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#21
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How to get maximum height on a winch launch?
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#22
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How to get maximum height on a winch launch?
Dan G wrote: Dan G wrote: how should the winch driver feed the power in? I received a very detailed reply in the Yahoo winch design group. With permission, I reproduce it below: Interesting ... but once more, beware of rules. Lots of factors can make differences to technique: whether the drive is direct, fluid flywheel or torque convertor, size of drums (and consequently inertia) and turbocharged vs normally aspirated. Ian |
#23
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How to get maximum height on a winch launch?
Ian wrote:
Dan G wrote: Should say that it's "full power all the way up" the main climb, of course we back off at the top. But there was some suprise at the launch point recently when a K21 waved off... the common thought was "K21s should be able to handle it". Maybe it could if it was no wind, but with a good headwind they just can't. And that was a surprise? Find yourself another club, man, because if headwinds on a winch launch are unexpected your current lot are going to damage someone one day. Easy now, my club has a long safety record any would be proud of. AFAIK there's never been a winch launch accident. In that particular example there was a very strong wind gradient. I was using as an illustration of the belief that K21s can take everything the winch gives. So far the sole answer to my original question has been "look at the wingtips during the launch and find what stick position will hold them at about 45 degrees to the horizon AND give a safe speed. If you have a low powered winch you may have to adopt a lesser angle" from Derek. Does everyone agree with this? No. I think it's bad advice. Yes, you should aim to have a proper climb angle and air speed, but the stick position is irrelevant. Just use enough to do what's needed. Well, "find what stick position will hold them at about 45 degrees to the horizon AND give a safe speed" and "you should aim to have a proper climb angle and air speed" seems to be the same thing... Dan |
#24
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How to get maximum height on a winch launch?
I have very little practical knowledge of winching and I
still do not understand what the theoretical factors are that determine max launch height. Neglecting wind speed and wind gradient for the moment, as well as safety concerns relating to rope breaks down low, what is the technique to max out height given the changing climb and tow line angles and geometry? Derek Piggot writes in his book "Ground Launches , 1996, pg 34.): “The optimum speed on the launch depends to some extent on the wind strength with a slightly higher speed giving the best launch height in calm conditions. Best L/D speed is close to the optimum, with the minimum cruising speed or speed for minimum sink being the lowest safe speed.” Derek Piggott, Ground Launches , 1996, pg 34. If the speed for best L/D is close to optimum, what is optimum? The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says the following about auto tow “The factor which determines the towing speed of the automobile is the indicated air speed (IAS) of the sailplane. It has been shown that the launching forces on the sailplane are at a minimum just below the IAS for maximum L/D of the sailplane” Just how far below the best L/D IAS is the point of minimum stress on the airframe? Once we have derived the factors to max out launch height, how do you then balance efficiency versus climb rate after that point in the launch when you have enough altitude for a safe recovery from a low rope break? Pete -- Peter D. Brown http://home.gci.net/~pdb/ http://groups.yahoo.com/group/akmtnsoaring/ |
#25
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How to get maximum height on a winch launch?
Pete Brown wrote:
Derek Piggot writes in his book "Ground Launches , 1996, pg 34.): “The optimum speed on the launch depends to some extent on the wind strength with a slightly higher speed giving the best launch height in calm conditions. Best L/D speed is close to the optimum, with the minimum cruising speed or speed for minimum sink being the lowest safe speed.” Derek Piggott, Ground Launches , 1996, pg 34. If the speed for best L/D is close to optimum, what is optimum? The optimum is the best glide speed at the wing effective load factor based on the takeoff weight of the glider and the cable tension. For example, for an ASK-21 with a best L/D speed of 49 knots at a weight of 1300 lbs, if the cable tension is 1900 lbs, the effective load factor is 1950/1300 or 1.5, the optimum launch speed is 49*SQRT(1.5) or 60 knots IAS. The American Soaring Handbook, Vol 3, Ground Launch, 1980, pg 32 says the following about auto tow “The factor which determines the towing speed of the automobile is the indicated air speed (IAS) of the sailplane. It has been shown that the launching forces on the sailplane are at a minimum just below the IAS for maximum L/D of the sailplane” Just how far below the best L/D IAS is the point of minimum stress on the airframe? What they appear to be discussing here is a rule of thumb for automobile speed based on the non-adjusted best glide IAS of the glider. Given a nominal climb angle of 45 degrees, if the automobile is driving at 50 mph (44 knots), the glider will be flying at 62 knots TAS. So, in this case, once the glider has rotated to climb angle you'd want to drive 6 or 7 MPH slower than the gliders best L/D speed at sea level density altitude. If the density altitude is higher, you'd want to drive faster. Once we have derived the factors to max out launch height, how do you then balance efficiency versus climb rate after that point in the launch when you have enough altitude for a safe recovery from a low rope break? Assuming the winch (or automobile) is providing a constant cable tension, the max launch height will be pretty close to optimal if the pilot maintains the optimum launch speed IAS as calculated above. This should result in a climb angle for most of the launch of about 45 degrees. Marc |
#26
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How to get maximum height on a winch launch?
Dan G wrote: Ian wrote: And that was a surprise? Find yourself another club, man, because if headwinds on a winch launch are unexpected your current lot are going to damage someone one day. Easy now, my club has a long safety record any would be proud of. AFAIK there's never been a winch launch accident. In that particular example there was a very strong wind gradient. I was using as an illustration of the belief that K21s can take everything the winch gives. I am glad to hear about the safety record. I only hope they can keep it. No. I think it's bad advice. Yes, you should aim to have a proper climb angle and air speed, but the stick position is irrelevant. Just use enough to do what's needed. Well, "find what stick position will hold them at about 45 degrees to the horizon AND give a safe speed" and "you should aim to have a proper climb angle and air speed" seems to be the same thing... No, they are completely different. Stick position is irrelevant. All that matters is whether you need to move it. Let me give a simple example. The exact position of the temperature lever on a shower doesn't matter, does it? If it's too hot you move it "cold" a bit and if it's too cold you move it "hot" a bit. You don't say "I'll have a position 5 shower, no matter how hot that is". Same with the stick. Ian |
#27
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How to get maximum height on a winch launch?
I thought that was exactly the point I was trying to
make! Stick position is type dependent. Where you hold the stick and how much you have to pull back, or push forward, may be different to what you are used to, if you fly a different type of glider. It depends on the hook position, the exact c of g and centre of pressure, and the trim position. The advice I gave was to hold the stick in a position that gives the optimum angle of climb for your type of winch. Full stop, period! Real life winch launching takes place using different types of gliders in differing wind strengths. No one throttle setting can cover all cases. If you launched a slow glider (e.g. a K8) into a 45 knot headwind, you could stop the winch altogether once it has got airborne and allow it to launch like a kite. The wind would be providing the energy while the winch would just act as an anchor point on the ground. Conversely launching a heavy two seater on a still day with a moderately powerful winch might require absolutely full throttle. Derek Copeland At 18:12 16 December 2006, Ian wrote: Dan G wrote: Ian wrote: And that was a surprise? Find yourself another club, man, because if headwinds on a winch launch are unexpected your current lot are going to damage someone one day. Easy now, my club has a long safety record any would be proud of. AFAIK there's never been a winch launch accident. In that particular example there was a very strong wind gradient. I was using as an illustration of the belief that K21s can take everything the winch gives. I am glad to hear about the safety record. I only hope they can keep it. No. I think it's bad advice. Yes, you should aim to have a proper climb angle and air speed, but the stick position is irrelevant. Just use enough to do what's needed. Well, 'find what stick position will hold them at about 45 degrees to the horizon AND give a safe speed' and 'you should aim to have a proper climb angle and air speed' seems to be the same thing... No, they are completely different. Stick position is irrelevant. All that matters is whether you need to move it. Let me give a simple example. The exact position of the temperature lever on a shower doesn't matter, does it? If it's too hot you move it 'cold' a bit and if it's too cold you move it 'hot' a bit. You don't say 'I'll have a position 5 shower, no matter how hot that is'. Same with the stick. Ian |
#28
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How to get maximum height on a winch launch?
On Saturday, December 16, 2006 at 3:33:08 PM UTC-8, Derek Copeland wrote:
I thought that was exactly the point I was trying to make! Stick position is type dependent. Where you hold the stick and how much you have to pull back, or push forward, may be different to what you are used to, if you fly a different type of glider. It depends on the hook position, the exact c of g and centre of pressure, and the trim position. The advice I gave was to hold the stick in a position that gives the optimum angle of climb for your type of winch. Full stop, period! Real life winch launching takes place using different types of gliders in differing wind strengths. No one throttle setting can cover all cases. If you launched a slow glider (e.g. a K8) into a 45 knot headwind, you could stop the winch altogether once it has got airborne and allow it to launch like a kite. The wind would be providing the energy while the winch would just act as an anchor point on the ground. Conversely launching a heavy two seater on a still day with a moderately powerful winch might require absolutely full throttle. Derek Copeland At 18:12 16 December 2006, Ian wrote: Dan G wrote: Ian wrote: And that was a surprise? Find yourself another club, man, because if headwinds on a winch launch are unexpected your current lot are going to damage someone one day. Easy now, my club has a long safety record any would be proud of. AFAIK there's never been a winch launch accident. In that particular example there was a very strong wind gradient. I was using as an illustration of the belief that K21s can take everything the winch gives. I am glad to hear about the safety record. I only hope they can keep it. No. I think it's bad advice. Yes, you should aim to have a proper climb angle and air speed, but the stick position is irrelevant. Just use enough to do what's needed. Well, 'find what stick position will hold them at about 45 degrees to the horizon AND give a safe speed' and 'you should aim to have a proper climb angle and air speed' seems to be the same thing... No, they are completely different. Stick position is irrelevant. All that matters is whether you need to move it. Let me give a simple example. The exact position of the temperature lever on a shower doesn't matter, does it? If it's too hot you move it 'cold' a bit and if it's too cold you move it 'hot' a bit. You don't say 'I'll have a position 5 shower, no matter how hot that is'. Same with the stick. Ian looking for a used winch or plans for making one. thank you ray |
#29
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How to get maximum height on a winch launch?
looking for a used winch or plans for making one. thank you ray Ray - join the Yahoo forum 'Winchdesign'. There you will find probably the most concentrated winch related knowledge in the world. Where in the world are you located? There are a good number of winches for sale on the German server: http://www.segelflug.de/osclass/inde...&sCategory=107 Uli Winchmeister of the Carolina Soaring Association |
#30
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How to get maximum height on a winch launch?
At 23:33 16 December 2006, Derek Copeland wrote:
I thought that was exactly the point I was trying to make! Stick position is type dependent. Where you hold the stick and how much you have to pull back, or push forward, may be different to what you are used to, if you fly a different type of glider. It depends on the hook position, the exact c of g and centre of pressure, and the trim position. The advice I gave was to hold the stick in a position that gives the optimum angle of climb for your type of winch. Full stop, period! Real life winch launching takes place using different types of gliders in differing wind strengths. No one throttle setting can cover all cases. If you launched a slow glider (e.g. a K8) into a 45 knot headwind, you could stop the winch altogether once it has got airborne and allow it to launch like a kite. The wind would be providing the energy while the winch would just act as an anchor point on the ground. Conversely launching a heavy two seater on a still day with a moderately powerful winch might require absolutely full throttle. Derek Copeland Yep, what he said. If you take a winch launch with a really skilled winch driver you will experience a constant speed all the way up the launch. How is that done? There are a combination of things that can give clues to the winch driver, the bow in the cable, if it is wire, does not work so well with dynema/sky rope. Marked throttle settings as on a Skylaunch winch can help but it really is a matter of feel and experience. For most glass gliders 55-65kts seems to work best. I always reduce power at the top, back to idle once the glider gets near to the max angle, instigating a back release, this avoids the possibility of a loop if the cable is released under tension. It is absolutely essential to ensure that the power is cut, before the glider is vertically over the winch in nil/very low headwind conditions, it is very unpleasant to have 1000ftl of cable drop on top of the winch, it it can take a bit of time to get it off as well. |
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