Method and device for detecting an overstepping of design loads of the fin of an aircraft

1. A method for detecting an overstepping of design loads of a fin of an aircraft, said method comprising:

A/ in a preliminary step, determining a first safety envelope representing a chart formed in a plane on the basis of pairs of values of bending moment and of twisting moment of the fin and defined so that, for any pair of such values which lies outside said first safety envelope and which is applied to the aircraft, there exists a risk of appearance of permanent deformations on the fin;

B/ in the course of a flight of the aircraft, carrying out the following steps repetitively and automatically:

a) estimating the current bending moment;

b) estimating the current twisting moment;

c) carrying out a first comparison by comparing the pair formed by said current bending moment and said current twisting moment with said first safety envelope; and

d) determining whether a structural inspection of said fin has to be performed, at least on the basis of the result of said first comparison.

2. The method as claimed in claim 1 ,

wherein in step d), an alarm is emitted in the flight control deck of the aircraft, if a structural inspection of said fin has to be performed.

3. The method as claimed in claim 1 ,

wherein in preliminary step A/, said first safety envelope is determined with the aid of load models of the aircraft.

4. The method as claimed in claim 1 ,

wherein in step B/a), the current bending moment Mx is determined with the aid of the following expression:

Mx =( A 1+ B 1.α).β+( C 1+ D 1.α).δ rsup +( E 1+ F 1.α).

δrinf+G1.p+H1.r+I1.ny+J1.dp/dt+K1.dr/dt+L1.δsp+M1.δail

in which:

A 1 , B 1 , C 1 , D 1 , E 1 , F 1 , G 1 , H 1 , I 1 , J 1 , K 1 , L 1 and M 1 are coefficients;

α is the angle of incidence of the aircraft;

β is the angle of local sideslip of the aircraft, at the level of the fin;

δrsup and δrinf are the angles of swing of upper and lower rudders of the aircraft;

p represents the roll rate;

r represents the yaw rate;

ny represents the lateral load factor;

dp/dt represents the roll acceleration corresponding to the derivative with respect to time t of the roll rate p ;

dr/dt represents the yaw acceleration corresponding to the derivative with respect to time t of the yaw rate r ;

δsp is the angle of deployment of spoilers of the aircraft; and

δail is the angle of deployment of ailerons of the aircraft.

5. The method as claimed in claim 4 ,

wherein said coefficients A 1 to M 1 depend on the dynamic pressure, on the mass distribution, on the Mach number of the aircraft and on lateral aerodynamic characteristics of the aircraft.

6. The method as claimed in claim 4 ,

wherein said coefficients G 1 and M 1 are zero.

7. The method as claimed in claim 1 ,

wherein in step B/b), the current twisting moment Mz is determined with the aid of the following expression:

Mz =( A 2+ B 2.α).β+( C 2+ D 2.α).δ rsup +( E 2+ F 2.α).

δrinf+G2.p+H2.r+I2.ny+J2.dp/dt+K2.dr/dt+L2.δsp+M2.δail

in which:

A 2 , B 2 , C 2 , D 2 , E 2 , F 2 , G 2 , H 2 , 12 , J 2 , K 2 , L 2 and M 2 are coefficients;

α is the angle of incidence of the aircraft;

β is the angle of local sideslip of the aircraft, at the level of the fin;

δrsup and δrinf are the angles of swing of upper and lower rudders of the aircraft;

p represents the roll rate;

r represents the yaw rate;

ny represents the lateral load factor;

dp/dt represents the roll acceleration corresponding to the derivative with respect to time t of the roll rate p ;

dr/dt represents the yaw acceleration corresponding to the derivative with respect to time t of the yaw rate r ;

δsp is the angle of deployment of spoilers of the aircraft; and

δail is the angle of deployment of ailerons of the aircraft.

8. The method as claimed in claim 7 ,

wherein said coefficients A 2 to M 2 depend on the dynamic pressure, on the mass distribution, on the Mach number of the aircraft and on lateral aerodynamic characteristics of the aircraft.

9. The method as claimed in claim 7 ,

wherein said coefficients G 2 and M 2 are zero.

10. The method as claimed in claim 1 ,

wherein:

in preliminary step A/, a second safety envelope is moreover determined representing a chart formed in a plane on the basis of pairs of values of twisting moment and of shearing stress of the fin and defined so that, for any pair of such values which lies outside said second safety envelope and which is applied to the aircraft, there exists a risk of appearance of permanent deformations on the fin; and

in the course of the flight of the aircraft, the following steps are carried out automatically:

a current shearing stress is estimated;

a second comparison is carried out by comparing the pair formed by said current twisting moment and said current shearing stress with said second safety envelope; and

in step d), one determines whether a structural inspection of said fin has to be performed, likewise on the basis of the result of said second comparison.

11. The method as claimed in claim 10 ,

wherein said current shearing stress Ty is determined with the aid of the following expression:

Ty =( A 3+ B 3.α).β+( C 3+ D 3.α).δ rsup +( E 3+ F 3.α).

δrinf+G3.p+H3.r+I3.ny+J3.dp/dt+K3.dr/dt+L3.δsp+M3.δail

in which:

A 3 , B 3 , C 3 , D 3 , E 3 , F 3 , G 3 , H 3 , 13 , J 3 , K 3 , L 3 and M 3 are coefficients;

α is the angle of incidence of the aircraft;

β is the angle of local sideslip of the aircraft, at the level of the fin;

δrsup and δrinf are the angles of swing of upper and lower rudders of the aircraft;

p represents the roll rate;

r represents the yaw rate;

ny represents the lateral load factor;

dp/dt represents the roll acceleration corresponding to the derivative with respect to time t of the roll rate p ;

dr/dt represents the yaw acceleration corresponding to the derivative with respect to time t of the yaw rate r ;

δsp is the angle of deployment of spoilers of the aircraft; and

δail is the angle of deployment of ailerons of the aircraft.

12. A device for detecting an overstepping of design loads of a fin of an aircraft, which device comprises:

a set of information sources;

first means for determining, with the aid of information emanating from said set of information sources, a current bending moment;

second means for determining, with the aid of information emanating from said set of information sources, a current twisting moment;

third means for carrying out a first comparison, by comparing the pair formed by said current bending moment and said current twisting moment with a first safety envelope representing a chart formed in a plane on the basis of pairs of values of bending moment and of twisting moment of the fin and defined so that, for any pair of such values which lies outside said first safety envelope and which is applied to the aircraft, there exists a risk of appearance of permanent deformations on the fin; and

fourth means for determining whether a structural inspection of said fin has to be performed, at least on the basis of the result of said first comparison.

13. The device as claimed in claim 12 , wherein: said device furthermore comprises: fifth means for determining, with the aid of information emanating from said set of information sources, a current shearing stress; and sixth means for carrying out a second comparison, by comparing the pair formed by said current twisting moment and said current shearing stress with a second safety envelope; and said fourth means ( 10 ) determine whether a structural inspection of said fin has to be performed, likewise on the basis of the result of said second comparison.

14. The device as claimed in claim 12 , which furthermore comprises seventh means intended to indicate to an operator, as appropriate, that an inspection of the fin has to be performed.