Electric Islander
Join Date: Jan 2016
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Serious back of the envelope calculation here with a little real world data thrown in:
Islander with 2x 300HP engines = approx 400kW peak power output
Lets assume that the whole 2 min sector is flown at full power to be nice and pessimistic...so 13.3kWh per sector.
Now the complete battery assembly for the Tesla Model3 is pretty representative of the bleeding edge of EV battery technology and it comes in at 168Wh/Kg.
The total mass of the ~75kWh battery pack in a Model3 is approx 470 Kg. Empirically I can tell you that my Model3 charges from say 20% to 80% in about 40 minutes from a DC fast charger.
That passes the smell test for me, it seems very practical for this particularly odd air route.
Islander with 2x 300HP engines = approx 400kW peak power output
Lets assume that the whole 2 min sector is flown at full power to be nice and pessimistic...so 13.3kWh per sector.
Now the complete battery assembly for the Tesla Model3 is pretty representative of the bleeding edge of EV battery technology and it comes in at 168Wh/Kg.
The total mass of the ~75kWh battery pack in a Model3 is approx 470 Kg. Empirically I can tell you that my Model3 charges from say 20% to 80% in about 40 minutes from a DC fast charger.
That passes the smell test for me, it seems very practical for this particularly odd air route.
300hp = 223 kw, not 400. Also the Loganair BN2s are carb so only 265hp, meaning 197kw.
Assume one of the longer legs, say 15 minutes. You would fly 1 min @ full power, 13 mins @ 75% and 1 min @ 50%. This equates to 14.8kwhr. You could satisfy this + 30 mins reserve with a 50kwhr battery pack, using tesla math is ~300kg.
Very achievable!
Hot swap battery packs on turnaround and charge them while they're not on the aircraft. Keep those sub 5 minute turnarounds going
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My bad! I also completely mucked up the calculation, the 15 minute trip would actually require 73.875 kwhr and 30 mins reserve would be 194 kwhr, giving total weight of ~1200kg which is definitely prohibitive.
James Watt will probably forgive you too (by convention, SI units named after real people get a capital letter).
Join Date: Nov 2005
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Once batteries hit approx. 3kw per kg then they can replace avgas/petrol even though avgas is approx.12kw/kg IC engines are only 25% efficient and electric motors are 90% approx. Lithium air batteries have in theory 11kw/kg max but difficult to achieve but could make 2.5kw/kg which would be close. So far Li air have only lasted a few recharge cycles on air and needs pure oxygen to last, so not practical but a total rethink by a university research group in the US, with help from a super computer group have come up with one that has lasted 750 recharge cycles and can run directly on air, so the holy grail of batteries may be in sight so I believe recharge time will be critical, not the weight for commercial operation.
Join Date: Feb 2008
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Seems like 200Liters of fuel is about all you can take with a maximum revenue load out on the conventional Islander so what battery mass would that give us?
Dry weight of an IO-540 is 200Kg. Dry weight of a Siemens SP260D (230kW+ rated) is 50Kg...so *2 (and wildly assuming similar mass add for props/coolant/lube/ancillaries etc) we save 2*150Kg=300Kg on our electric converted Islander, so we get that mass of battery for free to reach the ZFW of a conventional Islander, then another ~160Kg of battery to approximate the mass of 200 liters of fuel and reach MTOW.
Using my original Tesla based 168Wh/Kg number, thats 460*168=77kWh of battery, which doesn't sound close to viable if we assume 75% power for the hold and a conventional VFR reserve.
Will be interesting to see if this is something that the CAA will be willing to get engaged into to starting thinking through operational and regulatory issues associated with electric and hybrid commercial flight.
(All numbers from the internet, no POH data...your milage may vary etc etc!)
FWIW, for those who like to nerd out:
Siemens (incl SP260D) presentation
https://nari.arc.nasa.gov/sites/defa...FW-Aug2015.pdf
Pipistrel Alpha-Electro - Series production electric aircraft designed around a specific mission with some certification success. Answers some questions about what a modern slippery composite airframe can achieve and also datapoint on in-flight regen and noise.
https://www.pipistrel.si/plane/alpha-electro/overview
Last edited by ion_berkley; 19th Oct 2018 at 20:29.
Join Date: Feb 2000
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Well if we're going to be pedantic, let's do it properly.
KW/hr (more correctly kW/h) is a measure of the rate of change of energy consumption, so I'd be willing to bet that it's a quantity that makes sense to the power distribution companies.
KW/hr (more correctly kW/h) is a measure of the rate of change of energy consumption, so I'd be willing to bet that it's a quantity that makes sense to the power distribution companies.
Psychophysiological entity
Redesign the Islander so that it's really clean and pointy. Mag-lev it down the runway and up a ramp, both of which point at the destination. At some point c 3,000' with runway in sight, flick the engine's two big toggle switches to the ON position . . . preferably at the same instant.
As with all forms of transport - not needing the power in the first place is the best route to economy.
As with all forms of transport - not needing the power in the first place is the best route to economy.
Don't omit or underestimate the weight of required control gear.
These are not ancillaries, they are just as important as the motor and batteries and they tend to be weighty.
Me? I'd be looking to a hybrid as a practical solution in the short term.
Either electric augmented by a smaller engine or vice versa.
In either case I'd pick a single (Caravan?) and not a twin.
Shorter cable runs and no need for duplication.
Motor rotor built into the prop drive shaft passing through stator.
Regen on descent - maybe obviate the need for flaps?
Lotsa possibilities.
These are not ancillaries, they are just as important as the motor and batteries and they tend to be weighty.
Me? I'd be looking to a hybrid as a practical solution in the short term.
Either electric augmented by a smaller engine or vice versa.
In either case I'd pick a single (Caravan?) and not a twin.
Shorter cable runs and no need for duplication.
Motor rotor built into the prop drive shaft passing through stator.
Regen on descent - maybe obviate the need for flaps?
Lotsa possibilities.
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Where did it go?
Regarding this type, this is one of the funniest posts I've read in here: Britten Norman Islander
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I will have to concede that you have identified a narrow application where kW/h may not be gibberish. Beyond that though ....
Electric motors (at least when fitted with a sufficient mass of control gear 
) can operate efficiently at a broader range of RPMs than can most combustion engines. I wonder if that characteristic could be use to help mitigate propeller noise (e.g. operating a a somewhat lower RPM and higher pitch at low speeds).
) can operate efficiently at a broader range of RPMs than can most combustion engines. I wonder if that characteristic could be use to help mitigate propeller noise (e.g. operating a a somewhat lower RPM and higher pitch at low speeds).
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