Preparing for EASA Mod.15 Gas turbine engines . Then avoid reading lengthy books here are my personal short notes and explanations and important topics for Mod.15

1. G.T.E FUNDAS Part – ii
PERFORMANCE OF GAS TURBINE ENGINE. - Nisarg Mistry

2. THRUST AND FACTORS AFFECTING THRUST.
Thrust-
It’s a force which propel or pull or push acting on an aircraft .
Thrust is a Reaction force .
Thrust = mass X acceleration.
Its S.I unit is - lb (pound).
Its of two types –
1) - Gross Thrust.
2) – Net Thrust.

3. 1) – Gross Thrust -
Static Thrust is k/a Gross thrust.
Gross thrust is produced when the engine is running and aircraft isn't moving.
The Gross thrust is calculated when the engine starts on the ground.
In other words - “The total thrust of a jet engine without deduction of the drag caused by the
momentum of the incoming air (ram drag). This thrust is measured when the engine is static, or
not moving.”
•While computing gross thrust the velocity of air coming into the engine due to the velocity of
aircraft is disregarded.
Velocity of Gas leaving the engine is used as accelerating factor.
It is denoted by Fg .
Fg = W (V2 – V1 )
g
Where ,
Fg = Gross Thrust,
W = Weight of Air.
V1 = Intake velocity of Air.
V2 = Exhaust Velocity of Air.
g = Accln due to Gravity. = 32.2 ft/sec2.
Note – While calculating Fg the ‘g’ is not taken
in S.I system it is taken in Ft/sec2 .

4. 2) – Net Thrust -
Net Thrust is the amount of force which develops when aircraft is INFLIGHT.
The Net Thrust is the diff. b/w Gross thrust and Drag Thrust(RAM DRAG).
RAM DRAG (Fd )- The loss in thrust involved in taking the air from the front of
Engine is k/a RAM DRAG.
Net thrust is Denoted by Fn .
Fn = Fg - Fd

5. PERFORMANCE OF A G.T.E
Non-Designing factors affecting Thrust -
Speed of Air or Speed of Aircraft.
Temperature.
Pressure.
Density.
Humidity.

6. 1) – Speed of Aircraft and Speed of Air -
Increase in Fwd speed of the aircraft will result in Decrease of Thrust.
Faster the aircraft moves greater will be the initial momentum of air i.e the
Inlet velocity (V1 ) will increase. Hence , the Diff. in momentum change will
Become smaller so , Thrust will decrease.
THRUST
AIRSPEED
The slope justifies that with an
Increase in airspeed , thrust
Decreases.

7. 2) – Temperature -
In free air , a rise in temp. will cause the speed of the molecules to increase
So that they run into each other more vigorously . When they are further apart
a given number of molecules will occupy more space , fewer molecules pass
through the engine inlet area . Hence , as the air temp. increases thrust tends
to decrease.
%thrust
Increasing air temp.
Flat Rated Thrust – In some manufactured engines thrust it is found that they
“flat rate” their engines to a higher temp. That is the
engine is guaranteed to produce a min. specific thrust at
59o F (150 C).
PWR lever manipulation is required while operating at low temps.

8. 3) – Pressure -
When the pressure increases in free air , it is becoz there are more molecules per
cubic foot(meter) . When there are more molecules per cubic foot , more of them
can be directed through the fixed inlet area. More molecules going through the
inlet area per second means a greater weight , or mass, of air . As pressure goes up
, density goes up . Hence, due to a rise in density weight of air inside the intake
increases eventually producing more thrust.
Thrust
Increasing Air pressure

9. 4) – Density Effect -
Density is the mass per unit volume or the number of molecule per cubic foot (meter).
The amount of air directed into the engine per second is controlled by the fixed engine inlet area
, assuming constant RPM . Thus, the mass flow is determined by the density.
Density say its D.
Temp 1
D
Pressure
therefore , D = K P
T Temp.
Since, if there is an increase in temp. the density decreases , when density decreases less
molecules are present per cubic foot hence , a decrease in thrust.

10. A rise in altitude will effect the temp. to drop and so the pressure .
The effect of altitude change on thrust is really fxn of Density.
36,000ft
Thrust
Increasing Altitude
At about 36,000 Ft ( 10973 m) , essentially the beginning of the tropopause ,
the temp. stops falling and remains constant while the press. Continues to fall .
As a result (refer graph) , the thrust will drop off more rapidly above 36,000 ft becoz the
thrust loss due to the air press. Drop will no longer be partially offset by the thrust gain due to
temp. drop . Thus, 36,000 ft is the optimum altitude for long range cruising,
becoz at this altitude , even though the engine’s thrust is reduced , the relation b/w thrust
produced and drag minimized on aircraft is most favorable.
Most imp.

11. 5) – Humidity Effect -
Jet engines are built for cold dry air , and humid air has fewer o2 molecules to
burn per unit volume . Therefore, the engine combusts a little bit less air &
puts out slightly less thrust.
It has low effect on GTE coz the F.C.U ( Fuel Control Unit ) meters the fuel on
the basis of pressure (inlet & burner press) not volume and humidity effects
the volume so, this has negligible effect on thrust.
In case of Reciprocating Engine the fuel/air is controlled by carburetor , the carburetor
controls the volume of air/fuel inside the cylinder Hence , changes in humidity has a great
effect on Piston Engines.
To Be Continued in next presentation……