UAV Tech

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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