FES underpowered for 18m ship?

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).

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.

On Monday, September 14, 2020 at 4:06:32 PM UTC-4, Mana wrote:
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).

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?

On Wednesday, September 16, 2020 at 4:44:36 PM UTC+2, Darren Braun wrote:
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.

That is definitely the future and awesome idea Matthew,

Pasi

On Wednesday, September 16, 2020 at 8:34:19 PM UTC+3, Matthew Scutter wrote:
On Wednesday, September 16, 2020 at 4:44:36 PM UTC+2, Darren Braun wrote:
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.

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.