An engine disk and blade (52A,52B) combination comprising: a metallic disk having: a plurality of first blade attachment slots; and a plurality of second blade attachment slots, circumferentially interspersed with the first attachment slots; a circumferential array of first blades, each first blade comprising: an airfoil; and an attachment root, the attachment root received in an associated respective said first attachment slot; and a circumferential array of second blades, each second blade comprising: an airfoil; and an attachment root, the attachment root received in an associated respective said second attachment slot, wherein: the first blades and second blades are non-metallic; the first blades are radially longer than the second blades; the first slots are radially deeper than the second slots; tips of the first blades are at like radial positions to tips of the second blades at a given axial position; characterized in that the first blades have a characteristic tip longitudinal span (S A); and the second blades have a characteristic tip longitudinal span (S B), less than the characteristic tip longitudinal span of the first blades. a metallic disk having: a plurality of first blade attachment slots; and a plurality of second blade attachment slots, circumferentially interspersed with the first attachment slots; a plurality of first blade attachment slots; and a plurality of second blade attachment slots, circumferentially interspersed with the first attachment slots; a circumferential array of first blades, each first blade comprising: an airfoil; and an attachment root, the attachment root received in an associated respective said first attachment slot; and an airfoil; and an attachment root, the attachment root received in an associated respective said first attachment slot; and a circumferential array of second blades, each second blade comprising: an airfoil; and an attachment root, the attachment root received in an associated respective said second attachment slot, wherein: the first blades and second blades are non-metallic; the first blades are radially longer than the second blades; the first slots are radially deeper than the second slots; tips of the first blades are at like radial positions to tips of the second blades at a given axial position; characterized in that the first blades have a characteristic tip longitudinal span (S A); and the second blades have a characteristic tip longitudinal span (S B), less than the characteristic tip longitudinal span of the first blades. an airfoil; and an attachment root, the attachment root received in an associated respective said second attachment slot, wherein: the first blades and second blades are non-metallic; the first blades are radially longer than the second blades; the first slots are radially deeper than the second slots; tips of the first blades are at like radial positions to tips of the second blades at a given axial position; characterized in that the first blades have a characteristic tip longitudinal span (S A); and the second blades have a characteristic tip longitudinal span (S B), less than the characteristic tip longitudinal span of the first blades. the first blades and second blades are non-metallic; the first blades are radially longer than the second blades; the first slots are radially deeper than the second slots; tips of the first blades are at like radial positions to tips of the second blades at a given axial position; characterized in that the first blades have a characteristic tip longitudinal span (S A); and the second blades have a characteristic tip longitudinal span (S B), less than the characteristic tip longitudinal span of the first blades.