Drag Analysis for an Economic Helicopter

S. Schneider, S. Mores, M. Edelmann, A. D'Alascio and D. Schimke

2

Content

Numerical

Simulation vs. Measurement•

Wind Tunnel Setup

•

Numerical

Simulation Setup•

Discussion

of ResultsDrag Breakdown

of EC135•

Configurations

with

different level

of complexity•

Structured

Mesh

Generation•

Unstructured

Mesh

Generation•

Case

Description•

Discussion

of Numercial

ResultsOptimization

of Fuselage

Components•

Case

Description•

Configurations

–

Three

modified

backdoors•

Discussion

of Numercial

ResultsConclusion

Numerical

Simulation vs. Measurement Wind Tunnel Setup

Unsteady Kulites

(21) are placed on •

the backdoor,

•

the upper rear part of the engine fairing and •

the vertical fin.

The

measurements

are

performed

with

rotating

rotor

head.

Primary air inlets, outlets and the Fenestron®

duct have been closed.

Presently only global loads can be compared.

A detailed analysis of the unsteady pressure data and the PIV data is on going.

3

Wind tunnel model of the EC135

Wind tunnel measurements of a scaled EC135 model are being carried out at the Technical University of Munich between winter 2010 and spring 2011.

The experimental investigation is performed in open test section

mode.

The model is equipped with 128 steady pressure ports.

Numerical

Simulation vs. Measurement Numerical

Simulation Setup

Geometry

simplification: •

Only

the

first

element

of the

support

strut

is

modeled.•

All simulations

are

performed

without

the

rotor

head.

The hybrid mesh (commercial software: ICEM-Tetra) consists of 27 prism layers and the remaining volume is filled-up with tetrahedra.

The unsteady numerical simulation have been performed with the DLR TAU code.

Five different test cases have been analysed at constant angle of attack and different yaw angles.

4CFD model of the scaled EC135

Condition Value UnitM∞ 0.178 [-]p∞ 94465 [Pa]T∞ 284.85 [K]q∞ 2107.72 [Pa]ρ∞ 1.1555 [kg/m³]Δt 0.23*10-3 [s]Menter-SST unsteady [-]angle of attack 0 [°]yaw angle -20 / -10 / 0 / 10 / 20 [°]

Initial conditions and test cases

Landing skids

Cabin

Engine deck

Mast fairingExhaust

Tailboom

Stabiliser with Endplates

Fin, shroud, bumper

Strut

Numerical Simulation vs. Measurement Discussion of Results

Both the force and the moment coefficients show a good correlation with the experimental data.

The rotor head of the wind tunnel model exerts a higher drag and

lift force which causes the almost constant discrepancy between the experimental and the numerical data.

The maximum deviation of data lies at a yaw angle of +/-

20°, which might be also due to the different model geometry (is currently under examination)

A detailed analysis of the unsteady pressure data and the PIV data in on going.

5

Drag Breakdown of EC135 Configurations with different level of complexity

This section relates the drag breakdown over several components of the full-scale EC135 helicopter only by means of CFD simulations.

For this purpose, several configurations with different level of

complexity will be investigated.

6

Configuration 1 - Isolated fuselage with closed Fenestron® duct and

engine inlet and exhaust

Configuration 2 - Based on configuration 1 including landing skid components

Configuration 3 - Additionally simulation of air mass flow through the inlet of the engine

fairing and out of the engine exhaust

Configuration 4 - Additionally simulation of the influence of the main rotor on the fuselage by

using an actuator disc approach

Configuration 5 - Highest level of complexity

Drag Breakdown of EC135 Structured Mesh Generation

Configuration Part BlocksCells[Mio.]

1 Complete 135 8.270

2

Fuselage 135 8.383

LandingSkid (LK) (right) 48 2.037

LandingSkid (LK) (left) 48 2.037

LK-Connector (front) 24 0.274

LK-Connector (rear) 24 0.240

Complete 279 12.971

3

Fuselage 135 8.383

LandingSkid (LK) (right) 48 2.037

LandingSkid (LK) (left) 48 2.037

LK-Connector (front) 24 0.274

LK-Connector (rear) 24 0.240

Engine Exhaust (right) 18 0.274

Engine Exhaust (left) 18 0.240

Complete 315 12.971

7Structured sub-grids of configuration 3Mesh of landing skid (green) and front

landing skid conncetor (red)

Structured grid statistic

Structured multi-block approach using the HEXA module of the commercial grid generator ICEMCFD.

The structured mesh generation necessiates a different meshing strategy compared to the unstructured one.

The landing skid components and the engine exhaust components are embedded in several sub-grids communicating with the fuselage mesh through Chimera interpolations.

As the structured mesh generation of complex geometries is very sophisticated, only the first three configurations will be considered for comparison.

Drag Breakdown of EC135 Unstructured Mesh Generation

The unstructured grids are prepared with the commercial software CENTAUR of CentaurSoft.

Hybrid meshing technique using the four primarily element types (tetrahedra, hexahedra, prisms and pyramids)

Generation of a one block mesh without the need of applying the Chimera method.

Structured hexahedra elements mainly used on •

the stator blades,

•

the horizontal stabiliser, •

the backdoor and

•

the landing skids

This facilitate higher stretching ratio of the cells and therefore a reduction of mesh points.

Additionally the grid and solution quality is improved.

8

Configuration Blocks Points

1 1 53771182 1 79262263 1 112765814 1 110456835 1 32975146

Unstructured grid statistic

Surface mesh generated by CENTAUR

Cut through volume mesh at position y=0

Drag Breakdown of EC135 Case Description and Discussion of Numercial Results

Case Description:The considered flight state corresponds to a fast level

flight at a TAS of 140kts and an altitude of 5000ft (ISA condition).

Discussion of Numerical Results:The total drag is divided into three parts:

•

drag of the fuselage components, •

drag of the tailboom components and

•

drag of the landing skid components

Landing Skid Components:The drag analysis of the landing skid components

results in a good correlation between the several

configurations as well as the different applied flow solvers.

Moreover the low RMS deviations, indicated by the black error bars, suggest converged drag values.

9

Altitude and atmospheric condition 5000ft ISA

True Air Speed (TAS) 140kts

Helicopter pitch angle -1.5°

Helicopter side slip angle (configuration 1 and 2) -1.5°

Helicopter side slip angle (configuration 3, 4 and 5) 0.0°

Flight conditions

EC135 – drag breakdown

Drag Breakdown of EC135 Discussion of Numercial Results

Tailboom Components:The massive drag increase can be explained with the

additional Fenestron®

components and the flow separation in the front part of the Fenestron®

duct.

The flow separation occurs since the Fenestron rotor, represented by an actuator disc, produces only sparse thrust in the fast level flight condition.

In general the drag values of the tailboom components show a good correlation between the different configurations and the different flow solvers

10

Flow separation in the Fenestron duct

The drag value of configuration 3 (FLOWer) seems not to be fully

converged, since the error bars (tailboom components) show a wider bandwidth compared to the other drag values.

Fuselage Components:The results of the predicted drag of the fuselage components show the largest dispersion between the

different configurations and the flow solvers.

The increased drag of configuration 5 can be explained again by the additional components of the engine deck.

Change of the unsteady flow field in and around the engine deck.

Drag Breakdown of EC135 Discussion of Numercial Results

Fuselage Components:The integration of the windows and the more detailed floor of the

cabin also affects the unsteady flow field and accounts for the drag increase.

The landing skid components massively influences the flow field and the flow separation position at the backdoor.

The flow field behind the rear bending tube possess an intense turbulent character and flow separation occurs more upstream.

At each bending tube (configuration 5) the flow is interrupted which results in a completely different flow behaviour at the backdoor.

There is a reverse flow beginning at the flange of the tailboom and going upstream to the rear cross tube.

The flow field at the cabin floor and backdoor is very sensitive

which also arises in larger RMS deviations of the drag

An apparently contrary behaviour of the drag values between the configurations and the flow solver can be identified (is currently under examination).

11

Unsteady flow field in and around the engine deck

Flow field at the floor of the cabin and at the backdoor

Drag Breakdown of EC135 Discussion of Numercial Results

The reduction of the drag between configuration 1 or 2 and 3 can

be qualitatively explained by the different flow situation at the inlet of the engine deck.

In configuration 1 and 2 the inlet of the engine deck is closed and a retention effect of the air is formed.

This turbulent and unsteady air generates a vortex going downstream along the edge between the fuselage and the engine deck.

Simulating an air mass flow (engine boundary condition) through the inlet of the engine fairing reduces this effect and therefore the drag.

The assumed value for the mass flow is too small since the retention effect still can be observed.

Only when the simulating the complete engine deck the retention effect vanishes.

Introducing the main rotor represented by an actuator disc increases the drag mainly of the fuselage components.

The downwash effect of the main rotor slightly changes the flow field around the engine deck and therefore also the flow field of the remaining fuselage components are affected.

12

Different flow situation at the inlet of the engine deck between the different configurations

Optimization of Fuselage Components Case Description

This last section will give an outlook towards an economic helicopter by disclosing the potential of aerodynamic improvements of selected components.

For this purpose a study of passive shape modifications on the lightweight class helicopter EC135 was conducted.

Detailed aerodynamic investigations were carried out with main emphasis on the drag reduction.

Main focus was on the modification of the landing gear and the aft body region, which were identified as the main drag contributors.

13

Altitude and atmospheric condition 5000ft ISA

True Air Speed (TAS) 140kts (72m/s)

Helicopter pitch angle -1.5deg

Helicopter yaw/roll angle 0.0deg

Flight StateCase Description:The considered flight state is defined as a fast level flight at

a true air speed of 140kts and an altitude of 5000ft (ISA condition).

Both the rotor head and the components of the Fenestron anti-torque system are not considered.

However each of the four computations includes an engine boundary condition to represent a more realistic airstream around the aft

region of the fuselage

All unstructured meshes for this study were generated using the grid generator CENTAUR of CentaurSoft.

Optimization of Fuselage Components Configurations –

Three modified backdoors

In the context of the fuselage optimisation investigation three modified backdoors were investigated to determine the aerodynamic drag improvements.

14Baseline - EC135

Configuration A – sharp trailing edge closing the backdoor

Configuration B – truncated sharp trailing edge closing the backdoor

Configuration C – backdoor with defined flow separation edges

Faired cross tubes (the modified cross tubes and steps are marked green)

Optimization of Fuselage Components Discussion of Numerical Results

The main drag reduction contributors are the landing skids and the backdoor.

Introducing faired bending tubes results in a reduction of the fuselage drag for all three configurations.

Since the flow around the backdoor is significantly changed the tail unit is affected slightly negatively due to an increased dynamic pressure resulting from the separated vortices.

Configuration C shows the smallest drag reduction improvement as a result of the reshaped engine deck fairing in the area of the modified backdoor.

For the future development the engine fairing will be

investigated in further studies.

Modifying the backdoor and adding bending tube fairings an overall drag reduction benefit of approximately ~24% can be reached.

15

Relative drag breakdown of the main components

DES of a helicopter fuselage (ATAAC)

L. Paluszek, F. Le Chuiton

Experiment

angle of attack = 0 degrees

angle of side-slip = 0 degrees

upstream velocity V∞ = 40 m/s

Mach number M∞ = 0.1131 --

Reynolds number Re∞ = 2.27 106 m-1

Experimental setup

The experiment was carried out at the Technical University of Munich in 2009

Measured quantities: forces, unsteady pressures and averaged velocity components at 6 PIV windows behind the back door

PIV windows

Transition line

Location of the pressure taps and transducers (red)

Numerical model

3 grids considered:

•

12.2 mln cells, block structured grid (mandatory for ATAAC)•

9.9 mln cells, hybrid grid (mandatory for ATAAC)•

12.9 mln cells, hybrid hexacore grid

Solver settings (URANS):

•

Central scheme

with artificial

dissipation•

Preconditioning•

Least Square gradient reconstruction•

Menter SST turbulence model•

Dual time stepping•

Implicit relaxation solver for inner iterations•

FAS Multigrid

Computational domain

Geometry of the wind tunnel model of the EC145 helicopter fuselage

Predefined laminar zones

Mesh details

Block structured hexahedral grid (12.2 mln cells)ICEM CFD

Hybrid tetrahedral grid (9.9 mln cells)CENATUR

Hybrid hexacore grid (12.9 mln cells)ICEM CFD

Preliminary URANS results, lambda-2 iso-surfaces

Preliminary URANS results, lambda-2 iso-surfaces

Very strong mesh dependence observed

TAU averaging module

Testing of the ´on-the-fly´

averaging option in TAU

•

Means•

Variances

•

Averaged surface streamlines

A very useful tool for both steady and unsteady solutions

Instantaneous (top) vs mean Cp Pressure variance

Numerical

challanges

in Tau

Frequent

divergence

of the

omega

equation

in the

Menter

SST model

Divergence

at coarse

multigrid

levels

when

using

low

dissipation

schemes

Slow

residual convergence

(or

none

at all) for

the

dual time stepping

scheme

•

Convergence

was achieved

when

using

ΔT ~ global convective CFL = 1 and at last 100 inner iterations

Divergence

after

restarting

from

a solution

file

or

after

grid

adaption

Engine

inlet

boundary

condition sometimes

blows

air

into

the

domain

Tau user

guide

does

not

mention

that

‘Reference bl-thickness‘ parameter

is

used

when

initialising

turbulent quantities

in the

solution

and it‘s

default

value

is

1e+22 (which

means

that

all cells

within

1e+22 metres

from

the

laminar

walls

are

initialised

with

TKE and TI = 0)

Problems with

the

averaging

module

in TAU python

Suggestions

Green-Gauss

or

TSL gradient

calculation

option

for

coarse

grids

CFL reduction

factor

for

coarse

grid

levels

(instead

of a single

value

as it

is

now)

Normalisation

of all residuals

Especially for DES

Hybrid discretisation

scheme

(upwind

for

RANS, central

for

DES)

Different numerical

dissipation

settings

for

RANS and DES zones