Vertical rudder tube issue

27 posts in this topic

Posted (edited)

Just before I finished the plane I noticed thee fabric looking a little loose near the tail. I used a heat gun and heated it up very gently until it tightened. Just before the final inspection I noticed it again so I used the heat gun again. All seemed to be ok but after 50 hours of flying the fabric is odvious loose again and I can see the rubber post on the vertical Stab is deformed. I checked a spare Avid Mark IV airframe I have and there is little to no bracing in this area. Poor design. 

I need to get this figured out. It can’t go unchecked. I have though about removing the fabric ( see pic with blue tape) on both sides, reattaching it to forward tubes and make a cover to cover the area after repair. 

Here are a few pictures. I’d be interested to hear what you all think. 

 

8E125FD3-55A5-4410-A4D4-2D522199FC15.jpeg

09402617-5F62-468D-A756-248C62B87AFC.jpeg

7D16BF4B-2F81-49C4-B210-4052BEAE2A6B.jpeg

F6B73124-CE48-4B92-81ED-E48505D4D103.jpeg

Edited by NorthIdahoAvidflyer

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Posted

I can't imagine fabric tension bending the rudder post. Not 1.8 or 2.7 oz fabric. I have seen 3.8 oz bend some wood structure but not a steel tube.  You could replace that piece of fabric with a panel. Would not affect anything. I would be curious to know why that piece of fabric keeps releasing tension. Possible fabric drumming, not to full tension (350 deg), or something evil.

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Posted

You might  want to burn some drain holes in the bottom of your rudders low point,and lower   Fuselage at the tail wheel!!!!!!

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Posted

Cracked tube or weld?

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Posted

I can't imagine fabric tension bending the rudder post. Not 1.8 or 2.7 oz fabric. I have seen 3.8 oz bend some wood structure but not a steel tube.  You could replace that piece of fabric with a panel. Would not affect anything. I would be curious to know why that piece of fabric keeps releasing tension. Possible fabric drumming, not to full tension (350 deg), or something evil.

Allen, look at the photo with my two fingers pointing. The gap in this area is larger then in any other area between the rudder post and rudder. The rudder post is bending forward and releasing the tension on the fabric. Not sure why yet but I’m tearing into it to take a look. I will check for water inside the tubes, cracks or anything that does not look right. Maybe it needs some reinforcement in the area. As you can see from the picture of my spare frame there is not a lot of bracing in this area. I’d say there is probably some pretty good forces from the rudder controls in this area. Guess time will tell. I’ll be tearing it apart this week. 

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Posted (edited)

Wonder if you could epoxy a piece of tube or a piece of wood between tail post right in front of the rudder stop tabs, and the cross tube the horizontal stabilizer struts bolt to.   You could at least give a try and see if that light bending of the tail post would stretch the fabric enough to keep it tight?  

I wrote that and then went back and looked at your pictures again and didn't send it because I saw that small brace coming up from the bottom that has the rear trailer hitch hole in it.  You would think that that brace should keep the tail post from bending forward.  The rudder hinges are above and below enough and well braced also, so I don't think they can flex forward.  I would check that that small brace tube is not broken loose.  Just to be clear on what small brace I'm referring to, I mean the one that starts at the bottom of the fuselage, just behind the tail spring bolt, and ends just below the rudder stop tabs.  I would try to see if you can push the tail post back like I mentioned in my first sentence as well.  JImChuk

PS  edited part of the first sentence.  Don't know what I was thinking when I wrote it.  Only one beer all evening two....:-)

Edited by 1avidflyer
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Posted

It's your tail post bending from landing stress on the TW spring. On my original Avid I noticed that was happening not too long before my lower longeron tubes broke right in front of the TW spring channel.  What I did was to cut the fabric in the triangle below the area you marked out, then wrap and glue it around those tubes to minimize your fabric repair area.  Then straighten out the tubes and split a tube the next size larger, clam in over the weak tube areas and weld it on.  Then add vertical tubes from the collector where your horizontal braces connect and truss it with a couple more tubes up and down back to the tail.  And add another brace tube on the rudder post half way between the existing one and the hinge and tied back to where your first vertical tubes connect to the longerons.

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Posted

That is a more logical reason for the problem. I didn't think fabric tension would bend the tail post alone. Might be a good thing to enforce on mine before I cover it. I have a habit of bumping the tail wheel first on landing, anyway.

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Posted

It's your tail post bending from landing stress on the TW spring. On my original Avid I noticed that was happening not too long before my lower longeron tubes broke right in front of the TW spring channel.  What I did was to cut the fabric in the triangle below the area you marked out, then wrap and glue it around those tubes to minimize your fabric repair area.  Then straighten out the tubes and split a tube the next size larger, clam in over the weak tube areas and weld it on.  Then add vertical tubes from the collector where your horizontal braces connect and truss it with a couple more tubes up and down back to the tail.  And add another brace tube on the rudder post half way between the existing one and the hinge and tied back to where your first vertical tubes connect to the longerons.

SuberAvid, I believe you are correct. Before cutting the fabric off I was able to push up on the tail wheel and it would show movement in the tubing. I cut the fabric off and there is nothing that looks bad. I have a guy coming over this evening and we are going to make a plan on how to brace this up. Thanks for the input. 

 

tail3.jpg

tail1.jpg

tail2.jpg

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Posted

That is what's nice about a group like this.  Lot of experience and history of others who had similar problems or at least something related to it.  I did edit my post to show more correctly where I thought the braces could go.  Don't know what I was thinking when I wrote it yesterday.  JImChuk

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Posted

It wouldn't hurt to put a gusset to reinforce the tube running up and back from the tail Spring mount.we just found this to be broken on a recent inspection 

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Posted

Dusty, like in this area?? See gray shaded area.

tail4.jpg

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Posted

Weld in two tube ,about 3/8" or 7/16" OD from the center of the bend in the vertical fin post to the center of the lower longeron bays. THis will stiffen it a little bit. Ive seen the same thing super cubs in Alaska and that is how we fixed it....YOu dont need large tubes, just small one to add a little strength in the middle of  the long spans...

If you tig weld, have a spotter  with fire extinguisher in case you catch the adjacent fabric on fire.... Gas welding is easier for me but what ever you are comfortable with.

GLue on a patch with poly tak, let it dry, shrink it , apply the primer coat(poly brush) and the silver (poly spray ) and top coat and your done! OR what ever coating system you use. If the old fabric has enamel, try to clean it off with MEK so you can get a good glue bond with the poly tak...

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Posted

Hey northidahoflyer, 

 

could you edit the title please (rudder vs rubber) so that when we search for your post we'll find it in the future..

Thanks buckets

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Posted

Hey northidahoflyer, 

 

could you edit the title please (rudder vs rubber) so that when we search for your post we'll find it in the future..

Thanks buckets

Good catch Flywise. The auto correct on my phone is still kicking my ass. 

:BC:

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Posted

The visible rust on the bottom of the rudder vertical tube would be something to look at a little more closely. It could cause problems down the road. It might be just external or could be also internal, which isn't good. A common problem on tail draggers, especially on ones tied outside or in open hangars. Maybe a little snake oil in the tube might extend its life. I understand the no. 1 snake oil salesman is on here. Now is the time to inspect it. If you see the oil leaching thru the fabric later on then you would know there is a problem. But I am a little picky ( some use other words) when it comes to rust and tubing.

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Posted

welding a gusset  behind this broken tube would help to reinforce this area. This aircraft has 750 hours on it. It is an Avid mark 4 912

20190104_160708.jpg

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Posted

Thank you Dusty!! That's a good picture. I will make sure to gusset this area. Much appreciated :)

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Posted

Here are a few more angles. 

Tail 6.jpg

Tail 7.jpg

tail 5.jpg

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Posted

The more I look at this, pull on the various parts and look at the broken tube picture from Dusty it has become obvious that the tailwheel is causing a lever action on the rear rudder post. The little tube that runs up from the front of the rudder plate to the rudder post is carrying a majority the load/weight placed on the tailwheel. This lever action is pulling the rudder post forward and will eventually cause major issues. With two gussets and by triangulating the current structure from the lower horizontal strut carry through tube I believe it will better distribute the load through the whole structure.   

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Posted

Ideally you want the tail spring to take the shock loads. That is why I never was a big proponent of installing stiffer tail springs on Super Cubs. Those that did ended up putting more stress on the tubes in the tail area and eventually something would bend or crack.....

IN the case of the Avid, maybe someone dropped the tail in the past or got it to crow hopping on the runway or ran the tail thru a ditch, who knows... the poor tail wheel takes a beating... solid rubber tailwheels add more stress, pneumatic tail wheel tires take up some shock.....

I wouldnt be worried about it Vance, it is just a light weight structure but is you beef it up just a little, it will put the stresses back on the tail spring which is what you want...

About the only tubing primer I ever saw that could stand up to MEK based fabric coating systems was stits epoxy tubing primer, (green). There might be others out there but I know Stits works, spendy but works...

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Posted

pneumatic tailwheel, shorter tailwheel spring and longer mains will help keep from banging the tail so hard.  When I had the Avid "tall wide" gear on mine I would almost always drag the tail wheel first when coming in short and the mains were still a foot or so off the ground.  Landing full flaps and carrying power helps to keep the angle a bit flatter but still get you slowed down.

:BC:

 

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Posted

Keep your airplane light and ad power when in your landing flare . This eliminates most of your problems. 

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Posted

Like I mentioned, I added extra reinforcement tubing to my Avid MKIV after I broke the lower longerons, then added extra tubes to truss up the tail area on my Avid+ when I was building it.  So far have not had any issues or evidence of damage.  I would like to think I do a good job of always keeping my tail up when landing but the reality is that you don't get it perfect every time and the conditions are not always great.  (plus sometimes I am pretty rough on landings)  I hit an unseen rock one time that actually bent the rim on my tailwheel and I also had the spring bolt break once.  My latest addition to help prevent damage.

DSCN3195-1.jpg

DSCN3197-1.jpg

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Like I mentioned, I added extra reinforcement tubing to my Avid MKIV after I broke the lower longerons, then added extra tubes to truss up the tail area on my Avid+ when I was building it.  So far have not had any issues or evidence of damage.  I would like to think I do a good job of always keeping my tail up when landing but the reality is that you don't get it perfect every time and the conditions are not always great.  (plus sometimes I am pretty rough on landings)  I hit an unseen rock one time that actually bent the rim on my tailwheel and I also had the spring bolt break once.  My latest addition to help prevent damage.

DSCN3195-1.jpg

DSCN3197-1.jpg

Hey Superavid,

looking at your tailwheel installation it seems you have a negative castor angle which is supposed to be bad (see the picture below). Did you try different angles (shims installed)

? What did you find?

Thanks for your input, Laurent

 

tws.jpg

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