the boeing engineers were aware that nasa had carried out a series of "powered lift" studies some time earlier, including both externally blown flaps, as well an upper-surface blowing (usb), an unusual variation. in the usb system, the engine is arranged over the top surface of the wing, blowing over the flaps. when the flaps are lowered, the coandă effect makes the jet exhaust "stick" to the flaps and bend down toward the ground. they searched for additional research on the concept, and found that half-span upper-surface blowing research had been conducted in the nasa langley 12-foot (3.7 m) tunnel. an examination of the preliminary results suggested that the system was as effective as any of the other concepts previously studied. boeing immediately started to build wind-tunnel models to verify the nasa data with layouts more closely matching their own designs. by the end of 1971, several models were being actively studied.