UAV Tech Department of Aerospace, Power and Sensors
Transcript of UAV Tech Department of Aerospace, Power and Sensors
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Department of Aerospace,
Power and Sensors
Lecture 13 Developed with cooperation with Prof.Ray Whitford
Cranfield University
Defence Academy of the United Kingdom
Zdobyslaw Goraj, Aircraft Design Department, WUT
Warsaw, June 4, 2020
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Profile drag breakdown (fixed landing gear)
Wing
38%
Interference
2%
Misc
6%
Horiz tail
5%
Vert tail
3%
Fuselage
23%
Antennas,
fairings &
supports
5%
Wheels, etc
18%
Prowler II (Fixed mains/retract nose)
Cherokee 180
Cherokee
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Profile drag breakdown (retractable landing gear)
Wing
23%
Pods &
pylons
7%
Fuselage
27%
Wing tanks
10%
Horiz tail
7%
Vert tail
5%
Interference
14%
Roughness
& gaps
7%
Gates Learjet
Global Hawk
Learjet
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External weapon drag
Predator B
carrying 2 x GBU-12
(14 Hellfires or 6 x 500lb bombs) Armed Proteus
External weapons
cause big drag increase
X-45A weapon bays
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External weapon drag
(a) Clean aircraft
(b) Dirty aircraft
(c) Aerodynamic view of (b)
F-16
External weapons
cause big drag increase
Plus for a given MTOW,
reduces fuel load
Mach number
Zero-lift drag x 2
CD0
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Separated Flow (streamline objects)
V
Adverse Pressure
Gradient
Separation
Boundary layers cannot overcome adverse pressure
gradients and will “separate.” The separation point
is where the wall shear stress goes to zero.
0dx
dp
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Pressure gradients (streamline objects)
inc V
dec p dec V
inc p
FAVOURABLE - a region
of decreasing pressure ADVERSE - a region of
increasing pressure (spada prędkość)
0dx
dp 0
dx
dp
Euler’s Eqn: VρVp
x
2
21
0 ρVpp Bernoulli’s Eqn:
Min pressure
if V is decreasing
then p is increasing
The same conclusion:
because p0 is constant,
so if V is small, then p is high
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Separated Flow (streamline objects)
Wake of separated flow
This adverse pressure gradient, downstream of the minimum
pressure point will cause the flow to separate from the wing
surface giving rise to pressure drag or “drag due
to separation” even at low angles of attack.
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Pressure contributions to lift
-
- +
-
+
Low High
Cp Cp
-1.0
0
1.0
-4.0
-3.0
-2.0
-1.0
0
1.0
2
21p
V
ppC
ρ
Area within loop Lift
Steep adverse
pressure gradient May lead to flow separation
Min pressure
Wartość bezwzględna
ciśnienia przyrasta
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Flow separation leading to stalling
= 5°
= 10°
= 15° = 20°
Lift loss, High drag
Separation point (SP)
SP SP
SP
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Stalling
After stall Before stall
Flow separation
Chaotic wake
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Pressure contributions to lift
-
- + -
+
(A) Low
(B) High
-
+
(C) Beyond stall
A
B
C
Lift
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Aerofoil lift & drag curves
Pressure
drag
Skin friction drag slope) curve (lift
a/dαdCl
angle)
lift (zero
0stall
maxlC
lC
lC
Drag
coefficient
Lift
coefficient
dCLift coefficient
Stall
pressurefriction skin ddd CCC
Cambered aerofoil
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UAV aerofoil sections (highly cambered & thick)
Roughness caused by only
1-2mm diameter particles
of ice in a density of 1 per sq cm
on the wing upper surface
can lead to significant loss of lift
(20-30%)
Large – ve pitching moment = trim drag
(Ref: Z Goraj
WUT Warsaw)
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Viscous Drag
The total drag due to viscous effects is:
Dviscous = Dskin friction + Dpressure
This is only part of the drag story
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Wingtip (or Trailing) Vortices
TOP SURFACE - relatively low pressure
BOTTOM SURFACE - relatively high pressure
upper surface flow (inboard) lower surface flow (outboard)
The pressure imbalance at the wingtip sets up a spanwise
component of flow, strongest at the tips, weakest in the center
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Trailing vortices
Lanchester 1907
-
+
Pressure distribution
across wing span
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Formation of trailing vortices
Formation & Consequences
of Trailing Vortices
Pressure difference between
upper and lower surfaces
Trailing vortices
Downwash
Trailing vortex drag
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Trailing vortex (lift-induced) drag
The downwash will
reduce the AoA at
the tailplane and
hence its effectiveness
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Trailing vortex (lift-induced) drag
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Trailing vortex (lift-induced) drag
Consequences of wingtips: – Reduced lift
– Increased drag
Induced drag will be greatest when the pressure difference between upper and lower surfaces is greatest
– High angles of attack
– Takeoff and landing
Induced drag will be zero when there is no pressure difference (i.e. at zero lift)
But how to reduce trailing vortex drag?
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Aspect ratio
Aspect Ratio:
High A Low A
Note: A=b/c for rectangular wings.
Typical Values
Fighters: 2-5
Transports: 6-10
Gliders: 20-40
UAVs: 2-25
cb
b
S
b
Area
SpanA
222
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Total drag
Induced Profile Total
Aeπ
C C C
2
LDD 0
Total drag now has two components
– Profile drag (skin friction + pressure) at zero lift
– Induced drag
In coefficient form:
: Aircraft Profile Drag at Zero Lift (Parasite Drag), it
includes skin friction drag and pressure drag contributions.
e : Oswald Efficiency Factor - includes changes in profile
drag with angle of attack.
The lift term includes variation of profile (parasite) drag
( pressure and skin friction drag) with AoA
0DC
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Drag Polar (only for symmetrical aerofoil)
Aeπ
1kFor shorthand:
Which gives: 2
LDD kC C C0
Zero lift + Induced
drag drag
kC2
1
D
L
0Dmax
Max endurance/range
condition (see later)
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Drag force vs drag coefficient
2
2
2
D
2
22
D
2
2
2
L
2
LDD
b
W
eρV
2 SρVC
2
1
eASρV
2W SρVC
2
1DRAGbut
Induced Profile Total
SρV
2WC where
Aeπ
C C C
0
0
0
Thus for the actual drag, span loading is the important
term for minimising induced drag: hence long span
b
W
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Minimising trailing vortex drag
High Aspect Ratio
Schimpp-Hirth “Nimbus 3” Open Class Sailplane (circa 1983)
Wing Span = 24.6 m AR = 37 Max. L/D = 60 (flight test)
Global Hawk
A=25.1
GlobalFlyer
A=32.5.
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Minimising induced drag
Wing tip treatment
High Aspect Ratio
HERMES
PROTEUS SILVER ARROW
PREDATOR B
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Total Drag
C D
C L
Skin Friction Drag
Pressure Drag
Induced Drag
Total
0
Take-off
V low
CL high
Cruise
V high
CL low
L
2
21 SCρVWeightLift
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Drag polar (simplified)
Typical Values: Aircraft k (L/D)max
T-41 0.032 0.058 11.6
T-37 0.020 0.057 14.8
T-38 0.015 0.140 10.9
F-4 0.024 0.169 7.9
F-16 0.019 0.117 10.6
MiG 21 0.015 0.200 9.1
MiG 29 0.019 0.166 9.4
747 0.017 0.045 18.1
Global Hawk 0.012 0.013 35
0DC
2
LDD kC C C0
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L/D versus AoA (cambered aerofoil)
LC DC
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Drag breakdown
Parasite drag CD Si (reference area) CDi * Si /S
Wing 0.0068 44.4 0.0068
Fuselage 0.005385 44.4 0.005385
Vertical stabilizer 0.008 3.83 0.0007
Nacelle 0.06 0.67 0.0009
Total parasite drag 0.0138 44.4 0.0138
-0.4 0.0 0.4 0.8 1.2 1.6
CL
-10
0
10
20
30CL/ CD
maxD
L
(Ref: Z Goraj
WUT Warsaw)
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Typical (flying wing) drag polar
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Drag coefficient CD
-0.40
0.00
0.40
0.80
1.20
1.60 Lift coefficient CL
Ailerones deflection
= 0 deg
= -10 deg
= -15 deg
max
LD
L for C
26.7D
L
max
(Ref: Z Goraj
WUT Warsaw)
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(L/D)max versus Swet
0
10
20
30
40
50
60
1 1.5 2 2.5 3
Span / (Swet)^1/2
(Lif
t /
Dra
g)m
ax
B-52G
747
U-2A
FW-200
DarkStar
Global
Hawk
Altus
Strato 2C Voyager
Condor
Nimbus 2C
BWB-1-1
Small SWEPT wing
& large body
area Wetted
Span14.8
D
L
Large UNSWEPT wing
& small body
(Aspect ratio based on SWET)
Aerodynamic efficiency
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Drag= Thrust required
The two parts of the drag (profile and
Induced) are plotted as a function of velocity
V
D=T R
Induced (vortex)
Total
Profile ) V( 2
)V
1 (
2
Min drag
Need more thrust
to fly slower qS
kW qSCD
2
D0
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Effect of zero-lift drag, weight & alt increase
Changes in the curves are shown for increased:
Zero-lift drag, Weight and Altitude
V
D=T R qS
kW qSCD
2
D0
CD0
W
Altitude
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HALE configurations
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Conventional configuration
2(W/S)
KVVρCΔn
gustgustL
gustα
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General Atomics single mission aircraft? Wep/ISTAR
Common features
Rear engine/propeller
High aspect ratio wings
Canted tails
Retractable landing gear
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3-D Effects on Lift
CL
cl
a0
a
Aerofoil (2-D)
Wing (3-D)
Not a problem for high aspect ratio surveillance types (though their high lift curve slopes + very low wing loadings
make them susceptible to high gust loads)
but is a problem for UCAVs with low aspect ratio and sweep
Reducing aspect
ratio
A=6, =0°
A=2, =60°
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Penalty of high span loading (W/b)
b
W
eρV
2 SρVC
2
1DRAG
2
2
2
D0
• Co-operation
• Co-operability
• Expense
• Procurement Issues
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Boeing's X-45B is 40% larger than the initial X-45A The maximum take-off weight has grown from
21,400 lb to 25,000 lb
Original X-45A
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Not minimising induced drag (low aspect ratio UCAVs)
M=0.85 M=0.85
Wing extensions to reduce
span loading and
increase endurance/range
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Morphing vehicle (multi-role)
Highest
aspect ratio
=20°, A=7
High
speed
shape
=70°
A=3
Ref: AIAA-2004-6597
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Morphing vehicle
Manoeuvrable Killer - High drag
Hunter- Low drag
Stealth – Medium drag
CL
CL
CL
CD
CD
CD
Ref: AIAA-2004-6597
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Northrop morphing design
Northrop UAV due to morph in 2005-06
2.75m
Span +180%, L/D +44% Swept – 20%
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Tail & fin
Skin friction 11%
Fuselage
Skin friction 21%
Wing
Skin friction 23%
Induced 37%
Wave 3%
63%
Nacelle & pylon
Skin friction 5%
100%
Drag breakdown
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Ref: Z Goraj (WUT Warsaw)
HALE wing section (thick with shockwave)
0 0.2 0.4 0.6 0.8 1
-0.1
0
0.1
0.2 Global Hawk LRT-17.5
Shockwave
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Drag Breakdown
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Shock wave development
M=0.95
M=0.72
M=0.82 M=0.77
M=0.5
M=1.1
M=1.0
M>1
M>1
M>1
M>1
M>1
M>1 M<1
M<1
M>1
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Shock waves
M=0.7 M=0.75
M=0.84 M=0.88
M=0.97 M=0.98
M=0.775 M=0.82
M=0.90 M=0.95
M=1.10 M=1.18
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Aerodynamics Summary
4 types of DRAG: Skin friction, pressure, vortex and shock wave
but only 1 type of lift
LIFT: From pressure distribution, limited due to stalling
Both lift and drag depend on:
Aerofoil shape and angle of attack ()
Planform shape (including aspect ratio)
Wing area
Air density & viscosity
Speed
For long endurance use a high aspect ratio laminar flow wing
For high speed use a moderate aspect ratio + swept wing