Maybe not all the way through a large bone, but... A friend's 3XXmm rotor span indoor electric heli with flexible non-weighted plastic blades fractured his index finger. The heli's all-up weight is 300-some grams.
Now try an 700-900 class outdoor heli (I have not looked into the type of machine involved in today's mishap -- could have been small as a 450.
From an RC forum:
In the "Battle Axe" thread we saw what a 900 can do to a tree , assuming the head rpm was over 1500 rpm putting the tipspeed somewhere around 400mph. 400mph seems to be a magic number regarding tip speed on most 3d style helis today
700mm-800mm blades will cut to the bone at 900 rpm just as easily as at 1800. There's more than one injury thread starting off with "while the rotors were spooling down" or "after it had lost most of its headspeed".
the problem isn't that the heads are being operated at some unsafe speed and the problem cannot be fixed by reducing to a "safe" RPM.
The reason these are dangerous is because a rotor with the properties that allow it to efficiently cut through air and rapidly provide large amounts of force to the main shaft without failing also allow it to transfer large amounts of energy and efficiently cut through living tissue.
really, the problem is we are dealing with high-energy machines because to get them to perform the way people want them to perform requires high amounts of energy.
a little math:
using the values on the EDGE website for their 800 mm rotor, we have a mass of 270 grams. center of mass position is not indicated, but i'm going to assume it's around 2/3 (if someone knows better, feel free to tell me) at 600mm. this means that at 900 rpm, the center of mass will have a linear velocity of roughly 30 m/s.
if we assume worst case scenario, blade hits limb and is stopped dead, we can assume roughly 1/10 second to stop, which give us a deceleration rate of 300 m/s/s. using F=ma we have 0.27kg*300 = 81 newtons of force.
if we assume frontal area of the blade as it makes contact at 1/10=in wide (and this is a generous estimate, i would not be surprised if the effective contact was much thinner) along, say, 3" wide piece of forearm, that gives us a total contact area of 0.3 inches, for an impact pressure of roughly 270 psi.
Now, i was not able to find a solid number, but most sources indicate that as little as 100 psi can break the skin, and the tissues beneath require less force once skin has been broken. so the initial impact give us almost triple the skin-penetration requirement, and then the second blade come swinging around and adds another 80 lbs of impact to the mix... and this is at the "low" headspeed of 900 rpm as in the above video, and using very conservative figures for the math.
basically, it boils down to what i mentioned above, if you want to toss a 2-3kg machine around at 3-4Gs of acceleration, you are going to necessarily involve a large amount of force to do it. the interface which transfers that force into the air will just as happily transfer it into a person if a person ends up in the way.
I certainly don’t doubt one could get killed by an RC helicopter. It’s the specific method in the headline I take umbrage with, lopping the head involves cutting through a lot of skull, not just the initial impact point. A lot of factors, mostly revolving around weight, make that highly unlikely. Cutting through soft tissue I believe, breaking bones even. But not top lopping.