At 06:25 16 September 2020, Matthew Scutter wrote:
On Wednesday, September 16, 2020 at 4:07:16 AM UTC+2, 2G wrote:
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...rollers/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.