# What do you think about "e-soaring"?

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#### John.Roo

##### Well-Known Member
Nice dokument....
"When Dreams Take Flight is a film about dreams and dreamers, a film about people who are willing to risk life and limb to follow their hearts and chase a dream that has existed since the dawn of man. Watch as Todd Reichert, a student from the University of Toronto, trying to become the first man to construct and successfully fly a human-powered flapper wing aircraft."

#### John.Roo

##### Well-Known Member
LiFePO cells are more safe in compare with LiIon cells.
Unfortunately they are also more heavy...
So what would be weight difference?

For example this 100 Ah cell...

18 cells = 18 x 3,2 V = 57,6 V (nominal)
Voltage range / cell = 2,5 - 3,65 V
Battery voltage range 45 - 65,7 V
Capacity = 57,6 V x 100 Ah = 5 760 Wh = 5,76 kWh.
Weight = 18 x 2,30 kg = 41,4 kg.
Price = 18 x 137 USD = 2 466 USD

Li-Ion 18650 Sony VTC 6 cells....

Solding 15S 34P
Voltage = 55,5 V (nominal)
Voltage range / cell = 2,75 - 4,2 V
Battery voltage range 41,25 - 63 V
Capacity = 55,5 V x 34 x 3000 Ah = 5 661 Wh = 5,61 kWh.
Weight = 15 * 34 x 0,050 kg = 25,5 kg.
Price = 15 x 34 x 8,50 USD = 4 335,00 USD

Of course is necessary to add some weight for battery case, connectors etc.
Also would be good to check current limit for both cell types.
But is interesting to see difference

#### EzyBuildWing

##### Well-Known Member
Amazing eVTOL machine demo'd at OSH.......presumably looking for higher energy-density batteries.....
Some interesting comments below the vid.

#### henryk

##### Well-Known Member
Article says that he was "jumping from bell tower" and during his last flight he crashed in distance between 500 - 1 300 m from tower.
If he was "jumping from bell tower" than is difficult to imagine how he was using ground effect.
Tower height is not mentionned - you wrote 40 m.
3-4 km distance from 40 m altitude?
I really can´t imagine help of ground effect on that distance.
And if he really used ground effect, than fall from 1 m would be not catastrophic.

=the lowest level=DUNAJEC river...

=the highest=bell tower (+40 m over castle ground)

Vmax=(2gh)^0.5 =sqr 800=circa 30 m/s (100 km/h !)

then=

BTW=after BEKAS1-A car towing to 1-2 m high Bodek was flown 200-300 m !

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#### John.Roo

##### Well-Known Member
Off topic.....
One of most beautiful Czech glider
Aerobatic LF-107 Lunak (kite / hawk)

General characteristics
• Crew: one
• Length: 6.78 m (22 ft 3 in)
• Wingspan: 14.27 m (46 ft 10 in)
• Wing area: 13.38 m2 (144.0 sq ft)
• Aspect ratio: 15.22
• Empty weight: 205 kg (452 lb)
• Gross weight: 310 kg (683 lb)
Performance
• Maximum speed: 300 km/h (190 mph, 160 kn) - wooden construction
• Maximum glide ratio: 24 at 80 km/h (43kts)
• Rate of sink: 0.85 m/s (167 ft/min) at 65 km/h (35kts)
"The word, Luňák, translates into English as “kite”. Kites are predatory birds known for their mastery of both soaring and agile flight; they are also known for bursts of high speed when diving on prey. It was a very appropriate name for an aerobatics sailplane that had a maximum speed of 300 kilometres per hour.
The aircraft’s agility and speed led it to be nicknamed “Engineless Fighter” and similar by some people.
From the point of view of the experienced pilot, there was a lot to like in the Luňák beyond the aforementioned agility and speed.
The cockpit was spacious and the fighter style bubble canopy that covered it gave the pilot an excellent view all around the aircraft. In the style of fighter aircraft of the day, the canopy opened by sliding backward. This feature allowed the aircraft to be flown with the canopy open if the pilot wished.
"

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#### blane.c

##### Well-Known Member
Supporting Member
LiFePO cells are more safe in compare with LiIon cells.
Unfortunately they are also more heavy...
So what would be weight difference?

For example this 100 Ah cell...
View attachment 128630
18 cells = 18 x 3,2 V = 57,6 V (nominal)
Voltage range / cell = 2,5 - 3,65 V
Battery voltage range 45 - 65,7 V
Capacity = 57,6 V x 100 Ah = 5 760 Wh = 5,76 kWh.
Weight = 18 x 2,30 kg = 41,4 kg.
Price = 18 x 137 USD = 2 466 USD

Li-Ion 18650 Sony VTC 6 cells....
View attachment 128631
Solding 15S 34P
Voltage = 55,5 V (nominal)
Voltage range / cell = 2,75 - 4,2 V
Battery voltage range 41,25 - 63 V
Capacity = 55,5 V x 34 x 3000 Ah = 5 661 Wh = 5,61 kWh.
Weight = 15 * 34 x 0,050 kg = 25,5 kg.
Price = 15 x 34 x 8,50 USD = 4 335,00 USD

Of course is necessary to add some weight for battery case, connectors etc.
Also would be good to check current limit for both cell types.
But is interesting to see difference
Does the weight include soldering/welding them all together and ancillary items like a "bucket" to put them all in?

##### Well-Known Member
Supporting Member
Those are bare cell weights.

In general, LiFePO4 cells will need less overhead for packaging, cooling, management, and isolation than LiPo or LiIon — figure 100% overhead va 200%. Part of this is that larger cells are available, part of this is that the per-cell failure modes are milder and less likely to propagate so need less protection.

LiIon still generally wins on system weight when sized for energy, and basically always wins when sized for power.

I’d still rather sit on top of LiFePO4.

#### henryk

##### Well-Known Member
Nice dokument....

-iff compare works/effects/costs of prof. De Laurie/ Yves Rousseau / ing.Toporov...=

F thrust =m * a !

iff Fthrust=F drag (constant speed V)
the power for level flyigt =

N=F thrust * V

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#### John.Roo

##### Well-Known Member
Does the weight include soldering/welding them all together and ancillary items like a "bucket" to put them all in?
Good point
No, it is only comparizon of weight of "pure cells" with +- same capacity.
Prismatic LiFePO cells are easier to connect and only 18 cels are used.
To have same capacity would be necesary to sold together 510 Li-Ion VTC6 cells = much more work (and possibility of mistake).

For larger electric airplane is practically impossible to use LiFePO cels. Weight difference wll be huge.... However we need only small capacity of 5-6 kWh.
Question is.... is weight "penalty" acceptable?
16 kg difference is not small but we get higher safery, lower price, easy installation.
I am sure it will be possible to get also prismatic Li-Ion cells, but lower safety and higher price of Li-Ion (or Li-Po) technology will remain.

#### John.Roo

##### Well-Known Member
-iff compare works/effects/costs of prof. De Laurie/ Yves Rousseau / ing.Toporov...=

F thrust =m * a !

Students from Toronto University produced really flying prototype.
But still not practically usable - during takeoff they needed help of tow car and flight was only short.

Unfortunatelly videos you posted are not showing any practical advantage of "flaping wings".
On first one we see glider with moving wings - very far from first 3D video.
Third video is still same taxiing without any sign of flight....

Students from Toronto shown in their video nice story about dreamers dreaming, lot of work and finally really flying result.

#### John.Roo

##### Well-Known Member
Those are bare cell weights.

In general, LiFePO4 cells will need less overhead for packaging, cooling, management, and isolation than LiPo or LiIon — figure 100% overhead va 200%. Part of this is that larger cells are available, part of this is that the per-cell failure modes are milder and less likely to propagate so need less protection.

LiIon still generally wins on system weight when sized for energy, and basically always wins when sized for power.

I’d still rather sit on top of LiFePO4.
I agree
Idea was to show weiht difference and think about positives and negatives of both technologies.
I would say that if weight penalty is acceptable (like in or case) than is better to use LiFePO cells - especialy in amateur homebuild conditions.

However I have to answer one more question.... what is safe "C" rate of LiFePO cels.
We need at least 3C to get reasonable takeoff power from 5-6 kWh battery...

##### Well-Known Member
Supporting Member
It varies with the cell, of course. In general you will find that LiFePO4 capacity starts dropping above 1C, and drastically so above 2C. Bigger cells are generally worse. This matches the behavior of large cylindrical and prismatic LiPo and LiIon as well — only pouch cells are really happy at 5C and above. That said, draining a LiFePO4 faster than it likes tends to heat up the battery and hurt capacity… but the inherent robustness of the chemistry means that as long as your pack has adequate cooling you’re not going to damage things much. It’s not unreasonable to plan around 5C or more and just derate the capacity, especially for a liquid cooled pack.

#### John.Roo

##### Well-Known Member
It varies with the cell, of course. In general you will find that LiFePO4 capacity starts dropping above 1C, and drastically so above 2C. Bigger cells are generally worse. This matches the behavior of large cylindrical and prismatic LiPo and LiIon as well — only pouch cells are really happy at 5C and above. That said, draining a LiFePO4 faster than it likes tends to heat up the battery and hurt capacity… but the inherent robustness of the chemistry means that as long as your pack has adequate cooling you’re not going to damage things much. It’s not unreasonable to plan around 5C or more and just derate the capacity, especially for a liquid cooled pack.
Interesting
For weight calculation I used this cell...

Peak discharge current 2C - 200 A for 30 sec.
Max. discharge current 1C - 100 A.

For horizontal flight I need 5 kW of power and for safe takeoff I prefer 15 kW.
Capacity 5-6 kWh.
Liquid cooled pack is not possible.
Do you think it could be done from LiFePO cells?

##### Well-Known Member
Supporting Member
That peak current will be defined by a certain thermal rise with no forced cooling, probably at a 25 degree ambient. On the one hand you’ll want to derate for your environmentals (45 degree ambient or whatever), on the other hand forced air or water cooling will cover a lot of sins. What you’ll want to do is build a pack, stick it in the environment chamber, and just run a series of cycles at increasing drains with the cooling system running until you hit your delta t limit — for 45 ambient you probably will have a delta t limit around 20 or 30 degrees depending on how many cycles you want to get out of the battery. Of course the pack you test this wash probably isn’t going to be a flight-worthy pack, but the variation from cell to cell should be low enough that it will apply. As a wild hunch I’d suspect that 5C with liquid cooling would be fine indefinitely at 30 degree delta t. Forced air I have less experience with.

Honestly, the few electric aircraft I’ve worked with (with battery packs ranging from 12 kg to 330 kg) have all been sized for power, not energy. I don’t see anything wrong with LiFePO4 for this application, but I’d size the battery for 2C, especially if you’re doing forced air cooling instead of liquid. I think you’re likely to find that liquid cooling ends up lighter, though — the increase in pack size to permit air flow is not small. For a LiIon where you already have a lot of space between cells to protect against thermal runaway you can potentially run the air there (although you do need some ablative material or something for the fire containment) — but if you intend to take advantage of LiFePO4 by being a little bit more relaxed on containing failures (which is a risk — the cells are less likely to go boom, but if one does catch somehow, another one right next door isn’t going to have a good time!) then you lose that volume for air flow.

Also, unless you’re really okay with suddenly losing power in case of a cell failure, you’re going to want to put a bypass mechanism in your BMS. Since you’re talking about a pure serial string of cells, this not only hurts your total available energy, it also lowers your voltage — you can’t just drop a full parallel string like in a normal pack. This will hit your motor efficiency and you’ll need more energy once again.

Plus of course you don’t really want to run the cells below 2.8 volts (20% remaining capacity or so) if you want them to survive long. Combined, if you want 6 kWh for an actual operation, you’re going to be multiplying by 1.2 (1 / amount of rated capacity left at end of life) * 1.2 (1 / amount of total capacity you want to use) * 17/16 (one cell bypassed, unless you want to be able to bypass two) * 1.05 (motor efficiency derating at lower voltage) * 1.1? (rated capacity at worst case temp, probably -40 but maybe +75 / rated capacity at 25 c) which will end up somewhere just under 2x your 6 kWh — you can probably get away with 10 or 11, but 12 kWh rated for 6 kWh useful isn’t a bad estimate.

And at this point you’ll find that the energy sizing and the power sizings aren’t that different.

##### Well-Known Member
Supporting Member
Looking at some LiFePO4 cells with data sheets (don’t buy anything that doesn’t have charge / discharge curves at different temperatures etc!) it looks like at least some of them are even pickier on temperature than I expected. e.g from (randomly chosen) https://cebabattery.com/wp-content/uploads/2018/01/LiFePO4-60150208-3.2V-100Ah-Battery_DATASHEET.pdf you’re (a) limited to 60 degrees and (b) losing an eyeballed 20% of capacity already by 0 degrees, even at 1C, assuming a sane LV cutoff instead of the 2.0 V they plot. I’d definitely consider putting in a heater, or budgeting the energy to pre warm the battery (which if you’re nasty you can just do with a pulse discharge into your motor).

I suspect there are cells out there specified up to 80 C, and you might be able to also find some that are useful at lower temps, but judging from this a standard -40C isn’t going to happen.

In a normal plane once you’re running self-heating should keep the battery warm enough, but if you’re planning on soaring for long enough to cold-soak and then restarting for a go around you need to decide whether to try to keep the pack warm (energy budget) or have the pilot command it to pre-warm (possible pilot error) before use.

#### John.Roo

##### Well-Known Member
Wow - you have definitely much more experience
Our first electric airplane had LiFePO cells, but we quickly changed battery to Li-Po cells (year 2010/2011).

Most of flights I did with 27S 5P (pouch cells 11 000 mAh, 3,7 V nom).
Than we bought controller for higher voltage and I did some flights with 39S 5P.
Following project had 150 kg battery (a bit over 35 kWh) from Sony VTC6 Li-Ion cells.

Then I flown in small light one-seater with +-5 kWh battery...

... and I think that this could be a way for affordable electric airplane.
Simple, light, self launch e-glider.
Only I prefer not use retractable motor but propulsion in the front or in the fin.

However safety first - I would like to precede problems with propulsion system as much as possible.
Li-Ion cells are known and often used so they are acceptable for me. However if LiFePO cells could be used - only better. They seems to be more safe.

3C for takeoff (short time) and max. 1 C for horizontal flight.
Charging definitelly below 1C.
So far I had no problem with high temperature of cells (measured on multiple points) so I never used cooling / heating.

I totally agree with conservative discharge voltage. 15-20% of remaining capacity after landing is good way

#### henryk

##### Well-Known Member
prismatic Li-Ion cells,
"A123" , but Li Ion technology=replace for napalm bomb !

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#### Sraight'nlevel

##### Well-Known Member
Wing here looks interesting:

#### proppastie

##### Well-Known Member
Supporting Member
Then I flown in small light one-seater with
looking at middle video the takeoff elevator position is much more level than all the 3rd video ......were there CG issues in the configuration of the 3rd video?