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Aviation History
1959
1959 - 3474.PDF
•^ = 20 LB/FT2 1=50 FT FLIGHT, 25 December 1959 Fig. 77 f/e/f). Lift/drag ratios according to similarity rule 10 MACH NUMBER 15 Fig. 19 (below). Evolution of flat-top body Fig. 20 (centre). Estimated lift and drag of halfcones Fig. 21 (right). Trends in hypersonic aircraft 8, DEG HIGH DRAG HIGH LIFT-HIGH DRAG Fig. 18 (right). Range and heating of sub-satellite 100 r .5 1.0 1.5 C, ANC CD .25 .50 .75 RANGE/EARTH CIRCUMFERENCE in Fig. 16. The flat-top configuration has the highest L/D overthe Mach-range chosen, and at the design Mach number of 5 this ratio is only slightly less than 7, exceeding the flat-bottom L/D bymore than 15 per cent and the symmetrical configuration by over 20 per cent. At the design Mach number, theory and experimentare in essential agreement. Increasing the wing-tip droop angle was found slightly todecrease L/D for a slender configuration with sharp tips, whereas with a blunter configuration, having tips cut off normal to theflight direction, L/D at first increases and then decreases. Estimated maximum L/D over a wide range of Mach numbersare presented in Fig. 17. Base-pressure coefficients equal to 70 per cent of the vacuum value were assumed. The resulting base dragis the principal cause of reduced L/D at Mach number below about 7. At Mach numbers from 7 to 10, highest flat-top L/D areachieved (of the order of 6). With increasing Mach number above 10 there is a steady decline down to about 4 at M 20. This declineoccurs in spite of the increased slenderness of the vehicles, and it is due to the combined effects of decrease in lift-curve slope andincrease in the proportion of total drag which is friction drag. Accordingly, while flat-top configurations tend to retain theiradvantage in lifting efficiency at Mach numbers approaching 20, this advantage is small; it applies to reduced values of L/D; andit tends to be achieved with configurations which may be imprac- ticably slender. Low-L/D vehicles may be preferable here. Low L/D Configurations Rang: and heating per unit area of sub-satellites are shown inFig. 18. The results shown on the left of the figure are for a maximum glide speed equal to 0.9 satellite speed, and theindependent variable is L/D. The results shown on the right are for an L/D of 1, and the independent variable is maximum glidespeed. Increasing L/D from 1 to 3 at 0.9 satellite speed will not even provide semi-global range, whereas increasing the speed byslightly less than 10 per cent to near satellite value will easily provide semi-global range, or more. Of further importance is thefact that the heating penalty associated with this speed increase is far less than that ?ssociated with the increased L/D. Lift-drag ratio is important for other reasons related todecelerations and lateral manoeuvrability. Sub-satellite vehicles developing L/D near 1 are characterized by adequately lowdecelerations and adequately high lateral manoeuvrability. A high-lift, high-drag configuration for sub-satellite applica-tions can be derived by removing the upper half of a blunt body of revolution (Fig. 19). The lift and drag of half-cones, calculatedby Newton's impact theory, are shown in Fig. 20. Drag coefficient increases continuously with increasing cone angle while liftcoefficient reaches a maximum at a cone angle of 45 deg. As a result, L/D decreases with increasing cone angle from a valuenear 2 at 20 deg to zero at 90 deg. Heating per unit area increases markedly with increasing L/D, approaching values like 80,000B.Th.U./sq ft in the stagnation region and 40,000 on an average surface element at an L/D of 2. Therefore L/D should be nohigher than that required by considerations of deceleration and manoeuvrability. Equilibrium surface temperatures are observedto decrease somewhat with increasing L/D, being generally a little under 3,000 deg R. This is within the range of useful strengthsof ceramic, if not high-temperature structural, materials: and radiation cooling should—indeed must—be achievable to a highdegree. Even a small percentage of the heat load may amount to several thousand B.Th.U./sq ft of surface area, and so thecoolant required can add several pounds per square foot to an already relatively heavy structure. One method of converting a half-cone body to somethingresembling a flyable sub-satellite is to locate a small wing on top of the body, the portion of wing aft of the base being hinged toact as elevons. Stability about all three axes is derived primarily from the body, while damping in roll comes mainly from the wing.Generous nose-bluntness and filleting at the wing/body junction are provided to minimize heating problems in these regions.Experimental results, confirming Newtonian theory, give some assurance that high-lift, high-drag configurations of the flat-toptype developing L/D of the order of 1 may be designed with suit- able flying qualities at supersonic and hypersonic speeds. If theconfigurations are blunt to the degree of the one shown on the extreme right of Fig. 21, it seems unlikely that they will be capableof a conventional landing. If a conventional landing capability is important, then basically more slender configurations which candevelop higher L/D at low speeds may be necessary. It is essential, of course, to minimize hypersonic heating. In the sub-satellite portion of flight the vehicle flies at a high angle of attack, giving it apparent asymmetric bluntness to provide high lift andhigh drag. As speed is decreased and the trajectory steepens, the angle of attack of the vehicle is reduced to a value giving high L/D.The sinking speed is thereby reduced to allow normal landing. Summarizing, the trends of hypersonic aircraft configurationswith increasing speed and range are shown schematically in Fig. 21. The shaded area represents the speed/range corridor forthe glider-type aircraft. The boundaries of this corridor are largely determined by the requirement for reasonable payload. Flightabove the upper boundary markedly decreases payload, because L/D becomes so low as to require excessive take-off mass for agiven glider mass. Flight below the lower boundary has the same result at longer ranges because L/D becomes so high as to causeexcessive aerodynamic heating, with the result that substantial coolant must be provided to protect the vehicle. Flight below thelower boundary at shorter ranges is unattainable with known techniques for obtaining high L/D.The configurations that tend to fly within these boundaries are shown schematically in the shaded area. At first they increase inslenderness with increasing speed and range in order to maintain high L/D. At speeds up to about 18,000 feet per second, con-figurations with the body situated entirely beneath the wing may develop high L/D and may be evolved into something resemblinga complete aircraft configuration by drooping the wing tips. Configurations for higher-speed, longer-range flight may also havea flat top to provide high lift to decrease heating rates, along with high drag (by blunting) to reduce total heating.
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