Drag airplane1/3/2024 ![]() ![]() ![]() A basic rule of thumb is that if the junction occurs at an angle of less than 90, then a fairing is usually necessary. There are many airplanes with 90 or near-90 wing junctions that could benefit significantly from the addition of a root fairing. One recurring myth in the popular aviation press is that if the angle between the wing surface and the fuselage side (in the front view) is 90 or more, the junction does not require a fillet or fairing. While this is advantageous, experimentation has shown that, with proper design of the fuselage and the wing-to-fuselage junction fairing, low-wing airplanes can have interference drag that is no higher than that of high-wing airplanes.īecause the airflow over the upper surface of an airplane wing is so sensitive, careful design of the wing-to-fuselage junction area is vital in keeping interference drag to a minimum. The wing/fuselage intersection on most high-wing airplanes affects only the less-critical lower surface of the wing. In a properly designed high-wing configuration the fuselage side does not cut the upper surface of the wing, and so it does not interfere with airflow over it. The airflow over the upper surface of the wing is much more sensitive to interference, which might cause premature separation or stall.īetter wing-to-fuselage junctions are an argument in favor of high-wing airplanes. In general, junctions on the upper surface of the wing are more critical than lower-surface junctions. A poorly designed junction can cause a large increase in drag, particularly in climb and economy cruise where the lift coefficient is high. The junction between the root of the wing and the side of the fuselage is one of the primary problem areas for interference drag. Second, they can be minimized or eliminated by proper design. This example illustrates two important facts about aerodynamic interference: First, interference effects can be quite large. The addition of a fairing can reduce the interference drag to only a few percent of the total drag of the two struts. This is caused by flow separation induced in the corners, at the joint between the struts. The drag caused by interference is equivalent to the drag of a section of isolated strut that is 10 times the strut thickness in length. An example of how powerful this effect can be is the drag of a simple “T” formed by two intersecting streamlined struts similar to the intersection between the mail lift strut and jury strut on a high-wing airplane. A properly designed wing-winglet combination has less drag than the wing alone, due to the winglets’ effect on induced drag.ĭespite these exceptional cases, the effect of flow interference generally increases drag. Winglets are another example of favorable interference. This produces a reduction in induced drag that is large enough to offset the increase in parasite drag caused by the tip tank. The wingtip-mounted components act like endplates and increase the span efficiency of the wing. On certain military airplanes, drag is actually lower with wingtip-mounted missiles or fuel tanks in place rather than after they have been jettisoned. In a few cases, the interaction of components may be favorable and actually decrease drag. In most cases it will increase drag, sometimes dramatically. This interaction and alteration of the airflow affects the drag of the airplane. The interaction is especially large in the neighborhood of junctions between components. Each part of the airplane affects the air flowing over it, which in turn influences the airflow over the other parts of the airplane. The airflow over an isolated component of an airplane is not the same as when that component is combined with others to form a complete airplane. The interference drag of a poorly designed airplane can be much higher than this, but it is rarely less than 5%, or even 6%, for a well-designed, clean airplane. For preliminary design performance estimation, interference drag can be taken into account by calculating the drag of the airplane by summation of each component drag, and then adding between 6% and 10% to the parasite drag calculated by summation. The drag change caused by the aerodynamic interaction of the various components of the airplane is called interference drag, which can make up a substantial portion of the total drag. A fairing or fillet at the wing-fuselage junction can help keep airflow interference to a minimum. The reason for this inaccuracy is that the “summation” method of drag estimation ignores the effect of combining the isolated components into a complete airplane. They quickly learned that this method was not particularly accurate, and almost always underpredicted total drag. In the early days of aviation, designers believed that the total drag of an airplane could be calculated by simply measuring (or calculating) the drag of each component (wing, body, tail, landing gear, etc.) separately and then adding them together. ![]()
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