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Aviation Accident Investigator

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Real-Life Math -- Solution

The following method was used to calculate the speed of the aircraft at the time the blades struck the ground.

Nominal rotational speed of the Main Rotor Head (MRH) = 330 revolutions per minute (RPM).

Therefore, the number of times a blade passes a point on the ground =

330 (RPM) x 4 (4 blades on the main rotor) = 1320 (per minute)

Time between strikes = 60/1320 = 0.045 seconds

Distance between the first three strikes = 1 meter

Distance/Time = Speed

1 meter/0.045 = 22.22 meters per second

22.22 x 60 (seconds in one minute) x 60 (minutes in one hour) / 1000 = 79.99 km/h

To convert to knots:

79.99 x 0.54

= 43 knots

The speed of the aircraft at the time the blades struck the ground was 43 knots.

"Math plays an extremely important part in my work," says George Carney. He is part of a two-person team that investigates all British army air accidents throughout the world.


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