11
$\begingroup$

The fan blades in a fanjet are mounted loosely in a fir-tree shaped mount so they are free to lead or lag and balance themselves. The blades on fully articulated and semi-rigid helicopter rotor heads are mounted with a drag hinge that allows them to lead/lag as needed. Do any propeller aircraft have a method to allow the blades to lead/lag, or is this for some reason unnecessary?

$\endgroup$
2
  • $\begingroup$ Are you talking about GA aircraft or something bigger like a turboprop? $\endgroup$
    – Ron Beyer
    May 4, 2016 at 2:03
  • $\begingroup$ @RonBeyer Either. Just wondering if this principle is ever used for propellers at all $\endgroup$
    – TomMcW
    May 4, 2016 at 2:28

2 Answers 2

10
$\begingroup$

The two things are completely different and exist for different reasons:

  • Fan and turbine blades are mounted loosely, so they can self-balance. This is because at the very high RPM fans and turbines spin at they are extremely sensitive to imbalance. On the other hand, the self-balancing only works at high RPM.
  • Rotors have lead-lag hinges because the rotor spins parallel to the flow so in forward flight the advancing blade meets the air at significantly higher speed than the retreating blade.

Propellers spin slower, so they are not as sensitive to imbalance, and they don't have the shroud, so they can't be mounted loosely anyway. They spin perpendicular to the airflow, so all blades meet the air at similar speed and angle of attack, so they don't need to deal with as big differences in the aerodynamic forces. And they are smaller and more rigid.

$\endgroup$
2
  • $\begingroup$ Re "It also requires the blades to be connected at the tips with the “shroud"" -- what does this mean? The blade tips are physically scraping on the shroud? $\endgroup$ Sep 29, 2021 at 19:18
  • $\begingroup$ @quietflyer no; some have connecting links on the outer edge. But I think it was actually a braino—removing. $\endgroup$
    – Jan Hudec
    Sep 29, 2021 at 19:35
0
$\begingroup$

I don't think props do (usually) have lead-lag hinges. Those lead-lag hinges are only necessary as a complement of the flapping articulations in fully articulated rotors, as in many helicopters, and their role is to accommodate the periodic variations in angular velocity of the blades, since the tip-path blade orbit is always a circumference, but when that tip-path plane is at an angle w.r.t. the plane perpendicular to the engine shaft, its projection is an ellipse, and the resulting periodic variation in blade angular speed has to be 'accommodated' with the led-lag hinges.

When a rotor (or a propeller) has no flapping hinges, there's no need of lead-lag articulations...

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.