AIAA-98-3109

THRUST CORRECTION ON JET ENGINESIN SEA LEVEL TEST FACILITY

Ricardo Mareca RiosManuel Rodriguez Martin

Manuel Pablo Gonzalez San SegundoBelen Minguez Aguilar

Industria de Turbo Propulsores, S.A.,Madrid, Spain

AbstractWhen testing an engine on an outdoor stand, under

zero wind conditions, the measured thrust, corrected forinstrumented error, would be the true engine thrust.However not all the times it is possible to have this kind ofinstallation. The act of bringing the engine into anenclosed test bed has an effect on measured thrust. Thereare a lot of test cells around the world dedicated to Pass-Off engine which apply a scaling factor on the thrustmeasurement against the master test cell for cross-calibration purposes without knowing exactly the sourcesof such factor. In other cases some of these sources areconsidered of second order against the inlet momentumdrag which is typically the main one. All these effects socalled 'test cell factors' will be discussed in detail from thetheoretical point of view.

The momentum equation is particularized for steadystate conditions step by step. In most of the cases onlythree factors are taking into account: momentum drag,cradle drag and suction drag, however there are otherfactors appearing in the momentum equation that are notconsidered and could be of the same order as the suctiondrag.

An easy way to derive each of these factors arediscussed through the paper based on a control volume inthe secondary flow of the test cell applied from the inlet ofthe test cell to the middle of the engine hi front of thecradle.

In order to obtain test cell and engine data, a detailedinformation of the instrumentation required (anemometers,

Copyright © 1998 by Industria de Turbo Propulsores.Published by the american Institute of Aeronautics andAstronautics, Inc., with permission.

pressures and temperatures), the land and minimumnumber of tests that should be performed will be provided.

Additionally, test information will be presented aboutthe following topics.

- Stream tube by anemometers to derive inletmomentum drag.

- Cradle drag by anemometers, drag coefficient andmeasured area of the stand and mounting parts.

- Suction drag on the nozzle by static pressures.- Nozzle area variation effect in the case of engine

with afterburner.- Final correction factor as a function of the engine

corrected mass flow.The influence of the stoneguard in the thrust

correction is very important due to this effect does notappear directly in the gross thrust equation. Thiscorrection depends on the position and kind of pressure isused : ambient pressure or total pressure hi the bellmouth.

AAjnA9

CddsD

FgFm

Fvi

Nomenclature

Axial projection of an area elementCross sectional area of the engineEngine Area at the nozzle exitCross sectional area of the test bedDrag coefficientArea elementExternal Pressure DragAdditive DragCradle DragGross ThrustMeasured Thrust from load cellStoneguard DragSuction DragViscous ForceX-direction

American Institute of Aeronautics and Astronautics

P Static pressurePNOZZLE Static pressure on the external surface of

the nozzlePt Total pressuret Timeu Vector velocityV X-component of velocityW Engine mass flowp Density

Subscriptse Station at the inlet of the engineent Entraimentexhaust Back end surface of the enginef Venturi section of the bellmouthfree Without Stoneguarde-s Surface from the inlet to the exit of the

engine(exterior) Surface exterior of the enginei Cradle partsinlet Front end surface of the engine(interior) Surface interior of the enginemiddle Middle surface of the engineo Station at the plane of anemometersref Reference pressures Station at the exhaust of the enginestoneguard With stoneguardx X-component

Introduction

Sea level test cell, despite their limitations onenvironmental control fulfill an important role in assessingengine performance. The universally accepted datum is anoutdoor test stand, which when utilised in condition ofzero wind, measures gross thrust directly. The act ofenclosing the engine (Figure 1) and directing the airflowthrough the cell brings with it a number the effects thatrequire a number of corrections to the measured value ofthrust to obtain an uninstalled gross thrust.

Test cell influence factors are dependent upon theconfiguration of the cell, the airflow demand of the engine,positioning of the engine in the cell, stoneguard in thebellmouth, etc.

The ejector action of the jet stream from the engineexhaust as it enters the exhaust collector (detuner) inducesa flow of secondary air through the cell. The exhaustcollector diameter and its position relative to the enginenozzle affects the total amount of secondary air flow andthe local static pressure at the plane of the nozzle. Thecollector entrance is normally sized for minimum airflowconsistent with the engine/cell cooling requirements,allowable cell depression and minimum cell velocity.

This secondary flow gives rise to aerodynamiceffects which cause forces to act on the thrust frame whichgive rise to corrections in the measurement of thrust. Themain effects are:

• Inlet Momentum Drag (analogous to an inletmomentum effect in flight). This represents a dragforce due to the air entering in the bellmouth fromthe forward direction.

• Tailpipe Pressure Correction. With the enginetailpipe in close proximity to the exhaust collector,the secondary air must accelerate to pass throughthe annular gap. This accelerated flow causes thestatic pressure at the nozzle exit to be depressedrelative to the free stream static. The nozzleexhausts into a lower pressure area and generatesmore thrust.

• Bellmouth Form Drag. The thrust contribution ofthe bellmouth, which is usually attached to thethrust stand, is directly credited to the engine.

• Thrust Stand Form Drag. Secondary airflow alsoimpinges on the frontal area of the thrust stand andcreates an additional drag.

Buoyancy drag on the exhaust nozzle, on thebellmouth and skin friction drag can be quite small bykeeping the secondary airflow low.

The total avoidance of static pressure depression atthe engine nozzle requires large spacing between thenozzle and the capture the efflux, but secondary airflowincreases rapidly as collector area increases. Therefore,location of the engine relative to the exhaust tube is acompromise to obtain proper operation with minimalcorrections.

The Thrust Equation

To better understand all the forces that need to behandled in an enclosed sea-level cell, a momentumbalance can be drawn around the engine as shown on thefigure 2.

Figure 1

American Institute of Aeronautics and Astronautics

Vo —> Fstoneguard

Figure 2The momentum equation for a control volume may

be written in the form:Pressure force + viscous force

Rate of accumulation of momentum + Rate ofconvection momentum through boundaries

Each of these terms may be represented by anappropriate area or volume integral, but for simplicity ofpresentation, the viscous force will be written simply asFv. The above word equation can then be written:

^\

~\ J P * ds + Fv = — J J J pu • dv + jj puu • ds

It should be noted here that the vector area element,ds , is defined to be positive when directed outwardly.Thus, the pressure force from the surface onto the fluidmust have the negative sign identified with it. An engineoperating in steady-state conditions will be considered (or"quasi-steady-state"), so the integral denoting thevolumetric accumulation of momentum with time will bezero.

The thrust equation may then be written:

Fv - J J p • ds = J J pww • ds (1)

The above equation can then be applied to thecontrol volume defined in the figure 2.

The axial component of force is desired only, so dAis defined as the axial projection of an area element.

dA = ds i

Thus, applying equation (1) we obtain:

~Fvxe-s(mterior) + (Po ~ o ~ (Ps ~ Pref) ' As ~

- - J (P - pref) 'dA- J (P - Prefy dA - Fstoneguard =

o e(interior)

= -.W'V0 + W'Vs (2)

For simplicity in applying the equation, conditionsare assumed one-dimensional at entrance and exit.

The measured thrust from load cell is the net forceresulting from all pressure and viscous stresses in thesurfaces wetted by fluid passing through the engine or byfluid impinging on the thrust stand and on the outsideengine casing. Besides, the thrust, F, is defined as being

positive in the negative x-direction, so that the followingexpression is obtained for the thrust measured:

-Fm=e(irt erior) e(exterior)

wc e-s (irk erior) """Aa e-s(exterior)

It is assumed in the last expression that the bellmouthis attached to the thrust stand.

It has been agreed to define the gross thrust test bedcorrected as the thrust that would be measured in anoutdoor test stand, in conditions of zero wind, in thisconditions, the external force resulting from pressure andviscous stresses is zero.

With P0 = Pref and Pexterior = Pref equations (2) and (3)yield:

\(P -e(int erior)

' dA-

^stoneguard = " * "s vv

")' ""• ~*~ ̂ e-s(mt erior)e(int erior)

Combining equations (4) and (5):p _ p -W.Vrm rg " ys - Pr

So in an outdoor stand the measured thrust is directlythe gross thrust.

Then, the gross thrust can be written for an enginetested in enclosed test bed using equations (2) and (3),replacing Pref = P0 (cell pressure), if the cell velocity islower than 15 m/s, the error for assuming cell pressureequal to total pressure of the airflow coming into theengine, will be less than 0.1%.

(7)e(exterior)

Interpretation Of The Terms Appearing In TheThrust Equation

hi the equation (7) appears directly the measuredthrust from load cell, and other terms which will be

American Institute of Aeronautics and Astronautics

analysed in this section how to take in account its. Also, itwill be assessed about the weight of this terms in the totalresult from equation (7).• W-VO Inlet Momentum Drag:

This term (Inlet Momentum Drag) is due to theapproach velocity of the airflow. W is the engine massflow and V0 is the average velocity inside the stream tubewhich is captured by the engine.

Since the engine mass flow is an enginecharacteristic, the approach velocity determines theimportance of the Inlet Momentum Drag. This velocitydepends on the cell configuration. Normally, it is expectedapproach velocity values lower than 15 m/s, that meansthrust correction by Inlet Momentum Drag lower than 1%,for a typical engine.

Unless the size of the test bed is very large comparedwith the engine, this effect has to be taken into account inthe thrust correction.• Fcradle Cradle Drag:

The Cradle Drag is caused by the secondary airflowimpinging on those elements form the thrust stand andengine mountings. It could be estimated by the followingexpression:

-1. . V rw . A .v2^cradle ~ "T* P * 2-ii i * ' i

Where Cd; is the drag coefficient and Aj is the frontalmeasured area of the stand and mounting parts. V; is thevelocity ahead of the stand part.

The value of this term depends of the standconfiguration and, again, it is related with the velocityapproach. For typical velocity approach and standconfiguration in enclosed test bed, this term represents lessthan 1% of the measured thrust, and normally has to betaken into account in the thrust correction.

• FVX e-s (exterior) External Viscous Drag:This term is the force resulting from viscous forces

acting on the external engine parts by the inducedsecondary airflow. However, the values of the skin frictiondrag are usually negligible and can therefore be ignored.

.V

P - P ) • dA External Pressure Drag:e(exterior)

It is common practice to break the External PressureDrag into three components, Df, the drag associated withthe "front end" of the engine, Dm, the drag associated withthe "middle" of the engine, and, Db, the drag associatedwith the "back end" of the engine (see figure 3).

1 Dinlet Dmiddle Dexhaust

Figure 3

•Dm= \(P-P0)-dAmiddle

The argument of this integral is quite small and thevalue of Dm is negligible and can therefore be ignored.

•Df= \(P-P0)-dAinlet

Secondary airflow could give rise to horizontalpressure gradients along the engine behind the bellmouth.By keeping the secondary airflow low, the static pressureon the "front end" of the engine will be nearly thereference pressure, thus requiring no correction.Nevertheless, below the value of this term will beestimated in conjunction with another term of thrustcorrection.

• Db= \(P-P0)-dAexhaust

This term is named Suction Drag (Db = Fsuction) and isdue to the lower static pressure around the external surfaceof the engine final nozzle compared to the static referencepressure. Due to the close proximity of the nozzle exit tothe collector inlet, the secondary airflow accelerates pastthe exhaust nozzle into the mixing tube, and sets up astatic pressure gradient along the external surface of thenozzle. The generated net force can be calculated byintegrating the surface pressure along the nozzle andresolving the x-component. Nevertheless, this correctioncan be kept small and it can be approach supposingconstant depression along the nozzle:

^suction = ("nozzle ~ "o)* (-^engine ~ -^9)

Pnozzie is the measured static pressure on the externalsurface of the nozzle and Aengine is the cross section of theengine.

e• -I (P - P0 ) • dA Additive Drag

oTo estimate the Additive Drag (Dative), the axial

component of the momentum equation is considered for

American Institute of Aeronautics and Astronautics

the fluid flowing external to the engine between sections"o" and "e". For simplicity, the external pressure dragassociated with the "front end" of the engine, betweensection "e" and "f", is added. The momentum balance canbe drawn as shown on the figure 4. An uniform conditionsof the airflow are assumed at sections "o" and "f and theviscous stresses are ignored on the external surface of thebellmouth.

'Vf,P,,Wen,

Figure 4

The momentum balance yields (Pref = P0)e f

-(Pf-P0)-Af + \(P-P0)'dA +o e(exterior)

= -Went-V0 + Went.Vf (8)

where Went = WceU - Wengine and Af = AceirAengine

The additive and friction drags can be defined as:

e f-$(P-P0).dA- l(P-P0)-dA = DaMttve+Df

o e (exterior)

Substituting in the equation (8):

^additive + Df = -(Pf - P0) • Af + Went • V0 - Went ' Vf

Considering the continuity equation and assumingthat total pressure remain constant between sections "o"and "f.

9-(Acell-A0)-V0 = 'ent

Pf+ .p.y

Substituting and comparing with Inlet MomentumDrag:

D additive + D1 'entW" •engine 'engine

1 \ V f V1_1. _ /+±2_Vf .

Now the value of this rate will be estimated for ITPtest bed when a jet engine with afterburner is tested.

Typical values are:

VJ_V0

wt

= 0.8

ent _= 3.8"engineD additive + DJ

W • • V"engine 'o= -0.095

This means that the thrust correction due to theAdditive Drag and Front External Pressure Drag for ITPtest bed with the above parameter, represents less than the10% of the Inlet Momentum Drag Correction and can bethe same order as other terms of correction. Nevertheless,the above procedure is easy and recommended to dobefore rejecting the influence of this term.

The gross thrust for engine tested in enclosed testbed can be finally written as follows:

= Fm Vo + Fcradle + Fsuction (Daddi tive

Influence Of The Stoneguard InThe Thrust Correction

In the equation of the thrust correction derived in lastsection, it was noticed that the Stoneguard Dragintroduced at the beginning does not appear directly in thegross thrust. The raison is that term appears in themomentum balance of the total control volume and also itis directly credited to the measured thrust from load cell(assuming that the bellmouth is attached to the thruststand).

Considering an engine running in an outdoor teststand without and with Stoneguard. The thrust valuesmeasured in each case will be different:

(P s-P0)] f r e e

m(stoneguard) — [W Stoneguard

The Thrust is function of the inlet total pressure andthe discharging pressure. In the free conditions bothpressures are the same and equal to P0. In the secondsituation, the engine is working at the same dischargingpressure, P0, but the total pressure at the entry is lower dueto the pressure loss through the screen of the Stoneguard.

The Thrust is a function of the inlet total pressureand the discharging pressure. In the free conditions bothpressures are the same and equal to P0. In the secondsituation, the engine is working approximately at the samedischarging pressure, P0, but the total pressure at the entry

American Institute of Aeronautics and Astronautics

is lower due to the pressure loss through the screen of thestoneguard.

VELOCITY FIELD AT TEST CELL SECTION (NL4DLE)

e(irti.erior)s

~~ J \*~*exterior)

Assuming that in both cases the values of thefunction 'K' are similar, the thrust with stoneguard andinlet total pressure Pt< P0 can be extrapolated to the thrustthat is obtained with the inlet total pressure equal to P0.

r (fo>*o) * m(stoneguard) vM»"o/ ^ stoneguard

It is better to forget the Stoneguard Drag and to referthe thrust at the inlet measured total pressure downstreamof the stoneguard screen.

Test To Support Thrust Correction

As it was summarised above, four different sourcesof correction have been identified and each of these haveto be evaluated by means of different tests. Theinstrumentation required is at least as follows:

- 10 Anemometers (e.g. mini-ducted fan).- Calibrated Bellmouth to measure engine mass flow.- Static pressure around the nozzle.

The following test procedure is followed in ourcompany to perform the aerodynamic calibration of thetest cell.

• Inlet Momentum Drag Correction: In order tocompute the inlet momentum drag 5 short dry and reheat(for military engines) performance curves have to beperformed for five different positions of the anemometersin the spanwise direction, the center line and roughly ±lmand ±2m of the center line. Approximately an anemometereach 0.5m is recommended in the vertical direction. Thedistance from the stoneguard to these anemometers will bearound 8 diameters of the engine inlet.

Using this procedure the streamtube at this positionof the test cell can be obtained for different engine speed,(see figures 5,6 and 7)

Figure 5VELOCITY HELD AT TEST CELL SECTION <NLc75%}

Figure 6VELOCITY HELD AT TEST CELL SECTION |NL=90%)

Figure 7

Knowing the engine mass flow at different speedsand the anemometer position information is possible toderive the mean velocity in the streamtube and then theinlet momentum drag. An example of the velocity field isshown in the figure 8

6American Institute of Aeronautics and Astronautics

VELOCITY FIELD AT TEST CELL SECTION

• ANEMOA1• ANEMOA2

ANEMOA3v ANEMOA4x ANEMOA5• ANEMOA6* ANEMOA7-ANEMOA8- ANEMOA9-, ANEMOA10

——

it**

% ~i

t t

• J =~l-l-ii —

;.»'•• *

•A'Jt-i$**

4**

* « *I

5s s%

Engine Corrected Mass Flow (kg/s*K«0.5/KPa)

Figure 8In the above figure an additional effect is present,

due to this information is derived from an engine withafterburner system, when the throat nozzle area isincreased, the detuner cannot 'swallow' more airflowbecause of the area between the exhaust driven ejector andthe detuner area is lower than dry conditions, so thevelocity of the test cell begins to reduce (see figure 9).

TEST CELL VELOCITY AT DIFFERENT NOZZLE AREAS

—

* ANEMOA1* ANEMOA2

ANEMOA3N ANEMOA4x ANEMOA5• ANEMOA6- ANEMOA7. ANEMOAS- ANEMOA9•- ANEMOA10

—

iI

-

*

1

«'

tl ——>i 5

j

Athroat/Athroatdry (%)

Figure 9Finally,

INLET MOMENTUM DRAG

Engine Corrected Mass FLow (kg/s K«0.5/KPa)

Figure 10

• Cradle Drag Correction: It will be made as neededshort dry and reheat to obtain velocity measurements in allthe positions specified in the cradle. Anemometers must be

kept aligned with the flow, and placed at least threediameters ahead of the object, if possible. To evaluate theCd as a function of the shape, a practical information dragand hydrodynamic resistance can be found in reference 5.

CRADLE DRAG FOR DIFFERENT NOZZLE AREAS

Engine Corrected Mass Flow (kg/s K»0.5/KPa)

Figure 11• Suction drag correction: Static pressure probes have tobe mounted at the three azimutal positions of the nozzle(top, bottom and left or right) and at four positions in thestreamwise direction.

SUCTION DRAG FOR DIFFERENT NOZZLE AREAS

Suct

ion

Drag

/Max

Dry

Thr

ust (

%) • A8=DRY

*A8=120%*A8=130%

xA8=150%oA8=160%

m , m

Engine Corrected Mass Flow (kg/s K«0.5/ KPa)

Figure 12

• Additive drag correction: This term can be derivedfrom the above formulation.

ADDITIVE AND FICTION DRAG

Engine Corrected Mass FLow (kg/s K«0.5/ KPa)

Figure 13

American Institute of Aeronautics and Astronautics

• Final Thrust Correction Factors: Adding all the aboveterms, the final thrust correction factor can be derived.

FINAL THRUST CORRECTION FACTORS

a Inlet Momentum Drag• Cradle DragA Suction Dragx Additive Drag.Final Drag

Engine Corrected Mass Flow (kg/s K«0.5/KPa)

Figure 14

Conclusions

The terms which are necessary to take into accountfor thrust correction are:

• Inlet Momentum Drag• Cradle Drag• Suction Drag• Additive Drag + Inlet External Pressure DragThe final correction seems to be very small, just 1%

of the max dry thrust, but if the maximum tolerancebetween the test cell to be cross-calibrated and the mastertest cell is taken into account (±0.5% normally), theaerodynamic calibration have an important weight on that.

The correction by stoneguard depends if you areusing the ambient pressure or the measured total pressurein the bellmouth to refer the thrust. That means, If we wantto derive the actual gross thrust in the test bed [W-VS +(PS-P0)], we will not have to include the Stoneguard Drag,but, in a Pass-Off test to deduce the gross thrust referred tothe ambient or cell pressure, we will have to take inaccount the Stoneguard Drag and correct the measuredthrust with this effect, In fact, in a Pass-Off test we have totake into account other agreed considerations as:corrections by the Fuel Lower Heating Value, etc..

References

(1) P.F. Ashwood "Introduction and General Survey",AGARD-LS-132, Operation and PerformanceMeasurement on Engines in Sea-Level Test Facilites.

(2) D.M. Rudnitski "Performance Derivation of Turbojetsand Turbofans from Tests in Sea-Level Test Cells",

AGARD-LS-132, "Operation and PerformanceMeasurement on Engines in Sea-Level Test Facilites".

(3) Gordon Oates Chapter 6: "Component Performance",AFAPL-TR-78-52, The Aerothermodynamics of AircraftGas Turbine Engines

(4) R. Mareca ITP, "Test Cell No. II Thrust CorrectionFactor", P9001-640-CC3-F-007, ITP Technical Report.

(5) Sighard F. Hoerner "Fluid-Dynamic Drag", page 3-17.

(6) M. P. G. San Segundo, "Thrust Correction on jetengines", GOl.OO.BCO.OOl.Rev.O (A), ITP TechnicalReport.

8American Institute of Aeronautics and Astronautics