Conceptual Design and Configuring Airplanes

Some basic principles of airplane design

John H. McMastersTechnical Fellow

The Boeing Companyjohn.h.mcmasters@boeing.com

and

Affiliate Professor

Department of Aeronautics and Astronautics

University of Washington

Seattle, WA

April 2007

Ed Wells Partnership Short Course

Based on: American Institute of Aeronautics and Astronautics (AIAA) & Sigma Xi Distinguished Lectures &

Von Kármán Institute for Fluid Dynamics Lecture Series: “Innovative Configurations for Future Civil Transports”, Brussels, Belgium June 6-10, 2005

Airplane Design: Past, Present and Future – An Early 21st Century Perspective

John McMastersTechnical Fellow

Ed Wells Partnership

The central of several purposes of this course is to examine the co-evolution of our industry, aeronautical technology, and airplane design practice in a broad historical context. Attention then focuses on speculations on possible future trends and development opportunities within an unconventionally broad and multi-disciplinary context. It may then be shown that while aeronautics may be a “maturing industry”, there are numerous opportunities for further advance in our ever-changing enterprise. The emphasis throughout will be concepts and ways of thinking about airplane design in a systems sense rather than on the details of the methodologies one might use in design. The material for this course is a continuing work in progress and represents the instructor’s personal, sometimes idiosyncratic perspective which is in no way intended to reflect an official position of The Boeing Company or its current product development strategy.

Course Objectives:• Provide familiarization to non-specialists on the topics to be discussed

• airplane design,• systems thinking, • the value of very broad multidisciplinary inquiry)

• Present airplane design and its evolution in a very broad historical context• Present one perspective on a general approach to airplane configuration synthesis at the conceptual level• Provide a basic aeronautics and airplane design “vocabulary”• Stimulate thought and imagination about the future of aeronautics

Target Audience: Anyone interested in airplanes and aeronautical technology in a very broad, multi-disciplinary system sense.

WARNING

ITAR and EAR ComplianceImportant Security Information:

Registration for this course (the following notes for which contain no ITAR/EAR-sensitive information) does not enforce the International Traffic in Arms Regulations (ITAR) and Export Administration Regulations (EAR) in any discussions that may result from it. Each attendee is responsible for complying with these regulations and all Boeing policies.

EAR/Compliance Home Site: http://policyplus.boeing.com/PS/PDF/DDD/PRO-2805.pdf

ITAR/Compliance Site: http://policyplus.boeing.com/PS/PDF/DDD/PRO-174.pdf

Notation and Symbols Used

A Area (ft.2, m2)a Speed of sound (ft./sec., m/s)AR Aspect ratio, b/č = b2/Sb Wing span (ft., m)č Average wing chord (ft.,m)CF Force coefficients (lift, drag, etc.) = F/qSCℓ Section (2D) lift coefficientCM Moment coefficient = M/qSĉCp Pressure coefficient = Δp/qD Drag force (lb., N)E Energy (Ft.-lbs., N-m)e “Oswald efficency factor”ew Wing span efficiency factor (= 1/kw )F Force (lift, drag, etc.) (lbs., N)H Total head (reservoir pressure)I Moment of inertiakw Wing span efficiency factor (= 1/ew)L Lift force (lb., N)ℓ Length (ft., m)M Mach number (V/a)M Mass (kg)M Moment (ft. lbs., N m)P Power (ft.-lbs./sec., N-m/sec.)p Static pressure (lbs./ft.2)

q Dynamic pressure (lbs./ft.2) = ½ρV2

R Range (mi., km)Rn Reynolds number (ρVℓ / μ)S Wing area (ft.2, m2)T Thrust (lb., N)T Temperature (oF)u Local x-direction velocity componentV Velocity, Speed (ft./sec., m/s, mph, km/h)v Local y-direction velocity componentw Downwash velocity (ft./sec., m/s)ż Sink rate (vertical velocity) (ft./sec., m/s)

Greek:α Angle of attack (deg.)Γ Circulationγ Climb or glide angle (deg., rad.)γ Ratio of specific heats in a fluidε Wing twist angle (deg.)θ Downwash angle (deg.)φ Velocity potentialΛ Wing sweep angle (deg.)μ Dynamic viscosityν Kinematic viscosity (μ/ρ)ρ Fluid mass density (kg/m3)

•Conceptual Design and Configuring Airplanes• Some basic principles of airplane design

Presentation Overview

Caveat emptor, Amen.

The Book of GenesisFrom

The Aerospace System Designer’s BibleBy W.B. Gillette & J.H. McMasters

And on the first day there was gravity and the spirit of Newton said:

and Matter became weighty.

And then there was boundless energy and it was consolidated and Einstein quoth:

and there was Motion, but it was merely transverse.And on the third day, from the heavens, a voice cried out:

and there was Lift.

But on the fourth day, the Devil said:

and there was Drag.

On the fifth day a tiny voice from the wilderness cried out: “…don’t forget Stability and Control.”

And this was echoed by various multitudes crying: “…environmental control systems, ground support equipment, and etc.” far into the night of the sixth day.

And on the final day, the spirit of Maynard Keynes proclaimed: “He who controls the purse strings, controls the Policy!” and there was Economic Reality.

A completed airplane in many ways is a compromise of the knowledge, experience and desire of the many engineers that make up the various design and production groups of an airplane company.

It is only being human to understand why the engineers of the various groups feel that their part in the design of an airplane is of greater importance and that the headaches in design are due to the requirements of the other less important groups.

This cartoon “Dream Airplane” by Mr. C. W. Miller, Design Engineer of the Vega Aircraft Corporation, indicates what might happen if each design vs. production group were allowed to take itself too seriously.

Special Interest Groups

Dream Airplanes(One Person’s Dream may be Another’s Nightmare)

..after dining with Airbus.. Boeing

Sauna Piano lounge

Payloads Marketing

Weights

Manufacturing

Aft Super

computer

Flight Controls

Aerodynamics

Structures

Noise

Propulsion

J.H. McMasters (circa 1985)

Schizophrenia

FwdSuper

computer

The Boeing Company

Hecho en México y Chile

Engineering (Design) Isn’t Done For Its Own Sake, It Is Practiced in a Context

The “Design Onion”

Engineering(Design

&Analysis)

Manufacturing

Customers(OperationalConsiderations)

Marketing

SocietalNeeds &Implications

EnvironmentalImpact & Consequences

Politics

ResourceAvailability

Theology

Philosophy• Why are we here?• Why are we doing this ?

Economics

Business&

Finance

Nationalism

Tribalism

Tastes&

Fashion

History

“Everything in this world is connected to everything else”. Think “system of systems”.

Perspectives on Airplane System Design(With the specific or implicit objective of improving the air transportation system.)

Traditional System View A “System of Systems” Approach

AirplaneSystem

WingSub-system

High-LiftSub-system

Flaps

FlapActuators

Somewheredown here isa sub-system an individualdesigner candeal with.

Design requirements,objectives andconstraints

Life, theUniverse

And Everything

World EconomicSystem

World TransportationSystem

New AirplaneSystem ?

AlternativeSystem ?

A Suite ofSystems ?

If one doesn’t consider the whole system, jumping to the conclusion that a particular sub-system is the best solution may result in a dumb or futile design effort.

Design requirements, objectives, and constraints.

?

Airplane Design Taxonomy

• Conceptual Design

• Preliminary Design

• Detail Design

• Design Support

Design Objectives

• An optimized system, defined in sufficient detail to

– Offer to customers for sale– Allow performance, cost, etc. guarantees

to be written into legally binding contracts

• A complete design [the “drawings” ] including manufacturing

requirements, etc. that meets guarantees and allows production of the required hardware

• Derivatives, modifications, up-grades, in-service deficiency corrections, etc.

• A “configuration concept” that

appears to meet requirements

and constraints – as a system.

The Conceptual/Preliminary “Design Process”

Design Requirements

(“musts”) &

Objectives (“wants”)

“ A problem properly posed is half solved”

Aerodynamics

Structures

Propulsion

Systems

Controls

Software

Manufacturing

TradeStudies

&Testing

Integration

What would happen if:• Requirements change• Constraints change• Change assumptions

Resources

Marketing

Other external factors

Meets DR&Os ??

Yes !

No !

Reject ?

or

Proceed

TheDesign

Life Cycle Cost – Airplane Design Like Aerodynamics is an “Initial Value Problem”

Initial Decisions Affect the Slope of the “Locked In” Curve Initial Decisions Affect the Slope of the “Locked In” Curve

Production andOperational Support

EngineeringDevelopment

Program Definition

Concept Exploration

•

•

•

•

• •

100%

75%

50%

25%

0%

Cumulative Percent of Life Cycle Cost

Program Phase

175%

150%

125% Cost Avoidance Area

Potential Cost Overrun Curve

Locked In Curve

$ expenditure

The sum of a set of local optima is not necessarily a global optimum(e.g. an optimum wing doesn’t necessarily produce an optimum airplane)

“Performance”

“Size/Shape”

Δ

“Flat” Optimum

On the Nature of Optima

Exploring the Design Space

Range of PastExperience & Data

Terra incognita

“Configuration” (Size/shape)

Performance(or cost)

Boundary of the feasible design space

We have become slaves to our data bases.

What might lurk inplaces we’ve never been before?

The V/STOL Merry Go Round(A Now Classic “Configuration Matrix”)

The more ideas you have, the more opportunities you’ve got.

Messerschmitt Me 262 First operational jet fighter Arado Ar 234 First operational jet bomber

Heinkel He 162 Messerschmitt (Lippisch) Me 163Junkers Ju 287 Swept-forward wing jet bomber

Messerschmitt P. 1101Horton Ho 229

German Aeronautical Progress (1944-45)

Heinkel He 280DFS 228

Focke-Achgelis Fa 269 Tilt Rotor

Blohm und Voss P. 188 W-wing bomber

Focke-Wulf Ta 183

Focke-Wulf Ta 283Ramjet fighter

Lippisch P.13a Delta wing fighter

Sänger Antipodal Bomber

Blohm und Voss P.202Oblique-wing fighter

Messerschmitt variable sweep fighter

German Aeronautical Progress to 1945

http://www.luft46.com/

Some Basic “Laws” of Airplane Design• Innovation for mere innovation’s sake can be a great

waste of time (and money) – never invent anything if you don’t have to

• You never get something for nothing – someone, somewhere always pays for lunch– While the laws of economics are somewhat malleable, the

laws of physics are not; thus– “If it looks good, it will fly good” is a myth that is sometime

true

• Simplicity is the essence of true elegance – it can also save weight and/or cost

• If you can’t build it, you can’t sell or use it

• They who control the purse strings control the policy – to avoid exercises in futility, learn how to close a business case

• Grand concepts are easy – The devil is always in the details !

McDonnell XP-67 “Moonbat”

Fokker’s Rule: “If it looks good, it will fly good” is a myth that is sometime true…..

McDonnell XP-67 “Moonbat” Dornier Do 335 “Anteater”

A-10 “Warthog”

To disparage a camel as a “horse designed by committee” is to completely ignore the obvious advantages of the camel over the horse in the environment in which the camel is intended to operate.

Antonov An 2(over 12,000 built since 1947)

Boeing F-32 “Angry Frog”

Basic Laws of Airplane Design (cont’d)• In aeronautics, we live in a closed thermodynamic

system in a largely Newtonian universe, thus:

– Weight (W) < Lift (L) = ½ ρ V2 CL S

– Thrust (T) > Drag (D) = ½ ρ V2 CD S

• D = Dparasite + Dinduced + Dcompressibility + H.O.T. – 2/3 management requirements

• DP ~ f(SWet, CL , Re) x speed (V)2

• Di ~ k [Lift (L)/span (b)]2x speed (V) -2 ~ k (nW/b)2 x V-2

– The sum of the moments equals the time-rate-of- change of angular moment (in a vector sense)

– Rangejet = (M x L/D) x (tsfc)-1 x loge (Winitial/Wfinal)• Grand concepts are easy – The devil is always in the details !

ew = 1/kw = theoretical wing span efficiency factor

n = load factor = L/W

Flying iseasy; herelies the realchallenge.

(aerodynamics) ( propulsion) (structures/weights)

Force and Moments on an Airplane

Weight - W

Lift - L

Thrust - T

Velocity - V

Drag - D

Yaw

Pitch

Roll

Angle of attack

V

Airplanelongitudinal ref. axis

Power [P] = TV

If V = constant:

L = W T = D

AerodynamicEfficiency = L/D

LD

F α

Forces on an Airplane in Steady [constant speed] Climb or Glide

Angle of attack (α)

Weight (W)

Lift (L) ~varies with airplane angle of attack

Drag (D)

Flight Velocity (V)

Thrust (T)

In steady flight: Lift (L) = Weight (W) x cos γThrust (T) x cos α = Drag (D) + W sin γ

Pavalilable > Prequired = T x V = [D + W sin γ ] x V

By standard convention, the component of the total aerodynamic force on the airplane perpendicular to the flight path is the Lift (L) and that parallel to the flight path is Drag (D). The thrust need not alignwith with the flight path of the airplane reference axes, but by small angle approximations, the above relations hold well enough for conventional airplanes (or birds, etc.).

Flight path axis

Airplane geometricReference axis

Climb (or glide) angle (γ)γ

γ (radians) ≈ T/W – (L/D)-1

The Classic Breguet Range EquationInitial Weight (@ t =0) = Wi = W0 + Wpayload + Wfuel

Final Weight (@ t= T) = WF = Wi – Wfuel

For a jet aircraft:

Thrust specific fuel consumption = tsfc (lbs. fuel/ lbs. thrust/ hr.)

dR = V dt

dW /dt = - T x tfsc = - D x tsfc dt = - dW/ D tsfc

∫0 dR = - [ V tsfc] ∫Wi dW/D T/ W = D /L (L/D) / W = 1/D

R = + [V(L/D)/ tsfc] ∫ dW/W M = V/a

For a Propeller-driven airplane:

Power specific fuel consumption = psfc (lbs. fuel/unit power/hr.)

Power (P) = TV = DV dt = - dW/ DV psfc

0 T

Range – R @ V = constant

Lift (L) = Weight (W)Thrust (T) = Drag (D)

R = C2 [(L/D) / psfc] loge Winitial /WFinal (C2 is a numerical constant for range in mi., etc.)

Range - R

Payload

WpayloadMax. Wpayload

R WF

Wi

WF

R = C1 [M (L/D) / tsfc] loge Winitial /WFinal (C1 is a numerical constant for range in mi., km, etc.)

Max. range to include necessary fuel reserves.

Drag and Drag Estimation

Drag (D) = ½ ρ V 2 CD S CD = CDp + kwCL2/ π AR + CD wave

D = Dparasite + Dinduced + Dcompressibility + (trim, interference, excrescence,…)

Parasite drag: DP ~ f(Swet, CL , Re) x speed (V)2

“Induced” Drag: Di ~ kw [Lift (L)/span (b)]2 x speed (V) -2 ~ kw (nW/b)2 x V-2

ew = 1/kw = theoretical wing span efficiency factorn = load factor = L/WAR = b2/S

Parasite drag = Friction drag + “Form” (pressure) drag

The Parabolic Drag Polar

LiftCoefficient

CL

Drag Coefficient CD

0

Parabolic Drag Polar(Two-term polynomial curve fit )

Actual measured airplane drag polar

CD0

“Zero-lift” drag coefficient

L/Dmax CD = C1 + C2CL2

With: C1 = CDo

C2 = 1/π AR e

e = “Oswald” efficiency factore = ew (theoretical wing span

efficiency factor)

Lift (L) = ½ ρ V2 CL S

Drag (D) = ½ ρ V2 CD S

Drag and Power Required

XDrag

Speed – V Speed - V

Drag due to Lift

(Induced Drag) - Di ~ W2( b V)-2

“Parasite” (viscous) Drag

Dp ~ Swet V2

Total Drag = Dp + Di

Power ( P)required = D x V

V*prop V*jet V*prop

Power

V* = Optimum Speed to Fly for Maximum Range

Pavailable

Vmin Vmax

Power (P) = Thrust (T) x Speed (V)

Dreams of Leonardo da Vinci

Wilbur Wright Orville WrightOtto Lilienthal

Fathers of Human Flight

1871-1948 1867-1912 1848-1896