What are some alternative benefits to electric propulsion?

Question

The context I am asking is regarding if electric motors could ease the fuel cost, take off weight, or runway length if they were set up in 'hybrid' configuration with regular hydrocarbon engines.

If so, could potentially a military aircraft (IE, F35) have any weight savings or feature benefit of having an electrically boosted STOVL/VTOL system? Any fuel savings over regular flight? And of course, would this make sense at all from an engineering standpoint, today or ten years from now?

I am not asking the ability of electrical propulsion systems to replace the current main prop or jet systems, but rather to the alternative capacity that they might offer in other auxiliary or backup uses.

Answer 1

For aircraft, none.

Hybrid drive trains are used because:

• They offer, rather limited, efficiency improvement by being able to operate the engine closer to its optimal conditions.

  Because spark-ignition engines are more efficient at higher power settings, the hybrid can get advantage by having smaller engine, running it at (relatively) high power or not at all and cover the variations in required power using the electric engine.

  Cars accelerate and stop a lot and often cruise at small fraction of power, so they can make good use of this.

  The advantage only applies to spark-ignition, i.e. gasoline, engines. Compression-ignition (diesel) engines have more uniform efficiency over their operating range, so there is no or only negligible advantage. Which is part of the reason you don't see hybrid diesel cars.

• Batteries can be pre-charged with cheaper mains power. This is used in the hybrid cars, but it obviously requires lots of heavy batteries.

• Electric motors can provide high torque from standstill. Piston engines can only provide limited torque from standstill and it stresses the clutch heavily.

  This is particularly important for train engines. Train wheels have bearings lubricated by oil pumps attached to the axles that only operate when the train is moving, so when the train is stopped, the bearings are not lubricated and the force required to get it moving is much higher then when moving.

• At high power outputs, electric generator and motor are simpler and more reliable than a gearbox and clutch. For big train engines and ships, mechanical clutch is out of question. Hydraulic one, which is a turbopump and turbine in oil, can be used, but that still needs variable gearbox. The electric drive-train acts as almost ideal continuously variable transmission, being able to deliver almost constant power over wide range or rotational speeds.

In either case the hybrid drive-train is heavier than mechanical one.

Now for aircraft:

• The engine is run at constant speed and power setting, which is relatively high (usually cruise setting is 75% for piston and even higher for turbine planes). So there is no room to design a hybrid drive train to run the engine closer to optimal.

• Weight is critical for aircraft, so the weight of electric generator and motor is a show-stopper there. For cars adding weight causes smaller increase in drag, so its less of a concern and trains even need to be heavy to have enough adhesion for their traction.

• Since weight is critical, aircraft can't afford to take much batteries that could be pre-charged with cheaper mains electricity, the other benefit hybrid cars have.

• Propellers need almost no force to start spinning and are operated at narrow range of RPM, so the ability of electric motor to provide torque from standstill and across wide range of RPM is of no use to aircraft.

So none of the benefits for which hybrid drive trains are used in other vehicles is applicable to aircraft, so there is no point in building one.

Answer 2

I agree with the previous answer that there are a number of issues with implementing a serial hybrid (e.g., a chemically powered generator running electric motor(s)) on an aircraft, but I think there are a couple of positive sides to it as well...though we may have to design a new propulsion system rather than supplement an existing one. I found a NASA study looking at distributed turboelectric propulsion to replace cryogenically stored hydrogen scramjets on a BWB aircraft which shows the potential to actually save weight by moving over to the electric system (https://mdao.grc.nasa.gov/publications/IPLF08-Kim.pdf), but that's a pretty niche application. This, naturally, does not universally imply that that is the case, however: just for now and for that application.

However, the mention of distributed propulsion does highlight a major plus for electric propulsion systems. It's dramatically easier to route flexible power lines than mechanical shaft connections. Hence, if you want to have multiple engines...electric propulsion enables you to do that relatively easily. Also, say that you want to make an aircraft that can hover and cruise as a typical fixed wing aircraft -- typically, you'd have to design collective pitch change into your proprotors. Or, with electric propulsion, you could just have a bunch (I mean 12+) of proprotors, each of which you tailor with a twist distribution proper to a certain mode of flight. Look at a proposed NASA design (http://aero.larc.nasa.gov/files/2012/11/Distributed-Electric-Propulsion-Aircraft.pdf), a separate design by Aurora Flight Sciences that has advanced in the DARPA X-Plane program (http://www.darpa.mil/news-events/2016-03-03), or Lilium Aviation's recent design (http://lilium-aviation.com/). Given all of these are concepts, so nothing is proven by a working design as of yet...but it seems like a lot of people are looking into the possibilities afforded by a distributed electrical propulsion system. The expected advent of lithium-sulfur batteries should be a big deal here too. These batteries are expected to have a capacity of ~500 kWh/kg (about 2x the absolute best Li-poly available now), but I think that 2019 is the expected date for high-capacity Li-S batteries to start hitting the market.

Finally, one thing about engines is that their efficiency can vary dramatically depending on how they are being run (i.e., the load on the engine, RPM, etc.). This excerpt from a German textbook is, unfortunately, in German (https://books.google.com/books?id=QAGHZPVnnSAC&pg=PA540#v=onepage&q&f=false), but the basic idea is reflected on a Wikipedia page where the plot is reproduced in English (https://en.wikipedia.org/wiki/Consumption_map). On the vertical axis is power output of a spark-ignition engine, the horizontal axis is the RPM that the motor is being run at, the the contours are, essentially, fuel consumption. The long and the short of it is that, if you have any change in the engine's load or what RPM it's running at, it's not running at its most efficient. This would be the case for your typical non-hybrid propulsion, where load and RPM vary with flight condition. However, if we decouple flight condition (i.e., desired cruise speed, throttle setting, prop speed setting, prop pitch setting, etc.) from the chemical engine and let an electric motor (which has a much higher efficiency than a chemical engine) deal with these variations, we can, conceivably, run the chemical engine within an optimal performance range (some variation will probably be needed if power output is excessive, but that's a design problem wherein you'd try to size the engine for its most efficient in cruise, I'd think). That's a fuel savings for us, plus a decrease on engine wear because it doesn't have to see all the throttle variations typically present in normal flight.

As noted, this isn't the case for a typical airplane (small RPM bands)...but what if we decided to play the game that Aurora, Lilium, and NASA are playing and make VTOL/STOVL aircraft? The differences between flying a proprotor in edgewise and axial flight are dramatic, and having RPM control can make a lot of difference and might obviate a need for variable pitch propellers. Just design your twist distribution right, vary your RPM, and you just might be able to get acceptable efficiency in both regimes (at least for slower speeds). Or, like NASA, use two different sets of proprotors for a fixed-wing cruise and vertical flight, each one optimized for that specific flight regime. I'm not saying it's easy...just that electric propulsion makes the opportunity present.

But, yes, weight and technology are major concerns...and you can see in the electric aircraft that have been built to date. Firefly (Sikorsky's electric helicopter) has a max endurance, I think, of 15 minutes. Helios (https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-068-DFRC.html) is a neat aircraft, but look at how flimsy the design is in order to enable it work as a solar/fuel-cell hybrid (i.e., solar just doesn't provide a lot of power, so you need to make a light, high-aspect ratio aircraft). The Gamera-S is a solar-powered quadcopter under development at the University of Maryland (see below), but look at how sparse the frame is--weight is a HUGE problem for these aircraft, especially given how much power we can currently get from batteries or the sun. Serial hybrids don't get rid of that problem (since it introduces engine weight, fuel weight, lubrication system weight, etc.) but it can give you a couple of perks, as noted above.