They get their low span loading from a combination of light overall weight and the relatively large span that comes from their large wing area. Ultralight wings are large, but have aspect ratios similar to heavier light airplanes. This makes parasite drag relatively unimportant and places a premium on low wing loading. Part 103 ultralights are limited to a level-flight maximum speed of 55 knots and a stall speed of not greater than 24 knots. It’s interesting to note the difference between these two types as well as the similarity. Motorgliders and ultralights have large spans for their weight in order to be able to climb well with relatively small engines. These differences affect the span loading that is best for the airplane.įor low-powered airplanes that fly relatively slowly, the need to get acceptable climb performance tends to drive the preferred design toward large span. At the airspeed for minimum power required to fly, which corresponds to best rate of climb for piston-engined airplanes, three-fourths of the total drag of the airplane is induced drag.At airspeeds lower than that for best L/D, induced drag is larger than parasite drag.At airspeeds faster than the speed for best L/D, the parasite drag is larger than the induced drag.When the airplane is flying at its best lift-to-drag ratio (L/D), half of the total drag is induced drag.The importance of induced drag varies over the flight envelope. (See “Flight Test Results for an Advanced Technology Light Airplane,” David L. The reduction in span canceled out any improvements provided by the advanced technology and reduced wetted area. Its rate of climb was worse, and its cruise performance at any realistic cruise altitude was not much better. In flight test, the “advanced” airplane proved to be inferior overall to the stock airplane with the exception of top speed at near sea level. The “advanced” wing had just over half the area of the stock wing, and even though its aspect ratio was higher than the stock wing, the actual span was 12% less. The program started with a Cessna Cardinal and replaced the stock wing with a much smaller wing incorporating sophisticated high-lift devices to keep the stall speed down, an “advanced” airfoil, and spoilers for roll control. One example of this was a NASA-sponsored program to investigate advanced technologies for general aviation. This usually ends up with an airplane that may be faster at low altitude due to the decrease in wetted area, but climbs poorly and flies slower at cruise altitude due to the increase in induced drag caused by the reduction in wingspan. More than one designer has made the mistake of making the wing area of an airplane smaller to reduce wetted area and decrease drag and ended up reducing wingspan at the same time, even though the aspect ratio of the new, smaller wing is higher.
The important thing to remember is that for a given lift, density altitude and airspeed, all wings of the same span will generate the same amount of induced drag regardless of wing chord, area or aspect ratio. I will spare you the algebra here, but when all the manipulation is finished, we get the equation for actual drag shown above, in which the span remains but the aspect ratio does not. To get actual drag from drag coefficient, we need to multiply the coefficient by the wing area and the dynamic pressure. This nondimensional coefficient is what most of us are familiar with. The reason for the misconception is that the nondimensional induced drag coefficient is a function of aspect ratio: The actual induced drag of a lifting wing (in pounds, or Newtons depending on unit system) is given by:ĭynamic pressure is proportional to airspeed squared and the density of the air. It’s common to think that induced drag is determined by aspect ratio, but this is not the case and that misconception can be a trap for the unwary wing designer. It is a function of the span of the wing and the amount of lift the wing is generating. Induced drag is the portion of the drag of the airplane that is a direct result of the production of lift. The reduction in induced drag due to the span increase is greater than the drag of the added wetted area of the extensions. The highly tapered wingtip extensions on this RV-6 improve cruise performance at altitude.
0 kommentar(er)
