The speed is less important than energy delivered at site of impact. [...] Fan rotors move slower, but are more massive, and have more total energy behind them. You dont need something to be sharp or fast moving to cut you in half; it just needs to exert enough energy over a small area to cause mechanical shear of your body.
Okay, let's try to stay on topic.
Getting a loop of wire from a wench wrapped around a leg and slowly slooped up will chop it off just as surely as if the wire was moving fast but at less torque.
Unless the wire catches on a bone, and the wire is thin enough to snap there. But let's fucking stay on topic. We don't need ridiculous examples designed to distract people because they contain an injury to discuss the topic at hand, you sensationalist nutter.
In fact, there are multiple factors involved in determining how much force it's going to require to break the silly string, and in how much force is being delivered. One of the factors is the force applied, one of the factors is the time in which the force is applied and how that affects the material in question, one of the factors is the quantity of material the force is being spread out across. And in spite of being a lot smaller and less massive, the tip of a quadcopter blade is going to impart a lot more energy to a much smaller section of silly string than a metal fan blade is, because the fan blade is squared off (being stamped from a piece of metal) while the tip of a rotor is very sharp, and the rotor is moving much much faster and F=MA. There's a lot more A in the quadcopter system. But wait, there's more; you only have to consider the effect of its mass until the string breaks. If the string breaks at a level of force which can be achieved with the current acceleration at lower than the actual mass, then the additional mass of the rotor is irrelevant here.
There are lots of other factors, but these are the ones that we've been talking about in this discussion already.
To be a proper experiment, it needs to be a high speed metal bladed fan, with big heavy blades. I can probably find one if I look hard enough.
Only the speed is likely to be relevant, and it will still have a dramatically lower tip velocity than a quadcopter doing anything but hovering.