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Comment by deugtniet | Hacker News Reader

I watched the video, and only (maybe) saw STOL, not VTOL.

Looks like they use two Dyson style fans for propulsion, which means the fan blades are on the inside of the fuselage, could be very interesting for an urban environment, as there is less risk of hitting something with your blades.

That being said, they mention a gas turbine is necessary for propulsion, so it probably won't be quiet enough for urban envirnoments

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Deep in the article it mentions a 15 dB reduction in noise compared to a turboprop at the same thrust.

Unfortunately, though, they don't give hard numbers… If that's 120 dB versus 135 dB it's still too loud for urban environment. But 105 dB versus 120 would make a huge difference and likely be tolerable for a city if it's kept to landing pads on skyscrapers.

Putting landing pads on skyscrapers is mostly a fantasy. There are only a few such pads today. They can't easily be added to existing buildings because the roofs are already used for antennas and climate control machinery. Weight is an issue. And even relatively quiet aircraft are going to annoy tenants on the top floors.

> Putting landing pads on skyscrapers is mostly a fantasy.

Then let me share another fantasy with you: reading what you wrote, there's no need to land on skyscrapers then, drop a stairwell - similar to some private jets and helicopters.

Huh? You can't just drop a stairwell from an aircraft. Not sure what you're suggesting there.

> STOL

I suspect the "V" is still "in theory," because it probably relies on those fans being rotated, and it looks like the ones in the demonstrator are fixed.

Once the rotation happens, a lot more factors come into play, and those problems are not new (see "Osprey").

I think that SCRAMJet engines also use fluid dynamics, in a similar manner.

An issue with both the Osprey and the VTOL F-35 is that they each have two engines involved in VTOL. So, if one engine fails - then the plane becomes unbalanced and probably flips over. (That's why the F-35 has auto-ejection [1])

This technology is interesting because, if you can have one engine involved in VTOL (like the Harrier) but vector the thrust easily, then an engine failure during takeoff or landing can avoid spinning the plane.

[1] https://www.thedrive.com/the-war-zone/the-f-35b-can-eject-it...

>> An issue with both the Osprey and the VTOL F-35 is that they each have two engines involved in VTOL. So, if one engine fails - then the plane becomes unbalanced and probably flips over.

This is not correct for the V22 Osprey. Either engine can power both rotors through the wing driveshaft.

Disclaimer: I worked with the V22 engine control system at one point (AE1107)

Than you had one hell of a problem to solve!

Where do you get your information about the Osprey, because it is woefully incorrect. Part of the engineering challenge of the Osprey was specifically to overcome loss of engine. There is linkage connecting both props to either engine so if one engine were to fail, both props would still turn. It's one of the talking points about the damn plane. I'd be quite ashamed for posting such non-sense if I were you.

The F-35B is a different beast (lift fan plus tilting jet exhaust) than the Osprey (tilt rotor). The tilt-rotor Osprey has shafts that connect the engines so one can power the aircraft if the other fails. This design feature is also present in the V-280 Valor, the replacement for the Blackhawk helicopter.

The F-35B only has a single engine.

Not one bit of info in the GP's post is correct. It's like it was intentionally posted. If it wasn't such an old account, I'd suspect some sort of chatbot was behind it.

No the F-35B has only one engine just like the A and C versions. The big fan at the back is powered by a driveshaft from the engine, it's not a separate engine.

And like the other commenter said the Osprey can power both props from one engine (I wonder how that performs though, I'd be surprised if it can sustain a fully loaded hover)

From looking at their website [1], it looks to me like they are just using a jet turbine and routing the air through pipes to those ring-shaped outlets.

[1] https://jetoptera.com/technology/

Thanks for the link. I have a question. They say:

"The energy required to propel our smallest aircraft at 200 mph for 30 minutes is 100 kWh. This energy can be provided by 26 kg of jet fuel or 600 kg of modern Li-ion batteries."

Is really "26 kg of jet fuel" comparable to "600 kg of modern Li-ion batteries"?

Thanks!

According to Wikipedia [1] jet fuel has 43 MJ/kg and lithium ion batteries with silicon nanowire anode have 1.566 MJ/kg. With that you get get 714 kg of batteries for 26 kg of jet fuel. The Tesla Model S 85 kWh battery has 0.57 MW/kg [2] which gives 1961 kg. Looking only at the energy density, the numbers from the article seem quite optimistic for the battery weight, but it is not clear what they actually calculated and I guess their numbers might contain factors accounting for efficiency. Do they account for the difference in weight, of the energy source and energy conversion mechanism, 26 kg vs 600 kg is nothing were I would expect similar performance?

[1] https://en.wikipedia.org/wiki/Energy_density

[2] https://en.wikipedia.org/wiki/Tesla_Model_S#Battery

>The Tesla Model S 85 kWh battery has 0.57 MW/kg [2]

Note that your source is from 2015. The Model S battery of today has a higher capacity (100 kWh) and has undergone constant lightweighting improvements.

This article [1] says that it is now 0.65 MW/kg which is a 15 % improvement.

[1] https://www.teslaoracle.com/2022/02/18/model-s-plaid-battery...

Electric->Mechanical is much more efficient than Fuel->Mechanical so that is probably where the difference is coming from.

I do wish in those kind of comparison's they would make them equivalent: as in "26kg of jet fuel + XXXkg of turbine" vs "600kg of modern Li-on batteries + XXXkg of electric motor" since for a flying vehicle the all-up weight is the significant number.

1 gallon of regular gasoline is 33kWh and a gallon weighs 6lbs. For a 33kWh battery pack comprised of 18650 Samsung 35E cells, you’re probably looking at around 300lbs for just the cells alone.

Irregardless, it doesn’t matter because an electric motor will be far more efficient using the energy stored than a fueled engine (be it gas turbine or ICE). In automobiles for example, 80% of the energy from the gasoline is wasted.

But not necessarily more efficient at thrust.. a turbine is mixing fuel with oxygen from the env to both spin the turbine but also vents the exhaust and gets thrust that way. So it doesn't carry smth like 3/4 of its reaction mass. I know that turbofans duct in most of their flow, so not sure how this nets out on a large-scale device.

The efficiency of gasoline and Diesel engines is a lot better, over 30% and even over 40% for Diesel. I think the best gasoline engines get close to 40%.

Sure, the best diesel engines get 40-45% and some gasoline engines approach 40%, but that is definitely not the norm. The average is 20% for gasoline (hence my prior figure) and 30% for diesel, which is a far cry from the average electric motor’s efficiency of around 90% in automobiles.

Those are all roughly true numbers but you forgot charge efficiency: energy extracted out of battery / energy input into battery. That's also around 90% (assuming higher voltage charging, not 120v AC which would be closer to 60%). That would put total efficiency of battery electric closer to 80%.

A UAV is still dangerous around people even with no blades involved; a falling UAV is enough to kill. This is a real issue that companies (startups and giants from the domain) are trying to solve, which is required for unmonitored UAV in urban areas to start getting legally accepted (at least in France).

Ballistic parachutes have been around for a long time. I wonder if they can use that with an audible alarm to eeduce the likelihood of a person strike.

It's funny to me that the plane is driven by a compressor turbine but they are calling it "blade less"

Maybe there's a new way to build compressors I don't know about?

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