 Compared to the Yak-25, top speed was increased by 40 km/h (25 mph) and service ceiling was increased by 1,000 m (3,281 ft).However, the swept wing was found to be unsatisfactory with poor aileron authority.Although the modifications introduced by the Yak-30D eradicated several shortcomings displayed by the first prototype, the first production deliveries of the superior MiG-15 were already taking place by the time that the improved version of the Yakovlev fighter entered flight test, and the results of NII V-VS trials were, therefore, little more than academic.