Opportunities in Aircraft Design

Opportunities & Trends for Progress in Aircraft Performance Sina Golshany

Preface - Priorities • Since the late 1990s, fuel cost constitutes the largest fraction of the operating cost of a commercial airplane. • In a long-term time-average sense, fuel cost has been rising steadily since the 1960s. This trend continues, with local anomalies. • Fully digital systems, more electric systems, and improvements in engine reliability have led to lower maintenance cost, therefore: Making fuel-burn reduction the primary emphasis in

commercial aircraft design now, and into the future.

Preface - Opportunities Breguet Range Equation:

Ri→ f

Vtrue L  Wi = ⋅ ln TSFC D  Wi − W f

Propulsive Efficiency, 1:1 Aerodynamic Efficiency, 1:1 corresponding to Range, corresponding to Range, given a specific fuel load given a specific fuel load

   

Weight Efficiency, less than 1:1 corresponding to Range, given a specific fuel load

Preface - Opportunities • As the top priority remains to reduce fuel burn, commercial airplanes have progressed to accommodate this in the past 40 years.

• The most significant changes have occurred in areas that directly effect the Breguet’s range equation (as a surrogate for fuel burn) in the following order: 1- Improving the installed engine fuel efficiency (designing the airplane around the best engine available, or enabling better engines to be installed) 2- Increasing the lift-to-drag ratio, particularly at high-speed cruise 3- Reducing the empty weight of the airplane, mostly via improvements in materials

• Drawing on public domain material, this presentation explores 11 opportunities and trends that might be useful to a designer

Opportunity 1 – The high-speed fan • Fan propulsive efficiency is the most significant contributor to the overall fuel efficiency of a high BPR propulsion unit.

1.0 0.6

0.8

t/c = 6.0 %  2.0 %

FAN EFFICIENCY

0.9 0.8 t/c = 13.5 %  5 %

0.7

1.0 1.1

0.6

1.3 Mtip= 1.4

0.5 0.4

0

0.2

0.4

0.6

0.7

0.8

MACH NUMBER

1.0

1.2

Opportunity 2 – Turbine Inlet Temperature • Higher T4 temperatures are achievable in a simple cycle, but the PSFC improvements can be relatively small, especially considering the difficulties 0.8

PSFC LBM SHP - HR

Turbine Inlet Temperature [ ⁰F]

0.7

1700

1900

2100

2300

2500

4

0.6 6 0.5

8 10 12 14 16 20 18 Core pressure ratio

0.4 0.3

50

100

150

SHP SPECIFIC POWER LBM/SEC

200

250

Opportunity 2 – Turbine Inlet Temperature • For a ~ 50 % increase in T4, PSFC improves by 12 % • For a ~ 50 % increase in T4, Spec. Power increases by 84 % 0.8

PSFC LBM SHP - HR

0.7

1700

Turbine Inlet Temperature [ ⁰F] ~ 50 % 1900 2100 2300 2500

4

0.6

Core pressure ratio 6

0.5

8

~ 12 % 0.4

20

~ 84 % 0.3

50

100

150

SHP SPECIFIC POWER LBM/SEC

200

10 12 14 16 18

250

Opportunity 3 – Regenerative Cycles • Reheat, Recuperation, Intercooling, or wave-rotor equipped cycles can drastically improve fuel efficiency – note that pressure ratio relation is reversed.

0.8

PSFC LBM SHP - HR

0.7

20 1700

0.6

Turbine Inlet Temperature [ ⁰F] ~ 50 %

18 16

0.5

14 12 4 -6 108

0.4 0.3

1900

50

100

2100 2300 2500

150

SHP SPECIFIC POWER LBM/SEC

200

250

Opportunity 3 – Regenerative Cycles • Reheat, Recuperation, Intercooling, or wave-rotor equipped cycles can drastically improve fuel efficiency – note that pressure ratio relation is reversed.

PSFC LBM SHP - HR

0.75 Basic Cycle

0.50

~ 25 % With Regeneration

0.25 0 100

Basic Cycle

% FUEL FLOW

50

0

With Regeneration

0

20

40

60

% Power

80

100

Opportunity 4 – Engine/Airplane Matching • Better design coordination will allow engine & airframes to be designed to compliment each other – Example

40,000

30,000 POWER PLANT CRUISE EFFICIENCY

ALTITUDE [FEET]

η = 0.25 0.24 0.23 0.22 0.21

20,000

Power Plant Overall Efficiency: 10,000

ηα

TSFC θ 0

0.2

0.4

0.6

0.8

1.0

MACH NUMBER

1.2

1.4

Opportunity 4 – Engine/Airplane Matching • Better design coordination will allow engine & airframes to be designed to compliment each other – Example Max Power = Required Power

40,000

MAX Thrust = Drag

30,000

POWER PLANT CRUISE EFFICIENCY

ALTITUDE [FEET]

η = 0.25 0.24 0.23 0.22 0.21

20,000

10,000

0

0.2

0.4

0.6

0.8

1.0

MACH NUMBER

1.2

1.4

Opportunity 4 – Engine/Airplane Matching • Better design coordination will allow engine & airframes to be designed to compliment each other – Example

40,000

30,000 POWER PLANT CRUISE EFFICIENCY

ALTITUDE [FEET]

η = 0.25 0.24 0.23 0.22 0.21

20,000 L = 12 D 14 16 18

10,000

0

0.2

0.4

0.6

0.8

1.0

MACH NUMBER

1.2

1.4

Opportunity 4 – Engine/Airplane Matching • Better design coordination will allow engine & airframes to be designed to compliment each other – Example

40,000 INCREASED L/D

30,000 POWER PLANT CRUISE EFFICIENCY

ALTITUDE [FEET]

η = 0.25 0.24 0.23 0.22 0.21

20,000 L = 12 D 14 16 18

INCREASED η

10,000

0

0.2

0.4

0.6

0.8

1.0

MACH NUMBER

1.2

1.4

Opportunity 4 – Engine/Airplane Matching • Better design coordination will allow engine & airframes to be designed to compliment each other – Example • 2 opportunities emerge:

• Better Engine Design • Alternative Cruise Mach numbers

40,000 INCREASED L/D

30,000 POWER PLANT CRUISE EFFICIENCY

ALTITUDE [FEET]

η = 0.25 0.24 0.23 0.22 0.21

20,000 L = 12 D 14 16 18

INCREASED η

10,000

0

0.2

0.4

0.6

0.8

1.0

MACH NUMBER

1.2

1.4

Opportunity 5 – Aircraft Development Costs • There is a serious need for reducing the cost of design & production of airplanes.

MILLIONS OF DOLLARS

60 50 40 30 20 WW-2

End of Cold War

Korea

10 0 1920

Vietnam

30

40

50

60

70

80

90

2000

2010

2020

Opportunity 5 – Aircraft Development Costs • Although airplanes have become more complex, our capacity for efficient handling of complex data has increased much faster 2.50E+09

50 40

2.00E+09

30

1.50E+09

20

1.00E+09 WW-2

End of Cold War

Korea

5.00E+08

10 0 1920

CPU TRANSISTOR COUNTS

MILLIONS OF DOLLARS

60

Vietnam

1930

1940

1950

1960

1970

1980

1990

2000

2010

2020

Opportunity 6 – Natural Laminar Flow (NLF) • There is a valid case for use of NLF to reduce airplane drag • Developing better methods for transition prediction will be important SKIN FRICTION COEFFICIENT Cf =

τ0 q

10-2

10-3

Transition Band

10-4

10-5 103

105 U δ REYNOLDS #, Rδ = ν0 104

106

Opportunity 7 – NLF Wing Planforms • To make NLF practical, LE sweep instabilities (cross-wash induced) have to be minimized % MAC LAMINAR FLOW

60

AR= 15 AR= 14 AR= 13 AR= 12

50 40

AR= 11 AR= 10 AR= 9

30 20 10 0

0

5

10

15

20

QUARTER CHORD WING SWEEP [DEG]

25

Opportunity 7 – NLF Wing Planforms • To make NLF practical, LE sweep instabilities (cross-wash induced) have to be minimized % MAC LAMINAR FLOW

60

AR= 15 AR= 14 AR= 13 AR= 12

50 40

AR= 11 AR= 10 AR= 9

30 20 10 0

Reduced Sweep 0

5

10

15

20

QUARTER CHORD WING SWEEP [DEG]

25

Opportunity 7 – NLF Wing Planforms • To make NLF practical, LE sweep instabilities (cross-wash induced) have to be minimized AR= 15 AR= 14 AR= 13 AR= 12

50 40 30 20 10 0

AR= 11 AR= 10 AR= 9

INCREASED NLF

% MAC LAMINAR FLOW

60

0

Reduced Sweep 5

10

15

20

QUARTER CHORD WING SWEEP [DEG]

25

Opportunity 7 – NLF Wing Planforms • This presents interesting tradeoffs with wing aspect ratio, and most importantly, cruise Mach number MID CRUISE L/D @ OPTIMUM MACH #

25.5

AR= 15 AR= 14 AR= 13 AR= 12

25.0 24.5

AR= 11 AR= 10 AR= 9

24.0 23.5 23.0 22.5 22.0 21.5

-8

-3

+2 +7 +12 QUARTER CHORD WING SWEEP [DEG]

+17

+20

Opportunity 7 – NLF Wing Planforms • This presents interesting tradeoffs between wing aspect ratio, and most importantly, cruise Mach number MID CRUISE L/D @ OPTIMUM MACH #

25.5

AR= 15 AR= 14 AR= 13 AR= 12

25.0 24.5

AR= 11 AR= 10 AR= 9

24.0 23.5 23.0 22.5 22.0 21.5

Increased Sweep -8

-3

+2 +7 +12 QUARTER CHORD WING SWEEP [DEG]

+17

+20

Opportunity 7 – NLF Wing Planforms • This presents interesting tradeoffs between wing aspect ratio, and most importantly, cruise Mach number MID CRUISE L/D @ OPTIMUM MACH #

25.5

AR= 15 AR= 14 AR= 13 AR= 12

25.0 24.5

Increased AR

24.0

AR= 11 AR= 10 AR= 9

23.5 23.0 22.5 22.0 21.5

Increased Sweep -8

-3

+2 +7 +12 QUARTER CHORD WING SWEEP [DEG]

+17

+20

Opportunity 7 – NLF Wing Planforms • This presents interesting tradeoffs between wing aspect ratio, and most importantly, cruise Mach number MID CRUISE L/D @ OPTIMUM MACH #

25.5

AR= 15 AR= 14 AR= 13 AR= 12

Diminishing return on AR (weight)

25.0 24.5

Increased AR

24.0

AR= 11 AR= 10 AR= 9

23.5 23.0 22.5 22.0 21.5

Increased Sweep -8

-3

+2 +7 +12 QUARTER CHORD WING SWEEP [DEG]

+17

+20

Opportunity 8 – Aerodynamics of Close-Coupled Large Rotors

• A better understanding of the aerodynamics of close coupled large rotors will have large practical benefits for airplane efficiency • Pusher & Puller Installations need to be explored further CL

TC ’’

CL’’’

α -(CT+CD) αR

Opportunity 8 – Aerodynamics of Close-Coupled Large Rotor Installations 1.5

20 1.0 15

CL’’’(WING)

Net CL 10

0.5 TC ’’ 0 0.5 0.9

0 0

10

20

30

αR ≈ SIN-1[(1-TC ’’)0.5 SIN(α)]

40

5

50

-15

-10 -5 -(CT+CD)

0

Opportunity 9 – Leading Edge Active Flow Control • Large increases in CLmax can be achieved by temporarily energizing the flow near the leading edge, to delay leading edge separation

3.5 3.0

2.5

CL

2.0 1.5 1.0

No Flaps

0.5 0

4

8

12

α [DEG]

16

18

Opportunity 9 – Leading Edge Active Flow Control • Large increases in CLmax can be achieved by temporarily energizing the flow near the leading edge, to delay leading edge separation

3.5 3.0

2.5

CL

2.0 1.5

Flaps Deployed

1.0

No Flaps

0.5 0

4

8

12

α [DEG]

16

18

Opportunity 9 – Leading Edge Active Flow Control • Large increases in CLmax can be achieved by temporarily energizing the flow near the leading edge, to delay leading edge separation

3.5 3.0

2.5

CL

LE Active Flow Control Increased Flow Rate

2.0 1.5 1.0

No Flaps

0.5 0

4

8

12

α [DEG]

16

18

Opportunity 9 – Leading Edge Active Flow Control • Large increases in CLmax can be achieved by temporarily energizing the flow near the leading edge, to delay leading edge separation • The main consequence is reducing the required wing area, if sized by approach speed

3.5 3.0

~ 40 % increase in CLmax

2.5

CL

LE Active Flow Control Increased Flow Rate

2.0 1.5 1.0

No Flaps

0.5 0

4

8

12

α [DEG]

16

18

Opportunity 10 – Lower Sweep Wings • The high sweep of current wing planforms is primarily a function of the relatively high cruise speed and the airfoil technology implemented 1.5

1.0

∆P q

M=1.0

0.5 0

-0.5

M=0.6 -1.0

Opportunity 10 – Lower Sweep Wings • For a given airfoil technology, the effects of sweep on critical Mach number are well understood LEADING EDGE SWEEP [deg]

100 80 0.5 0.6 0.7 0.8 0.9 1.0 Mach Number Normal to Leading Edge (effective)

60 40 20 0

0

1.0

2.0

MACH NUMBER

3.0

Opportunity 10 – Lower Sweep Wings • For low-speed flight, a lower sweep wing can have significant reduction in wing area for the same approach weight 4.0

C L MAX

CLmax |Λ 0 = 1.0

3.5

CLmax |Λ 0 = 2.0

3.0

CLmax |Λ 0 = 4.0

CLmax |Λ 0 = 3.0

2.5 2.0 1.5 1.0 0.5 0

0

10 20 30 40 50 60 70

WING SWEEPBACK AT QUARTER CHORD [deg]

Opportunity 10 – Lower Sweep Wings • For low-speed flight, a lower sweep wing can have significant reduction in wing area for the same approach weight 4.0 A 25⁰ increase in wing sweep, causes a 20 % reduction in CLmax This corresponds to ~ 20% increase in required wing area for the same approach speed.

C L MAX

CLmax |Λ=0 = 1.0

3.5

CLmax |Λ=0 = 2.0

3.0

CLmax |Λ=0 = 4.0

CLmax |Λ=0 = 3.0

2.5 2.0 1.5 1.0 0.5 0

0

10 20 30 40 50 60 70

WING SWEEPBACK AT QUARTER CHORD [deg]

Opportunity 11 - Augmented High-Lift Systems • There is a significant opportunity for augmented high-lift systems, to reduce the required wing area for STOL airplanes of all sizes UNDISTURBED FLOW

TURBULENT WAKE FLAP JET

Opportunity 11 - Augmented High-Lift Systems

Capable of 2D buffet free CLmax of ~ 6.5

Opportunity 11 - Augmented High-Lift Systems UNDISTURBED FLOW

NO TURBULENT WAKE NO FLAP WAKE EXTENSION

40⁰ FLAP DEFLECTION

PROPELLER DISK

Capable of 2D buffet free CLmax of ~ 6.5

Opportunity 11 - Augmented High-Lift Systems • It is possible to optimally deflect thrust, to reduce the wing area required for a given approach speed & weight.

1.0

0.5

1.9

1.5

0.5

1.0

1.0

0.8

1.5 CL =1.9 AR

( WT ) 0.6 0.4 0.2

Optimum Jet Deflection No Jet Deflection

0

0

0.2

0.4

0.6

Low Speed Metric

0.8

1 CL AR

1.0

1.2

1.4

Opportunity 11 - Augmented High-Lift Systems • For Example 1.0

0.5

0.8

CL =1.9 AR

( WT ) 0.6

1.9

1.5

0.5

1.0

1.0 1.5

~ 30% Reduction

0.4 0.2

Optimum Jet Deflection No Jet Deflection

0

0

0.2

0.4

0.6

Low Speed Metric

0.8

1 CL AR

1.0

1.2

1.4

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