The same thing they do on the Mirage F1, MiG-25, Su-24, F-14, MiG-31, F-16, MiG 1.44, J-10, J-20 and so on and so forth: improve directional stability – particularly at high AoA – without increasing the height of the aircraft. Without them, you’d need a tailfin of Tornado/Tu-22M proportions 😉 As for the Su-34, it is quite trivially not expected to perform as much high AoA flying as the basic Su-27, so the ventral strakes (to give them their technically correct name) are deleted to save weight and decrease RCS.
Flanker_man and others visiting MAKS regularly will agree with me that the Su-34 can perform similar maneuvers including high AOA turns as other Flanker comrades putting to shame almost all you`ve mentioned before. The same trivial question, did they expect from the Mig-25, Su-24 or Mig-31 to perform at high AoA either? Let me please answer.
I think the difference maker here is the shape of the Su-34 forebody. While aircrafts like the Mirage F1, MiG-25, Su-24, F-14, MiG-31 have round and smoothsided forebodies creating asymmetrical flows around the fuselage at high AOA, the Su-34 has a fuselage cross section chine-shaped. When at high angle of attack asymmetrical flows creating aerodynamical forces and moments prevail over the tailfin which happens to be in the wing wake, the directional stability is lost and spin emerges. Therefore almost all aircrafts have ventral fins to improve directional stability and enhance spin characteristics and recovery.
Unlike round forebodies the chined one produces stronger and more symmetrical vortices over the fuselage and wings which have significant impact on lateral/directional stability of the aircraft. Hence the Su-34 high directional stability at high AOA, what later rendered ventral strakes useless not to mention the tailfins are even smaller than on the Su-35.
regards
M
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