Next Generation Civil Transport Aircraft Design Considerations for Improving Vehicle and System-Level Efficiency

Next Generation Civil Transport Aircraft Design

Considerations for Improving Vehicle and

System-Level Efficiency

∗

Diana M. Acosta

NASA Ames Research Center, Moffett Field, California, 94035 † ‡

Mark D. Guynn and Richard A. Wahls

NASA Langley Research Center, Hampton, Virginia, 23681 §

Rub´ en Del Rosario

NASA Glenn Research Center, Cleveland, Ohio, 44135 The future of aviation will benefit from research in aircraft design and air transporta- tion management aimed at improving efficiency and reducing environmental impacts. This paper presents civil transport aircraft design trends and opportunities for improving ve- hicle and system-level efficiency. Aircraft design concepts and the emerging technologies critical to reducing thrust specific fuel consumption, reducing weight, and increasing lift to drag ratio currently being developed by NASA are discussed. Advancements in the air transportation system aimed towards system-level efficiency are discussed as well. Finally, the paper describes the relationship between the air transportation system, aircraft, and efficiency. This relationship is characterized by operational constraints imposed by the air transportation system that influence aircraft design, and operational capabilities inherent to an aircraft design that impact the air transportation system.

Nomenclature

c Thrust specific fuel consumption h Fuel heating value F t Time C Coefficient of drag D C Coefficient of lift L D Aerodynamic drag force L Aerodynamic lift force S Wing planform area V Velocity, or airspeed W Aircraft weight W Weight of fuel consumed f W Operational empty weight OEW W Weight of payload payload W Weight of total fuel onboard totalf uel η Propulsive efficiency P η Thermal efficiency T ρ Atmospheric density ∗ Aerospace Engineer, Intelligent Systems Division, Mail Stop 269-1.

† Aerospace Engineer, Aeronautics Systems Analysis Branch, Mail Stop 442, Senior Member AIAA.

‡ Project Scientist, Fixed Wing Project, Fundamental Aeronautics Program, Mail Stop 267, AIAA Associate Fellow.

§ Project Manager, Fixed Wing Project, Fundamental Aeronautics Program, Mail Stop 5-3, Senior Member AIAA.

1 of 15 American Institute of Aeronautics and Astronautics

I. Introduction

he aviation industry is vital to the nation’s economic well-being. In 2009, civil aviation activity supported T 1,200,000 Americans with jobs. That same year, aviation provided the nation with a positive trade value of over $75 billion and operations generated a total output of $296 billion to the U.S. economy. Nevertheless, the aviation industry also has a negative impact on the environment and energy usage. In the U.S., air travel fuel use is 7% of fuel consumed for transportation, and jet fuel produces 65 million metric tons of CO per year, which is 4% of CO emission from energy usage in the nation. Fuel is also approximately 30% of operating costs for U.S. passenger airlines.

Growth in the aviation industry has been supported over the years through a diverse research and development portfolio in government, industry, and academia. This paper will review what is being done in research at National Aeronautics and Space Administration (NASA) today and could be done in the future to sustain growth in the aviation industry, with a particular focus on improving vehicle and system-level efficiency. First, the paper focuses on vehicle efficiency in Section II. The factors contributing to vehicle efficiency, aircraft design concepts for improved vehicle efficiency, and the emerging technologies critical to reducing thrust specific fuel consumption, reducing weight, and increasing lift to drag ratio are discussed.

Next, the paper focuses on system-level efficiency in Section III. The factors contributing to system-level efficiency and the vision for the Next Generation Air Transportation System are discussed. Section IV describes the relationship between the air transportation system, aircraft, and efficiency. This relationship is characterized by operational constraints imposed by the air transportation system that influence aircraft design, and operational capabilities inherent to an aircraft design that impact the air transportation system.

Finally, Section V highlights the fact that the impact of aviation extends beyond vehicle and system-level efficiency.

II. Vehicle Efficiency

II.A. Factors Contributing to Vehicle Efficiency The goal of improving vehicle efficiency corresponds with the goal of reducing fuel usage required to operate an aircraft. The amount of fuel consumed by the aircraft is related to the thrust specific fuel consumption, aircraft weight, and aerodynamic lift and drag forces experienced during operation. Eq. 1 defines the relationship between the weight of fuel consumed and these contributing factors.

∫ t D W = cW dt (1) f L t In this equation, W represents the weight of fuel consumed over time t to t , c represents the thrust f 0 specific fuel consumption, W represents the aircraft weight, D represents the aerodynamic drag force, and L represents the aerodynamic lift force.

The thrust specific fuel consumption, defined as the mass flow rate of fuel over engine thrust, can be represented by V c = , (2) η η h P T F where V is aircraft airspeed, η is propulsive efficiency, η is thermal efficiency, and h is the fuel heating P T F value. Some of the emerging technologies in the design of propulsion systems to improve η and η are P T discussed in Section II.B.3. Aircraft operations also has an impact, with V influencing c , Mach influencing η , and altitude influencing η .

P T The aircraft weight is a sum of the aircraft’s operational empty weight, W , the weight of total fuel OEW onboard the aircraft, W , and the weight of payload, W , as expressed in the following equation: totalf uel payload W = W + W + W . (3) OEW totalf uel payload The W is comprised of the structural weight, propulsion system weight, and weight of equipment required OEW to operate the aircraft, ranging from avionics to passenger seats. The emerging technologies discussed in 2 of 15 American Institute of Aeronautics and Astronautics Section II.B.1 strive to reduce W . Operators determine the W and W at the beginning of OEW payload totalf uel each flight, where the W is planned for the mission as well as extra reserve to accommodate inefficient totalf uel operations.

The aerodynamic lift and drag forces are defined as ( ) L = ρV C S (4) L and ( ) D = ρV C S, (5) D respectively, where ρ is the atmospheric density, V is aircraft airspeed, C and C are coefficients of lift L D and drag, and S is the wing planform area. An aircraft’s S and C and C characteristics are factors of L D the aircraft design, and improvements sought are discussed in Section II.B.2. Besides the direct influence of altitude (through ρ ) and V , L and D are also influenced by operations through the values of C and C .

L D The C and C are dependent on attitude (or angle of attack), which is adjusted to provide the required L L D given the V , Mach, vertical profile, and wind conditions.

The total time in operation is another factor in W , since W is defined as the weight of fuel consumed f f over time t to t . Inefficient operations, such as rerouting, vectors, holds, and slow speeds while in the air and delays on ground, will increase t for an aircraft’s mission, and consequently increase fuel usage and diminish the aircraft’s efficiency.

II.B. Emerging Aircraft Design Concepts and Technologies for Vehicle Efficiency Advanced aircraft design concept studies have been conducted in recent years to explore the promise of integrated system solutions and motivate research to attain that promise. Studies aimed towards enhanc- ing energy efficiency and environmental compatibility have highlighted the benefits of integrated solutions.

Three different aircraft design concepts that have emerged from studies conducted by or funded by NASA 5 5–9 include the truss-braced wing configuration, the hybrid wing body configuration, and the double-bubble configuration. A conceptual illustration of these configurations is shown in Figure 1. Success of these di- verse aircraft design concepts will be dependent on the development of advanced technologies. The research and development of these critical technologies tends to focus in three key challenge areas for improving or maintaining the targeted aircraft efficiency. These three challenge areas include (1) reducing vehicle weight, (2) increasing the ratio of lift over drag, and (3) reducing thrust specific fuel consumption. Emerging tech- nologies and research efforts in each of these challenge areas are discussed in the subsections that follow and highlighted in Figures 2, 3, and 4. Effective design and integration of these emerging technologies will rely on the ability to achieve simultaneous reduction of weight, increase in the ratio of lift over drag, and reduction in thrust specific fuel consumption with minimal impact on other environmental considerations, including noise and emission generation.

II.B.1. Reduce Vehicle Weight Vehicle weight will be reduced through the introduction of new structural concepts and designer materials that leverage tailored designs for the vehicle fuselage and wings with integrated control actuators. Research is focused on improving material and structural properties and manufacturability of advanced composites and metals, and on enabling control technologies.

The improvements in composite structures being pursued include the development of design tools and new concepts such as stitched composites and multifunctional skins with composite primary structures.

Tailored placement of fibers within composites will optimize structural properties with the aid of new design tools. Stitching through dry carbon fabric during panel fabrication offers benefits to the structural weight through the replacement of mechanical fasteners, reduction of de-lamination, and improvement in damage tolerance (Fig. 2(a)). Stitched composites also enable the construction of non-circular pressure vessels, which will be essential for some unconventional configurations like the hybrid wing body and the double- 11, 12 bubble. Multifunctional skins will also reduce weight though the combination of lighter gage composite primary structures with other functions that provide protection external to the skin. Among the functions investigated for inclusion in the protective skin are acoustic treatment, thermal insulation, lightning strike 10, 13 protection, impact detection and indication, and ice protection (Fig. 2(b)).

3 of 15 American Institute of Aeronautics and Astronautics Figure 1. Conceptual illustration of fuel-efficient aircraft, including the truss-braced wing configuration (right), hybrid wing body configuration (center), and double-bubble configuration (left). Image credit: NASA.

4 of 15 American Institute of Aeronautics and Astronautics The objective of research for metal structures is to reduce weight through the optimization of load paths using structural concepts like curvilinear stiffeners (Fig. 2(c)), new alloys, and novel manufacturing techniques. The manufacturing technique explored for this purpose involves additive manufacturing and 12, 14 is envisioned as a replacement for forging (Fig. 2(d)).

Active structural control will further reduce wing weight by adjusting the lift distribution and limiting the resulting structural loads during critical maneuvering and gust loads design cases using integrated distributed control actuators (Fig. 2(e)) and new control laws. Control system research under investigation includes techniques for gust load alleviation, load limiting control allocation, modal suppression, and distributed 10, 15 controls.

II.B.2. Increase the Ratio of Lift over Drag Increasing the ratio of lift over drag will be achieved through reductions in drag. Research emphasis is on reducing viscous drag, induced drag, and wave drag.

One approach to reducing the viscous drag, or friction drag, on an aircraft is the reduction of overall aircraft surface area, or “wetted” area. Configurations such as the hybrid wing body configuration (Fig. 3(a)) are designed towards this goal. Another technique to reduce the aircraft surface area is to reduce the size of stability and control surfaces by increasing their aerodynamic performance through the use of active flow control. As an example, one research effort is evaluating the effectiveness of active flow control to augment 12, 16 rudder performance at low speed conditions (Fig. 3(c)).

Viscous drag can also be reduced by preventing large portions of flow from transitioning from laminar to turbulent flow. Since local skin friction increases significantly in turbulent flow, drag is reduced by maintaining a laminar boundary layer. The transition from laminar to turbulent flow is influenced by many factors, including the leading edge wing sweep and Reynolds number. A number of passive and active approaches exists to control the transition. One passive technique under investigation involves discrete roughness elements positioned on the aircraft surface near the leading edge that modify the flow to inhibit 12, 16, 17 or delay transition (Fig. 3(d)).

Induced drag, or the drag created as a result of the lifting force on the aircraft, benefits from efforts to improve the span-wise lift distribution. Aircraft configuration plays a key role. An elliptical span-wise lift distribution attainable through novel configurations, such as the hybrid wing body configuration (Fig. 3(a)), reduces induced drag. Large wing spans and high aspect ratio wings that reduce the span loading also reduce induced drag, and serve as motivation for the truss-braced wing configuration (Fig. 3(b)). Span- wise lift distribution can also be improved in-flight using active controls. One concept to control the lift distribution utilizes active aeroelastic wing shaping control and variable camber continuous trailing edge flaps (Fig. 3(e)).

The reduction of wave drag, or the drag resulting from shocks over the wing upper surface at high subsonic speeds, is being investigated. Researchers are evaluating the ability of circulation control to modify 15, 19 circulation around the aft of the wing during cruise conditions (Fig. 3(f)).

II.B.3. Reduce Thrust Specific Fuel Consumption Reductions in thrust specific fuel consumption are being pursued through technologies that improve the propulsion system’s thermal efficiency and propulsive efficiency, or η and η , as seen in Eq. 2. Many of T P the concepts and technologies have been developed to improve thust specific fuel consumption, but research is needed to enable the concepts and technologies with minimal impact on weight, drag, noise generation, and emission production.

Improved thermal efficiency can be attained through turbomachinery operation at higher pressures and temperatures. One approach is the use of integrated Ceramic Matrix Composites and Environmental Barrier Coating systems for the combustion liner (Fig. 4(a)) and turbine vanes that will allow higher temperatures 20, 21 for new engines and better fuel/air mixing due to a reduction in cooling air flow required. Since high pressure and temperature engine environments also encourage emissions of nitrogen oxides, lean partial- mixed combustors and lean direct multi-injection concepts with advanced fuel flow control techniques are 20, 22 also being studied to simultaneously provide fuel efficiency and reduce emission production (Fig. 4(b)).

The introduction of advanced ultra-high bypass ratio propulsors and embedded engines with boundary layer ingestion are two strategies being pursued for propulsive efficiency improvement. Open rotor concepts that optimize propulsive efficiency with ultimate bypass ratio are being researched with counter-rotating 5 of 15 American Institute of Aeronautics and Astronautics rupted in the primary longitudinal loading direction. If such an arrangement were used for the ember (attached by a discontinuous shear clip to the skin) would be less effective in bending ately resulting in a non-competitive solution.

Current design (including the extrapolation to 2030 - 2035) of composit e structures requires designing to meet blem, the HWB PRSEUS fuselage panel has been designed as a bi-directionally stiffened load requirements and then overdesigning to provide capacity to absorb impact and operate in hot, humid e wing bending loads are carried by the frame members and the fuselage bending loads are environments . As part of the work in the N+3 Phase 1 program, when pushed to find a way to get to another seven percent fuel burn reduction, Cessna turned the composites design problem around and asked how to meet the . Features of this design include: loads paths that are continuous in both directions, skin and structural requirements without overdesigning the primary structure . What if the primary structure is designed are highly tailored, thin skins designed to operate in the post-buckled design regime, and without any weight penalties (“knock do wns”) , and protective skins are used to meet the impact and hot, humid rest damage propagation. The resu lting panel design is extremely effective in eliminating the requirements? What if multiple requirements are met by one material in the protective skins? What if the external ed with the non -circular HWB pressure cabin. (Ref. 2) impact absorbing material also provides the acoustical treatment for the cabin? What if the external protective skin can replace the internal thermal insulation? What if the impact damage is visible unlike many of today’s composite III. PRSEUS Structural Solution structures? What if paint is replaced by a n aesthetic film, allowing attractive decorative outer surfaces , smooth surfaces which facilitate natural laminar flow , and reduced lightning direct strike trauma due to the absence of paint ?

ed nature of PRSEUS is evident in the strategic placement of the carbon fibers (Fig. 4). The Could the result be a step change in weight and fuel burn reduction? Figure 1 shows the current composite skin re-cure d rods, and foam-core materials are assembled and then stitched together to create the 2 structure; Figure 2 shows the goal for the STAR - C protective skin.

etry for the HWB fuselage loading. Lo ad path continuity at the stringer-frame intersection is irections. The Protective Outer Skin ated pultruded over Primary Structure trength/stability

section while Elastic Wing

Conductive skin g the neutral (Lightning, EMI, skin to further Stringer Paint, smoothness for panel bending laminar flow) ents are placed

lly tailored wing structural designs with distributed

kin surface and Skin advantage of

g by placing drag on high aspect ratio wings

Energy ducive lay-ups Absorbing Foam (Impact, Sound, effective. The Thermal, Space to suppress Frame for wires,

antenna’s, etc.) system by 25% while enabling reduced drag

modes, which ee of tailoring Figure 4. Pultruded Rod Stitched E fficient Unitized Structure (PRSEUS).

Figure 1. Current composite skin/ Figure 2. Protective outer skin concept.

ossible using (a) Schematic of the Pultruded Rod Stitched Efficiency Uni- (b) Schematic of the Smoothing, Thermal, Absorbing, Re- structure.

materials.

tized Structure concept for stitched composites. flective, Conductive, Cosmetic concept for multifunctional

sign and Fabrication

gral structure is ideal for the HWB pressure cabin because it is a highly efficient The FAP/SFW Project has provided the funding through an N+3 Phase II contract for Cessna to conduct the skins.

ry in th ree directions that is damage tolerant, stitc hed to react pull-off loads, and also capable research to answer these questions . The goal of this research is the development of potential concepts for protective the post-buckled design regime which enables thin-gauge skin-stringer designs to be lighter skins which enable natural laminar flow and a signifi cant weight reduction in the aircraft’s primary structure . The dwich designs. The PRSEUS HWB airframe features large unitized wing and fuselage protective skin is needed to absorb impact damage and to provide environmental protection . The STAR - C concept efficient continuous load paths, higher notched design properties, and larger allowable aspect ratio, lower sweep wings as seen in most N+3 should be responsible for smoothing out bumps or gaps, providing thermal insulatio n, absorbing impact and acoustic energy , reflecting ultraviolet and infrared radiation, conducting large amounts of electrical current (for lightning hanced levels of survivability beyond those possible using unstitched designs (Fig. 5). The

h

strike), and providing a cosmetic or appealing surface.

mblies are PRSEUS-based designs (red region) and the non-pressurized areas (green region) Outcomes from the project will be an assessment o f the feasibility of the protective skin concept and Downloaded by NASA AMES RESEARCH CENTER on July 16, 2013 | http://arc.aiaa.org | DOI: 10.2514/6.2013-1899 mbination of metallic and

educed

recommendations on material properties which will best support the concept . Cessna has completed the first half of .

the program where requirements and metrics have been defined; a search for potential mat erials has been conducted;

he PRSEUS fabrication e

173 test articles (first generation) have been constructed and tested; and an initial assessment of feasibility has been ht and thinner airfoils will result in more flexible wings, e self-supporting stitched made.

can be fabricated without The following sections will describe the work done in each of these areas . The paper will end with t he

idered early in structural design

and then accurately conclusions from the assessment of the first generation of test articles and a description of the next steps to arrive at a second generation of test articles along with recommendations for material properties to support the protective skin oven-cure operation using concept .

moldline (OML) tooling.

ign and analyze high aspect ratio wings with

in the stitched assembly II. R equirements o out-time, or autoclave The critical or design requirements for the STAR - C skins are energy absorption (impact), smoothness, and preg systems, which can conductivity (lightning strike) . Other significant requirements include thermal, reflectivity, cosmetic, acoustic, sembly because it must be (c) Diagram of a curvilinear stiffener structural concept. (d) Schematic of an electron beam freeform fabrication system

rocessing envelope. Design optimization tools developed

ures

for additive manufacturing.

accomplished using a American Institute of Aeronautics and Astronautics

at VA Tech through NRA contract

atisfy

method where the bagging

e inner moldline (IML) ailored structural design

Figure 5. HWB stru ctural breakdown.

This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.

American Institute of Aeronautics and Astronautics

alloys

nocomposite skins

ers

ign

iloring

ure

terials

(e) Diagram of an aeroelastically tailored wing structural de-

nd control surface design

sign with integrated structural controls.

High toughness alloy at stiffener base

Figure 2. Concepts and technologies for the reduction of vehicle weight.

for damage tolerance, transitioning to

13 13

metal matrix composite for increased

stiffness and acoustic damping

12 12 6 of 15 American Institute of Aeronautics and Astronautics incorporate mission, airframe, operations, and propuls ion system design and optimization. This process has as its objective the global optimization of aircraft fuel burn, but the configurations that are being optimized are inherently low in noise. For example, the engines are above the aircraft so that there is

N Baseline N+3 Reference require integration of

engine noise shielding, the fan pressure ratios are low so that jet noise and fan noise are low, the fan tip “SUGAR Free” “Refined SUGAR” speed is near sonic so that the shocks (and combination tone noise) are weak, and there is ample room for acoustic liners for engine noise treatment.

meet fuel burn goals

The H3.2 aircraft design, depicted in Figure 94, was designed with a payload and range comparable to a Figure 4. The test section is 8.2 ft by 8.2 ft in cross section , and 25 ft in length. The test section B777-200LR. As will be discussed in Chapter 7, this larger payload capacity was chosen to reduce the empty weight fraction of the aircraft to an acceptable level for reduced fuel consumption. A wide range of 765-093 765-094 floor and ceiling are slotted , and the sidewalls are solid. Semi - span models are mounted in the propulsion system configurations, including both po dded and boundary layer ingesting engines, were

Analysis compared

considered in the design process, as was the use of cryognenic methane fuel. In addition to having higher specific energy, which improves the fuel efficiency of the aircraft, the low fuel temperature allows for the tunnel using the Sidewall Mounted Support System (SMSS). The wind tunnel is capable of an

strate usable and robust

use of superconducting materials. Such materials could be used in a distributed, turbo-electric propulsion

to NTF data with

N+3 High L/D “SUGAR High” N+3 High L/D “SUGAR Volt” system to distribute power from the gas turbines to ma ny engine fans without the need for gears. The low absolute pressure range from 1 atmosphere to 8. 8 atmos pheres , a practical temperature range temperatures could also enable laminar flow over the pressure surface of the outer wings. However, as will be explained in Section 6.2.5 the final H3.2 design is powered by conventional jet fuel and uses a from - 270°F to 130°F, a Mach number range from 0.1 to 1.2, and a maximum Reynolds number

r Natural Laminar Flow

NLF

bevel gear transmission system to distribute power from two gas turbines engines to four fans.

6 ° ° of 146x10 per foot at Mach 1. Typical test s use a temperature range from - 250 F to 120 F . For

aminar Flow Control (HLFC)

t ests that utilize the high - pressure air delivery station to enable flow control or propulsion 765-095 ° simulation systems , the lower temperature limit is set to - 50 F to simplify the testing technique .

ction to high-fidelity aero design

N+3 High L/D “SUGAR Ray”

Applied to Rudder

The dual flow high - pressure air

halen

station is coupled to the SMSS as shown

in Figure 5 . The two independently

Figure 94: H3.2 conceptual aircraft design.

f CF control through

Re = 6.7M

controlled air lines pass through the center

765-096

of the force an d moment balance, and

s Elements (DRE)

The nomenclature H3.2 refers to the second iteration of the Class 3 Hybrid Wing Body design. The H3.1 design 765-097 n - demand se on demand was presented in the January 2010 interim report. As will be discussed in Chapter 7, three different sizes were (a) Hybrid wing body configuration with reduced “wetted” (b) Truss-brased wing configuration with a large wing span considered for the HWB concept: a B737 size with 180 passengers which is denoted as H1.2, a 767 size with 256 6 5 couple to the model using a concentric area and elliptical span-wise lift distribution during cruise. and high aspect ratio wings.

passengers, plus cargo, which is denoted as H2.2, and a B777 size aircraft which is denoted H3.2.

Figure 3.18 – Final Five airpla nes selected for further study

monstration at relevant C demonstration at relevant C

bellows arrangement. The model is

L L

Alternatives

il sized for

Alternative #1 Alternative #2 Alternative #3 Alternative #4 Alternative #5 Alternative #6

protected from over pressurization by the

g

Flow Control Actuators

Number of Fuselages 0 1 2

relax surface quality

NASA/CR—2010-216794/VOL1 118 Wing-Body Blend None Fairing Moderate Blend Extreme Blend

model protection system . The maximum

N o. o f P assenger D ec k s 1 1 . 5 2 Fusela Number 1 2

Location Low Mid High Pylon Mount Low-High Low-Pylon pressure limit can be adjusted

Sensors

d tail size

High Lift System Conventional Triple Slotted Flap USB EBF IBF AFC Bracing None Strut Cable Truss

independently for both legs from 300 psi to

Join None Tip Mid Box

Wing dela y y

ise dra g

Folding None In Flight On Ground

1200 psi. If the set pressure matches or

or integration of NLF for integration of NLF , Morphing None Planform Variable Camber Both Wi ng l et N one C onvent i ona l R a k e d F eat h ers M orp hi ng ess with AFC where f is the continuous flap deflection of flap n , n 1 2 3, y is the BBL station, and a , i 1 2 3 4 5 are exceeds the safety limit for the wind tunnel Pitch Effecter Conv. Horizontal T-Tail V-Tail Canard Wing TE n i Yaw Effecter Conv. Vertical V-Tail H-Tail Winglet Drag Rudder the polynomial coefficients determined by S&C

R o ll Eff ecter Ail eron / S po il er Wi ng W arp i ng into flight weight systems

model , then the supply line into the SMSS

Vehicle Characteristics Figure 5 : Diagram of high - pressure air routing to a semi - span model Location Under Wing Mid Wing Above Wing Aft Fuselage

rudder 1

flow

5 4 3 2 7 Propulsor Type Propeller Open Rotor High BPR Fan Ultra High BPR Fan a y y y y y 1 0 1 2929 10 5 1 1 1 1 1 mounted on the sidewall of the NTF.

would be isolated and vented in less than

Propulsor Arrangement Discrete Distributed 4 3 2 5

tion ration

a 5 y 4 y 3 y 2 y 1 0 0 2 2291 10

trades through demonstration 4 1

Energy Conversion Brayton Const. Vol. Fuel Cell / Motor Piston Electric Motor 1 1 1 0.5 seconds.

nf Augmentation None Batteries Fuel Cell Brayton Propulsor 5 4 3 2 c 4 Integration a y y y y y 1 4 5450 10 3 2 2 2 P r i mary F ue l Li qu id G aseous H y d rogen B atter i es 2 2 3 nf (7)

DRE effect low M low Rn DRE effect , low M , low Rn c

tail 4 3 2 2

ATM 2008 NextGen a 5 y 4 y 3 y 2 y 1 0 0 3 4045 10 2 2 2 2 (c) Notional concept to augment rudder performance with ac- (d) Image showing the delay of transition from laminar to tur- Aircraft Class Regional Medium Large 5 4 3 2 1

p high Reynolds number

a y y y y y 1 0 2 8738 10

Notional AFC Approach 16

Formation Flight FALSE TRUE 1 3 3 3 3 3 tive flow control. bulent flow due to discrete roughness elements at low Mach

B. Model Description

and landin g g

4 3 2 1 In Flight Refueling FALSE TRUE Mission a 5 y 4 y 3 y 2 y 1 0 0 7 6635 10 0 3 and Reynolds number conditions. 3 3 3 Ground Refueling FALSE TRUE

lity

ideslip s 15 r , Figure 8 shows the BBL stations of the 12 flap segments. Aerodynamic vortex-lattice calculations show that Alternatives Selected for Analysis as Part of SUGAR

1 The FAST - MAC model shown in Figure 6

VCCTE flap concept could produce a drag reduction benefits ranging from 46% to 66% at cruise.

Figure 3.19 – Alternatives Selected for Analysis

has a modern supercritical wing and was

The ESAC with an optimized wing shape equipped with the VCCTE flap system is illustrated in Figure 9.

designed to become a n NTF standard for

evaluating performance characteristics of

integrated active flow control and propulsion

systems. Th e outer mold line of the model was

designed for a cruise Mach number of 0.85, a

lift coefficient of 0.50, at a Reynolds number

based on mean aerodynamic chord of 30x10 .

The wing was designed with the unstructured

Figure 9 - Elastically Shaped Aircraft Concept with Variable Camber Continuous Trailing Edge Flap Figure 6 : Planform view of the FAST - MAC semi - span model.

(e) Elastically Shaped Aircraft Concept with Variable Camber (f) Depiction of the Fundamental Aerodynamics

Navier - Stokes flow solver USM3D in

Continuous Trailing Edge Flap. Subsonic/Transonic-Modular Active Control semi-span

conjunction with the CDISC design code . A

model used to evaluate the effectiveness of circulation control I V. Nonlinear Aeroelastic Flight Dynamic M odeling for drag reduction during cruise conditions.

tangential blowing slot is located at the 85% chord location on the upper surface, and is directed To develop an understanding of the aeroelastic effects on aerodynamics and wing shaping control, a coupled Figure 3. Concepts and technologies to increase the ratio of lift over drag through the reduction of drag.

aeroelastic flight dynamic model is developed for the wing structure which is modeled as beam-rod finite elements. over a 15% chord simple hinged flap for both the cruise and high - lift configurations . For The nonlinear model includes the propulsive effects of engine mass and thrust-induced stiffness as well as the fuel transonic testing, the non - dimensional blowing slot height was set to h/c = 0.0019. The wing has usage management to account for mass variation during cruise.

° an aspect ratio of 5.0, taper ratio of 0.40, a leading edge sweep of 30 , zero dihedral , and a reference area of 6.06 ft . The CDISC design method pro duced a linear twist distribution with ° 5.0 of washout . The chord length at the side of the fuselage is 25.0 inches, resulting in a mean 7 of 15

American Institute of Aeronautics and Astronautics

American Institute of Aeronautics and Astronautics provide stability for lean burn systems. NASA ERA is contracted with General Electric ( GE ) Aviation and Pratt & storical*Baseline*blade*set 6*GE*Advanced*Designs

rocess has as its objective the global optimization of aircraft fuel burn with other NASA

Whitney ( P & W ) in a 50/50 cost share to develop the new lean burn concepts (see F igure 4 ) and demonstrate these in odern*Baseline*blade*set Pylon*wake*mitigation a sector rig test which simulates the pressures and temperatu res of a relevant engine environment. These sector rig e.g., field length) either appearing as constraints or being evaluated in a post processing 2*GE*Advanced*Designs tests will be performed at the NASA Glenn R esearch Center (GRC) Advanced Subsonic Combustor Rig (ASCR)

• ERA will focus on CMC durability

2*Snecma*Designs during the fi rst 9 months of 2012. The NASA ASCR facil ity has been upgraded to provide combustor entrance emissions) for reinsertion into the design loop. The D8 aircraft is targeted at a comparable ble 2 : Open Rotor Configurations Tested at NASA GRC Wind Tunn els.

conditions up to a pressure of 900 psia and temperature up to 1300F.

characterization and EBC

FY11-12 Plans: B737-800.

development ts are summarized below and details can be found in Refs. 8 - 13: CRESS EN I blade set , including Laser diagnostics, in the NASA 9x15 was completed in October

• Establish creep and TMF baseline

n - 1 blade designs have demonstrated acoust ic margin to Chapter (CH) 4 that is equivalent Downloaded by NASA AMES RES Radial Radia of the D8 series aircraft will be discussed and described in this report. The D8.1 aircraft is orary tube - and - wing aircraft with turbofan engines – see F igure 9.

1 l 2 Axial ems analysis team has developed a model to analyze a modern open rotor based propulsion ern airfram e. NASA system level assessments estimate a 25+% fuel burn and 15+ EPNdB imized for minimum fuel burn while incorporating technologies available currently . This

• Model thermal gradient cyclic durability

rgin – see F igure 10.

Axial Gen 2 high speed Open Rotor testing in the NASA GRC 8x6 was completed on 9/9/2011.

is of comparison as to the final impact of the inclusion of each advanced technology on the Axial a collaboration of FAA/CLEEN, GE, and NASA.

• Evaluate effects of combustion environment

the NASA 9x15 wind tunnel including multiple blade sets and integrated pylon tests. All Axial/Radial/Radial n regards to each metric considered. It also provides a sense of the overall contribution to plete Jan 19, 2012. The Gen - 2 blade designs offer the promise of additional acoustic Mixer Concept is and documentati on of the data is in progress and expected to be completed over the next Dual Main Mixer Concept Counter - Rotating

ment of the goals through the configuration alone. The D8.5 is the final advanced vehicle

• Quantify recession of CMC and coating

Externally Staged

optimized to meet the N+3 program goals. This includes the insertion of advanced

Swirler Concept rotor result show promise, more research on installation effects and certifications must be F igure 4 : Candidate lean burn injector /mixer concepts.

20 NASA/P&W Partnership on Geared Turbofan Technology Low emission combustor concept (a) Combustor concept utilizing Ceramic Matrix Composites (b) Drawing of candidate lean burn injector/mixer concepts. processes, and designs that may not be available or ready to include in an aircraft until the n rotor propulsion systems are installed o n commercial aircraft. Also, it is unlikely that ill be able to match the acoustic margin of ducted sys tems because Open Rotor systems by and Environmental Barrier Coating systems for the combus- 21 ct (and acoustic liner) and, as a result, have greater flow and acoustic inte ractions with the e.

tion liner.

section we will discuss the challenges of an ultra high - bypass ducted propulsor.

50% reduction of combustor cooling air would reduce NOx formation ~ 50%

American Institute of Aeronautics and Astronautics

aircraft concept includes a “double-bubble” fuselage that allows for the inclusion of a

www.nasa.gov Contact: Janet.B.Hurst@nasa.gov 216-433-3286

he D designation is provided as homage to the chief designer and originator of the concept

ark Drela.) The concept features embedded aft engines with pi tail arrangement while

nd a reduced Mach number operation that allows for a nearly-unswept wing and eliminates

urbomachinery Aeroelastic Analysis

E slats.

Lightweight Fan Blades

ackground: Future Hybrid Wing Body aircraft will use an Embedded Propulsion

ystem with Boundary Layer Ingestion to improve Fuel Burn. An embedded

• Thin, hollow composite blade

ropulsion system will lead to a persistent and severe inlet distortion reaching the

(c) Open Rotor Propulsion Rig installed in a NASA low speed (d) Diagram highlighting research technology areas for an r Propulsion Rig installed in the 9x15 Low Speed wind tunnel on the left and on th e right it is http://silentaircraft.org/ development for reduced weight 20 23 nic Wind Tunnel in preparation for performance testing at cruise Mach of ~0.8 .

wind tunnel. ultra-high bypass ratio geared turbofan.

n at all operating conditions, resulting in high dynamic stresses due to

• Aeroelastic tailoring required to locally eroelastic forced response and the possibility of flutter.

stiffen lightweight blades to avoid flutter American Institute of Aeronautics and Astronautics

pproach: A new high - fidelity full - rotor aeroelastic analysis is being developed to

• Elimination of metallic leading edges, replaced by functionally graded Nanofillers improve composite nable the computational modeling of a fan subjected to a distorted inlet flow. This nanocomposite to toughen blades to toughness

ew capability allows an arbitrary inlet distortion to be specified with both

survive wear and bird strike impact

ircumferential and radial variations of flow properties, together with prescribed

• Material developed also applicable to lightweight fan cases for blade - out lade vibrations.

containment and adaptive fan blade

esults: Substantial work has been done in the formulation and implementation

designs

f an arbitrary inlet distortion in the Aeroelastic Analysis code TURBO - AE. A

presentative fan configuration has been selected and a computational mesh of

Composite fan blade design

ufficient resolution has been generated. An inlet distortion pattern representing a

(e) Embedded engines located on the aft of the double-bubble (f) Diagram depicting research to enable embedded systems, showing regions of functional configuration for boundary layer ingestion. including an aeroelastic analysis of fan blades due to inlet dis- gradients to improve toughness oundary layer ingesting inlet has been prescribed. Simulations have been Light, Efficient Components for TeDP Must Be Subsonic Fixed Wing Project Fundamental Aeronautics Program tortion and composite fan blade design with aeroelastic tailor- 17 17

erformed with rigid - and vibrating - blade geometries subjected to clean and 10, 24

ing.

Cryogenic or Superconducting

istorted inlets.

ignificance: New aeroelastic analysis capability will ensure operability of fan in

TeDP Technical challenges are

mbedded propulsion system of Hybrid Wing Body aircraft.

soluble and being pursued: onic Fixed Wing Project amental Aeronautics Program 21 21 Researcher: Dr. Gregory Herrick (RXS) Contact: Milind Bakhle, bakhle@nasa.gov, 216 - 433 - 6037

Superconducting transmission lines

between generators and motors

Superconducting m

(g) Diagram of the turbo-electric distributed propulsion con-

Utilities & Air Force are working this

drive propulsive fan

cept, as applied to a hybrid wing body configuration.

Figure 35: D8 Series aircraft rendering.

Figure 4. Concepts and technologies for the reduction of thrust specific fuel consumption.

8 of 15

Cryogenic Inverter for

American Institute of Aeronautics and Astronautics

Turbine engine driven

variable speed fans

10-216794/VOL1 48

superconducting generator/motors th

Weight ½ SOA & ~1/10 SOA

open rotor systems (Fig. 4(c)). The counter-rotating open rotor systems must be matured to lower noise 20, 23 generation, which is a driving factor in development since noise suppression is not provided by a nacelle.

Ultra-high bypass ratio concepts, including geared turbofan (Fig. 4(d)), are also being matured through the development of small, high density cores that will enable higher bypass ratio engines without impacting the 20, 24 engine diameter, drag, or weight. Another concept under investigation is embedded engines. Embedded engines ingest boundary layer flows for improvements in propulsive efficiency and also offer drag reduction benefits (Fig. 4(e)). The research emphasis is on integrated inlet/fan embeddded systems that minimize the loss in fan performance and stability. One such effort is exploring the design of lightweight fan blades utilizing aeroelastic tailoring to withstand the high dynamic stresses resulting from the embedded inlet 10, 24 distortion (Fig. 4(f)).

Hybrid electric engine concepts also aim to improve thrust specific fuel consumption, with the turbo- electric distributed propulsion concept promising improvements in both thermal and propulsive efficiency (Fig. 4(g)). Development efforts to enable the turbo-electric distributed propulsion concept include the development of superconducting material, superconducting motors, cryo-inverters, and cryo-coolers.

III. System-level Efficiency

III.A. Factors Contributing to System Efficiency System-level efficiency refers to the ability of airports and airspace to accommodate air traffic demand, which corresponds to total throughput of the air transportation system rather than fuel efficiency. As such, many of the operational inefficiencies from the perspective of vehicle efficiency are imposed by air traffic controllers to increase system-level efficiency. Today, system-level efficiency is limited by the infrastructure and procedures that allow the air transportation system to function, and by the uncertainty in the flight environment that affects those procedures.

While the air transportation system has evolved over the years to accommodate the growth of aviation, the system relies on an infrastructure and set of procedures that are becoming strained by high demand.

Air travel is limited by the number of aircraft within a predefined sector and the mission operations an aircraft is permitted to execute. Since air traffic controllers are responsible for the safe flow of aircraft through a sector with only limited information about each aircraft and limited computer support systems, the controller’s cognitives limitations restrict the number of aircraft within the sector. Similarly, a dependence on voice radio systems for the communication of instructions and clearances between air traffic controllers and pilots limits the complexity and number of communications. As a result, current operations remain highly constrained, leaving margins for improvement to air traffic flow and system-level efficiency. Required spacing between aircraft also limits the density of aircraft and total capacity of the airspace. Spacing standards between aircraft are set to 3 miles for inland flights and 5 miles for oceanic flights to account for uncertainty in aircraft location introduced by the use of surveillance radar to track aircraft. The terminal area has additional constraints. Airport infrastructure, including runways, taxiways and ramps, limits the movement of aircraft into, around, and out of the airport. Nearby airports and interactions with associated traffic flows also limit aircraft movement within the terminal area. Furthermore, environmental concerns, such as noise and emissions, are limiting growth of air travel at certain airports Uncertainty in the flight environment, including the presence and severity of weather and aircraft- generated wake turbulence, also impedes throughput. Weather en-route that may present a hazard for aircraft requires traffic to be rerouted in-flight or delayed prior to departure. Uncertainty in the weather and weather forecast requires large margins of safety between the inclement weather and aircraft, leading to reroutes that may be excessive. Congestion in sectors with good weather ultimately limits the throughput of the entire air transportation system. Poor visibility and weather hazards in the terminal area often add ad- ditional spacing requirements or completely stop operations. Incoming and outgoing flights are subsequently put on hold, delayed or canceled until the weather is known to be safe. Due to the risk and uncertain nature of wake turbulence generated by aircraft, wake imposed spacing standards between aircraft in the terminal area becomes another limiting factor on throughput. These spacing standards are dependent on the aircraft types and sizes and on runway configurations, and typically range from 4 to 6 miles.

The effects of these factors, combined with high demand, can culminate into vast decreases in system- level efficiency and vehicle efficiency. In fact, it was found that domestic air traffic delays in 2007 resulted in a total cost of $41 billion and delayed flights consumed about 740 million additional gallons of jet fuel.

9 of 15 American Institute of Aeronautics and Astronautics III.B. Next Generation Air Transportation System The Next Generation Air Transportation System, or NextGen, is a vision shared by the Joint Planning and Development Office, Federal Aviation Administration, and NASA that describes the future air transportation 27, 28 system in the United States. Advancements introduced in NextGen will improve system-level efficiency and vehicle efficiency by increasing throughput, capacity, and flexibility, and by allowing aircraft to operate at their optimal conditions. A conceptual illustration of NextGen is shown in Figure 5.

Figure 5. Conceptual illustration of NextGen.

Technologies onboard and tools on the ground will enable better information exchange, communication between between air traffic controllers and pilots, and safe, precision operations by aircraft. The reliance on radar systems and voice communication is expected to end with the adoption of Automatic Dependent Surveillance-Broadcast (ADS-B) technology. ADS-B is an on-board technology that derives aircraft loca- tion information from a Global Navigation Satellite System, similar to the Global Positioning System, and provides greater positional accuracy and integrity than the current radar system. The aircraft location, along with additional information such as aircraft type, altitude, speed, heading, climb or descent rates, flight ID and intent, can be sent by ADS-B to ground stations and properly equipped aircraft within 200 miles, increasing awareness within the air transportation system. As a result, information is available for air/ground integrated technologies to provide advisories, air traffic controller to make more informed decisions, and pilots to eventually provide self-spacing and self-separation.

Integrated strategic and tactical planning tools for air traffic management will lead to improvements in surface operations, departures, flow and airspace planning, en route operations with weather avoidance, and dense terminal arrivals. Airport surface operations will be improved by information sharing and coor- dination among airport operators, flight operators and air traffic control facilitated by tools that provide control advisories to reduce overall delay on the surface. Algorithms pursued for airport surface operations management, such as Spot and Runway Departure Advisor with Collaborative Decision Making, will meter departure aircraft to reduce the number of aircraft in taxiways and runway queues and attempt to hold aircraft at the gate or preassigned holding pads with engines off. The sequence of aircraft arriving and departing will also be optimized to minimize delay. Planning tools, such as the Combined Arrival/Departure 10 of 15 American Institute of Aeronautics and Astronautics Scheduler, will consider minimum wake vortex separation and estimated time of operations in advising times of arrivals and departures at an airport. Departure times will also be planned to allow departing aircraft to efficiently merge into constrained en route streams of air traffic using tools such as the Precision Departure Release Capability.

En route, traffic flow management will benefit from integrated traffic flow and weather models, trajec- tory planning tools, and more accurate conflict detection and resolution algorithms available to air traffic controllers and pilots. The integration of traffic flow and weather models, for example, will enable aircraft to be assigned more efficient departure delays and routes around weather. In flight, updated weather infor- mation will be used to find more direct routes from an aircraft’s current location to a subsequent location on the aircraft’s flight plan. These new routes, called dynamic weather routes, will save time and alleviate congestion when the airspace is constrained by weather. Further relief for highly constrained areas will be provided by the concept of dynamic airspace configurations, which represents an airspace structure that can adapt to take advantage of available facilities and controllers to accommodate fluctuating demand.

Tailored operations will also be more prominent in NextGen. Optimized descent profiles, referred to as Continuous Descent Approaches (CDAs), are already implemented at a few airports in the United States, but are only feasible during periods of low to moderate traffic demand. Controller support tools and display enhancement will enable controllers to manage arrivals that satisfy time-based metering constraints and allow the execution of CDAs and other tailored operations under high traffic demand. The tools and display enhancements will range from early/late display indicators that augment timelines already used in air traffic control, to slot marker circle displays that provide spatial information on where an aircraft should be in 36 36 comparison to the actual location, to advisories that provide controllers recommendations with speed, altitude, and path stretching solutions that will keep aircraft on their assigned schedules and maintain safe separation.

IV. Relationship between the Air Transportation System, Aircraft, and

Efficiency

While vehicle efficiency corresponds the fuel usage of an aircraft and system-level efficiency corresponds to the total throughput of the air transportation system, the interactions between the air transportation system and aircraft can limit both vehicle and system-level efficiency. Operational constraints imposed by the air transportation system influence an aircraft’s vehicle efficiency. Meanwhile, operational capabilities of an aircraft influence the air transportation system’s system-level efficiency. This relationship is depicted in Figure 6.

Operational Constraints Vehicle Efficiency System-level Efficiency Air Transportation Aircraft System Operational Capabilities Figure 6. Diagram depicting the relationship between the air transportation system, aircraft, and efficiency.

For a given aircraft and destination, an optimal mission profile, path, and set of operating conditions exists to optimize vehicle efficiency. Deviations from this optimal mission, in the form of operational con- straints imposed by the air transportation system, increase fuel usage and diminish the aircraft’s efficiency by increasing drag, increasing thrust specific fuel consumption, and/or increasing the total time in operation, as discussed in Section II. Typical operational constraints include delays on the surface, non-wind-optimal routes, rerouting, vectors, holds, speed changes, altitude constraints, and vertical profiles. Fewer operational constraints are expected in NextGen, allowing aircraft to execute optimal missions and tailored operations, thus improving vehicle efficiency.

11 of 15 American Institute of Aeronautics and Astronautics One goal of NextGen is to accommodate a highly diverse aircraft fleet, yet aircraft operational capabil- ities will continue to influence the system-level efficiency of the air transportation system. Traffic flow is dependent on the sequencing, spacing and routing of aircraft. These factors must be in accordance with an aircraft’s ability to maneuver, an aircraft’s wake generation characteristics, and an aircraft’s sensitivity to wake turbulence and weather. These aircraft operational capabilities are discussed further in Section IV.B.

IV.A. Influence of Air Transportation System Operational Constraints on Aircraft Design Improved throughput and tailored operations in NextGen are expected to first impact aircraft operational decisions, followed by the design of aircraft concepts. Once confidence is gained in the ability of an aircraft to reliably operate efficiently through the airspace, aircraft operators and pilots are expected to gradually decrease the amount of mission and reserve fuel carried for each flight. The reduction in fuel will reduce the weight of the operating aircraft, thereby improving the aircraft’s fuel efficiency. Ultimately, airframe manufacturers are expected to design aircraft to carry less fuel for a particular mission. With a smaller fuel capacity, the aircraft size and weight will reduce accordingly to further improve fuel efficiency of aircraft design concepts. Researchers at Boeing Research and Technology recognized this benefit in a recent study, citing a 17.5% improvement in fuel per seat for an aircraft designed to a 2030 NextGen mission profile as opposed to a 2008 mission profile.

IV.B. Influence of Aircraft Operational Capabilities on the Air Transportation System Aircraft designs, and resulting operational capabilities, have the potential to both improve and adversely im- pact the system-level efficiency of the air transportation system. In some cases, improvements to system-level efficiency can be obtained through the introduction of diverse mission operations that expedite throughput for aircraft following the different operational procedures and relieve traffic congestion and associated delays for traditional operations. In other cases, improvements to system-level efficiency can be obtained through the uniformity and predictability of aircraft operational capabilities. Designing aircraft with operational ca- pabilities that will benefit the air transportation system, however, may reduce the vehicle’s design efficiency by adding weight, decreasing the lift to drag ratio, or increasing thrust specific fuel consumption. Such design considerations include maneuvering performance, wake generation and robustness, and robustness to the effects of weather.

IV.B.1. Maneuvering Performance Efficient and effective sequencing and flow of air traffic will be influenced by aircraft maneuvering perfor- mance. For arrival and departure at airports, the operational capabilities affecting sequencing and traffic flow include aircraft speed capabilities, turning capabilities, climb rate, descent rate, ability to decelerate in de- scent, field length requirements, and time and distance required to perform an operation such as deceleration, acceleration, or exit the runway onto a taxiway after touchdown. Aircraft that are agile and able to operate efficiently over a broad range of conditions will be easier for the air transportation system to accommodate into the air traffic flow within the terminal area. Aircraft that require a longer than average time or distance to turn, decelerate or accelerate, however, will require additional spacing between nearby aircraft, thereby limiting throughput. Similar constraints exist en route, where different cruise airspeeds, cruise altitudes, and times and distances required to climb and descent to the optimum altitude introduce complexities in managing air traffic. While designing an aircraft for reduced cruise airspeeds, increased cruise altitudes, and increased time and distance required to climb and descent leads to benefits in vehicle efficiency, the same design choices in maneuvering performance could impair system-level efficiency by restricting the movement of nearby aircraft.

An integral aspect of improving the capacity of the airspace system is to improve access to airports by “enabling better utilization of existing infrastructure and currently underutilized airports” through new technology and procedures. One consideration in sizing future aircraft will be the footprint needed for landing and surface operations to ensure the ability to maneuver within existing airport infrastructures.

Aircraft length and span, for example, should be responsive to runway, taxiway and ramp configurations at candidate airports. Novel configurations such as the truss-brassed wing with large wing spans will either need to limit wing span in accordance to airport infrastructure or incorporate advanced systems to reduce wing span during surface operations.

12 of 15 American Institute of Aeronautics and Astronautics Field length requirement is a unique operational capability in that a diversity in requirements may be advantageous to system-level efficiency, particularly if an aircraft is able to operate in and out of short field lengths. Operation from short field lengths can either allow the use of unused or underutlized runways at major airports or exploit the metroplex concept of expanding air service to non-hub airports. This allows aircraft with short field length requirements to be sequenced in and out of an airport separately from the remaining air traffic. Aircraft design considerations for reducing field length requirements, however, necessitate additional thrust and lift capabilities that reduce the vehicle efficiency.

IV.B.2. Wake Generation and Robustness Reduced separation requirements will be a standard mode of operation for the Next Generation Air Trans- portation System, both en route and within the terminal area, with the introduction of ADS-B, advanced estimation and planning tools, and conflict resolution technology. The limiting factor in aircraft spacing will be due to wake-based spacing requirements. Researchers in air traffic management would like to see incremental reductions in spacing standards leading to dynamic wake-based spacing. Such reductions in spacing standards would help maximize use of the highest-demand airports and airspace. Aircraft design concepts that generate modest amounts of wake turbulence and that are able to maintain stable flight when encountering wake turbulence will amplify these benefits by permitting small dynamic wake-based spacing standards. The aircraft design trends towards reductions in weight and induced drag will aid in the reduction of wingtip vortices and resulting wake turbulence. The opposite may be true for future aircraft’s robustness to wake turbulence encounters. In favor of system-level efficiency, aircraft design considerations should in- clude active or passive stability techniques for an aircraft’s flight dynamics, aerodynamics, structural and propulsion systems while in the presence of a wake turbulence.

Further benefits to airspace capacity and throughput can be gained by grouping aircraft together in a formation. With proper spacing and positioning within the formation, following transport aircraft can benefit from an average fuel flow reduction of approximately 7 to 8 percent. More rigorous design considerations, with emphasis on stability and durability of aircraft systems, will be necessary for aircraft in formation with separation optimized for fuel efficiency.

IV.B.3. Robustness to Effects of Weather Rather than delaying flights or vectoring flights around inclement weather, the Joint Planning and Devel- opment Office has proposed the operational concept that “operators of aircraft equipped with capabilities to mitigate the effects of weather may choose to tactically fly through certain weather-impacted areas.” The ability to fly through inclement weather would allow aircraft to maintain efficient point-to-point op- erations while alleviating air traffic congestion on alternative routes. The top three weather hazards that state-of-the-art aircraft avoid are thunderstorms, extreme turbulence, and severe winds. Another detrimen- tal hazard is volcanic ash. Designing aircraft to fly through these weather hazards is a challenge, both from an aircraft technology perspective and an aircraft safety perspective. Autonomous transport aircraft may serve as an enabler by alleviating ride quality requirements for aircraft design, yet the design of weather resilient aircraft and flight through weather conditions can be expected to add weight, decrease the lift to drag ratio, or increase thrust specific fuel consumption. Nevertheless, aircraft design concept studies should consider varying degrees of robustness to the effects of weather as a design criteria for improved system-level efficiency.

V. Impact of Aviation beyond Vehicle and System-level Efficiency

Sustaining long-term growth in the aviation industry relies on improving the overall impact on the environment and energy use. While improvements in vehicle and system-level efficiency are aimed towards this goal, the impact of aviation spans the entire aircraft life cycle.

One impact of aviation not captured within the definition of vehicle and system-level efficiency is the the energy used, emissions produced, and noise generated while servicing and maintaining aircraft. As new technology is introduced on aircraft, some of these technologies may require regular service. For example, a higher dependence on electrical power may lead to requirements to recharge or exchange batteries on the ground; laminar flow wings may require regular cleaning; and lightweight, flexible structures may require regular inspection. In response to these requirements, airport infrastructure will grow. Attention will be 13 of 15 American Institute of Aeronautics and Astronautics needed to ensure the growth of airport infrastructure is done in an energy and environmentally conscious manner. Further attention should be given during the design of aircraft concepts and technologies to minimize additional requirements imposed on airport infrastructure.

Acknowledgments

This paper was developed by members of the Subsonic Fixed Wing Project, and now Fixed Wing Project, within the Fundamental Aeronautics Program of the NASA Aeronautics Research Mission Directorate.. The authors thank Parimal Kopardekar, Project Manager of the Concepts and Technology Development Project within the Airspace Systems Program, and John Kaneshige for the valuable discussions and insight that influenced this paper.

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