Pilot Training - Theory of Flight: Parasite Drag

70. Skin friction and form drag.-a. Parasite drag is composed of two distinct elements, the drag of skin friction in the boundary layer, and the form drag due to disruption of streamline flow and the resulting turbulence. The skin friction component results from the interaction between a viscous fluid and the surface of the solid which may be either smooth or rough. The form drag due to disruption of the streamline flow depends upon the extent of the disruption of the streamlines and the resulting turbulence. Thus the drag of a flat plate is practically all skin friction when edgewise to the airstream but when normal to the airstream the drag is due almost entirely to disruption of the streamline flow.

b. A low parasite drag is of utmost importance in airplane design, for every pound of drag requires a pound of thrust from the propeller to maintain the relative motion necessary for sustentation. Each pound of thrust so used adds to the engine power required and to the fuel consumption as well.

c. Of the two components of parasite drag, that due to disruption of the streamline flow affords the greatest opportunity for reduction. On modern high performance aircraft, streamlining is so effective that the form drag is often less than the drag of skin friction. It is therefore important that skin friction be made as small as possible. The area of surfaces exposed to the airstream is fairly well fixed by structural requirements of strength, housing capacity, and the like. Skin friction can be minimized by employing a glossy flat finish to surfaces and eliminating protruding rivet heads, roughness, and other surface irregularities.

71. Parasite drag coefficient.-Parasite drag is a resistance and as such can be expressed by the same sort of fundamental equation as that employed for lift and drag of the wings:

where C_Dr, is the resistance coefficient for the particular item under consideration, A the projected area of the body in square feet or the area of its largest cross section taken in a plane perpendicular to the relative, wind, and V the velocity of the airstream in feet per second. The determination of the resistance of segregated items over a range of airspeeds would prove an exhaustive task where the total of such items represents the parasite members of an airplane. Fortunately a simplifying expedient is available which materially reduces the labor of parasite drag determination. This is the expression of all parasite drag items in terms of the equivalent flat plate area for a particular airspeed.

72. Equivalent flat plate area.-a. A flat plate, if approximately square and of some 12 square feet or more in area, will be found to have a resistance coefficient of 0.00152 when placed at right angles to an airstream. Therefore

Flat plate drag = 0.00152 AV²                                               (42)

b. By equivalent flat plate area is meant the area of a flat plate normal to an airstream which will give the same drag as the body or bodies placed in the same stream. Thus if the sum of all parasite items gives a drag of, say 228 pounds, at 100 ft./sec. the equivalent flat plate area will be

c. For any other airspeed, then, the parasite drag will be

and at 60 ft./sec is 0.00152 x 15 x 60 x 60 = 82 lbs. The labor of determining the individual resistance and summing these up to obtain the total parasite drag is thus saved for this and any other speed by employment of the equivalent flat plate area.

d. It will be noted that in the above calculations, standard air is assumed and the factor r/2 is incorporated in the parasite drag coefficient 0.00152.

73. Parasite drag and model tests.-a. The above method of determining the parasite drag serves well for preliminary design but it should be checked by resistance data on the complete model. Corrections must be made for parts eliminated from the model and the wing drag must be subtracted from the measured total to obtain the parasite value. This in turn must be scaled to full size or where the subscript m indicates the model values, and the scale, any linear dimension of the model to that of the full sized airplane.

b. Parasite drag is particularly sensitive to changes in Reynold's number, and an attribute of the successful airplane designer is a knowledge of the limitations of model test data in its application to the full scale airplane.

c. Since the total drag of an airplane must equal the thrust, its value for those actually built can be readily determined from flight test for the high speed condition where the engine is operating at its rated horsepower and the propeller at its maximum efficiency as discussed in later sections. By subtracting computed wing drag from the maximum thrust, the parasite drag is determined. This may be expressed in terms of equivalent flat plate area, and thereafter the parasite drag at other speeds may be readily calculated. The value of knowing this drag for airplanes in service lies primarily in being able to prognosticate closely that of similar ones in the process of design. By making due allowance for differences in trussing, fairing, etc., the experienced designer need not undertake the laborious calculations incident to summing up individual drag items

74. Interference.-a. None of the above methods of parasite drag determination take cognizance of specific interference effects between adjacent bodies. Two wires, one behind the other, may show less drag than a single wire. If side by side and spaced less than 6 diameters they will show more than twice the resistance of a single wire. The result depends entirely upon whether turbulence is reduced or increased by the close proximity of the bodies.

b.  Most pronounced, however, is the effect of bodies attached to or protruding from the wing. The turbulence resulting will give interference drags ranging from 50 to 200 percent greater than those of the parts tested separately.

C. The method usually applied by the Navy for estimating drag is by summing the drags of the component parts of the airplane at a specified speed. The Army prefers a shorter method which consists in the estimation of equivalent-flat-plate area of the complete airplane at high speed. This method consists of comparing airplanes being designed with airplanes already constructed whose equivalent flat plate areas have been determined by flight tests. Likewise an estimate of the variation of parasite drag coefficient depending upon the particular shape of fuselage to be used is made from an examination of photographs and detailed drawings of existing airplanes. These estimates are then checked against and compared with the data presented.

75. Slipstream effect.-The airplane propeller in producing a forward thrust must impart to the air a change in momentum in accordance with Newton's laws. The air affected is that which is drawn through the propeller and pushed aft, constituting the slipstream. The velocity of the slipstream varies with throttle setting and angle of attack, and is of the order of 10 to 15 percent more than the airspeed for normal flight at full power and some 40 percent for steep climb at full throttle. Obviously all parts of the structure exposed to the slipstream will have their drags increased over that which would obtain if no acceleration had been imparted to the air driven aft by the propeller. Account must be taken of this in determination of parasite drag. It is customary to compute A, from the high speed performance test of the complete airplane, and to consider that the velocity of the slipstream has a negligible effect on the parasite resistance at the high speed condition of flight.

76. Struts.-a. The struts of an airplane are the structural members which are primarily subjected to compression loads. Where exposed to the airstream, as are the interplane members of the wing cellule, struts should be streamlined to minimize drag. A streamline form is defined by its fineness ratio. The fineness ratio of a body is the ratio of its dimension in the direction of the airstream to the maximum dimension perpendicular to the airstream. Thus the strut section shown in figure 66 has a fineness ratio of 3.

b. Though tubing of circular cross section and built up struts of the so-called "Eureka" section shown in figure 67 give higher compressive strength values for a given weight they offer more resistance than a

streamlined shape such as depicted. The Eureka section can be ruled out as being bound to evidence more turbulence than a flat plate. The drag of the circular cross section per foot length is expressed by the equation

That of a streamline form is the same except for lower values of the coefficient which range from 0.000009 for a fineness ratio of 2.5 to0.000008 for a fineness ratio of 4.0.

Thus the drag of a strut of circular cross section is 0.00012 / 0.000008=15 times that of a streamline section of fineness ratio of 4. It is little wonder then that light fairings are employed on tubes of circular cross section where the cost of streamline tubing is not considered justified. Typical examples of fairing are shown in figure 68.

77. Wire and tie rods.-The usual tension members of the airplane structure are wires and tie rods, the distinction being in the terminal employed. The tie rod uses a threaded terminal whereas the wire uses a spliced, wrapped, or ferruled one. Tie rods or solid wires of circular cross section will have a drag calculable from the same formula as that of the round tubing. Stranded cable, however, owing to its rough surface will have a higher resistance coefficient, that is, 0.00015. Hence the latter offers practically 20 percent more drag than the solid wire or round tie rod. Streamline tie rods show a much lower drag than do those of circular cross section as would be expected after noting the differences in the case of struts. These tie rods are not of true streamline section, however, for rigging requires a definite tension which might place the trailing edge forward and ruin the low drag characteristic. In consequence, the cross section is lenticular as shown in figure 68 which indicates the usual proportions. Obviously, either edge may be the leading edge. The drag is only 15 percent that of the tie rod of circular cross section but this holds only as the long axis parallels the airstream. Should such a rod be placed athwartships, not only would the resistance increase excessively but serious vibration would be set up.

78. Fittings.-A fitting will have a drag varying from approximately 70 percent that of a normal flat plate of equal projected area to that of a flat plate of double its projected area. The higher value will occur where the fitting is so attached as to be subjected to considerable interference. Careful design of fittings has brought about a reduction of drag from 50 to 75 percent or where an obsolete fitting shows a 4-pound drag at 100 m. p. h. a modern one will show a little over a pound. It is no wonder even with this lower value that the present day tendency is to conceal fittings as much as possible to minimize the drag. This is indicated in figure 68.

79. Bare fuselage.-The bare fuselages of most modern airplanes show a resistance coefficient which is but a fraction of that of a flat plate. This is due to the low resistance form now generally employed. An average value of drag coefficient, with all projections, windshields, engine, radiator, and the like removed, is approximately 0.0002; where the lesser items are considered, the value of drag coefficient will average about 0.00025. However, when great refinement is undertaken the coefficient may be reduced to about one-tenth that of the flat plate, or 0.00015. Nacelles show resistance coefficients essentially the same as those for fuselages.

80. Fuselage with appendages.-The addition of windshields will ordinarily increase the resistance coefficient value somewhat though such is not always the case. Where the windshield is well formed, this increase may be negligible even with the pilot placed behind it. In general, miscellaneous projecting parts which are not streamlined may be treated as fittings. These may include hand fire extinguishers, compasses, airspeed meters, altimeters, etc. Obviously, wherever possible, such items of equipment should be housed inside the cockpit for each offers one or more pounds of drag at 100 ft./sec.

81. Fuselage with engine.-The contribution of appendages to the bare fuselage drag is practically nothing compared to that of an engine housed conventionally in the nose. An engine, so placed, requires a blunt nose and ordinarily takes such a form that the streamlines are badly disturbed and the resistance coefficient, in consequence, greatly increased. It has been found from full scale tests that the drag of a bare cabin fuselage with engine removed and nose rounded is less than one-third that evidenced when a radial air-cooled engine in place. In this instance the drag was 18 pounds as against 57 pounds at 100 ft./sec. In general, the drag of an open cockpit fuselage is some 10 percent higher.

82. N. A. C. A. low drag cowling.-The high drag of the static radial air-cooled engine, caused by its large frontal area and poor aerodynamic form, has militated against more universal adoption. The N. A. C. A. "low drag" cowling has overcome this disadvantage. This cowling consist of a sheet aluminum nose or hood  which completely covers the engine except for an air intake around the shaft as illustrated in the lower sketch in figure 71. In addition, sheet deflectors close to the crankcase divert the airstream to those parts of the engine which would otherwise run hot. An annular slot at the rear insures that the air in leaving will flow smoothly along the fuselage. Such a cowling offers as little disturbance to streamline flow over the fuselage as possible. The air for cooling the engine is separated from the general flow and then fed back smoothly through the slot. Consequently, it is not surprising that the reduction in drag is better than 2.5 times that of the conventional cowling with a large spinner. The drag of the fuselage with the engine so cowled in a particular instance was 34 pounds at 100 ft./see. or 1.87 times that of the fuselage without the engine. This reduction in drag through the use of N. A. C. A. cowling will increase the maximum horizontal speed at full power of conventional cabin airplanes from 5 to 10 m. p. h., and small open cockpit airplanes from 15 to 20 m. p. h. Furthermore, the power available for climb will be greater, the rate of climb improved, and the ceiling raised. Finally the fuel consumption will be less and the range increased in consequence.

83. Fuselage with radiator.-a. The liquid-cooled engine has a smaller frontal area and better aerodynamic form than the air-cooled radial, so it contributes less drag. This is more than made up, however, by the conventional radiator. This cellular or honeycomb radiator is usually placed in the nose or on the side or bottom of the fuselage. An average drag value is 8 lbs./sq. ft. at 100 ft./sec. though the resistance varies with the depth. Thus a radiator with 5-inch tubes has has a drag of approximately 3/8 that of a flat plate of equal frontal area and one with 9-inch tubes about 1/2. With the shutters closed, the coefficient will be the same as for a flat plate. Owing to the high drag offered by such radiators other types are sometimes employed. In the wing and float radiators used on racers, parasite resistance is practically eliminated since the water is cooled by contact with a surface already a necessary part of structure. This is indicated in figure 69.

b. Maintenance and operating problems of these radiators are such as to discourage their general use. rhe "core" type is the one preferred. It remains then to employ a radiator of such a type but of smaller frontal area and less depth. This is feasible only by using a cooling liquid which boils at a higher temperature. Such a liquid must, furthermore, have a relatively high flash point, a relatively high specific heat, and good surface wetting qualities. Finally, it must not attack the material with which it comes in contact nor be decomposed itself. Ethyline-glycol, marketed under the trade name of "Prestone," complies with these requirements and permits more efficient operation of the modern airplane engine than when it is water-cooled. c. While operation of an engine at 300° F. entails a loss of some 3 percent in power, this is more than counterbalanced by

(1) Better fuel economy.

(2) Reduction in radiator size.

(3) Reduction in the amount of cooling liquid necessary.

(4) Reduction in weight of engine installation.

(5) Reduction in the parasite drag of the airplane.

(6) Simple and positive provisions for ample heating of the inlet manifolds.

These advantages are all interrelated, but, with respect to parasite drag directly, certain facts are outstanding. The radiator normally offers about 15 percent of the total parasite drag of the airplane. With Prestone the radiator cooling surface can be 70 percent less than that required with water-cooling. Such a reduction in frontal area should result in a radiator parasite drag of 4.5 percent of the total instead of 15 percent. An idea of the relative sizes of water-cooled and Prestone cooled power plants can be obtained from figure 70.

84. Wheels.-a. The following table gives the approximate drag in pounds per wheel for standard sizes:

Obviously, the fairing of wheels is worth while for the reduction of drag.

b. For the conventional spoke type wheel, light metal sheets are employed in fairing. With the disk wheel, side plating serves as part of the wheel structure and constitutes its own fairing. Such fairing does not constitute the most that can be done in this respect. Figure 71 indicates the refinements resorted to in reduction of wheel drag. In general, however, it can be said that some 40 percent of the landing gear drag is due to the wheels.

85. Retractable and detachable landing gears.-a. A retractable landing gear is one which can be actuated so that it may be housed snugly in the structure to minimize resistance in flight. In one instance an .Increase of 55 in. p. h. with the landing gear housed as against what it was when extended was observed.

b. Detachable landing gears are not installed on service type airplanes at this time. Their use, however, might improve performance

in combat after dropping. This type of landing gear has been installed on a number of airplanes used in long distance and transoceanic flying to reduce weight and resistance after take-off. Justification for their use might lie in their minimizing overturning tendencies of landplanes forced to alight on the water.