Flying Aquila: Early lessons from the first full-scale test flight
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There's no landing gear, it takes off from a dolly, and it's only designed to stay up for 'months at a time.' Does this mean it needs extensive repairs after it crash lands on runways that are half its width? The ScanEagle UAS also has no landing gear but is small enough to be caught by a cable mid-air [1].
Having no journalists present for their test launch and only getting officially produced videos and non-critical reposts is not enough.
That's very little to go by, but this plane likely can fly very slowly. I guess they plan to glide it almost into ground, stall it to further decrease speed, and then 'crash' it in some fairly soft spot. Alternatively, they could land it on top of that dolly, but that might be more of a challenge.
(You know your landing gear is up when it takes full power to taxi.)
This is likely why landing footage was not included in the video.
Still, landing is the hardest part of flying, so omitting it raises some questions with me.
This is different from the recent NASA Prandtl-d aircraft which is of the Horten paradigm: lots of taper and no winglets.
It's possible the former is a lot easier to build, because of constant chord, especially as a solar powered platform.
With batteries you can distribute the weight very well so designing for things like wing root moment might not be the defining thing, instead some re complex aeroelastic design criteria.
Very fascinating to see this resurgence of flying wing again, popping up in many other places too.
Basically in a wing, there's overpressure on the bottom and underpressure at the top. This is all good. But it causes a problem at the wingtip, the air escapes and goes from bottom to top. The tip vortex. Does not contribute to lift, causes drag, so is bad.
So you can put a vertical winglet there to diminish the tip vortex, or to extract thrust from it. Or you can extend the wing but make it have a lower angle of attack near the tip, again extracting thrust, basically a horizontal winglet if you will.
Then there's the issue of stability. A plane is stable when, if you increase the angle of attack, the rearward lifting surfaces have more increase in negative nose down moment than the positive nose up moment of the forward lifting surfaces. So the aircraft tends to correct itself (move back to lower angle of attack).
In an ordinary plane it means a wing near the center of gravity and a tail with a lower angle of set incidence far behind.
In a canard, the center of gravity must be between the wings and the front wing must be set in a higher incidence.
In a flying wing it means sweepback and twist: the outer portion of the wing must act as a tail. The outer portion must be twisted nose down so it has a lower angle of incidence.
Flying wing design is harder since there are more functions it must do than in a more conventional plane.
That is a really really short version of it. It didn't mention positive moment airfoils etc.
But not too slow. You need to be able to stay on station when there is a wind. You would need some sort of powered airship. To be better than a wing it would have to have less drag.
I suppose it could be both. Maybe inflatable fixed wing, if it had the right dimensions.
Clouds often move in another direction than the wind near the ground. (is that some eddy current?)
Chances are it has to be faster at altitude, as it will need to push the same amount of air down there, while the air is less dense.
In the context of Challenge #3, they talk about Aquila's "25mph" speed comparing it to a commercial airliner at "200mph" - that's about takeoff speed for a 747, which then goes on to cruise at somewhere around 700mph, so if the Aquila has a similar operating flight envelope you'd expect a "cruise" speed of perhaps 80-90mph.
Also, it's entirely possible the Aquila is designed to have a wider efficient flight envelope that a 747, which a) was mostly designed using the best aerodynamics available in the 50s/60s, b) is limited by the sound barrier at the top of it's speed range (it'd hit Mach1 at ~750mph and things go strange aerodynamically there), and c) needs to be highly optimised for air-breathing jet engine fuel efficiency - which requires some tradeoffs electric power isn't constrained by (the thin cold air at 70,000feet doesn't change the power output of an electric motor the way it does a kerosine-burning jet).
Why not multiple balloons attached to the ground (to overcome wind drift), at not too high altitude, perhaps interconnected by line-of-sight communication.
[edit] I suppose though they fly high enough to be out of range of most inexpensive weapons systems.
I doubt people in a friendly country would shoot it down, although I could certainly see Iran having issues of someone flying one along the border providing "free" Internet to people inside their borders. No doubt they would be able to successfully jam it's electronics.
Perhaps more worrisome would be having these things fall out of the sky. Being a long as they are, structural failure by suddenly overloading the wing might turn them into the moral equivalent of the Maple Seed of Doom[1] on their way down. Probably reasonably low risk in rural areas but something to think about if they are taking off from airports in Kansas city for example to fly out over the great plains.
It's that latter bit that makes me even more curious. In the write ups so far, both the 'project loon' and the Aquila videos suggest this very expensive piece of equipment is going to be flying over sparsely populated areas to provide Internet. But with so few customers how do you cover the cost? Simple economics would suggest you'd want to fly it over a really densely populated city, then you would have a huge addressable market rather than over the corn fields of Iowa or the back roads of Oklahoma.
It suggests to me that its easier to spend a million dollars operating a couple of these over a small town than it is to get permission to pull a fiber along the power grid. The latter is more of a policy issue rather than a technology issue though.