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Aviation History
1955
1955 - 0246.PDF
246 FLIGHT TODAY'S RESEARCH FOR TOMORROW'S TRANSPORTS . . . that, under repeated loading within the elastic limit as ordinarilyunderstood, slip occurs in die crystals of the material. We have been able to observe a number of metallurgical changestaking place in the alloy before fatigue cracking occurs. One of these is the exudation of thin metallic ribbon from the crystalsurfaces as shown at a magnification of 1,500 times in Fig. 14 (a). These ribbons leave a crevice in the surface which may sub-sequently develop catastrophically. Fig. 14 (b) shows a crpck caught in the act of proceeding along such a region.It is well known that, in aluminium alloys, high temperature promotes precipitation of the alloying atoms, and produces a softmatrix of practically pure aluminium in which are embedded large aggregates of the precipitates. One explanation of the exudationphenomenon induced by repeated loading is • that the slip pro- duced in the crystals causes local high temperatures, which inturn cause local precipitation of the alloying atoms. The soft spots so produced are then extruded under the action of thealternating stress, and the crevice is formed, causing stress con- centrations, which excite the phenomenon further, so extendingthe crevice, which eventually becomes a fatigue crack. If this explanation be correct, a higher fatigue resistance couldresult were it possible to choose alloying constituents which will not precipitate under these circumstances.Noise from Aircraft.—In this section of his lecture, Sir Arnold Hall began by discussing the differences in noise made by airscrewdriven and iet propelled types of aircraft respectively. As cruising speeds are increased [he said], it is desirable to reduce therotational speed of propellers in order to avoid loss of efficiency, and it is likely that at take-off the external noise of high-subsonicpropeller-driven aircraft will not be greater than that produced by present-day types. The noise in die cabin whilst climbingand cruising may well be a problem, because much of the noise generated by a propeller comes from the low-frequency com-ponents of the rotating pressure-fields which are produced as the blades rotate, and such low-frequency sounds are not greatlyattenuated by sound insulation; their control to reasonable limits is likely to involve an appreciable weight penalty in soundproofing. Recent mathematical investigations which we have made havesuggested that the noise may be significandy reduced by changing the distribution of the thrust along the blade from mat at presentused, putting more of the load towards die root. A reduction of noise by about 5 decibels seems to be possible by diis means.The resulting blade may well be more difficult to manufacture than existing types, but there would be a substantial reward. The sources of noise from a jet, as suggested by mathematicalwork by Professor Lighthill, of Manchester, can be described in rough terms as follows. From a subsonic jet, there is a high-frequency component of noise, which has a directional maximum of about 45 deg to the axis of the jet; the frequency of the sound is about 0.7 — and more (when U is the velocity of the jet, and d its diameter) a typical figure being 500 cycles per second (1 octaveabove middle G) or above. In addition, there is a low-frequency component, which has a directional maximum mainly along the direction of the jet; the frequency is about 0.3 — and less— atypically 250 c.p.s. (middle C) and below. If the jet is supersonic —as it is likely to be during the climb and cruise—there is, inaddition, a furdier component with a directional maximum about 100 deg from die jet axis. All three are highly directional.The high-frequency 45 deg sound arises from the highly sheared regions at die edge of the jet, where violent mixing istaking place between the jet and the surrounding air. The low- frequency, along-axis sound arises from die turbulence withinthe jet itself. The additional sound at 100 deg from a supersonic jet arises from interference between the eddies and the standingshock-waves in die jet. The intensity of the noise is proportional to the eighdi or higher powers of the velocity—if the velocity is doubled, the noiseincreases at least 250 times. It will, therefore, be beneficial to use engines such as the by-pass type, which for a given thrustproduce a lower jet velocity dian the simple tunbojet. It has been found that useful reductions of jet noise can beobtained by inserting "fingers" into the jet at the lip of the jet pipe, or by using a pipe corrugated around its tip. These devicesprobably affect the noise by causing a spreading of the jet, so that die velocity is reduced more rapidly widi distance downstreamthan would be die case in their absence, widi die benefit to noise generation represented by the "eighth power of the velo-city" law. In addition, they probably tend to reduce the direc- tionality of me noise, so reducing die peaks of intensity. Navigation and Communication.—In introducing this sectionof his paper die lecturer remarked that die elimination of accident due to impact widi terrain, and accident or discomfort due toentering intense storm, would be considerably assisted by the use of micro-wave radar in civil aircraft. Suitable equipment, oper-ating on a frequency high enough to obtain good echoes from storm centres, was now becoming available. Such aids, togetherwidi long-range radio navigational facilities of increased accuracy, should eliminate accident due to navigational error and storm. A recent development in communication equipment which wehave been able to make [he continued] may prove of value to civil aviation. The traditional system for communication overdistances more dian a few miles has been manual telegraphy in die 3 to 30 mc/s band. Radio telephony, with its advantage ofbeing available for direct use by die pilot, radier than through a radio operator, has only recently been relied upon by some air-line operators, using more powerful amplitude-modulated equip- ment, identical in principle widi domestic broadcast equip-ment, than has hidierto been available. We have been experi- menting widi a system of a different type, which uses transmissionon a single sideband. The work has shown that considerable improvement is obtained by diis means, as compared with dieperformance of conventional equipment, the frequency of success- ful message transmission under difficult conditions being threetimes improved. . . . In die ordinary amplitude-modulated transmission, the carrierwave conveys no information, and the pair of sidebands duplicate the message. By suppressing one sideband, the band-width neededfor the transmission is halved, and die probability of jamming by odier signals is correspondingly reduced. Signal-to-noise ratiois improved, mere is absence of selective fading, and improved intelligibility at low signal to noise ratios. The penalty for diesefeatures lies mainly in die need for considerably improved fre- quency stability in the transmitter and receiver, but this require-ment is widiin the performance of mass-produced crystals, pro- vided diey are temperature-controlled. The Landing Operation in Bad Weather.—The basis of dietechnique which has been evolved, and is now widely used, for low visibility, or low cloud-base, landing is this. The aircraftmakes an approach to the runway down an approach path which is denned for die pilot either by a radio-beam (I.L.S.) or by verbaldirections from die ground, based on die observation of the aircraft by high-precision radar (G.C.A.). The runway is equipped wididie cross-bar lighting system, which we introduced a few years ago, and which has proved itself capable of giving the pilot dievisual data on which to position himself for his final approach and touch-down. The geometrical layout of diis lighting systemis shown in Fig. 17. The pilot changes over from die I.L.S. or G.C.A. system to the cross-bar lighting system, thereaftercompleting his landing visually as soon as he has adequate visibility of the lighting system; but if he has not acquired this visibilitybefore he comes down to a particular altitude known as the "critical height" he breaks-off his approach to the runway, andflies over the airfield, thereafter awaiting an improvement in visi- bility or making a landing at an alternative airport. The break-offheight is determined for each type of aircraft to ensure that a safe "fly-over" can be made. The pilot is informed, before startinghis approach, of die "runway visual range," this being a measure LINE DEFINED BY TAXYING LEADER CABLE SYSTEM LINE DEFINED BY LANDING LEADER CABLc SYSTEM Fig. 17. Phases of the approach and land- ing manoeuvre: A-B, inbound leg (manual, or automatic height control and auto-direc- tion from I.L.S.); B-C, final leg (manual on I.L.S. or G.C.A. information, or auto approach or approach coupler); C, critical height (transition zone); C-D-E, landing phase (manual, based on vision aided by approach and runway lighting and Fido, or auto-landing based on such system as leader- cable); E-F, taxying (vision or leader cable). APPROACH LIGHTING
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