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TRAINING CENTER FOR EXPERIMENT AL AERODYNAMICS

TECHNICAL NOTE 14

POWERED LIFT MODEL TESTING

FOR GROUND PROXIMITY EFFE CTS

by

P.E. COLIN

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RHODE-SAINT-GENESE, BELGIUM'

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TCEA TN 14

POWERED LIFT MODEL TESTING FOR GROUND PROXIMITY EFFECTS

by

P.E. COLIN

The research reported in this document has been made possib1e through the support and sponsorship of the U.S. Department of Arm~. through its European Research Office.

CENTRE DE FORMATION EN AÉRODYNAMIQUE EXPf:RIMENTALE,

72, CHAUSS~E DE WATERLOO, 72 RHODE - SAINT - GEN!SE (BELGIQUE)

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S U M MAR Y

=============

The effect of ground proximity on the

performance of powered l i f t vehicles has been investigated on simple models using two different testing methods. Single and double-jet models representing VTOL configurations and an air-cushion model with peripheral jet have been tested both in the wind-tunnel where a stationary plate immersed in the flow was used to represent the ground and on a special rig allowing the models to be moved over a fixed ground plate. The lift and centre of pressure loc at ion have been determined with both techniques for various model heights above the

ground-plate over a range of momentum coefficients. The results obtained with both methods are compared.

D D e H D g d d e h x S q v. J m. J ClJ G s G L s L = NOTATION ===============

Diameter of circular-disc VTOL models

Equivalent diameter for triangular VTOL model Height of triangular model (base of triangle to vertex)

Base diameter of air-cushion model

Nozzle diameter for single-jet VTOL models Equivalent nozzle diameter for double- jet VTOL models

Distance of models from ground-plate

Distance of centre of pressure from centre of models or from centre of nozzle for triangular model (positive upstream)

Model area

Dynamic pressure corresponding to tunnel or model velocity

jet velocity jet mas s flow m.v.

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Statie ground suction force on VTOL models

Ground suction force on VTOL models with relative wind

Statie lifting force on base of air-cushion model Lifting force on base of air-cushion model with relative wind

1 •

Introduction.

The VTOL aircraft which derive their l i f t at zero or low speeds from vertical or near vertical slipstreams may be subjected to large interference effects when operating in close proximity to the ground.

A jet emerg1ng from a lifting surface and impinging vertically on the ground will entrain ambient air between the surface and the ground and as aresuIt, low pressures will be induced on the lower surface. The lifting performance of such a simple system will therefore be reduced. However. the sign and magnitude of the ground effect on the lifting capability depends essentially on the configuration of the VTOL aircraft under study. A multi-jet model, for instance. may produce a favourable ground effect. At low speeds, conditions in ground proximity are usually less favourable than at zero speed, any favourable static effect being reduced or any statie ground suction being increased.

Because of the usually small margin of lifting thrust available over the aircraft weight, it is important to deter-mine accurately at an early stage the effect of ground prox1m1-ty on the performance of a projected aircraft.

Model tests are usually carried out 1n the wind-tunnel_a the ground being represented by a fixed-plate immersed in the flow. This method does not reproduce the full-scale flow

characteristics of an aircraft flying close to the ground 1n no wind condition because of the existence, on the stationary

~n front of the model due to the high pressures induced under i t by the lifting jet or jets.

The same argument applies to the air-cushion vehicle which is designed to operate very close to the ground to take

full advantage of a large favourable effect.

To investigate the validity of the wind-tunnel fixed-plate technique, a simple rig has been designed which allows the model to be moved over a flat surface representing the

ground. A description of the r~g has been given in a previous

note (ref.6). It operates like a pendulum, the model moving

along a circular path in the vertical plane. Comparative

experiments carried out on models representing VTOL

configu-rations and on an air-cushion model both with such a rig ~n

the low-speed tunnel with a fixed-plate are described.

Lift and pitching moment induced by the jet or jets have been determined. The results obtained with the VTOL

models show substantial differences for small distances of the models from the ground-plate.

For the air-cushion model, the differences are

considerable particularly in the higher range of relative wind speeds.

Model description and apparatus.

The models used 1n the experiments are shown 1n fi g.l •

3 •

configurations with single and double-jet nozzles . With one

exception. all plan-forms are circular : one has the shape

of an equilateral triangle . They are made of thin metal sheet

with the plane parallel to the direction of flow (zero angle

of attack). The position of the jet nozzle(s) is indicated in

fi~.l.

With reference to an investigation carried out at the

R.AoEo in the United Kingdom_t the size of the triangular model

was chosen to produce the same statie characteristics as the

single-jet circular model. An equivalent diameter De equal

to the diameter D of the circular model is obtained as shown

ln figol o For the double-jet models ~ the t otal nozzle area

is equal to that of the single-jet models 5 an equivalent

diameter de being defined in fig.l. The models are provided

with a small clearance around the nozzle(s ) to which they are

attached through a two-component strain-gauge balance (lift

and pitching moment). Only the aerodynamic loads induced by

the jet(s) are therefore recordedo

The fifth and l ast model of the serles tested

represents an air-cushion vehicle with a peripheral jet nozzle

inclined 30° inward as shown in fig.l.

The wind-tunnel tests were performed in the TCEA

low-speed tunnel Ll which has a circular open~jet working

-section of 3 mo di ameter . The board used for ground

representation was made of a metal plate 10 mm thick, with

a l m span and a length of 1.7 m with a 2 mm thick leading

The rlg designed for comparative measurements is shown schematically in fig.2. It consists essentially of a

pendulum of

4

m radius at the end of which the model is

attached. The pendulum rotates ln the vertical plane, being released. prior to a run, from the vertical position. The

model velocity at the bottom of the circular path at the first passage is approximately 16 m/sec, the velo city decreasing at each successive passage.

The arm of the pendulum includes the air supply plpe to the model with a flow-meter. Special bearings are fitted at the axis of rotation.

The end of the pendulum arm is turned by 90°, the

model being mounted vertically with the jet{s) horizontal .

Such an arrangement allows a flat vertical plate to be used for ground simulation and the centrifugal force on the model acts at right angles to the force to be measured.

The strain-gauge balance measurements are made at

the bottom of the circular path with a galvanometric recorder. A time counter connected to two photo-electric cells located

0.5

m on each side of the lowest point of the path is used to

determine the model speed.

Programme of tests .

1. VTOL models

same operating conditions for both testing methods.

Low-speed flight conditions were obtained by keeping the mass-flow through the jet nozzle(s) constant and varying the tunnel or model speed. The data were obtained for a range

of distances of the models from the ground-board of

1

.

5

to

4

nozzle diameters (or equivalent diameters) . The conventional

positive l i f t ~s considered here as negative i oe . , a ground

suction force is positive .

A non-dimensional coefficient ~s obtained by dividing

the ground suction force G by the momentum flow through the jet ( s) mj v j •

The results are plotted against the momentum coe ffi ci ent

m·v ·

ClJ

=

~

qS

Measurements of pitching moment were used to

determine the centre of pressure location with respect to the

centre of the model (centre of the jet nozzle for the triangu

-lar model) . The distance x along the model longitudinal axis

is considered positive upstream. The results are plotted in

non-dimensional form as distance in per cent of model diameter

D or length H against momentum coefficient ClJ o

Reference to a ratio of jet to tunnel velocity such as given by the momentum coefficient was shown to be suitable by tests performed at two different mass-flow rates. The data

obtained under these conditions were in very good agreement

technique were performed in the tunnel to investigate the surface flow pattern on the ground-plate in the vicinity of the model.

2. Air-cushjon model

The same testing procedure was used as for the VTOL models. The normal convent ion of a positive l i f t is used in

this case ~.e. a positive lifting force L on the model base

acts in a direction away from the ground-plate. A non-dimen-sional coefficient is obtained by dividing the l i f t L with

relative wind-speed by the statie l i f t L •

s

The distance of the centre of pressure along the

longitudinal axis from the eentre of the model is given in the

results as a percentage of the model base-diameter (positive

direction upstream). Data was obtained for 3 values of the height of the model above the ground-plate namely 5,10 and 20 per cent of the base-diameter and covered in each case a range of moroentum coefficients.

Surface-flow visualisation tests were also made on

the ground-plate in the wind-tunnel.

Discussion of results.

1. VTOL model s

Fig.3 gives the static ground suction characteristics of the four roodels tested plotted against the inverse of the

7 •

square of the distance ratio h/d or h/d • The variation is e

linear for the single-jet modeIs. Conditions are more favoura-bIe for the double-jet at the smaller values of the height h due to jet recirculation in the plane of symmetry. The

triangular model has, as expected,the same statie characteris-tics as the single-jet circular model.

Figs.4 to 11 show the ground suction characteristics with relative wind and the centre of pressure location plotted

against the momentum coefficient C~o No points are shown on the curves as these were obtained from intermediate plots of the experiment al data.

For the first three models~ the wind-tunnel values of ground suction are optimistic over most of the C~ range compared to the pendulum data. Only at small C~ values is there a tendency for the curveS to cross over.

The fourth model, with the double-jet nozzles on an aX1S perpendicular to the relative wind, shows opposite

characteristics, the wind-tunnel results being pessimistic over the whole range of momentum coefficients .

The differences between the wind-tunnel and pendulum

.

'

results are substantial for the smaller heights of the models above the ground-plate.

Differences of as high as 10

%

of jet-thrust have been obtained in certain cases. The differences become

generally much smaller as the height increases and tend to disappear for a height ratio of about

4.

The wind-tunnel and pendulum curves show a tendency

to cross at a C~ of approximately 2.5 or below for the first

three models. The surface flow pictures in figs. 12 and 13

obtained in the wind-tunnel show the existence of astrong horseshoe vortex ; the separation of the ground-plate boundary layer occuring some distance upstream. The position of this vortex with respect to the model depends essentially on the

momentum coefficient C~o At small values of C~, the effect is

such that the flow induced under the model is increased with a corresponding increase of ground suction compared to the

pendulum results o At the higher values of C~. the vortex is

further upstream of the model and produces a kind of blockage with a consequent change in the ground suction force towards its statie value. For the fourth model however, the flow

picture gives a vortex position which, for the same c~ range

as for the other models, is located much closer to' the model.

This corresponds to the higher induced flow case of the other

models with a resulting larger ground suction force than

given by the pendulum.

It should be mentioned that the surface flow

pictu-res show a pronounced similitary with those obtained with building models when tested on a fixed-plate in the wind-tunnel.

A previous programme of experiments (refo

6)

on a

circular -disc model with a central jet showed that the

boundary-layer thickness on the fixed-plate in the wind-tunnel did not appear to affect the ground suction force

9

•

separation of the boundary-layer with the resulting change 1n flow pattern.

2. Air-cushion model

Figs . 14 and 15 give the lifting characteristics and centre of pressure position in terms of momentum coeffi-cient for the air-cushion model. The surface-flow pictures taken in the wind-tunnel are given in fig .16. The difference between wind-tunnel and pendulum results is considerable in the lower C~ range and may be as large as 30 per cent and higher of the statie l i f t value . The flow pictures are similar to those obtained with the VTOL modeIs.

The l i f t obtained 1n the wind-tunnel is always

optimistic which means that the blockage effect mentioned for the VTOL models seems to apply throughout the C~ range tested. Unless momentum coefficients are reached i n the lower range

such that the upstream part of the peripheral jet is turned backward before the ground is reached. the front of the horse-shoe vortex is always located sufficiently upstream for the effect mentioned above to apply. The lift measured in the wind-tunnel tests thereforep tends toward the statie value

much more rapidly than in the pendulum tests.

CONCLUSIONS.

A simple r1g has been descri bed for testing VTOL or air-cushion types of models in ground proximity in a more

representative manner of the full-scale conditions than ~n the wind-tunnel when a fixed-plate is used to represent the ground.

Within the scope of the present investigation.

substantial differences are obtained for the VTOL models ~n

the ground suction force measured with the rig and in the wind-tunnel for small distances of the models from the ground plate. The sign and magnitude of the differences depend

essentially on model configuration and are related to the

position of the horseshoe vortex associated with the

separa-tion of the boundary layer on the fixed-plate in the wind-tunnel.

In the case of the air-cushion model considerable

differences are observed in the lifting characteristicsj and

wind-tunnel tests with a fixed ground-plate would appear to

REFERENCES 1. poisson-Quinton. Ph. 2. Poisson-Quinton. Ph. 3. W i 11 i am s. J . 4. Campbell. J.P.

5.

Colin. P.E. 6. Colin. P.E. 11 .

Influence de la Proximite du Sol sur les Caracteristiques Aerody-namiques d'Avions V/STOL Utilisant

des Jets - AGARDograph 46J part 2.

June 1960.

Etude en Courant Plan d'une

P1ate-forme à Effet de Sol - O.N.E.R.A.

Note Technique n° 57 (1960) .

Some British Research on the Basic Aerodynamics of Powered Lift

Systems - J.R.Ae.S •• July 1960

Ground Proximity Effects Associated

with V/STOL Aircraft - Paper pr-.

e-sented at the Eighth Anglo-American

Conference. London. Sept .1961.

Ground proximity and the VTOL

Aircraft - AGARD Report 409.

A preliminary Investigation on

V/STOL Model Testing for Ground

proximity Effects -

Wissenschaft-liche Gesellschaft für Luftfahrt

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fTCEA TN 1 4 1

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Training Center for Exper1menta Aerodynamics

POWERED LIFT MODEL TESTING FOR GROUND PROXIM1TY EFFECTS

October 1963

The effect of ground proxlmlty

on the performance of powered

lift vehicles has been investi

-gated on simple modele using

two different testing methods.

S1ngle and double-Jet models representing VTOL conf~gurations

I. COLIN, P.E.

11. TCEA TN 14

and an alr-cushion model w1th perlpheral Jet have been tested both in the wind tunnel where

a stationary plate immersed 1n the.flow was

used to represent the ground and on a special rig allow1ng the models to be moved over a fixed ground plate. The lift and centre of pressure location have been determined with both techniques for various model heights above the ground-plate over a range of momentum

coeffic1ents. The resulta obtained with both

methods are compared.

and an a1r-cushlon model w1th per1pheral Jet have been tested both 1n the wind tunnel where

a stationary plate 1mmersed 1n the flow was

used to represent the ground and on a special rig allowing the models to be moved over a fixed ground plate. The 11ft and centre of pressure location have been determ1ned w1th bath technlques for var10us model he1ghts above the ground-plate over a range of momentum

coefficients. The results obtalned w1th both

POWERED LIFT MODEL TESTING FOR GROUND PROXIMITY EFFECTS

October 1963

The effect of ground proximity

on the performance of powered

11ft vehicles has been investi-gated on simple models using two different testing methods. Single and double-jet models representing VTOL configurations

TCEA TN 14

Training Center for Experlmental Aerodynamics

POWERED LIFT MODEL TESTING FOR ROUND PROXIMITY EFFECTS

e ertect or ground proximlty

on the performance of powered

lift vehlcles has been

investi-gated on simple models using two different testing methods.

Single and double-jet mod~ls

representing VTOL configurations

11. TCEA TN 14

I. COLIN, P. E •

11. TCEA TN 14

a stationary plate immersed in the.flow was

used to repre&ent the ground and on a special

rig allowing the models to be moved over a

fixed ground plate. The 11ft and centre of pressure locatlon have been determlned wlth both technlques for various model heights above the ground-plate over a range of momentum

coefficlents. The results obtained with both methods are compared.

IITCEA TN 14

Training Center for Experimental Aerodynamics

POWERED LIFT MODEL TESTING FOR GROUND PROXIMITY EFFECTS

October 1963

The effect of ground proximity

on the performance of powered

11ft veh1cles has been 1nvestl-gated on s1mple models using

two different testing methods.

Single and double-Jet models represent1ng VTOL configurat10ns

TCEA TN 14

Training Center for Experimental

Aerodynam1cs

POWERED LIFT MODEL TESTING FOR GROUND PROXIMITY EFFECTS

October 1963

The ettect

ot

ground prox1m1ty

on the performance of powered

11ft veh1cles has been 1nvesti-gated on simple models us1ng

two different testing methods.

Single and double-Jet mod~ls

representing VTOL configurat10ns

I. COLIN, P.E.

II. TCEA TN 14

I . COLIN, P.E.

II. TCEA TN 14

and an a1r-cushion model w1th per1pheral jet

have been tested both in the wind tunnel where

a stationary plate immersed in the. flow was

used to represent the ground and on a special rig allow1ng the models to be moved over a f1xed ground plate. The 11ft and centre of pressure location have been determ1ned w1th both techn1ques for various model heights above the ground-plate over a range of momentum

coefflclents. The results obta1ned with both

methods are compared.

and an alr-cush10n model w1th peripheral jet

have been tested both in the w1nd tunnel where

a statlonary plate immersed 1n the flow was

used to represent the ground and on a spec1al rig allowing the models to be moved over a f1xed ground plate. The 11ft and centre of pressure 10cat10n have been determined with both technlques for various model he1ghts above the ground-plate over a range of momentum

coefficients. The results obta1ned w1th both

POWERED LIFT MODEL TESTING FOR GROUND PROX1M1TY EFFECTS

October 1963

The effect of ground proxlmity

on the performance of powered

l i f t vehicles has been lnvest!

-gated on slmple models using

two different testlng methods.

Single and double-jet models

representing VTOL conf~guratlons

TCEA TN

14

Training Center for Experimental

Aerodynamic5

POWERED LIFT MODEL TESTING FOR GROUND PROXIMITY EFFECTS

October 1963

The effect of ground proxlmlty

on the performance of powered

lift vehlo1es has been investl-gated on 51mple models uslng

two different test1ng methods.

Single and double-jet models representing VTOL oonfiguratlons

11. TCEA TN 14

I . COLIN, P.E.

11. TCEA TN

14

a statlonary plate 1mmersed in the flow was

used to repre&ent the ground and on a special rig allow1ng the models to be moved over a fixed ground plate. The lift and centre of pressure location have been determined with both techniques for various model heights above the ground-plate over a range of momentum

coefflcients. The results obtained with both

methods are compared.

and an alr-cushion model with peripheral jet have been tested both in the wind tunnel where

a stationary plate immersed in the flow was

used to represent the ground and on a special rig allowlng the models to be moved over a fixed ground plate. The l i f t and centre of pressure location have been determined with both techniques for various model heights above the ground-plate over a range of momentum

coefflcients. The results obtained with both