Hacker News Reader

Comment by mcculley | Hacker News Reader

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.

View on HN

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.

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.

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.

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.

Reply on news.ycombinator.com