[question] Two bladed rotor system
 

Hello

in a hangar talk discussion, this come an argument:

"...in a two bladed rotor system, the blades can change from the 180º position (lead lag)... to equalize the forces in each blade during forward flight ... this is accomplished by the hub itself ... "

using, as example, the rotor head of the R22, and what is written in the POH - "main rotor articulation : free to teeter and cone, rigid inplane",

i always have the idea, that 180º is always maintained and the forces are resisted by the hub itself, that "fights" to maintain it (resistance of the materials)

comments?

thanks
regards

CS-Hover,

I would have a look at the various types of rotorhead that exist:

Aircraft in Detail - Helicopter Rotorhead Gallery Index

You could also lookup "effective hinge offset" in Prouty.

2 bladed rotors "teeter" about a bush on the hub to keep the machine weight (and cost) down. The blades actually cone slightly above the plane so that centrifugal forces carry the weight of the heli.

CS-Hover, no, it is the fact that the two-blade hub is underslung that causes the blades to not have to lead and lag.

The lead and lag forces are absorbed by the hub, but as these forces change due to rotation, they are taken up by the mast.
The mast actually has a slight twisting and untwisting as the blades go around.
If that helps confuse the situation.

Shawn has given the answer that you are looking for. Undersling reduces these unbalanced in-plane forces.

The following sketch is of a Kaman intermeshing twin-rotor. It requires another means of handling the lead-lag, because the two rotors are mechanically interlocked. You can see that Kaman uses lag hinges and a friction damper.


http://www.unicopter.com/0433_4.jpg


Dave

the underslinging of the yoke, holding the blades on to the gymbal with pillow blocks, allows the yoke to rock under the head and this small action absorbs all the hunting stresses bought about by acceleration and deceleration of the blades. very simple and very effective.