AIRCRAFT PROPULSION

AIRCRAFT PROPULSION
JET ENGINE PERFORMANCE EXERCISE
Introduction
This exercise is concerned with a simple
jet engine and involves using
the NASA
EngineSimU program.
The program
accept
s
data about the engine such as compressor
pressure ratio, turbine inlet temperature and the altitude and speed of the aircraft, in which
the engine is installed. The fuel consumption and thrust from a jet engine depends upon
engine design and the operating
condition of the aircraft. The performance of a jet engine
has to be estimated at an early stage for the flight conditions for which the aircraft will be
used.
Further background on this will be found in the textbook ‘Gas Turbine Theory’ by Cohen,
Rogers
and Saravanamuttoo. A typical output is shown in the form of a performance map
on Fig.3.12. (In the fifth edition).
From: Cohen, Rogers and Saravanamuttoo ‘Gas Turbine Theory’
The calculations carried out by the CATU program
:
The CATU program
accep
t
s
the altitude and use empirical equations to determine the
corresponding ambient pressure and temperature.
The aeroplane Mach number is used with the above to calculate the stagnation temperature
and pressure. Next the stagnation pressure delivered by t
he intake is calculated using the
efficiency of the intake. The pressure and temperature rise through the compressor is
calculated and the combustion temperature is evaluated by means of a numerical scheme
which is equivalent to using the Rogers & Mayhew
‘Combustion temperature rise
-
fuel air
ratio chart’.
Following the combustion chamber the turbine calculation is carried out, knowing that the
power from the turbine has to be sufficient to drive the compressor and turbine mass flow is
the compressor mass
flow + the fuel flow.
The exhaust products from the turbine are expanded through the propelling nozzle which is
normally of the convergent type and the gross thrust from the nozzle is calculated.
The above information is used, with the intake momentum dra
g, to calculate the specific fuel
consumption, SFC or TSFC in the case of EngineSimU. This is the fuel flow rate divided by
the net thrust.
2
Using the NASA EnginSimU program
To use this program go

http://www.grc.nasa.gov/WWW/k

-
12/airplane/engsimu.html
(make
sure that Java is enabled on your web browser).
Choose the turbojet option and
set the
program
for metric units.
(i)
Record the variation of the specific fuel consumption (SFC) wit
h the specific net
thrust (SNT) for an altitude of
7
000 meters, aircraft Mach number of 0.
8
, ideal intake
and nozzle, efficiencies of 0.
9
for the compressor and turbine, pressure loss of 0.03 in
the combustion chamber and efficiency of one, while varyin
g
the compressor pressure
ratio
from 6 to 9 in steps of one and the combustion chamber maximum
temperature
from 1
1
00K to 1
4
00K in steps of 100K. The SNT is to be calculated by
dividing
the
net thrust by the mass
airflow
rate.
Plot SFC .vs. S
NT
for these con
ditions using Excel or similar, producing a graph
similar to that
on the first page
.
[30%]
(ii)
Carry out a hand calculation for one of the conditions of the
above;
assume the
compressor and turbine
efficiencies
are isentropic.
Compare
your results of SFC and
SNT to the results produced by the
computer
program
. Explain the difference.
[30%]
(iii)
For three compressor pressure ratios and three combustion temperatures, vary the
Mach number
(at least three Mach numbers, one supersonic)
and altitude
(at least
three alti
tudes, one above 11km),
and plot the variation of SFC and SNT with the
Mach number and altitude
[20%]
(iv)
Discuss the effects of the variation of SFC and SNT with the Mach number and
altitude.
[20%]

You are required to produce only a brief report that answe
rs the questions set above. No
need for introduction or table of contents.
Include references in your report if needed.

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