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Archive for September, 2017
Posted by mike_kelly | Sep 06, 2017 @ 01:42 PM | 23,416 Views

A quad with two motors per arm is a Coaxial Quad also know as an X8 and Octa-Quad. An X8 is inherently less efficient than a flat octacopter because the bottom propeller is blanketed or covered by the top propeller. Hence building an X8 is almost entirely for increased reliability. If one motor fails you have a second motor on each arm. An X8 is also more compact than a flat Octacopter. An Octo with same propeller size as an X8 would have to be much much bigger in diameter. An X8 can carry more weight than a flat quad, of the same prop size, but at the price of efficiency. The efficiency penalty is in the order of 15% but building an X8 is a black art. There are so many variables that getting it right with your available motors and props is difficult.

The top propeller increases the speed of the air moving down. This means that if the lower motor were running at the same speed it would just be idling. Since the air it "sees" from above is already moving as fast as it can accelerate air.

So the basic theory is that the bottom motor must run faster than the top motor. We can generally do that two ways. Increase the pitch of the lower propeller and or increase the KV of the lower motor which increases it's speed. General guess is that the bottom motor should run about 10-20% faster than the top motor. The problem is that it is difficult to get motors of any particular size or propellers of a particular length that are only 10-20% different than each other. It...Continue Reading