Would not expect it to be any worse than a bird strike, which I hear are common enough. Eventually the firmware of the drone would know to avoid airplanes at all cost and perhaps have some sort of equipment in there to limit them ever flying close to airplanes. The regulation should ideally force this, for drones to have sensors and firmware to avoid airplanes.
Bird strikes are bad enough. And lots of efforts made to lure birds away from airports.
Consider if you loaded the drone into a cannon and fired it at a stationary airplane, hitting at 250MPH. That's 366 feet per second. Like a low-velocity bullet. Into an aluminum airframe. A BB going that speed will puncture a pop can. A 5lb drone going that speed may puncture the skin of an airplane?
Can't help of thinking of the foam strike causing the Columbia space shuttle disaster
"Investigators into the Columbia accident have estimated that the dislodged foam was about 19 by 11.5 by 5.5 inches (48 by 29 by 14 cm), weighed about 26.7 ounces, or 1.7 lb (0.75 kg) and impacted the Shuttle at nearly 530 mph (850 km/h). For the sake of a rough comparison, this block of foam would be about the same size and weight as a large loaf of bread."
That report bothers me. How did the foam achieve such a relative velocity? It was part of the Shuttle; it dislodged; it hit the shuttle further along. Does the report mean "impacted the Shuttle while both were travelling at nearly 530"?
The shuttle was faster than mach 2 (and accelerating) at time of impact - I presume the foam had decelerated significantly between breaking off and hitting the wing. I suppose you could say the shuttle hit the foam rather than the other way round.
Picture a 5-pound drone hitting a stationary plane at 250 miles per hour and ask if the damage would be "negligible".