Gas turbine engine thrust measurement

The thrust produced by a gas turbine engine can be calculated from the rate at which fuel is consumed by the engine. At a constant air temperature and pressure the thrust produced by the engine is proportional to the rate of fuel consumption.

FIELD OF THE INVENTION 
This invention relates to gas turbine engine thrust measurement, 
particularly in aircraft gas turbine engines. 
It is desirable to be able to measure the thrust of a gas turbine engine 
because the thrust produced by the engine can be used as a measure of its 
mechanical condition, engines could, for example, be serviced when their 
maximum thrust drops below some specified level. Additionally before an 
aircraft takes off it would be useful to measure the thrust being produced 
by its engines in order to ensure that this is enough to allow a safe take 
off and flight. 
DESCRIPTION OF THE PRIOR ART 
It is easy to measure the thrust of a gas turbine engine running on a test 
bed by using a strain gauge for example, however, it has proved very 
difficult to measure the thrust produced by a gas turbine engine when 
installed on an aircraft. 
It has been attempted in the past to produce systems which measure 
parameters such as the pressure and temperature at various points in the 
engine and the speed of rotation of the or each spool in the engine, such 
systems are generally complex and are not very satisfactory because of the 
large random variations in these parameters which occur within the engine. 
These variations make it very difficult to relate the simultaneous values 
of different parameters or the same parameter at two different points in 
the engine in order to calculate the thrust being produced by the engine, 
and as a result the derivation of thrust is complex and unreliable. 
BRIEF SUMMARY OF THE INVENTION 
This invention was intended to produce a simpler method of finding the 
thrust of a gas turbine engine. 
In a first aspect this invention provides a method of gas turbine engine 
thrust measurement in which the thrust produced by the engine is 
calculated from the rate at which fuel is burnt by the engine. 
In a second aspect this invention provides a method for calculating whether 
or not a gas turbine engine is producing an acceptable amount of thrust 
employing, a graph having the rate at which fuel is consumed by the engine 
plotted on one axis and ambient air temperature plotted on the other axis 
and divided into an acceptable thrust area and a number of unacceptable 
thrust areas. 
In a third aspect this invention provides apparatus for calculating the 
thrust produced by a gas turbine engine comprising; a processor, a fuel 
flow meter providing a signal describing the rate of fuel flow into the 
engine to the processor, an air pressure sensing means supplying a signal 
giving the air pressure value to the processor and a thermometer supplying 
a signal giving the air temperature value to the processor, the processor 
being arranged to operate on said signals to calculate the thrust produced 
by the engine. 
It has been found that the thrust produced by each design of gas turbine 
engine in a steady state condition is proportional to the rate at which 
fuel is burnt within the engine provided the air pressure and temperature 
on entering the engine is constant. Furthermore this ratio is largely 
unaffected by wear or minor damage to the engine, the reduction in thrust 
caused by wear or minor damage being matched by a reduction in the rate at 
which fuel is burnt. 
The relationship between thrust and the rate of fuel burning will generally 
be different for different engine designs and may be different for engines 
of the same design in different airframes having different air intakes. 
Accordingly, once the relationship between thrust and rate of fuel burning 
at a given air temperature and pressure is known it is possible to 
calculate the thrust of an engine at that air temperature and pressure by 
simply multiplying the rate at which fuel is supplied to the engine by the 
appropriate constant. Indeed if all that is required is a measure of 
whether the engine is producing enough thrust for a safe take off it is 
not even necessary to do this, all that is required is to check that the 
fuel flow rate is within a band corresponding to a band of acceptable 
engine thrust levels. 
In reality temperature and pressure will not remain constant so rather than 
a simple multiplication by a constant a formula including pressure and 
temperature terms must be used. This formula can be obtained by running an 
engine on a test bed in varying conditions and measuring the thrust 
produced.

DETAILED DESCRIPTION 
Referring to FIG. 1 a graph of air temperature against fuel flow rate is 
shown. This graph is divided into three regions 1, 2 and 3 by two lines 4 
and 5. The lines 4 and 5 are the loci of the minimum and maximum 
acceptable fuel flow rates over a range of temperatures respectively. The 
graph is plotted for the fuel flow rates with the engine at maximum power 
and stationary at ground level. 
The region 1 below the line 4 corresponds to engine thrusts that are 
unacceptably low. The region 2 between the lines 4 and 5 corresponds to 
acceptable engine thrusts and the region 3 above the line 5 corresponds to 
unacceptably high engine thrusts. 
Generally an unacceptably low engine thrust will indicate that the engine 
is worn or damaged while an unacceptably high calculated thrust generally 
means that there is a fault in either the fuel meter or the temperature 
sensor or the engine is operating outside its normal limits. 
In use the pilot would prepare the aircraft for take off. When the aircraft 
was stationary at the end of the runway with its engine at full power the 
pilot would take the fuel flow rate and air temperature reading from his 
instruments and looking at the graph of FIG. 1 decide where the point 
corresponding to these two values was. 
If the point corresponding to the fuel flow rate and temperature values is 
in the regions 1 or 3 take off will be aborted because this will show that 
there is a fault in either the engine or the instruments. 
If the point corresponding to the fuel flow rate and temperature values is 
in the region 2 the pilot can proceed to take off knowing that he has 
sufficient engine thrust available. 
A third line 6 could be positioned between the lines 4 and 5 to divide the 
region 2 into two parts 2a and 2b. In this case, if the point 
corresponding to the fuel flow rate and the temperature is in the lower 
part 2b of the region 2 the pilot will proceed with take off but will 
later report that the engine was producing a thrust in this region so that 
appropriate maintenance action may be taken. 
Each graph of the type shown in FIG. 1 will only be correct for one 
specific air pressure, so it would be necessary to either produce a 
separate graph for each airfield or group of airfields at the same height 
above sea level or to provide a conversion table to allow the scales of a 
single graph to be converted for use at a number of different heights 
above sea level. In general variations in atmospheric pressure due to 
weather will be relatively small and can be ignored. 
Since the aircraft is stationary the air pressure and temperature on 
entering the engine will be the ambient air presure and temperature. 
An automatic system for sensing engine thrust is shown in FIG. 2. 
A gas turbine engine 7 is supplied with fuel from a tank 8 by a pump 10 
which pumps the fuel along a pipe 9. A fuel flow meter 11 measures the 
rate at which fuel flows along the pipe 9 and into the engine and produces 
an electrical signal dependent on this rate of flow. This signal is 
supplied to a processor 12. 
The power level of the engine 7 is controlled by a throttle 13, and a 
signal giving the throttle setting is also supplied to the processor 12. 
On each flight, the first time full power is selected on the throttle 13, 
which will be when the engine 7 is run-up for take off, the processor 12 
delays a short period for the engine 7 to reach full power and then, 
provided that the throttle 13 is still at full power, takes the fuel flow 
rate signal from the meter 11 and air pressure and temperature readings 
from a barometric pressure sensor 14 and a thermometer 15 respectively. 
This short delay before taking all of the reading is necessary because in 
gas turbine engines there is always a delay between a new throttle 
position being set and the engine reaching a steady state at the new power 
level. The length of the delay needed will depend on the design of the 
engine. 
The processor 12 operates on the reading from the fuel flow meter 11, 
pressure sensor 14 and thermometer 15 in accordance with an algorithm to 
derive a result that the fuel flow, and therefore the thrust, is too high, 
too low or acceptable. 
If the fuel flow is too low the thrust produced by the engine 7 is too low 
to allow safe take off, so the processor 12 switches on an engine fault 
indicator light 16 on the instrument panel, warning the pilot not to 
attempt to take off. 
If the fuel flow is acceptable the thrust produced by the engine 7 is 
sufficient for a safe take off, so the processor 12 switches on an engine 
acceptable indicator light 17 on the instrument panel clearing the pilot 
to attempt take off. 
If the fuel flow is too high, the engine 7 may be operating outside its 
normal limits or it may not have settled to a steady state or the fuel 
flow meter 11, pressure sensor 14 or thermometer 15 may have provided a 
false reading to the processor 12 for some reason. If the engine 7 has not 
settled to a steady state a further delay may allow it to do so, and if 
the meter 11, sensor 14 or thermometer 15 has given a false reading it may 
not do so again if the reading is re-taken. So the processor 12 checks 
that the throttle 13 is still at full power and then takes fresh readings 
from the meter 11 pressure sensor 14 and thermometer 15 and operates on 
them again using the same algorithm. If the new result is that the fuel 
flow is too low or acceptable the processor takes the action described 
above and switches on one of the lights 16 and 17. If the new result is 
again that the fuel flow is too high the processor 12 switches on an 
instrumentation failure or engine overated indicator light 18 on the 
instrument panel, warning the pilot not to take off. 
If the fuel flow is acceptable by less than a pre-set amount the processor 
12 switches on the light 17 but also places a low thrust message in a 
memory 19. The memory 19 is checked by maintenance personnel after each 
flight and if a low thrust message is found appropriate maintenance action 
is taken. 
The system described could be arranged to provide an actual thrust value to 
the pilot by employing the appropriate algorithm and having the product of 
this algorithm displayed on a numerical display on the instrument panel. 
This might be desirable if the minimum thrust required for take off varied 
due to some further parameter such as weather conditions, payload or 
runway length. 
The system shown in FIG. 2 can be used to measure engine thrust in flight 
with a few alterations. As explained above the relationship between thrust 
and rate of fuel burning varies with the air temperature and pressure at 
entry to the engine 7. When the take off thrust sensor of FIG. 2 is used 
the aircraft is stationary so the ambient air temperature and pressure is 
the air temperature and pressure of the air entering the engine 7, however 
in flight the temperature and pressure of the air entering the engine 7 
will be different from the ambient air temperature and pressure because of 
the ram effect of the movement of the aircraft through the air. 
In order to find the temperature and pressure of air entering the engine 7 
the thermometer 15 is positioned in the airflow just in front of the 
engine 7, while the barometric pressure sensor 14 is replaced with a 
pressure sensor positioned in the airflow just in front of the engine 7. 
The processor 12 can be set to take measurements each time full throttle is 
selected in flight, alternatively some other trigger such as a pre-set 
time or a command from the pilot could be used. 
Each time the processor 12 is instructed to take measurements it carries 
out the sequence of actions described above, to inform the pilot whether 
or not the engine7 is still functioning correctly. 
Aircraft do not usually operate for long periods at full throttle in 
flight, so it may be preferred to monitor the performance of the engine 7 
by measuring the thrust it produces in some other condition such as cruise 
for example. This would require the use of a different algorithm because 
the relationship between fuel flow, air temperature and pressure and 
thrust will generally vary across the power range of an engine. 
It may not be necessary to employ a pressure sensor in the airflow just in 
front of the engine 7, most aircraft already use a barometric pressure 
sensor for use as an altimeter and a pitot device measuring the difference 
between barometric and total pressure to determine airspeed, where 
available the signals from these two sensors could be used by the 
processor 12 to calculate the pressure of the air entering the engine 7. 
Instead of using an algorithm the processor 12 could employ look up tables 
to decide whether or not the fuel flow rate was acceptable. 
All of the thrust measurement methods described could be used in a 
multi-engined aircraft by either measuring the total fuel flow and 
deriving the total thrust or by measuring the fuel flow of each engine 
separately and deriving the thrust of each engine separately.