Weight Assessment of the Aircraft Turbojets at the Preliminary Design Stage

Аuthors

Ezrokhi Y. A.^*, Kalenskii S. M.^**

Central Institute of Aviation Motors named after P.I. Baranov, CIAM, 2, Aviamotornaya str., Moscow, 111116, Russia

*e-mail: yaezrokhi@ciam.ru
**e-mail: 30105@ciam.ru

Abstract

The presented article considers basic approaches to the weight assessment technique (WAT) forming of the aircraft turbojet at the initial stage of its designing. Analysis of various approaches to the engine weight assessment revealed their pros and contras.
The most widespread in practice approach is based on regularities, formed by the regressive analysis means, which interconnect the engine gas-dynamic and weight parameters (“regressive” techniques). These techniques reflect quite well the engine mass characteristics, which “took part” in the regression analysis or similar to it, but they cannot be applied for the weight forecasting for the newly designed prospective engine due to the great inaccuracy.
The “block-regressive” techniques, based on the engine splitting into several enlarged blocks are marked as more precise for the engine total weight determining. The value of each block weight in these techniques bears conditional character and does not correspond to real engine subassemblies weight. Thus, the engine weight changing assessment at its subassemblies modernization may yield incorrect result.
The authors put forward a new mass assessment technique, which employs the dependence of the blade machines weight on the specific work, performed by them. Mass of the outer channel, mixer and afterburner herewith depend on the specific working fluid consumption.
The following selected values for the takeoff mode were set as basic parameters at the preliminary design stage:
- Pressure ratios in compressor stages π*f  and π*hpc,
- The air consumption through the engine Ga,
- The bypass ratio m, 
- The gas temperature prior to the turbine Т*g.
The engine modules, which weight depends directly on these parameters, were selected for the proposed approach application while the weight assessment technique forming.
Conditional generator of energy was selected as the first module. It included gas the gas generator, as well as a part of the turbo-fan operating on the internal circuit. This module weight is assumed proportional to the compressor specific power with the total pressure ratio of π*Σ and relative air consumption of 1/(m + 1).
The bypass duct and a part of the turbo-fan, “working” on the second contour, are included on in the second module. The weight of this turbo-fan part is accepted proportional power density of the fan and a relative airflow value m/(m + 1). The weight the bypass duct weight is proportional to the duct corrected airflow.
The third module is formed by the the mixer, afterburner and jet nozzle. Its weight is being considered proportional to the engine air consumption through the engine, normalized by the fan exit parameters.
The fourth module includes all engine assemblies and systems. This module weight is assumed proportional to total weight of all other engine modules.
The proposed weight assessment technique was applied to the mass assessment of more than 40 aircraft engines of various generations and demonstrated good results. For most engines being considered the error of weight determining did not exceed ~3–4%, which is less than for the “regressive” techniques.
Besides, developed technique allowed estimating more adequately the basic design parameters change impact on the weight of the engine being developed. This is an important factor for correct accounting for these parameters effect on the engine weight during optimization process by the aircraft criteria.
Thus, the developed weight assessment technique for the turbofan with moderate bypass ratio and turbojets may be employed for solving many practical problems at the preliminary stage of the engine development. 

Keywords:

turbojet, engine weight assessing technique, blade machines specific work, engine assemblages weight

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