Flight Mechanics Issues presentation

APA Vehicle Aerodynamics Subcommittee:Flight Mechanics Issues for Aircraft, and underlying Fluid Dynamics Phenomena

Stephen McParlin APA TC ([email protected]) Robert Tramel APA TC ([email protected])

AIAA-2009-0744

Flight Mechanics Issues for Aircraft: AIAA-2009-0744

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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


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Contents







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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


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Contents







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Design drivers for aircraft

• 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


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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


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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)


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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


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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


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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


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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


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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


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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 Mmax


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Manoeuvre type

• Cruise Constant , M, zero angular rates

• Steady-state Constant , M, constant angular rates

• Transient Varying , M, varying angular rates


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Example flight envelope for air combat manoeuvres


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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


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Effect of design Mach number on configuration type


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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


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Contents







Fluid Dynamics phenomena

• 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

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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

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ATTACHMENT LINE

REYNOLDS NUMBER RELAMINARIZATION

PARAMETER

LAMINAR TURBULENTDEPENDS240R 560R ReR

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RELAM NOT LIKELY RELAM POSSIBLE

-63.0x10K

Attachment-Line Contamination, Transition and Relaminarization

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Swept wing CLmax: leading-edge scale effects (Yip,1993)

Chord Reynolds number25


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

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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

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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)

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Shockwave categories on supersonic slender wings

• Miller plot

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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

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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.)

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Flow control – successful or otherwise

Gloster Javelin with Vortex Generatorshttp://commons.wikimedia.org/wiki/Image:Gloster.javelin.xh903.arp.jpg

http://commons.wikimedia.org/wiki/Image:Gloster.javelin.xh903.arp.jpg


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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


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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


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• 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


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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


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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

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New Common Research Model (AIAA 2008-6919)

•Common Research Model is a potential geometry to support transonic buffet and trailing edge separation activities



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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


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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

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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


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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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