RESEARCH AVIATION FACILITY
BULLETIN NO. 21
National Center for A tm ospheric Research
PO Box 3000•B oulder, CO 80307-3000
Telephone: (303) 497-1036«T elex: 7402061
FAX: (303) 497-1092•O M N E T : (NCAR.RAF)
Revised 10/91
PRESSURE MEASUREMENT FROM NCAR AIRCRAFT
Forward
The R A F Bulletin series is intended to guide scientists in m aking effective use of
N CAR aircraft. We recognize that some o f the topics presented deserve m ore space
than is available here; however, we have tried to make the material m ost useful to
those having little or no experience in the use o f aircraft as an observing system.
W e invite com m ents from those who use our facilities on how we m ight im prove this
presentation.
General
This Bulletin describes the types of pressure measurements and the sensors and
transducers used to make them.
Perform ance specifications and inform ation on
transducer accuracy are also included.
Pressure Measurements
Both static (absolute) and differential pressure measurements are m ade at various
locations on the RAF aircraft.
Table I summarizes the m easurem ents, their
location, and the type of transducer used.
The static pressure measurements are made through a static pressure port, flush
on the side of the fuselage or on the side o f a pitot-static tube.
This port is
connected to either a Rosem ount1 M odel 1201 capacitive-type absolute pressure
transducer or a Rosemount M odel 1501 digital absolute pressure transducer.
The
Rosem ount M odel 120IF series absolute pressure transducer uses a diaphragm -driven
capacitance-m easuring circuit.
The Rosem ount M odel 1501 high-accuracy digital
absolute pressure transducer outputs a pressure-dependent frequency signal.
1
R osem ount Engineering Co., Post Office Box 35129, M inneapolis, M N 55435.
The National Canter fo r Atmospheric Research is Operated by the University Corporation
fo r Atmospheric Research under Sponsorship o f the National Science Foundation
D ifferential pressure measurements are made with either a pitot-static tube or a
pitot tube referenced to a static port.
The differential pressure measurements
are used to determ ine aircraft dynamic pressure and flow angle (angles o f attack
and sideslip o f the aircraft).
Dynamic pressure (qc) is the difference between the
total pitot pressure (Pt) and the static pressure (Ps).
The true airspeed of the aircraft (TAS) is calculated from measurem ents o f this
pitot-static difference, the static pressure, and the total air temperature using
Bernoulli's equation.
Flow -angle measurem ents are made with either a Rosemount M odel 858AJ flow-angle
sensor or a nose radome flow-angle pressure-sensing configuration.
In flow-angle
m easurem ents, differential pressures are m easured in the horizontal and vertical
axes, relative to the aircraft, at or near the nose o f the aircraft.
These pressure
differentials are used with the dynamic pressure at the aircraft nose or boom to
determ ine the angles o f attack and sideslip of the aircraft.
The RAF currently uses an electrically deiced flow-angle sensing probe (Rosemount
M odel 858AJ) that can be interchangeably mounted on any o f the aircraft. This flow
angle sensor is hemispheric, capping a cylindrical tube, as shown in Figure 1. The
attack and sideslip angles of the aircraft are determined from the R osem ount M odel
858AJ flow -angle sensor by the following equations:
ATDQC =
/5TTF
3
CR-QQ3
CELTP =
EPF
CR*CJB
Here A D IF (P a, - P a ;) is the differential pressure in millibars across the vertical
axis o f the probe.
BDIF (PP,- PP \ is the differential pressure in millibars across
the horizontal axis o f the probe. QCB is the dynamic pressure at the probe tip. GR
is the norm alized sensitivity coefficient for the Rosemount 858AJ probe.
This
coefficient is 0.079 for Mach numbers below 0.51.
The radom e technique for flow-angle sensing is similar, in principle, to that used
w ith the R osem ount M odel 858AJ probe. The major difference is that the nose of the
airplane itself is used as a probe instead o f the separate flow-angle sensor, as
In-situ calibration techniques are used for each aircraft to
shown in Figure 2.
determ ine
the norm alized sensitivity coefficients.
technique are discussed by Brown2, et al., 1983.
Figure 1 —
Details
of
the
radome
Schematic Representation o f the Rosem ount 858AJ FlowAngle Sensor.
A t present the radome flow-angle sensing configuration is available on the King
A ir (N312D, the Sabreliner (N307D), and the Electra (N308D) aircraft. The King Air
and the Electra radomes are heated and will function well under m ost atmospheric
conditions.
The Sabreliner radome is unheated; thus, the range o f atmospheric
conditions in which it will function properly is som ewhat limited.
(Icing
conditions and liquid water will adversely affect radome perform ance on the
Sabreliner.)
:
Brown, E.N., C.A. Friehe, and D.H. Lenschow, 1983: The use of pressure
fluctu-ations on the nose of aircraft for measuring air motion, J. Clim.
Appl. M eteorol., 22, 171-180.
3
(
Figure 2 —
Sketch of Nose Radom e Flow -Sensing Configuration on the
N CAR Sabreliner and Detail of Pressure Taps in the
Radome.
The R osem ount M odel 122IF series differential pressure transducer is used for
differential pressure measurement for determ ining high-rate winds in the gust
probe dynam ic pressure measurement on RAF aircraft.
Table I: Pressure Measurements
Parameter
Transducer Used
(Rosemount
Model)
Description
Line Length3 Sensor to
Transducer (m)
N307D
N308D
N312D
3.6
1.2
12.2
2.4
3.6
1.5
Static Pressure:
PSW
Static pressure at the wing tip
1201
PSB
Static pressure at the boom
1201
5.4
PSF
Static pressure at the fuselage
1201
4.9
PSFD
Static pressure at the fuselage
(digital)
1501
5.2
PCAB
Static pressure in the aircraft
cabin
1205
Differential Pressure:
QCW
Dynamic pressure at the wing tip
1221
QCB
Dynamic pressure at the boom
1221
5.4
QCF
Dynamic pressure at the fuselage
1221
2.9,
4.9“
3
These are nominal line lengths, all lines are 4.32 mm I.D.;
N307D = Sabreliner; N308D = Electra; N312D = King Air.
4
The high-pressure line length is given first.
4
2.0
QCR
Dynamic pressure at the radome
1221
2.1,
5.54
ADIF
Differential pressure in the
vertical plane on the boom w/Rosemount 858AJ
1221
5.4
BDIF
Differential pressure in the
horizontal plane on the boom w/Rosemount 858AJ
1221
5.4
ADIFR
Differential pressure in the
vertical plane on the radome w/radome
1221
2.1
1.7
0.9,
0 . 63
BDIFR
Differential pressure in the
horizontal plane on the radome w/radome
1221
2.1
1.7
0.8,
0 .63
5
1.7
Correction for Static Pressure Defect
Pressure m easurem ents on board an aircraft are affected by local flow field
distortions, and corrections are made for these pressure defects.
The corrections
are unique to both the aircraft and the position o f the m easurem ent on the aircraft
and are determ ined at RAF by a tower flyby or trailing cone tests.
Extensive
trailing cone calibrations have been com pleted on the RAF aircraft.
This
procedure and uncertainty analysis are discussed by Brown, 1988, N CAR TN 313+STR.
The correction for static pressure error for a given location on a given aircraft
m ay also be determined by flying by an instrumented tower (equipped w ith a
precision barom eter) at various air speeds (varying dynamic pressure, qc).
The
pertinent
m eteorological
data
(ambient
static
pressure,
etc.)
are
recorded
sim ultaneously on the aircraft and on the tower.
Pressure corrections ("PCOR's")
are determ ined from a regression between the pressure differences m easured
betw een the reference and the aircraft, AP, and the dynamic pressure, qc, m easured
on the aircraft.
An exam ple o f a PCOR calculation follows:
PdKQF) =q +q*QF
H ere C, and C2 are coefficients determined from the regression, in this case a
first-order regression. Thus, the corrected static pressure would be:
PSTC = PSF + PXKQEF)
For dynam ic pressure correction, the sign o f the PCOR is reversed as shown below.
PSFC = PSF + PCER
QTC = PHtc - (Pa .i c + KCR) = CJF - FdR
Table II lists the
aircraft.
"PCOR's," as determ ined by tower flyby tests, for each RAF
6
Table II: PCOR Tabulation
(
PCOR Function
Aircraft/Pressure
Measurements
PCOR Method
Sabreliner N307D
Trailing Cone
PSB, Q C B
PSF, PSFD, QCF, O C R
-0.1746 + Q C B *(-0.1988 - Q C B *0.0001275)
-0.8901 + Q C F *(0.046 + QCF*.000075)
Electra N308D
Trailing Cone
0.517 -0.0202 * Q C W
0.366 -0.0182*QCR
PSW, Q C W
PSF, PSFD, O C R
King Air N312D
PSW, Q C W
PSF, PSFD, Q C R
PSF, PSFD, O C R
-0.3112 - 0.0339*QCW
-1.781 + 0.0655 * Q C R - 0.00025 * QCR2
(Radome Gust Probe Package)
-.09356 + 4.47474 *Mach - .145661 * A K R D
Tower Fly-by
Trailing Cone
Transducer Specifications
Perform ance and environmental specifications for the pressure transducers used
on N C A R aircraft are shown in Table III.
This inform ation was obtained from
"Product D ata Sheets" provided by the m anufacturer, Rosemount, Inc.
7
(
Table HI:
Specifications for Rosemount Pressure Transducer Models
1501 Digital
Absolute
(FSP = 1,000 mb)
120IF Absolute
(FSP = 1,034 mb)
Operating
Accuracy
±0.042% FSP
(±0.42 mb)*
±0.30% FSP
(±3.0 mb)**
Refer to Table IV
±0.20% FSP
(±0.35 mb)***
Static Error
The static error
is the root sum
square of the
errors due to
nonlinearity,
repeatability,
hysteresis, &
resolution.
±0.026% FSP
(±0.26 mb)
±0.10% FSP
(±1.0 mb)
±0.10% FSP
±0.10% FSP
(±0.2 mb)
Lons-Term
Stability
Change in output
over period
indicated.
±0.025%FSP
(12 months)
±0.15% FSP
(6 months)
±0.15% FSP
(6 months)
±0.15% FSP
(6 months)
1221
Differential
1332 Differential
(FSP = ±172.4 mb)
(
Response Time
(63% response)
75 milliseconds
15 milliseconds
10 milliseconds
10 milliseconds
Operating
Temperature Range
-55°C to +81°C
-55°C to +71 °C
-55°C to +71 °C
(Electronically
compensated
range)
-18°C to +65 °C
(Electronically
compensated
range)
This is the root sum square error, which includes dynamic error (stated by
Rosem ount as ±0.021% FSP), the static error, and the long-term stability.
The reported operating
on the transducer.
tem perature changes on
errors o f 1.0 to 1.5 mb
tem perature around the
accuracy includes the effect of ambient tem perature
Laboratory tests at RAF indicate that ambient
the order o f 2°C per minute produce static pressure
in the 1201 series transducer. Thus, if the ambient
transducer is controlled, by locating the transducer
8
in the aircraft
improved.
(
***
cabin
for
instance,
the
operating
accuracy
This value is an estimated root sum square error obtained
(stated by Rosemount) caused by various environm ental factors.
includes the static error in the root sum square calculation.
is
greatly
from errors
This value
Table IV:
Differential Pressure Measurements and the Operating Accuracies
of the Rosemount 1221 Transducer Used for Each Measurement
(-55 °C to +71 °C)
A ircraft
Measurement
Transducer
Pressure Range
Used
Accuracy*
King A ir N312D
QCRC
±137.9 mb
±0.27% FSP (±0.4 mb)
ADIFR, BDIFR
±68.9 mb
±51.7 mb
±0.27% FSP (±0.2 mb)
±0.37% FSP (±0.2 mb')
QCRC
±206.8 mb
±275.8 mb
±0.27% FSP (±0.6 mb)
±0.23% FSP (±0.6 mb)
ADIFR, BDIFR
±68.9 mb
±51.7 mb
±0.27% FSP (±0.2 mb)
±0.37% FSP (±0.2 mb)
Sabreliner N307D
and
E lectra N308D
*
T he operating accuracy includes the static accuracy as well as calibration
tolerance and the effect of am bient temperature over the com pensated range
(-5 5 °C to +71 °C)
Error Propagation
Pressure m easurem ent error, either static of differential, will propagate into
param eters derived from those pressure measurements.
Errors propagated to
selected derived parameters are shown in Table V.
For illustration purposes, the
errors shown are those that would result for each 1.0 mb error in the corresponding
m easured pressure.
Details of the m easurem ent uncertainty o f true air speed.
9
(
angle o f attack, and sideslip applicable to RAF aircraft are discussed in Brown,
1991, N C A R TN 354+STR.
10
r
Table V: Error Propagation
Derived Parameters to Which Error is Propagated
Measured Parameter
(1.0 mb error)
Windspeed*
(m/s)
TAS
(m/s
W
(m/s
)
Static Pressure
Dynamic Pressure
Attack Differential
Pressure
Sideslip
Pressure
Attack
(deg)
Sidesli
P
(deg)
)
750 mb
35 mb q„
0.05
0.05
. . .
—
—
540 mb
81 mb q„
0.13
0.13
. . .
. . .
. . .
760 mb
93 mb q„
0.09
0.09
. . .
. . .
. . .
750 mb
35 mb q„
1.18
1.18
0.14
0.09
. . .
540 mb
81 mb qr
0.78
0.78
0.05
0.05
. . .
760 mb
93 mb q„
0.65
0.65
0.10
0.4
. . .
750 mb
35 mb
—
. . .
0.48
0.37
. . .
540 mb
81 mb q,.
—
. . .
0.42
0.16
. . .
760 mb
93 mb q„
. . .
. . .
0.35
0.14
. . .
750 mb
35 mb qr
. . .
—
. . .
. . .
0.37
540 mb
81 mb q„
—
. . .
—
. . .
0.16
760 mb
93 mb q.
. . .
—
. . .
. . .
0.14
A ssum es wind is along the longitudinal
attributable to pressure measurement.
11
axis
(worst
case
for
error