First Article from Aviation Week

NOTE:  The individual on the cover is Ken Jackson who was the 1st Shift assembly lead-man at Airesearch CA at the time this photo was taken.

reprinted from AVIATION WEEK & SPACE TECHNOLOGY October 7, 1968

Copyright 1968 McGraw-Hill Inc. All rights reserved

Garrett Tests Corporate Turbofan Engine

By C. M. Plattner

    Los Angeles - Garrett Corp. has been testing since last May a new turbofan engine in the 4,000-5,000-lb. thrust class which could become a basic powerplant for a new round of long-range corporate jet aircraft in the early l970s.

    The engine, designated ATF3 (AiResearch Turbofan No. 3) is being developed by Garrett's AiResearch Div. and tested at the company's Torrance, Calif., facility.

    Although no orders had been placed for the ATF3 as of late last month, it is being considered for re-engined versions of the North American Rockwell Sabreliner, Lockheed JetStar, and Dassault Falcon (marketed in the U.S. by Pan American. All three companies are actively studying advanced versions of these existing aircraft, including those with longer range for which the ATF3 is suited.
Decision by one or more of these companies to proceed with development of a re-engined aircraft is expected soon.

    A Pan American official, for example, said the question of advanced Fan Jet Falcon versions is beyond the study phase and is now at the point of a management decision.

    Present versions of the Sabreliner and JetStar are powered by Pratt & Whitney JTI2 engines rated at 3.300 lb. thrust. With a foot already in the door, United Aircraft of Canada is one of the main competitive threats to Garrett with a design proposal (AW&ST Aug. 12, p. 33) based on technology incorporated in its JTI5D 2,200-lb-thrust turbofan.

    Another source of competition is General Electric, probably with a fan version of its T64 turboshaft engine. Rolls-Royce also is believed to have competing designs.

    Beyond these immediate applications to corporate jets, Garrett is of the opinion that the ATF3 will be a strong contender for new short takeoff and landing (STOL) aircraft in the commuter transport size.

    The ATF3 has an unusual cycle involving two flow reversals before the exhaust gases are mixed in the outer duct with the fan air. The air flow cycle begins with a split of air from the single-stage fan around and through the engine. Under full power sea-level conditions, about 25% of the fan air goes to the engine while the remainder is ducted around the engine for a by-pass ratio of about 3 to 1. The air entering the gas generator passes first through a five-stage axial-flow low-pressure compressor. After leaving the compressor, the air flows through eight ducts to the rear of the engine where it is turned 180 deg. and enters a centrifugal-flow high-pressure compressor.

    From the centrifugal compressor the air enters a reverse flow annular combustor and after being heated passes through a single-stage turbine which drives the centrifugal compressor and an accessory drive shaft on the rear of the engine. The hot gases then pass in succession through two separate two-stage turbines, the first of which drives the front fan and the second of which drives the low-pressure compressor.

    This turbine discharge air is split into eight streams and is turned through a cascade of vanes into the fan discharge stream to complete the cycle.

    Garrett claims an over-all pressure ratio of 25 to 1 for the ATF3, an unusually high figure for a small engine. Generally, large engines are more efficient as air compressors than small powerplants because diminution in working parts degrades efficiency when the same working tolerances are maintained. For comparison, the 10,000-lb. Rolls-Royce Trent, believed the only other working three-spool design aside from the ATF3. has an over-all compression ratio of 16 to I. Only the more advanced engines such as the 43,500-lb-thrust Pratt & Whitney JT9D which has a 24:1 compression ratio approaches the ATF3 figures.

    Garrett says that use of a centrifugal compressor which is more efficient than an axial-flow compressor in this size is the key to obtaining the high pressure ratio. In addition, the centrifugal compressor has the advantage of minimizing the number of compressor stages and making a simpler engine.

    Determining where to put the centrifugal compressor presented a problem when design work began in early 1966. To put it in line with the first-stage axial flow compressor made it necessary to put a shaft through it, destroying its efficiency. The solution embodied in the ATF3 design was to turn the compressor around and locate it at the rear of the engine where it was disassociated from the machinery used in the early compression cycle (fan and low-pressure compressor). The shaft which connects the centrifugal compressor to its driving turbine wheel is completely separate from the concentric twin shafts in the front part of the engine.

    The disadvantages envisioned for the location chosen for the centrifugal compressor were chiefly the losses associated with reversing the flow of air twice-in reality there are four air flow reversals if the reverse flow combustor is considered.

    Anthony A. duPont. director of product planning for Garrett and chiefly responsible for bringing the ATF3 into being, admits that the high risk area an the engine is turning the air efficiently so that losses do not degrade performance. However, in approximately 15 hr. of testing during which time about 70% power has been reached, data shows turning efficiency to be as good or better than anticipated, he says.

    Test data also has indicated that compressor efficiency may be even higher than originally thought and company engineers are toying with the idea of raising the over-all pressure ratio as high as 30 to 1.

    During the first round of testing. some seal leakage and mismatch of components was encountered. This resulted in a loss in efficiency but over-all performance came surprisingly close to figures predicted from component testing, duPont said.

    Garrett's basic ATF3 is a 4,000 lb.-thrust-category engine, but the company also is offering a 5,000-lb-thrust version, designated the ATF3A. The higher thrust would he obtained by increasing turbine inlet temperature and improving the match of compressor components.

    Beyond the present thrust category, Garrett feels that growth potential up to 9,000 lb. is possible with only a small increase in basic weight.

    Growth generally will he accomplished by raising turbine inlet temperatures and increasing fan size. The company has undertaken a separate program to develop advanced cooled turbine blades for the small high-pressure turbines of the ATF3. It hopes to develop blades capable of operating at 2,400-2,500F.

    The company is working on hollow blade designs as well as cast fin and fabricated designs.
Garrett sees the ATF3 primarily as a civilian product, but the company has been continuously alert to military application. During the early VSX engine competition Garrett performed several design studies on a 9,000-lb. version, although it declined to enter the competition on a formal basis.
When the Air Force AX close air support aircraft was evolving in early 1967, Garrett actively worked with various manufacturers until turboprop propulsion became dominant.

    A turboshaft version of the ATF3 in the 4,000-shp. category also is under study. Potential military application for a turboshaft version include the tri-service heavy-lift helicopter (HLH) and a scaled-down version of about half the thrust for Army's utility tactical transport aircraft system (UTTAS).
In the commercial field, one unique possibility is providing a "half-size" third engine for twin-engine transports such as the McDonnell Douglas DC-9 and Boeing 737. This would make a 2½-engine aircraft with improved takeoff performance. The small engine also would act as an auxiliary power unit.

    During the lifetime of the ATF3, duPont sees a basic market potential of 1,000-2,000 engines. The lower figure relates to use only on civilian aircraft and the higher figure accounts for military sales potential.

    During the 2½ years Garrett has been developing the engine, details on design have been closely guarded.

    The company still is chary of discussing many aspects of the program, such as price and minimum orders needed for a production commitment because of the current round of sales discussions.
At present, Garrett has committed to build only six engines for the test program, which is expected to culminate in type certification in mid-1970. Flight test phase of the development program is expected to be conducted in whichever aircraft the engines will be used.

    Initial reliability goal set down for the ATF3 is to attain a 2.000-hr. time-between-overhauls (TBO) two years after service introduction.

    In its analysis of desirable performance attributes in the ATF3. Garrett determined that cruise would have to be done at approximately 40% thrust to obtain low enough fuel consumption for true transcontinental range in reengined corporate jets. None of the present smaller corporate jet aircraft have this range capability now.

    Correspondingly. Garrett devoted considerable effort toward achieving low fuel consumption at 40% thrust. Because of the pronounced thrust decay with altitude of higher bypass ratio fan engines compared with turbojet engines, a higher sea-level thrust rating results. Thus roughly a 4,000-lb-thrust turbofan of the ATF3 variety is necessary to provide the same thrust at cruise altitude as approximately a 3,000-lb. rated thrust turbojet.

    Although lower fuel consumption for extended range is a highly desirable sales point in the thinking of small corporate jet manufacturers, the added thrust at low altitude is believed equally valuable. This will permit takeoff distances approaching STOL performance and will improve climb rate.

    Among the key features Garrett claims for its design are:

  1.     Slim profile determined solely by front fan diameter. This is possible because accessories are grouped at the rear of the engine and no drag-producing humps in the pod are necessary.

  2.     Inherent sound suppression, which is expected to result in a very quiet engine. Mixing fan air with exhaust gases in the short outer duct tends to reduce normal jet exhaust noise. Additionally, compressor whine is reduced because of the shrouding effect of the engine casing. Extensive noise measurements are planned early in the test program to verify noise levels.

  3.     The unusual order of flowing gases through the high-pressure compressor turbine, the fan turbine and then the low-pressure compressor turbine tends to give a better cycle and turbine match at low power than the conventional order used in axial-flow turbo-fans. This results in better partial power specific fuel consumption.

  4.     Good maintainability because of fewer parts in the engine and a prevailing unit maintainability philosophy. Almost the entire engine can be disassembled by component, for inspection or replacement of parts. The fan, accessory section, high-pressure gas generator, fan turbines and combustor all can be removed for inspection or replacement in a relatively short time without taking the engine off the aircraft.

  5.     Engines can be started easily with battery power and a standard aircraft starter-generator because of the small high-pressure spool.

    An electromechanical fuel control with a solid-state computer will be used to meter proper fuel flow for the flight conditions and throttle setting. If this primary fuel control fails, backup is provided by a direct mechanical link between throttle and fuel valve for manual engine control.

    In an effort to circumvent some of the interface problems involved in new aircraft and to eliminate some of the traditional airframe-engine manufacturer disputes over blame for lower installed performance than projected, Garrett is designing its own engine pod.

    The pod will be available on an optional basis and will be complete with thrust reversers if desired. Thrust reversers will be simple target types consisting of two doors which rotate to block the exhaust stream and deflect it forward. An estimated 90-95% of the flow will be deflected by the stainless steel thrust reverser doors. Dimensions of the pod are still not completely fixed but the outer shell forming the annular duct from the face of the engine aft is expected to be about 6 ft. long and a maximum of 36½ in. in diameter. A tail cone fairing with a bullet-like shape will cover the engine accessory section.

    All-up weight of the combined engine and pod assembly will range from 1,000-1.400 lb. depending on optional items such as thrust reversers and accessories.


updated 12/26/2008

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