From time to time, drones fall from the sky—we've already seen a few people injured thanks to some combination of gravity, pilot error, recklessness, and equipment malfunction. But could a falling drone actually crush your skull like a rotten grapefruit on a hot summer day? We did the math.
Inspired by drone enthusiast David Kovar, who recently lamented the lack of scientific data available on drone-human collisions, I decided to take a deeper look. In a quest to spark conversation on that topic, Kovar presented a hypothetical model of what kind of impact force a DJI Phantom, which is the world's most popular consumer drone, might have if it free fell 200 feet directly onto your skull.
Kovar presents a "worst-case" scenario: a 1.2 kg DJI Phantom falling from a height of 61 meters. Using a handy worksheet from Georgia State University's Physics Department, we're presented with this calculation:
7,173 Newtons of force! That would definitely be enough to crush your skull and would almost certainly kill you.Kovar cites an experiment by University of Illinois neurologists that shows that just 2,300 N is enough force to crush even a helmeted skull. Except, the calculations are a bit too simplistic, as Kovar himself notes. Normally, when we think in theoretical terms about force of impact exerted by an object, we neglect air resistance. This example is no different. The problem is, in the case of a drone, we can't ignore it. A Phantom has a pretty large cross-sectional area, meaning that, as it falls, aerodynamic drag will absolutely become a factor.
If we're considering its potential impact force, there's no way we get to the 7,173 N of impact calculated by Kovar, unless it falls from much higher.
Secondly, see that .1 meter measurement I've circled? That's the distance traveled after impact. When an object's movement is stopped by its collision into another object, all that kinetic energy doesn't just go away.
Instead, it gets transferred. When a car hits a barrier going really fast, it either penetrates the barrier, rebounds and crumples in on itself, or a bit of both. The same theory would apply here. This calculation assumes that our falling drone hits a body and comes to a complete and immediate stop, transferring all that energy to your skull if you're standing up.
A body is elastic though, so realistically our potential bystander's neck and spine would flex and compress (which could present its own fatal problems) and, depending on the angle of contact, the victim would move with the direction of the force by falling over.
That .1m of movement after impact simply isn't very realistic. In fact, even if we kept every other variable the same and continued ignoring air resistance, just changing the penetration after impact to .5m reduces the impact force to 1,435 N, much less than the 2,300 N that it would take to crush your skull. In practice, your body is less a solid, immovable stopping point as it is a soft, pliable cushion.
A falling UAV could seriously hurt or kill you in plenty of other ways, but crushing your skull instantly is probably a long shot. You have more to fear from lacerations from its rotors than the actual impact itself. By design, their shape and aerodynamic profile don't fall particularly fast, so unless it's falling from something like the Phantom's maximum height of 6,000 meters directly onto your head, your precious skull should remain intact. As long as popular drones don't get larger, that is.