Voglewede, T J

English

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t;. vOpy 
NO. I-W 
NATIONAL ADVISORY COMMITTEE FOR AERONAunCS 
ORIGINALLY I$SlED 
May 1944 as 
Confidential Bulletin L4E25 
A PROPOSED METHOD OF MEASURING ENG mE CRfl-RGE 
AIR FLOW IN FLIGHT 
By T. J. Voglewede 
Langley Memorial Aero autical Laboratory 
LaTlgley F. p.l , 'l 3 . 
WASHINGTON 
NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of 
advance research results to an authorized group requiring them for the war effort. They were pre-
viously held under a security status but are now unclassified. Some of these reports were not tech-
nically edited. All have been reproduced without change in order to expedite general distribution. 
L - 200 
https://ntrs.nasa.gov/search.jsp?R=19930093237 2020-06-17T01:45:11+00:00Z
l 
NACA CB No. L4E25 
NATIQTAL ADVISOR CmmITT'::E FO-q AERO:JA -TIC '3 
CONF':;:DENTIAL BULLETIN· 
A PROPCSt:;D METHOD O? :I[F.ASU~P\·(} '-:FG II ~ CHARGE ' 
AIR FLOT,V IN FLIGHT 
Ey T. J . Voglewede 
A method is outlined for determining in flight the 
weight rate of air flow to the enfine equipped with a 
Bendix - Stro:n.berg injection- t:rpe carburetor . The method 
has the advantages that no additional equipMent need be 
inserted in the charge - air system and only a few simple 
measur6~ents are neceseary . The analysis of an air - box 
calibration of the carburetor to be used in interpreting 
the flight meaE'urement~ is 3hown by an examp le. 
ITTRODUCTION 
~~lch diffic Ilty haE' beEn experienced i n measuring 
the weifht rate 0 f air flo v to the engine in flight in 
order to determin~ fuel - air ratio or to investigate other 
phases of enrjne op e r at ion . When i t is possible to 
i~8ert an. orifice plate or a Venturi in the induction 
sy~tem , ' additional J0sges are introduced in the system . 
Pltot - static trav e rs es with a reasonable numbe r of tubes 
do not usual l y dpfine the velocity di~tribution with Buf -
f:i.cient aCCl.l..racy and, in S OJ1lE' ca Ses , are largely affected 
by turbulence . It has been found that , on airplanes ; 
equip=,ed with the Bend5x - '3t omberG inj e c t ion- type car- . 
buretor , charge air flow can be d~terMin~d accurately 
within certa:;,n lir:its v:ith a few relatively simple rreas -
urements . The method of i nterpreting these measurements 
is outlined herein . 
METI10D 
The air-mea~n r ing unit of the Be"ldix - Stromberg 
injection- type carburetor coneists of a double Venfuri 
l 
I 
2 C ONF IDZNT J AL NACA CB .Io . L4E25 
syste:n as S1oV\,n in fi gnre 1 . The presflure dtfference 
developed by this syste:r:n. ~s . ..r.edu~ec1 by a den~ity­
compensatinR.: system to a value called the cOI!1pensated 
metering pres~ure . T~e co~pensated ~eterine pressure , 
acting tbroufh an arrangeme.nt .QJ .?- diapbragm and a poppe t 
valve , controls the pressure drop across the fuel jets . 
The method of determining a i r flow that is outlined here , 
however , is indegen~ent o f the action of tbe density-
compensatine; sys ,em and the fuel - me terinr system . 
In a compressible fluid , suer as ajr , the relation-
ship between wei gh t rate of flow and the pres~ure dif -
ference developed by a Venturi system may be expressed as 
( 1 ) 
where 
W weight rate of flow of air, pounds per sec nd 
g acceleration of gravity , feet per second pe r 
second 
effective a rea o f boost Venturi throat , square 
feet 
density at entrance total pressure and temp era-
ture , slues Y)er cubic foot 
absolute ent ranc e total pressure, pounds per 
square foot 
absolute ptatic p rescure at boo Rt Venturi t~roat , 
pouncs pe r square foot 
For a . single Ventur the function o f t h e pres ure ratio · 
f(~~) can be expre s seel.. rna therrati c a~lY . . ,Fo r the system 
under consideration , h wever , tbe relatlye restriction.s 
oi'fe r ed by tbe boost Venturi and.·tbe :r:a:!.n Venturi are 
not constant, so that tbe proportion of t h e tota~ air 
flow that passes through t~e b oost Venturt t~ no~ con-
stant and the p ressure difference developed by the boost 
Venturi is a different funct ion o f the total air flow . 
It is t:herefore ne ce s sary to r 'e ClOl't to a calibrat ion of 
the Venturi ~ysten over the range of pressure ratio that 
will be encount ~ red in fli~ht . 
" . 
CONFIDEl'TTIAL 
Figure 2 shows a typical air-flow calibration of a 
13endi::-:::-Stromberg :"njection-type carburetor pIotted 
according to the generalized equation (1). Data p lotted 
In figu1'e 2 inclnde ai.r-box tests at ordinary room tem-
pe rature at the inlet obtained from Bendix Aviation Corp . 
Stromberg Avj a tion Carbure~or Dept . and vVright Field 
air - box testR both at room teITpe~ature and at approxi -
r1ately _40 0 F at the inlet (reference 1 ). Analyzed in 
this way , the characteristics of the Venturi system can 
be simply dp,term .:ned becau ~e only the ran g e o f pre s sure 
ratio need be inve~tigated; that is , it is unnecessary to 
simulate temperatures or all possible comb inations of 
density and air flow . F1gure 3 ilJustrates better than 
fi~ure 2 t~at t~e calibration is irdependent of tempera -
ture , when the data are analyzed in this manner . In 
figure 3 ( a ) some of the data included in fi gure 2 are 
shown in the u~ual presentation as the pressure differ -
ence developed by the Venturi system for a range of 
entrance density at a constant air flow . For a given 
entrance density, the p ressure difference varies with the 
temperature . In fIgure 3(b) the sarre data aI'e plotted 
in the manner suggested by equation (1) and figure 2 . 
The te;~erature effect disappears , and the relation 
between pressure rat i o and the var iable W2/2g2 PI H1 is 
shown to be independent of t~e temperature . The charac-
teristics of the Venturi system can therefore be est~b­
lished by calibration tests at any desired temperature 
and can be specified b,- 'a ~ingle curve of the air-flow 
parameter W2/2g2 PIHl throughout the de~ired range of 
pressure ratio . 
ACCURACY 
The eXDcrimenta l deviation of the ~easurement of a ir 
flow by the r~ethod outlined herein i 8 within ± l~ percent 
for values of pressure ratio g reater than 0 . 65 . This 
deviation is shown in f~~ure 4 , vhere ~he percentage 
devjation of the facto r W2/2g2pl~1 f~om the faired 
curve of fi gure 2 is plotted again2t the pressure ratio . 
~xcept at high pressure ratios for which the lew pres -
sure differences coul d not be read from the calibration 
curves with sufficient accuracy , scat t er of the data in 
the square of the a ir flow is of the order of ~3 pe rcent, 
CO TF I DENTIAL 
-- .---- -
4 COI\'7IDENTI;'.L 
or in the air flov is of t he order of ±l~ percent . This 
scatter i8 the same order of accur a cy usually achieved 
in the deter:r.1ination of the '&ir flow with th$ calibrat ing 
equipment u.sed. 
At press1re ratios less than 0 . 65 , the ~catter of 
the data increases until at a pressure rat io of ap roxi -
mately 0 . 40 the ,TerJ.turi system becolJ'es worthle~s as a 
measuring unit . Fifure 2 shows that the minimu.m pres -
sure ratio obta:1.nable ' s app roxil11at.e ly 0 . 40. Because 
theory indicates that pressure ratios less than 0 . 528 can 
be at.tained only in supersoriic flow in an increasing 
area, the point of measurement in the boost Venturi is 
probably so~ewhat dowTIstrew. ' of the effective throat of 
the V€nturi. Theory further indicates that , for the 
relative areas necessary to satisfy the fore Go ing condi -
tions , a pressure r et io of approxir:ately 0.65 will be 
attained at the point of mea~uremBnt when the speed of 
sound is just reached at the thr08.t of the boost Venturi. 
The definite chan~e in the character of carburetor cali -
brat:1.on below a pre ssu~e ratio of 0 . 65 corre ~ponds 
to the transition from subsonic to supersonic flow in 
the Venturi . It is evident from figure 4 that the flow 
in h e boost Tenturi cannot be definitely controlle~ 
under thi s condition and the accuracy of the unit as an 
air- flow measu ring device is lessened . 
According to refe~ence 2 , l a r g e changes in pre~sure 
distribution at the entrance to the carbureto r may cause 
a s 'UC" R S 5-~e rcen t error in the mea surement of ai r 
flo! by the )'J eth:)d oV.t11ned .:.'lerein . In order to take 
into account any Dosrible varia.tion of this nature , the 
carburetor is Lsually calibrated with the duct in p lace 
to si~ulate flight conditio~s . Smal l changes jn duct 
design wi ll no t appreciably affe ct the c~lib ration,and 
carburetor duct in~tallations that have effictent pre s -
s urE: r e c very and therefo re ne2.r·l y uniform pres~ure 
di~trtbut ion are almo8t equivalent to an ideal inlet as 
far as the carburetor calibratio'n is concerned. 
FLIGHT TfSA SliR£' rE~iTS 
The neces qarv me a surements to de te~mine air flow in 
fli~ht con~ist o f u a~ B o lute entrance press ure , total 
. I 
I 
J 
l;ACA CB No . L4E25 CONFIDENT!AL 5 
entrance temperature , and boost Venturi s u ction. Taps 
are ueually provided for measurements of these ~ressures . 
In addltion to tbe usva l carburetor- air tAmpe rature 
installatjon , an altimeter fo r absolute entrance pressure 
and an airspeed reeter for the ~re~su~e d i fference are 
the onl y equi:)ment needed. 
FUEL- FLOW MEASUREMENTS 
If an indication of f~el - air ratio is desired , fuel 
flow can be det ermined indirectly with t,...1-l'e Be:1dix -
Stromberg injection- type carbur etor by measure~ent of 
the cornpensa ted air - mete r ing p r e ssure , i na slTl1.lch a s both 
the fuel - metering pre~sure and the effective fuel - meter i ng 
jet a r ea are dependent upon t his pres~ure difference . 
The measurement Ie inadequate at low fuel flows when the 
t~rott l e is nea r ly closed because an additional restric -
tiOl1 in the form o~ a variable - area ialin€, jet is in ,s erted 
in the fuel -mea~urlng unit . Flight measurements , how -
ever , do not ordinari l y extend into this region . 
A flow - bench caJibration of the carburetor is neces -
sary for tran~ lating the ~easurements of air - metering 
pre~8ure into fuel flow . If adequate fuel pressure 
indicates that no vapor i q present in the fuel system, 
this measure~ent can be expected to give results as 
accurate a~ the ordinary flo wmeter . For greater accuracy , 
a measurement of fne] tem erature j . s desirable to take 
into account a c hang e in fuel den~ity from that used in 
calibr~tion . The error in fuel flow due to change in 
temperatur e is of the ord~ r of 3 percent for 1000 F . 
C ONCLUDING P..EJAt~K~ 
On the bapis of t~e calib r ation t e st deecribed herein , 
it lE' believed that an accura::;e determination of the weight 
rate of air flow to the engine iliay be obtained in fli ght on 
airpla?:les equipped with a Bendix- Stromberg injection- type 
c a r bur etor , wi t bO l1t the ne c es s1 ty for · nsertJon ;Yf an 
addi t ion,al meaAurinp- ctevlc e In t h e ic bar:g'e - al '1" du~ t s. 
Lang l ey ,,1emorlal Aerona lti cal I ,ab oratory, 
Na t ional Adv isory Co~~lttee for Aeronautics , 
Lang l ey Field , Va . 
CO/F I DENT I AL 
~--~---------- ---------------~,--------- - - ---
6 CO JF IDENTIAL NACA CB do . L4E25 
nEFERENC~S 
1. Vanden- Reuvel , ' T. R.: PD18Al Carburetor from the 
V- 1650-3 Packard-Built Engine of the Horth Ar1erican . 
P- 51B Airplane. Memo . Rep. No. DJG-57-430- 652 , 
Materiel Command Eng. Div., Army. Air Porces , 
Sept. 3, 1943. 
2 . Kinghorn, Ge orge F.: Effect of Inlet-Air Velocity 
Distribution on the Metering Pressure of an 
Injecti0n-Type Aircraft Carburetor. NACA ARR, 
Hay 1942. 
CO -FIDF.N'rIAL 
~~- ----~ - ~-
~l 
I 
!\A.CA CF No . L4E2S 
Boost Venttlri 
CONFIDENTIAL 
CO FIDEN TIAL 
NATIONAL ADVISORY 
COMMITTEE FOR AERONAUTICS 
Figure 1.- Schematic drawing of air-measuring unit 
of Bendix-Stromberg inject ion-type carburetor. 
Fig. 1 
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NACA CB No. L4E25 CONFIDENTIAL Fig. 3 
CONFIDENTIAL 
Figure 3.- Corr,parison of methods of presentation of air-fl·ow J 
calibrations for a typical Bendix-Stromberg injection-type 
carburetor. W = 12,OOC pounds per hour. 
NACA CB No. L4E25 CONFIDENT IAL 
NATIONAL ADVISORY 
COMMITTEE FOR AERONAUTICS. 
Fig. 4. 
CONFIDENTIAL 
Figure 4.- Deviation of air-flow parameter from faired curve 
of figure 2. 


A Proposed Method of Measuring Engine Charge Air Flow in Flight

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