FES underpowered for 18m ship?

[Mana]

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.

[Dave Nadler]

Post by Mana
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).

[kinsell]

Post by Dave Nadler

Post by Mana
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.

- keep a landing spot in easy reach, and
- never engage power until the landing is planned in you're in position.
...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.

[Dave Nadler]

Post by kinsell
"The FES propulsion system has no known reports of in-flight failures,
allowing you to stay confident in areas of no-lift."

That's just Nonsense. I know of an instance where the controller failed while
FES was under power and the engine quit. I hardly hear about all problems,
surely there have been others...

[kinsell]

Post by Dave Nadler

Post by kinsell
"The FES propulsion system has no known reports of in-flight failures,
allowing you to stay confident in areas of no-lift."

That's just Nonsense. I know of an instance where the controller failed while
FES was under power and the engine quit. I hardly hear about all problems,
surely there have been others...

Of course it's nonsense, but if you tell people exactly what they want
to hear, they gobble it up. Red meat for the base.

The Silent 2 that went through the roof in Connecticut made quite a
splash, I'd call that an inflight failure. Apparently if a battery
explodes on landing roll, that doesn't count as "in flight".

People say motors have been around forever, that's true. But inverters
that take high-voltage DC, convert it to three-phase power at over 25KW
using transistors, and doing it with limited space and cooling, well
that's not something you run down to Grainger to pick up.

I'm not familiar with the FES accident referenced by the OP, would like
more data on that.

-Dave

[Matthew Scutter]

I think something has been lost in translation, but I'm pretty sure you will not shut down from battery heat - my batteries barely change temperature regardless of use, and neither controller nor motor have temperature problems even running through most of the capacity at full power.
What does happen is the voltage quickly drops as used capacity goes up, such that while I can get ~20kW (>5kt climb) of power at full charge, at 30% charge remaining I can only get 8kW (1-2kt climb). I haven't discharged below that yet but I understand you reach the point where the voltage doesn't sustain level flight at approximately the same time you reach 0% charge.

The FES is most efficient at low power so it's much better to climb to lowest safe altitude, then go sideways at level flight power. I need about 3.5kW for level flight at 350kg (Diana 2-FES), which with the 5.3kWh batteries seems to get me about 150km. I haven't measured the full climb height at high power, but extrapolating my shorter climbs I think I should get 5000ft before I can only go sideways.
Range/altitude seems to scale linearly with weight if you're at 450kg or 500kg like some of those 18m ships with a heavy pilot, you can calculate the fractions accordingly.

I think you can plan on smaller margins with the FES since the engine-not-running drag is much lower with the FES than a stuck-out pylon. You'll have to make your own decisions of course.

Post by Mana
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.

[Mana]

Thank you Matthew, this is very interesting data, exactly what I was looking for. :-)

BTW battery low = lower voltage, so for same output power to the motor you need more amps, hence the battery warms faster (P=U*I = R*I*I).

Post by Dave Nadler
- keep a landing spot in easy reach, and
- never engage power until the landing is planned in you're in position.

Agreed, of course! But the whole point of a "turbo" is to be able to regain altitude :-) If FES only allowed level flight, then it may expand the area where you'll find the thermal you need, but since you need to keep a landing spot in reach for the exact reasons you pointed out, level flight only would be very limiting.

It could be that the pilot in question used FES regularly during his flights and found himself with battery low towards the end (vs. only using FES once with a fresh battery in case of trouble)? I asked for his contact data to get real facts vs. speculation and I'll report if I'm able to reach him.

[Dave Nadler]

...the whole point of a "turbo" is to be able to regain altitude :-)

Right, but many have extremely slow climb, especially in high/hot locales.
There electric of course doesn't loose much efficiency,
but batteries/motor may suffer.

Antares at Uvalde required low power settings and could only launch
to low altitude before battery temp hit max.

It could be that the pilot in question used FES regularly
during his flights and found himself with battery low towards the end...
I'll report if I'm able to reach him.

That would be most helpful, Thanks!

PS: Does FES motor ever overheat (again especially in hot locales)??

[Matthew Scutter]

Post by Dave Nadler

...the whole point of a "turbo" is to be able to regain altitude :-)

Right, but many have extremely slow climb, especially in high/hot locales.
There electric of course doesn't loose much efficiency,
but batteries/motor may suffer.
Antares at Uvalde required low power settings and could only launch
to low altitude before battery temp hit max.

It could be that the pilot in question used FES regularly
during his flights and found himself with battery low towards the end...
I'll report if I'm able to reach him.

That would be most helpful, Thanks!
PS: Does FES motor ever overheat (again especially in hot locales)??
BTW battery low = lower voltage, so for same output power to the motor you need more amps, hence the battery warms faster (P=U*I = R*I*I).

Right, but output power is continually coming down as the voltage comes down as it seems amps stay more or less constant or decrease.
I only have a tenuous understanding of electronics and battery chemistry, but I'm told because the FES uses pouch cells rather than cylindrical cells the internal resistance is much lower, which would explain why the battery doesn't seem to meaningfully heat. I have not seen my batteries get above 25c.

I'll be flying it in Australia later this year so I'll return with my experience on motor temperatures. So far in Europe it seems to plateau around 55c at high power. When I asked Luka about Australia, he seemed to think it wouldn't be an issue and told me there was a very large margin of safety on both the motor and controller temperature. Theoretically if you did manage to overheat it though, you could demagnetize the permanent magnets.

[Kevin Neave]

Definitely not underpowered for 18m D2c (400 kg)

I can climb quickly from 200m to 400m (500 - 1200ft) then have
enough left for 75km in stillish air. Low power is more efficient so
if you recognise the day is dying earlier you don’t need to invest
the charge in climbing so range is much better. It will never
match the range of a conventional turbo but will always get you
away from a farmer’s field to an airfield near home.

Once the FES is running you’re still “soaring” to maximise range.
Still playing the same game, making the same decisions, just
have way better L/D.

I have access to both and first choice is the FES every time.

Post by Mana
Thank you Matthew, this is very interesting data, exactly what I

was

Post by Mana
lookin=
g for. :-)
BTW battery low =3D lower voltage, so for same output power

to the motor

Post by Mana
yo=
u need more amps, hence the battery warms faster (P=3DU*I

=3D R*I*I).=20

Post by Mana
On Monday, September 14, 2020 at 9:31:25 PM UTC+2,

Post by Dave Nadler
And it will be dangerous and fool-hardy if you DO NOT

always:=20

Post by Mana

Post by Dave Nadler
- keep a landing spot in easy reach, and=20
- never engage power until the landing is planned in you're in

position.=

Post by Mana
=20
=20
Agreed, of course! But the whole point of a "turbo" is to be able

to

Post by Mana
regain=
altitude :-) If FES only allowed level flight, then it may expand

the

Post by Mana
area=
where you'll find the thermal you need, but since you need to

keep a

Post by Mana
landi=
ng spot in reach for the exact reasons you pointed out, level

flight only

Post by Mana
w=
ould be very limiting.=20
=20
It could be that the pilot in question used FES regularly during

his

Post by Mana
flight=
s and found himself with battery low towards the end (vs. only

using FES

Post by Mana
on=
ce with a fresh battery in case of trouble)? I asked for his

contact data

Post by Mana
t=
o get real facts vs. speculation and I'll report if I'm able to reach

him.

[jfitch]

Depends a bit on where you fly. In the western US (absent pyrocumulus to soar, which has been reliable of late....) the nearest airport may be 150 km away, and it you waited until 500 ft you might have to climb 4000 ft over a ridge to get there. There are a few reasons why ICE turbos still outsell electric by something like 4:1. I like the idea of electrics, but for my use it needs about 3x the energy storage currently available. Even the shortest simple retrieve where I fly (let's say Carson City to Truckee) will require a 4000 ft climb. More practical in the flatlands of the east or Europe where airports are close and ridges are lower.

Post by Kevin Neave
Definitely not underpowered for 18m D2c (400 kg)
I can climb quickly from 200m to 400m (500 - 1200ft) then have
enough left for 75km in stillish air. Low power is more efficient so
if you recognise the day is dying earlier you don’t need to invest
the charge in climbing so range is much better. It will never
match the range of a conventional turbo but will always get you
away from a farmer’s field to an airfield near home.
Once the FES is running you’re still “soaring” to maximise range.
Still playing the same game, making the same decisions, just
have way better L/D.
I have access to both and first choice is the FES every time.

Post by Mana
Thank you Matthew, this is very interesting data, exactly what I

was

Post by Mana
lookin=
g for. :-)
BTW battery low =3D lower voltage, so for same output power

to the motor

Post by Mana
yo=
u need more amps, hence the battery warms faster (P=3DU*I

=3D R*I*I).=20

Post by Mana
On Monday, September 14, 2020 at 9:31:25 PM UTC+2,

Post by Dave Nadler
And it will be dangerous and fool-hardy if you DO NOT

always:=20

Post by Mana

Post by Dave Nadler
- keep a landing spot in easy reach, and=20
- never engage power until the landing is planned in you're in

position.=

Post by Mana
=20
=20
Agreed, of course! But the whole point of a "turbo" is to be able

to

Post by Mana
regain=
altitude :-) If FES only allowed level flight, then it may expand

the

Post by Mana
area=
where you'll find the thermal you need, but since you need to

keep a

Post by Mana
landi=
ng spot in reach for the exact reasons you pointed out, level

flight only

Post by Mana
w=
ould be very limiting.=20
=20
It could be that the pilot in question used FES regularly during

his

Post by Mana
flight=
s and found himself with battery low towards the end (vs. only

using FES

Post by Mana
on=
ce with a fresh battery in case of trouble)? I asked for his

contact data

Post by Mana
t=
o get real facts vs. speculation and I'll report if I'm able to reach

him.

[[e-mail]]

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.

[waremark]

'I can climb quickly from 200m to 400m (500 - 1200ft) then have
enough left for 75km in stillish air. Low power is more efficient so
if you recognise the day is dying earlier you don’t need to invest
the charge in climbing so range is much better. It will never
match the range of a conventional turbo but will always get you
away from a farmer’s field to an airfield near home.'

Are you saying that you would cruise home level at 1,200 foot above the ground? Depending on the terrain you are flying over, how can that enable you always to have somewhere to land if the motor dies? In my petrol self-launcher, even over fairly flat land, I would typically climb at least to 3,000 foot before leveling off and cruising home at 85 knots (if I am not feeling impatient I generally climb as high as I need to glide home).

[Kevin Neave]

It's a different mindset. With your self launcher, and my conventiona
turbo, if you start the engine you drive home.

With my FES if I need to climb I only climb as little as possible, then I'
back in the "Soaring" game to get as close to home as I can. I just hav
the option of setting my glide angle to something more favourable. And i
the UK will almost certainly have the option of an Airfield on the way hom
if I need it

If the engine dies then I'm still in the same situation all of us are in
so I don't fly over unlandable terrain with or without the FES running
(I'll make an exception for the Solent)

Flying in the UK 1200ft normally gives a pretty wide choice of fields

KN

Post by waremark
'I can climb quickly from 200m to 400m (500 - 1200ft) then have
enough left for 75km in stillish air. Low power is more efficient so
if you recognise the day is dying earlier you don=E2=80=99t need t

invest

Post by waremark
the charge in climbing so range is much better. It will never
match the range of a conventional turbo but will always get you
away from a farmer=E2=80=99s field to an airfield near home.'
Are you saying that you would cruise home level at 1,200 foot above the
gro=
und? Depending on the terrain you are flying over, how can that enabl

you

Post by waremark
=
always to have somewhere to land if the motor dies? In my petrol
self-launc=
her, even over fairly flat land, I would typically climb at least t

3,000

Post by waremark
=
foot before leveling off and cruising home at 85 knots (if I am not
feeling=
impatient I generally climb as high as I need to glide home).

[Stephen Struthers]

At 13:50 24 September 2020, Kevin Neave wrote:
t's a different mindset. With your self launcher, and my conventional
turbo, if you start the engine you drive home.

With my FES if I need to climb I only climb as little as possible, then I'

back in the "Soaring" game to get as close to home as I can. I just have
the option of setting my glide angle to something more favourable. And
i the UK will almost certainly have the option of an Airfield on the way
home if I need it

If the engine dies then I'm still in the same situation all of us are in
so I don't fly over unlandable terrain with or without the FES running
(I'll make an exception for the Solent)

Flying in the UK 1200ft normally gives a pretty wide choice of fields

KN

Ok so the UK stops North of Hadrian's wall !!

Like to see you soar round my neck of the woods at 1200 ft AGL lol

(apologies for post hijack)

[Mana]

Post by Matthew Scutter
I think you can plan on smaller margins with the FES since the engine-not-running drag is much lower with the FES than a stuck-out pylon. You'll have to make your own decisions of course.

That's what I find so cool about FES. The JS3 RES has a pylon with motor in front, whereas the GP15 Jeta has the motor in the back, which at least allows to fold the prop and minimize drag in case it gets stuck. I'm not sure if the Antares prop folds if the pylon is stuck?

FES is a great illustration of KISS: keep it simple, then fewer things can go wrong - and the 1 point L/D penalty is OK by me. I'm on the market for a FES glider and currently debating between LAK 17 and Shark 304.

[Dave Nadler]

Post by Mana
I'm not sure if the Antares prop folds if the pylon is stuck?

Nope. But drag isn't too bad (engine bay doors closed, no radiator).

[Darren Braun]

With RC front electric gliders you need a soft brake to keep the prop from windmilling in flight. Is that the same engine-not-running drag referred to here? i.e do today's FES have a failure mode like this?

[Matthew Scutter]

With RC front electric gliders you need a soft brake to keep the prop from windmilling in flight. Is that the same engine-not-running drag referred to here? i.e do today's FES have a failure mode like this?

Yes it does, I have had it windmill from inadvertently hitting the power off rather throttling down first a few times. I have no idea how much drag it really is, but I was able to continue climbing in a thermal while I restarted it to then stop it properly.
What I'd really like to see is the ability to actually recharge the batteries via the propeller brake, imagine prestart after a self launch, waiting for the line to open for an hour at cloudbase, slowly charging back up. Maybe someone can do the numbers on the drag that would be induced to charge at 1kW to determine if that's a totally harebrained idea.

Flight manual again:
3.7.2 Power loss during flight
If power is lost during flight, the propeller will windmill. Push the control stick
forward gently, to sustain the desired airspeed! You can perform the following actions to
try and restore power:
1. Check first if you unintentionally switched OFF the power switch!
Warning: This can happen in gliders that thave the landing gear lever and
the power switch located on the same side of the cockpit when retracting
the landing gear, i.e. LAK17A&B FES.
If this happens, switch power switch ON and adjust the throttle.
Note: On earlier software versions (before v2.13), it was necessary to
reduce throttle bar to zero manually; otherwise motor did not start due to
safety. Motor restarted when the throttle was reduced to zero
New versions (from FCU v2.13) automatically reset the throttle!
2. If the power switch is ON:
• Switch OFF the “Power switch” and the FCU.
• Turn ON the FCU and check for strange behaviour.
• If the FCU has no issues switch the power switch ON and try to start the motor.
The motor starts but behaves strangely under power:
• Stop the propeller from the windmilling with the electronic brake.
• When the propeller stops, switch OFF the power switch and the FCU.
If you are not able to stop the propeller with the electronic brake, you will need to land
with a windmilling propeller. Note: it is not possible to stop the propeller by
reducing airspeed. Try to land on both landing wheels simultaneously, to avoid potential
damage of the propeller.
Note: It is probably better to use a grass runway in good condition if one is
available than a concrete runway. If the grass runway is in bad shape, use a
concrete runway if one is available.
Warning: Try to avoid landing into high grass or similar.
Note: The L/D of a sailplane with a windmilling propeller is reduced only by
a small amount. With enough altitude will have enough time to choose a
suitable landing field.

[[e-mail]]

That is definitely the future and awesome idea Matthew,

Pasi

Post by Matthew Scutter

With RC front electric gliders you need a soft brake to keep the prop from windmilling in flight. Is that the same engine-not-running drag referred to here? i.e do today's FES have a failure mode like this?

Yes it does, I have had it windmill from inadvertently hitting the power off rather throttling down first a few times. I have no idea how much drag it really is, but I was able to continue climbing in a thermal while I restarted it to then stop it properly.
What I'd really like to see is the ability to actually recharge the batteries via the propeller brake, imagine prestart after a self launch, waiting for the line to open for an hour at cloudbase, slowly charging back up. Maybe someone can do the numbers on the drag that would be induced to charge at 1kW to determine if that's a totally harebrained idea.
3.7.2 Power loss during flight
If power is lost during flight, the propeller will windmill. Push the control stick
forward gently, to sustain the desired airspeed! You can perform the following actions to
1. Check first if you unintentionally switched OFF the power switch!
Warning: This can happen in gliders that thave the landing gear lever and
the power switch located on the same side of the cockpit when retracting
the landing gear, i.e. LAK17A&B FES.
If this happens, switch power switch ON and adjust the throttle.
Note: On earlier software versions (before v2.13), it was necessary to
reduce throttle bar to zero manually; otherwise motor did not start due to
safety. Motor restarted when the throttle was reduced to zero
New versions (from FCU v2.13) automatically reset the throttle!
• Switch OFF the “Power switch” and the FCU.
• Turn ON the FCU and check for strange behaviour.
• If the FCU has no issues switch the power switch ON and try to start the motor.
• Stop the propeller from the windmilling with the electronic brake.
• When the propeller stops, switch OFF the power switch and the FCU.
If you are not able to stop the propeller with the electronic brake, you will need to land
with a windmilling propeller. Note: it is not possible to stop the propeller by
reducing airspeed. Try to land on both landing wheels simultaneously, to avoid potential
damage of the propeller.
Note: It is probably better to use a grass runway in good condition if one is
available than a concrete runway. If the grass runway is in bad shape, use a
concrete runway if one is available.
Warning: Try to avoid landing into high grass or similar.
Note: The L/D of a sailplane with a windmilling propeller is reduced only by
a small amount. With enough altitude will have enough time to choose a
suitable landing field.

[[e-mail]]

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.

[Eric Greenwell]

Post by ***@gmail.com
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.

[[e-mail]]

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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-controllers/

I'd be interested to hear FES's reasoning, or other owners' experiences on why the power dropoff is so significant.

Patrick Grady

[2G]

Post by ***@gmail.com
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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-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

[Matthew Scutter]

Post by 2G

Post by ***@gmail.com
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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-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.

[Paul T]

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

[krasw]

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.

[jfitch]

Again, this depends on where you fly. I fly out of Truckee usually, and a typical retrieve is from Carson City as I mentioned. It is only 20 miles away, but with a 4000 ft high ridge in between. 100% of the pilots flying cross country out of Truckee will have this problem eventually. It isn't dangerous - a perfectly good airport at Carson - but electric sustainers in their current state of development will not support that retrieve, but an ICE will. There are countless other similar examples in the Great Basin area of a "deep valley with limited exit" - and a good landing site at the bottom. If you don't fly over these, you don't fly in this area. That is not to say the electric isn't useful (and I didn't say that), just that it has limitations in some terrain that the ICE may not (as in the Alps example mentioned above).

Post by krasw
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.

[Ramy]

John, for the Carson relight you are correct if one waits until pattern altitude at Carson. However the strategy should be to relight over spooner the moment you down to your minimum altitude, say 9K. You will need a short run and only 1-2K climb to get to a safe final glide altitude. If you run out of battery before hand you can escape back to Carson.
At least this is what I would do if I have FES.

Ramy

[Cumungus]

Post by Ramy
John, for the Carson relight you are correct if one waits until pattern altitude at Carson. However the strategy should be to relight over spooner the moment you down to your minimum altitude, say 9K. You will need a short run and only 1-2K climb to get to a safe final glide altitude. If you run out of battery before hand you can escape back to Carson.
At least this is what I would do if I have FES.
Ramy

Hi Ramy. Is this what happened last month when you had an FES failure and landed on the golf course in incline?

Something like:
- flies over east lake tahoe
- "getting low, here we go!"
- goes to turn the FES knob
- "dang, did I really leave my FES at the factory?"
- wing meets bush

Just wondering since I can't think of any other plausible explanation to why someone would go where you went at the altitude you were at without some engine or a will to die.

[Eric Greenwell]

Post by Cumungus

Post by Ramy
John, for the Carson relight you are correct if one waits until pattern altitude at Carson. However the strategy should be to relight over spooner the moment you down to your minimum altitude, say 9K. You will need a short run and only 1-2K climb to get to a safe final glide altitude. If you run out of battery before hand you can escape back to Carson.
At least this is what I would do if I have FES.
Ramy

Hi Ramy. Is this what happened last month when you had an FES failure and landed on the golf course in incline?
- flies over east lake tahoe
- "getting low, here we go!"
- goes to turn the FES knob
- "dang, did I really leave my FES at the factory?"
- wing meets bush
Just wondering since I can't think of any other plausible explanation to why someone would go where you went at the altitude you were at without some engine or a will to die.

You sound upset. Did Ramy's explanation offend you in some way? I thought he
provided a good description of the situation, which is something we rarely get, as
the pilot is either dead or unwilling to talk.

[Ramy]

Cumungus, you forgot to sign your name, so I need to ask: Do you fly from Truckee? Are you familiar with the area, the return methods, ridge soaring spots and landout options? Did you walk the golf course before?
I believe I answered all your questions in my detailed “I screwed up” report. Will you do the same if you screw up, and sign your name?
And back to the subject, using the FES as I described will eliminate this risk completely. Just make your rule of minimum safe altitude at spooner and stick to your rule and you will never need to land in the golf course. And no, one should not enter as low as I did relying on engine. It is not any safer than relying on ridge lift.

Ramy

[Dave Nadler]

relying on engine... is not any safer than relying on ridge lift.

Its considerably less safe...

[Eric Greenwell]

Post by Dave Nadler

relying on engine... is not any safer than relying on ridge lift.

Its considerably less safe...

You must read the ridges much better than I do :^)

[jfitch]

Yes, that would work. You need a different strategy (and perhaps different discipline) with the FES vs. an ICE. I usually keep trying until I'm a bit over pattern altitude, then fire. There are some other scenarios such as storm cells over intended destination, land somewhere and wait it out, then continue but these are admittedly rare. I believe every retrieve I have done in flying my ASH at Truckee for 20 years would have been as easily done with an FES. Now it just needs to self launch to 3000 AGL and still be able to do those retrieves, and charge itself somehow on the line overnight. :)

Post by Ramy
John, for the Carson relight you are correct if one waits until pattern altitude at Carson. However the strategy should be to relight over spooner the moment you down to your minimum altitude, say 9K. You will need a short run and only 1-2K climb to get to a safe final glide altitude. If you run out of battery before hand you can escape back to Carson.
At least this is what I would do if I have FES.
Ramy

[Matthew Scutter]

Post by Paul T
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/2018/11/JS-MD_3_Jet_Sustainer_Supplement.pdf

Performance seems about the same, the biggest difference as I can tell is trading reliability for cruise speed.

[Dave Walsh]

A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

[Mana]

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

The technology of the motor is very mature and much simpler than of a jet or a combustion engine, which has a direct impact on maintenance needs and on reliability. I agree that the batteries do add cost, complexity and fire hazard (until we find a better technology than LiPo).

The FES has way few moving parts than a pylon. If it doesn’t work, you just keep flying the same glider. See Dave’s talk on motor-gliders for a good overview of all that can go wrong with pylons http://www.nadler.com/papers/2018_So_You_Think_You_Want_A_Motorglider_updated_2.pdf.

[Tango Whisky]

Mana,

I would never think about a FES for flying in Fayence.
I you want to escape the "cuvette", just take a higher tow. With a FES, you will eat up all capacity with nothing left for a retrieve (think about needing to motor over the Col d'Etache...).
Internal combustion engines nowadays all rely on the Solo 2350 engine, which is remarably reliable, and with 15 liters of fuel, you will have way more capacity to climb than a FES.

I don't think that a FES is a good choice for the Alpine terrain.

Bert
Ventus cM "TW"

Post by Mana

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

The technology of the motor is very mature and much simpler than of a jet or a combustion engine, which has a direct impact on maintenance needs and on reliability. I agree that the batteries do add cost, complexity and fire hazard (until we find a better technology than LiPo).
The FES has way few moving parts than a pylon. If it doesn’t work, you just keep flying the same glider. See Dave’s talk on motor-gliders for a good overview of all that can go wrong with pylons http://www.nadler.com/papers/2018_So_You_Think_You_Want_A_Motorglider_updated_2.pdf.

[[e-mail]]

As always, there is no absolute solution.
For a couple of flight missions, FES is unbeatable.
Depending on the pilot and how he want to fly you have a wide range of offers on the market.
Not only in propulsion systems but also in glider performances.

[Dave Nadler]

Post by Mana
The technology of the motor is very mature and much simpler than of
a jet or a combustion engine, which has a direct impact on maintenance
needs and on reliability.

But you're asking the wrong question. Whilst the TECHNOLOGY is mature,
the SPECIFIC UNITS we are using in gliders are engineered and built by small
firms in tiny quantities, and not subjected to the kind of testing
and reliability validation we take for granted in our cars, appliances, etc.

[Dave Nadler]

Post by Mana
See Dave’s talk on motor-gliders for a good overview of all that
can go wrong with pylons

http://www.nadler.com/papers/2018_So_You_Think_You_Want_A_Motorglider_updated_2.pdf.

I'm flattered but perhaps more relevant is the OSTIV talk I did in February:

[andy l]

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

The discussion seems fairly simple on here - some people know what they are talking about, some are pontificating or speculating, some have actually flown one and read the manual

[Kenn Sebesta]

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

I couldn't disagree more. Electric motors are amazingly simple and reliable, and LiPo batteries-- in general-- are extremely safe. The issue arises when you start pushing the technology to its maturity limits. At the power/weight/energy limits we want for the gliders we start making compromises which push into research territory. However, that is a far cry from assuming there is anything complex about the system. We just don't know how to use industrial-grade COTS parts to make the glider's system as light as we desire, so we wind up having to have some bespoke parts. The bespoke parts aren't as well tested because the fleet is small, and this is where problems creep in.

The motors themselves have 0 problems and a 20-30kW motor is tiny in the scheme of industrial COTS motors.

[u***@earthlink.net]

Post by Kenn Sebesta

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

I couldn't disagree more. Electric motors are amazingly simple and reliable, and LiPo batteries-- in general-- are extremely safe. The issue arises when you start pushing the technology to its maturity limits. At the power/weight/energy limits we want for the gliders we start making compromises which push into research territory. However, that is a far cry from assuming there is anything complex about the system. We just don't know how to use industrial-grade COTS parts to make the glider's system as light as we desire, so we wind up having to have some bespoke parts. The bespoke parts aren't as well tested because the fleet is small, and this is where problems creep in.
The motors themselves have 0 problems and a 20-30kW motor is tiny in the scheme of industrial COTS motors.

Given your apparent claimed knowledge could you please list available COT components and sources for such a glider.
Motor 25 kw
Controller
Interface
Battery system with BMS and charging system
How hard could it be?
UH

[Kenn Sebesta]

Post by u***@earthlink.net
Given your apparent claimed knowledge could you please list available COT components and sources for such a glider.
Motor 25 kw
Controller
Interface
Battery system with BMS and charging system
How hard could it be?
UH

I think we're saying the same thing. For the most part, industrial-grade COTS parts for a glider FES-style system don't exist. They are either an order of magnitude larger (cars) or smaller (robotics).

BTW, don't forget the propeller. There aren't many out there for cruising at 50kts while only absorbing 5kW of power.

[Eric Greenwell]

Post by Dave Walsh
A real problem is putting the word "KISS" and "FES" is the same
sentence or indeed "KISS" and "electric propulsion".
There is nothing simple about electric motors powered by LiPo batteries.

It should be obvious the "KISS" remark refers to a comparison with other means of
propulsion, and not to a sailplane without a motor. Based on my experience with a
mast mounted combustion engine and discussions with owners of FES gliders, I
believe the FES system is less complex to own and operate.

And, as we all know, a motorless glider has it's own complexities, such as getting
tow when and where desired, and needing a retrieve if it doesn't get home.

[Dave Nadler]

Post by Matthew Scutter
How does a jet help?

Obviously, a jet makes you look much cooler when you land out.

[Dave Nadler]

...I believe both the PSA jets and the JS jets have approximately
the same climb altitude, ~1500m, and range ~110km?

Paul Mander figured the utility of the jets in his ASH-25J were
about equivalent to electric (heavy load of jet fuel compared
to batteries, by total available climb height). Of course jet
cruises faster.

[Paul T]

Post by Matthew Scutter

Post by Paul T
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/2018/11/JS-

MD_3_Jet_Sustainer_Supplement.pdf

Post by Matthew Scutter
Performance seems about the same, the biggest difference as I can tell is
trading reliability for cruise speed.

A jet will get you out of bad air quicker due to higher cruising speed -
minimally more drag than FES and less when stowed.......simples -JS1 has
250 km range I believe .............. - Anything to say a jet is less
reliable
than FES, where is the data? or just simple speculation from a biased
observer?

[Dave Nadler]

Anything to say a jet is less reliable than FES, where is the data?

A bunch of my friends with jet have had problems (and landouts).
I believe reliability has been improved, but we'll see.

[2G]

Post by Matthew Scutter

Post by 2G

Post by ***@gmail.com
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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-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
• 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

[Matthew Scutter]

Post by 2G

Post by Matthew Scutter

Post by 2G

Post by ***@gmail.com
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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-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
• 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.

[Paul T]

On Monday, September 14, 2020 at 11:54:00 PM UTC-7, Matthew

Scutter

wrote=
:=20

On Tuesday, September 15, 2020 at 7:29:17 AM UTC+2, 2G

wrote:=20

On Monday, September 14, 2020 at 9:06:08 PM UTC-7,

=
wrote:=20

Hi Eric,=20
=20
I agree with you in principle, that for higher output powers,

thing=
s must get bigger/heavier. However, I don't think this is the case here.
Th=
e 2x batteries they use (datasheet below) are spec'd for ~40 kW

discharge

r=
ate. The more realistic limiting factor might be how quickly they can
dissi=
pate heat from the batteries' internal resistance out of the battery
compar=
tment, but according to Matthew, this hasn't been a problem.=20
http://www.front-electric-

sustainer.com/Manuals/FES%20BATTERY%20PAC=

K%20GEN2%2014S%2040Ah%20manual%20v1.25.pdf=20

=20
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.=20
https://www.mgm-compro.com/brushless-motor-controllers/33-kw-

medium=

-voltage-controllers/=20

=20
I'd be interested to hear FES's reasoning, or other owners'

experie=
nces on why the power dropoff is so significant.=20

=20
Patrick Grady=20

I am amazed that this is even being speculated upon. How hard

is it

t=
o do FES climb performance runs? You simply take off and climb until

the

ba=
ttery (or controller) shuts down. Then, you repeat this test 5-10 times.
Th=
en 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
fav=
orable to FES.=20

=20
There are many FES installations out there - if you have one, do

this=
test and report the results.=20

=20
Tom=20

Not readily available? It's in the flight manual. If I adjust for

5.3kW=
h vs 4kWh batteries and 350kg weight of the Diana 2, it's ~2000m,

which

mat=
ches my napkin math from partial runs.=20

As for why owners haven't tried it - it sounds boring...=20
=20
5.3.4 Powered flight performance=20
5.3.4.1 Rate of climb=20
The maximum rate of climb is available only for a few minutes

with

full=
y charged=20

battery packs. As battery voltage is reduced, the maximum

achievable

cl=
imb rate is lower.=20

The average rate of climb depends mostly on the type of sailplane

and

i=
ts take-off weight.=20

Maximum attainable altitude gain that in standard atmosphere

conditions=
depends on=20

the type of sailplane, its weight and aerodynamic qualities. To

achieve=
the maximum=20

altitude gain, use about 15kW of power. Do not use full power as

the

ef=
ficiency of the=20

system is lower. Usually, 80-85 km/h is best for the climb with

positiv=
e flap setting (the=20

same setting as used while thermaling). Here are rough

numbers:=20

=E2=80=A2 1600 m (5200 ft) for UL sailplanes at 300kg take-off

weight,

=
i.e. Silent 2 Electro=20

=E2=80=A2 1400 m (4500 ft) for the 18m class sailplanes at

400kg

take-o=
ff weight (without=20

water ballast), i.e. LAK17A FES=20
=E2=80=A2 1200 m (3900 ft) for the 18m class sailplanes at

450kg

take-o=
ff weight (without=20

water ballast); LAK17B FES, Ventus 2cxa FES, Discus 2c FES, HPH

304ES=

=20

=20
5.3.4.2 Cruise flight=20
The maximum range of powered cruising flight, without the water

ballast=
, is around=20

100km (62 miles), depending on lift-sink conditions.=20
The optimum cruise speed and flap position depend on the type of

sailpl=
ane. Usually,=20

it is about 90 km/h (48 kts) at around 3000-3300 RPM and 4kW of

power

w=
ith a positive=20

flap setting, as used in thermals.=20

=20
And you REALLY believe that? If so, I've got a bridge for sale. No, I

wan=
t to see the INDEPENDENT verification of this data.=20

=20
Tom

Only on RAS do you get called biased or asked if you really believe

your

ow=
n experiences owning and operating - couldn't make it up if you tried.

Your command of the English language is limited obviously. You have not
been called biased - it was posed as a question - since you have and
operate a FES glider and voiced an opinion. There is a lot of BS floated
about in gliding often not backed up with substantive data that's all.

[2G]

Post by Matthew Scutter

Post by 2G

Post by Matthew Scutter

Post by 2G

Post by ***@gmail.com
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.com/Manuals/FES%20BATTERY%20PACK%20GEN2%2014S%2040Ah%20manual%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-motor-controllers/33-kw-medium-voltage-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
• 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.

Oh, so you admit that the manual is wrong - my bad!

Tom

[andy l]

Post by 2G

Post by Matthew Scutter
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.

Oh, so you admit that the manual is wrong - my bad!
Tom

Are you actually interested in this subject or not?

A friend has a Ventus 2 with a FES

I asked him about time or distance endurance, and I'm inclined to believe someone I've known for over 30 years, without needing certified statements from other witnesses, plus half their curriculum vitae, as suggested by some over-argumentative type on here.

[2G]

Post by andy l

Post by 2G

Post by Matthew Scutter
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.

Oh, so you admit that the manual is wrong - my bad!
Tom

Are you actually interested in this subject or not?
A friend has a Ventus 2 with a FES
I asked him about time or distance endurance, and I'm inclined to believe someone I've known for over 30 years, without needing certified statements from other witnesses, plus half their curriculum vitae, as suggested by some over-argumentative type on here.

I am VERY interested in this subject. You have me waiting with bated breath - what are your friend's results?

Tom

[kinsell]

Post by Mana
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?

[2G]

Post by kinsell

Post by Mana
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

[kinsell]

Post by 2G

Post by kinsell

Post by Mana
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/2019/10/sportine-aviacija-lak-17b-fes-n830dk.html