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Take, for instance, the following UAV:

enter image description here

and suppose you have a payload with a very big volume that fits exactly in the fuselage (and there is no more space), but is extremely light. Thus, the heaviest part of the UAV becomes the propeller and its engine.

  • How do engineers make sure the UAV stays balanced and doesn't land right on its propeller?

  • Do they add some weights somewhere near the rudder and elevator to keep its center of gravity somewhere near the wing's trailing edge?

Thanks.

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    $\begingroup$ You should first and foremost be worried about aerodynamic stability of your plane (stability while in flight). There is static stability, where the location of the center of gravity is important, and then there is also dynamic stability. If you have a stable airplane (this can be by means of an autopilot) then you can go ahead and worry about how it comes down. For landing it should have a landing gear of some sort, or a real parachute (not a drag chute), or you can try and catch it in flight with a net near the ground. $\endgroup$ – user7241 Dec 16 '17 at 12:12
  • $\begingroup$ @jjack as you can see the UAV I posted above doesn't land using landing gear or anything like that: youtu.be/1hAWmmYUODU?t=39 . So I guess there must be another way to make it land safety? This video is not so clear, but actually it somehow glides down, lands on its belly and slides a bit on the ground. $\endgroup$ – LandonZeKepitelOfGreytBritn Dec 16 '17 at 12:16
  • $\begingroup$ I wouldn't call that gliding. Looks more like falling abruptly from the sky :-) But since it's just a computer graphics movie and not real film you can't tell how it lands. From what they say in the movie about where it comes down relative to the gps location, they could be using a parachute. I don't really now what the two black parts around the EO-sensor are, but since the sensor extends beyond them and can't be retracted it would be foolish to use them as landing skids. $\endgroup$ – user7241 Dec 16 '17 at 12:26
  • $\begingroup$ In the whole video, at around two seconds, the uav flies past something looking like antennae. It could also be a net held up by poles which serves to catch the returning uav mid-flight. $\endgroup$ – user7241 Dec 16 '17 at 12:31
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    $\begingroup$ @LandonZeKepitelOfGreytBritn: I think one point you're still not understanding is that landing is not the problem. Flying is the problem. If the plane is too nose heavy it will not fly and would simply crash on the propeller on take-off. I would never call crashing on take-off "landing". The only way to make the plane fly is to rebalance it by adjusting weight distribution. Because of this we don't need to consider landing conditions since it's impossible to fly with the nose being too heavy. $\endgroup$ – slebetman Dec 16 '17 at 17:54
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Due to center of gravity limitations, it would be impossible for any type of aircraft to be loaded so that the front became the heaviest part. The aircraft would not be able to fly.

These UAVs are meant to crash land and DO land on the front propellor. You can do a Google search and see multiple videos showing the “break apart” landings they are designed to do.

Here are some landing videos:

Puma landing 2

Puma landing 1

Coast Guard Conducts First Unmanned Aircraft System Deck Landings

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  • $\begingroup$ Can you maybe link to such a video? $\endgroup$ – user7241 Dec 16 '17 at 14:05
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    $\begingroup$ This seems to be a flight maneuver called "perching". It's a high angle of attack maneuver. It is also seen in the behavior of birds. The wing coming off on impact with the ground indicates that this might be an additional design feature to absorb the impact energy (which is not high since it's a light little plane). $\endgroup$ – user7241 Dec 16 '17 at 16:19
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    $\begingroup$ Here's a link to an academic publication about the maneuver google.de/url?sa=t&source=web&rct=j&url=http://… $\endgroup$ – user7241 Dec 16 '17 at 16:27
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    $\begingroup$ The "Puma landing 2" link goes to the same landing as "Puma landing 1". $\endgroup$ – user2357112 supports Monica Dec 16 '17 at 16:57
  • $\begingroup$ Link fixed. Thanks $\endgroup$ – Mike Sowsun Dec 16 '17 at 20:44
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For the Facebook Aquila, they recently added a mechanism to align the propellers horizontally so they don't totally wreck. From their second test flight report:

Aquila's second test flight took into account the lessons we learned from our first flight. In advance of the second flight, we incorporated a number of modifications to Aquila, including:

  • Adding “spoilers” to the wings, which help to increase drag and reduce lift during the landing approach

…

  • Installing a horizontal propeller stopping mechanism to support a successful landing

This, unfortunately, didn't work all that well.

Aquila Landing #2

A few seconds before landing, the autopilot stopped the propellers as planned in order to lock them horizontally. The propellers are meant to lock in the horizontal position to avoid damaging them when touching down. In this flight, the motors all stopped, but only one propeller locked horizontally.

Still, the article says that the aircraft was in great shape aside from some minor damage that occurred during landing.

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    $\begingroup$ Wow, I wonder how they implemented this horizontal locking. Did Facebook make their own motor controllers which allow powering certain coils within the motor in order to put it into a certain position? $\endgroup$ – Michael Dec 16 '17 at 19:00
  • $\begingroup$ @Michael Not sure, but in the original Aquila post (code.facebook.com/posts/268598690180189/…), they talk about the need to, "develop more efficient onboard power and communication systems" as a future challenge. I don't know if that means they have to go from-scratch or if they've done that, or what. $\endgroup$ – Brad Dec 16 '17 at 19:08
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    $\begingroup$ I don't see how this answers the question about the nose being to heavy compared to the tail. The question is: "Do they add some weights somewhere near the rudder and elevator to keep its center of gravity somewhere near the wing's trailing edge?", not whether they lock the propeller. As already commented, the problem is more how to fly than how to land. $\endgroup$ – mins Dec 16 '17 at 19:32
  • $\begingroup$ @mins It doesn't directly, I was just adding another example answering the broader question of not-wrecking-on-landing. $\endgroup$ – Brad Dec 16 '17 at 19:35

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