Innovative approaches to testing and validation: Enhancing the Design/Development Process

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

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)

• Case StudiesIII. Innovative approaches to testing and validation - Enhancing

the Design/Development Process

Presentation Overview

Case III. Just About Anything Can Be Made to Fly……..

Especially If a Pilot Isn’t Required In It.

Wind Tunnel Testing

Various Wind Tunnel Concept Trade SummaryAt a given Mach number (M):

• Reynolds number (Rn) = ρVℓ / μ ~ (pT /μ)[mγA/ TT ]1/2

• Model loads (σ) ~ dynamic pressure x model size ~ γ pT A

• Power required (P) ~ γ2/3ApT (TT /m)1/2

where m = molecular weight of test gasA = cross-sectional area of test section (A1/2 ~ model characteristic length ℓ)

Tunnel Option

• Baseline• Increase size (x 2)• Increase pressure• Increase size & pressure• Decrease temperature• Increase press., decrease temp. “Heavy gas” (SF6)

• Heavy gas + press.

Test Section pT TT Reynolds No. Loads Power required Area (w x h) (atm) oF Rn/Rnbase σ/σbase P/Pbase

A 3 100 1.0 1.0 1.0 4 A 3 100 2.0 4.0 4.0 A 6 100 2.0 2.0 2.0 2.25 A 4 100 2.0 2.0 3.0

A 3 - 120 2.0 1.0 0.78

A 4 - 50 2.0 1.3 1.12

A 3 100 2.4 0.81 0.35

A 5 100 4.0 1.35 0.6

Abstract

A Possible Alternative Aerodynamic Testing Method to Support New Large Fast Airplane Development

Studies conducted in connection with the New Boeing Wind Tunnel project circa 1988-90 never succeeded in fully answering the basic question: “How much Reynolds number is enough ” to allow reliable, straight forward extrapolation of wind tunnel test data to full-scale flight conditions. To approach those conditions judged “necessary”, however, it was clear that even half-model testing would require a suite of very large conventional pressure tunnels. The Boeing tunnels were never built, and the remaining, aging tunnels now available to us for possible large, high-speed airplane development have less than the desired capabilities for several major reasons. The option of flight testing with “modified” existing manned aircraft remains generally cost prohibitive and has a second suite of major limitations. All of this is exacerbated if the need is to validate a vehicle intended to cruise very close to Mach 1, and considerably different in configuration than those that make up our present well validated experimental data base. In considering testing problems circa 1988-90, it occurred to us that as a “third option” it might be possible to employ “large”, free flying remotely piloted models [based on extant cruise missile and similar technology] to get very high Reynolds number conditions at costs competitive with wind tunnel tests. This concept was based on the notion that the “wind tunnel design

Abstract

A Possible Alternative Aerodynamic Testing Method to Support New Large Fast Airplane Development (cont’d)

problem” could be “turned inside out” by viewing the atmosphere as a single wall [the ground] pressure chamber with the power required to be supplied to the model rather than to blowing air over the model.

Simple calculations showed that for an approximately 767 sized airplane intended to cruise at around M = 0.95 at an altitude of ~40K feet, flying a ¼ scale model of the machine at the correct Mach number and lift coefficient would produce ~ 80% of full scale Reynolds number conditions. Similarly, a 1/5 scale model of an HSCT intended to cruise at ~60K feet, gets to 100% full scale Reynolds number conditions when flown at about 20K feet. In the approach proposed, the power requirements are a small fraction of those required to run an equivalent large pressure wind tunnel, there are no wall or model mounting effects, and flow quality (other than wind) is not a major issue. It is also potentially possible to model full scale aeroelasticity and pressure instrumentation installation is easy. Further, one is never limited to half model testing and limited (due to flutter considerations) dynamic testing is possible. The purpose of this presentation will be to describe the concept, its virtues and limitations, and requirements for its implementation.

**

** at about 4000 ft altitude

1.001

10

100

Current Boeing CommercialAirplanes

Near SonicCruiserB 767

Mach Number

ReynoldsNumber

x 10(Based onwing meanchord)

-6

BTWT(full model)

NASA NTF(theoretical)

NASA Ames 11’x11’(half model)(full model)

Current Wind Tunnel Testing Capability for Commercial Airplane Development

Matching Reynolds NumberCryogenic Model

Aerodynamic Testing

• Most airplane development has traditionally relied on wind tunnel and/or flight testing as an intrinsic part of the process. CFD results still require validation.

• Most of the wind tunnels available for large airplane development are capable of matching full scale Mach numbers, but fall far short of matching full scale Reynolds numbers.

• Our current suite of wind tunnels are aging and present increasingly significant limitations when a new design is different in configuration from those in our established data bases, or cruise at conditions near Mach 1.0

• Traditional flight testing remains a very expensive complement to, rather than replacement for, wind tunnel testing. • What alternatives do we have with currently available technology ?

How Much Reynolds Number Is Enough ??• Attempts to define Reynolds number requirements for a proposed suite of new Boeing wind tunnels circa 1988-90 failed to establish definite limits for all anticipated new product developments

• The following hierarchy was established, however: • Full scale if possible• High enough to allow straight forward, unambiguous extrapolation to full scale conditions (i.e. high enough to assure that boundary layer conditions and transition locations were similar to those on the full scale airplane - on all critical flight surfaces)• As high as possible within practical tunnel test section size and power requirement limitations (if such levels provided “useable” test data).

• The final tunnels proposed would have provided a Reynolds number capability higher than that of any existing tunnel except the NASA NTF, but less than that required for reliable stability and control characteristics extrapolation. The new Boeing tunnels were never built.

A Possible Alternative to Either Wind Tunnel or Traditional Flight Testing

• New Boeing wind tunnels proposed circa 1988-90 required large tests sections, used of up to 5 atmospheres of pressure, and required huge amounts of power to provide about 40-50% full scale Reynolds number on a Boeing 767 class airplane.

• Flight testing remained an extremely expensive alternative with severe limits on “safe” test conditions and configuration deviations from those of the basic platform upon which the tests were to be conducted.

A casual examination of the tables of a “Standard Atmosphere” then suggested a possible “third option”. This was based on the parallel observation that while a wind tunnel is generally located at approximatelystandard sea level atmospheric condition, the actual airplane cruises ataltitudes well in excess of 30K ft. The third observation was that the power required to fly a “wind tunnel model” at sea level was much less than the power required to blow air around a proper wind tunnel circuit.

~36K ft.

Standard Atmosphere

Air viscosity varies with temperature.

“Hot” air is more viscous than “cold” air.

A Possible Alternative to Either Wind Tunnel or Traditional Flight Testing

Playing with the Standard AtmosphereReynolds number = Rn = f (gas properties, temp., density) x characteristic length (L) x Mach number ( M)

1.0

Stratosphere~36K feet

0

50

1.50.5Sea level

Altitude1,000 feet

T/T0 M/M0Rn/Rn0

Rn/Rn40K

Speed (V) and Size (L) = constant

3.2 @ SL

A Possible Alternative to Either Wind Tunnel or Traditional Flight Testing

0 0.2 0.4 0.6 0.8

Model Scale (Fraction of Full Scale)

FlightAltitude1,000 feet

50

0

Ref. Full scalecruise condition

1/4 scalemodel

80% Full scaleReynolds number

100% Full scaleReynolds number

4

Mach number = constant

Note: The numbers get betterwhen the reference cruisealtitude increases to 50K feet.

Model Size Comparison (to same scale)

Model Size Comparison (to same scale)

Where to Test?

• Need about 200 square miles of open space

• Area 51 NV (a favorite)

• Glasgow MT

• NASA Dryden CARocket boost orair drop

Test datarun during coast down to minimum speed

Recoverydevicedeployed

Recover

Altitude

Range

Safety Zone Maneuvers asrequired

Further ConsiderationsWhile the scheme proposed was developed originally as an alternative to wind tunnel testing for aerodynamic configuration validation, on further reflection it can be seen that more than that is possible. For example, as our technology matures (and becomes more expensive to develop) and the time between major new programs increases dramatically, a major concern for many in our technical community has been the issue of where and how future generations of designers and program managers will get the experience necessary to “be good at” putting together new airplanes when it really matters.

The scheme proposed here amounts to building (and flying) a small airplane, but at a cost nearer to that of sophisticated wind tunnel testing program rather than a prototype (manned) airplane development. By extending the objectives of such testing, one may conceive of the “flying model” approach as a means to do significant “process validation”. In this, the configuration validator may be used as a basis for regularly scheduled “process checks” on a larger suite of the analytic/computational tools and methodology to be used in the design of new airplanes. Aeroelastic prediction methodology can be verified, control laws developed, design cycle time reduction strategies tested etc. in a wide array of options.

The learning that can come from these “sub-scale, un-manned” airplane development exercises, conducted on a regular basis as part of our on-going R&D efforts, could be of huge value and obtained at an affordable price.

A Recent Japanese Variant

A candidate SST configuration to be tested in Australia in July-August 2002. [Note: The first flight attempt was a dramatic boost rocket crash and burn on July 14, 2002.]

“Please Professor McMasters, may I be excused. My brain is full.”

No !