The rotor blade that spins above the helicopter body is about six feet across.
Varying the pitch angle of a rotor blade changes its angle of attack and hence its lift.
"The helicopter was still alight, part of the rotor blade was on the roof of nearby buildings".
Like a fixed wing, a rotor blade provides lift proportional to the amount of air flowing over it.
"We have a real worry that if a drone is flown in conflict with a helicopter rotor blade, the blade will shatter.
Each rotor blade is 75 metres long and when rotating covers an area the size of two and a half football pitches.
"He'll get excited," O'Connor says, "and his tail will tend" – she whirls her index finger like a rotor blade – "to do this".
In flight, the relative wind is a combination of the rotation of the rotor blade and the movement of the helicopter.
One of his first inventions was the "servo-flap," which could be added to the edges of a rotor blade to help stabilize a helicopter.
The same is true in a helicopter, where the angle of attack is the angle at which the relative wind meets the chord line of the rotor blade.
A rotor blade broke off and shot into waiting passengers, killing four of them, as well as a pedestrian on the street.
