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)

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





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

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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 )

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/show...=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.

yoke.JPG

[Michael S]

Wow, thanks for your help!

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

@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.

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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. Finally, I´m a bit relieved that I´m not the only one who had this problem.

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

[Westozy]

Here's my "proven" solution!!!



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!!!!!!

make sure to combine springs and struts