I was AT Edwards in the late 70’s when the YC-14 and YC-15 were in flight test. ONLY the Boein YC-14 had blown lift flaps. The other was a conventional design … which LATER morphed into the C-17. That’s how much the USAF thought of the idea. To be fair … there were other reasons they went for the -17 … starting with 4 engines.