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It's pretty clear these are run on hobby shop model airplane engines, the kind my friends and I risk blowing our hands off with back in the late 70s. But precisely which one?
TPM Readers NO and RI both say it looks like a model made by Japanese RC manufacturer OS. NO thinks it's a two-stroke, either .46 or .55 cubic inches. RI comes in a little smaller; he thinks a .4. I've looked at the full line on the site. And they're extremely similar. But the NK drone seems to have a larger flat/non-grooved section between the engine proper and the muffler. Perhaps that's a customization. But I haven't been involved in this part of the defense world since I was about 11. So if you know this world, can you look at the OS line of model airplane engines and see if you can identify the exact model, based on these photos here?
Here's one but there are more at that link.
Late Update: Okay, we may have an explanation of that extended flat portion of the engine that doesn't seem to appear on any of the off the rack OS engines ...
@joshtpm It's an extension piece, like this pic.twitter.com/mm1uKCHCd0
— David Harvey (@davidharvey) April 2, 2014