Even without photography, the experimenter can simply remember the results, or he can sketch the results. Slow motion photography is certainly useful but far from necessary. Leonardo Da Vinci din't have photography available, but he drew incredibly detailed images of birds' wings.
Basically the serviseable heavier-than-air aircraft became possible only after the fluid dynamic were developed in early XX century. I agree, that the first flights were conducted without the full understanding of flow theory, but until it became apparent that early Newtonian-based theories aren't completely right, the airplanes have quite a lot of problems.
Of course, Merlin&OWL could just again pull the trick "forget everything about scientific method, we'll just force the knowlege into stupid natives, er, Safeholdians", but this would clearly be too much. They already are dangerously close to the edge, when their actions just started to confirm the Langhorne's teaching: that only the selected few could be blessed with refined, perfect knowlege from Heavens. If they crossed this trechold, their future aqctions would be tremendously hampered.
Beacuse really, look at things from the outsider point of view:
- The few "selected ones" (i.e. the Inner Circle members) could produce perfectly refined knowlege without any intermediate stages or long-therm faliures.
- The other "mere mortals" could not invent anything without a lot of testings, intermediate stages and imperfections.
The logical conclusion for the righteous follower of Holy Writ would be that the first group obtained knowlege exclusively as divine inspiration. And inside the Langhorne's teaching this would be just right. So they would just sit and pray and wait the divine knowlege to descend upon them.