I'm a helicopter pilot with limited physics knowledge (units in BSc and HNCs).
I have recently challenged an assertion that rotating blades are stiffened by centrifugal force. In My mind, stiffness refers to the resistance of a member to bending deformation, K. From the comments, perhaps this is my problem?
My counter argument is quite simple. A force can only affect the stiffness of a blade if it changes the physical characteristics of the blade and such force(s) can only be exerted as a result of centripetal acceleration and aerodynamic effects as the blade flies.
A more accurate statement might be that the "blade resists the bending moments since counter-moments are exerted on them arising from the centripetal and aerodynamic forces".
I am very happy to be wrong (since I then learn) but I am catching a lot of heat for this challenge and no-one on Aviation.SE has been able to explain why I am wrong.
I do understand that there is a certain amount of pendantry in my claim but precision, particular in answers on the stacks, is part of my motivation.
What am I missing?
Best Answer
For simplicity, let's model the helicopter blade as a simple massless beam with a point mass at the end. When there is no gravity, the beam will be straight.
We now introduce a force to the beam tip, which will cause the beam to deflect. The bending stiffness $k$ is equal to the ratio of the force to the deflection:
$k=\frac{F}{d}$
When we now put this beam in a rotating reference frame, like the blade of a spinning helicopter rotor, we have to introduce a centrifugal force on the mass to account for the constant acceleration of the beam tip. When the beam is deflected upward, the centrifugal force will cause a downward bending moment and hence the beam will deflect less than in the scenario without the rotation.
Since the bending stiffness is the ratio of vertical force to the vertical deflection $K=\frac{F}{d}$, the (apparent) bending stiffness is higher in a rotating blade.