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#1
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As someone who has experience working with EVs, this is alarming to read. Hearing this makes me skeptical of the FES's electrical design as a whole. Your electric car doesn't lose all of its performance if the battery gets to 30% (it does lose some, but not THIS much).
I believe the effect you're describing is the speed of the motor being limited by battery voltage. A well designed system should not have this problem, and this is an indication of a poor battery/motor/propping combo. As Emir said, these motors have a KV parameter, which describes how fast the motor will spin at a given input voltage. As the battery voltage drops, the maximum speed of the motor decreases as well. However, for a well-designed system, this voltage-limited speed, even at min battery voltage, is above the prop's 20kW speed. When the battery is fully charged and the system is capable of producing much more power, the software in the inverter limits it to 20kW for thermal protection. As Emir also stated, the inverter is most efficient running at 100% duty cycle, but the efficiency hit from running at partial power (switching losses) is on the order of 1-2%, which isn't terribly significant in context of the whole electrical system's ~90% efficiency. Electric cars have solved this problem, and they have to operate over much wider speed ranges and power ranges. This should not be a problem for props, since they operate over a much narrower speed range. |
#2
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wrote on 9/14/2020 7:55 PM:
As someone who has experience working with EVs, this is alarming to read. Hearing this makes me skeptical of the FES's electrical design as a whole. Your electric car doesn't lose all of its performance if the battery gets to 30% (it does lose some, but not THIS much). I believe the effect you're describing is the speed of the motor being limited by battery voltage. A well designed system should not have this problem, and this is an indication of a poor battery/motor/propping combo. As Emir said, these motors have a KV parameter, which describes how fast the motor will spin at a given input voltage. As the battery voltage drops, the maximum speed of the motor decreases as well. However, for a well-designed system, this voltage-limited speed, even at min battery voltage, is above the prop's 20kW speed. When the battery is fully charged and the system is capable of producing much more power, the software in the inverter limits it to 20kW for thermal protection. As Emir also stated, the inverter is most efficient running at 100% duty cycle, but the efficiency hit from running at partial power (switching losses) is on the order of 1-2%, which isn't terribly significant in context of the whole electrical system's ~90% efficiency. Electric cars have solved this problem, and they have to operate over much wider speed ranges and power ranges. This should not be a problem for props, since they operate over a much narrower speed range. There is a good benefit for the pilot if the designer takes advantage of the power available when the battery is fully charged and at a high voltage: the glider can take off sooner and climb faster during the critical few minutes near the ground. Yes, he could limit the initial power to be the same as the power near the end, but then to get that desirable strong takeoff, he must provide a larger, heavier, more expensive battery, ditto for the controller. For an FES glider, that may not be a desirable trade-off. The trade-off is likely different for gliders with mast-mounted motors and the batteries carried in the wings: the propeller can be larger and more efficient, and the batteries can be larger, as they are not constrained by the non-lifting weight limit on the fuselage, nor the weight the pilot can carry. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) - "A Guide to Self-Launching Sailplane Operation" https://sites.google.com/site/motorg...ad-the-guide-1 |
#3
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Hi Eric,
I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem. http://www.front-electric-sustainer....% 20v1.25.pdf They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries. https://www.mgm-compro.com/brushless...e-controllers/ I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant. Patrick Grady |
#4
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On Monday, September 14, 2020 at 9:06:08 PM UTC-7, wrote:
Hi Eric, I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem. http://www.front-electric-sustainer....% 20v1.25.pdf They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries. https://www.mgm-compro.com/brushless...e-controllers/ I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant. Patrick Grady I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES. There are many FES installations out there - if you have one, do this test and report the results. Tom |
#5
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On Tuesday, September 15, 2020 at 7:29:17 AM UTC+2, 2G wrote:
On Monday, September 14, 2020 at 9:06:08 PM UTC-7, wrote: Hi Eric, I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem. http://www.front-electric-sustainer....% 20v1.25.pdf They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries. https://www.mgm-compro.com/brushless...e-controllers/ I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant. Patrick Grady I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES. There are many FES installations out there - if you have one, do this test and report the results. Tom Not readily available? It's in the flight manual. If I adjust for 5.3kWh vs 4kWh batteries and 350kg weight of the Diana 2, it's ~2000m, which matches my napkin math from partial runs. As for why owners haven't tried it - it sounds boring... 5.3.4 Powered flight performance 5.3.4.1 Rate of climb The maximum rate of climb is available only for a few minutes with fully charged battery packs. As battery voltage is reduced, the maximum achievable climb rate is lower. The average rate of climb depends mostly on the type of sailplane and its take-off weight. Maximum attainable altitude gain that in standard atmosphere conditions depends on the type of sailplane, its weight and aerodynamic qualities. To achieve the maximum altitude gain, use about 15kW of power. Do not use full power as the efficiency of the system is lower. Usually, 80-85 km/h is best for the climb with positive flap setting (the same setting as used while thermaling). Here are rough numbers: • 1600 m (5200 ft) for UL sailplanes at 300kg take-off weight, i.e. Silent 2 Electro • 1400 m (4500 ft) for the 18m class sailplanes at 400kg take-off weight (without water ballast), i.e. LAK17A FES • 1200 m (3900 ft) for the 18m class sailplanes at 450kg take-off weight (without water ballast); LAK17B FES, Ventus 2cxa FES, Discus 2c FES, HPH 304ES 5.3.4.2 Cruise flight The maximum range of powered cruising flight, without the water ballast, is around 100km (62 miles), depending on lift-sink conditions. The optimum cruise speed and flap position depend on the type of sailplane. Usually, it is about 90 km/h (48 kts) at around 3000-3300 RPM and 4kW of power with a positive flap setting, as used in thermals. |
#6
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Just get a jet turbo far better - wouldn't trust FES for a climb in
mountain conditions. |
#7
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In order to climb out of deep valley with limited exit is problem that 99% of pilots will never have, and those 1% should reconsider another sport. This is not a problem meant to be solved with tiny engines.
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#8
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On Tuesday, September 15, 2020 at 9:30:06 AM UTC+2, Paul T wrote:
Just get a jet turbo far better - wouldn't trust FES for a climb in mountain conditions. How does a jet help? I believe both the PSA jets and the JS jets have approximately the same climb altitude, ~1500m, and range ~110km? See http://js3.at/wp-content/uploads/201...Supplement.pdf Performance seems about the same, the biggest difference as I can tell is trading reliability for cruise speed. |
#9
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On Monday, September 14, 2020 at 11:54:00 PM UTC-7, Matthew Scutter wrote:
On Tuesday, September 15, 2020 at 7:29:17 AM UTC+2, 2G wrote: On Monday, September 14, 2020 at 9:06:08 PM UTC-7, wrote: Hi Eric, I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem. http://www.front-electric-sustainer....% 20v1.25.pdf They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries. https://www.mgm-compro.com/brushless...e-controllers/ I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant. Patrick Grady I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES. There are many FES installations out there - if you have one, do this test and report the results. Tom Not readily available? It's in the flight manual. If I adjust for 5.3kWh vs 4kWh batteries and 350kg weight of the Diana 2, it's ~2000m, which matches my napkin math from partial runs. As for why owners haven't tried it - it sounds boring... 5.3.4 Powered flight performance 5.3.4.1 Rate of climb The maximum rate of climb is available only for a few minutes with fully charged battery packs. As battery voltage is reduced, the maximum achievable climb rate is lower. The average rate of climb depends mostly on the type of sailplane and its take-off weight. Maximum attainable altitude gain that in standard atmosphere conditions depends on the type of sailplane, its weight and aerodynamic qualities. To achieve the maximum altitude gain, use about 15kW of power. Do not use full power as the efficiency of the system is lower. Usually, 80-85 km/h is best for the climb with positive flap setting (the same setting as used while thermaling). Here are rough numbers: • 1600 m (5200 ft) for UL sailplanes at 300kg take-off weight, i.e. Silent 2 Electro • 1400 m (4500 ft) for the 18m class sailplanes at 400kg take-off weight (without water ballast), i.e. LAK17A FES • 1200 m (3900 ft) for the 18m class sailplanes at 450kg take-off weight (without water ballast); LAK17B FES, Ventus 2cxa FES, Discus 2c FES, HPH 304ES 5.3.4.2 Cruise flight The maximum range of powered cruising flight, without the water ballast, is around 100km (62 miles), depending on lift-sink conditions. The optimum cruise speed and flap position depend on the type of sailplane. Usually, it is about 90 km/h (48 kts) at around 3000-3300 RPM and 4kW of power with a positive flap setting, as used in thermals. And you REALLY believe that? If so, I've got a bridge for sale. No, I want to see the INDEPENDENT verification of this data. Tom |
#10
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On Wednesday, September 16, 2020 at 4:07:16 AM UTC+2, 2G wrote:
On Monday, September 14, 2020 at 11:54:00 PM UTC-7, Matthew Scutter wrote: On Tuesday, September 15, 2020 at 7:29:17 AM UTC+2, 2G wrote: On Monday, September 14, 2020 at 9:06:08 PM UTC-7, wrote: Hi Eric, I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem. http://www.front-electric-sustainer....% 20v1.25.pdf They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries. https://www.mgm-compro.com/brushless...e-controllers/ I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant. Patrick Grady I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES. There are many FES installations out there - if you have one, do this test and report the results. Tom Not readily available? It's in the flight manual. If I adjust for 5.3kWh vs 4kWh batteries and 350kg weight of the Diana 2, it's ~2000m, which matches my napkin math from partial runs. As for why owners haven't tried it - it sounds boring... 5.3.4 Powered flight performance 5.3.4.1 Rate of climb The maximum rate of climb is available only for a few minutes with fully charged battery packs. As battery voltage is reduced, the maximum achievable climb rate is lower. The average rate of climb depends mostly on the type of sailplane and its take-off weight. Maximum attainable altitude gain that in standard atmosphere conditions depends on the type of sailplane, its weight and aerodynamic qualities. To achieve the maximum altitude gain, use about 15kW of power. Do not use full power as the efficiency of the system is lower. Usually, 80-85 km/h is best for the climb with positive flap setting (the same setting as used while thermaling). Here are rough numbers: • 1600 m (5200 ft) for UL sailplanes at 300kg take-off weight, i.e. Silent 2 Electro • 1400 m (4500 ft) for the 18m class sailplanes at 400kg take-off weight (without water ballast), i.e. LAK17A FES • 1200 m (3900 ft) for the 18m class sailplanes at 450kg take-off weight (without water ballast); LAK17B FES, Ventus 2cxa FES, Discus 2c FES, HPH 304ES 5.3.4.2 Cruise flight The maximum range of powered cruising flight, without the water ballast, is around 100km (62 miles), depending on lift-sink conditions. The optimum cruise speed and flap position depend on the type of sailplane. Usually, it is about 90 km/h (48 kts) at around 3000-3300 RPM and 4kW of power with a positive flap setting, as used in thermals. And you REALLY believe that? If so, I've got a bridge for sale. No, I want to see the INDEPENDENT verification of this data. Tom Only on RAS do you get called biased or asked if you really believe your own experiences owning and operating - couldn't make it up if you tried. Regrettably my certification as an independent standards body is still processing. My only comment on the manuals data validity would be the 4kW is not applicable to all types, the cruising flight power estimate should be scaled like the climb altitude. The heavier HpH Sharks at eGlide this year told me they were cruising with 5-6kW. |
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