I received an email today from an aeronautical engineering instructor which reminded me that I promised a while back to post a very interesting helicopter video clip.
So here it is.
I found the clip on Fergworld, which also has a translation for the German narration you’ll hear on the video:
The rotortips absorb the turbulence with their flexibility. This recording was made, during flight, by a camera mounted to the leading edge of the rotorblade. At every turn you can see the tailrotor. If the flexible rotorblades wouldn’t absorb the the forces, they would be transmitted to drive mechanism and fuselage and the vibration would destroy the helicopter.
With all that bending, it’s no wonder the FAA has mandatory retirement times for the blades! If we conservatively assume that they flex five times per revolution and are turning at 2000 RPM, a 5000 hour lifespan would mean that they flex fifty million times.
Though I don’t have any hard evidence (yet) to back it up, I’d wager that if you were to attach a camera to a fixed-wing aircraft propeller or one of the fan blades in a turbine engine, you’d see some flexing in those parts as well.
Helicopters, however, are a particularly extreme case because the aircraft is moving through the air in the same plane as the rotor disk, so the advancing blade’s airspeed is very high while the retreating blade is moving through the air much more slowly. Helicopters have a ‘swash plate’ which constantly changes the angle at which each blade meet the oncoming air, and that movement is what causes the extreme flexing.
I’m not explaining it very well (a better description can be found here). But that’s okay, because I’m not a rotorcraft rated pilot.