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Page 1: High Alpha Basics Class Presentation

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Some High Alpha andHandling Qualities Aerodynamics

W.H. Mason Configuration Aerodynamics Class

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Issues in Hi-α Aero•  General Aviation

–  Prevention/recovery from spins–  Beware: Tail Damping Power Factor (TDPF)

»  Sometimes advocated for use in design»  Has been shown to be inadequate!

•  Fighters–  Resistance to departure from controlled flight–  Carefree hi-α air combat maneuvering

»  Fuselage pointing»  Velocity vector rolls»  Supermaneuverability»  etc.

•  Transports–  Control/prevention of pitchup and deep stall (the DC-9 case study)

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High Angle of Attack

• Aerodynamics are nonlinear- complicated component interactions (vortices)-  depends heavily on WT data for analysis-  This means critical conditions outside linear aero range

• Motion is highly dynamic - often need unsteady aero• Keys issues (from a fighter designer s perspective):

- adequate nose down pitching moment to recover- roll rate at high alpha- departure avoidance- adequate yaw control power- the role of thrust vectoring

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The Hi-α StoryLongitudinal

Typical unstable modern fighter – the envelope is the key thing to look at

α 90°

+

-

Cm

Max nose up moment

Max nose down moment

Cm*

Minimum Cm* allowable is an open questionIssue of including credit for thrust vectoring

Pinch often around α ~ 30°-40°

Suggested nose down req t: Pitch accel in 1st sec: -0.25 rad/sec2

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Example: F-16

NASA TP-1538, Dec. 1979

Found in flight, and also in tests at the NASA Full Scale Tunnel, other tunnels didn t find this, and so the design was made with this issue

An α-limiter was put in the control system to prevent pilots from going above about 28°

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The Hi-α StoryDirectional

Cnβ

+

-

α 90°0°

Stable

Unstable

Vertical Tail

ForebodyEffect

Config w/o V-Tail

V-Tail in wake

0

Directional problem alsooften around 30°

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Example: F-5

-0.0050

0.0000

0.0050

0.0100

-10° 0° 10° 20° 30° 40° 50°

Cnβ

α, deg.

Full Config.,Tail Off

Full Config.,Vertical Tail On

Forebody alone

F-5 WT data: NASA TN D-7716

Sketch from NASA TN D-7716, 1974

Chine forebodies are even more effective!

Above 30°, the directional stability comes entirely from the forebody!

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Forebody Vortices

F-5A forebody, α = 40°, β = 5°NASA CR 4465, Aug. 1992

I m looking for a better, color, figure

At high angles of attack, vortices form over the forebody, producing additional, possibly restoring, forces and often interacting with the rest of the airplane flowfield

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Illustration of Forebody Vortices and a Fix

• Laser Light Sheet Video of the RFC forebody

X-29 forebodyTaken at the USAF Museum,

WPAFB, Ohio

• And Flow Asymmetries: - an amazing story- solve with nose strakes

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Rudder Lock and the B-17 Vertical Tail Mod

Darrol Stinton, Flying Qualities and Flight Testing of the Airplane, AIAA, 1996

Dorsal fin added!

Originally

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Vertical Tail Shaping II: Subtle Tail Changes

Note that yaw is the negative of sideslip, thus the negative slope

Also note good use of labels to identify effects

Lockheed “Constitution”Aviation Age MagazineFeb. 1952, pg. 37

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Who s on first?

A true story from the Grumman STAC Airplane program

The Aerodynamics group plane

The Stability and Control Group Plane See Hendrickson, et al,AIAA Paper 1978-1452for the story

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Some High Alpha andHandling Qualities Aerodynamics

Again

W.H. Mason Configuration Aerodynamics Class

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The Hi-α StoryLateral

Clβ

+

-

α 90°0°

“Stable”

“Unstable”

0

Dihedral Effect(also due to sweep) Flow Separates

on wing

Drooping LE deviceshelps controlseverity

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Example: F-4

NASA TN D-7131, July 1973

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Example: F-16 Lateral/ Directional

Control

Conventional aero control surfaces lose effectiveness at high angles of attack

ControlEffectivenessChange with alpha

ΔCn

ΔCl

α, deg

δr = 30°

δr = 30°

δa = -20°δd= -5°

δa = -20°δd= -5°

Subscripts:r – ruddera – ailerond – differential horizontal tail

NASA TP 1538, Dec. 1979

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A way to get yaw: differential canard

Differential deflection of the canard creates differential pressures on the side of the forebody, and hence yawing moment

Lapins, Martorella, Klein, Meyer and Sturm, NASA CR 3738, Nov. 1983

Grumman STAC

See also, Re and Capone, NASA TN D-8510, July 1977

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Example: the Evolution of the SCAMP •  How and Why the modified F-16 supercruiser

SCAMP became the F-16XL

Talty and Caughlin, Journal of AircraftVol. 25, No. 3, March 1988, p. 206-215.

Dave Miller & Schemensky, Design Study Results of a Supersonic Cruise Fighter Wing AIAA Paper 1979-62, Jan. 1979

Original Concept The Final Airplane

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Low Speed Tests Mandated Planform ModsBaseline low speedconfiguration(note crank)

The Apex mod

The TE extension mod

One more not shown: a Wing fence

Grafton, NASA TM 85776, May 1984

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The F-16XL

Note Wing Fence called a sensor fairing

Original

All mods combined

WT data: Grafton, NASA TM 85776, May 1984

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Hi-α Flight Mechanics I•  Departure

–  Aircraft departs from controlled flight–  May develop into a spin

•  Wing drop–  Uncommanded motion: asymmetric wing stall

»  Typical spin entry for straight wing GA airplanes, a wing roll-off–  Became famous again as the F-18E Abrupt Wing Stall Problem

•  Wing rock–  Damping characteristics generate wing roll oscillations

•  Nose slice–  Yawing moments from airplane (mainly forebody) greater then

rudder power–  May develop into a yaw-type entry into a spin

»  Swept wing fighter, with totally different inertial characteristics•  So-called fuselage heavy , as oppose to wing heavy light

planes

•  Tumbling–  Typically associated with flying wings

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Hi-α Flight Mechanics II•  Becomes closely integrated with flight control system•  Requires extensive ground and flight simulation

From Johnston & Heffley (STI), Investigation of High-AOA Flying Qualities Criteria and Design Guides, AFWAL-TR-81-3108, Dec. 1981.

Determinants of High a Flying Qualities Approach Example

Aerodynamic Dominance

Aerodynamics must provide an all around solution with respect to performance, stability and controllability FCS may give fine tuning, but is not required for basic stability and control

F-5

Balanced Aerodynamics and FCS

Aerodynamic design favors performance with some regions of undesirable flying qualities. FCS necessary to enhance or provide acceptable stability and control

F-15

FCS Dominance Aerodynamic design favors performance FCS to provide good flying qualities up to maximum usable lift coefficient

F-16

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The problem: configurations have changed from WWII

The fuselage is longer and heavier, the wings relatively smaller

Figure from Joe Chambers, High-Angle-of-Attack Aerodynamics:Lessons Learned, AIAA Paper 1986-1774

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Departure Criteria I•  Really need dynamic data and piloted simulation

–  Needs to be connected to flight mechanics analysis–  Simulation math model requires massive amount of data from WT(s)

•  Issue: can we do anything w/o dynamic data?•  Some rough approximations to get started (note inertia ratio):

–  Open loop: (actually a very rough approximation): Cnβ dynamic > 0

»  Several forms depending on coordinate system, this is essentially body axis (actually principal axis) - see Calico, Journal of Aircraft, Vol. 16, No. 12, Dec. 1979, pp 895-896.

»  Comes from the lateral-directional equations of motion linearized about steady, rectilinear flight, and an examination of the resulting stability quartic (ignoring rate derivatives) – see Greer, NASA TN D-6993, 1972. Essentially a Dutch Roll Approx

Cnβ , dyn= Cnβ

cosα − Iz Ix( )Clβsinα

Note: the classic F-16 study is Simulator Study of Stall/Post-Stall Characteristics of a Fighter Plane With Relaxed Longitudinal Static Stability, by Luat Nguyen, et al, NASA TP 1538, Dec. 1979 (pdf avail)

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Departure Criteria II

• Where GARI is the ratio of rudder to aileron deflection for an airplane with an aileron-rudder interconnect (ARI)

• When yaw due to aileron gets large, commanded roll produces a motion in the opposite direction!

• See AFWAL-TR-81-3108 for examples

LCDP = Cnβ −Clβ

Cnδa+ GARICnδr

Clδa+ GARIClδr

⎝⎜

⎠⎟

• Closed loop: LCDP, the lateral control departure parameter > 0

Damping derivatives:• Cnr < 0 (yaw damping), but not too negative

• Clp < 0 (roll damping), to prevent wing rock

Note: the value (and drawback) of and LCDP are that they are based on static aero coefficients

Cnβ ,dyn

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F-16 Hi-α characteristics

NASA TP-1538, Dec. 1979

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F-16 LCDP and augmented LCDP

NASA TP-1538, Dec. 1979

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Departure Map

-0.015

-0.010

-0.005

0.000

0.005

0.010

-0.015 -0.010 -0.005 0.000 0.005 0.010 0.015

Integrated Bihrle-Weissman Chart

LCDP

Cnβ dyn

Highly departure and spin resistantHigh directional

instability,little data

F

Strong departure, rollreversals and spin

tendencies

BE

C

Strong departure,roll reversals andspin tendencies

Spin resistant,objectionableroll reversalscan inducedeparture andpost stall gyrations

Weak spin tendency, strong roll reversalresults in control induceddeparture

F - Weak departure and spin resistance, no roll reversals, heavily influenced by secondary factorsE - Weak spin tendency, moderate departure and roll reversals, affected by secondary factors

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Ways to get aero data, including dynamic data

Wind Tunnels•  Basic static, Hi-α WT data

–  Beware of support interference and Reynolds Number issues

•  Forced oscillation data

•  Virginia Tech s DyPPiR

•  Rotary balance data (typically use NASA Langley Spin Tunnel)–  Bihrle Applied Research

CFD•  not there yet in general, VLM gives low α damping derivatives

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Wind Tunnel Testing for Hi-α Data

The RFC in the Full Scale (30x60) wind tunnel at NASA Langley for static WT tests

Sue Grafton with the NASA-Grumman 1/7th scale Research Fighter Configuration (RFC) at NASA Langley - mid 80s

Note: orange surfaces can move, note also the nozzle thrust vectoring deflectors

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And Trying to Understand the Flowfield

Kurt Chankaya, in the Grumman 7x10 tunnel

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Forced Oscillation Testing

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Virginia Tech s DyPPiR for Dynamic DataA unique system for the study of unsteady aerodynamics

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Movies of the DyPPiR in motion

For these and other Quicktime movies, seehttp://www.aoe.vt.edu/research/facilities/dyppir/

F-18 Rapid Plunge and Roll Maneuvers

Submarine (0.3 sec) Pitchdown Maneuver

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Rotary Balance Acquires Aero Data

Typically obtained in a spin tunnel, with the air moving vertically, the balance rotates, and the alpha and offset can be varied

NASA Langleyspin tunnel

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French Rotary Balance Testing of a Fighter

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The Abrupt Wing Stall Problem

Movie from Modeling Flightby Joe Chambers, NASA SP 2009-575

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The F-18 E/F Abrupt Wing Stall Problem •  In the midst of the maneuvering envelope the

F-18E/F developed a sudden drop of a wing. The problem made the airplane unacceptable to the Navy

Kevin Waclawicz, MS, 2001Mike Henry, MS, 2001

The “solution”: a porous wing fold fairing

Nasty, complicated, unsteady, shock interactions and unsteady flow

Chordwise pressure distributions, showing effect of LE and TE device deflection

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But, CFD is starting to be useful

Jim Forsythe DES Calculations, NASA CP-2004-213028

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The next step

•  Build a math model of the aero•  Develop the flight control system•  Simulate Hi-α flight with free-flight testing

Disclaimer

Note: a lot of what has been shown is based on linear aerodynamic approaches – the real world requires complete nonlinear aerodynamics to be included in the simulations, not classical stability derivatives, but tables of coefficients as a function of α, β, etc.

Jacob Kay, of JKayVLM, does this for a living, and wanted me to emphasize this.

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Free Flight Setup: A complicated activity

This was the setup in the LaRC Full Scale Tunnel, now torn down

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RFC Model in Free Flight at Langley

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RFC Free Flight in the Full Scale Tunnel

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And Finally, Flight Test

Software Bugs revealed in flight: the Grippen

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Some High Alpha andHandling Qualities Aerodynamics

Again and Final

W.H. Mason Configuration Aerodynamics Class

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Spins•  Normally viewed slightly differently than Hi-α•  General aviation airplanes have often been the focus of the work

•  Types of simulations include spin tunnels, rotary balance data, and RC model testing

•  Bihrle Applied Research is the main center of excellence

•  Various airplanes demonstrate different (multiple) spin modes – different equilibrium motions

– The worst, the famous flat spin!

A place to start: Overview of Stall/Spin Technologyby Joe Chambers, AIAA Paper 80-1580, Aug. 1980

Mike Cavanaugh’s Recommendation: “Spin Prediction Techniques,” William Bihrle, Jr., Billy Barnhart, Journal of Aircraft, Vol 20, No.2 pages 97-101, February 1983.

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Conclusions from NASA TP 1009,“Spin-Tunnel Investigation of the Spinning Characteristics

of Typical Single-Engine General Aviation Airplane Designs.” Burk, Bowman and White, Sept. 1977

•  Do not use TDPF, but certain principles still valid: – Important to provide damping to the spin – Very important to provide exposed rudder for spin

recovery •  Tail configuration can have an appreciable influence on

the spin recovery (but so can just about everything else!)

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Tail-design characteristics for improved spin recovery

James Bowman, NASA TN D-6575 (1971)

Better Worse

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Simulating spins on the ground•  NASA has a spin tunnel at Langley: also does tumbles

Models tossed in and the spin modes are examined

Air flows vertically

A rotary balance rig is available to acquire forces and moments while the model rotates

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French Spin Tunnel

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A way to improve spin characteristics of General Aviation Airplanes

NASA Ames: The Interrupted Leading Edge• Terry Feistel and Seth Anderson, AIAA 78-1476

NASA Langley: The Discontinuous Leading Edge• Dan DiCarlo and Paul Stough, AIAA 80-1843

- Both done about the same time- Making the leading edge discontinuous allows the wing to behave like two lower aspect ratio wings as the wing approaches stall, and then stalls

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Physics of Interrupted Leading EdgesThe NASA Ames version The NASA Langley version

Feistel, Anderson and Kroeger, JA Feb. 1981

DiCarlo, Stough and Patton, JA Sept. 1981

The Cirrus as well as the Adam A500 and Columbia 400 have adopted this wing concept

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Cirrus wing – showing discontinuous LE

At the Virginia Tech Airport,Sept. 21, 2012

Discontinuous LE to improve stall characteristics

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Spin Recovery Controls Depend on Mass DistributionNote: specific controls differ for each airplane

James Bowman, NASA TN D-6575 (1971)

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Finally, flight tests required for Hi-α,including spins

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Tumbling: A Related ProblemShown in the NASA Spin Tunnel

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Another related problem: Coupling

•  Rolling the airplane especially at hi-α, leads to coupling

Kinematic coupling: roll 90°

Now α is β!

Inertia Coupling:rolling forces nose up!

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Famous case attributed to inertial coupling:the F-100 and the vertical tail size change

The F-100 suffered from inertial coupling problems and general lack of directional stability: George Welch, NAA Chief Test Pilot was killed Oct 12, 1954 – and the F-100s were grounded until fixed.

The fix: 27% more VT area, 26 inch wingspan increase

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F-22 Case Study

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Wind tunnels used for F-22

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F-22 Pitching moment

Note: no scales

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F-22 Lat/Dir

Note: no scales

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F-22 Roll

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Comment on Thrust Vectoring

• Thrust vectoring mainly provides moments at high thrust

- a problem if you don t want lots of thrust!

• Thrust vectoring moment is near constant with speed

• Ratio of aero to propulsive moments:

- propulsion dominates at low q

- aero dominates at high q

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Conclusion

Aerospace andOcean Engineering

• High-α is a somewhat special area in configuration aerodynamics

•  Development of good aero characteristics at high-α is a critical aspect of modern tactical aircraft design

Best basic tutorial: Joe Chambers and Sue Grafton, Aerodynamics of Airplanes at High Angles of

Attack, NASA TM 74097, Dec. 1977 (pdf avail.)

Johnston & Heffley, Investigation of High-AOA Flying Qualities Criteria and Design Guides, AFWAL-TR-81-3108, Dec. 1981Two Special Sections in the Journal of Aircraft on the Abrupt Wing Stall Problem (and suggested solutions) • May-June 2004 • May-June 2005