I'm willing to bet the name "Great Horned Owl" was purposeful for a vehicle where quiet is a priority. Owls have feathers which make them essentially silent in flight.

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

An owl still emits more acoustic energy per kilo-mile than a passenger jet.

That's just a variant of the square-cube law. An owl the size of a passenger jet wouldn't.

Let's all just be thankful that there are no owls the size of passenger jets.

That's, again, the same law. It would break under its own weight.

Can you explain this further?

If you doubled the length of an owl (or any other object) by scaling it up, then it would quadruple (2²) the surface area, and octuple (2³) the volume.

The parent comment seemed to be suggesting the noise would scale with the surface area, while mass scales with the volume. The latter is definitely true, but I don't know about the former

It looks like the flying wing! https://en.wikipedia.org/wiki/Northrop_YB-49

My grandpa worked on drones for Northrup from the late 60s to the late 80s, and he loved the flying wing design (not used on his drone though). He built op-amp based control systems, and flew jet drones with oscillators and large portable field plotters.

Inspired by the captured WW2 German flying wing, no doubt.

The idea of a flying wing predates WWII.

https://www.airforcemag.com/article/jack-northrop-and-the-fl...

Fascinating new (to me) stuff here...

"The resulting XRQ-72A has a general planform reminiscent of other Northrop Grumman designs, including that of the B-21 Raider stealth bomber"

You mean "platform"? Nope! It really does have a similar planform, a word I last heard in what, high school drafting/CAD? You mean "B-2" bomber? Nope, I just hadn't heard about it...

Also interesting that it has no landing gear and uses a fuel generator to power electric engines...

Planform is a syonum for shape, usually as viewed from above.

I wonder about the radar profile of those ducted fans. Why stick them on a stealthy airframe? Curious - what is the radar reflection of plastic? Is it feasible to use modern plastics along with a hydraulic drive system? Would avoiding metal(i.e. copper coils in electric motors, metal fan blades, etc) keep the external fans from producing a radar reflection?

Fans are behind the body when ground radars are located in the front and below.

You can tell from the wiglets and fans that this drone is not intended to be as stealthy as B-2 or drones without those features. For this type use it's probably enough to have optical stealth. In other words, avoid being detected by monostatic radars in specular reflection wavelengths. If the corners, surfaces and edges are not perpendicular to the incoming signal they don't send signal back to the radar.

I doubt that they even tried to give this a particularly low radar cross section, it appears to be a project about being stealthy from human ears. There is no mention of radar at all in the article, by the way.

I came here to say the same thing. Why would you bother going to the trouble of designing a stealthy aircraft if you're going to plop a load of decidedly unstealthy motors on the top of it. Really bizarre decision.

What makes you think they’re unstealthy? I doubt there’s even a tiny bit of metal in them. If they’re made out of Teflon, PVC, and exotic composites, they’ll have barely any radar return.

They’re just propulsors. The motor is inside the airframe.

If it's flying extremely high wouldn't they be hidden behind the body?

> A pair of fuel-powered generators inside the central fuselage produce the electricity that powers four ducted fan propulsors mounted on top of the aircraft's flying-wing fuselage.

Is this efficient?

Yes. The turbine engine wants to run really fast for more efficiency. And the fan wants to be big and run really slow for more efficiency. This is a mismatch. If you connect them with an axle (like in a turbofan), you have to make a lot of compromises, lowering efficiency.

You can put a gearbox in there but it's heavy, maintenance intensive and expensive. Efficiency of the gearbox is pretty good, but not 100%. Then you have a geared turbofan or a turboprop. Geared turbofans are rare for a reason.

With a generator and electric motors, you can have a different amount of fans and turbines, and they don't need to be even close to each other. You don't need the expensive and maintenance prone gearbox. You can optimize the propulsion system more -> more efficiency.

Turbines don't scale down very well - they lose efficiency the smaller they get. This means, electric propulsion is promising for small aircraft since you can have one or two turbines but can have more propulsors.

The big problem with gas turbine gearboxes is that they have to operate at such high RPM that only the best metalworking tech works.

Same with driveshafts and any kinds of transmissions. Any off-axis power diversion from turbines is extremely hard. Imagine a driveshaft spinning at 1/10 of the turbine RPMs, it will still require extreme stiffness or it will disintegrate from vibrations.

Very high RPM generators are not that efficient too because of eddy current losses. It's not so bad in big industrial turbines spinning at 5-7k RPM or so, but losses mount at 15k+ RPM.

This is why there is such a high interest in making superconducting generators for direct drive by turbines. They can be very efficient, and very small — two usually mutually exclusive qualities for an electrical machine.

And even with the best metalworking, materials, and lubricants, high speed gearboxes don't last long, and require frequent inspections for fatigue, and overhauls.

P&W 1000G series have 32000hp gearbox rated in excess of the core cycle life and hours, but still apparently needs mandatory defectoscopy.

I was also skeptical, given that each power conversion step will necessarily be less than 100% efficient. I was surprised to learn that high wattage electrical motors can commonly achieve better than 95% efficiency. I assume the generator can be similarly efficient, which just leaves the combustion engine itself.

Pretty much. The thermodynamics are the same if your turbine drives a propeller or a generator, and the losses due to friction and resistance are small. Diesel-electric locomotives have used the principle for 100 years.

More dense energy-wise than batteries can get. Its required for this concept to be a reality.

That would be true if they weren't using fuel to power it, which they are. Part of the article shows the alternator they were possibly going to use.

You and the GP are in agreement.

The GP posted a sentence fragment, the expansion of which is presumably "[These generators and fuel are] more dense enegry-wise than batteries can get".

I think the better question is: Is this efficient enough?

If this design brings the noise levels they want, perhaps they are only worried about small distances to be covered?

Jet-a or diesel or petrol is incredibly more energy dense in Wh/kg compared to li ion batteries, the very best of which are about 255Wh/kg. Even after losing 40% to waste heat.

it can be. If you can operate a thermodynamic engine at a single 'operational point' meaning fuel/air flow in and energy out then you can heavily optimize its efficiency in terms of thermal efficiency and noise produced at that specific point...or at least a few points.

You can also muffle it if your optimization point is not just thermal efficiency but overall system efficiency.

It's really difficult to muffle a turbjoet or turbofan...the exhaust flow is the energy source.

I think it's meant to have a low heat signature, so efficiency was probably part of the consideration (less heat to shed).

Reminds me of E.E. "Doc" Smith's diesel-powered stealth spaceship in First Lensmen (1950)

https://gutenberg.ca/ebooks/smithee-firstlensman/smithee-fir...

If it has a little battery it could for a short duration have an even lower heat signature by turning off the turbine.

I think the benefit here is to be able to build and fly an electric drone without going knee deep into battery issues.

Doesn't matter, the fuel gives a much longer loiter time over targets.

I live near an airport. It would be nice if this quiet first approach was taken on by passenger aviation. All I can see at the moment is gradual improvement in noise performance being countered by increased traffic. A stepwise improvement would be most welcome.

I think hybrid electric designs are 'coming' partly for that exact reason. Aircraft can takeoff and land using mostly battery power which would reduce noise considerably. Part of the motivation is also to increase the bypass ratio by running a multiple fans per set of engines. Like four fans and two engines. High bypass --> less noise.

I've wondered why catapult designs haven't been studied for airports, since they're used frequently for launching fighter jets from aircraft carriers.

It would cut down on fuel consumption, emissions and noise when taking off.

G forces and the fact that a lot of noise isn't from takeoff itself but just from the low flying that occurs near takeoff and landing?

They would put massive stress on airframes that are not designed for them. And (at a guess) they are probably far more accident prone than unassisted take-off, a trade-off that is not acceptable for passenger operations.

Airbus has studied that. We might see it eventually. But it would be tremendously expensive to build electric catapults long enough to keep G forces down to tolerable levels. And aircraft would have to be redesigned with a strong attachment point.

https://www.theregister.co.uk/2012/09/07/airbus_smarter_skie...

Do catapults on aircraft carriers cut down noise and fuel consumption? No because you still run the engines at max.

Ostensibly over a shorter period of time though.

Not at all, planes keep full afterburner until they reach a safe speed and level (varies by aircraft) and most planes are refueled again after the take off. The catapult just enables them to take off with serious costs for the pilot, plane and energy.

This approach is focusing on cruising, not take off and landing; the requirements are completely different, the solutions may be quite different.

I wonder what's the rationale behind releasing this information to the public ...

I understand when they release info about weapons, to let your enemies know what you're capable of or to just sell the tech to foreign customers.

But this thing is a surveillance drone that probably will never be sold, wouldn't it be better to keep it fully secret ?

It might be aimed at impressing and retaining investors, perhaps even with some urgency. From Motley Fool, discussing defense companies including Northrop Grumman:

"Assuming these companies avail themselves of the [USG's COVID-19] aid, therefore, it could be a good long while before investors can expect to receive any dividend income from them. That probably doesn't seem like such a big deal to investors in Textron, which only has a 0.3% dividend yield. But for shareholders in Northrop Grumman (with an 1.8% yield) or General Dynamics (3.6% yield), it could be a much more significant factor in your decision to buy or sell."

Also I do wonder just how secret this tech is. The shape has a stealthy look but other than that?

Anyway just some thoughts.

This is no longer cutting edge. This is B-2 tech coupled with electrical motors on a platform that is slow and only survivable in a low-tech environment.

Turkey is attacking Syrian assets with similar, albeit less stealthy drones as of a month ago.

This is an incremental improvement and not at all "secret".