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#111
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SAFE Winch Launching and automatic gearboxes
At 11:49 20 July 2009, brian whatcott wrote:
There is another factor worth considering in the matter of launches. An automatic gearbox / transmission is intended to transfer as much horse power as possible from engine to axle. Maximizing HP in this way involves a LARGE torque at slow speed, tapering to a low torque at high speed. You have mentioned that the objective of a glider launch is holding the tension constant. This is a different objective altogether! This calls for a ramped HP, so that either a ramped throttle application to achieve flight speed is needed, at which point that HP is held - or as a possible alternative: running an engine at constant (max) HP, but throwing away excess power during the ramp to flight speed. Brian W I don't think it's quite as simple as that. You need a lot of torque and power to accelerate the glider up to flying speed, then a slight reduction to allow for the safety climb and rotation and then lots of torque and power again for the full climb. As the glider nears the top of the launch, the climb flattens out and you need less power and torque. The cable speed reaches a maximum as the glider starts the rotation and then drops quite a lot as the glider enters the full climb, due to the water-skier effect, and then continues to drop thoughout the rest of the launch. The water-skier effect means in effect that the vertical speed is being added to the horizontal speed, so that the maximum winch launching airspeed would be vastly exceeeded if the cable speed is not reduced in some way. BTW, the water-skier effect is used by water skiers themselves, only in the horizontal axis, to gain speed for jumps and other tricks while the ski boat continues on at a constant speed. Oddly enough, the Skylaunch throttle stop system and automatic gearbox seems to automatically compensate for all the above effects, except that you have to back the throttle off near the top of the launch to avoid overspeeding the glider. Derek Copeland |
#112
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SAFE Winch Launching and automatic gearboxes
At 11:50 20 July 2009, brian whatcott wrote:
Don Johnstone wrote: ... a "tension controlled" winch is about the craziest idea I have ever heard, not least because, thankfully, it would never work. Why not? Brian W Because there is no direct relationship between the tension experienced at the winch and that experienced at the glider release using wire rope. Even with plastic rope the relationship is tenuous at best, the rope does have some mass and in addition has an elastic quality as well. The tensions experienced at each end of the cable can be vastly different and are different more often than they are equal, so measuring at the winch end tells you very little about what is happening at the glider, it may indicate what has happened but even this is not likely to be very accurate. |
#113
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SAFE Winch Launching and automatic gearboxes
I don't think it's quite as simple as that. You need a lot of torque and power to accelerate the glider up to flying speed, Lets see, you have an engine from a 7000 pound road vehicle and you are accelerating a 1,500 pound glider without even the possibility of wheel spin and you claim you need "at lot of torque"? That doesn't even pass a smell test. 1st and 2nd gears can/will develop so much torque at the drum you'll break the weak link or worse. Simple arithmetic shows the GM big block will develop 3000 pounds or more of rope tension in 3rd gear. 1st and 2nd gears are just a nuisance requiring the winch operator to gently advance the throttle to avoid trouble. That's just one of MANY issues related to using automotive components in a glider winch. then a slight reduction to allow for the safety climb and rotation and then lots of torque and power again for the full climb. As the glider nears the top of the launch, the climb flattens out and you need less power and torque. The cable speed reaches a maximum as the glider starts the rotation and then drops quite a lot as the glider enters the full climb, due to the water-skier effect, and then continues to drop thoughout the rest of the launch. The water-skier effect means in effect that the vertical speed is being added to the horizontal speed, so that the maximum winch launching airspeed would be vastly exceeeded if the cable speed is not reduced in some way. BTW, the water-skier effect is used by water skiers themselves, only in the horizontal axis, to gain speed for jumps and other tricks while the ski boat continues on at a constant speed. Oddly enough, the Skylaunch throttle stop system and automatic gearbox seems to automatically compensate for all the above effects, except *that you have to back the throttle off near the top of the launch to avoid overspeeding the glider. Derek Copeland * |
#114
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SAFE Winch Launching and automatic gearboxes
So how come do we hardly ever break weak links during the ground run????
Derek C At 16:19 20 July 2009, bildan wrote: I don't think it's quite as simple as that. You need a lot of torque and power to accelerate the glider up to flying speed, Lets see, you have an engine from a 7000 pound road vehicle and you are accelerating a 1,500 pound glider without even the possibility of wheel spin and you claim you need "at lot of torque"? That doesn't even pass a smell test. 1st and 2nd gears can/will develop so much torque at the drum you'll break the weak link or worse. Simple arithmetic shows the GM big block will develop 3000 pounds or more of rope tension in 3rd gear. 1st and 2nd gears are just a nuisance requiring the winch operator to gently advance the throttle to avoid trouble. That's just one of MANY issues related to using automotive components in a glider winch. then a slight reduction to allow for the safety climb and rotation and then lots of torque and power again for the full climb. As the glider nears the top of the launch= , the climb flattens out and you need less power and torque. The cable spee= d reaches a maximum as the glider starts the rotation and then drops quite = a lot as the glider enters the full climb, due to the water-skier effect, a= nd then continues to drop thoughout the rest of the launch. The water-skier effect means in effect that the vertical speed is being added to the horizontal speed, so that the maximum winch launching airspeed would be vastly exceeeded if the cable speed is not reduced in some way. BTW, the water-skier effect is used by water skiers themselves, only in the horizontal axis, to gain speed for jumps and other tricks while the ski boat continues on at a constant speed. Oddly enough, the Skylaunch throttle stop system and automatic gearbox seems to automatically compensate for all the above effects, except =A0th= at you have to back the throttle off near the top of the launch to avoid overspeeding the glider. Derek Copeland =A0 |
#115
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SAFE Winch Launching and automatic gearboxes
Del C wrote:
.... You need a lot of torque and power to accelerate the glider up to flying speed, then a slight reduction to allow for the safety climb and rotation and then lots of torque and power again for the full climb.... Derek Copeland Your note quoted above seems to treat torque (or thrust, or tension) and HP as synonymous. They are not. Somebody with a physics background could provide helpful input here.... Brian W |
#116
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SAFE Winch Launching and automatic gearboxes
Don Johnstone wrote:
At 11:50 20 July 2009, brian whatcott wrote: Don Johnstone wrote: ... a "tension controlled" winch is about the craziest idea I have ever heard, not least because, thankfully, it would never work. Why not? Brian W Because there is no direct relationship between the tension experienced at the winch and that experienced at the glider release using wire rope. Even with plastic rope the relationship is tenuous at best, the rope does have some mass and in addition has an elastic quality as well. The tensions experienced at each end of the cable can be vastly different and are different more often than they are equal, so measuring at the winch end tells you very little about what is happening at the glider, it may indicate what has happened but even this is not likely to be very accurate. That sounds like an interesting reply: in a comparable situation, a kite can only carry so much line aloft, and the tension at the kite is certainly at a different angle to that oseen by the kite flyer. These days, a wireless sensor could be provided to measure and relay the tension at the sailplane - but I think I am hearing that a constant tension is not in fact wanted??? Brian W |
#117
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SAFE Winch Launching and automatic gearboxes
On Jul 20, 11:28*am, brian whatcott wrote:
Don Johnstone wrote: At 11:50 20 July 2009, brian whatcott wrote: Don Johnstone wrote: ... *a "tension controlled" winch is about the craziest idea I have ever heard, not least because, thankfully, it would never work. Why not? Brian W Because there is no direct relationship between the tension experienced at the winch and that experienced at the glider release using wire rope. Even with plastic rope the relationship is tenuous at best, the rope does have some mass and in addition has an elastic quality as well. The tensions experienced at each end of the cable can be vastly different and are different more often than they are equal, so measuring at the winch end tells you very little about what is happening at the glider, it may indicate what has happened but even this is not likely to be very accurate. That sounds like an interesting reply: in a comparable situation, a kite can only carry so much line aloft, and the tension at the kite is certainly at a different angle to that oseen by the kite flyer. These days, a wireless sensor could be provided to measure and relay the tension at the sailplane - but I think I am hearing that a constant tension is not in fact wanted??? Brian W Constant tension during the climb phase is exactly what you want. Read George Moore's article in this month's Soaring Magazine. Tension telemetry is a great idea - someone please build it. A kite string or a winch rope forms a catenary arc due to it's weight and air drag. The tension on each end of a catenary arc is the same except for the rope/string weight difference if the ends are at different heights. If 2000 feet of Plasma rope were hanging vertically the tension due to its weight is zero at the bottom and only 20 pounds at the top - that difference doesn't matter much. |
#118
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SAFE Winch Launching and automatic gearboxes
Brian,
Torque is a measure of turning or rotational force in units such as lbs.ft or equivalent, horsepower is a measure of work done in a given period of time (1hp = 550lb.ft/sec) and tension is a stress that produces an elongation of an elastic physical body, usually from opposing forces. In that you can have torque and tension without any work being done, you really need (horse) power to make a winch launch happen. Derek Copeland At 17:23 20 July 2009, brian whatcott wrote: Del C wrote: .... You need a lot of torque and power to accelerate the glider up to flying speed, then a slight reduction to allow for the safety climb and rotation and then lots of torque and power again for the full climb.... Derek Copeland Your note quoted above seems to treat torque (or thrust, or tension) and HP as synonymous. They are not. Somebody with a physics background could provide helpful input here.... Brian W |
#119
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SAFE Winch Launching and automatic gearboxes
On Jul 20, 10:45*am, Del C wrote:
So how come do we hardly ever break weak links during the ground run???? Derek C See previous post about "little old lady" style throttle advance. |
#120
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SAFE Winch Launching and automatic gearboxes
At 23:36 20 July 2009, bildan wrote:
On Jul 20, 10:45=A0am, Del C wrote: So how come do we hardly ever break weak links during the ground run???? Derek C See previous post about "little old lady" style throttle advance. The reason we don't break weak links is because we take up slack at just over tickover revs, when the engine is not producing much power or torque. As the glider starts moving (all out) we advance the throttle over about a 2 second interval right up to the preset power stop. The automatic gearbox then does its job and allows smooth acceleration and smooth gearbox upchanges until a safe rotation speed is reached. The rest of the launch is done with the gearbox in top and essentially fixed ratio. The techniques and the way you think an automatic gearbox works, that you have suggested up to now, are more appropriate to drag racing than winch launching! Derek Copeland |
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