1、1 NASA TECHNICAL NOTE EFFECT ON BASE DRAG OF RECESSING THE BASES OF CONICAL AFTERBODIES AT SUBSONIC AND TRANSONIC SPEEDS by William B. Compton III Langley Research Center Langley Station, Hampton, Va. :* , NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. OCTOBER 1968 Provided by IHSNo
2、t for ResaleNo reproduction or networking permitted without license from IHS-,-,-TECH LIBRARY KAFB, NM I llllll IIIII IIIII lull Ill11 llpll IIII Ill Ill 0 L 3 L 7 L 8 / EFFECT ON BASE DRAG OF RECESSING THE BASES OF CONICAL AFTERBODIES AT SUBSONIC AND TRANSONIC SPEEDS I- By William B. Compton,III /
3、Langley Research Center Langley Station, Hampton, Va. /.“ .x NATIONAL AERONAUTICS AND SPACE ADMLJNSFfM4CON.- For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - CFSTI price $3.00 L Provided by IHSNot for ResaleNo reproduction or networking per
4、mitted without license from IHS-,-,-EFFECT ON BASE DRAG OF RECESSING THE BASES OF CONICAL AFTERBODIES AT SUBSONIC AND TRANSONIC SPEEDS By William B. Compton 111 Langley Research Center SUMMARY An investigation has been conducted to determine the effect on base drag of recessing the bases of truncate
5、d conical afterbodies. pared with the drag of recessed bases of equal size for afterbodies having boattail angles of Oo, 30, 50, and loo, and having ratios of boattail length to maximum diameter of 1.0 and 1.5. from a flat base to an open base. A fully conical afterbody with a boattail angle of 100
6、was tested also. ber range of 0.3 to 1.3. The Reynolds number based on model length was in the range of 8 x 106 to 16 x 106 depending on the Mach number. The drag of a flat base was com- For each boattail, the amount of base concavity was varied in several steps The tests were run at an angle of att
7、ack of 00 and through a Mach num- In addition to the base-drag information, boundary-layer profiles and afterbody- drag-coefficient plots are included. Results indicate that, in general, recessing the base gives an increase in base pressure coefficient of 0.01 to 0.03, depending on the boattail, and
8、 hence a reduction in base drag. For a given boattail, base drag decreases with increasing base concavity up to a certain point, but, beyond this point, further concaving the base has little or no effect. The ratio of the amount of base concavity to base radius necessary to achieve maximum base-drag
9、 reduction depends on the boattail angle and length. Recessing the base has practically no effect on boattail drag. INTRODUCTION The base drag of an aircraft, unpowered projectile, or missile with a blunt base Considerable work has been can represent a large portion of the total drag; and the effect
10、iveness of ways of reducing this drag has been the objective of many investigations. done on the reduction of base drag by the base-bleed and splitter-plate methods (e however, the rim width was kept as small as possible. The width is shown by the difference in R1 and Rb in figure 5. No attempt was
11、made to measure the pressure on the rims of the concave bases. As shown in figure 5, different bases were inserted into the boattails, so, of necessity, the outermost part of the cavity of the open-base configurations could not be kept parallel to the boattail. For the configurations used in the inv
12、estigation of the effects of recessing the base, the location of the pressure orifices on the surface of the boattail was measured axially from station B, and the location of those on the base was measured radially from the model axis. The locations of the orifices for these configurations are given
13、 in tables 1 and 2. The base orifices for the open-base configurations were located well within the 5 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-model and were not purposely placed at any special radial location since their exact placement was r
14、elatively unimportant. Figures 6 and 7 show photographs of the assembled model and of each type base. Instrumentation and Tests Pressures. were measured on the base, on the boattail surface, and in the model- shell gap with strain-gage pressure transducers. tion of the model aft of the gap were meas
15、ured by an internal strain-gage balance. The forces and moments on that por- The tests were conducted at Oo angle of attack and through a Mach number range of 0.3 to 1.3. For each run, data were taken at specific Mach numbers as Mach number was increased, and repeat points were taken as Mach number
16、was lowered. As each point of data was taken, the Mach number was held constant. The points taken as Mach number was decreased are identified in the plots by flagged symbols. For each data point, approximately five frames of data were recorded within 1 second and the average was used to compute the
17、values of force, pressure, and so forth. Data Reduction The main drag coefficients of concern in the report are the base drag coefficient CD,b, afterbody pressure-drag coefficient CD,a, and total afterbody drag coefficient CD a bal, with the afterbody defined as the entire model aft of the junction
18、of the cylin- drical section and the boattail (station B). As explained subsequently, the base drag was computed by pressure integration, whereas the afterbody drag was computed both by pressure integration, giving afterbody pressure drag, and from balance data, giving total afterbody drag. area to
19、each pressure orifice at = 0 and integrating. Skin-friction drag was not included in the afterbody pressure drag. All drag coefficients are based on the maxi- mum cross-sectional area of the model. The equation for pressure-drag coefficient is 97 Base drag and afterbody pressure drag were obtained b
20、y assigning an incremental n 1 qAm = - 1 (Pm - pi)Ai i= 1 For the base drag integration, the area assigned to the outermost orifice extended to the full radius of the base (Rb). Thus, the value of integrated base drag for the concave bases approached the value that would be obtained for R1 = Rb (see
21、 fig. 5). The incremental base pressure coefficients, used in comparing the average base pressures of all configurations, were calculated by using the following formula for 6 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-each boattail : “P,bi = (CD
22、,flat base - CD,concave basei)% (where i=l . The balance measured the total force on the model from the gap aft. To obtain total afterbody drag from the balance, the force measured by the balance was corrected for gap force and skin-friction drag between the gap and the break of the boattail (sta- t
23、ion B) . The skin-friction-drag calculation was based on turbulent-boundary-layer theory. The total afterbody drag coefficient was calculated from the balance as follows: r n i= 1 I I In this equation FA,f is the calculated skin-friction force between the gap and station B. Thus the total afterbody
24、drag computed from the balance measurement includes the effect of skin friction on the boattail and of asymmetry of pressures with +, whereas the inte- grated afterbody pressure drag did not. RESULTS AND DISCUSSION Cylindrical Afterbody Pressures and Boundary-Layer Profiles Figure 8 shows the boatta
25、il-surface-pressure-c.oefficient distribution on a cylin- drical afterbody with a flat base for various Mach numbers and values of +, x/Dm = 0 being located at model station 104.14 (station B for an afterbody with L/Dm = 1.0). (See fig. 1.) The general drop in pressure over the whole boattail at the
26、 subsonic Mach num- bers is due to expansion of the flow at the base. At the supersonic Mach numbers, where the base effects do not feed as far upstream, the pressures over most of the boattail are almost constant. These same trends are evident for the models used in reference 6. At most Mach number
27、s, the pressures of the different rows ol orifices are about equal except near the base where the pressures of the bottom row (at $ = 1800) are generally lower, an effect possibly caused by the strut wake. Between, but not including, Mach numbers 0.95 and 1.20, normal and reflected shocks, influence
28、d by the support strut, on the model or near its base cast doubt on the correct level of pressures on the boattail. Boundary-layer Mach number profiles for various free-stream Mach numbers are presented in figure 9. Since the boundary-layer rakes measured only total pressure, the static pressure in
29、the boundary layer was assumed constant in calculating the Mach num- ber profiles. The value used was equal to the static pressures on the model in the posi- tion of the rakes with the rakes removed. The profiles show the boundary layer to be about 1.52 centimeters thick (yDm = 0.1) and very consist
30、ent at all values of 4 7 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-except 1800, where there is a loss of total pressure in the wake of the model support strut. The experimental and power-law-calculated boundary-layer profiles are com- pared in
31、figure 10 for a representative subsonic and supersonic Mach number. To cal- culate the ratio of the local to maximum boundary-layer velocity, the total temperature throughout the boundary layer was assumed constant. The boundary-layer displacement thickness is approximately one-tenth of the boundary
32、-layer thickness and eleven- hundredths of the maximum diameter of the model. Variation of Pressures With and Base Concavity Figures 11 and 12 illustrate the reason for using the top row of orifices (at = 00) Figure 11 shows typical boattail-pressure-coefficient distribu- for pressure integration. t
33、ions at various Mach numbers and values of . The boattail-pressure levels of the different rows of orifices are practically the same except at a Mach number of 1.30, where the bottom row is at a higher pressure level than the others. Figure 12 shows typical base-pressure-coefficient distributions at
34、 various Mach numbers and values of . The base-pressure levels also vary with , particularly at Mach numbers of 0.80 and 1.30. This variation of pressure with is thought to be caused by the flow distur- bance of the model support strut, and hence the top row of pressure orifices should be the most i
35、nterference free. Therefore, only the top row was used when integrating the base and boattail pressures for drag. Typical boattail-pressure-coefficient distributions for various Mach numbers and values of base concavity are shown in figure 13. This figure indicates that any effect of recessing the b
36、ase on boattail pressures is practically nil; thus, the effect of base con- cavity is confined to the base. Base Drag and Pressures Figures 14 and 15 show the effect of concaving the base on base pressures, and hence base drag. Figure 14 presents the radial distribution of base pressure coefficient,
37、 with the abscissa being the ratio of radial location on the base to maximum model radius. Results for each configuration are plotted, in groups of bases relating to each boattail, at Mach numbers of 0.40, 0.80, 0.90, and 1.30. Since the exact radial location of the base orifices for the open-base c
38、onfigurations was relatively unimportant, the averages of the pressure coefficients of these orifices are plotted at r/Rm = 1.0 and the symbols are solid. Figure 15 is a plot of base drag coefficient as a function of Mach number for each configuration. The ratio of the base area to maximum cross-sec
39、tional area of the model is given for each boattail so that the relative.size of the bases can be taken into account when comparing the effects of recessing the bases of the different boattails. Because of 8 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IH
40、S-,-,-normal and reflected shocks on the model and near the base between Mach numbers 0.95 and 1.20, the data in this Mach number range were not faired. Figures 14 and 15 show generally that recessing the base gives a reduction in base drag at both subsonic and transonic speeds. As can be seen from
41、figure 14, the maxi- mum base-pressure-coefficient benefits derived from recessing the base varied from 0.01 to 0.03 depending on the boattail. Although not directly shown or discussed in ref- erence 7, a comparison of figures 3 and 9 in that reference would show the same results were obtained with
42、a turbulent boundary layer. When the base of the model in refer- ence 7 was concaved by fitting it with a cylindrical tube having the same diameter as the model and a length approximately equal to its diameter, and having thin, solid walls, the base pressure coefficient was increased by approximatel
43、y 0.02. To better illustrate the effect of varying the depth of base cavity, figure 15 is sum- marized in figure 16 for a representative subsonic and transonic Mach number. The change in base pressure coefficient derived from concaving the base is plotted for each boattail, with the abscissa being t
44、he ratio of base concavity to base radius. The incre- mental base pressure coefficients, plotted as the ordinate, were obtained for each boat- tail by subtracting the drag coefficients of the concave bases from the drag coefficient of the flat base and multiplying by the ratio of the maximum area of
45、 the model to the base area. Figure 16 shows that, in general, for a given boattail angle and length, base pres- sure increases with increasing base concavity up to a particular ratio of base concavity to base radius, but, beyond this ratio, further concaving the base has little or no effect. This r
46、esult is particularly true at the subsonic Mach numbers. It also appears that, in general, the steeper boattail angles require a greater ratio of base concavity to base radius to derive the maximum base-pressure benefits from recessing the base than do the shallower boattail angles. Again this effec
47、t is more evident at the subsonic Mach numbers. As seen from figure 15, at a boattail angle of 50, for a depth of base cavity of one- I I I half the base radius, and through the test Mach number range, the semitoroidally con- caved base offered no improvement over the base simply concaved to the sam
48、e depth. The pressures for the two types of bases are presented in figures 14) and 14(f). The semitoroidally concaved base had slightly higher pressures toward its edges but much lower pressures near its center, the result being no net improvement over the simply concaved base. Afterbody Drag Figure
49、 17, a plot of afterbody pressure-drag coefficient as a function of Mach number, is included to help compare the drag characteristics of the different base and 9 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-boattail combinations. Since recessing the base does not affect the boattail pressures, base concavity has the same effec
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