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#61
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Winch Launch - Fatal
At 12:09 14 December 2016, Don Johnstone wrote:
At 10:30 14 December 2016, Terry Walsh wrote: At 09:19 14 December 2016, Jim White wrote: At 01:59 14 December 2016, wrote: Don, you brought up the old pitch-up myth so I'll go there. Pitch-up on acceleration can only happen with the tailwheel in the ai and = that can only happen if the glider is slowly dragged into the air with feeb= le acceleration and THEN strong acceleration applied. It can't happe if s= trong acceleration is applied at the very start of the ground roll and main= tained until the glider reaches the normal airspeed for rotating int the c= limb. I could go on at length about this but I wrote a long paper on it available= here https://dl.dropboxusercontent.com/u/...cceleration.pd Rather = than take my word for it, I very strongly suggest the BGA and other nationa= l clubs do their own investigation using tension loggers. What you will se= e may terrify you. Hi Bill. If you follow URAS you will see that Don, whilst old and wise doesn't speak for the UK gliding community. Hi Bill, The theory may be correct but I can assure you that a K18 ALWAYS needs th application of almost full forward stck after lift off to prevent ove rotation. This is using a powerful V8 winch with about 2 seconds of groun roll. Yes the accleration is still happening after lift off. With a narro steel drum throttle response is never instantaneous due to inertia of th drum, even with the powerful (300 hp+) V8. I believe there is a solution to this, one which no one will like, but which would reduce the frequency of the accidents, unfortunately it may also very expensive and difficult to achieve on existing aircraft and would result in a reduction in winch launch height. Move the launch hook forward. I've picked this one up rather late, and probably like Don, I have many thousands of winch launches, in vintage gliders, modern gliders, gliders with compromise hooks, and with C of G hooks, initially on Tost, not exactly underpowered, and for the last 20 years on Skylaunch, certainly not underpowered. To comment on something Bill said, I have analysed quite a few, from 1 sec logger traces (perhaps not the most accurate way, but all I've got), and find initial acceleration to be nearer .6g/.7g, rather than 1g. Tost weak links are approximately 1.3 times auw, so should take 1g , at least when new. The problem with the wing going down is I think, that the pilot can be unaware. A long time ago I had a 19m Jantar, with a compromise well forward hook, and a loop of cord attached to the release toggle, because you could not reach it. One day I took a launch, when I got back a few hours later, a fellow instructor asked if I knew that the wing had gone down, so hard that everyone at the launch point turned to look. I probably had 1500 hours on that glider alone, and quite a few hundred launches in it, but hadn't a clue anything had happened. To me it was a normal launch, and I got away with it. I totally agree though that high acceleration is good, the opposite is not. Again, years ago, probably in the Jantar again, I had a very slow ground run, but got away. When I got back, I found that a subsequent launch in an ASW27, had had a wing down accident, substantial damage, fortunately no injury. Though not there to see it, I always put that down to a slow ground run like mine. Most of the early Schleicher gliders, K8, K18, K6, do need a forward stick position at the start, unlike most others, when neutral is best. The glider will aviate when ready, it doesn't need encouraging by the pilot. Winch launching is not inherently dangerous, any more than walking down the street is dangerous, unless something goes wrong. Not sure without checking how many are done in the UK each year without incident, but it runs into hundreds of thousands. Just a very few go wrong. What I would like to see is a re-think on launch speeds. High speed is safe, low speed is not. Yet we have ridiculous situations like the Puchacz, which I think is otherwise the best training glider available, with a maximum of 59 kts, yet not really safe on a winch launch below 55 kts. This is a fully aerobatic, inverted flight glider!!! With the Skylaunch, perhaps others, the throttle stop is set to suit the wind and the glider. The driver then advances the throttle lever to that stop over a second or two. I would like to see a completely computerised system, whereby the wind profile and the glider are selected, and the driver just presses a button to launch, and all the parameters are built in, including initial acceleration, surely not that difficult with modern technology. Obviously there must be a dead man's handle. Dave |
#62
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Winch Launch - Fatal
On Tuesday, December 6, 2016 at 6:15:05 AM UTC-8, Jock Proudfoot wrote:
When I started flying a 19meter Kestrel years ago (on aerotow) I quickly found I could not reliably keep my wings level at the start of the aerotow launch if I looked straight ahead, because the ailerons were so inneffective at low speed. So when we started to move, I looked directly left or right towards one wing, for the first few seconds. That made it much easier to keep the wings level, although it usually needed full aileron inputs initially.. My tailwheel Kestrel would initially roll straight ahead wth the tailwheel on the ground, so there was no need to look ahead. It just needed a few seconds to gain reasonable aileron effectiveness, then I could look ahead again. |
#63
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Winch Launch - Fatal
On Thursday, December 15, 2016 at 10:30:28 AM UTC-7, wrote:
On Tuesday, December 6, 2016 at 6:15:05 AM UTC-8, Jock Proudfoot wrote: When I started flying a 19meter Kestrel years ago (on aerotow) I quickly found I could not reliably keep my wings level at the start of the aerotow launch if I looked straight ahead, because the ailerons were so inneffective at low speed. So when we started to move, I looked directly left or right towards one wing, for the first few seconds. That made it much easier to keep the wings level, although it usually needed full aileron inputs initially. My tailwheel Kestrel would initially roll straight ahead wth the tailwheel on the ground, so there was no need to look ahead. It just needed a few seconds to gain reasonable aileron effectiveness, then I could look ahead again. OK, now that we're off on tangents, here's another. I once flew with a pilot checking out on winch launch who always applied full-right aileron as soon as the acceleration set in - often to the point the right tip nearly dragged. I asked why he was doing that. He replied that he "felt" the left wing was dropping. Video of the takeoff showed the wings level until he slammed the stick over to the right. He was baffled still insisting he "felt" a strong left wing drop. Eventually, an AME found this individual had a left-right asymmetry in his vestibular organs which, apparently, caused him to experience a sensation of roll under linear acceleration. This story shows why one must use visual cues and not "feel" to keep the wings level. It also shows the value of making videos of takeoffs. |
#64
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Winch Launch - Fatal
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#65
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Winch Launch - Fatal
On Sunday, December 18, 2016 at 9:45:10 PM UTC-7, B4soaring wrote:
Not really. Where you go wrong is assuming that the tailwheel is the centre of rotation - it isn't, it's an arm hitting a limit. Same difference. When an arm hits a limit, the center of rotation shifts to the contact point. |
#66
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Winch Launch - Fatal
On Mon, 19 Dec 2016 12:11:35 -0800, Bill.Daniels wrote:
On Sunday, December 18, 2016 at 9:45:10 PM UTC-7, B4soaring wrote: Not really. Where you go wrong is assuming that the tailwheel is the centre of rotation - it isn't, it's an arm hitting a limit. Same difference. When an arm hits a limit, the center of rotation shifts to the contact point. Here's a different approach to winch training. I learnt to fly on a winch (Supacat, with a 240 hp high-torque diesel V8 providing the grunt) initially in an ASK-21 and a G103 with an Puchacz added to the mix on my 20th launch. Pre-solo, the only aero tows I had were flight zero (the trial flight in Front Royale's ASK-21 that set the gliding hook) and flight 25 in the Puchacz for spin demos. The rest of my pre-solo spin training was off the winch followed by thermal climbs to a safe height. It was a good, thermally summer. But I digress. The way I was taught to launch nosewheel gliders on a winch was to start with enough back stick to raise the nose-wheel after a few metres and, once it was off, to ease the stick forward to balance on the main wheel until the glider lifts off. Putting the tail wheel down was frowned on as being an unnecessarily violent rotation. The same style works just as well for those gliders on the rather more powerful Skylaunch that replaced the Supacat. Similarly, in dry taildraggers I aim to lift off in the ground roll attitude and maintain that attitude until the airspeed passes 50 kts with definite acceleration. After that the glider is rotated into the full climb attitude: no or poor acceleration means don't rotate. -- martin@ | Martin Gregorie gregorie. | Essex, UK org | |
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