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FES underpowered for 18m ship?



 
 
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  #11  
Old September 15th 20, 02:12 AM posted to rec.aviation.soaring
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Default FES underpowered for 18m ship?


The power output can get lower with the state of charge because of two reasons:

Limit on the output current. Cables, heat dissipators, fuses and battery performances are related to the current. So it will be smart to restrict the current draw so you can save weight(read money also but mostly weight) on the components. So with less voltage and same current, less power.

Motor Kv is the ratio of RPMs vs Voltage and is constant under load. As the voltage of the packs gets lower the motor can turn at less speed.
Example: With full 120V battery the motor turns at 4500rpms and on near-empty state (85V), it will turn at 3200rpms.
You can arrange everything to make the motor turn at max RPM on any state of charge, but you will have some high-frequency energy losses and black magic stuff like that.
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  #12  
Old September 15th 20, 02:31 AM posted to rec.aviation.soaring
kinsell
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Default FES underpowered for 18m ship?

On 9/14/20 1:31 PM, Dave Nadler wrote:
On Monday, September 14, 2020 at 2:58:08 PM UTC-4, Mana wrote:
It would be disappointing if when you fly “by the rules” and remain above
a landing / outlanding spot until you turn power on, that the FES doesn’t
allow to regain altitude, but only to maintain level flight.
It changes the flight planning strategy altogether.


And it will be dangerous and fool-hardy if you DO NOT always:
- keep a landing spot in easy reach, and
- never engage power until the landing is planned in you're in position.

To quote a recent email I received from LBA/EASA:

...please let us point out that the pilot of a powered glider
shall always have in mind, that it might be necessary to operate
his aircraft as a pure glider. The engine of a powered glider,
predefined by airworthiness requirements JAR/CS-22, does not
meet the same safety standards as a "Part-E" engine of a motorplane.

The flight training for powered gliders shall take into account, that
loss of engine power may occur anytime, and result in a scenario, which is
comparable to a cable break during a winch launch or an aerotow.
This deviation to the operation of a motorplane is reflected in several
paragraphs of the airworthiness requirements JAR/CS-22, applicable for a
powered glider. Examples are the specifications for engines, used for
powered gliders (JAR/CS-22 Subpart H) that are less stringent than those
for powered aircraft (CS-23). Moreover, requirements for software are
not mentioned in the JAR/CS-22 at all - contrary to the specification
for large aeroplanes (CS-25).


Nice words, but I'm sure many FES pilots aren't going to heed them. If
they think the system is safer, they'll just push the limits that much
further.

From the mini-Lak brochure on lak.it:
"The FES propulsion system has no known reports of in-flight failures,
allowing you to stay confident in areas of no-lift."

We've seen the same mentality displayed in some of the Jeta discussions,
which has a more complex system.
  #13  
Old September 15th 20, 03:55 AM posted to rec.aviation.soaring
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Default FES underpowered for 18m ship?

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.
  #14  
Old September 15th 20, 04:35 AM posted to rec.aviation.soaring
Eric Greenwell[_4_]
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Default FES underpowered for 18m ship?

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
  #15  
Old September 15th 20, 05:06 AM posted to rec.aviation.soaring
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Default FES underpowered for 18m ship?

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
  #16  
Old September 15th 20, 06:29 AM posted to rec.aviation.soaring
2G
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Default FES underpowered for 18m ship?

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
  #17  
Old September 15th 20, 07:53 AM posted to rec.aviation.soaring
Matthew Scutter
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Default FES underpowered for 18m ship?

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.
  #18  
Old September 15th 20, 08:18 AM posted to rec.aviation.soaring
Paul T[_4_]
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Posts: 255
Default FES underpowered for 18m ship?

Just get a jet turbo far better - wouldn't trust FES for a climb in
mountain
conditions.

  #19  
Old September 15th 20, 08:53 AM posted to rec.aviation.soaring
krasw
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Default FES underpowered for 18m ship?

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.
  #20  
Old September 15th 20, 09:00 AM posted to rec.aviation.soaring
Matthew Scutter
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Default FES underpowered for 18m ship?

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.
 




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