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Author Topic: Line Tension Sensor  (Read 1520 times)

Offline Howard Rush

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Line Tension Sensor
« on: December 18, 2021, 12:36:40 PM »
What's a good onboard sensor for line tension or bellcrank axle load?  Pat will know. 
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Online Ken Culbertson

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Re: Line Tension Sensor
« Reply #1 on: December 18, 2021, 04:57:54 PM »
What's a good onboard sensor for line tension or bellcrank axle load?  Pat will know.
When it gets too much my pilot lets me know..... LL~

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Online John Rist

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Re: Line Tension Sensor
« Reply #2 on: December 18, 2021, 06:40:08 PM »
Looking at the thread on the Fiortty Timer it looks like it has line tension in the mix.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/fiortti-timer/

Looking at post 57, in the above thread, it talks about a function called G-force  "G-force adds RPM when less than calibrated force is noted.  The decrease from calibrated force is set by Rogerio based on his testing.  The loss of force can be overhead due to gravity or at the upwind part of the circle with wind pushing the plane in.  There is an arc at the end of the circle and overhead where RPM is added, and otherwise returns to set RPM."

It doesn't data log the force but rather adjust the RPMs up when G-force goes down.  Not sure if this is what you are looking for but hay whatever.   D>K
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Online Fred Underwood

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Re: Line Tension Sensor
« Reply #3 on: December 18, 2021, 06:57:59 PM »
https://stunthanger.com/smf/gettin-all-amp'ed-up!/fiortti-timer/

Looking at post 57, in the above thread, it talks about a function called G-force  "G-force adds RPM when less than calibrated force is noted.  The decrease from calibrated force is set by Rogerio based on his testing.  The loss of force can be overhead due to gravity or at the upwind part of the circle with wind pushing the plane in.  There is an arc at the end of the circle and overhead where RPM is added, and otherwise returns to set RPM."

It doesn't data log the force but rather adjust the RPMs up when G-force goes down.  Not sure if this is what you are looking for but hay whatever.   D>K

That is a nonengineer user's description of the function, and not relevant to this thread. 
« Last Edit: December 19, 2021, 03:44:33 PM by Fred Underwood »
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #4 on: December 18, 2021, 07:14:33 PM »
Looking at the thread on the Fiortty Timer it looks like it has line tension in the mix.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/fiortti-timer/

Looking at post 57, in the above thread, it talks about a function called G-force  "G-force adds RPM when less than calibrated force is noted.  The decrease from calibrated force is set by Rogerio based on his testing.  The loss of force can be overhead due to gravity or at the upwind part of the circle with wind pushing the plane in.  There is an arc at the end of the circle and overhead where RPM is added, and otherwise returns to set RPM."

It doesn't data log the force but rather adjust the RPMs up when G-force goes down.  Not sure if this is what you are looking for but hay whatever.   D>K

This isn't what I'm looking for.  Thanks.
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #5 on: December 18, 2021, 08:21:33 PM »
What's a good onboard sensor for line tension or bellcrank axle load?  Pat will know.


That's a good question Howard. Strain gages or load cells and ADCs are fairly readily available and easy to interface with. The real question is how would you plan to install it/them? AdaFruit has some load cells they make for cheap and you can find strain gains which are fairly large inexpensive. Miniature strain gages you could attach to the axle would be cost prohibitive on my budget. To use the cost effective ones you'd need to provision your bellcrank with a support that could push or pull on a load cell or make a leg that a strain gage could be attached to. The mount I have going in to my current design project could be used as a model example. The necked down zone would be a candidate if one side were not fixed to the surrounding structure allowing the load to pass through the instrumented structure. This kind of project would be easy to do on this particular model I have going. The entire control system is being design with the intent of being completely reconfigurable.

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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #6 on: December 18, 2021, 08:31:12 PM »
The entire control system is being design with the intent of being completely reconfigurable.

Looks like a good idea.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #7 on: December 18, 2021, 09:15:09 PM »
AdaFruit has some load cells they make for cheap ...

Thanks. One could be used as a bellcrank, but they look a little heavy.  You're a bad influence on me, sending me to the AdaFruit site.  Lots of cool distractions there. 
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #8 on: December 18, 2021, 09:21:12 PM »
Looks like a good idea.

I hope so. It just dawned on me that you should be able to strain gage the bellcrank arms and get some good results. It's worth doing a bench setup and seeing. That actually would be interesting as you would be able to confirm some of your analysis' for hinge moments as well. I would have interest in such results.
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #9 on: December 18, 2021, 09:31:14 PM »
Thanks. One could be used as a bellcrank, but they look a little heavy.  You're a bad influence on me, sending me to the AdaFruit site.  Lots of cool distractions there.

I know, right... They have so much of my money it isn't funny. That loadcell is too big and it's just some strain gages they glued to the bar. It wouldn't be real hard to strain gage a bellcrank and calibrate it on the bench. Doing that would be of interest to me as well. Unfortunately the current flying test plane is all sealed up and not accessible. This next one will have the control system accessible and the bellcranks and control system can be swapped readily. The mounting interface is intended to be a standard feature which different supports can be swapped in and out. An instrumented support combined with arm data would be particularly interesting. The flap interface is also reconfigurable by design. The hinges can be made in any length and plug in. The aft Log crank is also capable of that as well as ratio modifications. I finished flap horn design a little while ago and it will go in to the wing model next. 
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Offline pmackenzie

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Re: Line Tension Sensor
« Reply #10 on: December 18, 2021, 09:39:34 PM »
You can buy the strain gauge wires, and glue them onto whatever it is that is bending under load.
https://www.amazon.ca/DAOKI-BF350-3AA-High-Precision-Pressure-Resistance/dp/B07X87CJD8


Not sure these are the correct form factor, but if you dig around on ebay or Aliexpress you might be able to find some other types.

You would also need a load cell amp, but pretty sure they have those for Arduino, they sell them with the standard load cells on ebay

https://www.robotshop.com/ca/en/hx711-load-cell-amplifier.html?gclid=EAIaIQobChMIju_GpYXv9AIVP8qUCR2kgAuzEAQYAiABEgJZLfD_BwE

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Online John Rist

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Re: Line Tension Sensor
« Reply #11 on: December 18, 2021, 11:18:27 PM »
Once again I am not sure what the deal is here.  Ok so you can rig a strain gage to measure line tension.  What are you now going to do with this info. Don't you need some kind of a data logger to capture the info.  Assuming you can get all this rigged what do you do with the info?  When I fly I have the handle in my hand and can feel how hard the airplane is pulling.  I am not sure how accurate info on the subject would improve my flying.   ???  What am I missing?
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #12 on: December 18, 2021, 11:50:52 PM »
Once again I am not sure what the deal is here.  Ok so you can rig a strain gage to measure line tension.  What are you now going to do with this info. Don't you need some kind of a data logger to capture the info.  Assuming you can get all this rigged what do you do with the info?  When I fly I have the handle in my hand and can feel how hard the airplane is pulling.  I am not sure how accurate info on the subject would improve my flying.   ???  What am I missing?

Well, number one what you are missing is the opportunity to see a group of geek children spend many hours of effort to place cool to them instrumentation in an airplane to prove inequivalently that the airplanes does, in fact, pull on the bellcrank. Okay, humor aside, Howard has had an interest in hinge moments for much longer than I have on a measurable quantitative side. Having the data would validate his calculations. Qualitative is always subjective. So what you think is light I might think is heavy. I did an A-B test flying the spades which, to me, made a big difference in the control forces. However, communicating that qualitatively, means little to nothing as Mark isn't a known pilot quantity here. If I had an instrumented bellcrank with gross, up side and down side load measurements, there would be no dispute. The data would bear out the claim.

As a powerplant engineer performing a crash investigation I once spent $300k doing a test to prove unequivocally that, if you leave the fuel valve closed, the engine will quit. You know, that kind of thing. On the surface seems obvious but when you dig a little bit there's much more. Seems as though when the fuel valve was installed on the cartridge pump incorrectly it looked right but wasn't and had a little bit of opening to it which allowed the helicopter to take off and fly. The vapor bubble that formed in the fuel line during the take off didn't reach the engine driven fuel pump because of the short duration of the power demand. When the helicopter later entered an out of ground effect hover and remained there for a long time the vapor bubble reached the engine driven fuel pump causing fuel starvation to the engine and the engine quit. Another helicopter nearby observed the crashing helicopter billowing white smoke from the engine exhaust. The testing showed that the restriction allowed enough fuel for the flight but not the hover and after the engine flamed out the spool down and consequent reduced fuel flow let the fuel once again reach the engine driven pump.

With my flap and hinge offset efforts having real hinge moment data would be good and there is some significant probability that I will instrument my airplane as a result of Howards provocation. Having the data would substantiate any claims. Considering that large flaps could potentially improve the performance of the airplane if the control forces are maintained the same, this is how this effort can improve the sport.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #13 on: December 19, 2021, 02:20:40 AM »
You can buy the strain gauge wires, and glue them onto whatever it is that is bending under load.
https://www.amazon.ca/DAOKI-BF350-3AA-High-Precision-Pressure-Resistance/dp/B07X87CJD8


Not sure these are the correct form factor, but if you dig around on ebay or Aliexpress you might be able to find some other types.

You would also need a load cell amp, but pretty sure they have those for Arduino, they sell them with the standard load cells on ebay

https://www.robotshop.com/ca/en/hx711-load-cell-amplifier.html?gclid=EAIaIQobChMIju_GpYXv9AIVP8qUCR2kgAuzEAQYAiABEgJZLfD_BwE



Thanks, Pat.  I'll get me some. 
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #14 on: December 19, 2021, 07:18:58 AM »
Thanks, Pat.  I'll get me some.

Here's some smaller ones. Keep in mind these aren't bridge wired and you'll need 4 of them for good measurement.

https://www.amazon.com/gp/product/B08SLPCC3W/ref=ox_sc_act_title_1?smid=A30Y6WWS77DGEW&psc=1

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Offline CircuitFlyer

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Re: Line Tension Sensor
« Reply #15 on: December 19, 2021, 07:31:25 AM »
Sparkfun has an amplifier with I2C output https://www.sparkfun.com/products/15242
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #16 on: December 19, 2021, 08:45:33 AM »
Sparkfun has an amplifier with I2C output https://www.sparkfun.com/products/15242

That's an HX711 with a different connector. Actually many ADC's will work. The HX711 advantage is that it has libraries already in existence. There's also a bunch of knock offs for less money as well. In order to use them, the strain gages have to be wired in a Wheatstone bridge configuration. The ADC provides the excitation voltage and compares to the bridge voltage.
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Online Tim Wescott

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Re: Line Tension Sensor
« Reply #17 on: December 19, 2021, 09:07:14 PM »
... it talks about a function called G-force ...

That's the acceleration of the plane toward the inside of the circle.  It's the sum of acceleration due to line tension, any aerodynamic lift on the fuselage, and the acceleration due to gravity.

Howard wants to separate out just the line tension (to, as Mark mentions, keep the old techno-geeks off the streets).
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Online Teodorico Terry

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Re: Line Tension Sensor
« Reply #18 on: December 20, 2021, 12:21:30 PM »
I think that the easiest solution would be to use strain gauges at the handle;  it would be much easier to instrument than a model. You could actually add a "strain measuring link" between the handle and line connectors.  Such a sensor would need to be calibrated so that you can derive the forces from the strain measurements. If you record the data for the up and down lines you can also estimate the forces needed to move the control surfaces.  I suspect that the forces will be lower than we imagine.  Either way it could be a fun little side project for someone with more free time than they know what to do with.

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Offline Mark wood

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Re: Line Tension Sensor
« Reply #19 on: December 20, 2021, 12:25:48 PM »
I think that the easiest solution would be to use strain gauges on the handle;  it would be much easier to instrument a handle than a model. You could actually add a "strain measuring link" between the handle and line connectors.  Such a sensor would need to be calibrated so that you can derive the forces from the strain measurements.  It could be a fun little side project.

Teo

It depends on the goal of the measurement, but yes a link could be installed there.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #20 on: December 20, 2021, 04:00:40 PM »
I think that the easiest solution would be to use strain gauges at the handle;  it would be much easier to instrument than a model. You could actually add a "strain measuring link" between the handle and line connectors.  Such a sensor would need to be calibrated so that you can derive the forces from the strain measurements. If you record the data for the up and down lines you can also estimate the forces needed to move the control surfaces.  I suspect that the forces will be lower than we imagine.  Either way it could be a fun little side project for someone with more free time than they know what to do with.

Yes.  It's always better to keep stuff on the ground if you have a choice.  I intend to do that to measure control surface hinge moment.  The total line tension sensor is another matter. 

Either way, I could self pull test.  I won't have more free time than I know what to do with in 2022.  2023 I may.  The way I'm progressing, there's not much I'll know what to do with by then. 
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Offline Mark wood

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Re: Line Tension Sensor
« Reply #21 on: December 20, 2021, 08:31:09 PM »
Yes.  It's always better to keep stuff on the ground if you have a choice.  I intend to do that to measure control surface hinge moment.  The total line tension sensor is another matter. 

Either way, I could self pull test.  I won't have more free time than I know what to do with in 2022.  2023 I may.  The way I'm progressing, there's not much I'll know what to do with by then.

I think it would be better to instrument the pushrods. Cf might be a bit on the stiff side but a substitution to aluminum arrow shafts might be better for data collection. Or possibly control horns strategically machined to be good load cells might be even better. What do you think the ROM load level is?
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #22 on: December 21, 2021, 12:28:10 AM »
See Terry's post above.
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Online Ken Culbertson

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Re: Line Tension Sensor
« Reply #23 on: December 21, 2021, 10:09:54 AM »
I think that the easiest solution would be to use strain gauges at the handle;  it would be much easier to instrument than a model. You could actually add a "strain measuring link" between the handle and line connectors.  Such a sensor would need to be calibrated so that you can derive the forces from the strain measurements. If you record the data for the up and down lines you can also estimate the forces needed to move the control surfaces.  I suspect that the forces will be lower than we imagine.  Either way it could be a fun little side project for someone with more free time than they know what to do with.

Teo
In a perfect world using the clips could work but we don't fly in anything resembling a perfect world.  To get any comparative data you would have to factor in so many variables outside of the plane.  Line stretch, line drag, wind speed, wind direction, line wraps, altitude, air density, in addition to all of the bellcrank to hinge line leverages (which are measurable).  I also do not think that line tension translates into an increase or decrease in the force required to move the flaps but only the ability to apply that force.  I would keep measurements internal.  I will leave the where and how those qualified.

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Offline Mark wood

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Re: Line Tension Sensor
« Reply #24 on: December 21, 2021, 07:03:47 PM »
See Terry's post above.

 I did. And I agree, sorta. Yes it would be easier to instrument the handle, maybe. The Adafruit Feather has a version which is a data logger that can store data on an SD card and is very small. It uses Arduino IDE for programing which means it is relatively easy to use. The only downside is it has a maximum 5V power input so it would need it's own power source. The HX711 would be compatible with it on the same power source so it could work with the Feather. No difference for either installation. Maybe the strain gage prices would make a difference. The really compact half bridge ones I found were significantly more expensive but have the sensitivity for carbon fiber. The difficulty factor is really in the mounting it in the airplane. That is easy for my current project as I'm already provisioning for access. Yes, I am seriously considering the installation.

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Online Teodorico Terry

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Re: Line Tension Sensor
« Reply #25 on: December 22, 2021, 07:06:18 AM »
Interesting discussion.  I still think that the easiest solution would be to instrument the handle; the differences in tension between the up and down lines would provide an indication of the torque being applied to the bellcrank and with a little math an idea of the load applied to the pushrod.  The other thing to keep in mind is that we are measuring small loads and the portion of the control system to be instrumented will have to be designed accordingly (i.e. be flexible enough to give the gauges something to measure).  If instrumenting the handle the load measuring clips would have to be relatively thin or maybe be U-shaped.  I do not think that line stretch would affect the results since in the end, the input is manual which itself carries a lot of variation.

Many years ago I read an article on an old magazine where they were trying to measure the loads needed to move a control surface; this was in the early days of radio.  So they ended up mounting a section of a wing on a car or pickup, deflecting the surface a known amount (in degrees) and then driving at different speeds to see how much force was needed to keep the surface deflected.  Compared to a R/C model our planes fly in a relatively narrow speed band so the testing would be a little simpler.

You could do the same today using a servo to deflect the surface and measuring the current draw to estimate the load needed to deflect the surface a certain amount.  You would have to do a little testing ahead of time so that you can correlate the current draw to a given load but it is doable without being overly complicated (as in recording data using a pencil and a note pad).  Anything we do will be an approximation but then again, we are not trying to land a rover on Mars....

Teo

Offline Mark wood

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Re: Line Tension Sensor
« Reply #26 on: December 22, 2021, 09:18:05 AM »
Interesting discussion.  I still think that the easiest solution would be to instrument the handle; the differences in tension between the up and down lines would provide an indication of the torque being applied to the bellcrank and with a little math an idea of the load applied to the pushrod.  The other thing to keep in mind is that we are measuring small loads and the portion of the control system to be instrumented will have to be designed accordingly (i.e. be flexible enough to give the gauges something to measure).  If instrumenting the handle the load measuring clips would have to be relatively thin or maybe be U-shaped.  I do not think that line stretch would affect the results since in the end, the input is manual which itself carries a lot of variation.

Many years ago I read an article on an old magazine where they were trying to measure the loads needed to move a control surface; this was in the early days of radio.  So they ended up mounting a section of a wing on a car or pickup, deflecting the surface a known amount (in degrees) and then driving at different speeds to see how much force was needed to keep the surface deflected.  Compared to a R/C model our planes fly in a relatively narrow speed band so the testing would be a little simpler.

You could do the same today using a servo to deflect the surface and measuring the current draw to estimate the load needed to deflect the surface a certain amount.  You would have to do a little testing ahead of time so that you can correlate the current draw to a given load but it is doable without being overly complicated (as in recording data using a pencil and a note pad).  Anything we do will be an approximation but then again, we are not trying to land a rover on Mars....

Teo

Anything worth doing is worth doing well as they say. No we aren't going to Mars but it is just as easy to make good accurate data as it is to make garbage or questionable data. I am going to make some data. The following is me sorting through the options and discussing my decision process to that end. The only relevant question is whether to instrument the push rod or make a torque meter or a combination of both. The data collection package is the same regardless of where it is located, handle or airplane.   

Well, do you know what the contribution of the drag is on the wires well enough to be able to accurately calculate the small amount of tension required to move the controls to overcome the hinge moment out of the combined influences of flying in the elements? Yeah, me neither. My educated guess, having done that calculation for solid wires and realizing that I haven't a good idea of what the drag coefficients are for stranded wires, I'm gonna guess my level of confidence in that calculation, is plus or minus 25% at best. I could refine that by carefully reviewing the fuse cam videos, carefully measuring the line sweep from a less than optimal perspective and reduce that error by 10% ish. And I'd still be thinking about how to determine the contribution portion of those numbers from the elevator and from the flaps. I've just illustrated that there are three unknowns in this measurement method which require assumptions and calculations to estimate the levels from an alternate location when we could measure them directly with the same level of effort and not have to make any corrections.

I cannot talk to the other efforts but this is the kind of thing that requires some basic knowledge of instrumentation. I'm by no means the expert but I have spent a significant amount of my engineering years instrumenting engines and airplanes for flight test. This is the kind of thing that I do, make data. Good data, like Howards, is a tough one to get. In the years gone by they didn't have the same level of miniaturized ICs we have today. We have a computer hundreds of times more powerful than the best of the computers the space shuttle flew with in our cell phones.

 The instrumentation difficulty is the same on either end of the control lines with the exception that, on the airplane side, you have to be able to provision access to a place to mount the equipment. On both ends you need strain gages or load cells (an off the shelf strain gage in a package), an amplifier, a data logger and a battery.  If you can't find a load cell that will "clip in" you'll have to make one or two. 10, 20 or 30 years ago they didn't have this little beasty: https://www.adafruit.com/product/2796 , a microcomputer with data logging capability. If the ESC could work on 3.3V logic this thing would be the only processor you need on the airplane to do everything you'd like to do. It could connect to an IMU, a servo pot, a load cell amplifier, an ESC and the data could be extracted as easily as pulling the SD card out and plugging it in to your computer.

It is a fairly straight forward task to get the small signal out of the differential tension across either side handle to bellcrank. In both instances a load cell or place to mount strain gages is necessary. I searched for an hour or so and could not find anything suitable for this purpose which, a) easily fit between the handle and lines or b) cost less than $350. So, creating something is required. it's not too hard to do and calibration isn't difficult either. So, purchasing and using strain gages is necessary. These range all over the place in price and the price is function of sensitivity. But I can purchase strain gages sufficient for the job $37/5 enough to instrument a bellcrank or a handle or a pair of pushrods.

Picking off the differential tension between the lines on either the handle or the bellcrank is straight forward. Each link would be configured in a half bridge and each would receive the excitation voltage across the pair. Each pair is wired as a voltage divider on the link. The differential signal then would be pulled out between the two cells on each branch and compared.  Simple enough and it would certainly make some interesting data.

The alternatives to that are to instrument the push rod or the control horns. Neither one is any more difficult than creating the tension links before the model is assembled or if it is a take apart airplane. Carbon fiber is very stiff and we use it primarily in the control system because it is easy to get carbon fiber tubes and make pushrods with them. We do that more because of the bulk modulus of the section which prevents the PR from bowing. Stiffness isn't the primary problem. We get a bonus of an easy to bond very stiff material when we select CF as the material for a push rod. But the requirement doesn't force the use of CF for the pushrod and aluminum will suffice for load and modulus wise. Bonding the aluminum reliably is a bit more difficult than CF by a factor of about two requiring adhesives such as 3M DP460. What this means is, that if a good strain signal can't be gotten because the loads are so low, a material change in the PR could be substituted with less stiffness and the impact to the airplane would be minimal. I have 7075 arrow shafts, I used to make tail booms with from the ones I damaged shooting them at stuff. The primary advantages of instrumenting the push rod is that it can be accomplished in-situ and we don't have to perform any compensation calculations. We're directly measuring the force required to create the hinge moment. Calibration wouldn't be all that hard either.

In an ideal world where we are wishing to know the hinge moment we'd measure it directly. This is the last on my list of options and I would venture a guess that the data quality difference of this method to be insignificantly small when compared to using push rods. However this is an option to consider during the construction of an airplane which is being built as a flying test bed. Such as my FLCL model in the other thread. Yes, this diatribe is me thinking out loud on a thread. I am going to instrument that airplane for hinge moment measurement. The only question is how and where. Creating a torque meter is no more difficult than making a load cell link. It involves pretty much the same elements in construction. The only real challenge is in attaching the strain gages to a current control horn. A torque tube could be made in the same way as a push rod. My only reservations with this method is the pivots and the bonds. A pivot on the tube could be accomplished by...

--- dang, I hate when this happens ...  I had considered an alternate torque tube flap drive early on the in the design and decided against it. Now, I'm thinking a minor reboot as I just figured out how to do it and a good reason to proceed. ---

... The torque tube pivots would be easily done by making an insert which adapts a rod which is essentially a control horn cut in half. The torque tube would then be inserted between the two halves. This would result in the flap control link being moved outward to about the MAC of the flap This would allow for enough wind up to create a good signal. This is the most complicated method but most likely the one with the highest accuracy. I'm going to spend a little time on this in the CAD. Stay tuned.




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Re: Line Tension Sensor
« Reply #27 on: December 22, 2021, 11:56:04 AM »
FWIW, All of the ESC’s I have tried work just fine with a 3.3v logic signal.
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Online Ken Culbertson

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Re: Line Tension Sensor
« Reply #28 on: December 22, 2021, 02:30:11 PM »
--- dang, I hate when this happens ...  I had considered an alternate torque tube flap drive early on the in the design and decided against it. Now, I'm thinking a minor reboot as I just figured out how to do it and a good reason to proceed. ---
My 2c says go with the pushrod if the question being asked is "how much force is required to move the flaps". That is the part providing it.   I still think that the practical answer is "whatever it takes".

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Re: Line Tension Sensor
« Reply #29 on: December 22, 2021, 07:08:15 PM »
FWIW, All of the ESC’s I have tried work just fine with a 3.3v logic signal.

That's good to know. I've never really ventured to use one of the controllers with 3.3v logic. Most all of my boards are 5V. I have the Feather coming and will see what ESCs it will run. I kind of figure the ESC will run okay. The real concern I have is getting power from the BEC. Some of the Feather boards like the previous Feather state 5V is the maximum input it can take and BECs can drift above 5.5V. Maybe it's not a problem and I will find out I guess.
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Re: Line Tension Sensor
« Reply #30 on: December 24, 2021, 05:02:30 PM »
That's good to know. I've never really ventured to use one of the controllers with 3.3v logic. Most all of my boards are 5V. I have the Feather coming and will see what ESCs it will run. I kind of figure the ESC will run okay. The real concern I have is getting power from the BEC. Some of the Feather boards like the previous Feather state 5V is the maximum input it can take and BECs can drift above 5.5V. Maybe it's not a problem and I will find out I guess.

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Re: Line Tension Sensor
« Reply #31 on: December 24, 2021, 07:24:12 PM »
3.3v cmos logic outputs should work well to supply 5v ttl logic inputs just not the other way around. The feather boards aren’t designed to be supplied from an external 5v supply. What that basically means is there is no diode on the board to protect you computer usb power supply if you happen to connect both usb power and 5v from the esc bec at the same time.  Just don’t connect both and you should be fine.  Check the Adafruit schematic for your feather to see which regulator they use. It may accept up to 16v in. Hence the caution to not connect your computer usb at the same time.
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Re: Line Tension Sensor
« Reply #32 on: December 24, 2021, 09:28:04 PM »
3.3v cmos logic outputs should work well to supply 5v ttl logic inputs just not the other way around. The feather boards aren’t designed to be supplied from an external 5v supply. What that basically means is there is no diode on the board to protect you computer usb power supply if you happen to connect both usb power and 5v from the esc bec at the same time.  Just don’t connect both and you should be fine.  Check the Adafruit schematic for your feather to see which regulator they use. It may accept up to 16v in. Hence the caution to not connect your computer usb at the same time.

That's pretty much what I figured. It's not an issue as it can fly with a 1s battery and for data flights that a non issue as isn't a regular type event. I'm not an expert on the electronics. My background is more in the integration side.
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Re: Line Tension Sensor
« Reply #33 on: December 31, 2021, 07:49:00 PM »
I made a decision on how to instrument my airplane. The way the new airplane is being provisioned for modifiable control system the best place is in the control horns at the flaps and elevator.  I'm actually probably only going to instrument the outboard flap as any failure mode there is more than likely manageable. There is no simple method of taking an off the shelf device and inexpensively installing it. For $7 I got some strain gages off of Amazon and I machined a bar to make a load cell. This specific one is for testing. The one that goes in the airplane if this aluminum proof of concept one works will be machined from brass. I don't have any brass on hand the right size to machine in to a proper part. The reasoning behind making a brass part is that it can be silver soldered on the flap torque rod. This part is simply for bench testing to see what it takes to be able to measure the hinge moments and to get the software working correctly for the flight pack.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #34 on: January 01, 2022, 04:13:03 PM »
Not what I’d do, but interesting.
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Re: Line Tension Sensor
« Reply #35 on: January 01, 2022, 05:20:40 PM »
Not what I’d do, but interesting.

I haven't yet tested it. But me too initially. I went through a number of different solutions. This solution isn't retrofittable but the new airplane is provisioned for control system modifications and the way the flap root pivot is designed makes this an easy change. The brass version is intended to replace one of the horns on the flap drive. Since my intent with the new airplane is to be able to change the hinge line offset and other re-configurations, the hinges have to be made so they can be changed readily along with the root drive. I had thought about using  toque tube forward in the the wing and then driving the flaps with a rod and linkages ala RC old style ailerons. But I was sitting with those cheap strain $8/10 gages and this idea came up. Make a brass horn into a load cell by making it wider and cutting a slot in it so it can flex slightly. In terms of a load cell its pretty much how they are typically made. The torque strain gages are $37/5.

I thought seriously about instrumenting the push rods as well but the CF is super stiff and getting a good reading will be tough. And then I'd have to do some math to separate the loads. So direct measurement is always best. By tomorrow, I will have done some testing to see where this lies. I could make some S style load cells for the control handle and I considered that as well. I haven't yet discarded that as I think there is some valuable knowledge to be gained from that kind of data. After getting the HX711 and the Feather talking, and the filtering figured out, those kinds of projects will be easy. I was thinking it would be neato coolio to have it all in one instrumentation package with IMU and motor control data but I don't think there's enough program space. So it will have to be two separate packages.

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Re: Line Tension Sensor
« Reply #36 on: January 01, 2022, 07:53:42 PM »
Not what I’d do, but interesting.

I meant to ask, how would you do it?
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Re: Line Tension Sensor
« Reply #37 on: January 01, 2022, 08:06:12 PM »
I meant to ask, how would you do it?

He didn't say it was not how he would do it, but what he would do.

You want to measure control surface loads, he wanted to measure line tension.
 At least that is the way I took his comment :)

FWIW, a strain gauge on the axle could do either, depending on which way you orient it to measure the force.
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Re: Line Tension Sensor
« Reply #38 on: January 01, 2022, 11:21:12 PM »
He didn't say it was not how he would do it, but what he would do.

You want to measure control surface loads, he wanted to measure line tension.
 At least that is the way I took his comment :)

FWIW, a strain gauge on the axle could do either, depending on which way you orient it to measure the force.

We've been in a discussion regarding the hinge moment of the controls, particularly the flaps. Howard's statement; " I intend to do that to measure control surface hinge moment.  The total line tension sensor is another matter. " Measuring the line tension is a round about approximation way to that end although it is the end result of the hinge moments combined and has some level of interest. There are a number of options available including the push rods which I'm afraid are too stiff to get a good signal from. This sparked my interest specifically since much of the efforts I've been doing significantly reduce the hinge moments. No one here is going to accept much of what I say however, as I am not a known quantity, so actually measured data is valuable. 

Yes, a torsion strain gage, a very small one, as in expensive, could be used on the axle. I considered that to. The strain on the axle is likely sufficient to get a good reading. The strain gages I put on the arm are not really very expensive $8 for 10 as opposed to $37 for 5 to get the torsion although I'm not certain they are small enough to use on the 1/8" wire axle which is why I considered using a larger arrow shaft forward of the aft TE spar. I'm not entirely sure I'm prepared to fall that far down the rabbit hole. But you know how obsession and manias go...

These el cheapo gages might be able to do the job with the right set up. That's why I made the test part. I did some testing but broke one of the strain gages because my smallest hook up wire is still super large comparatively. Tomorrow, I'll remove the one that broke and try again. I ordered some more of these style gages in case I run out testing.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #39 on: January 02, 2022, 02:55:04 PM »
FWIW, a strain gauge on the axle could do either, depending on which way you orient it to measure the force.

Brilliant.

I initially asked about a line tension sensor because: a) I had the notion of using line tension in the throttle control law, and 2) I figured Pat would know what to do.  He is a fount of electronic component wisdom.  A line tension sensor for throttle control would necessarily be located in the airplane.  Amazon is bringing me strain gauges.  Later I decided to try something else first for throttle control. 

I have been working on hinge moments off and on since the B-17 project.  I've done some ciphering, but my measurement effort so far has been just to dissect some luggage scales.  Mark's notion of measuring hinge moment aboard the airplane seems to come from his assumption
Well, do you know what the contribution of the drag is on the wires well enough to be able to accurately calculate the small amount of tension required to move the controls to overcome the hinge moment out of the combined influences of flying in the elements? Yeah, me neither.

I think I do.  Therefore, I'd measure hinge moment at the handle or from a wing mounted on a car unless I can slick-talk my way into a wind tunnel.   
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Online Tim Wescott

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Re: Line Tension Sensor
« Reply #40 on: January 02, 2022, 04:24:10 PM »
... Therefore, I'd measure hinge moment at the handle or from a wing mounted on a car unless I can slick-talk my way into a wind tunnel.

Casa Rush should have its own wind tunnel.
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Re: Line Tension Sensor
« Reply #41 on: January 06, 2022, 07:09:59 AM »
I did some testing on the arm I crafted and managed to spark one of my oscilloscope probes when one of the grounds shorted to the supply voltage. In the process I damaged one of the strain gages. It's a fairly delicate process and I hadn't yet potted the gages and wires. I wanted to see if I could get a usable signal. I was able to get a signal but not measure it before the damage occurred.

I've got the airborne package pretty much to the working stage but haven't yet incorporated the strain gage side as I need to back up a step with the gage installation. I'm not really certain of how much more effort I'm going to put in to this other than being a wonderful intellectual exercise. What I can say is that today I have a much better appreciation for the instrumentation guys I used to work with. It was fun and interesting but my role was in definition and results not the doing part of the instrumentation.
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Re: Line Tension Sensor
« Reply #42 on: January 08, 2022, 06:34:20 PM »
Dude, I got a data maker... I finally got a load cell configured that can measure the small moments that can be expected from the flaps using the bar I showed the photo of. The photo is the serial data coming from the Feather data logger. The wiggles are not very large pressures on the end of the load cell. Right now my challenge is figuring out why the data logger is not writing the files correctly to the SD card. This particular device doesn't have enough memory to load and operate the IMU I have with the loadcell amplifier and it's code. I have another one coming that will have enough memory for the operation. I can make in flight hinge moment data.... #^
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Offline pmackenzie

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Re: Line Tension Sensor
« Reply #43 on: January 08, 2022, 06:44:27 PM »
Very cool  8)
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Re: Line Tension Sensor
« Reply #44 on: January 08, 2022, 06:49:02 PM »
So, I've had a bit of tinkering time and this load cell is sensitive enough to work in tension with a small modification or two. That means I can probably get it in the current airplane and do some A-B tests. It does have some temperature drift though.
 
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Re: Line Tension Sensor
« Reply #45 on: January 08, 2022, 06:49:34 PM »
Very cool  8)

I know. I'm freakin giddy.. Case you can't tell..
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Re: Line Tension Sensor
« Reply #46 on: January 16, 2022, 07:06:42 PM »
Closer to a flying configuration. I hacked out a couple load cells that can be installed in the primary pushrod to the flap horn. I futzing around with the bending cell helped me figure out if I could make the tension cell and I'm fairly certain this will work. With the right bridge configuration I get enough signal to drive the bending cell with the push of a finger. The plan is to use a tight fitting hole for the push rod and red Locktite plus a clamping force. It should hold.

I've got most of the pieces working. The load cell amplifier is making data and the Feather is reading it and at least writing to the serial port and the OLED. I have learned how to get the Feather to write to the SD card. Now all I gotta do is get the pieces put in to a single package.
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Offline Howard Rush

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Re: Line Tension Sensor
« Reply #47 on: January 16, 2022, 10:28:10 PM »
Cool.  I'm no stress man nor instrumentation guy, but it looks to me like the signal levels from the load cells would be tiny and you'd be operating in an electromagntically hostile environment.  How about mounting the amplifier and bridge right on the pushrod? You've no doubt considered this, and the rig you're showing is a necessary prototype.

The iterative nature of my control geometry program probably makes it too crude for figuring control force as a function of control surface deflection hinge moment.  I can refine it if you'd like.

I think you're putting the sensor in the right place and measuring total hinge moment of flaps and elevator.  What if you overbalance one to reduce total hinge moment, e.g. putting huge balance surfaces forward of the elevator hinge line?  Your instrumentation could tell you if that's a good idea and tell you if it's a big deal when the unstable surface passes through zero hinge moment. 

Edit:  corrected stupid statement above. 
« Last Edit: January 17, 2022, 09:33:57 AM by Howard Rush »
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Re: Line Tension Sensor
« Reply #48 on: January 17, 2022, 12:03:01 AM »
Cool.  I'm no stress man nor instrumentation guy, but it looks to me like the signal levels from the load cells would be tiny and you'd be operating in an electromagntically hostile environment.  How about mounting the amplifier and bridge right on the pushrod? You've no doubt considered this, and the rig you're showing is a necessary prototype.

The iterative nature of my control geometry program probably makes it too crude for figuring control force as a function of control surface deflection.  I can refine it if you'd like.

I think you're putting the sensor in the right place and measuring total hinge moment of flaps and elevator.  What if you overbalance one to reduce total hinge moment, e.g. putting huge balance surfaces forward of the elevator hinge line?  Your instrumentation could tell you if that's a good idea and tell you if it's a big deal when the unstable surface passes through zero hinge moment.

You're correct about the environment but there's a number of limitations, space being primary. What's in the picture is only the sensor and the balance half of the bridge. The interface amplifier is a big as the card and link which presents a real challenge trying to mount it on the pushrod. As it is the inline load cell is about as big as can fit in the space available. I didn't think this through fully and didn't provision a mount for the bridge board on the link when I went out and started making it. Maybe I'll make a plastic part to do that. What's another few hours?

I'm not sure about the direction your math tool should go. It's far cooler and sophisticated than I'd go with it. I'm kind of a back of the napkin kind of guy. My only real experience calculating hinge moments is for Part 23 loads analysis which is not vey useful for this effort. Basically with that you take the design flight load and assume a uniform load distribution tapering from the quarter point of the surface  to the TE. For this effort a better way would be to use the Cp curves from one of the analysis tools and do the moment integration around the hinge point. I'm way past what I originally set out to do which I never intended on doing any of the spade or hinge moment testing. It was one our dialogs that woke me up in the middle of the night with an idea. I think it'll be worthwhile though as we'll learn a lot. And, as you suggest, and I've planned on for the new airplane it will provide good insight into bad gradient trends.

I don't expect any trouble with the first configuration of the new airplane. The balance on that design is relatively conservative.
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Re: Line Tension Sensor
« Reply #49 on: January 17, 2022, 11:14:25 PM »
I've been struggling all night. BUT.... I got the loadcell reading and printing on serial and plotting.


Best of all I can write and read to the SD card.    #^

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