During lateral flight-test maneuvers of a V/STOL research aircraft, large errors in static pressure were observed. An investigation of the data showed a strong correlation of the pressure record with variations in sideslip angle. The sensors for both measurements were located on a standard air-data nose boom. An algorithm based on potential flow over a cylinder that was developed to correct the pressure record for sideslip-induced errors is described. In order to properly apply the correction algorithm, it was necessary to estimate and correct the lag error in the pressure system. The method developed for estimating pressure lag is based on the coupling of sideslip activity into the static ports and can be used as a standard flight-test procedure. The estimation procedure is discussed and the corrected static-pressure record for a typical lateral maneuver is presented. It is shown that application of the correction algorithm effectively attenuates sideslip-induced errors

Bach, Ralph E., Jr.

Parks, Edwin K.

Tran, Duc

English

NASA Technical Reports Server

NASA Technical Memorandum 102846
Sideslip-Induced Static
Pressure Errors in
Flight-Test Measurements
Edwin K. Parks, University of Arizona, Tucson, Arizona
Ralph E. Bach, Jr., Ames Research Center, Moffett Field, California
Duc Tran, San Jose State University, San Jose, California
June 1990
NASA
National Aeronautics and
Space Administration
Ames Research Center
Moffett Reid, California 94035-1000
https://ntrs.nasa.gov/search.jsp?R=19900018386 2020-03-19T21:22:03+00:00Z

SIDESLIP-INDUCED STATIC PRESSURE ERRORS IN FLIGHT-TEST MEASUREMENTS
Edwin K. Parks*
University of Arizona
Tucson, AZ 85721
by
Ralph E. Bach, Jr.**
NASA Ames Research Center
Moffett Field, CA 94035
Duc Tran?
San Jose State University
San Jose, CA 95192
Abstract
During lateral flight-test maneuvers of a V/STOL
research aircraft, large errors in static pressure were
observed. An investigation of the data showed a strong
correlation of the pressure record with variations in
sideslip angle. The sensors for both measurements were
located on a standard air-data nose boom. This paper
describes an algorithm based on potential flow over a
cylinder that was developed to correct the pressure record
for sideslip-induced errors. In order to properly apply the
correction algorithm, it was necessary to estimate and cor-
rect the lag error in the pressure system. The method
developed for estimating pressure lag is based on the
coupling of sideslip activity into the static ports and can
be used as a standard flight-test procedure. The paper dis-
cusses the estimation procedure and presents the corrected
static-pressure record for a typical lateral maneuver. It is
shown that application of the correctionalgorithm effec-
tively attenuates sideslip-induced errors.
L Introduction
In the flight-testing of aircraft, it is essential that
accurate measurements of static and total air pressure be
obtained. Usually, the pressure sensors are mounted ahead
of the aircmlt, neat the tip of a nose beom, to beuer sense
the "free sueam" conditions. However, unless a swivel
pitot static head is used, the pressure measurements will
be in error whenever the boom is yawed with respect to
the direction of airflow. The static-pressure measurement
is the most affected: its errors depend on the angles of
attack and sideslip of the boom as well as on the number
and location of static ports around the circumference of
the boom. Gracey discusses this effect as well as other
* Professorof Aemtpace Engineering,Associate Fellow, AIAA
** _ Engineer,Member, AIAA
?Gradamzstudent, Dept. of MechanicalEngineering
Copyright © 1990 by the Americanlnsdmte of Aenmaatics and
Asuonmfics,Inc. No copyright is asserted in the United States
underTitle 17, U_. Code. The U.S. Governmenthas a royalty-
free license to exercise all rights under the copyright claimed
herein for Goverranentaipurposes. All other rights are reserved
by me cep_sh, owner.
errors that occur in aircraft pressure-measurement
devices. I
The investigation reported in this paper was prompted
by the occurrenceof large errors in the measurement of
static pressure during lateral flight-test maneuvers of a
V/STOL research aircraft (VSRA). Observation of the
data revealed a strong correlation between the pressure
and sideslip-angle measurement records. An algorithm
Imsed on potential flow over a cylinder was developed to
correct the pressure record for sideslip-induced errors.
The algorithm was modified to account for separation of
the laminar boundary layer. In order to properly apply the
correction algorithm, it was necessary to estimate and cor-
rect the lag in the pressure system. The method developed
for estimating pressure lag is based on the coupling of
sideslip activity into the static ports and can be used as a
standard flight-test procedure.
The paper procee_ as follows: First, the cause and
effect of the pressure errors observed during flight lest are
indicated and then the correction algorithm is derived.
The flight-data procedures for applying the pressure cor-
rection and estimating the separation angle and pressure
lag are described, and results are shown for a typical
lateral maneuver. Finally, the implications of the method
are discussed in the concluding remarks.
II. Problem Statement
The VSRA is equipped with the industry-standard
flight-test boom shown in Fig. 1. The boom houses the
pitot static sensors for measurement of total and static
pressure, as well as the flow-angle sensors for measure-
merit of angles of attack (alpha vane) and sideslip (beta
vane). The staticpressureissensed at two closely space.A
ports locatedsymmetrically on the top and bottom of the
cylindrical tube about four diameters aft of the pitot head.
The tube diameter at lhe static ports is 0.62 inch, a value
for which, from a cross-flow standpoint, the Reynold's
number will remain subcritical, even at large yaw angles
and high airspeed. 2 Any separation of the boundary layer
from the cylinder near the static ports will therefore be
laminar in nature.
The beta-vane and static-pressure records shown in
Fig. 2 are observed to be strongly correlated. The effect of
sideslip-induced pressure error is evident in the record of
calculated pressure altitude, where it is seen that a sideslip
of I0" causes an altitude variation of about I00 feet. It
should be mentioned here that the nose-boom manufac-
turer offers a test program to provide individual calibra-
tion curves of pitot and static pressure as a function of
Mach number, angle of attack, and sideslip over a
customer-specified flight regime. It was decided, how-
ever, to try to express the static-pressure errors on a
yawed cylinder analytically by using a potential-flow
approach, suitably modified to include boundary-layer
separation. 3 The resulting algorithm could then be used to
correct the static-pressure record. This approach to the
problem is described in the sections that follow.
HI. Correction Algorithm
The effect of a yawed airflow over the cylindrical
nose boom will be modeled by assuming that the flow is
frictionless, and that the cross flow and axial flow can be
considered separately, It is also assumed that the nose
boom is aligned with the longitudinal body axis of the air-
craft. The plane containing the static ports is shown in
Fig. 3(a), where, to simplify the analysis, only one upper
and one lower port are indicated. The potential flow solu-
tion for the cross-flow component of tangential velocity at
the lower static port is given by 4
Vc = 2 (v 2 + w2) I/2 sin 0 (1)
where v, w are the components of the free-stream veloc-
iv/in the plane perpendicUlar to the cylinder, and 0 is the
angle between the line joining the stagnation points and
the line joining the static ports. Now, if u is the axial
component of free-stream velocity, the magnitude of the
tangential velocity at the lower port is
V ffi[u2 + 4 (v2 + w2) sin2 0 ]1/2 (2)
The error in static pressure at the lower port,
expressed in coefficient form, is given by
ACp= (p- p_) / ( pVt2 / 2) (3)
where p isthe pressure sensed at the port, p. isthe free-
stream static pressure, p is the density, and Vt is the
magnitude of the free-stream velocity. Note that
Vt = (u2 + v 2 + w2) 1/2 (4)
The use of Bemoulli's equation 4 to express the relation
between pressure and velocity allows Eq. (3) to be written
as
ACp -- 1 - V 2 / Vt2 (5)
Note that the flow-angle sensors mounted on the nose
boom near the static ports have the relations
tan13= v/u ; tana = w/u (6)
•where 13 is the angle sensed by the beta vane, and a is
the angle sensed by the alpha vane. It can be seen from
Fig. 3(a) that
tan0 = v/w= tan13/tana (7)
The expressionsof Eqs. (2), (4), and (6) can be used to
write Eq.(5) as
ACp = K (1 - 4 sin2 O)
K = (tan 2 _ + tan2 a) / (I + tan2 _ + tan2 ex) (8)
Were the flow over the cylinder truly inviscid, the
pressure sensed at the upper port would be the same as
that at the lower port. Empirical studies have shown,
however, that boundary-layer separation does occur (as
illustrated in Fig, 3(b)), so that the upper and lower ports
sense different pressures. For example, a port in the wake
of a separated boundary layer would contribute an error of
ACt,= K (I-4 sin2 0s) (9)
where 0s is the angle at which separation occurs. Now, if
it is assumed that 0s < 90", then for 0 < Os,only one port
is in the wake of separated flow. For this case the average
of the pressure envrs is given by
ACp = K [1 - 2 (sin 2 0 + sin2 0s)] (10)
It follows that for O> Os, both ports are in the wake of
separated flow, so that the average pressure error is given
by Eq. (9). The application of the algorithm given by
Eqs. (9) and (I0) for correction of the static pressure
record is described next.
IV. Application
The pressure-correction algorithmderivedin the
previous section is a function of the alphaand betavane
angles, and the flow-separation angle. A block diagram
outlining the application of the algorithm is shown in
Fig. 4. Note that any lag in the VSRA pressure system
mustbeaccountedfox,sincethesidesliperror is induced
at the static-port location, while the pressure transducer is
located 25 feet aft of the static ports, with the usual pneu-
matic tube interconnection. Before making the lag correc-
tion, however, the separation angle must be estimated,
and, if the pressure system lag is not known, it also must
be estimated from the data.
The procedure for estimating the separation angle is
simple: The angle 0s is varied until the "best" ctxzection
of the low-frequency errors is obtained. For the VSRA
nose boom, a value of 0s = 45" produced the best correc-
tion. It is interesting to note that for 0s = 45", Eq. (7) can
be used with Eq. (10) to obtain the expression
ACp--'-2 tan2 _ / (1 + tan2 _ + tan2 (x) (11)
where it is seen that no correction is necessary for zero
sideslip. Hence, it would appear that the boom design is
optimized for longitudinal maneuvers. This seems to be
the logical choice, since these maneuvers are the most
important in fiight-test performance studies.
The procedure for estimating the pressure system lag
is as follows: A f'Lrst-ordercurve fit to the raw pressure
coefficient produced residuals that enhanced the sideslip-
induced errors. The residuals were then correlated with
the correction coefficient ACp (with 0s ffi45"). The cor-
responding correlation curve is shown in Fig. 5. The pres-
sure-system lag is seen from that curve to be about
0.35 sec. A lag model 1 consisting of a pure (acoustic)
delay of 0.1 sec in tandem with a simple first-order lag of
0.35 sec removed the lag from the correlation curve and
realized a suitable attenuation of high-frequency errors
when the sideslip correction was applied.
The sideslip and corrected static-pressure and
pressure-altitude records are shown in Fig. 6_The raw and
corrected pressure records were also used to generate
records of pressure altitude (corrected for temperature 5)
that are shown in Fig. 7 with the inertial (radar-derived)
altitude for comparison. There the sideslip-induced errors
are seen to have been reduced by more than a factor of
five. Similar results have been obtained with other
maneuvers that have significant lateral excitation. It
should be noted that the coupling that exists between
sideslip and static pressure might be used in a standard
flight-test procedure for calibration of pressure lag. It
would be desirable, however, to perform a ground test of
the pressure system to validate the lag model.
V. Concluding Remarks
What appeared to be a serious cross-coupled static-
pressure measurement problem during lateral maneuvers
has been solved by applying an algorithm based on poten-
tial-flow theory. The algorithm accounts for viscous
effects that cause separation of the nose-boom boundary
layer. Furthermore, the coupling of sideslip activity into
the static-pressure port makes it possible to perform a
flight-data pressure-lag calibration that could be included
as a standard fiight-test procedure for research aircraft.
VI. References
[1] Gracey, W.: "Measurement of Aircraft Speed and
Altitude," NASA RP-1046, May 1980.
[2] Bursnall, W. J.; Loftin, L. K.: "Experimental Investi-
gation of the Pressure Distribution About a Yawed
Cylinder in the Critical Reynold's Number Range,"
NACA TN-2463, September 1951.
[3] Sears, W. R.: "The Boundary Layer of Yawed Cylin-
ders," J. Aeronaut. Sci., Vol. 15, No. I, January 1948.
[4] White, F. M.: Fluid Mechanics, McGraw-Hill,
New York, 1979. Ch. 7.
[5] Dunlap, E. W.; Porter, M. B.: "Theory of the Mea-
surement and Standardization of In-Flight Performance of
Aircraft," AFFTC, Edwards, CA, Rept. FTC-TD-71-1,
April 1971.
_ pitot heed
.20_
Static ports ._.._,,_ -2
// __ j Be,a vane 1500 Staticpressure
/.- ___ -
Alpha vane 1450
Fig. 1 VSRA air-data flight-cost nose boom.
lo,ooo_ d
° ,-°1 _'_
9000 i i I i I I
0 25 50 75 100 125
Time (Mc)
Fig. 2 Effec( of sideslip on static pressure and pressure
altitude.
(') /
Note points marked "l' are
stagnation points; those
marked "p" are ports.
_, Separated
,,'._ ,.-)wake
' ./7-, F
./ _.J// ")
Jr _,/n-_ .
rJ/I/r',- /P _,)/ II "',"/}'4
,'/,,///
Fig. 3 Flow over nose-boom cross section at the static ports: (a) potential flow; (b) viscous flow.
4
O
- Ipressure .- Lagcorrection
°s I Oynamic I
pressure
Sldeellp ,_Cp _ ,_P
corm_ton
8 _ es : ACp : K[1-2(singe ÷ elnge.)]
8 > es : &Cp : K(1-4 singes)
0 = arc'tan (tan IVtan a); K = (tan2 _ + tan2 (_)/ (1 + tan2 13+ tan2 a)
-O
Corrected
static
pressure
Hg. 4 Correction of static pressure for lag and sideslip-induced errors.
6 -
i 4
3 r i
l- : I
o Ii
I
if
,\
t
'\\
\
0 .25 .50 .75 1.IX) 1.25 1.50
Time delay (sac)
Fig. 5 Correlation of ACp with raw pressure coefficient
for Os = 45".
,4,5
1450 -_
10,000 -
.20
0 -__ - __'_ 8eta vane
j -
-20
Static pressm
Pressure altitude
I 1 l I ;
0 25 50 75 100 125
Time (sac)
Fig. 6 Sideslip with corrected static-pressure and pressure-
altitude records.
lO,OOO_ (a)
/
9200 I -- Pressure altitude
J ................ inertial altitude
9000_
10,000
l+
92O0
9OOO
(b)
o +,+ 5o +75 ',oo ,"
Time (ram)
Fig. 7 Pressure altitude plotted with inertial altitude: (a) unc(xrected; (b) corrected.
Nall_ _anaull¢=
S_teo Adm_llbq_'=_
1. Report No.
NASA TM- 102846
4. Title and Subtitle
Report Documentation Page
2. Government Accession No. 3. Recipient's Catalog No.
5. Report Date
Sideslip-Induced Static Pressure Errors in Flight-Test
Measurements
7. Author(s)
Edwin K. Parks," Ralph E. Bach, Jr., and Duc Tran t
9. Performing Organization Name and Address
"University of Arizona, Tucson, AZ 85721
Ames Research Center, Moffett Field, CA 94035-1000
tSan Jose State University, San Jose, CA 95192
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, DC 20546-0001
June 1990
6. Performing Organization Code
8. Performing Organization Report No.
A-90229
10. Work Unit No.
505-66-41
11. Contract or Grant No.
131 Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Point of Contact: Ralph E. Bach, Jr., Ames Research Center, MS 210-9, Moffett Field, CA 94035-1000
(415) 604-5429 or FTS 464-5429
To be presented at AIAA 8th Applied Aerodynamics Conference, Portland, OR, Aug. 20-22, 1990.
16. Abstract
During lateral flight-test maneuvers of a V/STOL research aircraft, large errors in static
pressure were observed. An investigation of the data showed a strong correlation of the pressure
record with variations in sideslip angle. The sensors for both measurements were located on a
standard air-data nose boom. This paper describes an algorithm based on potential flow over a
cylinder that was developed to correct the pressure record for sideslip-induced errors. In order
to properly apply the correction algorithm, it was necessary to estimate and correct the lag error
in the pressure system. The method developed for estimating pressure lag is based on the coupling
of sideslip activity into the static ports and can be used as a standard flight-test procedure. The
paper discusses the estimation procedure and presents the corrected static-pressure record for a
typical lateral maneuver. It is shown that application of the correction algorithm effectively
attenuates sideslip-induced errors.
17. Key Words (Suggested by Author(s))
Static pressure errors
Potential flow algorithm
Flight test
18. Distribution Statement
Unclassified-Unlimited
Subject Category - 06
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price
Unclassified Unclassified 10 A02
NASA FORM 1626 OCT_
For sale by the National Technical Information Service, Springfield, Virginia 22161



Sideslip-induced static pressure errors in flight-test measurements

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