The installation of generators on turbine aircraft is discussed. Emphasis is placed on the use of the samarium cobalt generator. Potential advantages of an electric secondary power system at the engine level are listed. The integrated generator and the externally mounted generator are discussed. It is concluded that the integrated generator is best used in turbojet and low bypass ratio engines where there is no easy way of placing generators externally without influencing frontal areas

Demel, H. F.

English

NASA Technical Reports Server

INSTALLATIONOF ELECTRIC GENEPATORS ON TURBINE ENGINES •
Herbert F. Demel
General Electric Company
Aircraft Engine Group
Cincinnati, Ohio 45215
This discussion of the installationof generators on turbine aircraft is
based on studies performed at General Electric, in particular the studies of
the samarium-cobaltgenerator.
Contributing technologieswere derived from three contacts: two fron,the
Aim Force, and one from the NASA Lewis Research Center. All three programs
had two common elements: the samarium-cobaltgenerator, and variable-speed,
constant-frequency(VSCF) solid-state technology. The generator-engine inte-
gration study was performed for the Air Ferce and finished in 1979. The goal
was to determine the feasibility of integral generators. A study based on the
Energy Efficient Engine (E_) program was performed for NASA Lewis. It sought
,. to define potential engine improvementsthat would provide performance in-
creases. The General Electric Aircraft Equipment Division performed a develop-
ment and hardware study for the Air Force on the 150-kVA _ermanent-magnet(PM)
VSCF starter-generator. The results of this study were applied to the other
two programs.
The potential advantagesof an electric secondarypower system at the
engine level are
(1) Improvedengine efficiency, by eliminationof constant bleed air
(electric-drivenenvironmentalcontrol system (ECS))
(2) Enhanced reliability because of the solid-rotorPM generator-
starter and fewer engine-mountedaccessories
(3) Improvedmaintenance b_cause there is only one type of secondary
power
(4) Improved accessibilitybecause the engine periphery is less congested
and the line replaceableunits (LRU) that remain are more easily replaced.
The seconaary power system must be evaluated on the overall aircraft level
to determine the net payoff. For example, weight added to the engine could be
offset by weight reductionson the aircraft; this is particularly true in the
integrationof generatorswith the engine. The applicationsfor the highand
• low-bypass-ratioengines are quite diffe-ent. In high-bypass-ratioengines
there is much more freedom to locate generators either inside or outside the
engine without increasingthe engine's frontal area. In low-bypass-ratioen-
gines the location is more difficult because the space in the front end of the
engine is smaller and also because we would like to minimize frontal area in-
crease. Existing engines would have to be analyzed for the effect on engine
• , dynamics. Of course, it is much easier to design an engine to incorporate
secondary power systems than to incorporatethem in an existing engine. A wide i
: spectrum of possible engine configurationswould have to be considered. The
all-electricsecondary power system that has only electric power extraction !
' would have electricallydriven accessories. A hybrid power extraction system 4
could still use some mechanical power (e.g., mechanically driven engine
accessories).
- The parametersfor system selection are performance,safety, reliability,
:_. maintainability,physical weight, volume, and initial and operational cost.
_" The higher efficiency of electric secondary power systems would reduce the
size of heat-exchanglngsystems, and result in a small improvement in the
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1984001988-073
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specific fuel consumpt;u, (_fc) of the engine. Permanent-magnet generators
require short-circuit protection s,'ch as a rotor disconnect. For hioh-speed
applications the generators would also require rotor burst protection. The
effect on engine efficiency in terms of specific fuel consumption would not be
as important since the secondary power level is r_atively small as compared
with the engine power, but has to be considered. Permanent-magnet-rotor gener-
ators would have to be safe in terms of electric safety, disconnects, or other
means and, in high-speed applications, burst protection. Reliability is one
of the highest priorities, especially if the generator is to be integrated
with the engine. Maintainability for an externally mounted generator would be
similar to that for current aircraft. However, the engine would ha_e to be
desigmed for easy access with _n integral generator. The engine physical
weight might not decrease. The volume available will depend on the Lype of
engine - high or low bypass ratio - _nd the power required by the aircraft.
Initial and operational costs would )e major trade-offs.
lwo different design concepts will be discussed: the integrated gen-
erator and the externally mounted generator. Each concept has advantages and
disadvantages.
Figure I shows a cross section of the TF-34 high-bypass-ratioengine with
a 120-kVA samarium-cobaltpermanent-magnetgenerate- integrated into the front
end. The generator,which also works a_ a starter, is placed on an extension
of the high-pressureturbine shaft; the stator is mounted into the frame struc-
ture. The generator is cooled by 200° F engine oii circulated through and
around the stator. To remove the generator, the engine must be removed from
the aircraft and split, which is of course a disadvantage. However, because
of the large dimensions and conservative design we expect high reliability,
ant thereforemaintenance would not be required frequently. The reliability
of this generator could be increasedmany times with improved technology. It
could then be viably integrated,especially in military aircraft. M_litary
aircraft have low-bypass-ratioengines or pure jet engines, where external
placement of the generators would increase the frontal area.
To remove and replace this generator would take 4 to 5 hours. It weighs
about 100 lb. Sixty kVA would be necessary to start the engine; the startup
procedurewould take about 23 sec, which is equal to pneumatic starting as was
desired.
The possible implicationsof engine dynamics have been _tudied, an_ in
this case it was not too great of a disadvantage to have the generator mounted
on the high-pressureshaft, although it resulted in a somewhat shorter bearing
life. This machine has about a 0.065-in. geometric radial gap. The sidE,
forces induced through eccentricity would be small.
• _igure 2 is also a cross section of a high-bypass-ratioengine. This is
the EJ engine, the result of the Erergy Efficient Engine program General
Electric is conducting for NASA, i .i_ design shows two 9enerators mounted
separatelyon an external gearbox located in the core area. Because the core
area is aft of the fan frame and between the core and the fan bypass flow duct,
cowl doors would have to be opened to get to thi_ area. The two generators
,_.' are rated at 150 kVA each; the use of two generators is the result of a study
for the all-electricaircraft where redundancywas required.
The accessory drive system can be made smaller by eliminating the hydrau-
lic pumps. If electric-drivenengine acuessoriesare used, the accessory drive
l system could be reduced even further. Electric-drivenengine accessories could
_: be located in strategic areas around the engine for easy access and maintain-
_"_. ability. One advantage of thls would be, for example, that a fuel pump could
_ be operated to get the flow desired withuut any bypass flow and with a decrease
in fuel temperature,
_ 74
.q
1984001988-074
FiqurE 3 is a frontal view of the area shown in fiqure 2, showin 9 the
]ocdliur_ af the generators mounted side by side. The generater speed is
24 000 rpm and the high-speed fuel pump speeo is 27 000 rpm. Each generatur
weighs about i00 ]b and is 10-1/2 in. in diameter and 14 in. long. The cvclo-
converters are mounted e _erna] to the e_glne and are readily accessible.
When the characteristics of the integrated engine-generator are con_Dared,
it is always to a base engine design that uses a Lonventional secondary power
system.
The ir,_egrated-engine-generator study was really a matrix of three studies
on three existing military engines: a large high-bypass-ratio CF6 engine; a
smaller high-bypass-ratio engine, the TF-34 used on the A-10 airplane; and a
high-performance, ]ow-bypass-rd_io engine, the F-404.
The stuay was conducted to determine the feasibility of an integrated
generator-starter. The ground rules were to use existing engines, a samarium-
cobalt generator, and VSCFtechnology. After the study one engine (the TF-34)
was chosen t_ be studied in greater detail. OnP requirement was the incorpora-
tion of a safety disconnect. Since a permanent-magnet generator cannot Le
deenergized as long as it is _utating, we had to disconnect it either mechani-
cally or electrically in case of a failure. An e_ectric fuse disconnect was
chosen as the best solution at that time, but others working on this problem
indicateother possible ways to safeguard against a short in the generator.
The results of this study show that the highest payoff of the integrated
generator-starterwould be in high-performanceaircraft, where it wJuld not
induce drag anJ therefoFe have a negative effect on aircraft design. The ap-
plication to high-bypass-ratioengines is not as advantageousbecause electric
power can be extracted through other means without affecting the frontal area
of the engine. For example, generators could be l_cated in the core area or
in the pylon.
The integratedgenerator-starterwould eliminate the accessory gearbox.
o _ There would not be a mechanical drive shaft extending to the outside of the
engine. Therefore the engine accessories wou_d have to be driven electrically.
An all-electricsecondary power system has only one snurce of power, in con-
trast to conventionalsystems, which have multiple sources of power. High
reliability results from fewer components and conservative design. Low mainte-
nance results from high reliabilityand the elimination ut subsystems. The
interface between the engine and the aircraft would _]so be simplified since
only fuel _nd electricity are transferredbetween them.
The disadvantagesare obvious: th= center of the engine is not readily
accessible since the engine has to be split and removed from the aircraft.
The redundancy Question had not been studied at that time, but it seems that
it would be difficult to achieve redundancy in the limited space available.
Development risk Js on any new system, wou_d be present.
The objectiJe o; the study using the Ej engines 'lasto define the opti-
" mal locationof the generator system on commercial aircraft. The se,_ction of
a twln-englne aircraft resulted in two generators per engine for redundancy.
The study showed that the externally mounted generators had more advantages
than integrallymounted generators for this application. The benefits of ex-
ternally mounted generators are as follows: th- generators can be optimized
since they are not limited _o engine speed, redundancycan be easily achieved
_vmounting two generatorson the accessory gearbox, the generators are easily
d:cessible for repair or replacement,and current-technologygenerators can be
used. The major disadvantageof exte,'nallymounted generators is that they
i! requlre amechanical drivesystemand an accessorygearbox, i_
In summary, the integratedgenerator is best used in turbojet and low-
bypass-ratioengines, where there is ,o easy way of placing generators
75
1984001988-075
externa]lywithout influencingfrontal areas. This weuld hp (he case for
high-perfcr_anc_dircraft, especially military, where the adjustment of engine
design on existing engines would bE feasible.
The h.ybridsystem, which wnuId use generators and mechanically driven
engine accessory systems, ha_ advantagesfor high-bypass-ratioengines. The
highest payoff would be on the overall aircraft level, where maximum power can
be extracted and used on the a_rcraft, for example, for all-e!_ctric environ-
mental control systems. The primary advantage of using samarium-cobaltgener-
ators, either integratedor exttrnal, would be their higher reliability as
compar d with cenventiona]wound-rotor generators. They also require _ess
maintenance and have lower life-cyclecosts. The techr,ologyof semarium-cobalt
generator starters is available now and could be incorporated into any existing
engine.
In conclusion, a comprehensivestudy would be reauired to show the advan-
tages and 6isadvantagesof all-electric secondarypower on the overall system
level. This multidimensionalmatrix would have to includ_ military as well as
tommercial aircraft. The electric power reauirements for engine-out and peak-
power conditionswould ha_a to be established and matched to the redundancy
requ red. The electric system Question of ac versus dc or the hybrid system
would have to be addressed, and the deve]_pment of end-user eqL'ipmentwould
have to proceed. Studies a_ the engine level would include the evaluation of
the locationand design of seconoary power systems, comparing integratedand
externallymounted generators. Systems have never been compared on an equal
basis: they have been compared with the baseline only, but not against each
other with the same ground rules. #dvanced engine accessory development would
have to continue, since fuel and lubricationpumps are components that actually
size the accessory gearbox because of their frontal area and mounting reouire-
ments. The use of electric power on the engine level would have to be inves-
tigated, inc]uding systems for thrust reversal and geometric actuation as w_ll
as new methods of electricallyanti-icing engine nacelles. New LU_,,,,v"-_--''-_-',u_,=_
must be pursu' _. nrovide protectin- against stator shorts in permanent-magnet
generators. ._ _ 'hine construction wouid allow redundant integraT genera-
tors to be t ._.
1984001988-076
_li41L ORIGINAl-PA_E IS
OF POOR QUALrrY
HARNESS
ASSEMmLY
COOLING
PASSAGE
IL-SLINGER
f
FUSE HOLDER ASSEMBLY
SHAFT
Figure I. - 120-kVA. perman_,%-magnet integrated engine starter-generator.
\
(2) 180 KVA ImCo PM-MACHINES E3
Figure 2. - Gearbox-drlven starter-generator.
1984001988-077
* ORIGINAL PACE IS
OF POOR QUALITY
r
AFT LOOKING FORWARD
I IEL BOOST PUMP
i 48oo RPM
ACCESSORYeNCt
LUBE & SCAVENGE PUMP
4800 RPM
)WL DOOR
180 KVA 8mCo
/
8YARTER/GENERATOR _
LMAIN FUEL PUMP
24000 RPM 27000 RPM
Figure 3. - Starter-generators and engine accessories.
78
1984001988-078


Installation of electric generators on turbine engines

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