Electric / hybrid aircraft propulsion (NOT cars)

Just come across this, regarding to above stuff, in one of the trade mags I get at work

Is turbine efficiency really so dependent on the power output, so as to make it worth building a hybrid power train, with the efficiency losses involved in that?

Wasn’t this the one on display in Friedrichshafen?

In order for this to work there would have to be significant electricity storage on board (batteries), in order to be able to decouple generating power from propulsion power. So basically a very significant part of the takeoff power would have to come from batteries, which would then be recharged in descent. You cannot simply switch turbines on and off, because they’re mostly cycle count limited…

Is turbine efficiency really so dependent on the power output, so as to make it worth building a hybrid power train, with the efficiency losses involved in that?

Yes.

Just think about it. The average airline trip in Europe is maybe 1 hour. 20 min climbing, 20 min cruising, 20 min descending. During climbing the efficiency is 10-15% at high fuel flow, in cruise 25% at moderate fuel flow, in descend 5-10% at low fuel flow. With a hybrid it would be possible to run 35-40% efficiency on the turbine continuously. Of that, you lose maybe 10% due to charging/emptying batteries, but you will still have 25-30% in cruise and also have that efficiency when climbing and descending, and the fuel flow is constant. Just as example, I’m sure Airbus and Siemens have used lots and lots of millions on calculations and scenarios like this alone. When they say 12 % total reduction in cost, I believe them. It could be more than that also.

Max power from batteries would only need to be 40% at take off. After that the batteries will be charged. The turbine would run at one single output from start to landing.

which would then be recharged in descent.

Unlikely. There are a few differences between aircraft and cars that make hybrid concepts very different and less compelling than for cars.

A car spends a large share of its energy accelerating. It can be recovered during braking. This is especially true in urban situations – hence hybrids are much more efficient than standard cars in cities, the advantage dwindles to almost nothing when cruising on motorways.

This also means that additional mass due to batteries has little impact on car efficiency. In start-stop situations the energy expended accelerating the battery can be recovered, and in cruise situations the mass of the car has next to no impact on energy needed to sustain the cruie speed, short of slightly increased tyre friction.

Aircraft spend most of their energy overcoming air resistance in cruise flight, which cannot be recovered. The energy spent in the climb cannot be recovered unless you descend much more quickly than is healthy, even in a pressurized aircraft. Anybody using beta range or reverse thrust in the descent? Would you like to? With, say, 25% reverse power? Didn’t think so. However, the engine could keep running at its design power and charge the batteries for a possible go-around.

Also, aircraft mass has a larger impact on energy required, more so during climb than during cruise, but still relevant during cruise. And more relevant for an airframe that is designed to be efficient at its cruise speed, as opposed the current generation of overpowered 90-110 kt aircraft overpowered to cruise at 150-170kt which are suffering badly from parasitic drag at these speeds (because of unsophisticated flap systems and the desire for low approach speeds).

Overall, this does not mean that hybrid aircraft make no sense. A battery/engine/generator combination which can only sustain current cruise power but still gets high take-off and climb power for short bursts could be more efficient at the same weight, give different engine location options and resulting more efficient airframe (especially in twins).

I for one like seeing some R&D money being spent on general aviation. And rather than developing yesterday’s Diesel engines into something viable in aviation, which over two decades has given us mediocre solutions for a small number of underpowered aircaft, why not leapfrog a generation and go hybrid?

[PS – a 300 HP Diesel SR22 would be great, too]

The claim of noise reduction requires proof. In my experience, noise comes from the prop as much as from the engine. I well remember seeing a trike taking off on electric power just as noisily as its 912-powered look-alike.

On top of that, it is claimed an advantage to use a smaller turbine, where we seem to agree here that the smaller the engine, the less efficient it is – turbines especially. Running at the most efficient regime may make up for that, though.

But for mass transportation, aircraft need to be more limited to the longer distances, where the speed advantage prevails over the overhead time of getting to/from the airport. The shorter the distance, surface transportation becomes more efficient. And any progress in propulsion technology will apply to all modes of transportation.

All of this having precious little to do with recreational aviation, our main subject here.

A car spends a large share of its energy accelerating. It can be recovered during braking. This is especially true in urban situations – hence hybrids are much more efficient than standard cars in cities, the advantage dwindles to almost nothing when cruising on motorways.

This is not how it works. Yes, energy recovery when braking adds to the overall, but by far the largest effect is the engine running at or around optimal efficiency. A normal car runs way out of anything that can be called optimal efficiency when driving in the city. It is either accelerating or decelerating and running at idle. It’s the acceleration and idle that destroys efficiency, and this is where the hybrid technology comes to help. The engine runs at optimal efficiency during acceleration, and the electric motor either is charged, or helps accelerating, depending on how hard you step. The battery in a Prius is tiny, a joke compared with the battery in my eUp. The reason this can be done is of course the energy storage capability of the battery, and this does not have to be that large. The same technology can also be used almost exclusively to increase acceleration and max power from smaller engines, as is used in F1 cars and top of the line sports car.

The newer generation of plug-in hybrids, takes the technology one step further. Here the battery capacity is increased several times (compared with a Prius). A Golf GTE can run 50 km on battery alone.

It’s the same concept that Siemens want to use in aircraft, only tailored to flight.

Apparently the V6 is slated for 2016 certification. Fingers crossed.

Interestingly enough, in spite of their design that renders them unable to produce high power for extended periods of time, the in-flight shutdown rate for the Continental diesel engine line-up is actually better than the initial 120min ETOPS certification turbine in-flight shutdown rate.

But for mass transportation, aircraft need to be more limited to the longer distances, where the speed advantage prevails over the overhead time of getting to/from the airport. The shorter the distance, surface transportation becomes more efficient. And any progress in propulsion technology will apply to all modes of transportation.

All of this having precious little to do with recreational aviation, our main subject here.

Actually I think it could take another route for GA, still using the same basic technology. Shorter bursts with lots of power, 5-600 HP or more, would make aerobatic aircraft and STOL type aircraft like we have never seen before.