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#81
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Hi Bill --
I 'member my O-Nine Two oil field piano wire breaking at the half way point during a tow in Roy Schlemeyer's old SGS 2-22. I yelled "Oh, line break" or something to that effect and recall the glider innards debris collected since the last ice age floating up and dancing before my eyes as I dumped the stick full forward. Was it half a second before I reacted? Can't believe I waited that long. And when the astronauts tell you that zero-g is exhilarating, believe it. Luckily, I had been well briefed by Roy as to what to expect. It has been theorized that some people are "allergic" to zero-g and they instinctively jerk the stick full back instead of push full forward. Unfortunately, this is the last mistake they ever make and we can no longer interview them as to why they did this. There, we're back to the original question, "Is winch launch safer than aerotow?" The answer? -- scroll down IT DEPENDS! Thanks and good to hear from you again. BJ Bill Daniels wrote: "Bob Johnson" wrote in message ... Eggert -- Those are really good numbers for your winch. With V8 300 HP (GMC 454 c.i., 7.4 L), and 5000 ft (1550 m) Plasma line laid out, we are getting the rule of thumb 1/3 cable length releases of 1700 ft (525 m). This is into 10-15 kt wind. Much over that, we leave the Blanik in the barn! BJ Midland, Texas Bob, When winching, the wind is your friend. Quitting at 15 knots is not necessary. I have winched into 35 knots and higher winds and the results are spectacular. Each 10 knots of headwind is the equivalent of about 40 additional HP. The thing that often severely limits the altitude gained is a slow pitch-up profile at the start of the launch. The final height achieved is largely determined by the profile flown in the first few seconds of the launch. Now, as everyone has pointed out, you need to be careful here. Safety at the start of the climb is a combination of airspeed, altitude and attitude. The more you have of the first, the faster you can get the second two and the higher you will get. I've done calculations, simulator runs (X-Plane) and flight test to prove the following point. If you have 60 knots in a glider with a stalling airspeed of 40 knots, you can be in full climb attitude at zero altitude and still have a large safety margin. Practice this way - at several thousand feet AGL, zoom the glider into a 50 degree nose-up attitude. As the airspeed decays to 60 knots, yell "WIRE BREAK", delay 0.5 seconds (simulating reaction time) and pitch forward at zero G. Watch the airspeed and altitude, you'll see what I mean. (For winch CFI-G's, this is a great way to teach how to handle wire breaks.) If you have (or simulate) a wire break at this point and start a zero G pitch over after a .5 second delay, the minimum airspeed during the parabolic ballistic trajectory will be about 50 - 55 knots when the glider reaches apogee at an altitude of about 100 feet AGL. So there you are at 100 feet and 55 knots in a normal gliding attitude - not exactly a problematic situation, just land straight ahead. The reason this works is that the glider's induced drag at zero G is minimal so the airspeed decay is mainly just due to gravity and the glider follows a parabolic trajectory until the pilot re-establishes one G at the normal glide attitude. I need to repeat that I am not advocating a rocket blast-off kind of climb profile but a smooth transition into the full climb without undue delay equipped with a full understanding of the safety margins. Bill Daniels |
#82
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Earlier, JJ Sinclair wrote:
Its been a long time now, but a group was auto-towing a Cherokee at Air Sailing, NV... I'm not sure, but I believe that was actually a Don Mitchell design called the Nimbus III-B (no relation to the later Schempp-Hirth product of similar name). That was in 1973: http://www.ntsb.gov/NTSB/brief.asp?ev_id=85473&key=0 Thanks, and best regards to all Bob K. http://www.hpaircraft.com |
#83
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"Bert Willing" wrote in message ...
Nothing worse than an additional communication step between pilot and winch driver. Did that for 10 years in Germany, happy to do it by radio ever since I left Germany, hadn't had an interference from others on the same frequencies for the last 13 years :-) Agree, the way they often do it in Germany is useless. Glider having to tell the guy at the phone what he's supposed to then tell the winch. It's so simple if the winch has a radio. Our winch has had a radio for decades. Even when we used to give the lauch signals with a flag! About fifteen years ago we switched to telephone to give the launching commands but as soon as the glider has left the ground the guy at the phone says _nothing_ more. Then only the glider pilot says either 'faster' or 'slower' over the radio. Since we have the wire tension indicated to the winch driver these commands have become very rare and so have cable breaks. |
#84
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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 Bob Johnson wrote: Hi Bill -- I 'member my O-Nine Two oil field piano wire breaking at the half way point during a tow in Roy Schlemeyer's old SGS 2-22. I yelled "Oh, line break" or something to that effect and recall the glider innards debris collected since the last ice age floating up and dancing before my eyes as I dumped the stick full forward. Was it half a second before I reacted? Can't believe I waited that long. And when the astronauts tell you that zero-g is exhilarating, believe it. Luckily, I had been well briefed by Roy as to what to expect. It has been theorized that some people are "allergic" to zero-g and they instinctively jerk the stick full back instead of push full forward. Unfortunately, this is the last mistake they ever make and we can no longer interview them as to why they did this. There, we're back to the original question, "Is winch launch safer than aerotow?" The answer? -- scroll down IT DEPENDS! Thanks and good to hear from you again. BJ Bill Daniels wrote: "Bob Johnson" wrote in message ... Eggert -- Those are really good numbers for your winch. With V8 300 HP (GMC 454 c.i., 7.4 L), and 5000 ft (1550 m) Plasma line laid out, we are getting the rule of thumb 1/3 cable length releases of 1700 ft (525 m). This is into 10-15 kt wind. Much over that, we leave the Blanik in the barn! BJ Midland, Texas Bob, When winching, the wind is your friend. Quitting at 15 knots is not necessary. I have winched into 35 knots and higher winds and the results are spectacular. Each 10 knots of headwind is the equivalent of about 40 additional HP. The thing that often severely limits the altitude gained is a slow pitch-up profile at the start of the launch. The final height achieved is largely determined by the profile flown in the first few seconds of the launch. Now, as everyone has pointed out, you need to be careful here. Safety at the start of the climb is a combination of airspeed, altitude and attitude. The more you have of the first, the faster you can get the second two and the higher you will get. I've done calculations, simulator runs (X-Plane) and flight test to prove the following point. If you have 60 knots in a glider with a stalling airspeed of 40 knots, you can be in full climb attitude at zero altitude and still have a large safety margin. Practice this way - at several thousand feet AGL, zoom the glider into a 50 degree nose-up attitude. As the airspeed decays to 60 knots, yell "WIRE BREAK", delay 0.5 seconds (simulating reaction time) and pitch forward at zero G. Watch the airspeed and altitude, you'll see what I mean. (For winch CFI-G's, this is a great way to teach how to handle wire breaks.) If you have (or simulate) a wire break at this point and start a zero G pitch over after a .5 second delay, the minimum airspeed during the parabolic ballistic trajectory will be about 50 - 55 knots when the glider reaches apogee at an altitude of about 100 feet AGL. So there you are at 100 feet and 55 knots in a normal gliding attitude - not exactly a problematic situation, just land straight ahead. The reason this works is that the glider's induced drag at zero G is minimal so the airspeed decay is mainly just due to gravity and the glider follows a parabolic trajectory until the pilot re-establishes one G at the normal glide attitude. I need to repeat that I am not advocating a rocket blast-off kind of climb profile but a smooth transition into the full climb without undue delay equipped with a full understanding of the safety margins. Bill Daniels |
#85
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JJ, you've had some bad experiences and I'm sorry for that. But you have to realize that there are hundreds of thousands, if not millions of winch launches all around the world that go off without a hitch simply because the people involved know what they are doing. The subject under discussion is a club that is thinking about trying a new launch system, winch launching and now pay-out winch. My point in posting on this subject is to show what can happen when unskilled club members try a new system. JJ Sinclair |
#86
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"JJ Sinclair" wrote in message ... JJ, you've had some bad experiences and I'm sorry for that. But you have to realize that there are hundreds of thousands, if not millions of winch launches all around the world that go off without a hitch simply because the people involved know what they are doing. The subject under discussion is a club that is thinking about trying a new launch system, winch launching and now pay-out winch. My point in posting on this subject is to show what can happen when unskilled club members try a new system. JJ Sinclair OK, JJ, If they are unskilled and insist on REMAINING unskilled, I'll give you the point. Everybody starts out unskilled, however - I don't hold that against them. I don't think you are dissing ground launch, you're just warning that any group needs to realize that there is a steep learning curve ahead of them. I'll agree with that. They've learned to fly gliders however, so they can't be dumb. I think they can learn to operate a winch safely and enjoyably IF they take the trouble to learn the art and science of it. There are lots of books to read and this medium, the internet, connects us with the world where there is a wealth of knowledge about ground launch. 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. Any volunteers? Bill Daniels I say learn and enjoy. Bill Daniels |
#87
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On Wed, 29 Oct 2003 16:48:38 +0100, "Bert Willing"
wrote: Nothing worse than an additional communication step between pilot and winch driver. Did that for 10 years in Germany, happy to do it by radio ever since I left Germany, hadn't had an interference from others on the same frequencies for the last 13 years :-) Don't forget that you rarely do 100 launches per day at your current airfield with "Vent arriere" messages each 3 minutes, interfering with launches... (Yes - I'm envious!) Of course you are correct - it's nice to have a direct connection between pilot and winch driver. My club at Landau has never had such a (radio) communication, and we seldom miss it. Sometimes the Ka-8 gets too fast (and releases early), but otherwise I cannot remember and disadvantages of not having a glider-winch connection. It's more a question of winch driver practice - drivers with little practice tend to judge the speed wrong and need corrections. To add some spice - I personally watched two bad winch accidents, each had to do with power failure of the winch/too low airspeed. In both cases there was direct radio communication between glider and winch. Bye Andreas |
#88
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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 |
#89
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To add some spice - I personally watched two bad winch accidents, each
had to do with power failure of the winch/too low airspeed. In both cases there was direct radio communication between glider and winch. Of course having radio connection does not mean you can just let the launch happen. You still have to fly it and react to whatever situation arises. First, you have to use the radio. Second you have to use it before it's to late and third and most important: never rely on the use of radio of having any effect on the launch. 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. Marcel |
#90
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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). |
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