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Comment by 11thEarlOfMar | Hacker News Reader

RE: electric flying things...

Is battery energy more efficient with air speed lift from a wing, like traditional aircraft, or from air suspension, like a helicopter? Does the mass/energy of a gasoline power train (including fuel & motor) exceed an electric power train?

I guess I'm wondering about the viability of electric power for flight at all. Drones are useful for some applications with light payloads and conducive environments, but when you need to lift a substantial payload, like me, isn't battery just too little power density?

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A few rules of thumb:

* fixed wing flight is faster, has a longer range, and is "more efficient" than rotorcraft

* gasoline, for now, has roundabout 50x as much energy per mass ("specific energy") as batteries.

* however, electric engines are 4 to 10 times more efficient.

* so basically, with electric power for now you can only fly about 10% to 20% as long/far as with avgas - ok for a few specific applications, but not yet good enough

* on the other hand, electric engines are much simpler, lighter, and more reliable than piston engines (turbine engines are also very reliable, though)

(* note that one needs sufficient specific energy to fly long/far enough, and sufficient specific power e.g. for takeoff)

* lastly, batteries appear to improve at around 4% to 8% per year (some incremental improvements, but mostly switching to new "chemicals", eg lead to NiMH to Li-...), so electric flight could become really interesting in the next two, three decades.

Thanks, that's what I was looking for.

Air suspension?

Helicopters are aerodynes just like fixed-wing aircraft, deriving lift from aerofoil speed. They are just less efficient because in traditional single-rotor designs only part of the rotor disk is generating lift at any one time ( and part is stalled ) and of that part needs to be directed as propulsive thrust.

Some helicopters have employed short wings to offload the rotor.

A wingless VTOL aircraft such as the Flying Bedstead, or the Lunar Lander testbeds, is a non-aerodyne and is even less efficient because it relies on a small volume of highly-accelerated air for lift versus displacement of a large quantity of slow ( roughly speaking ).

Ok, thanks for the explanation.

Still wondering if electric powered flight is commercially viable for general purpose human transport.

I guess you could think of a hovering helicopter vs a circling aircraft, both having the same weight and hence same lift. Both have to accelerate some air mass downwards. The lift comes from the velocity x mass = momentum of air, which has to be the same with both.

Since the airplane wings affect a larger air volume than the helicopter blades, the airplane accelerates a larger air mass, but to a lower speed. The helicopter: smaller air mass, higher speed.

Since energy required is proportional to the second power of speed, the helicopter requires more power for the same lift.

You could create a helicopter that was quite efficient in hover, with really big blades. It would then have some other problems, like limited speed.

> Air suspension?

Could be a reference to ground effect, and a mistaken belief that helicopter lift is driven by ground effect (ground effect does play into helicopter low-altitude behavior, but as you mention they are aerodynes.)

My understanding is that electric motors are necessary because they can change speed faster than an internal combustion driven motor in order to maintain balance. What I don't understand is why we don't see more drones or similar flying machines using a small gasoline generator driving electric propellers to get high energy density combined with maneuverability.

The issue, as I understand it, is all about weight. Putting two different power systems on board takes away all of the benefits of using one or the other exclusively.

I would expect to be a scaling point past which choosing gasoline over batteries becomes more efficient. Maybe common electric flying vehicles have not reached that size point yet.

I think that it is not a size point but an endurance point. A camera drone can be useful even with just five minutes on a single battery pack (your audience might actually be grateful if those overhead takes are less endless ;)). But a big commercial liner that has to be in final approach after only five minutes in the air would be very, very pointless.

At least for fixed wing aircraft, there is actually a bit of opening where hybrids could make sense: planes need big power reserves relative to their cruise power demand (or relative to moderate climb) for takeoff and landing abort capability. That extra power needs very little endurance and since electric is much heavier than fossil per Wh, but lighter per W, burning fuel for baseline demand augmented with batteries for peak can be a meaningful optimization.

Weight. Multirotors scale up poorly, since the mass of a bigger propeller makes the rapid speed changes required for control harder, or require more energy.

Finding an engine / generator / capacitor combo which is light enough and powerful enough to work well with a multirotor is expensive.

Not to mention that such machines would be looked down on because of their use of fossil fuels.

It's more R&D costs than anything else. Batteries and electric motors are really easy to use and scale up.

Micro/Nano Gas Turbine Engines easily surpass batteries over 1h, but are not cheap or easy to use. And, if you only need 10 minutes at a time for testing then batteries are significantly lower weight.

EX: Wren's MW44i 46 oz (1320g) and produces almost 7 hp or ~5kw.

Sure, batteries and motors scale up fairly linearly, but the physics don't. The energy required to change the rotational speed of a prop goes up exponentially as as the prop gets bigger.

Above a certain size, it's simply more realistic to use variable pitch rotors that don't rely on speed changes to maintain stability, at which point the traditional helicopter design becomes more efficient.

You can add more blades which changes things somewhat, but the transition point is around ~10lb with 30m of flight time. So, any man sized flying machine is going to quickly cross that threshold.

Can they attach a helium balloon on top to reduce the left requirement?

My question wasn't about changing the size of the propellers. I was wondering if one would get more range by substituting a battery pack for a gas tank and a tiny generator.

It's still a weight problem. Gas engines and generators are heavy. You also have to add a capacitor system to handle spikes in demand for sudden maneuvers or stabilization jumps in electric demands.

Increasing the weight increases the engine & prop size, which increases your energy needs.

EDIT: From the few multirotors with a gas engine that I've seen, the right combination of weight - maneuverability sweet spot appears to be around the 750mm size - around 2 feet in diameter. They also tend to have good flight times, but still in the sub-one hour range. Again due to weight.

I get that one still needs a capacitor or battery. My intuition is that this is still better energy density in watt hours per pound than a battery. Maybe I'm just too used to old battery tech.

I would expect fuel cells as an alternative power source but retain the electric motors at each point. part of a drone's appeal is that they are quiet and for military the combination of that and low thermal signature benefits. figure electric motors are also desired for simplicity in both design and mounting.

Since you're curious about power density, payload, and efficiency, the math behind it is really cool.

Check out this section on Electric/Eco friendly Planes from the famous "Sustainable Energy Without the Hot Air": http://withouthotair.com/cC/page_278.shtml

If you like mathematics you'll also enjoy the chapter on Planes: http://withouthotair.com/c5/page_35.shtml

if you interested in these type of vehicles register here http://bit.ly/2pdeml5 for the public co-founder program we will be doing a pre-salle soon, the electrical vehicle will cost around $40k.

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