Proprotor Systems for Tiltrotor Aircraft

1 . A proprotor system for tiltrotor aircraft having a helicopter mode and an airplane mode, the proprotor system comprising:

a hub; and

a plurality of proprotor blades coupled to the hub such that each proprotor blade is operable to independently flap relative to the hub about a flapping axis and independently change pitch about a pitch change axis; and

wherein, a first in-plane frequency of each proprotor blade is less than 1.0/rev.

2 . The proprotor system as recited in claim 1 wherein the first in-plane frequency of each proprotor blade is between about 0.6/rev and about 0.9/rev.

3 . The proprotor system as recited in claim 1 wherein, in the helicopter mode, the first in-plane frequency of each proprotor blade is between about 0.6/rev and about 0.7/rev.

4 . The proprotor system as recited in claim 1 wherein, in the airplane mode, the first in-plane frequency of each proprotor blade is between about 0.8/rev and about 0.9/rev.

5 . The proprotor system as recited in claim 1 wherein the plurality of proprotor blades further comprises at least four proprotor blades.

6 . The proprotor system as recited in claim 1 wherein the plurality of proprotor blades further comprises at least five proprotor blades.

7 . The proprotor system as recited in claim 1 further comprising a pitch control assembly having a positive delta 3 angle operably coupled to each proprotor blade.

8 . The proprotor system as recited in claim 1 further comprising a pitch control assembly having a positive delta 3 angle up to about 35 degrees coupled to each proprotor blade.

9 . The proprotor system as recited in claim 1 wherein, in the airplane mode, the proprotor system is operable at about 70 percent to about 80 percent of design RPM.

10 . The proprotor system as recited in claim 1 wherein, in the airplane mode, the proprotor system is operable at about 60 percent to about 70 percent of design RPM.

11 . The proprotor system as recited in claim 1 wherein, in the airplane mode, the proprotor system is operable at about 50 percent to about 60 percent of design RPM.

12 . The proprotor system as recited in claim 1 wherein, in the airplane mode, the proprotor system is operable at about 40 percent to about 50 percent of design RPM.

13 . The proprotor system as recited in claim 1 further comprising a lead-lag damper disposed between each proprotor blade and the hub.

14 . The proprotor system as recited in claim 13 wherein the lead-lag dampers further comprise elastomeric dampers.

15 . The proprotor system as recited in claim 13 wherein the lead-lag dampers further comprise fluid dampers.

16 . A tiltrotor aircraft having a helicopter mode and an airplane mode, the tiltrotor aircraft comprising:

a fuselage;

a wing extending from the fuselage;

at least one drive system supported by at least one of the fuselage and the wing; and

at least one proprotor system coupled to the drive system, the proprotor system including a hub and a plurality of proprotor blades coupled to the hub such that each proprotor blade is operable to independently flap relative to the hub about a flapping axis and independently change pitch about a pitch change axis;

wherein, a first in-plane frequency of each proprotor blade is less than 1.0/rev.

17 . The tiltrotor aircraft as recited in claim 16 wherein, in the helicopter mode, the first in-plane frequency of each proprotor blade is between about 0.6/rev and about 0.7/rev and wherein, in the airplane mode, the first in-plane frequency of each proprotor blade is between about 0.8/rev and about 0.9/rev.

18 . The tiltrotor aircraft as recited in claim 16 further comprising a pitch control assembly having a positive delta 3 angle up to about 35 degrees coupled to each proprotor blade and wherein the plurality of proprotor blades further comprises at least four proprotor blades.

19 . The tiltrotor aircraft as recited in claim 16 wherein, in the airplane mode, the proprotor system is operable at about 40 percent to about 80 percent of design RPM.

20 . The tiltrotor aircraft as recited in claim 16 further comprising an elastomeric lead-lag damper disposed between each proprotor blade and the hub.