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#11
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I won't speculate on the results of this type of testing, but, around here
the wind varies considerably with altitude, even with calm conditions in the morning. When doing performance work I would try to work crosswind. Wind shear effects are less influential. It is a great help to take a tape recorder and record every bump and time. A lot of data gets thrown out. If you take twenty minutes per point, you'll be spending a lot on tow fees. Dennis Brown John Sinclair wrote in message ... I asked Dick Johnson, why couldn't I fly in calm conditions (morning) and hold a given airspeed (say 60 knots) and a given heading (say west) for 10 minutes, then reverse heading to east (to cancel out any wind) and then analyze the GPS trace to determine my ships L/D at 60 knots. We have an accurate distance covered and fairly accurate altitude lost, so why can't we crunch the numbers? Dick said the GPS info wasn't accurate enough. I thought it was a good idea, but I defer to the master. JJ At 03:00 28 December 2004, Bob Gibbons wrote: On this topic of determining L/D from interthermal cruising, Dick Johnson did a fascinating and underappreciated study in the late 1970's of airmass behavior between thermals. Dick flew in mostly blue conditions and simply recoded his height loss versus distance covered between thermals. Dick's results are reported in SOARING, June 1979. Dick found that, on the average, the airmass between thermals has an average sink rate related to the upgoing thermal strength. The relationship Dick found was; the airmass sink is approximately 10% of the lift strength. I have always felt Dick's study explains why it is so difficult to fly cross country (in a blue conditions) with a ship having an L/D less than about 30:1. The probability of running into the next thermal purely by chance becomes too low as the L/D drops. For this discussion, I think Dick's study shows the inadvisability of trying to deduce flight performance from interthermal measurements. Bob On Mon, 27 Dec 2004 20:43:41 GMT, 'Papa3' wrote: Mark, My first post came across as a bit glib - apologize for that. But, I actually put a little thought into that subject recently while writing some batch analysis specs for GPS log files. The problem is that a good glider pilot will not encounter random vertical motions - even in cruise flight. He/she will stack the deck in his favor, seeking out cloud streets or connecting the best looking individual Cu. Thus, you can't just average out the L/D over time on specific segments (e.g. cruise flight). What you can do is deterimine which pilot does the best job of achieving highest L/D on a given day. Several of the popular flight analysis programs do this already. I'd certainly be interested in any detailed ideas you might have. P3 'Mark Zivley' wrote in message .com... Obviously ridge flight would not be conducive, nor wave, but put enough data together from cruises during thermal flights and I bet something could be put together. Papa3 wrote: Mark, How do you propose to isolate the impact of vertical air motion? For instance, I can fly the ridge at 100Kts and maintain altitude (same for wave or cloudstreets). I'm sure Rolladen-Schneider (ahem, DG) would love to publish the L/D of my LS8 from the average of my flights for a season: 'LS8, with a measured L/D of 800:1...' Cheers, Erik 'Mark Zivley' wrote in message m... We all know what the manufacturer's polars look like, but what about our individual planes. Has anyone done any work to develop a program that would look at some flight logs and determine what a particular glider's actual polar is? At one point Ball was making a vario system that would determine the aircraft's polar over time just by flying. For someone who already had some algorithms for computing wind from ground track drift during thermals could take this info and then be able to back figure from GPS ground speed what the IAS was during a particular phase of the flight. By isolating longer sections of cruise flight at varios airspeeds it should be do-able. Question is, has it been done. Mark extra 'hot' in the address to delay the spammers... |
#12
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Errors in measurement can be made up for with lots and lots of data, so
long as the errors are not biased one way or another. Thus, you should be able to get an accurate polar even in thermally air, without spending a fortune on tow fees. Turn on data recording, then fly absolutely straight and same speed, through thermals as well as sink, while taking data. Turn off data recording before thermaling back up to altitude. The key is to fly so that on average you're not biased toward flying in lift vs. sink. You should randomize heading (if you always go up/downwind you'll be in streets), randomize time of turning on/off the data recording (if you turn on after leaving a thermal and off when you find a new one, you'll be biased toward sink). If you do this for a season, for example getting 20 minutes of data in the 1-2 hours of prestart fooling around at contests, you might have a really good polar at the end of it. You could also do the opposite: A good pilot should be flying faster through sink and slower through lift, and should spend more time in lift than in sink. The difference between the "polar" measured in thermal conditions and the factory polar can be a basis of a measure of pilot skill. A good pilot should give a polar with a worse high speed end -- because he always flies fast through sink -- a much better low speed end -- becasue he always flies slow through lift -- and a positive bias -- the whole polar shifted up. In principle, all pretty easy to add to a glide computer. Of course we all have a long list of more important features. John Cochrane (BB) |
#13
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BB wrote:
Errors in measurement can be made up for with lots and lots of data, so long as the errors are not biased one way or another. Thus, you should be able to get an accurate polar even in thermally air, without spending a fortune on tow fees. Turn on data recording, then fly absolutely straight and same speed, through thermals as well as sink, while taking data. Turn off data recording before thermaling back up to altitude. The key is to fly so that on average you're not biased toward flying in lift vs. sink. You should randomize heading (if you always go up/downwind you'll be in streets), randomize time of turning on/off the data recording (if you turn on after leaving a thermal and off when you find a new one, you'll be biased toward sink). If you do this for a season, for example getting 20 minutes of data in the 1-2 hours of prestart fooling around at contests, you might have a really good polar at the end of it. You could also do the opposite: A good pilot should be flying faster through sink and slower through lift, and should spend more time in lift than in sink. The difference between the "polar" measured in thermal conditions and the factory polar can be a basis of a measure of pilot skill. A good pilot should give a polar with a worse high speed end -- because he always flies fast through sink -- a much better low speed end -- becasue he always flies slow through lift -- and a positive bias -- the whole polar shifted up. In principle, all pretty easy to add to a glide computer. Of course we all have a long list of more important features. John Cochrane (BB) Wouldn't it be simpler and much more scientific to arrange to do a series of test runs over a LIDAR site, and simply post process the radar and glider data recorder data ? |
#14
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Eric Greenwell wrote:
John Sinclair wrote: I asked Dick Johnson, why couldn't I fly in calm conditions (morning) and hold a given airspeed (say 60 knots) and a given heading (say west) for 10 minutes, then reverse heading to east (to cancel out any wind) and then analyze the GPS trace to determine my ships L/D at 60 knots. We have an accurate distance covered and fairly accurate altitude lost, so why can't we crunch the numbers? Dick said the GPS info wasn't accurate enough. I thought it was a good idea, but I defer to the master. How long ago did you ask him? GPS is much more accurate in the last few years, especially if using the WAAS ablities. But, let's say you know the distance to only +/- 100 feet (it's typically more like +/- 30 feet), then flying only a mile (5000 feet) would be a 2% error, or one L/D point for a 50:1 glider. Good enough for us, I think. I don't think this is quite right. Consider a 50:1 glider descending 100 meters: It will go 100 x 50 = 5000 meters I think vertical navigation errors are typically 1.5 times the horizontal. If horizontal accuracy is 10 meters, then vertical will be about 15 So when we've measured this exactly 50:1 glider sinking 100 meters, the two worst cases we would read for a measurement a (5000-Herror)/(100+Verror) = 4990/115 = 43.4 (5000+Herror)/(100-Verror) = 5010/85 = 58.9 With errors of 3 meters and 5 meters, it gets closer. 4997/105 = 47.6 5003/95 = 52.6 With errors of 1 and 2 meters 4999/102 = 49.0 5001/98 = 51.0 If you use the GPS for the altitude instead of the pressure altitude, you might have to fly off a 1000 feet or so of altitude, I suppose. Maybe Dick was referring to GPS altitude? Probably. If I understand the technique he uses correctly, he flies a test glider at a particular airspeed, timing the descent, to get a sink rate for that airspeed. Then he plots the sinkrates to get the polar. I can't see why he'd be interested in horizontal position at all. I don't know what kind of accuracy is possible from the barometric altimeters. It could be that a skilled pilot/data recorder can get accuracy below the typical 20 foot tick mark on the altimeter face. OTOH, you are correct, if I'm reading the FAA docs right, WAAS should be able to give vertical accuracy better than 2 meters 95 percent of the time. I don't know if the COTS handhelds can actually deliver that. On Sam Wormsley's GPS site, there's a link that suggests that a Garmin GPS 76 maintained about 6 meter vertical accuracy 95 percent of the time. That's very close to the 20 foot resolution on a barometric altimeter, and that's absolute position, rather than relative position. That is, we don't care where we started and where we ended, but rather how far we descended. If the error offset is pretty much the same at the beginning and end of a run, then the relative accuracy may be much better than the absolute position. It might be interesting to look at the GPS speed during a descent. If the airspeed and heading is being held constant, and the airmass is uniform, then the GPS speed should be constant. Excursions might indicate shears that would affect the quality of the data. You might be able to improve accuracy by throwing out segments that indicated non-steady-state behavior. Tim Ward -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#15
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At 18:30 28 December 2004, Eric Greenwell wrote:
How long ago did you ask him? GPS is much more accurate in the last few years, especially if using the WAAS ablities. But, let's say you know the distance to only +/- 100 feet (it's typically more like +/- 30 feet), then flying only a mile (5000 feet) would be a 2% error, or one L/D point for a 50:1 glider. Good enough for us, I think. At 50:1 flying a one mile distance should yield an altitude loss of 100', so I think a +/- 100' GPS error in altitude could yield a 'measured' L/D of between 25:1 and infinity. A single measured glide of 10 miles, assuming no other errors from pilot inputs or net airmass movements, would be good to only 10%, or +/- 5 L/D points, that's a bit better, but you'd still need to average a bunch of runs of 10+ miles to get any kind of accuracy. Best done in dead calm air the way Dick does. I have tried plotting speed versus glide angle over a number of flights. You need to adjust for wind speed and direction (estimated from thermal drift angle and/or ground speed differences around a circle), then you have to convert from TAS to IAS (2% per 100'). I plotted about 30 points and realized that I had a very low signal to noise ratio - that is the points made a big cloud that sort of sloped in the expected direction. I suppose with enough data points you could get a resonable average L/D performance for any given speed, but I did the experiment for a different reason, I was interested in the VARIANCE in performance that I could expect in real world conditions, including pilot technique and long stretches of good/bad air. If I know the difference between expected performance and 99% worst case performance I know how much cushion to carry on final glides - particularly as I get closer to home, and the ground, and have fewer options to recover. It also may help a bit to know how early you can start a long final glide, though this is much more subject to observable conditions like streeting and average thermal strength and therefore less informed by bulk statistical analysis. 9B |
#16
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Andy Blackburn wrote:
At 18:30 28 December 2004, Eric Greenwell wrote: How long ago did you ask him? GPS is much more accurate in the last few years, especially if using the WAAS ablities. But, let's say you know the distance to only +/- 100 feet (it's typically more like +/- 30 feet), then flying only a mile (5000 feet) would be a 2% error, or one L/D point for a 50:1 glider. Good enough for us, I think. At 50:1 flying a one mile distance should yield an altitude loss of 100', so I think a +/- 100' GPS error in altitude could yield a 'measured' L/D of between 25:1 and infinity. My example was for GPS distance, and pressure altitude, to indicate that a distance measurement wasn't a problem. That's why a I later referred to flying off at least 1000 feet if GPS altitude was used. What I don't know is how much error change one can expect in GPS altitudes taken 5 or 10 minutes apart. The difference (GPS start height minus GPS finish height) might have a much smaller error than the altitude itself, which would allow shorter glides (500 foot loss if the differential error was only 5 feet, for example). -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#17
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I have tried using a $10,000 carrier phase GPS receiver with 0.1m
precision (post-processed) for glide testing. The GPS data was so precise, you could clearly see the antenna move a few cm when the wing was raised for takeoff. Even when flying in the calmest conditions, with no discernable airmass movement, the vertical motions are significant. In analysing the data, I could not precisely fit a straight line to the data points from 1-2 min glides at constant airspeed, even after correcting for slight airspeed variations using total energy. Johnson does not have this problem because he only has two data points, one at the beginning and one at the end of each glide. When you have about a hundred data points, one every second, you can really see the problem. You need a lot more data points to average out the noise. Based on this, I doubt that you could get useful data from a less precise GPS, with a slower sampling interval, in uncontrolled conditions. There is just too much noise to get useful results without an impossibly huge data set. Eric Greenwell wrote: Mark Zivley wrote: We all know what the manufacturer's polars look like, but what about our individual planes. Has anyone done any work to develop a program that would look at some flight logs and determine what a particular glider's actual polar is? At one point Ball was making a vario system that would determine the aircraft's polar over time just by flying. For someone who already had some algorithms for computing wind from ground track drift during thermals could take this info and then be able to back figure from GPS ground speed what the IAS was during a particular phase of the flight. By isolating longer sections of cruise flight at varios airspeeds it should be do-able. Question is, has it been done. I haven't heard of it being done, and I can't imagine how one would compensate for air motion, both vertical and horizontal, just using the GPS info. Both motions change with location, altitude, and time. Perhaps if the flight record included the airspeed, like some varios can supply, there would be some hope of doing it. I don't think you could count on the vertical motion averaging to zero during the cruises, since we typically adjust our path to include as much up air as possible. You can get some good info using a flight recorder, but you have to do it when the air is calm. If you are really interested, invest in a few high tows and make the measurements. Take a look at this test done on a DG 800: http://groups.yahoo.com/group/ASA-NewsGroup/message/59 You don't have to be a group member to read the message. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#18
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"Tim.Ward" writes:
the distance to only +/- 100 feet (it's typically more like +/- 30 feet), then flying only a mile (5000 feet) would be a 2% error, or one L/D point for a 50:1 glider. Good enough for us, I think. I don't think this is quite right. Consider a 50:1 glider descending 100 meters: It will go 100 x 50 = 5000 meters I think vertical navigation errors are typically 1.5 times the horizontal. If horizontal accuracy is 10 meters, then vertical will be about 15 So when we've measured this exactly 50:1 glider sinking 100 meters, the two worst cases we would read for a measurement a (5000-Herror)/(100+Verror) = 4990/115 = 43.4 (5000+Herror)/(100-Verror) = 5010/85 = 58.9 To find and eliminate the random errors just have a second GPS receiver on the ground under the area of the flight test. Comparing of the records would show the GPS errors. (Any motion on a stationary receiver is GPS error.) We are able to get better than millimeter accuracy (for earth plate motion studies) using basically this technique, as you don't care where you are absolutely only relative to the other point/receiver. We are interested in the polar at higher speeds than plate motion speeds so some accuracy will be lost. Steve |
#19
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At 00:00 29 December 2004, Eric Greenwell wrote:
My example was for GPS distance, and pressure altitude, to indicate that a distance measurement wasn't a problem. That's why a I later referred to flying off at least 1000 feet if GPS altitude was used. What I don't know is how much error change one can expect in GPS altitudes taken 5 or 10 minutes apart. The difference (GPS start height minus GPS finish height) might have a much smaller error than the altitude itself, which would allow shorter glides (500 foot loss if the differential error was only 5 feet, for example). Clarification noted - but distance measurement is a problem with GPS with respect to polar calculations. Without knowing the technique Dick Johnson uses, or the specs on a specific pressure transducer, it's hard to know if measuring pressure altitude through a digital transducer is more or less accurate than the traditional method. I'd guess it's a close call, but that has nothing to do with GPS. The main source of error, is being able to turn GPS ground speed (or distance) into IAS reliably by subtracting wind speed and adjust for altitude. An even greater source of error is trying to use fixes from a typical soaring day with airmass movements and pilot control inputs, airspeed changes and flightpath deviations. The empirical evidence is that there is way too much randomness from the above noted effects to tease out a anything much beyond just how much randomness there in fact is on a typical flight. Maybe if you did fifty 10-mile runs on a dead calm day across five different airspeeds, you'd get less scatter - but I think that's more or less what Dick does, except he measures IAS directly, rather than having to figure it out from ground speed. 9B |
#20
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Andy Blackburn wrote:
Without knowing the technique Dick Johnson uses, or the specs on a specific pressure transducer, it's hard to know if measuring pressure altitude through a digital transducer is more or less accurate than the traditional method. I'd guess it's a close call, but that has nothing to do with GPS. I don't know the specifics of Dick's instrument, but a digitally logged unit has got to be better, maybe a lot better, than trying to read a mechanical altimeter with a little vibrator on the panel! The main source of error, is being able to turn GPS ground speed (or distance) into IAS reliably by subtracting wind speed and adjust for altitude. I think the pilots trying this are not doing that, but instead rely on their airspeed to give them IAS. This does require calibration of the ASI for real accuracy. An even greater source of error is trying to use fixes from a typical soaring day with airmass movements and pilot control inputs, airspeed changes and flightpath deviations. I agree completely, and expect the best data to come on the same kind of day Dick uses; that is, not a soaring day! It's this air mass movement that drives the Akafliegs to using "sacred cows" to measure the motion of the test glider relative to the "cow". -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
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