Flight Mechanics Issues presentation

APA Vehicle Aerodynamics Subcommittee: Flight Mechanics Issues for Aircraft, and underlying Fluid Dynamics Phenomena
Stephen McParlin APA TC ( [email_address] )
Robert Tramel APA TC ( [email_address] )
AIAA-2009-0744

Contents
Introduction
Design drivers for aircraft
Airworthiness requirements
Flight Mechanics issues
Taxonomy approach and factors considered
Fluid Dynamics phenomena
Causality
Sources of data
Preliminary conclusions
The way forward
Suggested areas of interest
Working group definition
Flight Mechanics Issues for Aircraft: AIAA-2009-0744

Introduction
This is a product of the APA TC
Identify/tackle capability gaps in CFD for aircraft
Evolution of the concept from ‘moment prediction’
Increased emphasis on understanding fluid dynamics
Lessons learned from the DPW series
Increased role for experimental analysis
Case for ‘Building Block’ approach
Review historical evidence
Flight, wind tunnel test experience
Consider flow control successes
Recommend workshop topics/structure
Engage Fluid Dynamics community in/outside AIAA

Contents
Introduction
Causality
Sources of data
The way forward

The basis for our consideration and prioritisation of issues is driven by application pull, rather than technology push
Impact on the design and operation of aircraft
Civil transports
Combat aircraft
Based on meeting end user requirements while containing:
Cost
Complexity
Risk
Looking to establish industrial-strength processes and tools
Where do we need to mature CFD methods to make an impact?
Focus on aircraft performance, stability and control

Airworthiness requirements (1) ‏
Military requirements for flying qualities
“ The aircraft shall be…resistant to departure from controlled flight, post-stall gyrations and spins. Adequate warning of approach to departure shall be provided. The aircraft shall exhibit no uncommanded motion which cannot be arrested promptly by simple application of pilot control.” – US MIL-STD-1797A
“ It is desirable that the specified flying qualities should be achieved by good aerodynamic and mechanical design. However automatic devices may be used where an overall benefit accrues provided that the system as a whole meets the requirements.” - UK DefStan 00-970

Airworthiness requirements (2) ‏
Civil requirements for flying qualities
“ It must be possible to produce and to correct roll and yaw by unreversed use of aileron and rudder controls, up to the time the aeroplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls” (CS 25.203) ‏
“ Stall warning with sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position must be clear and distinctive to the pilot in straight and turning flight” (CS 25.207)
“ The aeroplane must be demonstrated in flight to be free from any vibration and buffeting that would prevent continued safe flight in any likely operating condition” (CS 25.251)

Flying/Handling qualities
Customers require:
Predictable and consistent stability characteristics
Well-defined departure boundaries
Adequate warning of departure
Easy recovery
But, we have potential challenges:
Abrupt, non-linear stability changes and divergent modes
Lack of control power at or beyond departure
Solutions are multidisciplinary, a blend of:
Aerodynamic design
Flight Control System design
Flow Control, where necessary

Flight mechanics issues
Longitudinal
Static
Pitch up, tuck under, Mach tuck
Dynamic:
unstable SPO and phugoid (‘Falling leaf’) – limit cycle behaviour
Lateral/directional
Static
Wing drop, nose slice (“yaw off”) ‏
Dynamic
unstable Dutch Roll/Wing Rock
Classical linear stability modes become non-linear as the underlying aerodynamic forces become non-linear
Prediction of non-linear changes in aerodynamics is the key

Contents
Introduction
Causality
Sources of data
The way forward

Taxonomy approach
Identify causation of non-linear stability characteristics
Which problem happens, to which classes of configuration, at which operating condition
Consider the nature of flows at these conditions
Look at available experimental evidence
Postulate driving Fluid Dynamics phenomena
Investigate fundamental Fluid Dynamics
Validate CFD against ‘building block’ experiments
Use experimental knowledge base to inform CFD use

Factors considered in taxonomy
Flight mechanics mode
Longitudinal/lateral, static/dynamic
Flight regime
Low speed/subsonic/transonic/supersonic
Manoeuvre type
Cruise/steady-state/transient
Configuration type
Unswept/Swept/Slender/Hybrid/non-slender

Flight mechanics mode
Evidence for Flight Mechanics Issues comes from:
Real aircraft experience
Flight test of prototype/production aircraft
Dynamic wind tunnel tests
Experimental programmes
Purpose-built and properly instrumented flight vehicles
Wind tunnels
Static test data need appropriate analysis
Access to data for current aircraft is usually either proprietary, covered by security issues, or both
Need to look in the archives
Data need to be recorded and archived
A little Knowledge Management is very valuable

Flight regime
Low speed
M=0.3 and below
Subsonic
Up to onset of locally supersonic flow
Transonic
From local supersonic onset to peak drag rise
Supersonic
From the peak of the drag rise until M max

Manoeuvre type
Cruise
Constant  , M, zero angular rates
Steady-state
Constant  , M, constant angular rates
Transient
Varying  , M, varying angular rates

Example flight envelope for air combat manoeuvres Flight Mechanics Issues for Aircraft: AIAA-2009-0744

Configuration type
Unswept
e.g. Sailplanes, U-2, turboprop-powered transports
Swept
e.g. B-47, F-86, turbojet/fan-powered transports
Slender
e.g. F-106, SR-71, Concorde, SSBJ concepts
Hybrid swept/slender
e.g. F-16, F/A-18, F-22, MiG-29, Su-27
Non-slender
e.g. F-4, Avro Vulcan, Eurofighter Typhoon

Effect of design Mach number on configuration type Flight Mechanics Issues for Aircraft: AIAA-2009-0744

Nature of the taxonomic matrix
The matrix is not dense:
Factors interact
Mach number drives configuration shape
Mach number drives manoeuvre type
Configuration shape drives flight mechanics modes
Mach number vs. configuration matrix
Approximately triangular
Mach number vs. manoeuvre type matrix:
Higher Mach: thrust and/or structural limits
Low Mach: lift and control power limits at low q
Preliminary analysis indicates areas of interest

Contents
Introduction
Causality
Sources of data
The way forward

Boundary-layer transition
Flow separation (no shock waves)
Shock-wave/boundary-layer interactions
Vortex stability, bursting and interactions
Mixed-flow regions: spanwise segmentation of attached and separated flow regions
Flow control

Boundary layer transition
Streamwise (Tollmien-Schlichting) transition
Attachment-line contamination, instability and transition on swept wings and slender fuselage noses at high angle of attack
Relaminarization (and cessation of relaminarization) of turbulent attachment line flow
Crossflow transition on wings (and fuselage) with sufficient sweep (body angle of attack)
Taylor-Görtler instability / transition on concave surfaces
Shear-layer instability, transition and reattachment in laminar separation bubbles in steady and unsteady flows

Attachment-Line Contamination, Transition and Relaminarization Flight Mechanics Issues for Aircraft: AIAA-2009-0744 ATTACHMENT LINE REYNOLDS NUMBER RELAMINARIZATION PARAMETER LAMINAR TURBULENT DEPENDS RELAM NOT LIKELY RELAM POSSIBLE

Swept wing C Lmax : leading-edge scale effects (Yip,1993) Flight Mechanics Issues for Aircraft: AIAA-2009-0744 Chord Reynolds number

Crossflow transition for swept wings and non-axisymmetric body flows
First discovered in flight in 1952
Instability waves propagating spanwise
Impose maximum sweep limit on natural laminar flow
Have subtle but significant effects on the flow topology for swept leading edges
Postulated as a factor in flow separation from rounded wing leading edges and slender bodies
Hot topic in the laminar flow control world

Boundary layer flow separation (no shock waves)
Smooth surface versus “sharp-edged” shear layer separation
Leading edge or trailing edge separation (switchover depending mostly on geometry of airfoil and wing sweep as well as Reynolds number)
Laminar or turbulent state of boundary layer upon separation
Laminar separation bubble and bubble “bursting”: steady and unsteady separation
2D vs. 3D type separation; open vs. closed separation topologies (vortex vs. bubble)
Junction and secondary flow separations
Off-surface flow reversal (in wake flows over multi-element airfoils). Impingement of wake-like flows on downstream lifting surfaces
Separation in periodic flow field, including hysteresis effects
“ Unsteady” and “quasi-steady” separation

Shock-wave/boundary-layer interactions
Laminar boundary and turbulent boundary layer approaching the shock wave
Smooth surface vs. discontinuous surface (e.g. transonic shock vs. corner shock at supersonic onset Mach number)
Interactions on swept wings and non-swept wings
Separation bubble near the foot of the shock is closed or open (or local vs. global separation)
Instability of flow field (‘steady’ vs. ‘unsteady’ interaction)
Buffet onset (global instability of the turbulent flow field that forces the (flexible) wing and fuselage structure)

Shockwave categories on supersonic slender wings
Miller plot

Vortex stability, bursting and interactions
LEX/Chine vortex bursting and resulting fin buffet on hybrid and slender wings
Forebody vortices from slender bodies interacting with downstream wing or empennage.
Foreplane tip vortex over downstream wing/empennage
Vortex from nacelle chine/strake flowing over unswept/swept wing with highly deflected flap settings
Shock/vortex interactions at transonic and supersonic conditions

Mixed flow regions – spanwise segmentation of flow separation
Using geometric or flow control devices, the flow on wings can be segmented into regions of attached and separated flow. Often the presence of a strong vortex can allow suitable segmentation of wing flow.
Some possible segmentation examples:
Strake/swept wing with strong vortex on inboard strake and unswept type flow further outboard
Küchemann type tip flow field, where a stable vortical flow is generated on the outboard aft-swept wing tip, while the flow further inboard may be separated
Spanwise discontinuities in leading-edge geometry to affect span loading and formation of local vortices to provide spanwise containment of separated flow (drooped outboard leading edge, leading-edge notches, fences etc.)

Flight Mechanics Issues for Aircraft: AIAA-2009-0744 Flow control – successful or otherwise Gloster Javelin with Vortex Generators http://commons.wikimedia.org/wiki/Image:Gloster.javelin.xh903.arp.jpg

Contents
Introduction
Causality
Sources of data
The way forward

Causality – which flow phenomena?
Sources of data:
Compendia of real aircraft experience
AGARD studies into buffet and manoeuvre limits
Abrupt Wing Stall program
Experimental flight/wind tunnel test programmes
Numerous X-types
Collaborative testing/analysis programmes
Historical sources are important (incl. people)
Examples of successful (and otherwise) flow control
Which are the most significant problems?
Relevance to operational use hugely important

Causality – preliminary conclusions
Non-linear flight mechanics are driven by development of flow separations
Rapid changes in flow topology represent highest risk
Predicting flow separation onset is key
What fluid dynamics phenomena do we need to capture?
Operational relevance:
Most transport/combat aircraft operate predominantly at high subsonic/transonic conditions
Transport aircraft cruise/cruise-climb
Core of combat aircraft manoeuvre envelope
Low-speed high-lift for launch/recovery

Different types of flow separation
Significant configuration dependencies
'Designed' shapes have more subtle pressure gradients than simple models
Simple geometries may produce unrepresentative physics
Combat aircraft are more prone to leading edge flow separation
Thin wings, high adverse pressure gradients near l.e.
Large impact on drag over whole flight envelope
Transport aircraft are more prone to trailing edge flow separation
Thicker wings, strong adverse pressure gradient in recovery to trailing edge
Shock-induced flow separations are common to both
Buffet margin is a design/certification issue
Frequently the source of abrupt wing stall
Predicting flow separation at transonic conditions Flight Mechanics Issues for Aircraft: AIAA-2009-0744

Flow separation at low-speed and high-lift
Leading edge flow separation is the predominant issue for thin or highly swept wings
Problem common to that at transonic manoeuvre conditions
Loss of leading edge thrust produces significant drag penalty
Changes in flow topology have consequences for stability characteristics
‘ Real’ aircraft have designed leading edges or high lift systems
Trailing edge separations are the predominant issue for thick or low sweep wings
Cumulative momentum loss under the influence of pressure gradients
Less problematic for drag and stability than leading edge separations
Complex viscous flows on high-lift systems
Major design area for transport aircraft wings

Contents
Introduction
Causality
Sources of data
The way forward

The way forward (1) ‏
Suggested areas of interest:
Transonic buffet prediction
Relevant to most relevant classes of air vehicle
'Building block' experimental data available (UFAST)
Potential for investigation on a full configuration (CRM) ‏
Flow separation onset from rounded leading edges
Very high impact for military configurations
Configuration-level activity starting in RTO
Candidate for developing 'building block' experimental program to support configuration-level activity elsewhere
Trailing edge flow separation

New Common Research Model (AIAA 2008-6919) ‏
Common Research Model is a potential geometry to support transonic buffet and trailing edge separation activities

The way forward (2) ‏
Establish Working Group across APA/FD TCs:
Develop basis for AIAA predictive workshops
Focus on specific issues identified
Invite participation of topic specialist “Greybeards”
Develop understanding of experimental evidence
Identify necessary level of flow modelling
Engage with CFD community
Determine best practice
Demonstrate CFD capability against model problems
Refine methods/guidance to users

Summary
Provided an initial taxonomy of Fluid Dynamics causation for Flight Mechanics problems
Suggested three areas of primary interest
Prioritised on Operational Impact
Applied Aerodynamics and Fluid Dynamics TCs are forming a joint Working Group to develop a strategy to improve industrial capability
Invited sessions by topic specialists and a workshop are planned for the next two years

Acknowledgements
Thanks are due to the current and past members of the APA TC who contributed feedback:
Darren Grove, Jeff Slotnick and particularly Paul Vijgen
External comments gratefully received from:
Bram Elsenaar, John Fulker and Frank Lynch
N.b. Feedback and revision of the proposed activities is an ongoing process
Contributions gratefully received for consideration by the Working Group

Proposed framework for workshops
Propose Topic of Workshop to relevant TC’s
Identify experiments that exhibit relevant fluid-mechanics feature
Identify available (publishable) experimental and CFD data for this geometry
Provide geometry (and available test data) to community together with flow conditions for analysis
Obtain list of participants to conduct CFD (and possible experiments).
Obtain CFD results
Collect CFD results and prepare initial Workshop with results (invited papers etc.) ‏
Define need for additional CFD and experiments in follow-on Workshops

Common Research Model (CRM) – see AIAA 2008-6919
CRM (Common Research Model) is defined within NASA’s a Subsonic Fixed Wing project (SFW) by Aerodynamics Technical Working Group (TWG) ‏
TWG representatives from Boeing, Lockheed-Martin, Northrop-Grumman, Gulfstream,Cessna, Hawker-Beechcraft, Pratt and Whitney, Air Force, Navy, and NASA
SFW CRM (Common Research Model) geometry to be released in public by Fall 2008. Boeing design
Mach 0.85 design
Modern airfoils
With horizontal tail to allow trim
Wind-tunnel data (NTF-cryo and Ames 11-ft pressure tunnel) to be taken in Spring – Fall 2009

Transonic Buffet and Trailing-Edge Separation Data from CRM
Planned CRM data may be suitable for first Workshop on topics suggested in White Paper
Planned test data (AIAA 2008-6919):
Mach 0.7 to 0.92
Angles of attack into pitch up and buffet onset
Rec = 5 million (Ames 11-ft TPT) and Rec = 3 – 30 million (Langley NTF) ‏
Measurements:
Balance (6-component), surface pressures, wing-shape under loading (Ames and Langley) ‏
PIV and skin-friction interferometry (Ames) ‏
Flush wing-mounted Kulites in outboard wing

Flight Mechanics Issues presentation

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