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 including at least four blade arms;

first and second bearing assemblies coupled to each blade arm, each first bearing assembly coupled to an inboard location of the respective blade arm and each second bearing assembly coupled to an outboard location of the respective blade arm;

at least four proprotor blades each coupled to one of the blade arms of the hub such that each proprotor blade is operable to independently flap relative to the hub and independently change pitch, each proprotor blade including a spar and a skin, each spar including an integral pitch horn and an integral cuff, each spar coupled to the respective first bearing assembly and the respective second bearing assembly forming a centrifugal force retention load path for the proprotor blade through the spar via the respective first bearing assembly to the hub; and

a pitch control assembly including at least four of pitch links, each pitch link coupled to one of the integral pitch horns such that the pitch control assembly is configured to control the pitch of each proprotor blade;

wherein, the pitch control assembly has a positive delta 3 angle up to 30 degrees; and

wherein, the spar and the skin of each proprotor blade are formed from a carbon-based material enabling the proprotor blades to have a stiffness to mass ratio such that, in the airplane mode, a first in-plane frequency of each proprotor blade is greater than 2.0/rev and less than 3.0/rev.

2. The proprotor system as recited in claim 1 further comprising a flapping joint disposed between each proprotor blade and the hub.

3. The proprotor system as recited in claim 1 wherein the first in-plane frequency of each proprotor blade is between 2.2/rev and 2.8/rev.

4. The proprotor system as recited in claim 1 wherein the first in-plane frequency of each proprotor blade is between 2.4/rev and 2.6/rev.

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

a mast;

a gimbaled hub coupled to and rotatable with the mast, the hub including at least four blade arms;

first and second bearing assemblies coupled to each blade arm, each first bearing assembly coupled to an inboard location of the respective blade arm and each second bearing assembly coupled to an outboard location of the respective blade arm;

at least four proprotor blades each coupled to one of the blade arms of the hub such that each proprotor blade is operable to independently flap relative to the hub and independently change pitch, each proprotor blade including a spar and a skin, each spar including an integral pitch horn and an integral cuff, each spar coupled to the respective first bearing assembly and the respective second bearing assembly forming a centrifugal force retention load path for the proprotor blade through the spar via the respective first bearing assembly to the hub; and

a pitch control assembly including at least four of pitch links, each pitch link coupled to one of the integral pitch horns such that the pitch control assembly is configured to control the pitch of each proprotor blade;

wherein, the pitch control assembly has a positive delta 3 angle up to 30 degrees; and

wherein, the spar and the skin of each proprotor blade are formed from a carbon-based material enabling the proprotor blades to have a stiffness to mass ratio such that, in the airplane mode, a first in-plane frequency of each proprotor blade is greater than 2.0/rev and less than 3.0/rev.

6. The proprotor system as recited in claim 5 wherein the first in-plane frequency of each proprotor blade is between 2.2/rev and 2.8/rev.

7. The proprotor system as recited in claim 5 wherein the first in-plane frequency of each proprotor blade is between 2.4/rev and 2.6/rev.