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Aviation History
1959
1959 - 3473.PDF
778 FLIGHT, 25 December 1950 HYPERSONICS . . . A rather longer paper was contributed by A. J. Eggers Jr., ofthe Ames Laboratory of the U.S. National Aeronautics and Space Administration. He opened by suggesting that the hypersonicglider may be an attractive long-range aircraft. A boost/glide trajectory is shown schematically in Fig. 12. Inthe boost phase relatively little range is achieved, while speed and altitude are increased in such a manner that at the end ofboost the sum of aerodynamic lift and centrifugal force (due to the curvature of the flight path) just counterbalance weight. Themajority of the weight is supported by aerodynamic lift at speeds less than about three-fourths minimum satellite speed (i.e.,18,000ft/sec) while the majority of the weight is supported by centrifugal force at higher speeds. This fact is fundamental. Variation of maximum speed with range, of gliders developinglift/drag ratios from 1 to 6, is shown in Fig. 13. Increasing L/D reduces the speed required for a given range—substantially, forranges of the order of one-fourth the circumference of the Earth, but by decreasing amounts for longer ranges to the point wherethe reductions are relatively minor at global range. Increased L/D reduces the ratio of take-off mass to glider mass; but thisreduction, like the reduction in velocity, is largest at ranges less than semi-global and falls off with increasing range. The most promising technique for minimizing the quantity ofheat which must be absorbed by a hypersonic glider type is to radiate it from the surface at a rate approaching, if not equalling,that of the convection process. For a vehicle with L/D = 1, calcula- tions have shown that increasing L/D may markedly reduce unitarea heating (and hence heat absorbed) at ranges less than one- fourth the Earth's circumference, whereas at longer ranges itincreases this heating (particularly at semi-global distances and above). At ranges up to one-fourth the Earth's circumference thelarge velocity reduction with increasing L/D does, in fact, tend to dominate the heating problem. At longer ranges the reductionin velocity due to increased L/D is substantially smaller, and the fact that the lower-L/D vehicles fly at, higher altitudes in thinnerair to achieve a given range is the dominant factor, with the result that they experience substantially less heating. Since increased coolant means decreased payload, high-L/Dconfigurations have relatively attractive payload capabilities in hypersonic flight over intercontinental distances. Over longerdistances increased L/D loses much of its effectiveness in increas- ing glider mass, and payload may actually have to be reduced inorder to accommodate more coolant. High-L/D Configurations An aircraft developing high L/D ratios will be slender, and willdevelop these ratios at small angles of attack. The body should, of course, have low pressure-drag and be shaped (as far as possible)to achieve high in-flight stability. Bodies in which the cross- section area continually increases from nose to tail have the virtueof low drag at hypersonic speeds, along with the flare effect which contributes to stability. For simplicity, one may consider such abody of revolution mounted symmetrically on a thin wing at zero angle of attack. A front view of this arrangement, along with thedisturbance velocities created by die body, is shown at the top of Fig. 14. In order to achieve high L/D, the components of anaircraft should be individually and collectively arranged to impart the maximum downward and the minimum forward momentumto the surrounding air; the upper half of the body should there- fore be eliminated. The wing now serves the important functionof preserving the downward momentum of the air disturbed by the lower half of the body. The elementary momentum principlesuggests that the wing leading edge should coincide with the shock wave created by the body. The wing should extend downstreamtoward, but not beyond, the line along which the body ceases to impart downward momentum to the fluid. Accordingly, thewing trailing edge should, like the leading edee, be swept back and it should join with the body at its base. The configurationshown on the lower right of the figure satisfies the condition of high leading-edge sweep to minimize heating in this region, andthe low aspect ratio is favourable to minimizing structural weight. Something more may be learned, however, by again viewingthe configuration from the front. Such a view is shown in the upper left of Fig. 15. It is observed that the body imparts lateralas well as downward momentum to the surrounding air. Accord- ing to the momentum principle, this lateral momentum should beconverted into downward momentum, and this may be done by deflecting the wing tips downward about hinge lines in the streamdirection (as shown in the upper right). In this location the drooped tips can serve two functions. One, of course, is to increaselift. Also, and perhaps more important, they are suitably located to provide directional stability and control for the configuration.This gives the crude semblance of a complete aircraft configura- tion, of the flat-top type with a laterally symmetric fuselage. The most important question is, of course, do configurations of this type actually develop high L/D at hypersonic speeds*Maximum L/D of flat-top conical configurations have been calculated for high Mach numbers. In the calculations, base dragwas neglected and a 5 deg half-cone body was used. The win». trailing edges were formed by straight lines swept back from the-body base and intersecting the leading edges 1.4 body lengths afr of the vertex. Plan-form changes with design Mach number. With increasiritMach number and skin friction, there is a reduction in maxirruur L/D. But if skin-friction cofficients are less than 0.006, the flat-top configurations should be able to develop maximum L/d of the order of 6, if not greater, at Mach numbers up to 10. The flat-topconfigurations are consistently capable of achieving higher L/D than corresponding flat-bottom configurations, and this advantageincreases substantially with decreasing friction drag. Some measured maximum L/D for this type of configuration are shown GLIOE L=W(l-V2/v|AT) VSAT = 26,000 FT/SEC BOOST Fig. 12. Boost/glide flight trajectory Fig. 13. Variation of speed with range 25 .50 .75 RANGE/EARTH'S CIRCUMFERENCE Fig. 14. Evolution of flat - top wing/body combination WING Fig. 15. Evolution of flat-top aircraft configuration Fig. 16. Measured maximum lift-drag ratios 6 (ttMAX / 25.2T
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