A SIMPLE ANGLE OF ATTACK SENSING SYSTEM USING A PHOTO-ELECTRIC PRINCIPLE By William E. Brown (EAA 10669) 1806 S. Mission Rd. Wichita, Kansas 67207 INTRODUCTION
This article describes an instrument aid, "A Simple Angle of Attack Sensing System Using a Photo-
Electric Principle." This device is 1. Aerodynamic Vane
intended to provide angle of attack
data as an aid in the control of aircraft where a simple, reliable system is required to measure the relative angle of flow past a body or vehicle in motion through a fluid or atmosphere. GENERAL DESCRIPTION
This instrument aid consists of a system of components, which, together sense a relative angle of flow past a sensing device, provide an electrical current proportional to the flow angle and provide a display
of the measured angle. This system includes the following components
Electrical 4 Components
which are shown in Figure 1.
1. Aerodynamic Vane — The aerodynamic vanes are immersed in the fluid flow and are free to move so as to always be aligned with the fluid flow within the limits of the
unit. The vanes are inertially balanced so as not to be influenced by accelerations of the vehicle perpendicular to the fluid flow. 2. Flow Sensing Head — The flow sensing head contains the pivoting mounting shaft of the aerodynamic vanes as well as the internal photoelectric sensing elements which provide an electrical current signal which is proportional to the angular position of the aerodynamic vane. The details of the flow sensing head are described in a later section of this article.
Meter 5 Display
6. Wiring
FIG. 1 SYSTEM COMPONENTS
3. Mounting Boom — The mounting boom provides for the proper
positioning of the flow sensing head in relation to the vehicle. 4. Electrical Components — Electrical components for the regulation of voltages and for the electrical matching of the components are grouped together in a common mounting. 5. Meter Display — An electrical current ammeter, suitably marked in angular angle of attack units, is used to display the sensed flow angle. 32 SEPTEMBER 1975
6. Wiring — Electrical wiring connects the elements of the system
electrically. INSTALLATION
The entire system is mounted on the vehicle in a manner such that the flow sensing head is positioned in the fluid flow for proper measurement of the flow angle. The compon-
ents are mounted in a convenient place in the vehicle and are provided with a suitable direct current electrical source. A typical installation on a light aircraft is shown in Figure 2. FLOW SENSING HEAD
The flow sensing head internal details are shown in Figure 3. The
Aerodynamic Vane
head is of tubular construction and contains in the forward end a light source and further aft a photoelectrical cell whose electrical resistance varies inversely with the light impinged upon it. The pivoting mounting shaft of the aerodynamic vanes pierces the sensing head between the light source and the photoelectric cell. A shutter is affixed to this shaft and moves so as to modulate the amount of light reaching the photo-electric cell so that the light reaching the photo-electric cell is proportional to the angle of the aerodynamic vanes. The vanes and shutter geometries are arranged such that a m a x i m u m amount of light reaches the photo-electric cell at the one extreme of angular vane motion and a minimum at the other extreme.
Aerodynamic Vane Instrument
Panel Mounted
Lignl Sou
Meler Display
Electrical
Components
Behind Panel
PRINCIPLES OF OPERATION
The flow sensing head is positioned so that the aerodynamic vanes are immersed in the fluid flow past the vehicle. The aerodynamic forces cause the vanes to align themselves with the relative fluid flow in the vicinity of the flow sensing head. This causes the shutter to modulate the amount of light reaching the photo-electric cell to be proportional to the relative angular position of the vane between the extreme angular position limits of the vanes. The photo-electric cell electrical characteristics are such that the resistance is inversely proportional to the light reaching the cell. This functions to vary the electrical current flowing through the cell in a manner proportional to the light reaching the cell when an electrical voltage is applied to the photo-electric cell. The electrical source voltage for the light and the photo-electric cell is regulated by a suitable device such as a zener diode/resistor combination so that variations in electrical power supply voltage do not influence the device. A simple electrical current ammeter is used to measure the current flowing through the photo-electrical cell, and is marked to show the angular position of the vane corresponding to the current values associated with the angular positions. The electrical components are chosen so that the maximum current on the ammeter corresponds with the extreme vane travel limit associated with the maximum light reaching the photo-cell. A variable resistance is connected across the ammeter to provide adjustment for minor component variations so that the needle of the ammeter can be adjusted to read maximum when the associated vane
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