That is a good question, when the engine was sent back he said it looked as if it had had a prop strike but, that was not the case. He then mentioned it could have back fired and could have caused it. I still find that hard to believe and we would have heard the back fire. They fixed the crank and replaced the bearings, he also replaced the ignition timing sensor. I found that interesting, maybe that is the real reason it happened, they did not fire at the same time. Wouldn't this be an ignition box issue and not the timing sensor on the crank? The RPV we are flying is 42lbs and is designed to carry 20 pounds possibly 30. But the airframe is taking the stress, it is not being flown to perform aerobatics just haul weight and get to altitude quickly, 45-50 degree angles. So far the engine and aircraft have done this with ease at about 1/2 throttle. That is why we have found it hard to believe that we should run a smaller prop at the manufactures suggestion. It is being flown now with a Bolly 28-10 which seems to work great. The only thing we can figure is they are thinking that the prop bite is to much under that load and the crank is giving up before the prop, so by going to a smaller prop will cause the prop to loose bite and relieve stress on the crank. It would not make a huge difference in performance, it would just run our of juice at a lower altitude under load. Unfortunately, that is not what we want.