Indeed, however to say anything about damage you need to know the area where the force is applied and more about the materials. If the bottom of the copter is padded the duration of the impact coud be much longer and spread over a larger area.
Also, F = m * delta V / delta t is only valid if force is constant in this context. I would expect acceleration to vary during the course of the impact so this relation would likely not hold [EDIT] in terms of determining damage I mean.
Yes, in this case the equation refers to the average force over an interval of time: if at time instant A we had a speed v1 and at a later instant B we have some speed v2 then a force with a magnitude of at least m * (v2 - v1) / (tB - tA) must have acted, therefore the analysis should hold.
As you point out the area is very important in the analysis, since Pressure = F / A. For large areas the force "spreads out" and the damage is less.
The fatality of the accident can vary due to many circumstances however I think we can safely assume that for the purposes of our initial analysis, 5 lbs traveling at 30mph and crashing into my head is not something I am looking forward to ;)
Also, F = m * delta V / delta t is only valid if force is constant in this context. I would expect acceleration to vary during the course of the impact so this relation would likely not hold [EDIT] in terms of determining damage I mean.