$3.00 PER COPY
`$1.00 TO ASME MEMBERS
`
`72-GT-61
`
`The Society shall not be responsible for statements or opinions
`advanced in papers or in discussion at meetings of the Society
`or of its Divisions or Sections, or printed in its publications.
`Discussion is printed only if the paper is published in an ASME
`journal or Proceedings.
`Released for general publication upon presentation.
`Full credit should be given to ASME, the Professional Division,
`and the author Is).
`
`Variable Pitch Ducted Fans for STOL
`Transport Aircraft
`
`R. M. DENNING
`
`Chief Engineer—Advanced Projects,
`Rolls-Royce (1971) Ltd.,
`Bristol Engine Division,
`Filton, Bristol, England
`
`The Variable Pitch fan has a number of features which make it attractive as the
`basis for ultra-high bypass ratio ducted fans designed primarily for STOL aircraft.
`The variability imposes certain design constraints, particularly on fan pressure
`ratio, and leads to differences in engine geometry relative to equivalent fixed
`pitch engines. The merits of such engines are discussed under the headings of
`Performance, Noise, Engine Control, Thrust Modulation, Provision of Air Bleed
`for High Lift, Reverse Thrust and Development Flexibility.
`
`Contributed by the Gas Turbine Division of the American Society of Mechanical En-
`gineers for presentation at the Gas Turbine and Fluids Engineering Conference & Prod-
`ucts Show, San Francisco, Calif., March 26-30, 1972. Manuscript received at ASME
`Headquarters, December 20, 1971.
`
`Copies will be available until January 1, 1973.
`
`THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y. 10017
`
`Copyright © 1972 by ASME
`
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`Variable Pitch Ducted Fans for STOL
`Transport Aircraft
`
`R. M. DENNING
`
`INTRODUCTION
`With the rapidly diminishing availability
`of suitable land for new large airports has come
`the search for new systems of Air Transport which
`would be less profligate in land use and which
`might also increase the capacity of existing air-
`ports. As a result, the next major advance in air
`transport seems likely to be a radically new form
`of Short Haul transport which may well be the STOL
`aircraft. It almost goes without saying that such
`aircraft must have a high degree of acceptability
`to local communities. This will not be made easi-
`er by the fact that such aircraft will generally
`operate closer to such communities. While the
`reason for STOL is not lower noise, it cannot suc-
`ceed without achieving major reduction in subjec-
`tive noise levels. Conventional takeoff and land-
`ing commercial transport aircraft are now coming
`into service with Airline Operators which use
`aero-engines that, for the first time, are specif-
`ically designed to meet Noise Certification legis-
`lation.
`The fundamental feature, common to these new
`low noise engines, is the use of the single-stage
`front fan without inlet guidevanes and with large
`spacing of rotor and exit stator. By extracting
`most of the energy from the gas generator flow and
`reducing the hot jet velocity to around 1400 fps
`at takeoff, major reductions in noise, coupled
`with a significant improvement in fuel consumption
`have been achieved. Such fans are run at higher
`tip speeds than has been customary in order to
`produce a high fan pressure ratio and to minimize
`the number of low-pressure turbine stages. As a
`result, the fan unit is highly stressed, difficult
`to design for blade containment, and more critical
`to foreign object damage.
`Looking beyond the achievements of this new
`generation of single-stage fan engines, the Aero-
`space Industry is now seeking to produce engines
`with noise levels some 10 to 15 PndB lower and
`which also have the required performance charac-
`teristics for short field length aircraft. What
`these characteristics should be is not yet clear,
`and it appears likely that several competing de-
`signs will be evaluated up to the prototype stage.
`
`It is in this context that the variable pitch fan
`concept is being studied.
`
`STOL POWERPLANT REQUIREMENTS
`
`Although these have been discussed in many
`papers over the past few years, it is useful to
`recapitulate them. Summarized, they are as fol-
`lows:
`1 An increased takeoff to cruise thrust ratio
`2 Lowest practical noise level
`3 Low, near zero, axial thrust component on ap-
`proach to land to allow steep approach paths
`combined with good acceleration character-
`istics
`4 Reverse thrust usable down to low forward
`speeds
`5 Ability to provide energy for high lift sys-
`tem if required.
`It is the last item which has the most pro-
`found effect on the engine philosophy. If, for
`example, the engine is required to supply bleed
`air for high-lift wing systems, rather than using
`separate air supply units, then the thermodynamic
`cycle is dictated to a major extent by the amount
`and pressure ratio of such air. Even with the
`bleed thrust as low as 10 percent of the total
`, thrust, an oversize compressor with permanent by-
`pass is essential for efficient operation. There
`are a number of different high lift systems re-
`quiring bleed air ranging from boundary-layer con-
`trol via the jet flap, to the Augmentor wing. The
`Externally Blown Flap is a special case where most
`of the engine thrust is vectored by the flap which
`then becomes the primary thrust control. The Aug-
`mentor wing has been described in a number of pa-
`pers by D. C. Whittley (1, 2 inter alia), 1 and it
`requires the largest amount of bleed air with
`blowing thrust requirements which range from as
`low as 30 percent up to 80 percent of the total
`engine thrust. The concensus of view is that the
`1 Numbers in parentheses designate References
`at end of paper.
`
`2
`
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`

`ie4R7.4,64"
`F-.4N
`
`,4C,04/s7-/C L /AUNGS
`
`2E0c/C77cw 6E.-4Arecoe
`
`/17.45 raAS cgEA/Az:47-tr;,,t.
`
`0,4 ce,:7L,4t
`Fig.l M45S (RB.410) basic engine layout
`
`bleed air pressure ratio should be in the range
`2:1 to 3:1.
`
`ARGUMENTS FOR AND AGAINST THE USE OF VARIABLE
`PITCH
`
`Recent papers (4-6) have discussed the use
`of variable fan geometry.
`In its simplest form, the variable pitch fan
`engine is illustrated in Fig. 1. For noise and
`stress reasons, the variable pitch fan unit is
`driven at a low tip speed via a step-down gearbox
`between it and the gas generator. The gas genera-
`tor may be of the single shaft type as on the
`Turbomeca Astafan or be a compound compressor as
`proposed for the M45S (RB.410) derivative of the
`M45H engine. Because the variable pitch fan has
`a fundamentally limited pressure ratio capability,
`it produces a high bypass ratio which, combined
`with a low tip speed, leads to a very low-speed,
`high torque fan shaft. In this situation, the
`gearbox is essential if very large, low-speed,
`low-pressure compressor and turbine systems are to
`be avoided.
`The arguments in favor of variable geometry
`can be summarized as follows:
`1 Appraoch thrust modulation -- using fine
`pitch and high rpm values
`2 Reverse thrust available down to zero forward
`speed
`3 Optimization of specific fuel consumption
`4 Maximization of thrust
`5 •Improved acceleration control
`6 Removes necessity for a variable fan exhaust
`nozzle
`7 Provision of near-constant air bleeds at vary-
`ing thrusts
`8 Optimization of noise characteristics
`9 Built-in development potential.
`
`Fig.2 Blow fan air distribution modes
`
`Against these can be set a number of disad-
`vantages which are immediately apparent, e.g.,
`1 The low solidity of the fan system which lim-
`its the design pressure ratio
`2 The mechanical limitations on tip speed set
`by the variable pitch mechanism which again
`limit pressure ratio relative to a fixed
`pitch fan
`3 The requirement for a gearbox which almost
`inevitably arises from the limitations on tip
`speed
`4 The increased control system complexity
`5 The more complex reverse thrust engine aero-
`dynamics
`6 The general increase in mechanical complexity.
`
`POSSIBLE ENGINE CONFIGURATIONS EMPLOYING
`VARIABLE PITCH
`
`The subject can be approached from two dif-
`ferent viewpoints, that of the Propeller Designer
`who is offering a "Bolt-on-Augmentor" which is
`coupled to a shaft power generator or from that of
`the Engine Designer who is integrating variable
`geometry into his ducted fan philosophy. The
`first approach, if genuinely carried through, must
`result in relatively low-loaded fans which have
`pressure ratios that do not invalidate, complete-
`ly, the entry flow conditions for which the gas
`generator has been cleared. This probably implies
`fan pressure ratios in the region of 1.1 to 1.2
`and is typified by the prop-fan proposals set out
`by Rosen in reference (3). This paper is con-
`cerned with the second approach where the fan is
`an integral part of the engine and the designer
`is attempting to maintain the high flight speed
`characteristics of current short-haul aircraft by
`minimizing any increase in frontal area for a
`given cruise thrust.
`The variable pitch fan has one particular
`
`3
`
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`BLOWING PRESSURE 3.1
`FAN PRESSURE RATIO 1 27'1
`
`MACH 0.8
`CRUISE CONDITION
`30,000 FT ISA
`BLOWING PRESSURE RATIO 3:1
`
`0 9
`
`0.8
`
`CRUISE
`SFC
`LB /LB./HR
`
`0 7
`
`0 6
`
`THREE STREAM
`BLOWING
`ENGINES
`
`LIMIT
`BLOWNG
`I ENGINE
`
`Fp, RFAANTIOPRESSURE
`
`40% BLOW
`
`20% BLC
`
`FIXED PITCH VJH OPTIMUM
`I TWO STREAMSI
`
`FIXED PITCH VJH 900 FT / SEC
`(TWO STREAMSI
`
`VARIABLE PITCH TWO STREAMS
`
`I
`t
`( I.
`40
`30
`20
`50
`TAKE-OFF SPECIFIC THRUST
`I ISA . SEA LEVEL STATIC 1
`
`60
`
`Fig.5 Blow fan cruise performance -- SFC
`
`= COTHApL c FAFNLOWFLOW 1)
`
`A lp _ (TOTAL FAN FLOW 1 )
`IPC FLOW
`
`12
`
`10
`
`8
`
`6 4 2 0
`
`BY-PASS RATIO
`
`20
`40
`60
`PERCENTAGE BLOWING THRUST
`
`80
`
`q o
`Fig.3 Blow fan -- bypass ratios
`
`•■•
`
`CI
`
`BLOWING PRESSURE RATIO 3.1
`
`FAN PRESSURE RATIO 1 27 : 1
`
`TAKE-OFF THRUST
`
`no%
`
`DATUM 100%
`
`CONSTANT FAN SIZE
`
`CONSTANT GAS GENERATOR SIZE
`
`60
`40
`20
`PERCENTAGE BLOWING THRUST
`
`80
`
`HOT JET VELOCITY AT TAKE- OFF CONSTANT AT 900 FT /SEC
`
`Fig.4 Blow fan -- thrust sizing
`
`capability which may in the end turn out to be its
`raison d'etre. It can maintain high engine speeds
`and high compressor air bleeds at very low thrust
`levels, i.e., the approach to land condition. An
`engine of this "Blow Fan' type is illustrated in
`Fig. 2. Differing aircraft concepts have require-
`ments for bleed air thrust at takeoff and landing
`which vary from zero to 80 percent of the installed
`takeoff thrust. If the gas generator exhaust ve-
`locity is specified for noise reasons, then there
`is a defined energy output of the gas generator
`which must first be used to produce the required
`bleed air and then the remainder must be absorbed
`in a suitable fan system, in this case the vari-
`able pitch fan. As the bleed thrust increases,
`the amount of power remaining for the fan reduces,
`and, hence, the fan becomes smaller. When the
`bleed thrust reaches 80 percent of the total
`thrust, the fan has reduced to the unrealistic
`point where it becomes the first stage of the
`bleed air compressor. This is illustrated in Fig.
`3 which shows the variation in flow division be-
`
`4
`
`tween bypass duct, bleed air, and gas generator.
`Looked at from another viewpoint, such blow fans
`will have varying requirements for core engine
`size and fan size to produce a given thrust as
`shown in Fig. 4. Blow fans can be criticized on
`the grounds that they are special purpose engines.
`However, if the advantages are great enough and
`the market big enough, such a criticism is diffi-
`cult to sustain.
`PERFORMANCE OF "BLOW FAN" ENGINES
`The use of an engine which must deliver part
`or all of its bypass airflow at about 3:1 pressure
`ratio must inevitably produce some performance
`penalty compared with a fully optimized engine cy-
`cle. Probably the simplest method of comparing
`thermodynamic cycle performance is on a basis of
`takeoff specific thrust which is a measure of the
`power plant frontal area and implies similar in-
`stallation features. Fig. 5 gives SFC at a typi-
`cal cruise condition for engines having blowing
`thrusts varying from 0 to 80 percent of total
`thrust and compares these values with the optimum
`performance for simple, two-stream ducted fans.
`Also shown on the graph are the values for two
`stream engines whose hot jet velocity has been re-
`stricted to 900 fps at takeoff for noise reasons.
`This latter curve links up with blow fans at the
`extreme point where the variable pitch fan has
`shrunk to the blowing compressor size and beyond
`this point would give engines of fan pressure
`ratio above 3:1.
`On a basis of common hot exhaust noise char-
`acteristics, it would be unfair to compare the
`blow fan with the thermodynamic optima but rather
`with the restricted hot-jet velocity engines. At
`a given specific thrust, the 40 percent blowing
`thrust engine is about 6 percent worse on this
`
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`MACH 0,9
`CRUISE CONDITION
`BLOWING PRESSURE RATIO 3 1
`
`30,000 FT. ISA.
`
`N - SHAFT SPEED - RPM .
`
`A - FAN ANNULUS AREA - IN 2
`
`0 30
`
`LIMIT
`BLOWING
`ENGIN
`
`THREE STREAM
`BLOWING ENGINES,'
`•
`
`0 25
`
`20%8LO
`
`CRUISE THRUST
`TAKE-OFF THRUST
`
`I/
`
`FIXED PITCH VJH OPTIMUM
`
`FIXED PITCH VJH 900 FT / SEG
`
`0 20
`
`I/ VARIABLE PITCH (TWO STREAMS)
`
`/1---------
`
`20
`
`50
`40
`30
`TAKE-OFF SPECIFIC THRUST
`(ISA SEA LEVEL STATIC)
`
`60
`
`Fig.6 Blow fan cruise performance -- cruise
`thrust/T.O. thrust
`
`basis, while the 80 percent engine is, of course,
`equal, as is the zero blow engine. Only the zero
`blow engine is equal to the thermodynamic optimum,
`while the 80 percent blow fan is about 13 percent
`worse.
`Fig. 6 shows the associated ratios of cruise
`to takeoff thrust for these engines, indicating
`that the blow fans improve in cruise thrust rela-
`tive to the optimum engines as some compensation
`for their worse fuel consumption.
`FAN AERODYNAMIC DESIGN
`The obvious basis for an initial design of
`variable pitch fan is to bring together the exper-
`tise of the propeller and engine designers. Fig.
`7 indicates that if blade root stress levels are
`to be maintained at propeller values, then a tip
`speed of around 1000 fps is required at a hub:tip
`ratio of 0.5. This figure illustrates the benefi-
`cient effect of using a high hub:tip ratio and the
`reduction in stress levels relative to current
`single-stage fan engines. A further reason for
`using hub:tip ratios is to provide the space for
`the variable pitch mechanism. In the past, much
`of the impetus to lower hub:tip ratios has arisen
`from requirements to achieve maximum mass flow per
`unit of frontal area, an important requirement for
`supersonic aircraft and many military aircraft.
`On a low noise engine which has a given hot jet
`velocity, the basic requirement of the fan is to:
`1 Absorb a given horsepower
`2 Produce a fan pressure ratio compatible with
`overall performance requirements.
`Fig. 8 illustrates relative power absorption
`obtained with three different philosophies of fan
`design, all at a given tip speed of 1050 fps.
`
`CURRENT SINGLE STAGE FANS
`0.1
`0.3
`0-5 HUB/TIP RATIO
`
`0 . 7
`
`100
`
`80
`
`CENTRIFUGAL 60
`BLADE STRESS
`10 - g ) 40
`(N 2 A
`
`20
`0
`
`700
`
`1300
`1100
`900
`FAN TIP SPEED - FT./SEC
`
`1500
`
`Fig.7 Propellers and variable pitch fans -- hub
`stress comparison
`
`CONSTANT TIP SPEED
`
`CONVENTIONAL
`FIXED PITCH FANS
`
`DATUM
`
`1 00
`
`090
`
`RELATIVE 0 80
`POWER
`ABSORPTION 0.70
`
`0 60
`
`0 • 50
`
`FIXED PITCH
`
`VARIABLE PITCH
`( NON REVERSING)
`
`VARIABLE PITCH
`WITH REVERSING)
`
`PROBABLE RANGE
`FOR VP REVERSING FANS
`
`03
`
`0 4
`
`05
`0.6
`07
`HUB/TIP RATIO - 4A
`
`0 8
`
`Fig.8 Comparison of fan performance, fixed pitch
`and variable pitch -- power absorbtion
`
`These are:
`1 Conventional tapered annulus, fixed pitch
`design
`2 Variable pitch design which does not allow
`selection of reverse thrust through fine pitch
`(i.e., root pitch:chord ratio .< 1)
`3 Variable pitch design which allows selection
`of reverse thrust through fine pitch (i.e.,
`root pitch:chord> 1.0).
`Designs 2 and 3 require a parallel outer casing
`for geometrical reasons, while design 3 has the
`additional constraint on root pitch:chord ratio
`which tends to dictate the blade root:tip taper
`ratio. Design 1 is free from these constraints.
`It is evident from Fig. 8 that the minimum
`fan diameter to absorb a given power occurs at a
`hub:tip ratio of 0.5. This implies a mean fan
`pressure ratio (Fig. 9) of 1.27 for the reversing
`
`5
`
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`

`1 5
`
`1 4
`
`FAN
`PRE SSURE
`RATIO
`
`1 3
`
`1 2
`
`TIP SPEED 1050 FT./SEC
`
`CONVENTIONAL
`FIXED PITCH
`FANS
`
`FIXED PITCH
`
`VARIABLE PITCH
`I NON REVERSING
`
`VARIABLE PITCH
`
`REDUCTION
`GEAR
`RATIO
`
`''""P---\__PROBABLE RANGE
`VARIABLE PITCH
`FANS
`
`30
`
`2 5
`
`20
`
`15
`
`I
`0-6
`0-5
`0.3
`0.4
`HUB TIP RATIO — 0
`
`0.7
`
`0.8
`
`Fig.9 Comparison of fan performance, fixed pitch
`and variable pitch -- fan pressure ratio
`
`PREFERRED
`CHOICE j
`
`lv
`
`7>>,„
`4No
`
`EPICYCLIC
`REDUCTION
`GEAR
`
`FIXED LAYSHAFT
`REDUCTION
`GEAR
`
`L
`
`---■ I
`
`I
`10
`
`I
`I
`I
`30
`20
`40
`PERCENTAGE BLOWING THRUST
`
`I
`50
`
`Fig.lO Reduction gear ratio versus percentage
`blowing thrust
`
`favored method of driving the mechanism is by oil
`fan compared with 1.32 for the non-reversing fan pressure in a vane motor system. A high hub:tip
`and 1.37 for the fixed pitch fan. At a given pow- ratio is preferred to reduce blade taper ratio to
`er absorption fan casing diameters are 17 percent
`lower stress and produce the best hub and blade
`root form compromise.
`greater for the reversing fan and 7 percent
`The modern propeller is designed on an in-
`greater for the non-reversing fan at a hub:tip
`tegrity basis, rather than for containment as on
`ratio of 0.5, although these differences would be
`reduced if the fixed pitch fan had a lower hub:tip the fan engine. This is, of course, forced on the
`ratio. The choice of hub:tip ratio of 0.5 or more propeller designer by the nature of the propeller,
`but it does raise the question whether this design
`becomes inevitable on the variable pitch fan when
`philosophy can be applied to the variable pitch
`blade taper ratio and tip pitch:chord considera-
`tions are allowed for. This implies a more diffi- fan. The fundamental differences, which might
`cult entry duct design for the gas generator simi- justify such an approach, are the lower stress
`levels in the blade and the better round shank
`lar to that which arises on a propeller turbine.
`root form of the variable pitch blade. Addition-
`It has been assumed that reverse thrust is
`ally, the variable blades tend to be of larger
`achieved by turning the blade through fine pitch.
`chord due to higher bypass ratio and smaller blade
`This results in the camber and twist on the blade
`numbers and, hence, should be less liable to for-
`being in the wrong sense and implies that the
`eign object damage. The elimination of blade con-
`outer portions of the blade must produce most of
`tainment would make a significant reduction in en-
`the reverse thrust. This also requires that the
`gine weight of the order of 5 percent.
`blade tip should have minimum camber to reduce
`negative camber in reverse. Reversing through
`A further consideration where variable pitch
`has a possible advantage over conventional fans is
`feather pitch would avoid these particular diffi-
`culties but requires the blade to pass through the in the optimum employment of composite materials
`for blading. Again, the stress levels, root form,
`stall and flutter regions and is undesirable from
`and large chord blades are the justification. The
`the thrust modulation viewpoint.
`large reduction in blade weight, up to 60 percent,
`and the nature of the probable blade failure would
`MECHANICAL DESIGN -- BASIC CONSIDERATIONS
`reduce the blade containment problem to relative
`unimportance.
`The variable pitch fan engine both simpli-
`fies and complicates the work of the designer. On
`Reduction Gear
`the one hand, it re-introduces the gearbox, cool-
`The difference between the variable pitch
`er, and variable pitch mechanism, and on the other
`hand, it allows freedom to design the fan and power fan and the propeller is well illustrated by the
`differing gearbox requirements. The propeller nor-
`turbine system separately without imposing the
`mally requires a step down ratio of between 10:1
`constraints of one upon the other.
`and 14:1 to achieve its tip speeds of about 850
`The Fan Unit
`fps, whereas the variable pitch fan is likely to
`require a step down ratio of between 2.5 and 3.0
`This can be designed as a module consisting
`for the zero-blow design.
`of the fan and its pitch change mechanism. The
`
`6
`
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`As the percentage bleed thrust is increased,
`the size of I.P. compressor increases, and some re-
`duction in its shaft speed may be necessary if
`compressor stress level is critical. At the same
`time, the fan size reduces, and to maintain its
`tip speed, it is necessary to increase its shaft
`speed in inverse ratio to fan diameter. Both
`these effects tend to reduce the required gear
`ratio. Fig. 10 illustrates this effect and shows
`the constraints imposed by various stress limita-
`tions. This shows that the preferred ratio drops
`from about 2.5 at zero bleed down to 1.8 at about
`40 percent blowing thrust.
`There are a number of different types of
`gear design which have been considered for these
`types of engine. For the zero-blow engine, the
`epicyclic is still a contender, but is close to
`the limits of practicability from the planet gear
`centrifugal stress viewpoint. Below a ratio of
`2.5:1, the rotating annulus or a "Star" gear be-
`comes an attractive solution. An alternative to
`the Star gear is the bevel gear of the piston en-
`gine era. For a consistent gear box policy cover-
`ing all blow-fan engines, the latter types are
`clearly to be preferred.
`As power is increased, then the gear becomes
`more difficult to design on a weight and heat dis-
`sipation basis. While gears in the range 10,000
`to 25,000 hp would seem to be feasible, some upper
`limits on size may emerge on closer study.
`The Gas Generator
`The design of the gas generator presents few
`new problems and is, in general, simplified. The
`requirement for the low-speed fan shaft has been
`eliminated by the gearbox, and this avoids the ma-
`jor problem of passing such a shaft through the
`middle of the gas generator which is itself
`shrinking in relative size as flame temperatures
`rise progressively.
`The Turbine System
`The freedom to select the turbine shaft
`speeds to optimize the gas generator and bleed
`compressor system in isolation leads to the mini-
`mum number of turbine stages, i.e., 4 or 5, as on
`a propeller turbine. References (4) and (6) indi-
`cated that at a bypass ratio of 10, the ungeared
`engine would require around 12 stages of turbine.
`This should lead to significant reduction in cost
`and weight.
`APPROACH THRUST DESIGN CONSIDERATIONS
`It appears probable that STOL aircraft will
`progressively develop by maximizing aerodynamic
`lift at specified approach speeds which will be
`
`120
`
`100
`
`80
`
`NET THRUST -
`(EXCLUDING BLEED)
`60
`
`40
`
`20
`
`o -
`
`40
`
`BLADE
`-10° ANGLE
`RELATIVE
`TO
`15° DATUM
`
`100
`80
`60
`GROSS BLEED THRUST - %
`
`Fig.11 Blow fan performance approach to land
`
`set by runway retardation limits and cross-wind
`landing requirements. The likelihood of air bleed
`being required, at least on approach, should,
`therefore, increase with time. Residual propulsive
`thrust requirements are of the order of 0 ^J 10 per-
`cent of the total engine thrust, and these have to
`be provided and modulated while the bleed air sup-
`ply is at its maximum requirement.
`The process of moving the fan blades into
`fine pitch very rapidly reduces fan thrust and also
`decreases core engine exhaust thrust. The bleed
`air thrust is unaltered, except for the loss of
`supercharging due to the reduced fan pitch, and
`this can be offset to some degree by designing the
`bleed system to absorb the excess flow not re-
`quired by the gas generator in the fine pitch con-
`dition. Fig. 11 indicates a typical variation of
`engine thrust, less bleed thrust, with change in
`fan pitch angle. An important implication is that
`the fan still has a significant positive tip set-
`ting at approach thrust setting.
`
`PERFORMANCE OPTIMIZATION
`The presence of variable geometry, in the
`form of variable fan pitch, can be used to opti-
`mize engine performance. There are two particu-
`lar regimes where this advantage might be taken.
`These are:
`1 To maximize cruise thrust
`2 To optimize part-load fuel consumption.
`The variable pitch mechanism would be used
`to select the appropriate fan and intermediate
`pressure speeds, while the fuel throttle controls
`
`7
`
`Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/84052/ on 04/21/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use
`
`GE v. UTC
`IPR2016-00952
`GE-1030.007
`
`
`

`

`a
`
`0
`
`20dB
`
`9 I
`
`KEY
`
`a MG5 H
`
`• VARIABLE PITCH FAN
`• R-R MODEL FAN
`• NASA QUIET RAN (REF 7)
`
`Ku2AT
`NON-DIMENSIONAL FAN LOADING - p__
`
`50%
`
`CURRENT
`FAN LOADING
`
`150%
`
`Fig.12 Variation of single-stage fan intake noise
`levels with loading
`
`the gas generator high-pressure spool speed.
`The results to be obtained will depend very
`much on the engine component characteristics and
`the resultant matching achieved in the various
`parts of the speed range. On a particular design
`using semi-empirical component data, it has been
`shown that maximum cruise is always obtained when
`the fan speed is set to the maximum design limit
`and the turbine entry temperature is treated in
`like manner. It should be remembered that on a
`fixed pitch fan engine, the components are gener-
`ally matched to achieve a given maximum thrust and
`the part-load performance is the result of the con-
`sequent arbitrary combination of part-load charac-
`teristics which, in any case, presents a difficult
`performance prediction problem at low engine
`speeds. Hence, no general proposition can be
`stated regarding the magnitude of any part-load
`performance improvement.
`Again, the Development Engineer has, in ef-
`fect, an infinite set of fans at his disposal as
`he works to achieve his performance and reliabil-
`ity goals. The choice of the final geometry can
`be made during the test bed running phase on the
`basis of a measured optimum combination of fan
`blade angle and compressor speeds. In addition,
`it is likely that the fan pressure ratio and flow
`can be developed to higher power absorption during
`the later thrust improvement phase, possibly even
`without change in blade geometry.
`NOISE
`
`Current single-stage fan engines have jet
`noise levels which are close to those generated by
`the fan; hence, the employment of large amounts of
`sound absorbant duct lining will produce limited
`returns since they would leave the jet noise ob-
`
`8
`
`truding. The higher bypass ratio, lower jet ve-
`locity STOL engine enables linings to be used to
`maximum advantage and holds out the prospect of
`achieving the STOL noise target levels of 95 PNdB
`or EPNdB at 500 ft from the aircraft.
`At its design blade setting, the variable
`pitch fan should have noise characteristics which
`follow similar laws to those of fixed pitch fans,
`since conventional design rules have been applied
`and the blades have similar aerodynamic loading.
`The overall noise levels for the engine should be
`reduced because of the lower blade speeds and low-
`er jet velocities, but these are not necessarily
`peculiar to the variable pitch fan.
`Fan blading noise could be expected to be de-
`pendent on the parameters -- local dynamic head,
`local velocity, and blade lift coefficients. As
`blade incidence is reduced, then the steady-state
`pressure loads reduce and the fluctuating local
`pressures also should follow suit. Hence, it
`might be expected that noise created by unsteady
`flow over the blading at constant tip speed will
`also reduce. At the same time, the flow through
`the fan disk will be lower and the incident ve-
`locity onto the stator row will reduce substan-
`tially. Hence, overall noise may be expected to
`reduce for a number of reasons. In reference (4),
`it was shown that forward arc noise of a variable
`pitch fan does, in fact, drop rapidly at a rate of
`about 1 PNdB per 2 deg of pitch change. The total
`pitch change from the design value on approach to
`land may be of the order of 30 deg. This is fur-
`ther illustrated by Fig. 12 which plots fan for-
`ward arc total sound power, corrected for the ef-
`fects of tip speed and compressor size, as a func-
`tion of fan nondimensional power loading. This
`shows that variable pitch noise data exhibits sig-
`nificant downward trend with blade loading which
`is consistent with the results from fixed pitch
`fan designs. Hence, the philosophy for minimizing
`approach noise for a blow-fan engine would be to
`tune absorbent linings to the frequencies associ-
`ated with the higher engine speeds and reduced
`pitch settings.
`CONTROL SYSTEM CONSIDERATIONS
`The fan variable geometry creates a new
`degree of freedom similar to that obtained with a
`variable area exhaust nozzle. To use that free-
`dom to the best advantage requires a further ele-
`ment of complexity in the engine control. The
`two basic elements of the system are blade angle
`and fuel flow. These control fan speed together
`with associated compressor rpm and turbine entry
`temperature together with independent high pres-
`sure compressor rpm, respectively. The essence of
`
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`
`GE v. UTC
`IPR2016-00952
`GE-1030.008
`
`

`

`HIGH SPEED
`
`Fixed pitch fan
`
`DOWTY ROTOL TESTS
`
`..gismnaPP--
`Addffiftommowd:2221
`1. olak
`701114
`
`frf-
`
`LOW SPEED
`
`//
`
`"-- —
`
`GROUND LINE
`
`EXTERNAL FLOW
`-- —co-- FAN FLOW
`
`HIGH SPEED
`
`Variable pitch fan
`
`LOW SPEED --
`
`GROUND LINE
`
`Fig.13 Reverse thrust flow patterns
`
`any such control system for a compound compressor
`engine reduces to an attempt to schedule low-
`pressure shaft speed as a function of high-
`pressure shaft speed or vice versa.
`The different modes of steady-state opera-
`tion, optimum performance, minimum noise, and ap-
`proach with bleed may require several different
`schedules of compressor speed. The Olympus 593 in
`the Concorde already has such a system for use
`with its variable nozzle employing an analog elec-
`trical control. Alternatively, this is perhaps
`where the new digital control systems might be em-
`ployed to maximum advantage both for steady-state
`and transient operation. The Rolls-Royce Tyne
`Propeller turbine engine has, for many years, oper-
`ated with a hydromechanical system with broadly
`similar control requirements.
`
`REVERSE THRUST
`
`The constant speeding propeller was devel-
`oped to produce reverse thrust during the landing
`ground roll by allowing the blades to pass through
`zero pitch into negative pitch or, less ambitious-
`ly, to produce drag by "disking" in fine pitch.
`The airflow in these conditions is certainly not
`well ordered, and circulatory flows occur at the
`tips under forward speed conditions. Both blade
`camber and twist are in the wrong sense for maxi-
`mum performance. A typical propeller result would
`be a maximum reverse thrust of 30 percent.
`The use of a ducted variable pitch fan to
`
`100
`
`80
`
`60
`
`40
`
`FAN
`THRUST
`% MAX 20
`
`0
`
`-20
`
`-40
`
`-60
`
`INCREASING FAN BLADE ANGLE
`
`60
`40
`ENGINE POWER - °/o
`
`80
`
`100
`
`MEASURED
`— TARGET
`
`Fig.14 Variable pitch reversing fan typical
`power -- thrust relationship
`
`produce reverse thrust by passing through zero
`pitch in the manner of the propeller leads to a
`somewhat similar situation. The airflow is en-
`trained into what is normally the fan nozzle, with
`the assistance of supplementary intakes to reduce
`intake loss, and is ejected forward out of the nor-
`mal intake duct. The flow through the fan is
`again not well ordered and, hence, is difficult to
`predict. The