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#1
October 29th 03, 08:38 PM
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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 
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#2
October 30th 03, 10:11 AM
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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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#3
October 30th 03, 01:50 PM
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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 :
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#5
November 3rd 03, 05:28 AM
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In power planes I often wondered how high I would need to turn around,
and the biggest difference seemed to be how well I was climbing. In a heavy Piper Arrow on a hot day, We couldn't climb fast enough to ever glide back regardless how high we went if we did a straight out or 45 departure. Departing downwind was another story, of course. On hot days with heavy loads at Avenal, the tug sometimes turns very gently at low altitudes to downwind. I recall flying a two seater open cockpit on a very hot day and seeing miserable climb out of us and the 150/150. In our case not a problem with all the flat ground, but still a bit disconcerting to be so far out and low...
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#6
November 3rd 03, 03:52 PM
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"Mark James Boyd" wrote in message news:3fa5f571$1@darkstar... In power planes I often wondered how high I would need to turn around, and the biggest difference seemed to be how well I was climbing. In a heavy Piper Arrow on a hot day, We couldn't climb fast enough to ever glide back regardless how high we went if we did a straight out or 45 departure. Departing downwind was another story, of course. On hot days with heavy loads at Avenal, the tug sometimes turns very gently at low altitudes to downwind. I recall flying a two seater open cockpit on a very hot day and seeing miserable climb out of us and the 150/150. In our case not a problem with all the flat ground, but still a bit disconcerting to be so far out and low... Back in around 1970 a power instructor who had witnessed glider 200 foot 180 turns back to the runway wanted to try some in a Cessna 150. I rode with him as we tried a few at a safe altitude. The 150 was one of the old ones with a straight tail and manual flaps. The glider technique of a 45 degree banked turn was very marginal. We then experimented with some more aggressive maneuvers. The best seem to be a sort of diving 180 degree rolling turn with a pullout from the dive on the reciprocal heading. We came out of the dive headed towards the runway at high airspeed but in ground effect which got us to the runway with ease. Once we were confident of our technique, we tried a few at Caddo Mills, TX. At the time, Caddo Mills was just abandoned runways with no buildings or fences and surrounded by alfalfa fields. The 45 degree turn never made it back but the diving roll did work if you were quick and aggressive. We both agreed that the conventional instruction of not trying a 180 below about 500 feet was best. Bill Daniels
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#7
November 3rd 03, 10:41 PM
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"Bill Daniels" wrote:
"Mark James Boyd" wrote in message news:3fa5f571$1@darkstar... In power planes I often wondered how high I would need to turn around, and the biggest difference seemed to be how well I was climbing. In a heavy Piper Arrow on a hot day, We couldn't climb fast enough to ever glide back regardless how high we went if we did a straight out or 45 departure. Departing downwind was another story, of course. On hot days with heavy loads at Avenal, the tug sometimes turns very gently at low altitudes to downwind. I recall flying a two seater open cockpit on a very hot day and seeing miserable climb out of us and the 150/150. In our case not a problem with all the flat ground, but still a bit disconcerting to be so far out and low... Back in around 1970 a power instructor who had witnessed glider 200 foot 180 turns back to the runway wanted to try some in a Cessna 150. I rode with him as we tried a few at a safe altitude. The 150 was one of the old ones with a straight tail and manual flaps. The glider technique of a 45 degree banked turn was very marginal. We then experimented with some more aggressive maneuvers. The best seem to be a sort of diving 180 degree rolling turn with a pullout from the dive on the reciprocal heading. We came out of the dive headed towards the runway at high airspeed but in ground effect which got us to the runway with ease. Once we were confident of our technique, we tried a few at Caddo Mills, TX. At the time, Caddo Mills was just abandoned runways with no buildings or fences and surrounded by alfalfa fields. The 45 degree turn never made it back but the diving roll did work if you were quick and aggressive. We both agreed that the conventional instruction of not trying a 180 below about 500 feet was best. Bill Daniels Many years ago a highly respected aerobatic pilot in UK (still working today) wrote an article in our Pilot magazine on this subject. He reminded us that three things were required: 1. A rapid 180 degree change of heading. 2. Minimum loss of height. 3. Normal airspeed at the end of the manoever. Controversially, he maintained that a highly banked slipping turn satisfies all three criteria. The rate of decent is very high during the turn, but the duration is so short that it results in less height loss than either of the two alternatives (eg:slow and gentle or fast and furious), and the airspeed is normal throughout. Just don't forget to keep loads of top rudder on. - Colin
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#8
November 4th 03, 10:49 AM
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Colin wrote:
Many years ago a highly respected aerobatic pilot in UK (still working today) wrote an article in our Pilot magazine on this subject. He reminded us that three things were required: 1. A rapid 180 degree change of heading. 2. Minimum loss of height. 3. Normal airspeed at the end of the manoever. Controversially, he maintained that a highly banked slipping turn satisfies all three criteria. The rate of decent is very high during the turn, but the duration is so short that it results in less height loss than either of the two alternatives (eg:slow and gentle or fast and furious), and the airspeed is normal throughout. Just don't forget to keep loads of top rudder on. - Colin I don't see why the turn should be a slipping one, you certainly will loose more height in a slipping turn than in a normal one. And height loss is what make the turn possible or not.
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