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Minimizing Fuselage Drag BY BRUCE CARMICHAEL INTRODUCTION
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ow that wing designs are available with less than half the profile drag coefficient of the old favorite 23015 (Ref. 1), the fuselage drag of even a relatively clean airplane looms very large. The aircraft designer finds considerably less drag data for fuselages than are available for wings. This lack is true for bodies having completely turbulent boundary layers and even more so for those having partial laminar flow. The reason for the latter is that the most popular aircraft configurations have a multitude of boundary layer trippers on the front end including the tractor propeller. The subject of partially laminar fuselages has thus been considered by many to be academic. This article presents theory and experimental results for both fully turbulent bodies and extensively laminar bodies. A typical fuselage drag minimization study is included. The practical problems of achieving extensive fuselage drag reductions in practice are covered and the present state of achievement on actual aircraft is mentioned. Finally, a performance estimate for an extreme application of fuselage drag reduction is presented.
FUSELAGE DRAG MINIMIZATION CONCEPTS Total minimization by elimination of the fuselage, while perhaps of interest for gigantic thick wing, span loader, cargo or passenger craft, is not practical for the modest sized manned aircraft of interest to us. Useful drag reduction concepts include: 1. Reduce fuselage surface area with pod and boom designs. 2. Use lowest possible pod, length to diameter ratio, to reduce the ratio of wet64 AUGUST 1996
ted area to the controlling frontal area. 3. Where practical, encourage extensive laminar boundary layer flow. Concept (1) and (2) will be shown to promote (3). 4. Fuselage shaping in the vicinity of the wing trailing edge may possibly reduce fuselage pressure drag.
FORMULAS Minimizing fuselage drag means minimizing the drag area which is the product of the drag coefficient and the area on which it is based. Calculation using the theory charts involves the product of the wetted area drag coefficient and the wetted area. Most designers favor presenting the result as product of frontal area drag coefficient and front area. The Young theory (Ref. 2) gives wetted area drag coefficient as a function of: S = Wetted Area in sq. ft. S = Frontal Area in sq. ft. 1 = Length in ft. d = Effective Diameter in ft. Body length/diameter, length Reynolds number and fraction of length laminar. The wetted area can be estimated from the formula and Figure 1. Length RN = 9354 x speed in mph x length in ft. Kw = Wetted Area Coefficient Cw = Wetted Area Drag Coefficient 05 = Frontal Area Drag. Coefficient DA = Drag Area = CWS = C&S
THEORETICAL FUSELAGE DRAG The drag of a smooth, streamlined isolated fuselage can be rapidly estimated by use of the charts of Young (Ref. 2). These charts are reproduced here as Figures 2 and 3 for the case of length/diameter = 3.33 typical of a pod/boom design, and 6.67 typical of a
conventional fuselage. One enters on the horizontal scale, projects vertically upward to the percent of length assumed laminar line, then horizontally across to the wetted area coefficient on the vertical scale. Note the very large drag coefficient reduction as transition moves aft from the nose to 60% of length. You will note that the wetted area coefficients for a given length RN and transition location are lower for the higher length/diameter case. However, later in our illustrative example you will find that the drag area or product of the wetted area coefficient and wetted area will be lower for the low length/diameter case as will the frontal area drag coefficient. It is the drag area that must be reduced. By the way, the length RN for standard sea level conditions can be calculated as 9354 x speed in mph x body length in ft. Another look at the magnitude of possible drag reductions is given in Figures 4 and 5 for 1/d of 3.33 and 6.67 in the form of ratio of drag with partial laminar flow divided by the drag for complete turbulent flow vs. the transition location. Curves are presented for length RN of 1,10 and 100 million. It is possible to cut the drag more than in half.
ESTIMATING POSSIBLE EXTENT OF LAMINAR FLOW-IDEAL CASE The boundary layer transition location for the ideal case of amplification of infinitesimal disturbances can be estimated by a complex computational method not available to many designers. I have presented my best guess for isolated smooth bodies in terms of the maximum transition length RN as a function of the length/diameter ratio (Figure 6). Dividing the transition length RN by the length RN then gives the fraction of length ideally laminar.
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