ENGINEERING RESEARCH INSTITUTE
UNIVERSITY OF MICHIGAN
AUTOMATIC REDUCTION OF WIND TUNNEL DATA
Department of the Air Force
Contract No. AF 33(038)-20806
Progress'Report No. 1 Interim Report
for the period
February 17, 1951 to May 17, 1951
Project Submitted for the project by:
~~M.~~~~~~~-938 Vincent S. Haneman

PERSONNEL EMPLOYED ON THIS PROJECT
G. M. Corcos Research Assistant (Part time)
V. S. Haneman Project Engineer (Full time)
L. M. Harrison Secretary (Part time)
R. J. Leite Research Associate (Part time)
L. L. Rauch Supervisor
D. V. Theofil Research Engineer (Part time)

TABLE CF CONTENTS
INTRODUCTION. 10**@............3
INTRODUCTION........................................1
BACKGROUNDITA.......1.................1
RESULTS OF INITIAL STUDIES...................3

LIST OF DRAiINGS AND CURVES
Fig. 1 Schematic Block Diagram of Force, Moment System
Fig. 2 Schematic Block Diagram of Pressure System

ENGINEERING RESEARCH INSTITUTE
Page
UNIVERSITY OF MICHIGAN
Progress Report No. 1
AUTOMATIC REDUCTION OF WIND TUNNEL DATA
INTRODUCTION
This progress report will be concerned with the work on this
project during the period February 17, 1951 to May 17, 1951. It should
be noted that this project is operating under a letter of intent dated
17 February 1951, and the contract has not been signed as of the date of
this report.
Generally, the work for this period has been restricted to
initial studies of systems requirements, subdividing.:the -project.into
tasks, and placing certain orderss that will,require several months for
completion.
Included in this report will be a certain amount of background
material and presentation of the problems involved so that future reports
may refer to this information and will not require restatement.
BACKGROUND
This project has as its goal the development of equipment for
the automatic reduction of wind tunnel data. The specific aims for this
year are the development of equipment to:
a. Compute normal force, pitching moment, center of
pressure, and drag force from static wind tunnel tests.

ENGINEERING RESEARCH INSTITUTE Page 2
UNIVERSITY OF MICHIGAN
b. Compute pressure coefficient directly
from static wind tunnel tests.
c. Have self-balancing -of all reduction and
computing equipment.
d. Obtain items a) and b) above for various
Mach numbers during one run.
e. Obtain items a) and b) above for various
angles of attack during one run.
f. Compute normal force, drag force, and moment
coefficient directly from static wind tunnel
tests.
g. Integrate pressure coefficient over a model
to eventually obtain center of pressure and
force coefficients.
To accomplish these objectives, two basic problems present
themselves. These problems are the computation of a straight line
least squares fitting for n test points in the evaluation of force and
moment data and the computation and integration of a series of variables
for the pressure data.
The solution of these problems will be by analogue computor
techniques.
The equipment will be designed to have as sensing units strain
gages, differential transformers or other elements which will yield
voltage input to the computor and to apply the computed data to ordinary
voltage or current recording equipment such as Brush, Miller, Leeds and

ENGINEERING RESEARCH INSTITUTE Page
UNIVERSITY OF MICHIGAN
Northrup, etc. All equipment will be designed to have a maximum interchangeability of components for operational efficiency.
RESULTS OF INITIAL STUDIES
The results of the initial studies of the problems involved
and possible solutions have led to the system schematically presented
in figures 1 and 2.
It has been decided to use identical systems for the force,
moment computation and the pressure computation up to the point where a
DC signal proportional to the actuating quantity is obtained. This
decision has simplified the construction and has decreased the operational
complexity of the system.
In general, the equipment will use the strain gages or differential transformers in bridge circuits. The output of a bridge will be
applied as the input to an AC amplifier where the gain will be set
according to the requirements of the particular gage or transformer by
calibration. This information will then be supplied to the demodulator
and filter. The AC amplifier will have a class B section which will
drive a servo motor to balance the bridge when the system is not in
operation.
In the demodulator section, an AC component will be added so
that the demodulation and the filtering can be accomplished with greater
ease. The component that has been introduced will be taken out in the
last stage.
In the computation of the least squares fitting or in the force,
moment problem the equation can be placed in a form such that the measured

ENGINEERING RESEARCH INSTITUTE
UNIVERSITY OF MICHIGAN
quantity is the only variable and the constants are functions of position
of the gages. Using this form, the DC equivalent of the actuating
quantity is introduced into ordinary DC operational amplifiers of the
Reac type, through appropriate resistors, and are summed. The output of
this system is then the slope (force) and the zero intercept (center of
pressure).
In the computation of pressure coefficient and integration of
the coefficient over the body the DC equivalent of the pressure is
applied through a resistor (which is a function of the constants of the
tunnel, position and area over which this pressure is assumed to be acting)
and a commutator to a DC operational amplifier connected as an integrator.
The AC systems will operate on a two kilocycle carrier supplied
from an oscillator regulated in amplitude as well as frequency.
The AC amplifier and demodulator design, engineering and
construction will be under Mr. Richard Leite. Mr. Gilles Corcos will
supervise the design of the filter, balance control circuits and bridge.
The power supplies and DC operational amplifier design will be the
responsibility of Mr. D. V. Theofil.
Future progress reports will present the status of these
sections.

2 Kc Carrier
Moment or I
Bridge Amplifier Filter Factor 11
Zero Balance
Voltage
Balance Servo Balance Weighting Slope Force or
Servo Amplifier Switch Factor 12 um p. Mosnt
2 Kc Carrier
Moment or I l l Balanee
Force AC Weighting Switch
ero Balance
2 Ke Carrier Voltage
Moment or
Force
# 3 Presure
FBalance Servo Balance Weighttngc
Seroy Amplifier Switch Factor 32
FtGURE I SCHB7MATIC ELOCK DIAGRAM OF FORCE
MOMENT SYSTEM

2 [c Carrier
Pressure
Pressure Coef 1
Pre8Bure i ger AC Weighting
_I ridge B~plfier Filter Factor 1
Coutator
Balance I ervo Balance
Serv I I fier Switch
L1~~ Amp l i f w~~it cher~~ l l |Zero PEalance
Pressur Voltage
2 Kc Carrier Coef. 2
Pressure Bridge AC Filter Weighting Integrate Force,
~~2 Bridge Filter Bpli rCl
Amplifier Factor 2
Bal~e
Switch
Balance S1 ealancel
~ Balance S w ~ac
Servo Ampllfier A Switch
2 Kc Carrier Pressure
Coef. 3
Pressure
Bridge Filter
3 ~Lmp~fier Factor 3
Balance I I Snrw, I Balance
Sery Maanlifier ) Switch
FI G U RE 2 SCHEMATIC BLOCK DIAGRAM OF
PRESSE SYSTFq

UNIVERSITY OF MICHIGAN
lllI#1lr i il