Letīs see... the propeller diameter on the NK12 engine is 5,6m, which gives a radius of 17,6m. Now if the book I read is correct, the props run at 750rpm during cruise, this would make the tips of the engines run at 220m/sec. This is only the velocity along the circumference, so the vector of the arriving air has to be added to this as well. Without further calculation - at cruising altitude this easily should be supersonic.