Comment by captainperl | Hacker News Reader

Comment by captainperl | Hacker News Reader

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> DC motors are multiple times smaller and lighter than the equivalent powered turbine engine when you consider the entire system.

Airplanes this size can use cruise missile engines, which are very light. See the F-5 for more info (7:1 thrust to weight ratio.)

> On top of that you totally eliminate the need for variable pitch props

Why is that? Variable pitch props have nothing to do with engine type. They're variable to be more efficient depending on RPM and speed.

> You also negate a lot of hydraulic systems (and their redundant backups) without the need for things like fuel pumps, thrust reversers,

Not really. A lot of flight controls are boosted with hydraulics, and thrust reversers are for increasing drag (descending faster or slowing down.) None of these depend on engine type.

The real issue with electric planes is that they're a fire hazard. Already people have been killed when their electric plane's battery caught fire mid-air:

https://www.flyingmag.com/two-people-killed-in-first-crash-e...

The JAL 787 battery fire was a near-tragedy and resulted in interruption of service to SJC for 2 months:

https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...

I find the most erroneous posts on HN to be about airplanes and databases.

>Why is that? Variable pitch props have nothing to do with engine type. They're variable to be more efficient depending on RPM and speed.

Variable pitch props have everything to do with engine type. They exist because turbine and piston engines have a very specific power/torque curve which requires them to operate at certain RPMs for certain conditions to achieve peak efficiency. With electric motors you have no need for this because you get a constant increase of efficiency across the entire power curve.

>Not really. A lot of flight controls are boosted with hydraulics, and thrust reversers are for increasing drag (descending faster or slowing down.) None of these depend on engine type.

Thrust reversers have nothing to do with descending faster or slowing down in flight, they are used solely for landing retardation. But an electric aircraft wouldn't even need thrust reversers. You literally just reverse the propeller immediately at full power when touching down. You can even save on less flaps required because the motors will be able to "windmill" on the descent and slow the aircraft as well as regen power.

Variable pitch props are useful for other design conditions. The prop RPM is typically limited to a speed that prevents the prop tip from reaching supersonic speeds, for noise certification. On fixed-pitch props at this fixed RPM, there's a tradeoff between climb and cruise performance, because these operate at two very different advance ratios. Constant-speed props allow the blade to adjust some of this difference.

There's also the engine-out design case, it's much easier to add prop feathering than add more power to the operating engines.

This design doesn't make a lot of sense, the props are located at literally the maximum arm from centerline, which requires bigger control surfaces for engine-out design (heavier and more drag) and gives very little ground clearance in crosswind landings. On the economics side, fuel costs are a small part of operating light turboprops/jets. The biggest costs are airplane depreciation/capital and crew. The real breakthrough would be an autonomous turbine plane that could eliminate crew cost and achieve 2000 hours/year utilization (versus ~200 for some privately-owned jets).

Your post exaggerates the danger from batteries. Tesla's billions of miles with fewer car fires than gas cars per mile driven is at the very least evidence that they are no more dangerous than conventional gas cars. The infamous boeing battery fire came about from poor design, and the battery wasn't used to run the engines. Tesla heats and cools their batteries and also tolerates batteries failing; boeing's design was a single large cell and did not tolerate failure, ie it was poorly designed.

Tesla has shown battery tech is a safe and reliable energy source. However, this doesn't mean there can be no problem - of course there can! Airplanes also have had many problems with catching on fire, crashing, burning in the air.

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