Aero Chapter 05/06 (TS) and 05 (ADV & IUT), Thrust and Thrust Curve Review
T-45 Aerodynamics Student Workbook
increase in altitude will provide a decrease in
temperature and partially offset the decrease
in airmass flow. Above the tropopause,
temperature becomes constant and the
reduction in density and thrust is more rapid
Tr SEA LEVEL
(Figure 84).
Tr FL 350
While thrust is reduced at altitude, drag at
THRUST
altitude is also reduced due to reduced air
Vmr
density. In addition to increased engine
FL 350
efficiency, an aircraft realizes increased true
airspeed in the less dense air at higher
Vmr
altitudes (Figure 85).
SEA LEVEL
Summarizing the effect of temperature alone
on the thrust output of a turbofan engine, as
KTAS
temperature increases, density, mass flow,
and thrust available will decrease. As density
Figure 85: T-45 TRUE AIRSPEED VS THRUST
decreases, mass flow (Q) and thrust
available will decrease. According to the
- 42 F. Day
thrust available equation, Ta=Q(V2 -V1), as Q
Standard Day
goes, so goes Ta. Density will increase with
105 F. Day
a temperature decrease and Ta will increase.
(Figure 86) illustrates the difference in thrust
6,000
that can be expected at sea level with -42F,
5,000
59F (standard day), and 105F
temperatures.
4,000
HUMIDITY
3,000
High humidity will cause a reduction in thrust
because the density of moist air is less than
2,000
that of dry air. Water vapor molecules have
less mass than air molecules and replace
1,000
the air molecules resulting in less mass flow
through the engine.
0
30
40
50
60
70
80
90
100
110
% RPM
Relative humidity, which affects
reciprocating engine power appreciably, has
Figure 86: T-45 TEMPERATURE VS THRUST
much less effect on turbojet and turbofan
engine thrust, fuel flow, and rpm. Therefore,
humidity is not normally taken into consideration when determining fuel flow and rpm for normal jet
operations.
(7-99) Original
Page 53
Previous Page
