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Electric / hybrid aircraft propulsion (merged)

Having been peripherally involved in this, I’d make a few observations

  • the case for hybrid vehicles hands wholly on the intermittent power delivery and on the really crappy efficiency of an ICE at low powers (maybe 90% of urban usage)
  • nobody has figured out how to establish how much energy is really in any battery that’s usable for any advanced applications – partly because there is no good way to measure the internal leakage, or to characterise the loss of capacity over age
  • the long term case for electric vehicles (of any kind) hangs wholly on the powers to be not having yet found a way to tax the stuff The most likely approach is a mileage based one, apparently, because it avoids the need to screw around with peoples’ existing domestic electricity bills
Last Edited by Peter at 09 Mar 18:03
Administrator
Shoreham EGKA, United Kingdom

I’d say the only case for anything except a gasoline engine (or thermals) powering an aircraft is tax avoidance.

My job for the past 8 years or so has been running R&D for electrification of existing machinery. It mostly makes little economic sense except when considering artificial political constraints – tax avoidance being primary, also arcane permitting issues. Occasionally it does makes sense on a technical basis, for instance on the ocean floor.

Last Edited by Silvaire at 09 Mar 19:54

But how do you tell how much capacity the battery still has after a few charge/discharge cycles?

In the radio control world, we tend to monitor our batteries by discharging until the voltage starts to drop – generally during normal flying, then filling them back up again. My charger tells me exactly how many milliamp hours I’ve put in. The A123 batteries I use still have 90% of their original capacity a few hundred cycles and half a decade after I started using them, which is normal. If any individual cells started losing capacity, you’d know about it because they would end up reverse biased whilst the others were still putting out good amounts of power. You could easily monitor this, and I suspect a few high power diodes would be enough to prevent anything too catastrophic from occurring, at least in A123 cells. Lithium polymer and some other chemistries can be a bit more finickity. In flight, you tend to time your flights to use 80-90% of the theoretical available capacity and in practice this tends to be pretty reliable.

That would likely mean much higher landing speed and thus energy, thus would run afoul CS23 and be a safety concern.
But you could design a more streamlined cowling if the relatively large ICE needn’t sit in front.

No, you’d be landing vertically with minimal groundspeeds so this wouldn’t be of concern. But it would be so thoroughly ‘different’ that I’m sure certification would be difficult.

Have a look at this for something with some similar features to what I’m thinking about:

http://www.gizmag.com/hybrid-quadrotor-hq-uav/28767/

IC powered VTOL aircraft have largely failed because, whether you use piston or jet engines, they end up heavy (due to dedicated lifting engines) or complex. A good jet engine may have a thrust/weight ratio of 5. People who’ve tested my toy helicopter (460g motor and a few hundred grams for the blades) can apparently lift over 14kg (including its own weight) and possibly quite a bit more. I’m too realistic to think we’ll see these strange contraptions carrying people around the sky tomorrow, but sooner or later somebody will explore the new possibilities these high powered motors offer.

Occasionally it does makes sense on a technical basis

In radio control flying, electric motors are just so simple and reliable compared with IC that they’ve largely replaced them. Here’s an aerobatic aircraft with only 4 moving parts:



In radio control flying, electric motors are just so simple and reliable compared with IC that they’ve largely replaced them.

Electric power works well for a model because of scaling laws and limited range. Square-cube scaling makes any scaled miniature aircraft very light in relation to its power and wing area. It can then carry its batteries more easily than a larger aircraft, while still having useful performance and the limited range required of a model.

Is the scaling really the problem though?

Battery powered ‘full size’ aircraft normally seem to have an endurance of half an hour to an hour, which is obviously pathetic in our terms. However, it’s actually very good compared to most model aircraft, even ‘sports’ models that aren’t ridiculously overpowered, especially when you consider they have to carry the weight of a pilot relative to the weight of a few servos and a receiver.

What electric flight really needs is a x5 increase in energy density. This would be great for us, and make little difference to model flying – who really wants to keep flying helicopter aerobatics for a whole hour?

Last Edited by kwlf at 10 Mar 04:40

OK, let’s assume the technology in batteries in going to take a big jump over the next years, in terms of weight/power ratio. Let’s also assume that there is a good way of measuring the amount of energy left in them (from what I read, the indications on the state of the batteries/range left on a BMWi3 are accurate…).

Now take an Austro engine. Assume 170 HP max power (will become 180 or more for the DA52). This engine is certified to run at 92% MCP, so that is 156 HP. That is equivalent to a 210 HP AVGAS engine running at its typical 75% cruise setting. Add 70 HP (or whatever number) of peak electric power for take-off and initial climb. So now you have an AVTUR aircraft that consumes 6-7 GPH in cruise, turbo, comparable to one with 210 HP, AVGAS, but in take-off it would have 240 HP available. Yes, heavier, and nose heavy, but in the initial design you can play with the position of the batteries to compensate like Achim said. And you take less fuel (mass) with you for a given mission..

Anyone care to imagine what this propulsion combo would mean on existing or new airframes?

Private field, Mallorca, Spain

let’s assume the technology in batteries in going to take a big jump over the next years, in terms of weight/power ratio. Let’s also assume that there is a good way of measuring the amount of energy left in them (from what I read, the indications on the state of the batteries/range left on a BMWi3 are accurate…).

I think those assumptions are sensible. At the Geneva car show, the BMW CEO said that they expect to double the energy density within 4 years (well, he said “double the range” actually but I very much doubt one can reduce the kWh/100km number by more than a tiny bit). Regarding measuring the energy, I am not concerned. This is not a problem with the special purpose battery packs used in modern cars.

I’d say the only case for anything except a gasoline engine (or thermals) powering an aircraft is tax avoidance.

That is a more common point of view in your part of the world. In Europe, many people take a different view on the present and future of fossil fuels. And I have to say that I absolutely love my electric car and cannot imagine ever going back to a ICE car.

I’m not so sure I do believe that battery technology is poised for a big advance, but it also strikes me that you could switch different banks of batteries in order to ensure you don’t run out of juice at the wrong moment – it’s fairly straightforward to tell when current lithium batteries are either full or empty; it’s the in-between bit that’s difficult. So certainly not a show-stopper.

That said I’m hoping to have the space to start building a Luciole soon… And I’m not going to start with the power plant.

from what I read, the indications on the state of the batteries/range left on a BMWi3 are accurate…

How long are they in the market? Nov 2013? Wait until the batteries get old…

What is the life expectancy of the battery pack? How much is the replacement? And what is the range?

At the Geneva car show, the BMW CEO said that they expect to double the energy density within 4 years

The car industry seems to be having high hopes for Redox Flow batteries. Let’s hope this works out…

The advantage of hydrocarbons as fuel over batteries (with the exception of Zinc Air batteries) is that only one of the two reactants needs to be carried around. Furthermore, the waste products can be thrown overboard.

Lithium Ion cells have around 0.5MJ/kg, Zinc Air is around 3 times higher, but this completely pales to the ~46MJ/kg for fossil fuels.

LSZK, Switzerland

There’s also Aluminium-air and Lithium-air. The problems so far have been recharging and power density. I’m sceptical about improvements to lithium ion because the chemistry puts a limit on the energy density which isn’t all that much higher than we’re achieving today. I’m sceptical about the air-batteries more because they seem a long way, even in the labs, from being a mature technology.

The problem with fossil fuels is that they’re not used efficiently, so that 46 Mj/kg comes in at just 12 Mj/Kg when compared with 90% efficiency for an electric motor so the difference isn’t quite as great as it looks. Given an order-of-magnitude improvement that the metal-air batteries could theoretically achieve, you’d be in the same ballpark.

How accurate is a Cessna 172 fuel gauge? I’m sure a battery monitor could do better than that.

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