Helicopter rotor blade system detection approaching objects
You might want to take a look at wide angle photodetectors.
For example, the OP993 has a viewing angle of 118 degrees and is fairly cheap. 
If you look deeper on google and on online stores like mouse or digikey you might be able to find some with 180 degrees viewing angles.

This is just a guess, but I would avoid ultrasonic sensors since the vibration of the rotor/helicopter might be in that frequency range. Even if it is not, large non-ultrasonic vibrations could interfere with the sensor.