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#91
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On Thu, 30 Oct 2003 10:11:18 +0000, Robert Ehrlich
wrote: Bob Johnson wrote: As long as we're sitting around the campfire and also to show you I can go both ways, I 'member a time only a couple of years ago that I experienced the dreaded aerotow line break at 200 ft and 60 kt over the outbound fence. I looked out front and it wasn't too exiting, so I gingerly turned ninety to the left and the scenery looked some better, but not the greatest, so I REALlY gingerly gave it another ninety to the left and was really impressed this time as I found I was perfectly lined up with the takeoff runway. And I recall I hadn't lost too much of my original 60 kt. Will wonders never cease!! I thought "OK now God, you've made it possible for me to do this little magic trick perfectly the first time I tried it in front of all my friends and hangar bums, give me your hand again and let's try just one more." So I pulled spoilers, checked gear down and rolled up to my exact takeoff spot. And as I popped the canopy, my good friend who shall remain nameless, said "S--t!, you've gone and lost us another Tost ring, somebody go back to the hangar and see if they can find another tow rope". It was his turn to tow, so I pushed back. Nobody else said a word. Safety lecture from a dummy follows: I don't remember to this day why I ninetied to the left. During the previous year's biannual when Juan Batch pulled the plug on me over his outbound fence, turning right was the correct choice because in that direction lay the wind, which blows one back over the airport. This improves the scenery like you wouldn't believe. When I tried the trick for real solo, the wind lay to my left. I'd like to think it was instinct. But I believe it was a coin toss. Anyway, thank God. And Juan. It Depends BJ This raises the interesting question of the height loss during a 180 degrees turn in a glider or an airplane with a dead engine. I recently had a dicsussion about that with a friend who is a power pilot and on this occasion made again a small computation I had already made on this matter. As I never have seen these results elsewhere, I think it may useful to show that here. Assume you fly your turn wit an angle of attack which correspond to the speed V when flying straight and wings level, and that the vertical sink speed in the same conditions wuold be Vz, then during this 180 degrees turn flown with a bank angle phi, the height loss is pi*V*Vz/(g*sin(phi)*cos(phi)), and the turn is flown at speed V/sqrt(cos(phi)). The optimum (minimal height loss) is when sin(phi)*cos(phi) is maximum, i.e. phi = 45 degrees, and the product V*Vz is minimum. A glance on a typical glider polar will show that this last thing is obtained with V just below min sink speed, but as it is not easy to find how many below, let's assume the turn is done at min sink speed, this is not very far from the optimum. For a typical glider with min sink of .6 m/s at 80 km/h (22.2 m/s) the height loss is 8.5 m, for a typical airplane with min sink of 3 m/s at 120 km/h (33.3 m/s) the height loss is 64 m. This explains why the 180 degrees turn back to the runway over the outbound fence succeeds in a glider but not in a power plane. In the case mentioned above, the speed (60kt) was far over the optimum, however the result is as expected not catastrophic. Assuming a bank angle of 45 degrees, the equivalent speed in straight flight would be multiplied by 1.18, this gives 26 m/s or 93 km/h. Assuming the sink speed is 1 m/s in these conditions, we get a height loss of 16.6m. This is for a poor glider (L/D = 26 at 93 km/h). Thanks for that. A most informative calculation and certainly matches my most recent relevant experience. The last time I was having a supervised aero-tow refresher in our Puchacz I was doing a running commentary for the instructors benefit and as soon as I said "400 ft - no problem now from a rope break" BANG as he pulled the release. We had a touch over 60 kts and as soon as I saw the rope go I pulled a 45 degree banked 180, keeping the 60 kts just as Bob described, and was amazed at how easily we got in over the fence. In fact, once I'd rolled out it looked like a normal approach, so I opened the brakes and did a typical Puchacz approach and landing. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#92
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If someone wanted to do the world of ground launch a great favor, they would
start a web site where the collective wisdom of the world could be displayed so that anyone wishing to undertake winch launch could go there and get an education. Ok it may not be the collective wisdom of the world. But there is quite a bit of information on winch launching at: http://www.northwestsoaring.com/sitemap.shtml Let me know if I am missing anything really important about Winch Launching here. Brian Case CFIIG/ASEL |
#93
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"Brian Case" wrote in message om... If someone wanted to do the world of ground launch a great favor, they would start a web site where the collective wisdom of the world could be displayed so that anyone wishing to undertake winch launch could go there and get an education. Ok it may not be the collective wisdom of the world. But there is quite a bit of information on winch launching at: http://www.northwestsoaring.com/sitemap.shtml Let me know if I am missing anything really important about Winch Launching here. Brian Case CFIIG/ASEL Good site - congratulations. About your 2003 procedures - you must have had a bad experience with gusty wind conditions. This is why I have been advocating airspeed telemetry. Bill Daniels |
#94
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About 20 years ago I was doing an aerotow from the
front seat of a Janus at Lasham with Derek Piggot in the back during a cross country course. We were just off the deck within the airfield waiting for the tug to start to climb and I heard Derek say '60 knots'. Later I asked him why he said that out loud and he told me that he always made a note of when the speed reached 60 knots because he reckoned that at that speed he could pull up and turn back without loss of height. John Galloway At 14:00 30 October 2003, Martin Gregorie wrote: On Thu, 30 Oct 2003 10:11:18 +0000, Robert Ehrlich wrote: Bob Johnson wrote: As long as we're sitting around the campfire and also to show you I can go both ways, I 'member a time only a couple of years ago that I experienced the dreaded aerotow line break at 200 ft and 60 kt over the outbound fence. I looked out front and it wasn't too exiting, so I gingerly turned ninety to the left and the scenery looked some better, but not the greatest, so I REALlY gingerly gave it another ninety to the left and was really impressed this time as I found I was perfectly lined up with the takeoff runway. And I recall I hadn't lost too much of my original 60 kt. Will wonders never cease!! I thought 'OK now God, you've made it possible for me to do this little magic trick perfectly the first time I tried it in front of all my friends and hangar bums, give me your hand again and let's try just one more.' So I pulled spoilers, checked gear down and rolled up to my exact takeoff spot. And as I popped the canopy, my good friend who shall remain nameless, said 'S--t!, you've gone and lost us another Tost ring, somebody go back to the hangar and see if they can find another tow rope'. It was his turn to tow, so I pushed back. Nobody else said a word. Safety lecture from a dummy follows: I don't remember to this day why I ninetied to the left. During the previous year's biannual when Juan Batch pulled the plug on me over his outbound fence, turning right was the correct choice because in that direction lay the wind, which blows one back over the airport. This improves the scenery like you wouldn't believe. When I tried the trick for real solo, the wind lay to my left. I'd like to think it was instinct. But I believe it was a coin toss. Anyway, thank God. And Juan. It Depends BJ This raises the interesting question of the height loss during a 180 degrees turn in a glider or an airplane with a dead engine. I recently had a dicsussion about that with a friend who is a power pilot and on this occasion made again a small computation I had already made on this matter. As I never have seen these results elsewhere, I think it may useful to show that here. Assume you fly your turn wit an angle of attack which correspond to the speed V when flying straight and wings level, and that the vertical sink speed in the same conditions wuold be Vz, then during this 180 degrees turn flown with a bank angle phi, the height loss is pi*V*Vz/(g*sin(ph i)*cos(phi)), and the turn is flown at speed V/sqrt(cos(phi)). The optimum (minimal height loss) is when sin(phi)*cos(phi) is maximum, i.e. phi = 45 degrees, and the product V*Vz is minimum. A glance on a typical glider polar will show that this last thing is obtained with V just below min sink speed, but as it is not easy to find how many below, let's assume the turn is done at min sink speed, this is not very far from the optimum. For a typical glider with min sink of .6 m/s at 80 km/h (22.2 m/s) the height loss is 8.5 m, for a typical airplane with min sink of 3 m/s at 120 km/h (33.3 m/s) the height loss is 64 m. This explains why the 180 degrees turn back to the runway over the outbound fence succeeds in a glider but not in a power plane. In the case mentioned above, the speed (60kt) was far over the optimum, however the result is as expected not catastrophic. Assuming a bank angle of 45 degrees, the equivalent speed in straight flight would be multiplied by 1.18, this gives 26 m/s or 93 km/h. Assuming the sink speed is 1 m/s in these conditions, we get a height loss of 16.6m. This is for a poor glider (L/D = 26 at 93 km/h). Thanks for that. A most informative calculation and certainly matches my most recent relevant experience. The last time I was having a supervised aero-tow refresher in our Puchacz I was doing a running commentary for the instructors benefit and as soon as I said '400 ft - no problem now from a rope break' BANG as he pulled the release. We had a touch over 60 kts and as soon as I saw the rope go I pulled a 45 degree banked 180, keeping the 60 kts just as Bob described, and was amazed at how easily we got in over the fence. In fact, once I'd rolled out it looked like a normal approach, so I opened the brakes and did a typical Puchacz approach and landing. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#95
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#96
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#97
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What Eric describes has happened, but the way to eliminate that event is
to sheathe the cable with plastic hosepipe of suitable size at the glider end, so it is too stiff to wrap round the axle (or even enter the wheel box area). It is one of many things where the technology and operational procedures have been modified and developed over the years to reduce accidents and incidents to the minumum, leaving only pilot error/failure to act as trained as the remiaining significant factor. Chris N. Eric G: "I seem to recall some launches where the cable became tangled in the main wheel, when the winch jerked the glider forward, then paused very briefly. This is caused the glider to pitch up too fast at the start, and the pilot was unable to release, leading to a crash. [snip]" |
#98
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Hi Bruce --
That's a very welcome correction to my hazily-remembered version of a second-hand report of what the 454 c.i. engine torque/hp/rpm chart looks like. This is the kind of info I was looking for and thanks for providing it! In all our past kicking around of the ideal winch prime mover, here's one that sounds silly but might rate at least an engineering investigation -- a recip steam engine! If I recall correctly, the steam engine develops max torque at stall. What got me thinking about that was the fact that the Navy gets 66,000 lb Super Hornets flying in about the same three seconds that it takes us to get airborne. And they do it with steam, not because it's handy, but that's probably the only practical way to get it done. BJ Bruce Hoult wrote: In article , Bob Johnson wrote: Our engine is petrol fueled. Despite Google's best efforts, I have not yet located a Torque/HP/RPM curve for our very common 7.4 L engine, but have heard that it develops max torque and HP at about 3000 RPM and further that the curves are fairly flat at this point. You are making some totally contradictory and inconsistent claims there. If max torque and max HP occur close together then they must both drop off precipitously after that. If the torque curve is flat then HP will be increasing linearly with RPM, max torque and max HP will be very far apart. It is quite likely that you do have maximum torque at around 3000 RPM, but if for example the torque curve is flat enough that the torque at the 5000 RPM redline is still 60% or more of that at 3000 RPM then that (redline) is exactly where maximum power will be. -- Bruce |
#99
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On Thu, 30 Oct 2003 07:49:51 -0800, Eric Greenwell
wrote: In article , says... I still fail to understand why any winch or cabel failure should lead to an accident. With or without radio. With or without wind. With or without water ballast. Be prepared. I seem to recall some launches where the cable became tangled in the main wheel, when the winch jerked the glider forward, then paused very briefly. This is caused the glider to pitch up too fast at the start, and the pilot was unable to release, leading to a crash. Perhaps this is not what you mean by a winch failure? That should be a recoverable situation provided that the signalling channel between launch point and winch can convey three messages: - take up slack - all out - STOP If, as it appears to be the case at Torrey Pines, the headlamp signals can't be used to signal STOP then you have an accident waiting to happen. The launch marshal must ALWAYS be able to signal STOP and be obeyed without question. Doesn't matter whether the channel is radio, telephone, coded light flashes or signalling bat provided that it can transmit those three commands unambiguously. On the sites where I've winch launched an immediate STOP is signalled if the glider overruns the cable for any reason. The reason we use the three phrases listed above (repeated continuously) is that they have three, two and one syllable and so can be distinguished despite noise in the winch cab and/or wind noise in the launch marshall's microphone. -- martin@ : Martin Gregorie gregorie : Harlow, UK demon : co : Zappa fan & glider pilot uk : |
#100
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Bob Johnson wrote:
Robert - ... You may be using a which could account for your good performance at lower revs. ... Yes, it's a Diesel, with no gearbox, only a reduction box and and hydraulic (or should be called "oilic" :-) couple converter. |
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