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Comment by goodcanadian | Hacker News Reader

This article has me thinking about hybrid electric aircraft (is anyone working on that?). A small internal combustion engine could generate electricity to charge the batteries to run electric motors. With this set up, you could still get the efficiency advantage of the alternate propeller placement. The engine could be much smaller and lighter than it is conventionally because the additional power needed for take-off and climb thrust could be temporarily drawn from batteries. Further, in the event of engine failure, you would still have battery power available to the propellers for a short time in order to make an emergency landing.

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I've looked at it before, in the context of R/C aircraft (my hobby). The problem is the same one everyone else has named: airplanes need to maximize power-to-weight ratio and hybrid-electric systems are heavier for a given power output.

My conclusion was that a battery-based system was a bad approach. Instead you would want to use a hybrid motor/generator approach - you can use a glow motor to spin an electric motor, and if you couple the electric motor to another identical motor they will spin lockstep. It's like a diesel-electric transmission on a ship or locomotive. And the funny thing is glow engines are actually a kind of diesel engine too.

https://youtu.be/6kgzrXFSDwA?t=13

This would be most advantageous for multi-engine setups because you would be able to use more energy-dense liquid fuels instead of batteries, use a single larger/more efficient motor as the prime mover, and avoid the complexity of multi-engine operation. With gas engines, you need to tune each engine to perform similarly, have a throttle servo with identical/aligned travel for each, independent fuel supplies (or at least header tanks), etc. Whereas electric motors are dead simple, but the problem is the batteries are hard to lift. So instead of batteries you have a prime mover - and on top of that you get liquid fuels too.

Ideally you'd probably want a single larger generator-motor attached to multiple small drive motors, but they would need to be chosen so that the electrical characteristics of the motors were appropriate for each other throughout the RPM range.

The downside is that it probably doesn't scale well to full-size aircraft - many small aircraft are relatively underpowered even with gas engines. And in both cases, having multiple independent power sources enhances safety, as this creates a single point-of-failure.

It's definitely being worked on. Google "hybrid electric aircraft" and you'll find some discussion of the efforts.

It's less advantageous than it is for a car. For one, the difference between maximum power and cruise power is far lower in a typical light airplane than it is in a typical car. A Prius might max out at 130hp but cruise at 25hp. A light airplane often cruises at around 75% of maximum power. Another problem is that the hybrid powertrain adds significant weight, which hurts efficiency much more in airplanes than it does in cars. I don't think these kill the advantages entirely, but it's probably why it's not as far along in airplanes as it is in cars.

A light airplane often cruises at around 75% of maximum power.

Good point . . . I was probably thinking of larger, heavier, faster aircraft where cruise power is significantly lower than max power. As an extreme, I seem to recall that the Concorde cruised at about 20% power.

Obviously I don't know anything about these sorts of things, but 20% power may have been all the engines were capable of producing with the much thinner air at cruising altitude.

I guess that depends on whether by power it's generally meant "the max power the engines are capable of under ideal circumstances" or "max capable under current circumstances". I think the latter is generally what is assumed when talking about operating (since it's hard to know the former). For example, in a car, you would probably assume that meant the gas pedal half depressed (and even that would be grossly wrong to the actual meaning of the latter interpretation, given gearing and the RPM power curve).

Jet engines actually perform better at higher altitudes, bypass and propeller notwithstanding.

No, they are more efficient at higher altitudes, mainly due to the colder intake air, but they have lower performance (as in maximum available thrust), due to the air density being lower and there being less available to burn the fuel.

Right, the first thing to always remember when you're doing a hybrid is that you're stating up-front, "I want to have two engines in this machine -- that's a weight and space cost that I want to pay up-front." As mentioned by the article, with airplanes when you increase weight everything has to get bigger to accommodate it. That doesn't make it impossible, but does make it a bit tricky.

Another interesting question: the fuel cell design in the original post is probably not reversible, is it? It would be really cool if given 10-20 more years of work we then used variable-pitch propellers with reversible fuel cells allowing for a plane which, during descent, could regenerate most of the energy used to make its ascent. It might also make pilots less worried about limited range, because they'd know that in a pinch they could get some more energy to the battery.

Flight takes a lot of energy. Gliding let's you fly for longer at the cost of speed or altitude. You can also trade altitude for speed, but there is little waste in a normal descent.

> allowing for a plane which, during descent, could regenerate most of the energy used to make its ascent

The Pipistrel aircraft do almost exactly this. They claim about 10-15% energy recovery rate for thier regenerative systems. No need for variable pitch either; they simply windmill the main prop.

Max vs. cruise is a huge difference for VTOL though, which you need to replace cars for daily commute.

It's interesting that I read 'Google "hybrid electric aircraft"' without your quotes for a second and assumed Google was working on a hybrid electric aircraft, rather than you suggest people search for it with Google. Google is diversifying enough that just saying "google $term" without quotes (as you provided) can be ambiguous. Perhaps this is the long-term curse of google as a verb.

That's pretty funny, and I can totally see why you might read it that way. I guess technically now it would be an Alphabet hybrid electric aircraft? Not that I'm at all careful about saying Alphabet instead of Google.

Me neither, but I think this incident is illustrative as to why they changed it. "Google" has multiple meanings now (a site, a service, an action, and a company), and they probably want to distinguish their corporate element from those others.

I think one potential advantage is that turbofan bypass ratios are topping out due to the dimensions of the fans needed. A hybrid system would allow you have multiple fans per engine and also reroute power in case an engine failed.

I think turbofans also have issues with the difference between the rotational speed that is optimum for the turbine and that for the fans. Hybrid design would allow one to decouple the two.

Pipistrel is trying: http://www.panthera-aircraft.com/technical-data

I am working on a two passenger hybrid electric aircraft we decided for parallel operation.

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