Originally Posted by
paco
The underslung design of the head puts the rotor’s pivot point more into line with the blade’s normal C of G when in flight, and makes it more stable:
If it were otherwise, when the blade flapped up, the C of G would be more inboard:
In this way, most of the bending stresses in the lead-lag plane caused by coriolis effect and geometric imbalance when flapping are relieved (that’s from Bell - the exam answer is: preconing relieves bending loads in the plane of rotation). Preconing is the setting of a blade angle to a slight incline when the helicopter is level - on the Bell 206, it is 2½°. The centre of mass stays about the same distance from the mast after the rotor is tilted.
Phil
Thanks Phil. Removing lead-lag forces by underslinging hadn't occured to me.
Originally Posted by Slowrotor
The 206 has fixed undersling of course, but at low Rrpm with a heavy load the 2.5 degree undersling would be less than needed. It is my understanding that for this case the blade is made extra strong to absorb the additional lead/lag load.
Slowrotor, teetering for variable RPM is only possible if you consider Dave's CVJ hub - i'm sure he'll link it. This design would neatly remove both lead-lag forces and the "universal joint" joint forces at the hub.
Even for rigids the dynamics can become tricky, due to flexing modes coinciding with aerodynamic loads at certain RPMs. I've only scaped at the surface of blade flexural dynamics, to improve my understanding, but am realising more and more why S-69 ABC chose a 2 speed strategy (thanks Nick).
Mart