As general guidelines, from the German study referenced at the end of this answer:
At a closing speed of 200kt, a drone of any size pierces the airframe of an aircraft.
At a closing speed of 300kt, a drone of 1.4kg creates severe damages to the aircraft structure.
Actual tests
Independently of the study mentioned above, actual tests were performed in 2018 at Dayton University (Ohio, USA) by a group in charge of routinely performing bird-strike testing on aircraft wings, windscreens and engines. Such tests are not done in flight but on the ground using air guns (nicknamed chicken guns).
A 1kg quadcopter (DJI Phantom 2) was projected onto the wing of a Mooney M20 aircraft at 380km/h, a speed taking into account the regular speed of both aircraft in a head-on collision:
The drone did not shatter on impact, but tore open the leading edge of the wing as it bore into the structure, damaging its main spar. “While the quadcopter broke apart, its energy and mass hung together to create significant damage to the wing,” said Kevin Poormon, group leader for impact physics at UDRI.
Snapshots from the related video.
As the engineers noted in the report, the drone was similar in size and weight to gel birds they used for other tests, but effects were more important, e.g. the drone was able to damage the main spar in the wing, gel birds aren't. This is not really a surprise since birds are soft bodies, but drones contain hard components like motors and magnets made of dense materials.
Note: Years ago, DJI sent a letter to the university claiming their drones cannot be involved in such a collision, for multiple reasons. Whether DJI are correct or wrong is not relevant here, nor whether the same can happen with birds.
This test shows what a collision with a 1kg drone produces on a wing, regardless of the brand or how this can happen. This is the point which answers your question. In spite collisions are indeed unlikely, they happen like shown in the video of the test and illustrative examples of actual holes in wings after collisions have been included in @WPNSGuy's answer.
Impact force is determined by kinetic energy (speed and mass)
It's almost exclusively a matter of kinetic energy calculated as $\frac 1 2 mV^2$, where speed $V$ is squared. Without surprise increasing $V$ quickly changes the result. For comparison with a 50g pebble hitting the windshield of a car at 100km/h:
- 1kg is 20 times 50g. Energy is multiplied by 20.
- 130kt is 2.4 times 100km/h. Energy is multiplied by 5.76.
Hence a drone hitting a landing aircraft combines both multipliers and can transfer an energy 115 times larger than the pebble. Said otherwise, the effect is similar to 115 pebbles hitting the windshield simultaneously. This leaves little chance of not breaking it. Two main factors could limit the effects:
This energy might be absorbed by the drone during the impact, if the drone is broken and spread upon impact. But this is not the case, the drone is dislocated, but the components remain compact and act as as a whole.
The impact area being larger, the impact force per unit area is reduced, hence materials can counter a larger total kinetic energy.
Still the figures speak for themselves: It requires only a few percents of the total kinetic energy to break a windshield, regardless it is a car or an aircraft.
Heavy objects are hardly deflected by airflow
The third potential factor, the aerodynamic deflection you mention, is not very significant, even for the wing which airfoil is specifically designed to create a laminar layer around the airframe (the boundary layer).
Airflow deflection works because air is very light and has little inertia, so viscous forces can direct air away from the surface. On the contrary, a drone is dense and heavy, with a lot of inertia. Inertial forces dominate, the drone maintain a straight trajectory and hits the aircraft after little deflection.
This is exactly like the pebble meeting the aerodynamic profile of a modern/fuel efficient car: Small animals like mosquitoes are deflected, stones are not.
The theory behind this behavior is linked to the Reynolds number.
More factual details
A study by German researchers is available online:
