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Aviation History
1956
1956 - 1352.PDF
504 FLIGHT, 14 September 1956 CORRESPONDENCE . . . the Viscount, Vanguard, Britannia and Comet 4A. If we canmake four medium-haulers, surely we can produce just one long- hauler? And whilst on the subject let us remember that thelong-hauler of today often becomes the medium-hauler of tomor- row. Dakota, Constellation, DC-4 and 6, all were the pride ofthe trunk routes but are now frequently used as medium- and short-haulers; and now the Come: and Britannia join in. Itwon't be many years before we see the DC-8 and Boeing 707 on the shorter routes. If the Comet can be made into an economi-cal proposition over 200-mile routes, then why not the DC-8 or Boeing 707? And as the American air seems to be overcrowded,the answer to expansion there seems to be the same number of bigger machines, and not more 60-seaters. Agreed thai this maybe where we score, as the rest of the world may not be ready for 120-seat aircraft yet; but if we want to stay in the forefront, wemust build for the trunk routes. It should not be very difficult for de Havilland to produce a Comet 5 for sale in 1961; and theyhave the advantage of knowing the opposition, as they can build better landing and take-off performance into their aircraft—andperhaps other advantages, such as larger freight holds. Rumour already has it that the Comet 5 is in hand. Many operators,including Commonwealth lines, have not placed orders for big jets yet. Do not prejudice a possible Comet 5 by your remarks. As for B.O.A.C., their equipment policy is very difficult tofollow. They order Britannias and then order Seven Seas as an insurance policy against the Britannia—which didn't help Bristolsin their sales drive. Now they will be introducing the Britannia and Seven Seas simultaneously, one wonders if they intend tocompete against each other to find the best one. Next, they wish to order American jets for delivery in '61, and one can but wonderif they would order a British jet just in case the DC-8 fails! Admittedly B.O.A.C. have stood their ground with the Comet,and credit must be given to them for that, but if they won't play ball with the big jets, then we must do as "Favonius" says andremember that there are other long-haulers in the Empire— Australia, South Africa, India, Pakistan and Canada. Surely thereis enough market here to encourage the British industry? I am forced to the conclusion that your editorial idea that wemust leave the big jets to the Americans is solely sour grapes. If we had a big jet coming along the Editor would be the first tocheer. Remember that our industry was large enough to pioneer the turboprop and the 100-seater aircraft, as well as pure-jet trans-ports. We have made mistakes, but we have learnt much and must put this new-found knowledge to work. We can and mustrival the DC-8 and Boeing 707. Does any one really believe that the big jet market is half sold out already? The market isgrowing and there will be many repeat orders. Supersonic travel may be just around the corner, but before it comes we shall see the200-seater subsonic jet built in large numbers. Let us have a share in that market. London, W.6. AMATEUR. Identification by Contrails T CANNOT agree with a number of the statements in the AircraftA Recognition Society's paper on contrails, reproduced in your August 3 number.Briefly, persistent trails can be formed at all low altitudes down to sea level (I have seen them formed by light training aircraftdoing circuits) but they are virtually impossible above 45,000ft or at most 50,000ft. At heights up to about 20,000ft the exhaustplays a negligible part in their formation, which is entirely due to the passage of the aircraft causing condensation in super-saturatedair. Such trails start at the aircraft; their shape depends on the wing loading pattern. Frequently they are twin-headed, trailingfrom the wing tips and occasionally from the airscrews, but the really persistent trail developed from super-saturated air startsas a broad band from every part of the aircraft, except perhaps for dark bands where the heat of the exhaust jets re-evaporatesthe condensation. Above 20,000ft the water vapour in the exhaust begins to playa part in forming contrails and above 35,000ft is almost the sole cause of these trails, but there is an important difference betweencontrails formed by initially supersaturated air and by exhaust vapour. Exhaust formed contrails never start less than about50ft behind the jet pipes and sometimes the contrail head may be 100 yards or so behind the aircraft. A turbine engine burning heptane will theoretically produce1.441b of water vapour for every pound of fuel burnt. The air/fuel ratio of the combustion is about 60 : 1 so that the weight of watervapour in the exhaust is about 165 grains Der lb of air. This seems a lot of water but the exhaust is very hot, 500-600 deg C,and can only be cooled by mixing with the ambient air. The weight ratio of the ambient/exhaust air mix will have to be about100 :1 to bring the mixed temperature to within about 5 deg C of the ambient temperature and this will reduce the excess waterto about li gr/lb. - i.. . It is quite common, in mixing two masses of air (both initiallyfairly dry and giving a final mixture which is relatively dry) to cross the saturation line during the mixing process. This is oneof the bugbears of cabin conditioning. It is frequently impossible to inject even completely dry cold air into a cabin without a fogarea during the mixing process. In considering whether exhaust water vapour can form condensation when mixing with ambientair it is really necessary to go through each step of the mixing process to find if the saturation line is approached or crossed, butsome generalizations are permissible, and a broad picture of the possibility or otherwise of contrails can be formed by consideringsay a standard I.C.A.N. day in which the temperature falls 2 deg C per 1.000ft up to 36,100ft, above which it remains constant at— 56.5 deg C. The following table gives the vapour pressure of ice in mm Hg and the grains of water-vapour/lb dry air for thestandard day: — Vap. press, ice (mm Hg) ... grains/lb Height (ft • 1,000) 20 •48 6 25 •17 2-6 30 •057 1-1 35 •017 •41 36 1 •014 •36 40 •43 45 •55 5u 70 60 1-13 70 1-84 80 2-96 It will be observed that the quantity of water vapour which theair can hold at saturation reaches a minimum of 0.36 gr/lb at 36,100ft and then increases continuously at higher altitudes reach-ing 3 gr/lb at 80j000ft. In practice it is exceedingly unlikely that the moisture content ever exceeds 0.5 gr/lb in the stratosphere.There is continual exchange of air between the upper and lower regions of the atmosphere and any air in the stratosphere must havepassed through a minimum value to get there. In passing through this point the excess water would normally be condensed out intocloud. Now consider the mixing process between exhaust and ambient. At 20,000ft an increase of 5 deg C in temperature wouldincrease the saturation capacity of the air by about 7 gr/lb, but at this point the exhaust mix would only have an excess of aboutli gr/lb. The saturation curve would be approached only during the mixing process if the air were initially on the point of satura-tion and the exhaust mixed with 500 to 1,000 times its weight of ambient air, and in this region the excess of water vapour abovethe saturation level, if any, would be very small and unlikely to form a visible trail. The position is quite different at 30,000 to35,000ft, where the temperature is much lower and the saturation curve will be crossed at a comparatively low ratio of the ambient/exhaust mix; but at 50,000ft, where the air will be of low relative humidity, the saturation curve will not be crossed except possiblyduring a very short period of the mixing and any temporary condensation would rapidly re-evaporate and disappear. It is a well-known law that adiabatic compression of air driesit or reduces the relative humidity and adiabatic expansion increases the relative humidity. An aircraft with a normal wing loadingmay have negative pressures of \ lb/sq in over portions of the wing and in the wing-tip vortices. At 10,000ft such a suctionwould correspond to a temperature drop of 4 deg C and a drop of 6| from 25.2 to 18.7 gr/lb in the possible water content of theair without super-saturation or condensation. At this height relative humidity of the air is normally fairly high and condensa-tion in the wing-tip vortices would be almost normal except for an aircraft with low wing-loading. Such condensation wouldre-evaporate very rapidly if the ambient air was below saturation but if any degree of super-saturation existed the persistent typeof contrail would be formed. Andover, Hants. T. P. DE PARAVICINI. FORTHCOMING EVENTS Sept.!0-16. Battle of Britain Week. ?ept. 12. Kronfeld Club: Talk on Popular Flying. Sept. 14-16. International Air Rally, Innsbruck. Sept. 15. Battle of Britain Day Sept. 16. All-Britain Model Aircraft Rally, Radlett. Sept. 17. Aircraft Recognition Society: Lecture by Capt. 0. P. Jones, O.B.E.. M.V.O. Sept. 18-23. Aero Club of Italy: International Air Week. Sept. 19. Kronfel'd Club: Film Show. Sept. 19-21. R.Ae.S. and Institute of Physics: Conference, Cranfield. Sept.22-23. Women Pilots' Association: Flying Meeting, Sandown. Sept. 23. Air Display, Frankfurt. Sept. 25. Society of Instrument Technology: "Accurate Calibration of Flowmeters," by E. A. Spencer, B.Sc.(Eng.), Ph.D., A.M.I.Mech.E., and A. T. J. Hayward. Sept. 25-29. 2nd European Aeronautical Congress, Scheveningen. Sept.27-28. Aerodrome Owners' Association: Annual Conference, Southampton. Oct. 1. R.Ae.S.: Main Lecture at Derby: "First Sir Henry Royce Memorial Lecture," by H. I. F. Erernden, M.B.E. Oct. 4-5. Air Traffic Control Convention. Southend. Oct. 6. British Interplanetary Society. "Study of the High Atmos- phere in the International Geophysical Year," by Prof. H. S. W. Massey, F.R.S. Oct. 9. R.Ae.S.: Section Lecture: "The Pressure Jet Helicopter," by A. Stepan, Dipl.-lng. Oct. 11. R.Ae.S.: Main Lecture at Hentow: "Rocket Motors," by S. Allen, F.R.Ae.S.
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