Fuel-Burn Penalties Caused by Irregularities in Aircraft

Surfaces (ASTEROID Project)

A presentation to the Airbus DiPaRT Flight Physics Symposium 2017

22nd November 2017

Andrew Shires (UoL) a.shires@leeds.ac.uk

Ben Hinchliffe (UoL)

Kevin Hackett (ESDU)

David Philpott (ESDU)

Project part funded by:

Aircraft Surface Tolerances for Enhanced Repairs, OperatIons & Design

• Excrescence Drag is the additional

drag due to the sum of all deviations

from a smooth sealed external

surface

• Boeing estimates that excrescence

drag represents ~4% of total drag of

the 737 aircraft1

• Furthermore, excrescence drag increases as aircraft age due to a

deterioration in surface finish, leaking seals and repairs.

1. ‘Fuel Conservation: Airframe Maintenance for Environmental Performance’, Dave Anderson, Flight Operations Engineering, Boeing

Commercial Airplanes, September 2006

Examples of new aircraft surface irregularities

• Antennas, masts, lights

• Steps at skin joints, around windows, doors, control surfaces, and

access panels

• Internal air leaks through gaps, holes and aerodynamic seals

• Non-flush fasteners, surface roughness and waviness

In-service excrescence drag will be influenced by maintenance standards.

Average total airframe drag deterioration ~ 0.65%, composed mainly of1:

• Control Surface Rigging ≈ 0.25%

• Deteriorated Seals ≈ 0.20%

Also influenced by installation of repair patches.

1. ‘Fuel Conservation: Airframe Maintenance for Environmental Performance’, Dave Anderson, Flight Operations Engineering, Boeing

Commercial Airplanes, September 2006

Current Airbus processes2

Acceptable tolerances relating to surface discontinuities are specified in the

Technical Design Directive (TDD) for each aircraft

Parasitic drag determined using Airbus EDDI tool (Excrescence Drag

Determination and Investigation), based on;

• ESDU Aerodynamics Data Items

• AGARDograph no 264 'Aircraft Excrescence Drag‘, 1981

• Fluid Dynamic Drag Dr. Ing. S. F. Hoerner.

EDDI gives excrescence drag for basic 2D & 3D shapes. Where a significant

pressure gradient exists, e.g. on lifting surfaces, a magnification factor is

applied

For new aircraft the TDD is generally based on that for previous aircraft,

hence A350 tolerances are very similar to A300 ! 2. ‘Review of the methods and processes used to determine the effect of excrescences, including ventilation, on aircraft aerodynamics’,

ASTEROID contractor report, Adrian Miller, November 2016

• ESDU data items are largely based on extensive measurements in the

RAE 8ft wind tunnel by Gaudet et al.3

• Various excrescence heights, up to 5% of ∂, were mounted onto a flat

plate and a force strain balance used to measure drag with and without

the excrescence present

3. L. Gaudet and K. Winter. ‘Measurements of the drag of various two-dimensional excrescences immersed in turbulent boundary layers at

Mach numbers between 0.2 and 2.8’, from Proceedings of AGARD Conference on Aerodynamic Drag, 1973

• Plots show the drag increment

(CD) relative to the undisturbed

local skin friction (Cf) in a zero-

pressure gradient.

• Logarithmic dependence on

Reynolds roughness

• Distinct correlations for sub and

supersonic regimes

Project aims

• Primarily to update current ESDU data sheet methods

• Low and High speed (M = 1.4) wind tunnel tests to measure

boundary-layer velocity profiles using laser optic (LDV) and/or hot

wire techniques for different excrescence shapes and heights

• Obtain integral BL parameters (∂, ∂* and θ) and therefore the

viscous drag increment

• Develop empirical ‘User Defined Functions’ in CFD (Fluent and

VGK) to explicitly include the effects of excrescences

• Develop an ‘Operations Tool’, a web-based interface to the ESDU

excrescence drag software to allow maintenance engineers to quantify

drag penalties to prioritise maintenance strategies

Operations tool findings

Defect Fuel mass

per year

US Gallons

per year

Fuel Cost US$

(1.54 US$ per

USG)

Inboard slat not fully retracted, increasing

the groove at the slat trailing edge from 10

mm to 15 mm.

72649 1b

330233 kg

11043 $17,010

Spoiler panel number 2 slightly lifted by 3

mm

18162 lb

8255 kg

2761 $4,250

Wing lower surface access panel set 1mm

above wing surface.

4541 lb

2064 kg

690 $1,060

Adjustable aileron rubber seal gaps set with

3mm leakage gap.

31784 lb

14447 kg

4831 $7,440

Undercarriage door protrudes 2 mm at

inboard edge.

27244 lb

12384 kg

4141 $6,380

Outboard-flap end seal with 3mm leakage

gap.

81731 lb

37150 kg

12423 $19,130

Operations tool findings

Following several visits to aircraft under maintenance, the excrescences

we identified that could be addressed by the airline/operator were relatively

minor and would produce only small savings in fuel burn.

The most significant excrescences were inherent in the aircraft design

Most rectification work would have to be completed during scheduled

withdrawals from service. These are infrequent, and would require

expenditure in man hours that would offset any consequent fuel savings.

Getting the aircraft back into service on time is always the economic and

operational priority, regardless of the presence of any feature that may

cause a higher fuel burn.

Consequently, the operations tool development has halted !

• Consider a small excrescence located on a flat plate at E. Although

the local flow at E may be complex, the downstream velocity profile

returns to that corresponding to a flat plate but with an increase in

momentum thickness (θ) due to the excrescence drag.

• Consequently velocity traverses must be made upstream and

downstream of E.

Momentum thickness

(no excrescence)

Momentum thickness

(with excrescence)

Excrescence

A B

x

E

Low-speed test results

• Wind tunnel tests have been completed in the Cranfield Environmental

Tunnel with a range of forward facing steps and a step-groove.

• BL profiles were measured

using a pitot rake.

• Due to the size of the rake and

mounting, measurements could

not be taken down to the

surface.

• Additional low-speed testing

currently underway at City University

using a hot wire traverse

High-speed (M = 1.4) test results

• Wind tunnel tests have also been completed in the Cambridge University

tunnel with a range of forward and backward facing steps using LDV

• Excrescence heights h/∂ = 2%, 9% and 73%

1. Obtain time averaged profile

2. Remove erroneous points at the wall

3. Curve-fit profiles to produce a

composite curve comprising;

1. Outer layer

2. Log-layer

3. Buffer-layer

4. Viscous sublayer

4. Integrate composite profile to get δ*

and θ

5. Apply a compressibility correction4

High-speed (M = 1.4) test results

4. ESDU data item 68020

Comparison of BL profiles using a NACA (report 772) empirical

method with fitted profiles from the test (flat plate).

High-speed (M = 1.4) test results

FFS, h = 0.0001 mBFS, h = 0.0005 m

Forward step height, h = 0.0001 m

Forward step height, h = 0.0044 m

High-speed (M = 1.4) test results

A concern was that freestream Mach number fluctuates significantly !

High-speed (M = 1.4) test results

There were further concerns over the consistency of δ* and θ

distributions obtained from integrating the fitted (composite) profiles.

Causes may include;

• Fluctuating freestream Mach number

• Reflected Mach waves

• A difficulty of the LDV setup is ‘finding the wall’ with the laser system

– can lead to errors in the expected vertical position

• A further limitation of the LDV method is the low seeding levels very

close to the wall.

Consequently, another high speed test is scheduled for 4th December

2017 to address some of these issues

High-speed (M = 1.4) CFD results

• Ansys Fluent density-based solver

• 2D, RANS k-ε turbulence model

High-speed (M = 1.4) CFD results

• Initially achieved convergence of

continuity and u-momentum to 10-6

• However, when checking the convergence of q, ∂* and Cf it was

found that these were not well

converged

• Also, Cf converged to

approximately one order lower than q and d

• Therefore the convergence of Cf at

several points along the surface

was achieved to 10-8

X=-0.16m X=-0.0025m X=-0.0088m

X=0.044m X=0.1m X=0.125m

FFS h=0.0005mHigh-speed (M = 1.4) CFD results

X=-0.16m X=-0.044m X=0.0088m

X=0.044m X=0.1m X=0.125m

BFS h=0.0044mHigh-speed (M = 1.4) CFD results

High-speed (M = 1.4) CFD results (FFS)

Previously extrapolated lines fitted before and after excrescence to

determine Δθx=0, but switched to equivalent plate method

ESDU Δθ based on drag due to excrescence that will include wave drag

CD wave

contribution

CD wave

contribution

High-speed (M = 1.4) CFD results (BFS)

Previously extrapolated lines fitted before and after excrescence to

determine Δθx=0, but switched to equivalent plate method

ESDU Δθ based on drag due to excrescence that will include wave drag

Summary

Low speed wind tunnel tests

• We have some wake rake measurements from Cranfield still to process

• Have switched to City University due to issues with hot wire traverse

High speed wind tunnel tests

• Repeating experiment with new plates to hopefully address fluctuating

freestream Mach number as well as vertical positioning issues

RANS CFD

• Results are looking promising compared with experiment and importantly,

are more consistent

• Currently determining wave drag contribution

• Will then use CFD results to update current ESDU data sheet methods

and derive improved empirical models