So I will start this off with a more formal question than is implied by the thread title. From an efficiency (aka endurance) perspective what is the most efficient engine-aircraft combination. Now I know that I'm going to get the usual "it depends" answer so I'll elaborate. We are in the process of designing an experimental UAV for Boise State University and are having some difficulty in determining the best engine size(s) for our aircraft. We would greatly like to use a twin engine setup for enhanced reliability (which is why I'm posting here and not on the UAV forum) and greater power. Currently the design will be in the ballpark of 12-15 lbs with a 100" wing span and approximately 1030 sq in wing area for a wing loading between 25 and 30 oz/sq ft. Obviously these will change depending on the outcome of the engine recommendations.

So from what limited knowledge I have on the subject, I seem to recall that it is always desirable (more efficient) to swing a large prop slower as opposed to a small propeller at ten million rpm. This would seem to point to using a larger engine running at a low throttle setting. However, I would also assume that an engine running slower is not operating in it's maximum power or efficiency range which would then point to using a smaller engine with a bigger prop? AHH! Help! Im stuck in the endless trade-off design loop.

I also need some help in determining what size range of engines I should be looking at. At first glance the plane seems to be sized for engines in the 46 range, but I'm not sure. Thanks!!!