Complex sentence: More influential than the 707 in the 698's design was wind-tunnel testing performed by the Royal Aircraft Establishment at Farnborough, which indicated the need for a wing redesign to avoid the onset of compressibility drag which would have restricted the maximum speed.
Split into simpler sentences: More influential than the 707 in changing the design of the 698 was the result of wind-tunnel testing by the Royal Aircraft Establishment at Farnborough. The outcome was that the wing needed redesign to avoid the onset of compressibility drag which would have restricted the maximum speed.