I recently saw that the dust on the airplane's wings are frequently cleaned by some human workers (generally after every trip, workers clean the fine dust on wing's surface by hands). But I have a doubt. I thought that when the airplane is travelling at a very high speed in air, the dust will cleaned off due to the heavy force of air stream. But it doesn't go like that. But when someone cleans the surface gently, the dust will be removed with out any effort. WHY?
I thought that when we clean the surface of the wing, we move our hands tangentially to the surface of the wing creating a torsional stress. The threshold torsion strain is very weak to hold bonds of the dust particle with the surface of the wing. But the same thing happens with the air, it hits the surface almost tangentially. So my intuition says dust should be removed off.
This can be seen in our regular fans also. See the below figure.
Can someone explain this?