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Aviation History
1954
1954 - 1583.PDF
702 FLIGHT POWER FOR AIR TRANSPORT . . . effect of all possible design variables were too cumbersome to handle other than for checking final specific layouts. Also, for the present purpose one was not concerned with detail weight breakdown, but only in establishing reliable trends within a related set of completely furnished and equipped aircraft. In support of a generalized approach it could be demonstrated from available weight data that, while there was considerable variation in the component weights of different aircraft, surprisingly con sistent values for the complete article could be deduced with the aid of quite simple correction factors. In the end results, although it was desirable that the final aero planes emerging from this analysis should each be reasonably close to those found by a full individual engineering study, com plete reality was not essential. The prime objective was to show relative sizes of aircraft when fitted with different types of engine, in each case the size of body and the load carried being the same. For weight estimation, all the aeroplanes considered might be split into four main elementary groups: — (1) Wc, a constant weight which was determined by the con dition necessary to carry the specific load. Items in this group included the complete furnishings, sound proofing, air conditioning, flying, navigational, radio and emer gency equipment; the crew and the payload. The size of the body, already used in the drag estimates, was such as to provide rather better than present-day standards of comfort, with crew rest places and, obviously, crew stations. Clearly much of the body structure would be independent of aeroplane gross weight, although the exact proportion was uncer tain. Several values were tried and a reasonable compromise was found to be given by 8,000 lb + 0.025 WG, where WG was the aeroplane gross weight. Then, as examples, for two widely separated gross weights, 120,000 lb and 240,000 lb, the percentage body weight was 9.1 and 5.8 per cent respectively. The whole group denoted by Wc amounted to 45,000 lb, including 8,000 lb of body structure weight. (2) Ws, the remainder of the structure which varied with WG. This group included the wings complete, the tail unit and undercarriage, the fuel and hydraulic, electric and other systems. (3) WE, total installed engine weight. This could be taken as proportional to the engine design mass flow necessary to pro duce the required thrust. The constant appropriate to the type of engine employed must be taken. (4) WF, fuel weight, which was the difference between the gross weight and the sum of the weights in groups (1), (2) and (3). Then the gross weight WG = Wc + Ws + WE + WF. After describing the method of application of the range/gross weight calculations and before passing on to further aircraft analysis with different types of engine, Dr. Russell summarized the conclusions to be drawn from the foregoing discussion. From the initial general aerodynamic considerations, it was suggested that the trend for maximum economy would favour larger aircraft with powerful engines flying at high altitude at the maximum available cruise lift coefficient, having wings of the highest possible aspect ratio. Taking the argument a stage farther and making reference, although not in detail, to previous work, it was concluded that when the effect of wing structure weight was considered, the trend shown by the aerodynamic analysis was misleading on the question of aspect ratio. By advancing the argument still farther and citing examples in the design of jet aircraft, it was found that, although in fact the trend towards the large aeroplane was correct in relation to the cruise lift/drag ratio, while also leading to lower wing and power loadings, and hence improved take-off performance, the con sequent structure weight penalities in the larger aeroplane incurred an overall loss in fuel economy. The most economic aeroplane was therefore one with high wing and power loading, which would not fly very high and had the worst acceptable take-off performance. This was another way of saying that the smallest and lightest aeroplane for a given duty was the most efficient. Effect of Type of Engine.—The point had been made that a high ratio of take-off thrust to cruising thrust was an advantage and an attribute to be added to the engine characteristics affecting aircraft economy. Table III. Approximate gross weights for four alternative types of aircraft (payload 25,000 lb) intended for the North Atlantic route. Route London- New York Amsterdam- New York Stand off 1 hr 2hr 1 hr 2hr Turbo jet II Ducted Fan A = 0.75 194,000 209,000 203,000 218,800 A=2.0 192,500 204,500 201,000 212,500 Turbo prop 162,500 170,000 166,000 173,500 The basic engine data showed the turbojet engine to have the lowest value of Ts/To (where Ts = static sea level thrust; To «= thrust/relative density at the tropopause), somewhat less than unity, and the turboprop engine to have the highest value, considerably in excess of unity, ducted fans lying intermediate according to their by-pass ratio. But the effect of this and other engine characteristics on aircraft performance emerged when analyses similar to that already made were followed. Aircraft Analysis.—Attention could first be given to aircraft fitted either with turbojet engines or with ducted fan engines, in the latter case with alternative by-pass ratios. The figures apply ing to the root installed turbojets were the same as those derived earlier. In the case of the two aircraft with engine pod installations advantage had been taken by employing a higher aspect ratio. A number of interesting conclusions could be drawn from the calculations:— (1) From the performance point of view, for turbojet aircraft cruising at 0.85 M, there seemed little ground for controversy as to whether engines should be buried in the wing root or slung in pods. For the present purpose, there seemed no need to differentiate between the two arrangements (although personal recollections suggested merit in uninterrupted space around tlje engines). However, for ducted fans the greater pod dimensions necessary to house the engine imposed a relatively greater drag penalty, which was reflected by adverse trends in gross weight, engine power and fuel required. So, in deference to many susceptibilities, no further reference needed to be made to pods, and buried engines might tacitly be assumed. (2) The possible reduction in aircraft gross weight by using ducted fan engines was disappointing, especially at the longer ranges. At the minimum gross weights, which were related to the worst take-off performance and the smallest acceptable size of engine, the proportion of fuel required was adversely affected by a lower operating L/D ratio. It seemed that a more attractive aeroplane might be had by using a larger engine, so permitting a lower wing loading. A significant improvement in take-off performance followed with quite a small increase in aeroplane gross weight and a small reduction in the amount of fuel required. (3) Somewhat surprisingly, it appeared that the use of the higher by-pass ratio did not lead to lower aircraft gross weight. In effect, at any given cruising range the combined weight of engine plus fuel remained substantially constant. From this it might be inferred that the aeroplane empty weight less power plant also remained unchanged. (4) Aeroplanes with ducted fan engines could, with advantage, be given a better take-off performance than could those with turbojets. Specific aeroplanes from the group would not be identified for economic analysis until the appropriate still-air ranges which fitted certain operational route conditions had been established. The same methods could again be applied to derive a family of aircraft fitted with turboprop engines. For these aircraft a straight wing was selected; at the lower cruising speed and with substantially smaller quantities of fuel, sufficient volume could be provided within the wing, even at a thickness/chord ratio which satisfied the aerodynamic drag rise condition at zero sweep-back. An aspect-ratio of 8.0 was chosen after a structural analysis had shown this value to be consistent with the structure weight data. The same characteristic curve profiles (to which by now we should have become accustomed) could be observed. For the turboprop aircraft an additional factor had to be taken into account. When the design was adjusted to the maximum field length, the high propeller thrust resulted in a rather high wing loading. As the proportion of fuel required for a given range was lower than that for the other engines, it followed that the wing loading in landing was substantially higher than the values applying to the other aircraft. Although a better lift coefficient could be achieved with efficient flaps on a straight wing, the approach and touch-down speeds were greater than those in the other cases. While advantage might be taken from reverse thrust Table IV. Relative weights for turbojet, ducted fan, and turboprop air craft designed to carry 25,000 lb payload and sub-normal fuel reserves on the London-New York route. Engine Gross weight (lb) Cruising speed (m.p.h.) Power unit weight (lb) Airframe weight (lb) Fuel weight (lb) ... Turbo jet 215,000 560 15,600 74,450 98,500 Ducted Fan A = 0.75 194,000 560 15,400 66,500 84,700 A=2.0 192,500 560 17,400 67,300 81,300 Turbo prop 162,500 500 19,800 61,300 54.600
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