Use of Custer Channel Wings–Wing Ducts on Small UAVs

The strong variations in lift that occur with changes in forward speed lead to one of the fundamental difficulties in aircraft design: how to provide sufficient lift at landing and take-off without having oversized wings for cruise conditions. This fundamental problem is generally tackled using two approaches. First, by providing flaps and other high-lift devices, extra lift can be generated during landing and take-off, albeit at the cost of extra drag and complexity. Second, by using long and smooth runways, higher landing and take-off speeds can be tolerated, closing the gap between these speeds and those of operational flight. Even so, it is common for aircraft flying in the cruise condition to be operating with rather small main wing angles of attack (AoAs) compared with those at stall, implying that smaller wings would be desirable if acceptable landing and take-off could be achieved. A number of designers have attempted to tackle this problem with various forms of powered lift augmentation. This paper re-examines the idea of the Custer wing duct, also known as a channel wing, here applied to small unmanned air vehicles (UAVs). Such aircraft are generally not operated from long smooth runways and rarely have complex high-lift systems in their wings. It is shown that by using suitable ducts around the propellers, startlingly good take-off and landing performance can be achieved, and that suitable ducts can be readily incorporated into small UAVs with the use of 3D printing (selective laser sintering) for their manufacture. Computational fluid dynamics (CFD) analysis, wind tunnel tests, and flight trials of a Custer channel wing UAV are described.