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1 Challenge the future
Introduction to Aerospace Engineering
Lecture slides
15-12-2012
Challenge the future
Delft University of Technology
Introduction Aerospace Engineering
Flight Mechanics
Dr. ir. Mark Voskuijl
2 Flight Mechanics
3.&4 Horizontal flight performance
3 Flight Mechanics
Flight mechanics
Hours 1 & 2 : Introduction, general equations of motion
Hours 3 & 4 : Horizontal flight performance
Hours 5 & 6 : Climbing and descending flight
Hours 7 & 8 : Flight envelope
Hours 9 & 10: Example questions and solutions
4 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
5 Flight Mechanics
Equations of motion General equations of motion in symmetric flight (1)
Free Body Diagram Kinetic Diagram
L
D
W
T V
T
Zb
Ze
Xe
V
Zb
Ze
Xe
dmV
dt
dVm
dt
Xb Xb
6 Flight Mechanics
Equations of motion
: cos sin
: cos sin
T
T
F ma
dVV T D W m
dt
dV L W T mV
dt
General equations of motion in symmetric flight (2)
7 Flight Mechanics
Propulsive force
Thrust is assumed to be independent of airspeed
Typical analytical assumption – Jet
8 Flight Mechanics
Propulsive force
Pa is assumed to be constant and independent of airspeed
Typical analytical assumption – Propeller
9 Flight Mechanics
Propulsive force
Jet: Thrust is (assumed) independent of airspeed
Propeller: Power available is (assumed) independent of airspeed
Analytical assumptions - summary
airspeed airspeed
thrust power available
jet prop
10 Flight Mechanics
Summary previous lecture
• You should be able to derive the
equations of motion for 2
dimensional flight
• The thrust of a jet aircraft can be
assumed independent of airspeed
• The power available of a propeller
aircraft can be assumed
independent of airspeed
• The complete aircraft
aerodynamics can be represented
by 1 equation; the lift drag polar
// : cos sin
: cos sin
V T
V T
W dVF T D W
g dt
W dF V L W T
g dt
0
2
LD D
CC C
Ae
11 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
12 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
13 Flight Mechanics
Introduction
• How fast can an aircraft fly?
• How slow can a given aircraft fly?
• At what speed should be flown to be able to fly as far as possible?
(Interesting for airliners)
• At what speed should be flown to stay in the air as long as
possible? (Search and rescue, military purposes)
Horizontal flight performance
14 Flight Mechanics
Introduction
Most important are the lecture sheets!!!
Background material:
Introduction to Flight (Anderson)
Par. 6.1 – 6.2
Par. 6.3 – 6.6
Par. 6.12 – 6.14
What to learn?
15 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
16 Flight Mechanics
Equations of motion General equations of motion in symmetric flight (1)
Free Body Diagram Kinetic Diagram
L
D
W
T V
T
Zb
Ze
Xe
V
Zb
Ze
Xe
dmV
dt
dVm
dt
Xb Xb
17 Flight Mechanics
Equations of motion
: cos sin
: cos sin
T
T
F ma
dVV T D W m
dt
dV L W T mV
dt
Flight Condition: The aircraft is performing a horizontal (straight) and steady flight. Assumption: The thrust is assumed to be in the direction of flight (T = 0).
General equations of motion in symmetric flight (2)
18 Flight Mechanics
Equations of motion
• Straight flight: flight in which the centre of gravity of the
aircraft travels along a straight line (d/dt = 0)
• Steady flight: Flight in which the forces and moments acting on
the aircraft do not vary in time, neither in magnitude, nor in
direction (dV/dt = 0)
• Horizontal flight: The aircraft remains at a constant altitude
( = 0)
• Symmetric flight: flight in which both the angle of sideslip is
zero and the plane of symmetry of the aircraft is perpendicular to
the earth ( = 0 and the aircraft is not turning)
definitions
19 Flight Mechanics
20 Flight Mechanics
21 Flight Mechanics
http://www.youtube.com/watch?v=TCUHQ_-l6Qg
B52 landing in crosswind
22 Flight Mechanics
Equations of motion
: cos sin
: cos sin
T
T
F ma
dVV T D W m
dt
dV L W T mV
dt
Horizontal, steady, symmetric flight
= 1 = 0 = 0
= 1 = 0 = 0
:
:
V T D
V L W
Note: Clearly this is the most simple form of the equations of motion
How do these forces (aerodynamic, propulsive, gravitational) vary with airspeed?
23 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
24 Flight Mechanics
Performance diagram
• For a given altitude, configuration and engine control setting, the
drag or power required and thrust or power available are plotted
against airspeed
• Also called Pernaud diagram
• Very useful to determine aircraft performance parameters
25 Flight Mechanics
Performance diagram
• Typical shape:
26 Flight Mechanics
Performance diagram
a
r
P T V
P D V
27 Flight Mechanics
airspeed airspeed
Force Power
Jet (thrust / power) Propeller (power) Aerodynamics (power / drag)
28 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
29 Flight Mechanics
Horizontal flight performance
• Minimum airspeed (How slow)
• Maximum airspeed (How fast)
• Range (How far)
• Endurance (How long)
• Speed stability / instability
Assumptions: Horizontal steady symmetric flight
( L = W, T = D, Pa = Pr ), except for the speed stability section
30 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
31 Flight Mechanics
Minimum airspeed
212
2 1
L
L
L W
C V S W
WV
S C
max
min
2 1
L
WV
S C
32 Flight Mechanics
Minimum airspeed
Flaps
33 Flight Mechanics
Minimum airspeed Performance diagram
Vmin
34 Flight Mechanics
Minimum airspeed
• Question: An aircraft has a wing loading (W/S) 2400 N/m2 and
CLmax = 1.4. Find the airspeed at which stall occurs (minimum airspeed)
at (i) sea level ( = 1.225 kg/m3) and (ii) at 5000m ( = 0.737 kg/m3)
• Answer:
21
L 2
min
L,max
L W
C V S W
W 2 1V
S C
min
min
2 1(i)V 2400 51.8 m/s
1.225 1.4
2 1(ii)V 2400 66.8 m/s
0.737 1.4
Example calculation
35 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
36 Flight Mechanics
Maximum airspeed
• Pilot applies maximum throttle
(max. thrust or Power available)
maxT D
L W
Vmax
Force
Maximum thrust
Drag
Dmax
L
CLT D W
L C
Jet aircraft
0
2
D D LC C k C
0D maxL
L
C Tk C
C W
Solution
0
2 maxL L D
TkC C C 0
W
2
L
ax bx c 0
C ...
L
W 2 1L W V
S CV
37 Flight Mechanics
Maximum airspeed
The following data is known of the Cessna Citation II (subsonic jet)
Calculate (1) the maximum airspeed of this aircraft when flying at H = 0 m and (2) the corresponding Mach number
Example
Aircraft Weight : W = 60 kN, Wing area : S = 30 m2, (Parabolic) Lift-Drag polar : CD = CDo + kCL2; CDo = 0.022, k = 0.047, CLmax = 1.35, Maximum Thrust at 0 m ISA :T0 = 12 kN. The aircraft is flying at an altitude of H = 0 m in the International Standard Atmosphere (0 = 1.225 kg/m3) Thrust is assumed to be independent of the airspeed
38 Flight Mechanics
Maximum airspeed
2 21 1L D2 2
Steady symmetric horizontal flight:
L W, T D
With
L C V S, D C V S
maxmax
Maximum Mach number?
VM
a
Solution
0
L LD
D
2
D D L
Thrust must be at maximum to achieve Vmax
Combining the above yields:
C CW 600005 C
C T 12000 5
Lift drag polar:
C C k C
0D2 LL
2 LL
CCC 0
5k k
C 0.022C 0
5 0.047 0.047
LC 0.11
max
L
W 2 1 60000 2 1V 172.1 m/s
S C 30 1.225 0.11
a RT 1.4 287.05 288.15 340.3 m/s
max
172.1M 0.5
340.3
39 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal steady symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
40 Flight Mechanics
Range
• How can we fly as far as possible for a given amount of fuel?
• How can we fly a given distance with the minimum amount of fuel?
/
V m sVm N
F F N s
Specific range is defined as the ratio V/F, where F equals the fuel flow
Dimensional analysis:
Or, put in words, specific range is the distance that can be flown for 1 Newton of fuel. Clearly, this variable must be maximised
41 Flight Mechanics
Range
a
P br P
j
PF c P F c
(steady horizontal flight)a rP P
1
rP P
j j
j
P
P DVF c c
V
F c D
Maximum range propeller aircraft
j P
max min
max max
and c can be assumed to be constant as a function of airspeed
(over the range of cruising speeds) for propeller aircraft
V
FL
D
CR D
C
42 Flight Mechanics
Range Maximum range propeller aircraft
43 Flight Mechanics
Problem!
Result:
Prop: maximum range at maximum CL / CD
Dear pilot, could you please fly at CL / CD max?
We must give an airspeed to the pilot
44 Flight Mechanics
Solution Result:
Prop: maximum range at maximum CL / CD
( )D LC f C y f x
2 1
L
L W
WV
S C
2
( ) ' '0
( )
L
L D
Cd d g x hg gh
dC C dx h x h
2
1
0
DD L
L
D
dCC C
dC
C
D D
L L
dC C
dC C
0
2
2L
DL
L
CC
C Ae
Ae C
0L DC C Ae
0
2
LD D
CC C
Ae
45 Flight Mechanics
Range
• Charles Lindbergh (1927)
• First solo, non stop flight across the
Atlantic ocean
Example: Spirit of St. Louis
46 Flight Mechanics
Performance diagram Spirit of St Louis (ref: NASA)
Note: The Pr curve shifts down when weight decreases. This means that Charles had to decrease airspeed during the flight
47 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
1. Propeller
2. Jet
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
48 Flight Mechanics
Range
So, at what speed should the pilot fly to maximise specific range?
TF c T
Typically, cT can be considered to be constant
T D
Jet aircraft
T TF c T c D
T
VV F
c D
max max min
V F V D D V
49 Flight Mechanics
Range
max max
max max
1
1D
L
D DL
V L V
L W
CDW
V C V
212
2 1L
L
WC V S W V
S C
2
1
2 1 2
D
L L
L D
CD WW
V C W CW
S C S C
Jet aircraft
Performance diagram (jet)
2
min max
L
D
CD
V C
50 Flight Mechanics
Problem!
Result:
Jet: maximum range at maximum CL / CD2
Dear pilot, could you please fly at CL / CD2 max?
We must give an airspeed to the pilot
51 Flight Mechanics
Solution Result:
Jet: maximum range at maximum CL / CD2
( )D LC f C y f x
2 1
L
L W
WV
S C
2 2
( ) ' '0
( )
L
L D
Cd d g x hg gh
dC C dx h x h
2
4
1 2
0
DD L D
L
D
dCC C C
dC
C
12
D D
L L
dC C
dC C
0
2
12
2L
DL
L
CC
C Ae
Ae C
0
13L DC C Ae
0
2
LD D
CC C
Ae
52 Flight Mechanics
How environmentally (un)friendly are
aircraft?
• Example: Boeing 747:
• W = 3000 kN
• S = 510 m2
• b = 60 m
• hcr = 11000 m
• Mcr = 0.85
• Estimated:
• CD0 = 0.02; e = 0.85; tot = 0.40
53 Flight Mechanics
How environmentally (un)friendly are
aircraft really
• W/S = 5882 N/m2; A = 7.06
• a = 295 m/s; V = 250 m/s (=900 km/hr)
2
2 10.517L
WC
S V
0
2
0.0342LD D
CC C
Ae
15.1L
D
C
C
198.4 kND
L
CD W
C
49591 kJ/sa rP P DV
54 Flight Mechanics
How environmentally (un)friendly are
aircraft really?
0.40 estimated (in the order of efficiency power plant)atot
P
Q
28.3 N/sa
tot
PgF
H
Fuel consumption: 12980 l/hr (in accordance with data manufacturer)
Every hour 900 km is travelled and 12980 l of fuel is used:
69 m/l
With 400 passengers: 27.7 pax km/l
3
43000 kJ/kg
0.8 kg/mfuel
H
55 Flight Mechanics
Comparison to a car
• Car driving 1:15 and 4 passengers: 60 pax km/l
• Conclusion: consumption of car is half the consumption of a 747
despite the speed ratio 9:1
• In practice ( seat occupation < 100% ) consumption per
passenger km is similar for car and aviation traffic
56 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
57 Flight Mechanics
Endurance
• Endurance: time duration spent
in flight
• Do not confuse endurance with
range!
• Solution: maximum endurance for
minimum fuel flow
• Fmin
58 Flight Mechanics
Endurance
min min
max
Horizontal steady flight
Assumption: constant
T
T
T
T
L
D
Fc
T
F c T
T D
F c D
c
CF D
C
Jet aircraft
Performance diagram (jet)
59 Flight Mechanics
Endurance
a
P P
br j
PFc F c
P
(steady horizontal flight)a rP P
r
P P
j j
P DVF c c
Propeller aircraft
23
3
2 1
2 1 2
P
j L
P D P D
j L L j L
D WF c
S C
c C c CW WF W
C S C S C
j P
3
max min ,min2 min
max
and c can be assumed to be constant as a function of airspeed
(over the range of cruising speeds) for propeller aircraft
F Lr
D
CE DV P
C
60 Flight Mechanics
Endurance Propeller aircraft
Pr should be minimal!
61 Flight Mechanics
Problem!
Result:
Propeller: maximum endurance at
maximum CL3 / CD
2
Dear pilot, could you please fly at CL3 / CD
2 max? We must give an airspeed to the pilot
62 Flight Mechanics
Solution Result:
Prop: maximum endurance at maximum CL3 / CD
2
( )D LC f C y f x
2 1
L
L W
WV
S C
3
2 2
( ) ' '0
( )
L
L D
Cd d g x hg gh
dC C dx h x h
2 2 3
4
3 2
0
DD L L D
L
D
dCC C C C
dC
C
32
D D
L L
dC C
dC C
0
2
32
2
LD
L
L
CC
C Ae
Ae C
03 L DC C Ae
0
2
LD D
CC C
Ae
63 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
64 Flight Mechanics
Speed instability
• General equations of motion
• Equations of motion horizontal
symmetric flight (thrust parallel
to airspeed
: cos sin
: cos sin
T
T
F ma
dVV T D W m
dt
dV L W T mV
dt
:
:
F ma
dVV T D m
dt
V L W
65 Flight Mechanics
Speed instability
Force
Drag
Thrust
Airspeed Vdesired
Disturbance (gust of wind for example)
D>T
T - D = mdV/dt
dV/dt < 0
Conclusion: the aircraft is ‘speed unstable’ and will deviate from the original position
following a disturbance. This makes it difficult for the pilot
66 Flight Mechanics
Speed stability
Force
Drag
Thrust
Airspeed Vdesired
Disturbance (gust of wind for example)
D>T
T - D = mdV/dt
dV/dt < 0
Conclusion: the aircraft is speed stable and the aircraft will automatically
return to the original airspeed. Very nice for the pilot!
67 Flight Mechanics
Speed stability - instability
Force
Drag
Airspeed
Speed unstable Speed stable
68 Flight Mechanics
Contents
1. Summary previous lecture
2. Introduction
3. Equations of motion horizontal symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
69 Flight Mechanics
Summary - Jet
Force
Drag
Airspeed
Max thrust
Max. Endurance (CL / CD)max
Max. Range (CL/CD
2)max
Max. Speed (depends on Tmax)
Min. speed CL,max
70 Flight Mechanics
Summary – Propeller Aircraft
Power
Power required
Airspeed
Power available
Max. Endurance (CL
3 / CD2 )max
Max. Range (CL/CD)max
Max. Speed (depends on Pamax)
Min. speed CL,max
71 Flight Mechanics
Summary – Horizontal flight
,maxLC min
,max
2 1
L
WV
S C
max
,max
(Jet)
(Prop)a r
T D
P P
Minimum airspeed
Maximum airspeed
Maximum range 2
max
L
D
C
C
L W
T D
...LC max
2 1
L
WV
S C
max
V
F
max
L
D
C
C
0
13L DC C Ae
0L DC C Ae
2 1
L
WV
S C
prop
jet
72 Flight Mechanics
Summary – Horizontal flight
Maximum endurance
max
L
D
C
C
L W
T D
min
F
3
2
max
L
D
C
C
0L DC C Ae
03L DC C Ae
2 1
L
WV
S C
prop
jet
73 Flight Mechanics
Contents
1. Previous lecture – Propeller - simplification
2. Introduction
3. Equations of motion horizontal symmetric flight
4. Performance diagram
5. Aircraft performance in horizontal flight
1. Minimum airspeed
2. Maximum airspeed
3. Range
4. Endurance
5. Speed instability / stability
6. Summary
7. Example exam question
74 Flight Mechanics
Example exam question (homework)
The Gulfstream IV, indicated in Figure 1 is a twin-turbofan executive transport aircraft. Data for this aircraft are given below: S = 88.3 [m2] b = 23.7 [m] A = 6.36 [-] e = 0.67 [-] CD0 = 0.015 [-] CD = CD0 + CL
2 / Ae W = 311000 [N] This aircraft has two Rolls Royce Tay turbofan engines. Fuel consumption can be represented with the following equation: F = cTT, where cT = 0.69 [N/hr/N]. The thrust specific fuel consumption is assumed to be independent of airspeed and altitude Question: What is the maximum specific range of this aircraft when flying at 8000m (ρ = 0.5252 kg/m3) ?
Problem
75 Flight Mechanics
Example exam question (homework)
2
2 1
1
2 1 2
D
L
L
D
L L
L D
CLD D W
L C
WV
S C
CD WW
V C W CW
S C S C
Solution (part 1) Specific range = V/F (m/kg) = V/cTT Steady horizontal flight: T = D; L = W V/F = V/cTD cT is a constant so maximum specific range when (D/V) is minimal So CL / CD
2 should be maximal. This is the case when CL = √(CD0Ae/3 ) (you should derive this on the exam, see sheet 48)
0
13
2 2
0.015 6.36 0.67 0.26
0.260.015 0.02
6.36 0.67
L
LD D
C
CC C
Ae
76 Flight Mechanics
Example exam question (homework)
2 1 311000 2 1227 [m/s]
88.3 0.5252 0.26L
WV
S C
0.02311000 23.9 [kN]
0.26
D
L
CT D W
C
Solution (part 2) From the lift coefficient, we can calculate the airspeed
The aerodynamic drag can be calculated with the drag coefficient. This must be equal to the thrust
22750 [m/N]
(0.69/3600) 23900T
V V
F c T
Now, all parameters are known to calculate the maximum specific range for this altitude and aircraft weight