slip below me to the left, so I release just before someone on the radio calls "abort, abort". I make a safe and smooth landing on the right side of the runway (to avoid the cable), and roll out about two thirds of the way down toward the winch. This is what we call a "PTT" or "Premature Termination of Tow". It's a long walk back with the glider behind the truck.

After a few Blanik launches and our club pres in his PIK, I'm up again in the PW5. This time my takeoff is smooth and I rotate up and start a pretty good climb. Someone on the radio calls to the winch driver "more power, more power". Before long my airspeed is 67 and flirting with 70 knots. The maximum allowed according to the manual is 65, so I start pulling back to try to adjust the speed (forgetting for a moment the statement "During the steep climb the stick forces are of small value."). Probably my next move would be to call for less speed, but suddenly the speed drops off and I hear/feel the cable release. Bummer - PTT #2. Hmmm... I can't see the chute or the line by this time, but I can't imagine that I overran it at this angle. But there's no time to analyze it: I'm at 400' AGL. That's high enough to turn around for a downwind landing, so that's what I plan to do. I have had a rope break at 300' during auto-towing solo in a Blanik, so I've done this before.

I nose over and ensure I have flying speed. There was very little crosswind when I took off, so it doesn't matter much which way I go. Since the normal pattern is left-hand, I turn left and do a 270-degree turn and get lined up on the runway. Hmmm... The next Blanik is already lined up at the launching point. I know that when you have a PTT from an auto-tow or winch launch, where you go up steeply, you can't really land in the distance available back to the starting point. You would have to descend at the same 40+ degree angle you went up at, and that's not likely in a slippery glass ship. Maybe in a Blanik with flaps and a big slip, but not in this glider. Hmmm... passing over the launching point to the other end of the runway is not an option either, because our pres is landing his PIK there. There's a nice wide taxiway, just as long as the runway, so that's the obvious choice. Since I'm already lined up on the runway, I S-turn to the left and line up again. There's nothing in the way, and I make a nice smooth landing and roll to a stop about 100 feet beyond the launching point. No problem. But there's no way I could have landed back on the runway without overrunning the launch point. It's nice to have options!

So far today I have employed two of the three PTT landing procedures.

I discuss the flight with S, and can't quite figure out why it released. People on the ground thought my climb angle looked good. My slight pulling up and backing off on the stick to adjust the speed shouldn't have dropped the line very much. I'd be ready to go again after a waiting for Blanik or two to launch. But then the winch guys call in on the radio: we're done for the day due to a failed universal joint. Later on I see the U-joint pieces in the hangar - an old repair failed and it spun itself apart. That occurred during my launch, so now at least I know the cause of the sudden power loss and back-release.

By now it's 2:00 in the afternoon, so we break for lunch and tear down the gliders... and change a flat tire on one of the trailers. It was fun while it lasted, but the lift was weaker than expected so no one really "got away" today. The last of us leave the airport about 6:00. Hot and dusty, and grateful to the host club, we drive off into the sunset.

5 comments:

smith said...

Fascinating posts Roger. I'm keen to try winch launches some day. My club hasn't done them for decades and a club in Western Australia that used a winch had a crash off the winch after a cable break the weekend before I was planning to visit them (when I was working there last year), and stopped operations for a while.

Someday maybe.

I was interested in your comment about actions after an early release or break. Talking to Brit pilots who have winch-launched a lot, apparently there's an issue with people stalling and spinning off an early release or a cable break because the glider is not above stalling speed before they start a turn back to the strip. Their point seemed to be that the pilots put the nose down, but didn't wait sufficiently long for airspeed to build up - the nose needs to be a long way down for this to happen.

How did things happen in your PTT incidents? Did it take a long time to transition from pitched up to nose down and flying speed? Were you worried about spinning?

regards

Smith

Roger said...

You're right, it's critical to be at a safe flying speed before starting the turn, or you risk stalling and spinning. It's a bit different between winch launching where the nose is sharply up, and aerotowing where the nose is nearly level. In either case it's not enough to simply nose over, you need to ensure the airspeed is actually up. Nosing over takes time, and airspeed can decay before it picks up again, depending on how steeply you do it... and how quickly you react. You need to check the ASI as well as other sensations of speed such as the noise level.

Our instructors are very good about teaching a sequence to dealing with a PTT: ensure flying speed, decide the best way to turn, ensure that the direction of turn is clear, and THEN turn. Students seem to be in a hurry to get turned around, afraid of being too low to get back. But once you do get turned around and line up your approach, more often than not you have too much altitude and need to slip it away. So it's OK to take a couple of seconds to ensure a safe turn. And our CFIs do pull the rope from time to time even on private pilots, to ensure they stay in practice.

We (members and CFIs) have had endless discussions about the safe altitude at which to begin pitching up when winch or autotowing. If you pitch up very early, and then have a PTT and need to nose over, you may not have enough altitude to regain flying speed and level out - you may fly into the ground. If you go up gently to 100-150 feet and THEN rotate aggressively, you have enough altitude to recover from any PTT, pull out, and land straight ahead. So we're fairly conservative about starting the climb, which means we may get 100-200 feet less than is technically possible.

But as we saw with the lighter ship, you only have a few seconds before you overrun the line, so you do have to start climbing, if gently, very soon after liftoff. IMHO, launching a lighter ship requires better timing and more accurate pitch control than launching a heavier one. I haven't actually timed it, but with some quick math I estimate you get to 60 knots in 3 to 3.5 seconds and need to start gently rotating before that, at the 40-45 knot mark. The heavier ship accelerates a little more slowly, and presents a larger drag to the winch, so you probably have another 1.5 to 2 seconds in a Blanik.

The climb takes 25-30 seconds to get to 1500 feet. The whole thing is over in 30 to 40 seconds.

smith said...

Thanks for the reply Roger. Sounds like a fine line. Your article inspired me to do some research on winching issues. Not only can you get in strife with a PTT, you can also pitch up too steeply early in the winch, stall on the cable and end up spinning into the ground *still on the cable*! Scary.

I watched a few videos of winch launches and they seem hectic to say the least.

Roger said...

I destroyed my first radio control glider that way on a bungee launch (not so different from a winch). Pitched up too steeply, or maybe stalled one wing and spun over. Nose down into the ground at full speed. Nothing at all I could do about it once it went awry. So I've approached winch launching very cautiously!

Anonymous said...

Hi great blog you have here, my club is operating a PW-5 and I have taken about 100 flights by winch in ours. One thing you mention is that you "start pulling back" to try to control the speed when the glider is being towed too fast. At least in my club's Tost Winch this is not possible because the PW-5 will not overload the winch to slow it down, instead I would need to release the stick a bit to reduce pressure and give a signal to slow down "wave wings" and/or release the wire. In our two seaters we can control the speed of the winch with the glider, ee. slow it down by pulling up but the PW-5 is so light that it does not matter. Also I have learned at height above about 900' if you pull too aggressively the hook will auto release. We have had two instances of the glider prematurely releasing in winch launch and in the first case we found out a plastic casing that is around the release cable beneath the seat had become dislodget out of it's socket so the cable was more tensioned than it should. After this happened the second time we taped the casing to the socket so it would not shake loose and that seems to have solved the issue.