|
【范围】
|
This SAE Aerospace Information Report (AIR) provides a methodology for performing a statistical assessment of gas-
turbine-engine stability-margin usage. Consideration is given to vehicle usage, fleet size, and environment to provide
insight into the probability of encountering an in-service engine stall event. Current industry practices, such as ARP1420,
supplemented by AIR1419, and engine thermodynamic models, are used to determine and quantify the contribution of
individual stability threats. The statistical technique adopted by the S-16 committee for performing a statistical stability
assessment is the Monte Carlo method (see Applicable References 1 and 2). While other techniques may be suitable,
their application is beyond the scope of this document. The intent of the document is to present a methodology and
process to construct a statistical-stability-assessment model for use on a specific system and its mission or application.
1.1 Purpose
The purpose of the Statistical Stability Assessment (SSA) is to provide a quantitative tool for optimizing engine stability
margin requirements and provide greater visibility of off-design conditions. It can be applied at the inception of a new
project to perform trade studies and support risk assessments; or to a mature project to assess the impact/risk of
modifications or changes in operational usage.
This document provides guidelines on the procedures and data that are required to perform the statistical stability
assessment. A number of illustrative examples are presented.
1.2 Field of Application
The compatibility of a gas turbine engine with its operating environment is a major concern in all gas turbine applications.
The installation of an engine, be it in an aircraft, ship, or ground facility, determines the quality of airflow provided to the
compression system of the engine and has a significant influence in the setting of the engine stability margin. This has a
direct impact on the performance, operability, and mechanical integrity of the installed engine. It is the operability aspects
that the SAE S-16 committee has devoted its main effort: understanding and accounting for the effects of intake flow-field
distortion on engine stability.
The current industry guideline for assessing the effect of intake flow-field distortions has been the subject of previous SAE
publications, notably ARP1420 and AIR1419. The loss of compression-system stability margin due to inlet flow distortion
is combined with the losses from other factors to produce a stability margin stack-up. Traditionally, stack-ups are
performed at a number of flight conditions that are judged to be the most critical by the engine company, based on
information available from the airframe company and the operator. Destabilizing factors are judged to be either random
or non-random and summed accordingly to provide an estimate of the likelihood of encountering engine instability at the
chosen flight condition. In practice, stability margins are usually set such that engine instability is predicted not to occur.
While this methodology continues to serve the industry well, it is based on the assumption that the random factors are
normally distributed and also offers limited insight into the likely stall rate for a fleet of aircraft. Further, the growing
number of factors that are accounted for in the stability stack-up, the increased use of variable geometry, and advanced
control modes have made it potentially more difficult to identify a small number of critical conditions that would indicate the
engine to be truly stall free.
This document presents a statistical approach to model the engine operating environment with a view to identifying critical
operating points and predicting a probability of compression system instability. Consideration is given to atmospheric
variations, vehicle usage, and installation effects. The resulting stability assessment can provide improved insight into
engine stability and be used to support trade studies, risk assessments, and design optimization.
To adequately capture the engine operating environment a detailed knowledge of the aircraft utilization is required. This
extension of the stability assessment, therefore, demands much greater participation from both the operator and the
airframe designer. The technique has its genesis rooted in military fighter applications, but is equally applicable to
commercial aircraft/propulsion systems, helicopters, business aircraft, auxiliary power units, and other gas turbine
installations.
As discussed previously, the Monte Carlo method was selected as the statistical tool because of its inherent ease of use
and flexibility in handling a large number of variables with various distribution types that can be treated either as
statistically independent or, if required, as having a statistical dependency.strRefField
|