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An example seal shaft includes a cylindrical body establishing a bore that extends axially between a first end portion of the body and a second end portion of the body. Apertures are also established in the body. The apertures extend radially between an outer surface of the body and the bore. The se

An example seal shaft includes a cylindrical body establishing a bore that extends axially between a first end portion of the body and a second end portion of the body. Apertures are also established in the body. The apertures extend radially between an outer surface of the body and the bore. The seal shaft further includes a first group knife edge seals and a second group of knife edge seals. The first group of knife edge seals have a diameter, and a ratio of the diameter to the total cross-sectional area of the apertures is between 58.79 and 78.77.

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1. A seal shaft comprising: a cylindrical body establishing a bore extending axially between a first end portion of the body and a second end portion of the body;a plurality of apertures established in the body, and extending radially between an outer surface of the body and the bore;a first group o

1. A seal shaft comprising: a cylindrical body establishing a bore extending axially between a first end portion of the body and a second end portion of the body;a plurality of apertures established in the body, and extending radially between an outer surface of the body and the bore;a first group of knife edge seals; anda second group of knife edge seals, wherein the first group of knife edge seals have a diameter, and a ratio of the diameter to the total cross-sectional area of the apertures is between 58.79 and 78.77. 2. The shaft seal of claim 1, wherein the plurality of apertures are positioned axially between the first group of knife edge seals and the second group of knife edge seals. 3. The seal shaft of claim 1, wherein the plurality of apertures comprises six apertures circumferentially distributed about the axis. 4. The seal shaft of claim 3, wherein the six apertures are equally spaced about a circumference of the cylindrical body. 5. The seal shaft of claim 3, wherein the six apertures each have the same cross-sectional area. 6. The seal shaft of claim 1, wherein the first group of knife edge seals each have an axial width, and a ratio of the axial width to the total cross-sectional area of the apertures is between .3658 and 1.1422. 7. The seal shaft of claim 1, wherein the cylindrical body has an axial length and the first group of knife edge seals have a diameter, and a ratio of the axial length to the diameter is between 48.04 and 64.62. 8. The seal shaft of claim 1, wherein the first group of knife edge seals includes four knife edge seals. 9. The seal shaft of claim 6, wherein the second group of knife edge seals includes four knife edge seals and have the same diameter of the first group of knife edge seals. 10. The seal shaft of claim 1, wherein both the first group of knife edge seals and the second group of knife edge seals include radially extending knife edge seals. 11. A method of installing a seal shaft in an air cycle machine comprising the steps of: (a) providing a seal shaft configured to limit flow of fluid between a compressor section and a turbine section of an air cycle machine, the seal shaft having a cylindrical body establishing a bore extending axially between a first end portion of the body and a second end portion of the body, the seal shaft further including a plurality of apertures established in the body and extending radially between an outer surface of the body and the bore, a first group of knife edge seals, and a second group of knife edge seals, wherein the first group of knife edge seals have a diameter, and a ratio of the diameter to the total cross-sectional area of the apertures is between 58.79 and 78.77;(b) positioning a portion of a turbine rotor within the bore;(c) positioning a portion of a compressor rotor within the bore; and(d) positioning a portion of a shaft of the air cycle machine within the bore. 12. The method of claim 11, wherein the plurality of apertures are positioned axially between the first group of at least three knife edge seals and the second group of at least three knife edge seals. 13. An air cycle machine for supplying air to an aircraft cabin comprising: a compressor section having a compressor rotor configured to rotate with a main shaft;a turbine section having a turbine rotor configured to rotate with the main shaft; anda seal shaft configured to limit flow of fluid between the compressor section and the turbine section, the seal shaft having a cylindrical body establishing a bore extending axially between a first end portion of the body and a second end portion of the body,a plurality of apertures established in the body, and extending radially between an outer surface of the body and the bore,a first group of knife edge seals, anda second group of knife edge seals, wherein the first group of knife edge seals have a diameter, and a ratio of a cross-sectional area bounded by the diameter to the total cross-sectional area of the apertures is between 44.54 and 59.73. 14. The air cycle machine of claim 13, wherein the plurality of apertures are positioned axially between the first group of knife edge seals and the second group of knife edge seals. 15. The air cycle machine of claim 13, wherein the plurality of apertures comprises six apertures circumferentially distributed about the axis. 16. The air cycle machine of claim 15, wherein the six apertures are equally spaced about a circumference of the cylindrical body. 17. The air cycle machine of claim 13, wherein the first group of knife edge seals and the second group of knife edge seals each include four knife edge seals. 18. The air cycle machine of claim 13, wherein the first group of knife edge seals and the second group of knife edge seals are configured to contact a seal land. 19. The shaft seal of claim 1, wherein a ratio of a cross-sectional area bounded by the diameter to the total cross-sectional area of the apertures is between 44.54 and 59.73. 20. The method of claim 11, wherein a ratio of a cross-sectional area bounded by the diameter to the total cross-sectional area of the apertures is between 44.54 and 59.73.