Replies to This Discussion

Permalink Reply by Matt Heintz on November 27, 2009 at 7:11am

Mike,
replacing the SS rivets with aluminum rivets is better.

Regarding possible corrosion with SS rivets. That is always a concern even though the SS rivets are in the cabin area with a minimum amount of humidity.

Once ready to rivet, put zinc chromate in and around the hole.

Replacing the complete center section webs to 0.063 is 100% acceptable. No need to go to 0.090. We are already significantly increasing the strength in the whole area.

Please note that the latest upgrade drawings calls for a new type of bolt. We are now using NAS6205 bolts which are significantly stronger. They are needed as we are adding 0.125 in material at the root, so a stronger bolt is needed to match the extra strength.

Other bolts with similar NAS6205 strength are acceptable.

Permalink Reply by Herbert E. Heaton on November 27, 2009 at 2:19pm

Matt,

My spars and center spar have been reamed for 3/8 dia bolts. Will I need to go to a NAS6206?

On page 6-ZU-2, in the top right corner, there is a dimension of 0.790 for the spar thickness. I come up with 0.25 + 0.040 + 0.25 + 0.125 = 0.665. Where am I missing the 0.125 difference?

Thanks for the great work.
Herb

Permalink Reply by Matt Heintz on November 28, 2009 at 7:38am

Herbert,
Your AN 3/8" bolts are perfectly fine. No need to change them.
You are correct. the drawing needs to be updated to 0.665

Permalink Reply by Stephen R. Smith on November 27, 2009 at 11:16pm

Mathieu,

I am looking forward to having an airplane that is comfortably overbuilt. I will gladly suffer the weight of a few more pounds of aluminum to gain an airplane that is stronger in critical areas. I think that is a good tradeoff. I plan to install the upgrade kit as soon as one is shipped to me via Quality Sport Planes.

With the theme of “overbuilt” in mind, I would like to run two design options past you. I am less worried about the modification being “unnecessary” (not something I want to debate) and more interested in making sure I am not doing something unsafe. The amount of weight my modification would add is negligible so I don’t see a significant downside there, but I am no engineer.

So what am I contemplating? First some background.

I have 630 hours on my Zodiac. About 160 of these are at full gross. Maybe I am a bit harder on my Zodiac than others are. I am perhaps less concerned with turbulence than other flyers. In turbulence I make sure I am well below maneuvering speed, but basically as long as I can keep the plane right-side up I keep flying. Out here in the west, with our plentiful mountains and heat, afternoon thermals are the rule so we encounter plenty of bumps in our afternoon flying.

My airplane has developed 4 smoking rivets on each wing. They are on the bottom along the spar line. The smokers are the first 4 rivets at the wing root.

I have looked at quite a few other Zodiacs and shared emails with other builders. Most don’t have this problem at all but they also tend not to have as many hours on them at full gross. Also I have encountered plenty of pilots who prefer not to fly in afternoon heat with the bumps it brings.

The Zodiac I learned to fly in has 500 hours on it. Almost all of these have been at full gross. This plane has a far more severe smoking rivet problem, also along the bottom spar at the wing root. That Zodiac has an empty weight of 905 pounds!!! It is used as a flight trainer. It has led a tough life. I don’t think its condition is anything most builders will encounter in there own Zodiac so folks should not be alarmed by my report. The owner has fixed his smoking rivets two times according to generally accepted guidelines only to see them come back. Something else needs to be done….

I know of one other Zodiac with 200 hours which has one rivet starting to smoke in the same place. That Zodiac has lived a sheltered life and rarely sees a passenger or turbulence.

I know of a forth Zodiac with rivets which have started to move – the paint is cracked around the same 4 rivets on both wings but no smoke is visible yet.

Now about the modification I want to make.

First, I do know that part of the upgrade is to double the rivets along the spar both top and bottom near the root. This will help I am sure. The new beefy web doubler may also help by reducing the spar’s tendency to bend near the root. But here is the thing: I for sure don’t want to take my wings apart again to fix this problem should it re-occur so I want to be absolutely sure it won’t re-occur. I want an “overbuilt” airplane in this area! One which I can fly for thousands of hours without seeing another smoking rivet.

Now that I know I will have my wings apart and now that I know I will need to remove the solid rivets on the bottom spar near the root to add the beefy web doubler, what I want to do is add a second bottom spar angle bracket. I plan to add this in front of the spar. I would run this new angle out to the end of the new web doubler and would use the same bolts to fasten it to the spar. This second angle would provide an additional way for the tensile load in the bottom skin to transfer to the spar and would further reduce the per-rivet shear-load. The only negative I see here is that the rivet line will only engage the nose skin because the bottom skin does not extend that far forward.

An alternate idea would be to add a second angle bracket on top of the existing back angle bracket to increase its effective thickness and spread out the load the rivet sees on the thin existing bracket. Again the plan would be to run this angle bracket doubler out as far as the new web doubler. I propose adding a angle doubler here instead of a single thicker angle because I don’t want a splice in the angle anywhere near the wing root and I don’t want to go to the work of extending a larger angle out on the bottom like we are doing on the top. (I LOVE the new extruded angle on top! I was very happy to see that in the upgrade kit.)

Incorporating one of both of the above solutions should for sure fix my problem and give me the “overbuilt” airplane I desire. I will perhaps need longer spar bolts (or longer solid rivets if that is what we are using down there).

Presuming proper execution, do you see any dangers with these ideas?

Thanks, Steve

P.S. Because most builders have not seen any smoking rivets on there wings I have encountered a fair amount of skepticism about this. It has been suggested that I don’t know what a smoking rivet looks like. Here are some Zodiac smoking rivet pictures from my VERY small collection.

Permalink Reply by Matt Heintz on November 28, 2009 at 8:01am

Stephen
Good points. "smoking rivets" are a serious issue and need to be changed when one is found and not 2 or 4. One "smoking rivet" will typically always lead to more.

We believe that the reason you are getting these "smoking rivets" is because of a vibration on the bottom skins. This vibration is possibly coming from the flaps, so it is important to make sure your flaps are tight against the flap stops in the up position and that when down, there is not a lot of slop in the system.

With the upgrade kit, the inboard area is made a lot more rigid by reinforcing the rear spar area and by adding a lot of extra angles at the main spar/rib area, including 6-ZU-1-4 (second bottom spar angle bracket).

Additionally, in drawing 6-ZU-3, we state " SPAR RIVET LINE - Double up rivet line thru spar extrusion (top) and spar angle (bottom) up to RR#5 A5 pitch 20".

Permalink Reply by Stephen R. Smith on November 28, 2009 at 8:28am

OK, I am a happy camper now. The original upgrade drawing did not have the second bottom spar angle bracket and I failed to notice its addition on the latest drawing. Bravo!

Thanks very much, Steve

Permalink Reply by Jeff Webb on November 29, 2009 at 6:02pm

Very well written. I'm not even flying yet but I have had the same concerns. The response by Mathieu made me feel better as well.

Thanks for the excellent post.

Permalink Reply by Gary Ray on November 28, 2009 at 11:34am

Quick notes on my findings during dissassembly:

When removing the center wing spar on my 601XL, one of the wing spar upright
brackets on the rear side of the spar in addition to both front brackets
had to be removed or there was not enough room to maneuver the center section
out of the cabin.

It is a good time to check the wing spar bolt holes in each spar upright
bracket for concentricity and replace it if it is not perfect. Mine were
not, so I will make new ones for the backside as well as the upgrade front replacements.

Check the main spar caps for nicks or imperfect edges that might have
occured during wing installation/removal and smooth them out.

My wing spar cap rivets are all really tight. Definitely harder to
remove than on the demo. Right wing even harder.
Required a bucking bar on the back side.

I think solid rivets is a good way to go. It is not likely
objectionable even for the inexperienced. There is always an experienced
helping hand at the airport for the limited number of rivets needed. Most of
the time is spent in preparation.

I also noted about 5 rivets on the flanges of the center section spar floor rivets that
were on there way to becoming smoker rivets. They did spin when drilling
the heads off. 113 hrTT most at 1200 lbs gross.

Everything else I have encountered has been as per Zenith Blog.
Ready and waiting for the upgrade kit.

Gary Ray
davgray@sbcglobal.net
601XL, 113 Hr TT

Permalink Reply by Matt Heintz on November 28, 2009 at 12:32pm

Gary,
Sound like you are doing a great job.

Permalink Reply by Ian McClelland on November 28, 2009 at 1:08pm

Hi Mathieu. I think you are doing a great job with the structural upgrade project and geting the information out there. I would like to know if any other majour changes to the draft drawings are being contemplated and when you expect to have the final drawings available to download?

Permalink Reply by Matt Heintz on November 29, 2009 at 7:16am

Ian,
So far, we have no major changes to the present DRAFT upgrade. Only minor changes. The front wing uprights 6-ZU-2-6 could change somewhat as there could be an interference issue with the side skin present holes on some aircraft. One option is to put a new fuselage skin doubler in 0.032 on the outside of the aircraft. This doubler would be riveted to the front and rear wing uprights so that new holes could be drilled in 6-ZU-2-6, but this is not final.

Permalink Reply by Gary Ray on November 29, 2009 at 3:01pm

I was looking at my aircraft with respect to the possibility of a skin doubler in the area of the
main wing spar cut-out. I liked the idea of starting at the forward gear upright bracket above the main landing gear and taking it forward to the front wing spar upright bracket (6-B-13-1) or possibility slightly beyond to terminate over an additional L-angle on the inside. Which my
eye-ball engineering concluded would stiffen the gear attachment, wing spar attachments and allow for new holes for the wing spar uprights and cut-out if needed.

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