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Aviation History
1960
1960 - 0864.PDF
872 FLIGHT, 24 June 1960 loaded Comet 4B is about 35kt at breakaway, dropping slightlyafter the initial roll. Blast while taxying in, being normally a function of landing weight, drops to less than 25kt at the samedistance. These figures assume nil surface wind, a smooth, level f^ apron, and the use of the minimum power required for themanoeuvre. Factors other than weight must also be taken into accountwhen an estimate is made of the blast likely to result from a given aircraft. As an example, the effects of engine installation caneasily be shown by a comparison of the Comet 4B with the Caravelle. Present gross weights for these two aircraft are158,0001b and 95,8001b respectively, yet there is only a small difference in the resultant blast; the gentler performer on theapron is in fact the Comet. It will be realized that if breakaway thrust required is 3 per centof weight, this thrust is the total produced by all engines. The number of engines available therefore requires each Avon in theCaravelle to produce some 2501b more thrust than does each Comet Avon. The rear position of the Sud aircraft's enginesfurther increases the blast velocity at a given distance behind the Apron Planning for the Jets FACING THE BLAST PROBLEM: WHAT PRACTICAL TESTS SUGGEST By R. A. TWOMEY* A IRPORT apron development the world over is b sing bedevilled/^ by jet blast. We read of tunnels, mobile lounges, towing *• •*- and blast screening, all proposed as solutions to a sup-posedly enormous problem, but all of them enormously expensive solutions! Is blast a big problem, or is it a bogy? With the jet re-equipment programmes of long- and short-haulairlines now well advanced, it becomes essential to answer this question. For jet blast is undoubtedly causing some airportadministrations planning difficulties, and a glance at the 1959 issue of IATA's Airport Buildings and Aprons-f will show thatapron layout has become a complicated matter. However, some national administrations and airlines have done a good deal ofresearch, practical or theoretical, into the blast characteristics of present and projected aircraft; and in spite of the many variablesin the equation a fair estimate is possible of the size of the problem, with the result that we may expect to reach reasonable decisionson die best way to lay out and use our aprons. Basic "Blastics" The speed and extent of jet blast dependon a number of factors: aircraft weight; the number, type and grouping of engines; apron surface and slope; ambient wind;radius of turn; and taxying technique. One of the most important parameters is the operating weight, which is the chief cause of thegreater blast of a Boeing 707 in comparison with a Comet. The average thrust needed for "breakaway" (the initial roll fromstandstill) is about 3 per cent of weight, and the average for taxying 2 per cent of weight. It has also been found that a simplesquare root law governs the relationship between thrust and blast velocity. As a result, the blast speed 150ft behind the jetpipes of a fully ^Reviewed in "Flight" for October 23, 1959. 281IO PIER FACE + 5 3D GPU PIER FACE +2S±IO Fig 7a. Comet ground handling trials: blast speeds (including effect of surface wind) during taxi-out. Averages and ranges in knots van &°ws*j>/> CD Cleaning//* *The author of this article is a memberof the BEAfTght operations department, but the opinions he expresses are per- sonal ones. Fig 1b (left). Comet blast speeds dur- ing taxi-in. Conditions as in Fig la
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