Aerodynamic design of low-speed aircraft with a ... - CAFE Foundation

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Max. D i ameter 50.88 a - .- FIG.^ - SCHEMATIC LAYOUT OF NASA WIND-TUNNEL TEST ~KODEL, SHOWING HULL, GONDOLA AND TAIL, REPRODUCED FROVI FIG, 1 OF NASA TN D-1026. .1c .08 CP .06 .04 .02 0 .6 .E 1.0 .1 .2 __~. ~ i'IG.2 - AXIAL FORCE CO€FF.Cn VS.PROPELLER ADVANCE RATIO. REPRODUCED FROM FIG.^ D) OF NASA TN D-1026 " c .6 .a 1.0 1.2 FIG,3 - PROPELLER POWER COEFF. Cp Vs. PROPELLER ADVANCE RATIO. REPRODUCED FROM FIG.^ B) OF NASA TN D-1026. 3
Table I NASA Test Body Coordinates Lenqth, in. Diameter, in. 1.4 1.2 1.0 24.624 36.304 36.936 42.564 49.248 47.020 61.560 48.885 73.872 50.149 86.184 50.753 98.496 50.880 110.808 50.880 123.120 50.633 135.432 50.642 147.745 160.056 48.888 47.200 172.369 49.732 184.68 41.299 196.993 36.904 209.305 215.461 225.000 227.773 233.929 240.085 246.46 31.426 27.867 24.564 20.977 16.677 9.493 0 Figure 2 presents the net body/propeller drag coefficient Cg = F/qOVo.@j against propeller advance ratio a = Uo/nd for 3 blade angles 15*, 20" and 25'; it is reproduced from Fig. 4d) Of Ref. 17. It can be noted that negative drag Coefficient Co values represent net thrust; it can be seen that more than enough net thrust was generated to tow two other identical bodies. Indeed the maximum thrust coefficient was Cg = -0.053 and the drag coefficient of the body (without propeller) was Cg = 0.021! Figure 3 presents the propeller power Coefficient Cp = HP/pn3d5 against the propeller advance ratio a = Uo/nd for 3 blade angles 15", 20' and 25O; it is reproduced from Fig. 4b) of Ref. 17. It must be noted that this power Coefficient Cp does not include any body dimensions as such; it has to be translated into the volume power coefficient CHP = HP/qoUoV0.66 so as to be used for a parametric fuselage analysis. Figure 4 presents the propeller efficiency 'I = ToUo/HP against the propeller advance ratio a = Uo/nd for 3 blade angles 15", 20" and 25'; it is reproduced from Fig. 4c) of Ref. 17. To is the actual measured thrust of the propeller, Uo is the measured free-stream velocity and HP is the measured shaft power to the propeller. The seemingly impossible evidence is that propeller efficiency is up to 122% for advance ratios yielding zero net thrust (equilibrium flight of body): this is conclusive proof that the wake-propeller does indeed operate within an area of lower velocities and that the efficiency does not have to decrease, as predicted by Lan and R~skam.'~ r .8 .6 .4 .2 0 .6 .8 1.0 1.2 1.4 FIG.4 - PROPELLER EFFICIENCY VS PROPELLER ADVANCE RATIO. REPRODUCED FROM FlG,q C) OF NASA TN 0-1026 W' However, the propulsive efficiency n = FoUo/HP is down to a mere 103% at equilibrium gecause the propeller-induced flow field aver the body causes additional drag, as compared to the bare body, or a so-called "thrust-deduction" of the propeller thrust. The same propeller, mounted in a conventional free-stream installation, achieved 73% propeller and propulsive efficiencies. Thus despite the "thrust-deduction." the power gain of the wakepropeller was an impressive 41% over the freestream propeller. This illustrates the power that can be extracted from the fuselage wake's kinetic energy and which is going to waste today in the general aviation aircraft. It can be added that the "thrust-deduction" is not a necessary evil: it can be eliminated by designing the body shape through a complex iterative process that includes the propeller's effect on the body pressure distribution and boundary-layer development. The final body shape is such that maximum pressure recovery is achieved on the fuselage's aftbody with the propeller, while the aftbody flow would be fully separated without the propeller's effect. This maximum pressure recovery is assumed to be that given by the Goldschmied turbulent separation criterion.27 The NASA experimental test data are tabulated below in Table I1 for convenient reference; in addition, the volume power efficient CHP has been computed from the propeller power coefficient Cp. w 4
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Aerodynamic design of low-speed aircraft with a ... - CAFE Foundation

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