FES underpowered for 18m ship?

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

I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem.
http://www.front-electric-sustainer....% 20v1.25.pdf

They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries.
https://www.mgm-compro.com/brushless...e-controllers/

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

Patrick Grady

I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES.

There are many FES installations out there - if you have one, do this test and report the results.

Tom

On Tuesday, September 15, 2020 at 7:29:17 AM UTC+2, 2G wrote:
On Monday, September 14, 2020 at 9:06:08 PM UTC-7, wrote:
Hi Eric,

I agree with you in principle, that for higher output powers, things must get bigger/heavier. However, I don't think this is the case here. The 2x batteries they use (datasheet below) are spec'd for ~40 kW discharge rate. The more realistic limiting factor might be how quickly they can dissipate heat from the batteries' internal resistance out of the battery compartment, but according to Matthew, this hasn't been a problem.
http://www.front-electric-sustainer....% 20v1.25.pdf

They would have to have a bigger inverter to handle the 40% higher input current when the batteries discharge from 4.2v-3.0v, but these ~20 kW class inverters weigh nothing (1-2 kg) compared to the batteries.
https://www.mgm-compro.com/brushless...e-controllers/

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

Patrick Grady
I am amazed that this is even being speculated upon. How hard is it to do FES climb performance runs? You simply take off and climb until the battery (or controller) shuts down. Then, you repeat this test 5-10 times. Then you repeat that test for a different glider. Why isn't this data readily available? I can only guess that this test has been done and it is not favorable to FES.

There are many FES installations out there - if you have one, do this test and report the results.

Tom

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.

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

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.

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).

On Tuesday, September 15, 2020 at 12:53:30 AM UTC-7, krasw wrote:
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.

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

On Tuesday, September 15, 2020 at 11:11:26 AM UTC-7, Ramy wrote:
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.

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.

On Tuesday, September 15, 2020 at 11:11:26 AM UTC-7, Ramy wrote:
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

On Tuesday, September 15, 2020 at 9:30:06 AM UTC+2, Paul T wrote:
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/201...Supplement.pdf

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

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.