They look like they are flying wrong.
If you want to impress your friends, it’s called anhedral or negative dihedral. Basically, anhedral increases the plane’s maneuverability at the expense of stability. Drones and military fighter craft with anhedral compensate for this instability by using a computer to correct the plane’s attitude. Without computer control, such planes would be difficult if not impossible to fly.
Modern aircraft with a zero or negative dihedral angle and no natural vertical stabilization surfaces (think the F117 Nighthawk) require automatic stabilization controls systems and fly-by-wire controls; there’s just no way a linear control system can compensate for the innate instability due to unavoidable negative feedback loops (i.e. going a little tipsy causes you to go even more tipsy, until you’re rolling or yawing or pitching all over the place).
I believe, but will stand corrected by someone who knows more about these craft than I do, the reason for this degree of instability isn’t so much flight maneuverability–although you’d obviously like them to avoid ground fire, their stealthy characteristics and low cost make that an issue of secondary concern, and their slow speed makes evasive maneuvers largely pointless–but rather their ability to take off and land on very short runways; the MQ-1 Predator UAV can essentially come from top speed and into a stall, dropping to a landing in the spand of a few dozen feet, which is useful when operating from remote field sites with little infrastructure.
Stranger
I don’t know of any UAV with a pronounced negative dihedral. Perhaps the OP is talking about the inverted-V tail on the Predator?
I agree with this observation. Doing a quick survey of UAVs airframes at FAS.org would indicate that most/all have virtually zero dihedral - as one might expect. Aerodynamically you’d like to maximize lift/drag at low speed (where UAVs operate), and you can tailor flight dynamics through your choice of control law and gain parameters. Some of the aircraft have “non-conventional” appearances, but that is a consequence of either 1) being much smaller than a conventional airplane (e.g. Exdrone) or being built for low-observables (e.g. Darkstar and Predator). BTW, it appears that the tail surfaces of Predator serve multiple roles - directional stability, pitch control, and protection for the pusher-prop at high AOA during landing - that’s why they have that curious configuration of extreme negative-dihedral.
I don’t believe that low stall speed is achieved by anhedral, or that one would want low stability margins during the landing phase. UAVs, by their very nature, tend to have benign handling characteristics and not only redundant flight-control algorithms but also fall-back modes that have a constrained maneuverability envelope. Looking at the UAVs that are shown explicitly with STOL capability, they look very conventional (rocket-launch assist and large wing area) - e.g. Pioneer and Hunter. They look virtually like scaled-down OV-10s.
Philosophically - UAVs are generally low-maneuverable airframes - they are designed to go up, observe something (establish a forward presence, as it were), and come back / not crash. If one looks at much of the development going on in UAV research, it’s all about sensor systems and increased autonomy, unless you get to the extreme small-end of the size scale, and then it’s how to make something operate efficiently with decent payload - that’s where the novel airframes tend to come in (flying wings, mini-helicopters, even flapping-wing bird/insect mimics). Air-to-air missiles may be highly maneuverable (think Sidewinder), but then they look nothing like a airplane and just like a long tube with flight control surfaces (which is all they are - sensors, payload, propulsive plant, and flight control). Cruise missiles are low-stress/low-maneuverable (possibly low-observable) and may have just very abbreviated wings and flight control.
Maybe the OP could link to the particular examples of anhedral that he/she was considering. Thanks.
“Why do wings on our drone spy planes bend down instead of up?”
Specifically regarding low-observable UAV X-planes - it doesn’t look like they have significant anhedral: Stealth UAVs.