Just wondering why very few people are using a twin-engine design for a drone airplane? Seems to me that it would be nice to have some redundancy to be able to RTL if a motor goes out. Don't you risk losing the aircraft if your only motor dies and you are quite far from the landing area? Or do modern brushless motors never die? What about the ESC?
Or is it the case that controlling an airplane with an engine out on one wing is impossible anyway?
What about tandem mounted motors, or a single counter-rotating motor installation?
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The main issue, I believe, is with the addition of another engine, you also have to increase the battery capacity to power that engine or suffer a reduction in flight time due to the increased power consumption.
Twin engines can certainly be done, but it's a trade-off and you have to weigh the options.
I know this thread is pretty old but thought you might be interested in this:
http://diydrones.com/forum/topics/giant-telemaster-twin-engine?xg_s...
Its a giant telemaster converted to twin engine.
I think the relative scarcity of twin engine DIY Drones is simply a function of the relative scarcity of twin engine RC planes. There are simply a lot more single engine choices.
There are quite a few twin engine/motor DIY Drones, though. The Multiplex TwinStar has been a popular choice.
From a reliability standpoint, if you have an appropriately sized motor, esc, and battery, it is a pretty reliable setup.