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Electric / hybrid aircraft propulsion (merged)

This article asserts that 100% electric VTOL is likely to prove impossible, and that a hybrid approach may be needed.

I am not surprised.

Administrator
Shoreham EGKA, United Kingdom

I will be quite surprised if 100% electric VTOL proves to be impossible, especially as it is clearly in existence today. Agree, hybrid will be the approach needed for extended range or endurance. The advantages of the electric propulsion system remain even with the additional complexity, combustion, fuel, vibration, waste heat etc.

The larger paradigm shift, away from legacy designs such as R22 exemplifies, is to be applauded and is way overdue. The helicopter is a marvelous multi-role utility, the new eVTOLs are necessarily constrained in role, but the viable roles are very suitable for new transport ecosystems and are perhaps more marvelous. The weight of research and industry activity is telling. 20 years ago I heard the same negative assessments of autonomy / electric / vtol / drones. Pretty sure that paradigm changed.

Here is some more background;

Boeing conservatism,

and

Broad Research and Industry Presentations

Other

Very important to read this document from Airbus too.

Although you may have enjoyed the current paradigm of effortless and safe GA for hundreds of years, things do tend to change in the aviation industry.

Other

That’s an interesting article from Airbus ( local copy ) … one has to start somewhere, identifying the challenges.

the current paradigm of effortless and safe GA for hundreds of years

A bit less sarcasm perhaps? Sarcasm and irony don’t travel well over the internet.

Administrator
Shoreham EGKA, United Kingdom

For those of you who want to follow what’s buzzing this may be interesting, updated 4 times per year:

http://www.e-flight-journal.com/efj3j2018-efj2besichert.pdf

Private UL field, Mallorca, Spain

This
might be of some interest.

Although it is obvious that this is a project high on spending grant money and low on real life applicability.

It would be nice if they provided some numbers, especially about the weight of the systems
- the diesel generator (which one? 110kw does not really match DA-300, nor CD-135 or CD-155)
- the all-up-weight of the electric motor system (sans batteries)
- the weight of the batteries (2×12kWh, so I guess maybe 150kg in total, uff!)

I suspect not much remained from the DA-40 useable load.

I wonder why they went for the CriCri configuration and how that changed the flying characteristics.

Hybrid can make sense only if the enroute power requirement is well below the takeoff power requirement.

That might be the case in things like surveillance (UAV or manned) but is not the case in any sort of “normal” aviation.

“Pure electric, the aircraft has an endurance of approximately 30minutes. The hybrid system extends this to 5 hours”

So the battery mode is basically useless, and if the engine is in the nose it is likely they are flying the 5hrs at a pretty low power.

Everything in that press release ( local copy ) is standard grant-application lingo, to take advantage of grants being preferentially given to collaborative projects.

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Hybrid can make sense only if the enroute power requirement is well below the takeoff power requirement.

That might be the case in things like surveillance (UAV or manned) but is not the case in any sort of “normal” aviation.


Well, of course, what is ‘normal’ can change over time. :-)

The current ‘normal’ is a consequence of the way the current GA airplanes are designed and operated (have the power, might as well use it; also the main bragging point is speed), which itself is a consequence of the properties of the piston engines (don’t like operating at low %).

An extra clean airplane with longish wings can have lower cruise power requirements (at a cost of somewhat lower cruise speed) – in such case, the main driver for selecting engine power is adequate takeoff and climb performance – which will result in an engine oversized for cruise – so the current ‘solution’ is to shorten the wings and fly faster using most of the available power.

An alternative would be to have the sustainer sized for cruise power (+ a bit more for recharging) and use the battery juice for STOL-like takeoff and super-fast climb (e.g. through icing layers). Given the required battery weight even with modest batteries, this won’t be lighter, but would result in significantly less fuel burn, while having superior performance where it matters most (takeoff and climb).

I mean something like a 4 person aircraft cruising at 150knots on 100hp Rotax, but with 300+hp available for takeoff and climb.

Another case would be an ‘ultralight’ cruising on a cheap 33hp industrial engine, with electric boost to 80-100hp to have safe and pleasurable takeoff and climb experience.

Yeah, right, electric stuff needs to become more common and fall down in price.

Peter wrote:

“Pure electric, the aircraft has an endurance of approximately 30minutes. The hybrid system extends this to 5 hours”

So the battery mode is basically useless, and if the engine is in the nose it is likely they are flying the 5hrs at a pretty low power.


They quote 110kw generator, which is not pretty low power for a cruising DA-40.
Regarding the battery mode basically useless: here I disagree the most (disclaimer: my belief is that the electric stuff can be much more reliable than the IC mechanical piston machinery). The main advantage is significantly increased safety: if the IC generator fails, you have 30min to find a suitable airport/landing place. Unless you are crossing a vast expanse of water/hostile terrain, this is an non-event safety-wise (granted, an annoyance).

Peter wrote:

Everything in that press release ( local copy ) is standard grant-application lingo, to take advantage of grants being preferentially given to collaborative projects.

Total agreement.

An extra clean airplane with longish wings can have lower cruise power requirements (at a cost of somewhat lower cruise speed)

Agreed, but you might be looking at flying at say Vbg which is ~95kt for most 150-170kt planes; I am not sure anybody would settle for going that slowly. But yes you can do it with an electric motor; an IC engine would be very inefficient at 30-40% power.

Jets fly close to Vbg and achieve their high TAS by flying high, but that implies pressurisation. But if you built a pressurised DA42 and got it up to FL300, ATC would kick you out. I used to know a guy with a Commander (twin TP, like Adam has) and he said he got sick of it being useless for its intended profile in Europe because he was getting pushed down to FL250 or less.

my belief is that the electric stuff can be much more reliable than the IC mechanical piston machinery

Electronics should last for ever if it never gets warm, and the manufacturing quality is superb (at work we see a ~1000 year MTBF on some stuff, based on actual field data). But I don’t think it is for power electronics, because that gets pretty warm (look at the extensive cooling for the Siemens motor drive electronics) and thermal cycling is a killer.

Also GA is never going to see the sort of R&D investment which the car business gets. There, millions are poured into making crappy and cheap stuff reliable. For example they use really crappy connectors but make them “reliable enough” with o-rings which stop water getting in.

What you get in GA is a load of tinkerers. Hence we got stuff like the Aspen EFD whose failure rate was legendary (probably close to 100%) for the first few years.

Administrator
Shoreham EGKA, United Kingdom

The very high power density of batteries/motors makes VTOL possible, so if regulatory requirements keep up you could design for a very high wing loading and a high Vbg to keep the cruise speed up with low power requirements.

Last Edited by kwlf at 09 Nov 02:46
EGCW
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