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Propulsion System Analysis
Answers


 Why do different aircraft have different types of
propulsion systems?
Different propulsion systems produce thrust in different ways. Some ways are
more fuel efficient than others. Depending on the mission of the aircraft,
a certain propulsion system is better suited for a given aircraft. High thrust
aircraft typically have worse fuel economy than low thrust (cruise) aircraft.
 When would you use a ramjet
on an airplane?
For very high speed aircraft, access to space, or hypersonic cruise.
 Find the Mach number
for a subsonic airplane flying at 650 mph.

M = Mach

M = V/A

V = Air Velocity

M
=.85

A = 762 mph (speed of sound)


V = 650 mph

 Find the Mach number for a subsonic airplane flying at 525 mph.
M = 525 mph/762 mph =
.69
 Find the Mach number for a suubsonic airplane flying at 725
mph.
M = 725 mph/762 mph =
.95
 Find the velocity of a subsonic airplane flying at Mach number
of .65.
M =V/A

M = Mach

.65 = V/762

V = Air Velocity

V = 495.3
mph

A = 762 mph. (speed of sound)

 Find the velocity of a subsonic airplane flying at Mach number
of .80.
M = V/A
.8 = V/762
V = 609.6 mph
 Find the velocity of a supersonic airplane flying at Mach number
of 1.5.
M = V/A
V = M * A = 1.5 * 762
V = 1143 mph
 Using the interactive
Atmosphere Calculator,
find the speed of sound, pressure and temperature at the following
altitudes using English Units.
ALTITUDE

SOUND SPEED

PRESSURE

TEMPERATURE

1,000 ft.

1113

14.17

55

5,000 ft.

1097

12.23

41

10,000 ft.

1077

10.11

23

20,000 ft.

1037

6.76

12

25,000 ft.

1016

5.46

30

30,000 ft.

995

4.37

47

 Produce three graphs from the data recorded above, showing
speed of sound, pressure, and temperature versus altitude.
 What effect does altitude have on the speed of sound?
The speed of sound decreases
with increasing altitude.
 What effect does altitude have on pressure?
The atmospheric pressure decreases
with increasing altitude.
 What effect does altitude have on temperature?
The atmospheric temperature decreases
with increasing altitude.
 Flying at a constant speed of 600 feet per second, find the speed of
sound and the Mach number at the following altitudes.
ALTITUDE

SOUND SPEED

MACH NUMBER

1,000 ft.

1113

.539

10,000 ft.

1077

.556

20,000 ft.

1037

.578

30,000 ft.

995

.602

 Graph the changes in Mach Number
at the different altitudes.
 How does the change in altitude affect Mach Number when
flying at a constant speed?
The Mach number increases
with increasing altitude for the same speed.
 The thrust of a jet engine is also affected by changes in altitude.
Calculate the thrust of a Pratt & Whitney JT8D17 jet engine (17,000 pounds at sea level)
at different altitudes using the temperature and pressure results
from Question 9, and the following equation:








F = thrust at altitude









F sl = sea level static thrust at
takeoff (17,000 pounds)









P = static pressure at
altitude









P sl = sea level static pressure (14.7
psi)

F

=

F sl

x

P/P sl

x

sqrt(T sl/T)


T = absolute temp(temp + 460) at altitude









T sl = sea level absolute temperature
(520 R)

ALTITUDE

PRESSURE

TEMPERATURE

factor

THRUST
(psi)

1000 ft.

14.17

515

.97

16490

10,000 ft.

10.11

483

.71

12070

20,000 ft.

6.76

448

.49

8330

30,000 ft.

4.37

413

.33

5610

 Graph the change in thrust with altitude for the Pratt &
Whitney JT8D17 jet engine.
 Give your conclusions on thrust and flying at different
altitudes.
Thrust will decrease with the change
in higher altitude.

Related Pages:
Standards
Activity
Worksheet
Propulsion Activity Index
Propulsion Index


