Method for improving a closed wing aircraft, the method comprising the steps of: a) designing an aircraft fuselage (1, 12) and designing three main wings on each side of the fuselage, wherein the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) is backward swept and extending from the low front portion of the fuselage; the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) is dihedral, located horizontally behind and vertically above the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and structurally connected to the upper middle portion of the fuselage as a shoulder wing, a high wing or a parasol wing; the 3rd wing (4, 13, 29, 33, 38, 45, 57, 71) is forward swept, located horizontally behind and vertically above the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and structurally interconnected to the fuselage in a way comprising at least one tail fin ( 5 or 14); b) designing each backward swept 1st wing (2, 18, 23, 31, 36, 49, 61, 65) with an upward-bent tip section comprising a wingtip; c) designing each 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) with an upward-bent tip portion comprising a wingtip; d) designing a structural interconnection between the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) on each side of the fuselage, a configuration comprising a bracing side-wing entity ( 30 or 26), connected to the wingtip of the 1st wing, and also connected to the wingtip portion of the 2nd wing, wherein one end of said bracing side-wing entity is connected to the wingtip portion of the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67), and its other end is connected to the wingtip of the 1st wing (2, 18, 23, 31, 36, 49, 61, 65), forming a bracing framework between the 1st wing and the 2nd wing, such that a closed frame, forming an aerodynamic channel, is obtained; e) designing a structural interconnection between the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and the 3rd wing (4, 13, 29, 33, 38, 45, 57, 71) on each side of the fuselage, a configuration comprising a wingtip-fence entity ( 7, 22 or 34), interconnecting the wingtip of the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and a wingtip or wingtip portion of the 3rd wing (4, 13, 29, 38, 45, 57, 71), forming another bracing framework between the 2nd wing and the 3rd wing, such that another closed frame, forming a corresponding aerodynamic channel, is obtained, wherein said wingtip-fence entity ( 7, 22 or 34) is different from the structural interconnection between the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67). a) designing an aircraft fuselage (1, 12) and designing three main wings on each side of the fuselage, wherein the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) is backward swept and extending from the low front portion of the fuselage; the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) is dihedral, located horizontally behind and vertically above the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and structurally connected to the upper middle portion of the fuselage as a shoulder wing, a high wing or a parasol wing; the 3rd wing (4, 13, 29, 33, 38, 45, 57, 71) is forward swept, located horizontally behind and vertically above the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and structurally interconnected to the fuselage in a way comprising at least one tail fin ( 5 or 14); b) designing each backward swept 1st wing (2, 18, 23, 31, 36, 49, 61, 65) with an upward-bent tip section comprising a wingtip; c) designing each 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) with an upward-bent tip portion comprising a wingtip; d) designing a structural interconnection between the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) on each side of the fuselage, a configuration comprising a bracing side-wing entity ( 30 or 26), connected to the wingtip of the 1st wing, and also connected to the wingtip portion of the 2nd wing, wherein one end of said bracing side-wing entity is connected to the wingtip portion of the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67), and its other end is connected to the wingtip of the 1st wing (2, 18, 23, 31, 36, 49, 61, 65), forming a bracing framework between the 1st wing and the 2nd wing, such that a closed frame, forming an aerodynamic channel, is obtained; e) designing a structural interconnection between the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and the 3rd wing (4, 13, 29, 33, 38, 45, 57, 71) on each side of the fuselage, a configuration comprising a wingtip-fence entity ( 7, 22 or 34), interconnecting the wingtip of the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67) and a wingtip or wingtip portion of the 3rd wing (4, 13, 29, 38, 45, 57, 71), forming another bracing framework between the 2nd wing and the 3rd wing, such that another closed frame, forming a corresponding aerodynamic channel, is obtained, wherein said wingtip-fence entity ( 7, 22 or 34) is different from the structural interconnection between the 1st wing (2, 18, 23, 31, 36, 49, 61, 65) and the 2nd wing (3, 16, 24, 32, 39, 47, 55, 67).