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The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

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Journal Archive (Newest First)
Main | FMC Software and its Relationship with LNAV and VNAV »

String Potentiometers - Are They Worthwhile

A flight simulator enables us to fly a virtual aircraft in an endless number of differing scenarios.  The accuracy of the flight controls when the aircraft is flown manually (hand flown) comes down to how well the aircraft’s flight controls are calibrated, and what type of potentiometer is being used to enable each control surface to be calibrated.

LEFT:  Custom-made box housing Bournes string potentiometer.  Note cable, dog lead clip, and JR Servo connection wires (click to enlarge).

This article will examine the most common potentiometers used.  It will also outline the advantages in using string potentiometers instead of inexpensive linear and rotary potentiometers.

What is a Potentiometer

A potentiometer (pot for short) is a small sized electronic component (variable resister) whose resistance can be adjusted manually, either by increasing or decreasing the amount of current flowing in a circuit.

The most important part of the potentiometer is the conductive/resistance layer that is attached (printed) on what is called the phenolic strip. This layer of material, often called a track, is usually made from carbon, but can be made from ceramic, conductive plastic, wire, or a composite material.  

The phenolic strip has two metal ends that connect with the three connectors on the potentiometer.  It’s these connectors that the wires from a control device are soldered to.  The strip has a wiper-style mechanism (called a slider) that slides along the surface of the track and connects with two of the potentiometer’s connectors. 

The strip enables the potentiometer to transport current into the circuit in accordance with the resistance as set by the position of the potentiometer on the phenolic strip. 

As the potentiometer moves from one position to another, the slider moves across the carbon layer printed to the phenolic strip.  The movement alters the current (electrical signal) which is read by the calibration software.

Types of Potentiometers (linear, rotary and string)

Potentiometers are used in a number of industries including manufacturing, robotics, aerospace and medical.  Basically, a potentiometer is used whenever the movement of a part needs to be accurately calibrated. 

LEFT:  Inexpensive rotary potentiometer.  This pot used to control the ailerons.  The pot was inserted into the base of the control column and held in place by a fabricated braket (click to enlarge).

For the most part, flight simulators use adjustable type potentiometers which, broadly speaking, are either linear or rotational potentiometers.  Both do exactly the same thing, however, they are constructed differently.  Another type of rotary potentiometer is the string potentiometer.

A linear potentiometer (often called a slider) measures changes in variance along the track in a straight line (linear) as the potentiometer's slider moves either in a left or right direction.  A linear potentiometer is more suitable in areas where there is space available to install the potentiometer. 

A rotary potentiometer uses a rotary motion to move the slider around a track that is almost circular. Because the potentiometer's track is circular, the size of a rotary potentiometer can be quite small and does not require a lot of space to install.

Potentiometer Accuracy

The ability of the potentiometer to accurately read the position of the slider as it moves along the track is vital if the attached control device is to perform in an accurate and repeatable way. 

LEFT: A very inexpensive linear potentiometer ($3.00).  The tracks on this pot are made from carbon and the body is open to dust and grime.  They work quite well, but expect their life to be limited once they begin to get dirty (click to enlarge).

The performance, accuracy, and how long that accuracy is maintained, is governed by the internal construction of the potentiometer; in particular the material used for the track (carbon, cermet, composite, etc).  Of particular importance, is the coarseness of the signal and the noise generated (electrical interference). when the potentiometer has power running through it.

For example, cermet which is composite of metal and plastic produces a very clean low noise signal, where as carbon often exhibits higher noise characteristics and can generate a course output.  It’s the coarseness of the signal that makes a control device easy or difficult to calibrate.  It also defines how accurately the potentiometer will read any small movement.

Potentiometers that use carbon form the mainstay of the less expensive types, such as those used in the gaming industry, while higher-end applications that requite more exacting accuracy use cermet or other materials. 

Essentially, higher end potentiometers generate less noise and produce a cleaner output that is less course.  This translates to more accurate calibration.  This is seen when you trim the aircraft. 

A quality mid to high-end potentiometer, when calibrated correctly, will enable you to easily trim the aircraft, insofar that the trim conditions can be replicated time and time again (assuming the same flight conditions, aircraft weight, engine output, etc).

Simulators, Dust, Grime and Other Foreign Bodies

Flight simulators to control a number of moving parts, generally use a combination of linear and rotary potentiometers.  For example, a rotary potentiometer may be used to control the flight controls (ailerons, elevator and rudder) while a linear potentiometer may be used to control the movement of the flaps lever, speedbrake and steering tiller. 

Any component that has a current running through it will attract dust, and the location of the potentiometer will often determine how much dust is attracted to the unit.  A potentiometer positioned beneath a platform is likely to attract more dust than one located behind the MIP or enclosed in the throttle quadrant.

A rotary potentiometer is an enclosed unit;  it is impervious to dust, grime and whatever else lurks beneath a flight simulator platform.  In comparison, a linear potentiometer is open to the environment and its carbon track can easily be contaminated.  Once the track has become contaminated, the potentiometer will become difficult to calibrate, and its output will become inaccurate.

Sometime ago, I had a linear potentiometer that was difficult to calibrate, and when calibrated produced spurious outputs.  The potentiometer was positioned beneath the platform adjacent to the rod that links the two control columns.  When I removed the potentiometer, I discovered part of the body of a dead cockroach on the carbon track. 

This is not to say that linear potentiometers do not have a place – they do.  But, if they are to be used in a dusty environment, they must have some type of cover fitted.  A cover will minimise the chance of dust adhering to the potentiometer’s track. 

I use linear potentiometers mounted to the inside of the throttle quadrant to control the flaps and speedbrake.  The two potentiometers are mounted vertically on the quadrant’s sidewall.  This area is relatively clean, and the vertical position of the mounted potentiometers is not conducive to dust accumulation.

Ease of Installation

Both linear and rotary potentiometers are straightforward to install, however, they must be installed relatively close to the item they control.  Often a lever or connecting rod must be fabricated to enable the potentiometer to be connected with the control device.

String Potentiometers (strings)

A string potentiometer (also called a string position transducer) is a rotary potentiometer that has a stainless steel cable connected to a spring-loaded spool. 

LEFT:  Cross section diagram showing internals of string potentiometer (click to enlarge).

The string potentiometer is mounted to a fixed surface and the cable attached to a moveable part (such as a control device).  As the control device moves, the potentiometer produces an electrical signal (by the slider moving across the track) that is proportional to the cable’s extension or velocity.  This signal is then read by the calibration software. 

The advantages of using string potentiometers over a standard-issue rotary potentiometer are many:

(i)        Quick and easy installation;

(ii)       Greater accuracy as you are measuring the linear pull along a cable;

(iii)      Greater flexibility in mounting and positioning relative to the control device;

(iv)      No dust problems as the potentiometer is enclosed;

(v)       No fabrication is needed to connect the potentiometer to the control device (only cable and dog clip) and,

(vi)       Greater time span before calibration is required (compared to a linear potentiometer).

The importance of point (iii) cannot be underestimated.  The string can be extended from the potentiometer within a arc of roughly 60-70 degrees, meaning that the unit can be mounted more or less anywhere.  The only proviso is that the cable must have unimpeded movement. 

Attachment of the string to the control device can be by whatever method you choose.  I have used a small dog lead clip.  As the potentiometer is completely enclosed dust is not an issue, which is a clear advantage in that once the potentiometer calibrated, the calibration does not alter (as dust does not settle on the track).

I have used string Potentiometers to calibrate the axis on the ailerons, elevators and rudder (one potentiometer per item).  I have also used a dual-string potentiometer in the throttle quadrant to calibrate the two thrust levers.

Fabricate Your Own String Potentiometer

Whilst you can purchase ready-made string potentiometers, their cost is not inexpensive.  As a trial, a friend and I decided to fabricate our own string potentiometers.

LEFT:  String potentiometer.  This pot connects to the ailerons.  The stainless cable can be seen leaving the casing that connects with the aileron controls.  An advantage of string pots is that they can installed more or less anywhere, as long as there is unimpeded access for the cable to move (click to enlarge).

The potentiometers used are manufactured by Bourns (3590S series precision potentiometer).  These units are a sealed, wire-wound potentiometer with a stainless steel shaft.  According to the Bourns specification sheet these potentiometers have a tolerance +-5%. 

LEFT:  Diagram showing spring-loaded spool (click to enlarge).

The potentiometer is mounted in a custom-made acrylic box in which a hole the size of the potentiometer's end, has been drilled into the lid.   Similar boxes can be purchased in pre-cut sizes, but making your own custom-sized box enables the potentiometer to be mounted inside the box in a position most advantageous to your set-up. It also enables you to place the mounting holes on the box in strategic positions.

Another small hole has been drilled in the side of the box to enable the stainless steel cable to move freely (see image at beginning of article).  If you want to allow the cable to move through an arc, this hole must be elongated to enable the cable to extend at an angle and move unimpeded. 

The cable (string) is part of a self-ratcheting spool (also called a retractor clip) which is screwed to the inside of the box and connected directly with the stainless steel shaft of the potentiometer.  The cable when attached to a solid point is kept taught by the tension of the self-ratchet spool (an internal spring controls the tension).   Ratchet spools are easily obtainable and come in many sizes and tensions.   Three standard JR servo wires connect the potentiometer to a Leo Bodnar BU0836A 12 bit Joystick Controller card.  A mini dog lead clip is used at attach the cable to the control device.

One of the major advantages when using string potentiometers is that the actual potentiometer does not have to be mounted adjacent to, or even close to the device it controls.  The line of pull on the cable can be anything within roughly a 70 degree arc. 

Additional Information

This website (no affiliation) has an excellent overview on potentiometers.  The video is very interesting.


Final Call

Previously, I used inexpensive linear and rotary potentiometers to control the main flight controls.  I was continually plagued with calibration issues, and when calibrated the calibration was not maintained for more than few months.  Furthermore, manual flight was problematic as the output from each of the  (cheap) potentiometers was course, which translated to less accuracy when using the ailerons and elevators.  Trimming the aircraft in any condition other than level flight was difficult.

Without doubt, the use of quality string potentiometers have resolved all the earlier calibration and accuracy issues I had been experiencing.  With the replacement potentiometers, the aircraft is easily hand-flown and can be trimmed more accurately.

Perhaps in the future I will ‘up the anti’ and purchase two commercial high-end string potentiometers and dedicate them to the ailerons and elevator, but for the time being the Bourne potentiometers suit my requirements.

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Reader Comments (2)

Hi Willow
Thanks for this interesting article. These pots are an excellent choice. your observation regarding the Pot you used having a Resistance Tolerance of ±5 % is quite correct but this is not the most important fact when used in potentiometer mode. These 10 turn pots have a linearity of ±0.25 % and a temp. stability of ±50 ppm/°C which is excellent and ideal for a position sensing application.
P.S Some while ago I asked you if you could let me know some dimension of the Fire Handles I wonder if you can help me.
July 16, 2018 | Unregistered CommenterMichael Dufall
Thanks Michael for adding to the article.

I sent an email to you concerning the FSP. Best, F2A
July 16, 2018 | Registered CommenterFLAPS 2 APPROACH

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