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AIR1419 “Inlet Total Pressure Distortion Considerations for Gas Turbine Engines” documents
engineering information for use as reference material and for guidance. Inlet total-pressure distortion
and other forms of flow distortion that can influence inlet/engine compatibility require examination to
establish their effect on engine stability and performance. This report centers on inlet-generated total
pressure distortion measured at the Aerodynamic Interface Plane (AIP), not because this is
necessarily the sole concern, but because it has been given sufficient attention in the aircraft and
engine communities to produce generally accepted engineering practices for dealing with it. The
report does not address procedures for dealing with performance destabilizing influences other than
those due to total-pressure distortion, or with the effects of any distortion on aeroelastic stability. The
propulsion system designer must be careful to assure that, throughout the development process, other
forms of inlet flow distortion, which can have just as serious effects on system stability and
performance, have been effectively addressed.
The report deals with spatial total-pressure distortion, as defined by an array of high-response total-
pressure probes. Time-variant total-pressure distortion, synthesized from statistical data, can provide
useful information. However, the consensus of SAE S-16 is that such techniques are not developed
sufficiently to permit general guidelines to be formulated.
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Concepts which are fundamental to this report are:
a. Inlet flow quality can be characterized, in a form relevant to engine distortion response, with
numerical descriptors derived from an array of high-response total-pressure probes;
b. Propulsion system stability can be controlled by the aircraft and engine designers;
c. Engine stability can be demonstrated by tests using equivalent levels of steady state distortion.
The report is organized into seven sections, expanding upon the ideas and recommended practice set
forth in ARP1420. The first two sections deal with surge margin, loss of surge pressure ratio, and
procedures for correlating the loss of surge pressure ratio with total-pressure distortion. Through use
of the terms and procedures discussed earlier, Sections 5 and 6 develop engine stability and
performance assessment techniques for handling total-pressure distortion by putting them into context
with other destabilizing influences and performance detriments. Section 7 describes various test
procedures, equipment, and methods currently available for generating the information needed to
apply distortion assessment techniques. Section 8 discusses interface instrumentation, data-
acquisition system accuracy, frequency response, record length, recording systems, and the data
management procedures necessary to minimize communication errors among participating
organizations. Section 9 provides a short overview of “state-of-the-art”, a brief discussion of other
forms of distortion at the inlet/engine Aerodynamic Interface Plane, a summary of other considerations
involved in assessments of inlet/engine compatibility, and brief summaries of probable future activity in
each of these areas.
The distortion descriptor is the vehicle by which engine reaction to inlet distortion is forecast and
assessed, from program outset well into field use of the system. ARP1420 defines the distortion
descriptor as a non-dimensional, numerical representation of the measured inlet pressure distribution,
and provides a means for identifying critical inlet flow distortions and for communicating during
propulsion system development. Central issues are the distortion descriptors, methods of correlating
them with performance and stability changes and test and information acquisition techniques. Use
and accuracy of the descriptors vary, depending upon the stage of the engine development, but their
definitions and purpose remain constant - to assess status, forecast stability and identify required
engineering activity.
The activities associated with distortion descriptor use can be categorized for convenience in phases
(Table 2), recognizing that there is little consensus concerning the definitions of these phases and that
no clear lines of demarcation exist between them.
1.1 Conceptual Studies Phase:
This phase, the initial step in the life cycle for an aircraft system, is characterized by analytical
evaluations of candidate aircraft/propulsion system configurations. Generally, no new testing is
planned for this phase and information for the evaluations is based on historical sources.
Recognition of and planning for stability assessment during concept evaluation serves to assure that
1) distortion effects are a prime consideration in the selection of the candidate propulsion system, 2)
those conditions that are considered areas of risk are given particular attention during the
subsequent design and development phases, and 3) distortion patterns, inherent to the type of
aircraft inlet, are defined to enable the engine to be designed with consideration for predominant
patterns.
The distortion descriptor is used to determine the relative standing of several candidate inlet
configurations. Specific items to be evaluated include: design concept and location, inlet
performance, aircraft maneuverability–as affected by distortion, armament location, approximate
inlet/engine matching characteristics, overall distortion trends with inlet geometry and primary and
secondary airflow requirements. The descriptor is used to evaluate the stability characteristics of
candidate compressor and engine configurations, their sensitivity to distortion, the surge margin
available for distortion, and potential problems peculiar to the various thermodynamic cycles and
engine control modes.
This phase should end when the general aircraft and the propulsion system configurations that can
best meet generalized mission requirements are defined. Limiting operating conditions within the
anticipated flight envelope should have been identified, including those due to inlet total-pressure
distortion, unstart, buzz, temperature distortion, water ingestion, armament-exhaust-gas ingestion,
and unusual amounts of engine bleed or power extraction. The consequence of surge associated
with these conditions should have been assessed and a procedure established for continuously
tracking compatibility throughout the development program.
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1.2 Preliminary Design Phase:
This phase is the second step in the development cycle during which mission requirements are
defined in more detail and a baseline vehicle has been selected. Airframe-inlet integration and inlet
component testing have begun with small scale models to update compatibility estimates and to
define the extent of required development work. Preliminary indications of the extent of the distortion
problems to be expected are identified using data obtained from steady state and dynamic probes
located at the inlet/engine aerodynamic interface plane. Fan and compressor performance maps,
flow path geometry and blade sizes are estimated. Where appropriate, applicable engine
components are tested with distortion patterns characteristic of the inlets being considered by the
airframe contractor.
The distortion descriptor is used to aid in selecting those inlet/airframe components that result in a
favorable flow field. Preliminary distortion characteristics are determined for critical aircraft operating
conditions. The distortion descriptors are used to assess the effects of distortion on the engine and
its components. Engine simulations, with transient capability and control logic, are initiated and used
to perform preliminary engine stability audits, to define engine surge margin utilization and to aid in
establishing distortion goals for the inlet.
Engine distortion tolerance estimates are used to establish the allowable airflow range for inlet/
engine matching, supercritical or subcritical limits, start/unstart procedure, control criteria, bleed
configuration, boundary layer diverter design, noise requirements, maneuver capability, lip shape
and radius, low-speed flow augmentation, and low-speed crosswind capability.
At the end of this phase, initial stability audit coordination has occurred between engine and airframe
companies. The basic inlet and engine configurations have been defined but require further
refinement. The vehicle mission has become well-defined. Provisions for coordinating any mission
changes will have been made. Agreement should exist between airframe and engine contractors on:
a. The distortion goals for the inlet and the engine at specific points within the flight envelope.
These represent the maximum level of distortion the inlet will generate and the level the engine
will tolerate.
b. The type, severity and number of equivalent classical patterns to be used during initial
development testing.
c. The distortion patterns from subscale inlet testing to be used for engine development tests.
d. A well-defined compatibility program including the definition of the data transmission formats, test
sequence, criteria for decision making, the scope of dynamic distortion tests, demonstration
points, type of instrumentation and location of aerodynamic interface plane.
1.3 Development Phase:
This phase starts when system mechanical configurations are defined and terminates when the
propulsion system is ready for field use. The airframe, inlet and engine configurations are refined
through extensive test programs. The performance and compatibility of the airframe/inlet system are
developed through wind-tunnel tests of large-scale models, and of the engine, through engine and
component tests using suitable distortion generators. A full complement of dynamic instrumentation,
located at the agreed AIP, is utilized during testing. Prior inlet distortion and engine tolerance
commitments are updated based on realistic test data and changing requirements.
Updated surge margin and surge pressure ratio changes due to distortion are obtained from
compressor rig and engine tests with inlet distortion. Distortion stability coefficients are adjusted
correspondingly, and the process is repeated as engine design changes occur. The descriptor is
used to verify inlet distortion levels, and design variables such as throat height, ramp position, cowl
shape, bleed and bypass are examined for their effects on distortion. Updated distortion patterns,
obtained from wind tunnel and flight tests, are used to refine stability and performance assessments.
Descriptors and engine simulations are used to focus attention on components requiring further
development. At the end of this phase, the airframe and engine have demonstrated compatibility
throughout the required flight envelope.
1.4 Engine Qualification or Certification Phase:
This phase represents the period during which tests are performed to clear the engine for initial flight
testing, for limited production, and eventually, for full production. Qualification or certification
requires quantitative assessments of engine performance and stability at a number of selected
conditions. Distortion patterns are used during this phase to define the inlet/engine interface test
conditions. The test conditions are defined in terms that include:
Inlet/Engine Interface Conditions
Airflow
Total-Pressure (local point-by-point values)
Total-Temperature
Altitude Ambient Pressure
Installation Interface Conditions (aircraft service requirements)
Customer Bleed
Power Extraction
Engine Operating Conditions
Engine Power Setting
Engine Service Bleeds (intercompressor, anti-ice)
Control Trim Status
At the conclusion of this phase, the engine configuration will have demonstrated acceptable
performance and stability for the specified sets of operating conditions.
1.5 Flight Test Phase:
Distortion testing through the early part of the development phase defines necessary design
changes and provides an assessment of system performance and stability well in advance of flight.
Flight testing may identify design changes and may uncover problems requiring further development
because ground test facilities are limited in their ability to simulate the full flight and maneuver
envelopes.
The primary engine uses of the descriptor are to correlate flight and ground test data, and to identify
sources of flight-revealed performance and stability problems. Comparisons are made with previous
engine stability predictions at specific steady-state and transient operating conditions. Stability
assessment procedures may be updated and improved. Flight stability limits are identified and
tracked in terms of descriptor level, aircraft Mach number, altitude, attitude, and inlet and engine
control parameters.
The flight test phase is complete when adequate aircraft propulsion system performance and stability
have been demonstrated over the flight and maneuver envelopes and there are no further
requirements for flight-generated compatibility information prior to certification for full production.
1.6 Operational Phase:
During this phase, system engineering changes, alterations in aircraft usage, maintenance effects,
and aging effects are assessed for their impact upon inlet/engine compatibility. If additional testing
and instrumentation are required, use of the distortion descriptor would be identical to that previously
described in the flight test phase.
The complexity and expense of a compatibility program to execute the multi-phased process
described will depend on system requirements. The program for a re-engining of an existing aircraft,
or development of a podded installation for which significant background data exist based on similar
designs and similar applications, may be uncomplicated and inexpensive to execute. More stringent
mission requirements may force severe departures from experience, thus incurring added risk and
therefore added program complexity and expense. The information in this report can be used as
necessary to create a development method to minimize the risk of inlet/engine compatibility
problems. The degree to which information regarding descriptor formulation and use, assessment
techniques, and testing outlined in this document is applied to a specific program should be
consistent with the expected severity of the compatibility problem.strRefField
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