FES underpowered for 18m ship?

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.

On Monday, September 14, 2020 at 8:58:08 PM UTC+2, 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.

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).

On Monday, September 14, 2020 at 9:31:25 PM UTC+2, wrote:
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.

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.

On Monday, September 14, 2020 at 3:52:25 PM UTC-4, Mana wrote:
...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)??

On Monday, September 14, 2020 at 10:07:18 PM UTC+2, wrote:
On Monday, September 14, 2020 at 3:52:25 PM UTC-4, Mana wrote:
...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.

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.



At 19:52 14 September 2020, Mana wrote:
Thank you Matthew, this is very interesting data, exactly what I
was
lookin=
g for. :-)

BTW battery low =3D lower voltage, so for same output power
to the motor
yo=
u need more amps, hence the battery warms faster (P=3DU*I
=3D R*I*I).=20

On Monday, September 14, 2020 at 9:31:25 PM UTC+2,
wrote:
And it will be dangerous and fool-hardy if you DO NOT
always:=20
- keep a landing spot in easy reach, and=20
- never engage power until the landing is planned in you're in
position.=
=20
=20
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
landi=
ng spot in reach for the exact reasons you pointed out, level
flight only
w=
ould be very limiting.=20
=20
It could be that the pilot in question used FES regularly during
his
flight=
s and found himself with battery low towards the end (vs. only
using FES
on=
ce with a fresh battery in case of trouble)? I asked for his
contact data
t=
o get real facts vs. speculation and I'll report if I'm able to reach
him.

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.

On Monday, September 14, 2020 at 1:45:06 PM UTC-7, Kevin Neave wrote:
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.
At 19:52 14 September 2020, Mana wrote:
Thank you Matthew, this is very interesting data, exactly what I
was
lookin=
g for. :-)

BTW battery low =3D lower voltage, so for same output power
to the motor
yo=
u need more amps, hence the battery warms faster (P=3DU*I
=3D R*I*I).=20

On Monday, September 14, 2020 at 9:31:25 PM UTC+2,
wrote:
And it will be dangerous and fool-hardy if you DO NOT
always:=20
- keep a landing spot in easy reach, and=20
- never engage power until the landing is planned in you're in
position.=
=20
=20
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
landi=
ng spot in reach for the exact reasons you pointed out, level
flight only
w=
ould be very limiting.=20
=20
It could be that the pilot in question used FES regularly during
his
flight=
s and found himself with battery low towards the end (vs. only
using FES
on=
ce with a fresh battery in case of trouble)? I asked for his
contact data
t=
o get real facts vs. speculation and I'll report if I'm able to reach
him.

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.

'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).

It'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'm
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 in
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



At 21:37 22 September 2020, waremark wrote:
'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 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=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 enable
you
=
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 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).

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)