433
12.3 Performance Parameters of Aircraft Propulsion Systems
with Dv the effective velocity increase. In the formula, the mass flow increase due
to fuel injection is neglected. Specific thrust is thrust divided by the mass flow rate
aspirated by the propulsion system. With the simplified formula for thrust, this is
The unit thus is m/s. The extension to double-jet propulsion systems is immediate. It
is then the total thrust divided by the sum of the aspirated mass flows. With turbofan
engines, the order of magnitude is 300 m/s at take-off and 150 m/s at cruise. With
propellers, the value varies from 30 to 50 m/s in cruise flight.
12.3.2 Dynamic Power
Dynamic power is the power corresponding to the kinetic energy increase in the
flow through the propulsion system:
(12.6)
Again, extension of the notion to double-jet systems is immediate.
12.3.3 Gas Power and Dynamic Efficiency
In a single-jet engine, the dynamic power originates from the flow acceleration in
the nozzle. The gas entering the nozzle is delivered by a gas turbine, which thus
functions as a gas generator. The nozzle may then be defined as the jet generator.
Gas power denominates the power that would be delivered to the propulsive jet
formed in the nozzle in absence of losses, thus by an isentropic expansion. The gas
power is thus the ideal value of the dynamic power. It may be interpreted as the net
output of the gas generator and the net input to the jet generator. The efficiency of
the jet generator, or the dynamic efficiency ( η d ), is then defined as the ratio of dynamic power to gas power.
12.3.4 Thermal Power, Thermodynamic Efficiency and Thermal
Efficiency
Thermal power is the power released by combustion. Thermodynamic efficiency
( η td ) is the ratio of gas power to thermal power. Thermodynamic efficiency thus
characterises the efficiency of the gas generator. Thermal efficiency ( η t ) of the propulsion system is defined as the ratio of dynamic power to thermal power.
T T m
v
s =
=
/
.
∆
(
)
=
−
2
2
1
dyn
j
0
2
P
m
v v .
12.3 Performance Parameters of Aircraft Propulsion Systems
with Dv the effective velocity increase. In the formula, the mass flow increase due
to fuel injection is neglected. Specific thrust is thrust divided by the mass flow rate
aspirated by the propulsion system. With the simplified formula for thrust, this is
The unit thus is m/s. The extension to double-jet propulsion systems is immediate. It
is then the total thrust divided by the sum of the aspirated mass flows. With turbofan
engines, the order of magnitude is 300 m/s at take-off and 150 m/s at cruise. With
propellers, the value varies from 30 to 50 m/s in cruise flight.
12.3.2 Dynamic Power
Dynamic power is the power corresponding to the kinetic energy increase in the
flow through the propulsion system:
(12.6)
Again, extension of the notion to double-jet systems is immediate.
12.3.3 Gas Power and Dynamic Efficiency
In a single-jet engine, the dynamic power originates from the flow acceleration in
the nozzle. The gas entering the nozzle is delivered by a gas turbine, which thus
functions as a gas generator. The nozzle may then be defined as the jet generator.
Gas power denominates the power that would be delivered to the propulsive jet
formed in the nozzle in absence of losses, thus by an isentropic expansion. The gas
power is thus the ideal value of the dynamic power. It may be interpreted as the net
output of the gas generator and the net input to the jet generator. The efficiency of
the jet generator, or the dynamic efficiency ( η d ), is then defined as the ratio of dynamic power to gas power.
12.3.4 Thermal Power, Thermodynamic Efficiency and Thermal
Efficiency
Thermal power is the power released by combustion. Thermodynamic efficiency
( η td ) is the ratio of gas power to thermal power. Thermodynamic efficiency thus
characterises the efficiency of the gas generator. Thermal efficiency ( η t ) of the propulsion system is defined as the ratio of dynamic power to thermal power.
T T m
v
s =
=
/
.
∆
(
)
=
−
2
2
1
dyn
j
0
2
P
m
v v .