| 'Aspect
ratio' is the length of the working portion of the rudder blade (or any
lift generating wing or fin) divided by its width (chord). The higher the
ratio (within structural possibilities) of an airfoil or hydrofoil the more
efficient the wing or fin is. Efficiency, in this instance means getting
the most lift for the smallest amount of supplied power or conversely, to
achieve the largest amount of lift with the smallest possible resistance
(drag). A great example of this ratio and its effect can be found in birds, for example. A duck with its short and wide wings (low aspect ratio 3:1) must flap its wings without pause to generate enough power for lift to stay in the air for any period of time. An albatross can stay aloft on its long and narrow wings (high aspect ratio 10:1) without a single wing flap almost indefinitely. This principle is perfectly valid for foils in any fluid. Low aspect ratio foils are said to have 'high loading' which means that the amount of lift per surface area is higher. This also means a lot of power to generate lift and to overcome high drag. High loading foils must be built shorter and stronger. Examples of high loading foils are: Jet plane wings, racing power boat propellers, bumble-bee and canard wings. Examples of low loading foils: unpowered gliders, para-gliders, human powered airplane propeller, Albatross wings, wind-mill power generators, kayak rudders In general, most foils generate lift only over a small portion of width of the foil shape. If the width of the foil increases, drag or resistance starts to increase in much bigger proportion than lift. High aspect ratio foils 'multiply' this lift by increasing their length, not width. |