Aerodynamic coordination, selection of wing profiles in combination with the high thrust-weight ratio made it possible to attain highest cruising speed in comparison with other passenger machines of analogous type - to 950 km/h during the simultaneous guarantee of good stability parameters and controllability in entire speed range and flight altitudes. The task of the combination of maximum efficiency with the maximum flight safety stood before the creators of aircraft. From the point of view of the efficiency most preferable there was twin-engined diagram, and accordingly the requirements of accepted in those years concept, it was considered that safest is four-engine, for Tu-154 was selected intermediate three-engine diagram.
The design of aircraft Tu-154 is distinguished from the majority of contemporary passenger aircraft by the high thrust-weight ratio of 0,35-0,36 (compared to 0,22-0,27 - in the majority of main aircraft). Position with the selection of this parameter it is not indisputable: from one side this can lead to reduction in the efficiency of aircraft, but from the other side margin of thrust guarantees the operation of aircraft in the airports with a length of VPP of 1500-1800 m and in the airports, which are located in the high mountain region and the regions with the hot climate. In contrast to the western analog Boeing-727, Tu-154 it was optimized to the flights at the cruising altitudes of 11000-12000 m (for Boeing - 7600-9150), for this was accepted the wing of the relatively larger area of 180 sq. m. (for Boeing - 145 sq. m.). Combination of both parameters made it possible to as a result obtain the optimum cruising consumptions of fuel of aircraft.
Airplane performance is achieved through a combination of aerodynamics and propulsion. In terms of the propulsion system, airplane configurations -- be they powered by propellers or jets--are developed around the characteristics of a specific engine. For this reason, the integration of the powerplant and the airframe is truly the cornerstone of the aircraft design. The introduction and continued development of the turbine engine has only served to emphasize the importance of achieving a successful engine/airframe interface. In May 1955, the French entered the commercial transport field with the Sud-Aviatlon Caravelle. The original patent for the Caravelle design was filed in November 1951, and was entitled "improvements in Aeroplanes Propelled by Several Jet Engines." The Caravelle, with its two jet engines mounted on the aft fuselage, represented a design innovation that was still in vogue twenty years later.
The most distinguishing recognition feature of the Tu-154 is probably the mounting of the three engines, which are located at the aft end of the fuselage. Some advantages and disadvantages of this aft-engine arrangement are as follows:
- The short lateral distance between the engines results in relatively small yawing moments following the loss of an engine. The required vertical-tail size is accordingly reduced as compared with that of an aircraft with wing-mounted engines, such as the Boeing 707.
- The rear location of the engines results in a relatively low engine-noise level through most of the cabin.
- Removal of the engines from the wing results in a small increase in the maximum lift coefficient and elimination of wing-pylon-nacelle interference drag. The integration of engines at the aft end of the fuselage, however, requires careful design in order to minimize interference drag in this area.
- The location of the engines at the aft end of the fuselage, as compared with the underwing position, reduces the problem of interference between the engines and the ground, a problem that becomes particularly important as the size of the aircraft is reduced.
- Mounting the engines on either side of the aft portion of the fuselage prevents location of the horizontal tail in a low position. In the case of other aircraft, the tail is mounted at some location between the root and tip of the vertical-tail surface. The Tu-154 utilizes the T-tail position in which the horizontal tail is mounted at the tip of the vertical surface. The use of a high tail position offers several advantages: If the vertical tail is swept back, the horizontal-tail moment arm is increased as the tail is moved toward the tip of the vertical surface. The horizontal-tail size, and hence the weight of the tail, may therefore be reduced for a given level of static longitudinal stability. In the T-tail arrangement, the horizontal tail acts as an end plate and reduces the required size of the vertical surface for a given level of static directional stability. Again, a reduction in tail weight may be realized. Structural and aeroelastic problems may, however, cause some increases in weight of the vertical tail. Whether the overall empennage weight is reduced by the use of the T-tail arrangement, as compared with the more conventional low tail position, however, is debatable and depends on the detailed design requirements of the particular aircraft.
- The high tail position also has some disadvantages. Certain inherent aerodynamic problems are encountered in the design of an aircraft with a high tail location. Careful attention to the detail design of such a configuration is required in order to achieve reasonably acceptable longitudinal aerodynamic characteristics. Lack of proper care in the design process can result in an aircraft with highly undesirable longitudinal aerodynamic characteristics.
- The rear engine location results in large concentrated weights that are a long distance behind the aircraft center of gravity. This arrangement, therefore, causes some problems in balancing the aircraft in certain loading configurations. However, these balance problems have been overcome in a large number of highly successful aircraft that employ the aft-engine arrangement.
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