Note: this is interesting because the propellers will actually do the talking!
"Accordingly, the controller may determine and cause to implement modulations of the rotational speed of a propeller, thereby causing the propeller to produce a series of sounds that are audibly perceptible as “Watch out!”
"Accordingly, the controller may determine and cause to implement modulations of the rotational speed of a propeller, thereby causing the propeller to produce a series of sounds that are audibly perceptible as “Watch out!”