control column pitch centering using gas struts

[Michael S]

Hi :)

I hope this is the right forum section for my problem.

yesterday I introduced my 737 control column. (http://www.mycockpit.org/forums/showthread.php?t=18209 (http://www.mycockpit.org/forums/showthread.php?t=18209))

Today I tried to do the centering with gas struts. This first try looked like this:

http://img4.imageshack.us/img4/2572/pic0369e.jpg

http://img4.imageshack.us/img4/8490/pic0370p.jpg

It does not look nice but for a first test I thought it should do the job. But it didn´t. The problem is that I can push or pull the control columns pitch axis into every position I want to and it will just remain there. No centering. :???:

Am I missing something here? I really hope, someone can help.

Thanks in advance

------------------

Edit: I want to add a few things:

The gas struts are 110N both

In central position both gas struts are half way compressed. So when I move the control column one gas strut is compressed and the other decompressed. (I hope you understand what I´m talking about :cool:)

I don´t have any experience with gas struts and I thought if this works for others it should work for me to. I don´t have any idea what I am doing wrong so I would highly appreciate any help.

[Crescent]

The best explanation I can give is Westozy's. http://www.mycockpit.org/forums/showthread.php?t=12672&page=10

Scroll down and you'll find a drawing he posted explaining what he did on the Spitfire column. I ran into the same problem you have with my Lancaster column. I tried gas shocks just like you, door closers, bungies, and finally I settled on springs about 8inches long and half inch wide.

They allowed the yoke to be pushed or pulled fore and aft giving the correct tension on one spring while the other relaxes. As you ease off pressure the spring in use pulls the column back to center. Hope this helps.

Chris.

[spitfire9]

The best thing to do is mount the gas struts in tubes. The end of the tube is mounted to the floor (or wherever), The struts are slid into the tubes . For example, when you pull back on the yoke ,the front strut is all the way in the tube and is compressed ,while the rear strut slides out of its tube ,offering no counter resistance. And vise versa when the yoke is pushed forward the rear strut slides back in the tube until it hits the back of the tube and starts being compressed ( at yoke center),, while at the same time the front strut starts to slide out of its tube.

This allows good centering and fixes the problem of having one strut pushing with equal resistance to the one being compressed.

Wish I had some pictures to show you but I don't at the moment.
If you have trouble understanding that rushed explanation. I could post a drawing or someone else here can offer some pictures or help.

[Geremy Britton]

I experienced the same problem. It is because your yoke is heavy and there is no counter balance on the other side of the column... so naturally the column will droop back slightly and won't have the power to push back up to the central position due to the weight (yoke) on the end.

Simple maths the further from the pivot the 'heavier' the mass. So as a control column height is around 800mm (don't quote me on that) then there is going to be a fair amount of force on the pivot wanting it to drop back. Therefore you need either springs which has been mentioned. Or more push force on the side of the yoke at the pivot to push it back to the central position.

Hope that made sense. :)

[spitfire9]

Please excuse my quick drawing but this is what I mean. Worked great with my rudder pedals.

2519

[Michael S]

Wow, thanks for your help!

@spitfire9: Yes, I understand what you mean and it really makes sense. Thanks alot. :D

@Geremy: Thanks. I honestly don´t think that weigth is the problem here. When i built the first version, I used 6 springs and they were not strong enough because of the weigth and the long lever arm. So for this second version i have done the maths and 110N should be strong enough. Also, if the yoke was to heavy, it would drop back from the center position. But I can push or pull it into every position I want and it will remain there. So weigth can´t be the problem.

@Chris. Thank you too. This link to Westozy's explenation was really helpfull.

--------
So, if I got it right, i will have to mount the gas struts in a way, that either the outer or the inner end is free to slide.


I was just wondering: when researching for this project, I relied for example on Ian`s page:
http://www.737ng.co.uk/Boeing_B737_Control_Column.pdf
His gas struts are not free to slide. So I don´t really understand how he achieved the centering. I´m just curious. ;)

BTW: I´m really stunned. Great forum here. Thanks so much for your help and for sharing your knowledge. :eek: Finally, I´m a bit relieved that I´m not the only one who had this problem. :cool:

I will see what I can do and keep you posted. ;)

[Westozy]

Here's my "proven" solution!!!

http://www.mycockpit.org/photopost/data/597/Joystick_centering.jpeg

Cheers, Gwyn

[Michael S]

Thank you. The only problem with this is that my gas struts are pushing and not pulling. But I think I see the point. Maybe I can design something similar. :)

[spitfire9]

Westozy has the same idea as the tubes there,, only in reverse because the screen door closers are made to be pulled apart to get the force where as the gas struts are compressed for force. The opposite of the screen door closers.

[David Withers]

few golden rules that seem fine in the head but physics wont allow it.

scratched my head for ages a few years back.


when springs or struts are in tension and against each other they will not centre as equal force is applied by each strut!!!!!

one side/spring/gas strut has to be completely disconnected or taken out of play or the column will just sit there.

we are trying to force a long heavy column a foot either way. but trying to push it very close to its pivot. so the springs have to very strong... practice will help here.

need a combination of heavy springs and screen door closers or whatever hydraulic type struts available.

gas struts slow the spring return down...and the springs help to return the struts.

make up a simple linkage that has two legs and a pivot....when the strut is pulled, the piston has to extend and hold the column. when the column returns, the linkage folds and the strut moves the linkage rather than the strut.

when springs or struts are in tension and against each other they will not centre as equal force is applied by each strut!!!!!

this is the way to do it.....have a look at the image!!!!!!:D

make sure to combine springs and struts

http://i228.photobucket.com/albums/ee209/brissydave/gasstrut.jpg

[David Withers]

sorry i'm not yelling...just love my exclamation marks.:D:D

pm if you need help

[Michael S]

Thank you, David.

Sorry. Imho your draft only makes sense when using gas struts, that are pulling. I´ve got gas struts, that are pushing. They are compressed when they move in. This is how they work:

http://img35.imageshack.us/img35/6113/gasstruts.png

Maybe I misunderstood it but I think you are refering to pulling gas struts.

I also don´t really understand why I should use both gas struts and springs.

I do understand that the gas struts equal each other when both are compressed and thats why my centering didn´t work. But why should I use springs additionally?

Thanks anyway. :D

[Radar]

:pMayby this could help. Ian P.Sissons did a awsome job on his column

[David Withers]

Yep no problem.. You are correct. Most gas struts and the screen door closers gwyn was on about are pulling ones. They are nowhere near
powerful enough so we add springs for the return cycle.

Something that has sorta been perfected here in Australia and works like a charm.
The spring pulls and the gas strut dampens the pull.

Your pushing struts are another matter. Good luck on that one.
Btw the springs have no connction to the column.. They simply help the piston retract .

[Michael S]

:pMayby this could help. Ian P.Sissons did a awsome job on his column


Thanks. I know his documentation. But I don´t understand how he did the centering because his gas struts are also compressed both. That´s what I mentioned before:



I was just wondering: when researching for this project, I relied for example on Ian`s page:
http://www.737ng.co.uk/Boeing_B737_Control_Column.pdf (http://www.737ng.co.uk/Boeing_B737_Control_Column.pdf)
His gas struts are not free to slide. So I don´t really understand how he achieved the centering. I´m just curious. ;)



--------


They are nowhere near
powerful enough so we add springs for the return cycle.


Thank´s alot, David, for your help. My gas struts are really powerfull, 110N both. So I think I can do it without springs.

[Michael S]

I have now designed something working similar to the drafts that you guys posted. I would like to know what you think of it and if it will work.

The outer ends of the gas struts are mounted into a rail. In center position both gas struts are decompressed and fitting at the outer ends of the rails. If the column is moved to one side, one gas strut will be compressed because it already sits at the outer end of the rail. The other gas strut is free to slide towards the column. I hope you understand what I mean. Just have a look at the picture.



http://img18.imageshack.us/img18/61/gasstrutsidea.png


Thanks in advance. :)

[Crescent]

This system should work fine. I came up with almost the exact same design when looking at my own, but it didn't give me the correct tension I needed for the Lancaster, otherwise I would have used it. I had to go with a spring system instead. Good work.

Chris.

[Michael S]

Thanks a lot to all who helped. :)

I will try it like shown above and keep you posted. ;-)

[ak49er]

Make a very gradual "v" block, dead center under the desired column centering point. Then add a spring loaded rubber wheel onto the bottom of your column. Then when it rolls back and forth, it will lightly catch the bottom of the "v" shape. If you make the spring loaded rubber wheel adjustable, you can then adjust it for feel, and for if you add anything else later like switches, checklist holders, etc.

[spitfire9]

Michael S,, your last post is just about exactly what I was getting at with my very poorly drawn tube method.

[Michael S]

@ak49er: I do not fully understand how this should work. But as you can see I have found the method probably working best for me. So I will try it the way I described above. Anyway thanks for your help. Sounds interesting too. :)

@spitfire9: Yes. Basicly it´s the same functional principle. Thanks for your help. ;)

[verticallimit]

Have not even made control column yet, but have had these thoughts about the design.
If you connect the 2 rails together and mount them on a different track system, this system towed by a motor controlled by the autopilot.

http://img689.imageshack.us/img689/2280/forslag.jpg

[David Withers]

thats it Micheal s...as long as the struts are free to run when not being extended you will be fine. exactly the same principle we use down here but in reverse.

now you just need to mount pots.

[chris0518]

Hi Michael,
I just red your post about your centering issue. Maybe you want to take another approach on that. I am a real world aircraft technician working on commercial airliners. On almost every aircraft with a conventional hydraulic actuating system the flight controls are centered with a so- called feel and centering unit. The pilot input moves a cam. On that cam runs a roller which is attached to a link. The link is hinged to the fixed aircraft structure on one side and attached to a spring on the other side. The other end of the spring is also attached to the fixed structure. When the control collumn is moved, the spring will extend. That simulates a progressive force (depends on the shape of the cam) on the controls and also provides centering. In my opinion that would be an easier way to achive centering, than doing it with gas springs. The feeling should also be more realistic.

I attached a sketch to this post.
Should you decide to cosider this solution and should you need further information on that, just let me know.

Chris

2531

[Michael S]

Hi, Chris.

It´s very interesting to know how it´s done in real world´s airplanes. Now that I have the gas struts and also an almost finished control column, I will go with the gas struts as described. But it also might be of interest for a future version of my yoke. Who knows, maybe one day I will "upgrade" my project to a full size sim. Then I would have to build dual linked yokes so it´s very interesting anyways.

Thanks a lot. ;)

[Westozy]

Great stuff guys, there are obviously "many ways to skin a cat"! Discussions like this where quite a few valid solutions are placed in view of the forum will be appreciated by many other builders. I think we need an 'ideas' gallery where everyone's sketches can be displayed, I'll talk to Matt Olieman about it.

Thanks guys,

Forum Manager

[spitfire9]

Very good idea Westozy,, I'll second that .

[Michael S]

Thanks to all who helped. Now I´ve got it working. I still need to add stop points for the forward and backward stop positions and I need to conect the pot. But - thanks to you guys - I´ve got a working elevator centering and let me say one thing: it really feels good. :D So thanks again. Here are a few pictures:

http://img21.imageshack.us/img21/3525/yokeelevatorcenter.jpg

http://img697.imageshack.us/img697/6262/yokeelevatorfwd.jpg

http://img121.imageshack.us/img121/8736/yokeelevatorbwd.jpg

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Edit: As I originally planned the control column to be centered with halfway compressed gas struts, there was not enough space at my base plate. The piece of wood, that you can see in the front of the control column will be replaced with the base plate of my rudder pedals at a later point. I just wanted to add that.

[goof2092]

Hi Chris,

Old post here, but I guess this is the reason these things are documented to help newer builders;)

I am at the stage of constructing the dual linked columns for my B737. I have read your post and seen the sketch, but I am technically challenged to could not interpret other connections very well. Do you have more drawings I can look at to help with the fabrication?

Thanks

Anthony

[verticallimit]

Hi Here a little video from the project.

http://www.youtube.com/watch?v=38Y72Xna43M&feature=share&list=UUTmfxyMIfyyC4TfERJScx4g
And yes... It's first this year i have build it in the cockpit. A long time under its way ;-)

[tiburon]

Great work ! Could you tell me where you got the actuators from ?

Cheers

Martin

[goof2092]

Wow Claus,

This looks fantastic. What force are the gas springs? Those actuators look interesting indeed. Source?

Anthony

[Tripacer]

Michael,
Very nice installation. What size are your gas springs and where did you get them? I think something like that would wwork on my sim and I can put it beneath the floor board (out of the way).

Tripacer

[verticallimit]

Sorry for the late reply.
My gas springs are 200 newtonmeter. You can find this gas spring in a kitchen store. It's used for vertical hinged doors. Or here https://shop.angst-pfister.com/ishop/Antivibration%20Technology/Spring%20elements/Spring%20elements/Gas%20springs%20APSOvib%C2%AE/category/node/14679.xhtml;jsessionid=571055CEAA2F433F94E844843AB13436

Then, the cockpit has been reassembled, and 2 nights has been used with small short flights from one airport to another, to test and to make small adjustments on the flight controls.
It become really good, and the trip to fitness center can now be skipped when you have to pull and push with approx. 6 kg on the steering gear, almost like the real airplane. So you quickly learn to use the trim button.
The autopilot dons't move the flight controls yet, this function must be integrated later on. But who can also get the idea to fly on autopilot .......
897489768977

[Shawn]

Have not even made control column yet, but have had these thoughts about the design.
If you connect the 2 rails together and mount them on a different track system, this system towed by a motor controlled by the autopilot.

http://img689.imageshack.us/img689/2280/forslag.jpg

this is basically the same design I am going with for my dual controls but with springs for centering. I have an electric linear actuator that will drive the shaft inside a linear bearing. The linear bearing will mount inside the 2" square tube that the actuator is currently sitting on. I plan to use a L shaped control arm so I can adjust the throw of the actuator.


http://youtu.be/af5jlWKuk7Q

Shawn

[Shawn]

9013

This is is the linear bearing that will be installed in the square tube. A shaft will pass through it and springs at each end will attach to a lever on the main control axle. If anyone sees an issue with this plan I'd be interested in hearing your thoughts.

Shawn

[verticallimit]

Be aware that there may be major forces in the different anchor points.
Your design looks good.

[Shawn]

Thanks Claus, the weak link will probably be the actuator itself. It's rated for 135lbs but I'm not sure what the holding power will be?