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#1
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Engine noise recording in Motor Gliders
In an earlier post, Ray Lovinggood said:
From the following webpage: http://www.aerokurier.rotor.com/akNachr/aeNewsE.htm Antares too quiet. Lange Flugzeugbau faces an unusual hurdle with the type certification of its electric glider, Antares. The required noise level measurement according to Chapter X was not possible. The noise emissions from the 20-meter self-launching glider are so low that it can't be heard above the ambient sound of the surrounding environment. ..... An aircraft that is actually too quiet. Greg Arnold responded: I wonder if it makes enough noise for engine operation to be recorded by the ENL function of a flight recorder. Greg Arnold puts a very good question. Now that Antares is in production, the IGC GNSS Flight Recorder Approval Committee (GFAC) has been able to look at a selection of IGC files that have been kindly sent to us by some of the owners. Previously, GFAC had seen very few IGC files with noise signatures for Antares and these were not of the "critical cases" described below. Here are some figures: In an Antares flight in August 2006, ENL with engine off was recorded in the IGC file from 130 to 448 when cockpit panels were open and some sideslip was present. In comparison, in a run with level-flight engine power and a well-sealed cockpit, recorded ENL was from 019 to 120 and about 50 feet was gained. In another August Antares flight, climb with engine at about a 200 ft/minute gave ENL between 155 and 398 with an average of about 280. In this flight, ENL at full power was recorded between 294 and 769 with an average of about 450. The ENL system that produced these figures was that designed by LX Navigation and the recorder was an LX7000. These engine-on numbers are substantially less than those for an internal combustion engine. Also, the lower engine-on figures are well within the range that we have seen for non-powered flight, for instance thermalling with cockpit panels open. As an example of the "noisy glide" case, in a test on two new types of recorders that I flew on 8 August 2006, an ENL of 393 was recorded in gliding flight with the cockpit panel open using a Aircotec XC Profi recorder as a "control". The Aircotec ENL system is particularly good at differentiating between use of engine and other cockpit noises. Although 393 was a peak glide reading in deliberately high cockpit noise conditions, it shows you the potential problem in distinguishing the noise of a quiet engine from conditions that can be met in the glide. GFAC is in correspondence with Lange Flugzeugbau, the manufacturer of Antares, and is collecting more data from Antares flights before coming to a view on how recording of the operation of electric engines should be handled. Meanwhile, independent of the "electric engine" issue, two new types of recorder are being evaluated by GFAC, each with a new type of ENL system that is being tested in order to optimise the frequency and gain settings before IGC-approval can be given. This emphasises the point that there are many different types of ENL system with slightly different characteristics and we simply do not know at this stage how they will all respond to electric or other "quiet" engines. It may be that we have to look at recording extra variables such as engine or prop RPM or battery current to the engine, whichever is easier to record. It might be worthwhile re-stating what an ENL system is supposed to do. Fundamentally it must differentiate between any running of engine that generates forward thrust, and any flight condition encountered in normal soaring flight without such use of engine. The key word is "differentiate". The critical engine-on case with all motor gliders (in terms of "differentiating") is not when the engine is run at high power, that is the "easy one" in terms of ENL numbers in the IGC file. A combination of engine and propellor noise at high power should give ENL figures over, say 800 out of the maximum ENL in the IGC file of 999. Most two-stroke systems give over 900 and some ENL systems give the maximum of 999. Four stroke engines give lower figures but still enough to differentiate between power-on and power-off. Wankel rotary engines are fitted to several types of motor glider and have been no problem either (although at least one installation requires the engine to come out in order to change the spark plugs, but that is a different problem!). CRITICAL NOISE-LEVEL RECORDING CASES POWER-ON. The critical power-on case in terms of ENL differentiation is cruising power, that is, just sufficient power for level flight. You could say that there is also the case where a trickle of engine is used to extend the glide angle, but we have to draw the line somewhere! POWER-OFF. The critical ENL power-off case is not a quiet, well-sealed cockpit but is a noisy one, typically thermalling with air vents and cockpit panels open. This can certainly produce ENL figures up to 300, more if sideslip is present and 400 has been seen on some recorders. Another particularly noisy case is high speed flight with the cockpit panel(s) open, but this is probably not as realistic as thermalling with panels open and a bit of sideslip as controls are applied to adjust the circle. ENL SYSTEM DESIGN The three ENL numbers recorded with each fix in IGC files must therefore differentiate between the "quiet engine" and the "noisy cockpit" cases. This is done by carefully selecting the frequency and gain at which the ENL system is most sensitive. The ENL system is then tested by GFAC in a range of motor gliders, gliders and powered aircraft. Frequencies between 75 and 100Hz give good (high) response to engine and propeller noise and less response to cockpit noises. With all sorts of internal combustion engines the ENL system has proved reliable ever since the concept was invented by David Ellis, the original owner of Cambridge Aero Instruments, back in about 1994. Because it is all inside the recorder, an ENL system is easier for the pilot, compared to microswitch-based or other systems requiring some sensor hookup to the recorder. It is also better than vibration-based systems because with ENL the recorder does not need any special mounting in the cockpit or system checks by Official Observers before or after flight. In addition, there is not just one design of ENL system. Several companies have designed their own systems that are tested by GFAC before they are IGC-approved. Some companies have more than one ENL system, each optimised for different types of recorder. Different ENL systems in IGC-approved recorders include those by Aircotec, Cambridge (separate systems for the 300 series and the legacy models 10, 20 & 25), Garrecht, LXN (including recorders by Filser and SDI), NTE, Print Technik and Zander. Each of these ENL systems has slightly different characteristics. ENL figures for particular flight conditions will vary between these systems. For internal-combustion engines this has not been critical because they all produce a substantial amount of noise in the cockpit. However, we simply do not know how electric engines will register ENL values until we have a good database of IGC files from the different types of IGC-approved recorders. Annex B to the IGC-approval document for each type of recorder includes typical ENL numbers that were obtained during testing in the various flight conditions including power from internal-combustion engines. These documents can be viewed on http://www.fai.org/gliding/approvaldocs So you can see that we have our work cut out in determining how the use of engine in quieter electric powered motor gliders should be recorded in the IGC file and clearly differentiated from common conditions encountered in non-powered flight. Any bright ideas will be gratefully received, as will IGC files from flights by electric-powered motor gliders to add to the GFAC database. But please don't just send files which have the engine at full power followed by gliding with all cockpit panels closed (although we would like these files as well). It is the critical ENL cases described above that we need to look at. A solution for the owners of electric-powered motor gliders that conforms to IGC standards of evidence will no doubt come out in due course. I am sorry that this post is so long, but the issue is not a simple one and quite a lot of words are needed to give a full picture. Ian Strachan Chairman IGC GNSS Flight Recorder Approval Committee (GFAC) |
#2
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Engine noise recording in Motor Gliders
Ian, here's an idea to toss into the mix.
No propeller can be perfectly balanced and even if it were, the propeller blades will pass through different airflow layers as they rotate generating vibration at a characteristic frequency. It would seem that an accelerometer inside the logger tuned to the frequency of a rotating prop could detect the vibration. In this case, one may think of the accelerometer as just a different type of microphone tuned to extremely low frequencies. Bill Daniels "Ian Strachan" wrote in message oups.com... In an earlier post, Ray Lovinggood said: From the following webpage: http://www.aerokurier.rotor.com/akNachr/aeNewsE.htm Antares too quiet. Lange Flugzeugbau faces an unusual hurdle with the type certification of its electric glider, Antares. The required noise level measurement according to Chapter X was not possible. The noise emissions from the 20-meter self-launching glider are so low that it can't be heard above the ambient sound of the surrounding environment. ..... An aircraft that is actually too quiet. Greg Arnold responded: I wonder if it makes enough noise for engine operation to be recorded by the ENL function of a flight recorder. Greg Arnold puts a very good question. Now that Antares is in production, the IGC GNSS Flight Recorder Approval Committee (GFAC) has been able to look at a selection of IGC files that have been kindly sent to us by some of the owners. Previously, GFAC had seen very few IGC files with noise signatures for Antares and these were not of the "critical cases" described below. Here are some figures: In an Antares flight in August 2006, ENL with engine off was recorded in the IGC file from 130 to 448 when cockpit panels were open and some sideslip was present. In comparison, in a run with level-flight engine power and a well-sealed cockpit, recorded ENL was from 019 to 120 and about 50 feet was gained. In another August Antares flight, climb with engine at about a 200 ft/minute gave ENL between 155 and 398 with an average of about 280. In this flight, ENL at full power was recorded between 294 and 769 with an average of about 450. The ENL system that produced these figures was that designed by LX Navigation and the recorder was an LX7000. These engine-on numbers are substantially less than those for an internal combustion engine. Also, the lower engine-on figures are well within the range that we have seen for non-powered flight, for instance thermalling with cockpit panels open. As an example of the "noisy glide" case, in a test on two new types of recorders that I flew on 8 August 2006, an ENL of 393 was recorded in gliding flight with the cockpit panel open using a Aircotec XC Profi recorder as a "control". The Aircotec ENL system is particularly good at differentiating between use of engine and other cockpit noises. Although 393 was a peak glide reading in deliberately high cockpit noise conditions, it shows you the potential problem in distinguishing the noise of a quiet engine from conditions that can be met in the glide. GFAC is in correspondence with Lange Flugzeugbau, the manufacturer of Antares, and is collecting more data from Antares flights before coming to a view on how recording of the operation of electric engines should be handled. Meanwhile, independent of the "electric engine" issue, two new types of recorder are being evaluated by GFAC, each with a new type of ENL system that is being tested in order to optimise the frequency and gain settings before IGC-approval can be given. This emphasises the point that there are many different types of ENL system with slightly different characteristics and we simply do not know at this stage how they will all respond to electric or other "quiet" engines. It may be that we have to look at recording extra variables such as engine or prop RPM or battery current to the engine, whichever is easier to record. It might be worthwhile re-stating what an ENL system is supposed to do. Fundamentally it must differentiate between any running of engine that generates forward thrust, and any flight condition encountered in normal soaring flight without such use of engine. The key word is "differentiate". The critical engine-on case with all motor gliders (in terms of "differentiating") is not when the engine is run at high power, that is the "easy one" in terms of ENL numbers in the IGC file. A combination of engine and propellor noise at high power should give ENL figures over, say 800 out of the maximum ENL in the IGC file of 999. Most two-stroke systems give over 900 and some ENL systems give the maximum of 999. Four stroke engines give lower figures but still enough to differentiate between power-on and power-off. Wankel rotary engines are fitted to several types of motor glider and have been no problem either (although at least one installation requires the engine to come out in order to change the spark plugs, but that is a different problem!). CRITICAL NOISE-LEVEL RECORDING CASES POWER-ON. The critical power-on case in terms of ENL differentiation is cruising power, that is, just sufficient power for level flight. You could say that there is also the case where a trickle of engine is used to extend the glide angle, but we have to draw the line somewhere! POWER-OFF. The critical ENL power-off case is not a quiet, well-sealed cockpit but is a noisy one, typically thermalling with air vents and cockpit panels open. This can certainly produce ENL figures up to 300, more if sideslip is present and 400 has been seen on some recorders. Another particularly noisy case is high speed flight with the cockpit panel(s) open, but this is probably not as realistic as thermalling with panels open and a bit of sideslip as controls are applied to adjust the circle. ENL SYSTEM DESIGN The three ENL numbers recorded with each fix in IGC files must therefore differentiate between the "quiet engine" and the "noisy cockpit" cases. This is done by carefully selecting the frequency and gain at which the ENL system is most sensitive. The ENL system is then tested by GFAC in a range of motor gliders, gliders and powered aircraft. Frequencies between 75 and 100Hz give good (high) response to engine and propeller noise and less response to cockpit noises. With all sorts of internal combustion engines the ENL system has proved reliable ever since the concept was invented by David Ellis, the original owner of Cambridge Aero Instruments, back in about 1994. Because it is all inside the recorder, an ENL system is easier for the pilot, compared to microswitch-based or other systems requiring some sensor hookup to the recorder. It is also better than vibration-based systems because with ENL the recorder does not need any special mounting in the cockpit or system checks by Official Observers before or after flight. In addition, there is not just one design of ENL system. Several companies have designed their own systems that are tested by GFAC before they are IGC-approved. Some companies have more than one ENL system, each optimised for different types of recorder. Different ENL systems in IGC-approved recorders include those by Aircotec, Cambridge (separate systems for the 300 series and the legacy models 10, 20 & 25), Garrecht, LXN (including recorders by Filser and SDI), NTE, Print Technik and Zander. Each of these ENL systems has slightly different characteristics. ENL figures for particular flight conditions will vary between these systems. For internal-combustion engines this has not been critical because they all produce a substantial amount of noise in the cockpit. However, we simply do not know how electric engines will register ENL values until we have a good database of IGC files from the different types of IGC-approved recorders. Annex B to the IGC-approval document for each type of recorder includes typical ENL numbers that were obtained during testing in the various flight conditions including power from internal-combustion engines. These documents can be viewed on http://www.fai.org/gliding/approvaldocs So you can see that we have our work cut out in determining how the use of engine in quieter electric powered motor gliders should be recorded in the IGC file and clearly differentiated from common conditions encountered in non-powered flight. Any bright ideas will be gratefully received, as will IGC files from flights by electric-powered motor gliders to add to the GFAC database. But please don't just send files which have the engine at full power followed by gliding with all cockpit panels closed (although we would like these files as well). It is the critical ENL cases described above that we need to look at. A solution for the owners of electric-powered motor gliders that conforms to IGC standards of evidence will no doubt come out in due course. I am sorry that this post is so long, but the issue is not a simple one and quite a lot of words are needed to give a full picture. Ian Strachan Chairman IGC GNSS Flight Recorder Approval Committee (GFAC) |
#3
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Engine noise recording in Motor Gliders
Hi Ian - The Antares has been in production for a while now !
Sepp Holzapfel won the Open Class DMSt 2005 in his Antares 20E. See: http://www.lange-flugzeugbau.de/htm/...news/news.html Conversely, in a different motor glider, I I had a flight rejected last year because the noise level was so high I must have started the motor (unfortunate ventilation design). Flight was OK'd when I showed them this was during cruise at 25 knots over motor redline... Of course, I didn't have Bumper's vent mod installed... See ya, Dave Ian Strachan wrote: Now that Antares is in production... Ian Strachan Chairman IGC GNSS Flight Recorder Approval Committee (GFAC) |
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