Comment by j9461701 | Hacker News Reader

Comment by j9461701 | Hacker News Reader

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The thing that sinks most of these flying car or flying taxi initiatives is the noise problem. Because airplane engines are so darn loud, all flying cars are forced to operate away from built up areas whether or not they're capable of vertical takeoffs and landings. Which makes them inherently redundant, as you're already having to drive out of the city to get to your flying car - at which point you could just drive slightly further to an airport and get on a conventional plane that boats vastly higher cruising speeds and fuel efficiencies.

If not for the fact that airplanes are noisier than a fox stuck in a trashcan rolling down hill, we'd already have flying taxis of a variety of different forms that take off and land on top of sky scrapers. The youtuber mustard did a recent video exploring one such concept:

https://www.youtube.com/watch?v=dkJOm1V77Xg

Using ducted fans and an electric motor both reduce noise, so time will tell if we've finally achieved a "city acceptable noise level" out of one of these designs at long last.

FWIW, most (if not all) of the big tech companies working on flying cars or UAV are aware of the noise problem and have in-house teams working with research labs & consultant on the problem. Now whether or not they succeed is another issue because it's a hard problem.

There is at least one prominent tech firm that appears to think they can mitigate these issues in a year or so, which is... unrealistic.

Anybody familiar with how the military creates stealth choppers that aren't as noisy? I'm assuming it has to do with blade design, but would these changes be applicable at smaller scale and would it be enough of a sound reduction?

The most promising theory is that the "stealth hawk" used actuated micro flaps on its blades' trailing edges to reduce noise and vibrations. I would put money on this being true. Flapped blades have been studied in academia for over a decade, it's definitely an engineering problem now and has clear benefits.

http://www.helijapan.org/pdf/general/general05/Friedmann/Jap...

There's no such thing as "stealth" really, but they can optimize blade shape for noise reduction and put mufflers on engines. Those measures reduce aircraft performance (no free lunch in aviation).

From what I heard 3rd/4th hand/speculation, the "stealth" helicopters are quieter than normal helos but at the cost of efficiency and maneuverability. There are always tradeoffs.

edit: what nradov said.

Some first order guesses: -Have lots of blades on the same rotor so the assembly can turn slower -Put the tail rotor inside a shroud -Having a smaller rotor might help, since the spread in air speed across the length will have less variation (less sure if this would help) -Turbine engines can be pretty loud, I'm guessing the same is true of other powertrain components. Maybe put the loudest stuff on the roof.

Kitty Hawk, Larry Page's pet-project flying car company, claims to be working on the noise problem:

"The lack of noise while in flight is another thing Kitty Hawk is highlighting about its new aircraft. In a video posted on its site, Kitty Hawk notes that a helicopter hovering at 1,500 feet emits about 80 dBA, while Heaviside only puts out 38 dBA."

https://www.theverge.com/2019/10/3/20897336/kitty-hawk-elect...

Heaviside is impressive. One of the most elegant eVTOL aircraft I've seen so far. Looks efficient, taking a lot of cues from sailplanes instead of just slapping together a quadcopter with some wing surfaces (or slapping some clunky lifting pods onto an airplane).

I think it's a feasible design. Hopefully they actually start flying it crewed and scale it up to at least 4 seats. If they can execute, I can believe it'd be significantly cheaper to operate than a helicopter.

The lack of safe landing sites also sinks the flying car initiatives. Only a few skyscrapers have helipads today. Adding them after construction is tremendously expensive. Roofs aren't designed to support the weight and are full of equipment (antennas, HVAC).

> Roofs aren't designed to support the weight

A Surefly Octocopter is 1850 pounds; what does a large HVAC unit weigh? Surely a roof that's strong enough to support large HVAC units is strong enough to hold an Octocopter.

You have to factor in the weight of the landing pad too

> Because airplane engines are so darn loud

It always appeared strange to me when countries have strict noise limits on aviation, but most countries do not seem to mandate noise limits for cars

Remove your truck's muffler and take note of how long it takes to get a ticket.

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