Avid Horizontal Stab Rigging

18 posts in this topic

Posted

Hey Everyone!

We have a B model and we found a horizontal stabilizer to replace our old one which was corroded badly. The bolts and everything for the stab braces line up perfectly, but compared to the cross members of the fuselage the new stab sits at a pretty good angle with the right side significantly lower than the left. Is that normal?

Unlike most other birds I’ve worked on, there does not seem to be a way to adjust it, nor does the build manual seem to help.

Any suggestions? Thanks in advance!

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Posted

On my KF1 I found that the v braces were interchangeable between right and left and the stab would be properly aligned either way but there is a front and back to the v braces if that makes sense. If you have the leading tube of the v brace on the trailing tube of the stab then the whole stab will be cockeyed.

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Posted

On my KF1 I found that the v braces were interchangeable between right and left and the stab would be properly aligned either way but there is a front and back to the v braces if that makes sense. If you have the leading tube of the v brace on the trailing tube of the stab then the whole stab will be cockeyed.

We checked this first thing and if we had them forward to backwards nothing lined up at all. We did try swapping the braces side to side without any change!

Thanks for the help!

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Posted

I believe there is a twist from side to side of about 1/4" or something from the factory.  How much difference are you actually seeing.  Give a dimention, and for that matter, a picture or several.  JImChuk

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Posted

I believe there is a twist from side to side of about 1/4" or something from the factory.  How much difference are you actually seeing.  Give a dimention, and for that matter, a picture or several.  JImChuk

are you taking about the offset of the vertical stab? My plane has a small plate that allows the horizontal stab to be centered on the fuselage not the vertical stab. 20191002_065038.thumb.jpg.ac0e49cf069237

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Posted

No, I agree there was a factory twist to the H.S. on earlier models. Won't lay flat on a table. But I don't recall offhand which model(s), which way or what amount it is. 

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Posted

Sorry for the delay everyone! Thanks for all your help! I finally made it to the hangar to get some pictures. The first picture is of the original stab which is corroded internally. The second is of the replacement (currently green). The original is fairly close to level, but still slightly out in comparison with the top cross member of the fuselage. The new one is significantly more askew. Again, the holes all line up perfectly with the struts and everything. Thoughts?

I have access to a machine shop and was thinking about cutting off the ends of the stab struts, machining an internally threaded 10-32 insert that would be welded inside the strut tube and accept a AN-42 eyelet which would allow me to adjust the rigging. Would that be a good idea or is there some other underlying issue?

2019-11-01 Avid Original Stab.jpg

2019-11-01 Avid New Stab.JPG

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Posted

Now I'm going to have to climb up on a ladder and have a look at mine.   If your plane is level sideways at the doors, is the verticle stabilizer then exactly plumb?  (Incase you aren't familiar with that term, in carpentry lingo, level is for horizontal, plumb is for verticle.)  I'm wondering if the fuselage is at all twisted???  Probably not, but something to check anyway.  Kitfoxes do have  heim joints on those struts but Avids never did. And for that matter, if my memory is correct, just one heim joint on each side, not both, cause it was just for adjusting the incidence of the horizontal stabilizer.  I suppose you have compared the distances between the holes and such on the two different stabilizers.   JImChuk

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Posted

Does the replacement horizontal stab sit level without the struts installed? If it does, I would say the strut is too long. Did you get struts with the replacement stab? If you did, might try them.

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Posted

Now I'm going to have to climb up on a ladder and have a look at mine.   If your plane is level sideways at the doors, is the verticle stabilizer then exactly plumb?  (Incase you aren't familiar with that term, in carpentry lingo, level is for horizontal, plumb is for verticle.)  I'm wondering if the fuselage is at all twisted???  Probably not, but something to check anyway.  Kitfoxes do have  heim joints on those struts but Avids never did. And for that matter, if my memory is correct, just one heim joint on each side, not both, cause it was just for adjusting the incidence of the horizontal stabilizer.  I suppose you have compared the distances between the holes and such on the two different stabilizers.   JImChuk

Thanks for the help! I'll have to take the digital level to the hangar and see what I can figure out about this. I presumed there wasn't any twisting in the fuselage since the original stab is pretty close, but who knows? I did compare the distance to the holes on the to stabilizers--they look so close to identical by placing them on top of one another. I might have to measure really accurately to see for sure. They're obviously out a little bit otherwise it would mount the exact same as the original.

Does the replacement horizontal stab sit level without the struts installed? If it does, I would say the strut is too long. Did you get struts with the replacement stab? If you did, might try them.

Yeah, you can have it sit any which way without the struts since it is just pivoting on the front and rear mounting bolts. If one strut is too long, then the other strut is identically too short as the holes all line up perfectly--and I mean perfect.

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Posted

I suppose you tried reversing the struts as well??  JImChuk

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Posted

I suppose you tried reversing the struts as well??  JImChuk

Yeah. I tried that--put the left side on the right side. They are exactly the same length and bring the exact same results, unfortunately. If you flip them front to back, they don't even come close to lining up to the holes.

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Posted

If your plane is level sideways at the doors, is the verticle stabilizer then exactly plumb?  (Incase you aren't familiar with that term, in carpentry lingo, level is for horizontal, plumb is for verticle.)  I'm wondering if the fuselage is at all twisted???  Probably not, but something to check anyway. 

I checked with a digital level today and found the vertical stab out a little less than 2 degrees when compared to the vertical stab, so it's not perfect. How close did they build them?

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Posted

mine is out too when you put a level on it.  I can see it and it bugs me, but it seems to fly just fine.  The elevator also has a twist in it, I think it was to keep it loaded and decrease the potential for flutter.  I think that's what I have read some where anyway.

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Posted


here is my unfinished Model C from the same perspective.. hope that helps

 

horizontal_stab.jpg

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Posted


A little off topic here but I see you have triangle gussets on the front spar carry through? Is this something you came up with?

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Posted


no, that's something the swiss FOCA (equivalent to the FAA in the us) came up with... together with the same kind of gussets at the rear carry through, mounting brackets for the window which are welded to the carry throughs so you don't have to drill into them and some other reinforcements.. as i went through the differences between the mk IV and the model C i actually noticed, that some of these reinforcements seem to be standard on the Mk IV.

the story behind all this is in the way experimental aircraft are built and registered in switzerland. The first bilder to build a new design, be it a kit or a plans built airplane or a one of a kind airplane has to go through quite some additional effort to have FOCA engineers verify the design by Swiss standards. It doesn't matter if this design is flying elsewhere in the world, they want to make sure it complies to our regulations for strength etc..

when the first builder of an avid did the whole verification, the FOCA engineers found chance to prove that they are indeed needed, so they came up with some modifications to both the fuselage and also the wings (both the front and rear spars have doublers installed towards the center to reinforce them) in order to make the airplane compliant with their ideas of engineering minimas :)

as a result, every avid registered in switzerland (HB- Reg) has these gussets and the other mods installed. we all had to take our fuselage out of the crate and drive it straight to a certified welder to have the mods done.

Luckily, the previous owner took good care of doing all this and complying with all those mods.

After discussing in another thread about an Apex installation, i went through the mods yesterday evening and thaught that i could scan and upload them to the forum.. it would be interesting to see how much of these changes made it into the MK IV.

the FOCA is usually in direct contact with the kit manufacturer and also reports their findings and calculations back to the manufacturer..

just to be precise.. today it is no longer the FOCA doing all this, this work has been delegated to the EAS which is the equivalent of the EAA in the us.. however, the process is still similar. We also have to get all modifications approved the same way, so stretching a fuselage is a big deal around here (for the first guy, but it is very well documented afterwards for others to follow) ..

cheers

Pascal

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Posted


I think the answer to most of the questions on the rigging of the horizontal stab. and elevator questions are in the construction manual. As I read the comments I seemed to recall some directions I followed when building my Avid Plus, and when I dug out the manual there it was. In Section D Fuselage/ ch.4 Rudder pedals, tail  page 5, para. 7. gives some very good directions on the rigging. The thing that I remembered was using a carpenters square to set the horizontal to the vertical stabilizer, and there it was too, picture F4-7shows  exactly that. The horizontal is rigged perpendicular to the vertical stab.

In para. 3 on page 4, the information on the twist built into the elevator is discussed.

Nice to actually remember something once in a while.

Hope this helps.

Jackak

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