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

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

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

On 9/14/20 12:58 PM, Mana wrote:
I just had a chat with a fellow pilot whose friend sold his LAK 17b FES because he was disappointed by the ability of the FES to gain altitude. The problem being that when applying sufficient power to gain altitude, the battery would get warm to the point where the controller shuts the motor down. I read recently an accident report tied to the same issue, in that case pilot too low and not in reach of a landing spot (pilot mistake) who counted on FES to regain altitude, but as the battery was too hot it wouldn’t power the motor and the pilot crashed. On the other hand FES works reliably to maintain level flight at a lower power ratio.

Does any LAK 17 or Shark 304 FES (or other glider of similar weight) pilot have any feedback on this? In real life, how much altitude are you able to gain, with fresh and semi full batteries? Is it a matter of being patient and accepting a low rate of climb?

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.

I find this surprising and I’ll try to get in touch with the pilot who sold his LAK, but thought I would ask the question in the forum in the meantime.


Sorry to bump an old thread, but is the accident report mentioned here
readily available?

On Sunday, December 13, 2020 at 8:21:42 AM UTC-8, kinsell wrote:
On 9/14/20 12:58 PM, Mana wrote:
I just had a chat with a fellow pilot whose friend sold his LAK 17b FES because he was disappointed by the ability of the FES to gain altitude. The problem being that when applying sufficient power to gain altitude, the battery would get warm to the point where the controller shuts the motor down. I read recently an accident report tied to the same issue, in that case pilot too low and not in reach of a landing spot (pilot mistake) who counted on FES to regain altitude, but as the battery was too hot it wouldn’t power the motor and the pilot crashed. On the other hand FES works reliably to maintain level flight at a lower power ratio.

Does any LAK 17 or Shark 304 FES (or other glider of similar weight) pilot have any feedback on this? In real life, how much altitude are you able to gain, with fresh and semi full batteries? Is it a matter of being patient and accepting a low rate of climb?

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.

I find this surprising and I’ll try to get in touch with the pilot who sold his LAK, but thought I would ask the question in the forum in the meantime.


Sorry to bump an old thread, but is the accident report mentioned here
readily available?

The NTSB doesn't list any accidents for a LAK17. The only ones for "Lithuanian Aviation" are the Genesis 2.

Tom

On 12/13/20 6:02 PM, 2G wrote:
On Sunday, December 13, 2020 at 8:21:42 AM UTC-8, kinsell wrote:
On 9/14/20 12:58 PM, Mana wrote:
I just had a chat with a fellow pilot whose friend sold his LAK 17b FES because he was disappointed by the ability of the FES to gain altitude. The problem being that when applying sufficient power to gain altitude, the battery would get warm to the point where the controller shuts the motor down. I read recently an accident report tied to the same issue, in that case pilot too low and not in reach of a landing spot (pilot mistake) who counted on FES to regain altitude, but as the battery was too hot it wouldn’t power the motor and the pilot crashed. On the other hand FES works reliably to maintain level flight at a lower power ratio.

Does any LAK 17 or Shark 304 FES (or other glider of similar weight) pilot have any feedback on this? In real life, how much altitude are you able to gain, with fresh and semi full batteries? Is it a matter of being patient and accepting a low rate of climb?

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.

I find this surprising and I’ll try to get in touch with the pilot who sold his LAK, but thought I would ask the question in the forum in the meantime.


Sorry to bump an old thread, but is the accident report mentioned here
readily available?

The NTSB doesn't list any accidents for a LAK17. The only ones for "Lithuanian Aviation" are the Genesis 2.

Tom


There was a 17B FES lost in Pennsylvania Oct 2019, wasn't sure if that
was what Mana was talking about.


http://www.kathrynsreport.com/2019/1...es-n830dk.html