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Aviation History
1947
1947 - 0944.PDF
•55O FLIGHT JUNE 12TH, 1947 As in the longitudinal motions, theamplitudes of response of an aircraft in lateral motion are probably as importantas the damping rates in determining free- flight characteristics. All-wing aircraftseem slightly rougher in turbulent air than conventional aircraft of similarweight. This is due chiefly to the re- duced wing loading, but high effectivedihedral and low weathercock stability may have an added effect. This is amatter of interest in fixing upon analytical criteria for the description offree-flight qualities. As mentioned above, increasing the weathercockstability for all-wing aircraft has a slight effect on the damping rates; however, itaffects the amplitudes of response to gusts materially. The application of automatic pilotcontrol to an all-wing aircraft has certain difficulties which are associated primarilywith the low value of C v/8. In conven-tional applications the fact that the air- craft is side-slipping is detected by eithera lateral acceleration or an angle of bank. In an all-wing aircraft neither of ithese indications exists except in an almost undetectable amount. Accord-ingly, it is necessary, in order to fly the aircraft at zero sideslip, and thereforein the direction of its centre line, to provide a yaw-vane signal to which thepilot or automatic pilot will respond. This introduces some difficulty in auto-matic pilot design because for small disturbances the sideslip angle withrespect to the wind, and the yaw angle with respect to a set of fixed axes, arenearly equal and opposite for a flying wing. The customary automatic pilotcontrol on azimuth angle therefore tends to oppose the necessary control on side-slip. To avoid this difficulty it is neces- sary only to reduce the rate of controlon sideslip to approximately one-third that on azimuth. This modification toa conventional automatic pilot was flown on the N9M with complete success.As has been pointed out previously, the permissible range of C.G. locationis not overly critical in all-wing aircraft. It is, nevertheless, of great advantage tobe able to load the aircraft almost at will, without concern as to how the use-*ful load is disposed and the swept-back configuration lends itself most suitablyto such loading. XB-35 Load Distribution In the case of the XB-35, the usefulload, consisting largely of bombs and fuel, can be readily disposed in suitablepositions about the C.G. While some fuel is located well forward and somewell aft of the desired C.G. location, under normal operating conditions theproper balance is readily maintained. In case of failure of one or more engines,it is necessary to pump the fuel from unused tanks to those supplying theremaining engines, but a simple mani- folding system provides this facility. Based on a great many studies ofvarious types and applications of the all- wing principle, some practical limita-tions may be approximately defined. Where very dense (high specific gravity)payloads are contemplated, such as war- heads or similar munitions, quite smallunits are practical, as demonstrated by the all-wing buzz bombs to whichreference has been made. Mediurn-sized units having a span of perhaps 100ftand a gross weight of 50,000 to 60,000 1b appear entirely practical for medium ALL-WINC AIRCRAFT bombers and freighters. Here again thedensity of the useful load, both in pay- load and fuel, is comparatively high. An aircraft of the XB-35 configurationand size can carry 50 passengers in com- fort in the existing aerofoil envelope withadequate headroom for all, and with vision forward through the leading edge,downward through windows in the floor, and upward if desired. Passenger visionin a flying wing may be more satisfac- tory than in conventional types it we getused to the idea gi forward vision rather than that provided by side windows.The really interesting views are likely to be forward and downward rather thanto the side. An aircraft like the XB-35 will have cargo space for 40,000 to Thickness a; RootTaper Ratio Thickness RatioAspect Ratio Root Chord Ratio Span Ratio VoLme Ratio CONVENTIONAL1 2:1 15% 1 1 1 DELTA1 00 75% ' 1.S 2 0.75 0.833 Fig. 14. Comparison of subsonic and supersonic all-wing aircraft. 50,000 !b of air freight at a density of10 to 15 lb per cubic ft in addition to the necessary crew and space for 50 pas-sengers "Turning to future possibilities," Mr.Northrop went on to say, " considerable further aerodynamic refinement can bemade over that already accomplished in all-whig types. Particularly if turbo-jets are used as the motive power, the minimum parasite drag may be reducedto 0.008 or less. Boundary layer re- moval and the use of somewhat thinnerwing sections may further appreciably reduce this figure. '' A maximum trimmed lift coefficientof 1.9 for the all-wing configuration seems attainable by methods alreadysuggested and possibly may be further increased by judicious use of boundarylayer control in combination with turbo- jet power plants. It is our opinion thatthe ratio of C Lmax to Cj>m,u may be in-creased to a value of 235 within the not- too-distant future from our presentactual achievement of about 130. In contrast, the years of intensive develop-ment of the conventional types already passed promise an improvement of lessmagnitude within a comparable time. In our judgment a trimmed maximum liftof 2.8 vs. a minimum drag of 0.020 seems reasonable to expect for large, long-rangetransport and bombardment aircraft of conventional type. "These estimates are, of course, com-pletely arbitrary and controversial. However, if one cares to assume theirvalidity, the following conclusions may be reached, based on methods and cal-culations used in the early part of this paper. The total minimum profile dragon the all-wing aircraft in terms of the conventional will be from 40 to 59 percent. The power required by the all- wing to maintain the same cruising speedas the conventional will be from 70 to 80 per cent and, conversely, the maxi- mum range of the all-wing, at thecruising speed of the conventional air- craft, will be 143 to 125 per cent. Themaximum range of the all-wing aircraft at its best cruising speed will be 158 to130 per cent of the conventional, and the most economic speed will be from 125 to115 per cent faster. '' Under high-speed conditions corre-sponding to full power of reciprocating, turbo-prop or turbo-jet engines, wherethe induced drag is assumed to be 20 per cent and the parasite drag 80 percent of the total, the power required to drive the all-wing aircraft at the speed ofthe conventional aircraft will be 52 to 67 per cent and, conversely, the rangewill be 192 to 149 per cent of the conven- tional aircraft. The maximum speed ofthe -all-wing aircraft at comparable powers will be 124 to 114 per cent of itsconventional counterpart. These values are shown in Fig. 13, to give an idea ofwhat possibilities for improvement appear reasonable in the next few years." So far in this discussion we have pur- posely avoided transonic and supersonicconsiderations. This neglect is possibly a reasonable one when discussing com-mercial ventures, in view of the cost of higher and higher speeds. A reasonabledegree of sweepback, such as is required in the type of aircraft under considera-tion, will permit speeds up to about 500 m.p.h. without involving great com-pressibility drag increases. For military aircraft, however, we cannot ignore thesonic ' barrier' and its implications, and it is a reasonable assumption that •sooner or later improved fuels will permit higher and higher operational speeds,even in commercial aircraft. Based on present knowledge of super-sonic flight, it will always be more diffi- cult to carry a given payload for a givenrange at supersonic speed because of the additional wave drag encountered atthese speeds. At transonic or compara- tively low supersonic speeds a plainswept-back wing appears to be one of the best possible configurations, providedthat sufficient volume is available within the wing. Since the flow normal to theleading edge is subsonic over almost the entire wing surface, subsonic aerofoilswith reasonably good subsonic flight characteristics can be used at thesespeeds. The all-wing design eliminates wing-fuselage interference as well asadverse interference between the tail sur- faces and wing or body. Supersonic Types '' At higher supersonic speeds the prob-lem of providing adequate volume is more yl difficult because more and more fuel fsjlj*required for a given range, and the per-1 centage thickness of aerofoils suitable for 'such use is much less than that satisfac- tory for subsonic flight. Save for onecompensating factor, this problem of " volume and size might well rule out theall-wing aircraft for supersonic use, and certainly does limit its usefulness for low-altitude flight. However, an attractive field of operation exists at very high alti-tude where air densities are low and. therefore, wing areas must be comparablygreat if suitable lift coefficients are to be maintained. If we design a franklysupersonic aircraft to fly at, say, a Mach number of 1.6 with supersonic diamond-section aerofoils, the maximum cruising lift coefficient will probably be no greaterthan 0.15, and the corresponding loading must be held to 40 lb per sq ft. The above figures are based on assumed
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