Rotorcraft skid undercarriage and a rotorcraft including such an undercarriage

1. A rotorcraft ( 21 ) having a antero-posterior midplane ( 33 ) and a transverse axis ( 34 ) arranged perpendicular to each other, comprising:

a fuselage ( 22 ) and a skid undercarriage ( 20 ) having skids ( 24 , 25 ) interconnected by a first cross-member ( 26 ) and a second cross-member ( 27 );

first and second connection means for connecting the first cross-member to the fuselage; and

third connection means ( 28 ) for transmitting forces from the second cross-member to the fuselage and vice versa,

wherein said first connection means ( 30 , 31 ) comprise two first parts bearing against the first cross-member ( 26 ), said first connection means alone transmitting forces from the first cross-member ( 26 ) to the fuselage so long as the deformation of the first cross-member ( 26 ) remains less than a determined non-zero level of deformation in rolling or in vertical sinking, said two first parts being spaced apart along a length of the first cross-member on opposite sides of the antero-posterior midplane of the rotorcraft, and

wherein said second connection means ( 29 , 32 ) comprise two second parts that are spaced from the first cross-member ( 26 ) until said determined non-zero level of deformation of the cross-member is reached or exceeded, said two second parts bearing against the first cross-member only when the deformation of the first cross-member ( 26 ) reaches or exceeds said determined non-zero level of deformation, said two second parts being spaced apart along the length of the first cross-member on opposite sides of the antero-posterior midplane farther from the antero-posterior midplane than respective ones of said two first parts.

2. A rotorcraft according to claim 1 , wherein the second connection means present angular stiffness in roll that increases with increasing sinking (E) of the undercarriage as a result of contact with the ground ( 23 ).

3. A rotorcraft according to claim 1 , wherein each of said two first parts of the first connection means straddles the first cross-member.

4. A rotorcraft according to claim 3 , wherein each of said two first parts further comprises a resilient blade that cooperates therewith to surround the first cross-member.

5. A rotorcraft according to claim 1 , wherein each of said two second parts of the second connection means straddles the first cross-member.

6. A rotorcraft according to claim 1 , wherein said two first parts and said two second parts are disposed symmetrically relative to the antero-posterior midplane.

7. A rotorcraft according to claim 1 , wherein an angular stiffness in roll of the first connection means when the fuselage is resting on the first cross-member ( 26 ) of the undercarriage via the first connection means ( 30 , 31 ) is at least about 10% less than the angular stiffness in roll of the second connection means when the fuselage is resting on the first cross-member of the undercarriage via the second connection means ( 29 , 32 ).

8. A rotorcraft according to claim 1 , in which the first cross-member ( 26 ) presents a second moment of area that varies along its length.

9. A rotorcraft according to claim 8 , wherein the second moment of area of the first cross-member ( 26 ) is smaller in a central portion than in two lateral portions extending on either side of the central portion.

10. A rotorcraft according to claim 1 , in which the first cross-member is tubular.

11. A rotorcraft according to claim 1 , wherein a vertical stiffness along the vertical axis ( 33 ) of the first connection means when the fuselage is resting on the undercarriage via the first connection means ( 30 , 31 ) is at least about 10% less than a vertical stiffness of the second connection means when the fuselage is resting on the undercarriage via the second connection means ( 29 , 32 ).

12. A rotorcraft according to claim 1 , in which said second connection means extend respectively substantially in line with two longitudinal reinforcing structures ( 37 ) forming part of the bottom structure of the fuselage.

13. A rotorcraft according to claim 1 , wherein the undercarriage has a resonant frequency in rolling, ωx, that is less than the absolute value of the difference between nominal Ω and ωδ:

ωx<|Ω−ωδ|,

Ω is a frequency of rotation of the at least one rotor, and

ωδ is a frequency of oscillation in drag of the at least one blades of the at least one rotor.

14. A rotorcraft according to claim 1 , further comprising means for adjusting along the length of the first cross-member ( 26 ) the positions of the first connection means and the second connection means.

15. A rotorcraft according to claim 1 , wherein said third connection means comprises a single connection to the fuselage at a middle of the second cross-member.

16. A rotorcraft according to claim 1 , wherein the first cross-member is a front cross-member extending in front of the second cross-member, which is a rear cross-member.