Yes; 12 posts up.
This thing is all over social media so the objective has succeeded already 
Just trying to get a hang of the Siemens achievements applied to a 300hp GA aircraft (i.e. GA8 Airvan or C206).
Shoot some holes in this please (with accurate figures)
Take-Off power: 300hp = 224 kW
Cruise power 65% = 195hp = 146 kW
Throw away the Lycoming IO-540 reducing weight by 199kg
Throw away AvGas reducing weight by 240kg.
Add:
Electric motor 300 hp weight required: 43 kg (based on 5.2 kW/kg)
Cruise Turbine in fuselage 146kW weighing 32kg (based on RR AE2100 at 4% scale, probably not linearly scalable?)
Cruise Generator 146kW on turbine: 28kg (reverse of 5.2 kW/kg)
Fuel to run turbine 300 kg (based on 5 hours cruise)
Take-Off battery at 2.6 kWh (delivering the additional take-off power of 78 kW over 2 minutes)
Take-Off battery weight: 9 kg (300 Wh/kg)
Weight shaved: 439kg. Weight added: 412 kg.
Ignore cost for the time being… as it’s probably very expensive…. today!
- turbines really do not scale down very well
- battery weight is overly optimistic (both energy density and power density)
Anyway, it totally misses the point of electric
- you want to design say around 300hp takeoff/climb power
- but much, much, much less (say 80-100hp) cruise power
- so, you can have quite large MTOW and still good takeoff/climb
- but need very clean airframe and efficient wings to get a decent speed out of that cruise power
- with normal engine, such a design makes no sense
– you can’t really run piston engine efficiently at 30% power (maybe diesel?)
– if you are carrying the strong/heavy engine for your takeoff/climb, you may as well use it for cruise
- with electric, the weight penalty for having motor with large power is very small
- the penalty comes from the weight of the batteries
– how much batteries do you need to supply 200kw?, even if for only 5min?
– even assuming 400w/kg specific power, you will need 500kg Hmmm….
- what about props? To be efficient both at 300hp and 100hp, maybe you want a cruise prop in front (or back :-) ), and two foldable takeoff/climb props on the wings…
Having huge amount of extra power for takeoff/climb would be very advantageous .. both for STOL and for climbing through icing layers…
Hmmm, so maybe have a large turbine for takeoff/climb and Rotax 915 for cruise… :-)
esteban wrote:
- turbines really do not scale down very well
They may scale down better than we think. The RR AE2100 achieves 4.5kW/kg. A micro turbine achieves 3.75 kW/kg. That’s not too big a difference.
esteban wrote:
- battery weight is overly optimistic (both energy density and power density)
Even at 200 Wh/kg the battery only weighs 13kg.
esteban wrote:
- the penalty comes from the weight of the batteries
That’s where the turbine in the back comes in. It runs continuously eliminating the need for a large battery pack (and bringing the charging time to zero).
1. Re: turbine scaling down: But what is specific fuel consumption for such small turbines?
2. I am talking about power density (watts per kilogram, not Wh/kg). High discharge rate batteries tend to have low capacities and lifetime…
What are the parameters (energy density, # of cycles) of commercially available batteries with 30C discharge rate?
3. At the end, why do you bother with batteries if you have the turbine for cruise? That extra 78kW for takeoff? Does it really change things significantly?
Turbines are fine, but $$$, both in price and fuel.
What you want is an electric that can give you significant takeoff/climb boost (and insurance against loss of power at the worst moments), while still being able to cruise cheaply on your (undersized) piston engine.
I remember reading about a valveless 2 stroke engine made using a tuned exhaust and intake. It was said to be a curiosity as it only ran at one speed. I could imagine a similar engine being quite practical as an aviation ‘booster’ engine.
esteban wrote:
1. Re: turbine scaling down: But what is specific fuel consumption for such small turbines?High, but micro turbines are the extreme end of the scale. If you look at a turbine generating 146kW that’s about a 45% scale PT6. Applying the specific fuel consumption you need about 300kg of fuel to run the turbine for five hours cruise.
esteban wrote:
2. I am talking about power density (watts per kilogram, not Wh/kg). High discharge rate batteries tend to have low capacities and lifetime…I’m not sure. But say we reduce the discharge rate to 10C, theoretical battery weight increases to 27kg, no?
What are the parameters (energy density, # of cycles) of commercially available batteries with 30C discharge rate?
esteban wrote:
3. At the end, why do you bother with batteries if you have the turbine for cruise? That extra 78kW for takeoff? Does it really change things significantly?Take-off performance! I can’t imagine taking off with a cruise power setting, it would eat up runway like a fully-laden Ilyushin.
Because the extra power is only required for take-off I added a small t/o battery pack rather than carrying an oversized gas turbine/generator around.
Hmm, rather than taking a non-existing turbine, what about taking an almost existing Rotax 915? :-)
Start with DA-62 (42 if you are cheap :-) ).
Throw away the austro engines.
Put in Rotaxes.
Now you are about 100kg lighter on each wing, but have lost 2×24kW power.
Already this might be a nice flying plane (with all the lower weight), but there is never enough takeoff/climb power for those short airfields and freezing layers.
So, put in 2×60kW electric motors and 2×15kWh battery packs (60kW for 15 min = 15kWh).
60kW motor + power electronics = hopefully 25kg (ex batteries)
15kWh @ optimistic 200wh/kg = 75kg (but things are moving that way … and 15min is perhaps overly generous and half of that might be entirely sufficient).
So, you end-up with cca same weight, but with 2×160kW = 2×215hp for 15min, which should climb up very nicely indeed and get you up there in those 15 mins if not much sooner.
You do lose some cruise speed, but then gain something on smaller cowlings.
Mogas vs jet-A is a nuisance, agreed.
If done right, the electric motor might turn the prop even if an engine fails – would help immensely in those low-speed failure cases around takeoff/landing (this aspect alone might be a winner).
That might almost add up if you only do one takeoff per flight 
So it won’t work for what many think is the most obvious business case: flying schools. Private owners are too few and they don’t do enough hours to recover the capital cost.
No matter how you shake this stuff, hybrid power trains are no good for aircraft. The attempts to develop them are mainly development grant harvesting schemes.
And that’s before you look at existing and likely future (in our lifetimes) technology for batteries and charging them, etc.
Also people won’t pay for a backup for an engine failure, if the chances are low enough. It’s like backup for GPS – there is no market for it.
I would think flying schools can do full-on electric (like Pipistrel Alpha Electro) with battery swaps if needed.
The case where you need gas burner is when you are really going places… but then you are not taking off/climbing in quick succession.
The only case which is not satisfied by either full-on batteries or hybrid+15min batteries is if you want to do both touring and plenty of circuits … then again, for how long do you need extra power if you are not climbing to cruise level, but just taking off to circuit height? You can do quite a few of those even with 15min of battery power … if you want a bit of practice.
Regarding the future technology:
- I don’t believe in truly great breakthrough in battery energy density (i.e. 200wh/kg → 3000+wh/kg), but I believe we will get to 400wh/kg in not so distant future, and with some luck also to 1500wh/kg. Not relevant for big commercial iron (those will live on turbines, nothing with even remotely close power/weight is available in foreseeable future), but getting there for private GA.
- what I would like to see is free piston generators – they have a real potential to bring a revolution to hybrid vehicles: without crankshaft, things can get truly compact/lightweight/efficient. Together with lightweight electric motors, this may allow novel configurations that are simply not feasible today.
Btw, there are backups for engine failures and they have proven to be market success – BRS, for whatever it is worth. Silent noisemaker is a bit more worrisome than a dead GPS. Yes, the shute is backup for other things as well, and actually not so great for engine failures at critical phases of flight … but still we have all heard the mantra ‘dead engine – pull the shute’.
Another backups for engine failures = the second engine. Not so popular as in the old days, but still many like them… :-) I would not say the market is dead, it just needs to be propely educated. :-)
Another fancy configuration:
- sort-of normal configuration aircraft (but extra clean so can cruise on quite low power) with just a slightly undersized piston engine in the nose, simple fixed cheap lightweight cruise prop, + 2 streamlined electric engines on the wing with fixed (but foldable) climb props just for takeoffs and climb. Can takeoff/climb like an angel … and cruise efficiently … and if the engine fails, you do have some more time to find the place to land … and can have some power to better control the landing… won’t save you if you are crossing the Atlantic … but for things like Channel crossing and island hopping it can make quite a difference…
