FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1953
1953 - 0563.PDF
320 340 360 380 TAS.(kt) 400 420 440 Fig. 3 (above) shows typical performance-curves based on T.A.S. Fig. 4 (right) illustrates the variation of cruising performance with the wind component. O 16 OI4 0-12 OIO 0-18 0-06 \ \ \ \n \ \ ^> ^^^ I ^ste 'SO*-T - «tai _T^ i ^^i^ 140 160 180 200 E A S (kt) 220 240 260 557 CIVIL JET OPERATIONS the importance of small changes in specific range when expressed in terms of payload. It will be appreciated, however, that quite a lot of flexibility exists in the choice of a satisfactory procedure. It must also be remembered that the choice of a cruising procedure cannot be divorced from the climb fuel needed to reach its starting point. A small gain in specific range at altitude on a short sector can be com pletely discounted by the extra fuel needed to climb to a higher initial cruising level. To some extent the optimum procedure has consequently to be related to stage length. Figure 3 shows specific range against true air speed for similar tem perature conditions. It will be seen that there is a very heavy fuel penalty if an operation is attempted away from the best range conditions, to gain a worthwhile increase in speed. On current operations with the Comet 1, B.O.A.C. is using a constant incidence cruise procedure, achieved by flying set values of indicated air speed against aircraft weight, an adjust ment being made every half hour. For a given incidence and set engine r.p.m. it is possible to consolidate all the cruising information on to a single graph. From this it can be seen that variations in tempera ture have surprisingly little effect upon cruising endurance; they do, however, have a considerable effect upon cruising height. It might be supposed from this that flight under conditions of pronounced horizon tal temperature gradient, causing rapid changes in cruising level, would show marked deviations in performance from normal conditions. So far it has not been possible to detect any such effect. Under constant incidence cruise conditions at set engine r.p.m. it can be shown quite simply that both true air speed and Mach number are uniquely related to ambient temperature only, irrespective of aircraft weight or cruising height, provided that the flight is under stabilized conditions. These relationships enable useful simplifications to be made in the tabulation of cruise control data. It should be noted that Mach number increases as ambient temperature falls, and that operation below a given temperature may lead either to an excessive drag rise, or to con trol difficulties. Such a condition is not encountered on the Comet 1, but if it were it would then probably be best to abandon the constant ircidence technique upon reaching a suitable Mach number, thereafter ooeratine at this constant Mach number. CRUISE CONTROL IN PRACTICE.—Although operations can be conducted from a suitable set of performance graphs, similar to the figures already shown, this does not provide a very good method of presentation for practical use. Experience has shown that tabular information is less likely to be misinterpreted, and is generally easier to use. The effect of a wind component upon the cruising procedure is shown in Fig. 4. Even a headwind component of the order of 100 knots makes very little difference to the optimum cruising conditions and, as far as Comet I operations are concerned, it is current practice not to alter the standard procedure even when extreme conditions are encountered. It might be expected that there would be a potential gain in crflising at a different level under strong wind conditions, particularly if the wind were rather localized, as in a jet stream. This is not so as this type of air craft is particularly inflexible as far as altitude is concerned. The normal cruising level is very close to the absolute ceiling, precluding any attempt to climb out of strong wind conditions, and the range penalty for descending is so heavy that the headwind component would have to fall off by considerably more than 10 knots per 1,000ft loss of height to justify descending. The only correct technique to avoid jet stream winds is by flying out of their effect horizontally. Before leaving the subject of cruising some reference must be made to the subject of engine failure. If an engine fails at cruising height the air craft will then be above both its three-engined ceiling and the optimum level for a three-engined cruise, which for the Comet is about 10,000ft lower, for a given weight. Although specific range under stabilized three- engined cruise conditions is seriously below that for a normal operation, being about 20 per cent less, the benefit of the excess height at engine failure considerably reduces the penalty. In order to obtain the maximum benefit from the extra height, a carefully controlled descent to the three- engined cruise level is made, taking about an hour; for ease of reference this controlled descent is now known as a "drift-down." CLIMB AND DESCENT.—A much larger proportion of a normal flight is devoted to climbing to cruising level and descending from it than is normally the case with a piston-engined propeller aircraft. The operation of a jet aircraft bears a much closer resemblance to the flight of a projectile than does that of traditional types. For this reason the overall efficiency can be considerably affected by using different techniques for climb or descent. As specific range is extremely poor at low altitudes, there is an obvious requirement to climb as fast as possible directly after take-off, and maximum permissible engine r.p.m. is used for this purpose. Considerable latitude exists in the choice of climbing speed as far as performance is concerned; a fast air speed on the climb being theoretically better than a slow one at any given point for specific range at a reasonable rate of climb, but demanding a prolonged period of flattened climb at low level, and excessive engine consumption, in order to gain the initial speed. Such a climb also suffers from the dis advantage of possibly providing an operation at speeds well above those recommended for use under turbulent conditions, which are not infre quently encountered in the earlier stages of climb after take-off. As far as descent is concerned, the most efficient method is a long shallow operation, which, on range alone, provides a gain of about 20 per cent over normal cruising for an equivalent distance. However, if height is held until nearly overhead destination, and a rapid descent is then made, there is a penalty in view of the fuel burnt during this rapid descent, it being impracticable to shut down the engines owing to the requirements of pressurisation and other ancillary power demands. This further penalty lifts the total fuel requirement for a delayed descent to a figure about 40 per cent above that needed in the optimum case, not taking into account the maximum descent rate which can be tolerated in the pas senger cabin of a public transport aircraft, which would still further increase the penalty. The importance of this limitation will be under stood from the example of the Comet I, which is pressurised to a maxi mum working differential of 8 J lb/in2, providing a cabin altitude of 8,000ft at a cruising level of 40,000ft. It follows that the time taken to descend from 40,000ft must not be less than that needed to bring the cabin from 8,000ft to surface pressure. A seriously delayed descent, making allowance for this requirement, can impose an even greater penalty than the examples already given. Nevertheless, such a delayed descent has the operational advantage of preserving altitude, and there fore diversion range, to a much later stage in a given operation should weather conditions for landing be doubtful at destination. As in the case of the climb, speed on the descent is not very critical, and a fair latitude exists in making a suitable choice of procedure. TAKE-OFF.—Take-off technique on a civil airliner is, to a large extent, governed by mandatory performance requirements laid down in WHEEL BRAKE LIMIT- TAKE-OFF DISTANCE CHOSEN WEICHT Fig. POWER FAILURE SPEED RATIO V, /, 5 shows the curves from the Comet flight manual—in simplified form—upon which are based take-off calculations.
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
