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#111
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I don't think the positition of the horizon should
make any difference. In mountains it's irrelevant and different tugs will climb at different rates - a powerful tug will be more pitched up and adopt a higher position relative to the horizon than a low power one (relative to the thrust line, the prop-wash and the best position of the glider just above the prop-wash). One of the advantages of being as low as possible is not just the extra time that it takes to get to 'upset' position (which is small) but the fact that you are less likely to lose sight of the tug in the first place. Once you can't see the tug, things can go wrong very quickly. I've flown in Australia with their low tow and whilst I'm sure each method has its merits, I am personally much more comfortable with the UK position, don't like having to transition through the propwash at low level and find that teaching a 'correct' position that looks almost identical to the position on the ground before All Out is easier too. Rob At 16:48 09 January 2004, Eric Greenwell wrote: Andy Durbin wrote: As a US instructor I have flown with many pilots that received their initial training from other instructors. I have often been surprised at the tow position taken by such pilots. I usually urge them to keep lowering the tow position until they feel the wake, then to move just high enough that the wake is not a factor. That tow position may be 10 or 15 ft lower than that initially used. Where is the towplane, relative to the horizon? A pilot flying a high high tow has less time to react to a potential upset than one flying the UK recommended low high tow. The fact that US pilots seem to be trained to use high high tow may explain why tug upsets continue to happen even when a nose or forward hook is used. 10 or 15 feet doesn't sound like it would give much extra time, not like the low tow position Australia uses. -- ----- change 'netto' to 'net' to email me directly Eric Greenwell Washington State USA |
#112
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In article ,
Ian Strachan wrote: In article 3ffee053$1@darkstar, Mark James Boyd writes snip If anyone decides to modify their experimental glider to be towed tail first on a tailhook, I guess we have two volunteers to be the tow pilots... The main question is, in how many seconds would it be being towed tail-first ...... Another question would be insurance, but we are in "reductio ad absurdum" land, here, aren't we? -- Ian Strachan Lasham, UK Ian, Exactly right. So if we can all agree that "given the appropriate level of experience and skill, a pilot can fly any flyable aircraft in any conditions," we can avoid this obvious truth and focus on the relative risks and costs (including the cost of acquiring training and experience). |
#113
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The reson, I think, is that the Pirat has quite a high-set wing and a belly hook as low as it can get. As a result, the lever arm, and therefore upwards pitching moment from the winch cable about the wing is relatively high compared to the typical glass glider with mid-set wing and belly hook offset and up a bit. So with a good fast winch launch, yes, this moment can overcome anything the elevator is trying to do. However, this is an intrinsically stable situation. As the cable angle relative to the glider's longitudinal axis increases, the lever arm reduces (by approximately 50% when the glider is 30 degree nose up and the cable is 30 degrees down). So as the glider pitches up the effect of the cable reduces, the effect of the elevator (all other things being equal) stays the same and a point of equilibrium is reached. To put it simply: I think you forgot to account for the increasing load that pitching up produces. Sure, the lever arm is reduced, but the glider is at a higher angle of attack, increasing it's lift. This increases the force much more than the lever arm is reduced, and the pitch up continues. This eventually stabilizes with the glider in the normal nose high attitude of a winch launch, but this is not a good attitude for aerotowing! For a nose hook, the lever arm is much less to begin with, and a small pitch up reduces it to zero - quite a different situation. -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#114
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Robert John wrote:
I don't think the positition of the horizon should make any difference. It's the position I typically use, so I'm trying to get an idea of how different it is from what I normally do. Unfortunately, I can't go out and experiment right now, with a foot of snow on the runway! So, with a 180 hp Super Cub or Pawnee, say, is the tug canopy on the horizon, the wing root, wheels, or maybe the tug is an entire "tug" distance above the horizon? In mountains it's irrelevant and different tugs will climb at different rates - a powerful tug will be more pitched up and adopt a higher position relative to the horizon than a low power one (relative to the thrust line, the prop-wash and the best position of the glider just above the prop-wash). -- ----- change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#115
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As a US instructor I have flown with many pilots that received their initial training from other instructors. I have often been surprised at the tow position taken by such pilots. I usually urge them to keep lowering the tow position until they feel the wake, then to move just high enough that the wake is not a factor. I also agree with Andy that the optimal tow location is just above the wake. I like it because it's more efficient for the tow plane since less up elevator is required to maintain proper airspeed. In the flat lands, if the tug wings are on or above the horizon, you're too high, IMHO. I've also been known to fly a little to the left on tow to give the tow pilot's right leg a little rest. Tony V. |
#116
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In article 3ffee053$1@darkstar, Mark James Boyd
writes snip If anyone decides to modify their experimental glider to be towed tail first on a tailhook, I guess we have two volunteers to be the tow pilots... The main question is, in how many seconds would it be being towed tail-first ...... Another question would be insurance, but we are in "reductio ad absurdum" land, here, aren't we? -- Ian Strachan Lasham, UK |
#117
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Tony Verhulst wrote in message ...
In the flat lands, if the tug wings are on or above the horizon, you're too high, IMHO. I think you got something reversed. The lower the glider goes, the higher above the horizon the tug will appear to be. When I tow behind a 235 Pawnee all the tug is above the horizon. Andy |
#118
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if you are towing that low out here in the summer time... you are going to
find yourself in the wake from time to time.. and being in the wake increases the drag on the tow plane and reduces the climb rate and increases the down pull, forcing the tow pilot to push on the stick.. we "train".. tow plane on the horizon.. granted there are hills/mountains all around so the horizon is a "relative" term... keep all 3 wheels of the tow on the same horizontal line.. the tail wheel between the mains.. works just fine for the Pawnee 235 if you are low.. "to just above the wake".. it is more difficult for the tow pilot to see you in the mirrors.. and he may think your going down to box the wake. Also, with summer thermals.. the tow plane enters the thermal 200ft before the glider, and a 400fpm rate of climb jump to better than 1000fpm for the tow plane.. and you are in the wake.. granted.. in a couple of seconds or so.. you'll be in the thermal and going up just as the tow plane exits it and hits the sink it's hard enough fighting the thermal drafts.. but to add an unplanned excursion into the wake? a couple of those.. and at 2K AGL.. I'm off... BT "Robert John" wrote in message ... I don't think the positition of the horizon should make any difference. In mountains it's irrelevant and different tugs will climb at different rates - a powerful tug will be more pitched up and adopt a higher position relative to the horizon than a low power one (relative to the thrust line, the prop-wash and the best position of the glider just above the prop-wash). One of the advantages of being as low as possible is not just the extra time that it takes to get to 'upset' position (which is small) but the fact that you are less likely to lose sight of the tug in the first place. Once you can't see the tug, things can go wrong very quickly. I've flown in Australia with their low tow and whilst I'm sure each method has its merits, I am personally much more comfortable with the UK position, don't like having to transition through the propwash at low level and find that teaching a 'correct' position that looks almost identical to the position on the ground before All Out is easier too. Rob At 16:48 09 January 2004, Eric Greenwell wrote: Andy Durbin wrote: As a US instructor I have flown with many pilots that received their initial training from other instructors. I have often been surprised at the tow position taken by such pilots. I usually urge them to keep lowering the tow position until they feel the wake, then to move just high enough that the wake is not a factor. That tow position may be 10 or 15 ft lower than that initially used. Where is the towplane, relative to the horizon? A pilot flying a high high tow has less time to react to a potential upset than one flying the UK recommended low high tow. The fact that US pilots seem to be trained to use high high tow may explain why tug upsets continue to happen even when a nose or forward hook is used. 10 or 15 feet doesn't sound like it would give much extra time, not like the low tow position Australia uses. -- ----- change 'netto' to 'net' to email me directly Eric Greenwell Washington State USA |
#119
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With the supercubs and Robin DR400s at LGC the tug
wheels are generally on or a little above the horizon. I wouldn't generally be so low as to be catching the wake though in strong thermal conditions I can see that you might. If I tow through a strong thermal I'm going to release anyway! Rob At 18:00 09 January 2004, Eric Greenwell wrote: Robert John wrote: I don't think the positition of the horizon should make any difference. It's the position I typically use, so I'm trying to get an idea of how different it is from what I normally do. Unfortunately, I can't go out and experiment right now, with a foot of snow on the runway! So, with a 180 hp Super Cub or Pawnee, say, is the tug canopy on the horizon, the wing root, wheels, or maybe the tug is an entire 'tug' distance above the horizon? In mountains it's irrelevant and different tugs will climb at different rates - a powerful tug will be more pitched up and adopt a higher position relative to the horizon than a low power one (relative to the thrust line, the prop-wash and the best position of the glider just above the prop-wash). -- ----- change 'netto' to 'net' to email me directly Eric Greenwell Washington State USA |
#120
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Eric Greenwell wrote in message ...
I think you forgot to account for the increasing load that pitching up produces. Sure, the lever arm is reduced, but the glider is at a higher angle of attack, increasing it's lift. This increases the force much more than the lever arm is reduced, and the pitch up continues. Good point. I have along train journey today, and will have a ponder on this. I suspect the outcome will be along the lines of "the glider has stick fixed stability in free flight anyway: the effect of the added winch cable force is to increase this stability". This eventually stabilizes with the glider in the normal nose high attitude of a winch launch, but this is not a good attitude for aerotowing! For a nose hook, the lever arm is much less to begin with, and a small pitch up reduces it to zero - quite a different situation. Agreed completely. Ian |
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