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SB 00036 (cracked horiz stab spar) Installation Notes

Sam Buchanan

been here awhile
As the service bulletin kits are arriving in the field and installations occur there may be information that is useful for those contemplating or planning the repair. Please post any details you encounter with the installation that need to be shared.
 
Double Check the Cherry Max Rivets!

As is customary for Vans, all the rivets for the SB00036 kit are shipped in one little bag. Included are the driven rivets for reattaching the skin.....and TWO sizes of Cherry Max rivets.

A...uhh....friend was installing the kit on his non-cracked pre-punched RV-6 stab and failed to take note of the two sizes of pulled rivets (even though this detail is in the documentation....). A quick check with the borescope after riveting the hinges on one side of the stab revealed a major uh-oh. Three of the pulled ends looked good but five were atrocious.

It then dawned on my sometimes dense friend that the longer rivets intended for use with the doubler on the forward side of cracked spars had been used with the non-cracked and undoubled spar. So I, uh, my friend got to drill out the Cherrys with a #40 bit so the heads could be knocked off.

Separate the Cherry Max rivets when you receive your kit and put the ones you don't need in an unsafe place...... ;)
 
That's a huge heads up. Wondered why I got 32 cherry max.

Yep, the rivets are only one dash number apart, anything less than a very close look and they all appear the same. Cherry Max's need to be the exact correct length to set properly and provide rated strength.
 
Trouble with installing Cherry Max rivets

After borescoping my horiz spar bracket repair I found that half of my Cherry Rivets had the mandrel recessed into the rivet head, and sticking out of the spread rivet (see attached photo). The Vans engineers say this is unacceptable because if the mandrel should vibrate out there is little strength to the rivet. This problem apparently exists with the RV-12 Cherry Max rivets as well. There is a Vans YouTube video on dealing with the problem, and the engineers recommend heating the rivet while soaking in lubricant to “prep” it for installation. This means I need to remove the elevators again, drill out the bad rivets, and reinstall.
 

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Are you guys doing the repair on the plane? Or is everyone pulling the HS to work on the bench?
 
Are you guys doing the repair on the plane? Or is everyone pulling the HS to work on the bench?

I'm planning to do it on the plane, with a sawhorse supporting the tail cone. The height is convenient and everything is just as easy to get to as if it was done on the bench (where it would also need to be held steady somehow).
 
After borescoping my horiz spar bracket repair I found that half of my Cherry Rivets had the mandrel recessed into the rivet head, and sticking out of the spread rivet (see attached photo). The Vans engineers say this is unacceptable because if the mandrel should vibrate out there is little strength to the rivet. This problem apparently exists with the RV-12 Cherry Max rivets as well. There is a Vans YouTube video on dealing with the problem, and the engineers recommend heating the rivet while soaking in lubricant to “prep” it for installation. This means I need to remove the elevators again, drill out the bad rivets, and reinstall.

Bit of a pain in the butt that. I guess we should order extra rivets just in case.
 
After borescoping my horiz spar bracket repair I found that half of my Cherry Rivets had the mandrel recessed into the rivet head, and sticking out of the spread rivet (see attached photo). The Vans engineers say this is unacceptable because if the mandrel should vibrate out there is little strength to the rivet. This problem apparently exists with the RV-12 Cherry Max rivets as well. There is a Vans YouTube video on dealing with the problem, and the engineers recommend heating the rivet while soaking in lubricant to “prep” it for installation. This means I need to remove the elevators again, drill out the bad rivets, and reinstall.

That looks familiar.......

I decided to forget the Cherry Max rivets and drilled out the rivets on the bottom skin so I could access the front of the spar. It only took a few minutes, wish I had done that first instead of messing with the Cherry's. Driving the hinge rivets went well and now I know the hinges are secure.

My A&P friend told me the Cherry's can't be drilled out but I was able to use a #40 drill to remove enough of the mandrel so the heads could be popped off with a flat drift and hammer without damaging the hinges.
 
Are you guys doing the repair on the plane? Or is everyone pulling the HS to work on the bench?

Did mine on the plane with the tail propped up, think it is easier than wrestling with a stab sliding around on a bench not to mention the additional time required. The kit installation really isn't a job to dread, just be methodical.
 
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After borescoping my horiz spar bracket repair I found that half of my Cherry Rivets had the mandrel recessed into the rivet head, and sticking out of the spread rivet (see attached photo). The Vans engineers say this is unacceptable because if the mandrel should vibrate out there is little strength to the rivet. This problem apparently exists with the RV-12 Cherry Max rivets as well. There is a Vans YouTube video on dealing with the problem, and the engineers recommend heating the rivet while soaking in lubricant to “prep” it for installation. This means I need to remove the elevators again, drill out the bad rivets, and reinstall.
After drilling out Cherry rivets and lubricating new ones as indicated in Vans YouTube video reinstall went smoothly. Since old mandrels were recessed in head it was easy to drill out head and then punch mandrel out.
 
Anyone who uses the Cherry Max rivets to install the kit would be well served by borescoping the rivets after installation. It can be difficult to judge the quality of the installation otherwise especially for those of us who don't have prior experience with them.
 
I too was bit by the poorly documented fact that two different sized Cherry Max rivets are in the repair kit. The ONLY place it's called out is in a note on figure 5 which starts out "If there are no cracks". I have cracks so crossed out the note, hence, didn't see that there are two sizes. Plus the kit didn't come with a contents list.

Enough whining.

So, on inspection I have determined that the depth of the grip falls between 3 & 4. The -3 rivets pulled up to a "minimum" depth and the -4 rivets showed as the "maximum". Cherry's come in 1/16" increments so it's possible to see no stem or 1/16" stem protruding from the set end. This is explained in the Cherry Max literature. I decided to leave well enough alone in order to not do any damage trying to drill out the difficult to drill Cherry rivets.

Note: the q-tip was something I stuck in the hole to identify bottom vs top in the boroscope picture. It can be confusing.
 

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This is only based on personal experience, but likely half the cherry max rivets I've set by hand in the past have broken early. Using the pneumatic puller, I think I've had 1 set incorrectly. You should definitely boroscope after install, but also consider the tool to decrease side loading and bending on the stem while setting the rivets.
 
Proper CherryMax Rivets

I don't intend to get in an argument with anyone here and I appreciate the sharing of knowledge and experience by everyone.

But, I need to point out the proper selection method for CherryMax rivets and my recommendation for adherence to the instructions in the Service Bulletins.

According to the CherryMax Process Manual CA-1015, page 11, when using the CherryMax 269C3 Grip Gage, Step 4 says "Select the fastener with a grip equal to the next higher number. If the number reading is directly on a line you may use either that grip or the next higher one." It does not call for the use of a shorter rivet when in-between marks on the gage. See their example #1.

Also, take note, when the Service Bulletin calls for a specific fastener (or an alternate), the use of those specific fasteners are the only approved method of compliance.

So in this case, when the Repair Doubler is used the only approved fasteners are AN470AD4-6 or the CherryMax CR3213-4-4.

My personal preference is, if I have the bottom skin opened up to install the Repair Doubler, I would choose solid rivets over the CherryMax rivets every time. I'm sure not everyone feels the same way.

The biggest mistake I see is not keeping the rivet puller totally perpendicular to the surface being riveted, this will almost always cause premature breakage of the stem, resulting in an improperly set rivet. CherryMax rivets do not tolerate any misalignment when setting the rivet. Be very aware of this if your rivet puller has interference with the part being riveted.

The CherryMax Process Manual has great information on the installation and removal of CherryMax rivets, so it should be required reading for everyone using them. KAI Section 5 also has extensive information on using CherryMax rivets.

Have a great weekend!
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River numbers

Anyone know how many rivets needed to be removed on the 8 in order to get a bucking bar on front side of spar. After reading posts, I don’t want to try the cherrymax.
 
Anyone know how many rivets needed to be removed on the 8 in order to get a bucking bar on front side of spar. After reading posts, I don’t want to try the cherrymax.

The -8 has the same stab as my -6. I drilled out the bottom skin trailing edge rivets from the outside corner to the first interior rib. The tip rivets were drilled out from the trailing edge corner to the trailing edge of the fiberglass cap. In other words the rivets that can be easily installed with a squeezer. This allows the skin to be pulled down enough to squeeze your hand and a bucking bar into the stab so the hinge bracket rivets can be driven. It only took a few minutes to drill out the rivets, well worth it to be able to use solid rivets in my opinion.
 
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Parts arrived in mail today. I’ve already primed and will let the primer cure for a few days unless I put the parts on top of the heater and bake them.

The black coating on the non-powder coated brackets comes off easily with a solvent like naphtha.

I’m going to follow Sam’s lead and lift up skin to put in solid rivets.
 

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I ordered a 6 non prepunched power coated and i got it yesterday. I will definitely use solid rivets. Drilling out the skin rivets is easy and only takes a minute. Even easier to re-rivet the skin with the squeezer.
 
Access to accomplish SB-00036

Not a builder...patience please. Also I tried, really...to find a thread about opening another access point to evaluate SB-00036 (rear outboard elevator hinge bracket area on rear spar).

Question - Can you drill out a new hole closer to the mid point for easier access than the current tooling hole? If yes...any engineering analysis and directed location?

Thanks
Meegs

RV-8
 
Question - Can you drill out a new hole closer to the mid point for easier access than the current tooling hole? If yes...any engineering analysis and directed location?

Yes. Referenced in Step 1 in the service bulletin and Figure 15 on the last page. Describes the options to enlarge the existing tooling hole, but you can also add another hole if needed for inspection in the outboard rib. Just be sure to put it the new hole on the centerline and leave sufficient material from an edge-distance perspective when choosing where to drill on the centerline.
 
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Parts arrived in mail today. I’ve already primed and will let the primer cure for a few days unless I put the parts on top of the heater and bake them.

Not to ignite a new primer war but what primer did you use?
 
...........
What's the method for ordering a kit? An email to support or some other method?

New kits are available on the Van's online store for the following kits (each link below will display the kits for that model(s). NOTE that we are currently shipping ONLY the non-powder-coated brackets (you will prime with a self-etching primer of your choice). If you order a powder-coated kit, we will not ship a kit to you until the parts are available, and we do not yet know when that will be. If you need to repair an airplane now, order the non-powder-coated kit.

 
One side done. Time was 2 hours. Not a big deal. Solid rivets installed. Should take about an hour on other side due to learning curve.

Helpful notes for others:
- Remove rivets on the top or bottom skin all the way to the middle rib and front spar to allow room to get hand in. I used a #42 drill and drill stop. Rivets came out with no damage to skins.
- Brackets were easy to remove. Use a #40 and drill all the way thru the rivet.
- Break the rivet tail/head off.
- Use a punch and lightly tap the rivet out. If stuck, use a #30 drill and drill a smidge out and re-punch. All came out and the holes in the spar all looked good.
- After buggering up 2 heads trying to squeeze and drilling out, the rivet gun was superior to the pneumatic squeezer due to clearance.

With 32 cherry max rivets for later use, the kit was a great bargain.
 

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Rivet Removal Warning

Just a warning to anyone removing rivets for SB-00036 (or any other reason). Experienced builders should be familiar with this information, but not all current RV owners are the original builder. This is not intended to offend anyone, simply to educate the uninformed.

DO NOT drill through the entire rivet when drilling from the shop head side of the rivet! I have removed hundreds of rivets (during major structural repairs) and I can say this with certainty, if you drill through the entire rivet from the shop head side you WILL enlarge the holes in the structure!

The reason being, the center of the shop head is not coaxially aligned with the actual center of the hole, because the shop head has a tendency to form off-center due to the angle the bucking bar (or gun) was held while setting the rivet.

If you have an expectation of reusing any of the parts being disassembled you should always avoid drilling through the entire length of the rivet!

Remove the rivet head and drive the rivet shank out using a drift punch (or an automatic center punch that was ground down like a drift punch tip) or use the modified side-cut method described in KAI Section 5. Always backup the back side with a block of wood with a hole drilled in it to receive the rivet tail when driving the shank out.

Also note, the illustration in Section 5 shows drilling from the manufactured head side of the rivet WITH A SMALLER DRILL BIT - this can still lead to oversized holes if you are not extremely careful.

This warning also applies to removing CherryMax or pulled-rivets (RV-12 builders). There is a risk of structural damage when drilling through ANY rivet. You wouldn't believe how many calls we get after the damage is done - "How do I fix this (or many!) oversized hole(s)?".

Yes, it will take a little bit more time to do it right, but it sure beats replacing the spar.
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Tony, I agree with you under normal circumstances. Having a steel drill guide (AKA hinge bracket) isn’t a normal circumstance. And I was extremely careful as the info stated.

Trying to drill out the aft side manufacturers head, no direct sight of the head, not enough room to get my arm to drill without fear of creasing the skin, and no way to get both hands in to steady a drill precluded the decision to use a #40 bit on a AN4 rivet.

The risk of damaging the spar, in my opinion was much greater should I try to drill from the forward side thru an aluminum rivet head instead through the aft steel bracket which makes a dandy drill guide and I could see what I was doing. And those rivets were set by me using a squeezer, not bucked.

The smaller bit stayed inside the shaft of the larger rivet. Rivets snapped nicely and when a #30 bit was used should the rivet not punch out, never went below the bottom of the steel hinge bracket. And since the steel bracket is a drill guide that doesn’t wander if clecoed or rivets still in place should drilling deeper be required.

Now that I’ve shared my how and why, what should I have done differently in this not normal circumstance having steel drill guide?
 
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Webb, I agree with your methodology, you followed the procedures I outlined above by not drilling entirely through the rivet using a final size (#30) drill bit. This is the sign of an experienced builder using the knowledge he learned through experience.

That's the main point I was attempting to make here. I'm simply trying to caution people who perform this SB that taking your time, like you did, will lead to a successful outcome.

You are correct for drilling from the aft side, regardless of which type of rivet head you have to tackle. It would be very difficult, if not impossible, to remove the rivets from the forward side without causing spar damage.

You are also correct that this is not a normal circumstance, and having the steel hinge bracket as a drill guide allows more assurance that you won't bungle up the spar.

Thank you for the through explanation on how you successfully performed the SB. It will be helpful to everyone who reads it if they ever need to perform the SB.

Hopefully, I don't receive any more calls about oversized holes from drilling out rivets. :)
 
Thank you Tony. We’ve talked before and I thank you for your advice when needed on this rebuild of mine.

Folks can also grind the tails off if they are uncomfortable drilling the rivets out since the steel will protect the spar.
 
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I'm like so many other with glassed in tips that drilled an inspection hole. It was pointed out to me today that Cirrus also has an inspection hole drilled in the outside of their HS and they install a piece of tape over it. It's a required preflight item to make sure the tape is there. I can see an open hole being a great place for water to get in, and with fully glassed in ends,


Edit: The only way for the water to escape is by going inboard and draining into the fuselage.

Pic of cirrus attached for reference.
 

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I have no way for the water to escape. If enough water were to get in, I could find myself in an unrecoverable aft cg condition because my HS is full of water. I'll be adding a piece of 3M tape as well and adding it to the preflight.
.

You need drain holes in all compartments throughout the airplane as water will always find a way in. This is especially true in all control surfaces for reasons you point out.
 
You need drain holes in all compartments throughout the airplane as water will always find a way in. This is especially true in all control surfaces for reasons you point out.

I forgot about the holes on the inboard side. So it would drain inboard and fill the aft most portion of the fuselage until it got high enough to drain out the tail wheel spring. Still probably easier to tape over the hole to prevent ingress on the first place.
 
Empennage fairings

Water will get into fuselage around the empennage fairing ….not discounting the importance of the tape ,but it is not a substitute for good drainage .
 
00036 rivit removal

First, thank you, Tony Kirk for your support of our attempts to do SB 00036.
I was concerned about trying to drill out the shop end of the hinge bracket rivets (factory head forward) because of the difficulty of accurately centering the punch on the hole. So I used a Dremel tool to grind off the shop head. It was then easy to tap out the rivets with a drift. No hole damage!:D
 
I've had to put off inspecting the HS on my -4 as it's still in the paint shop. I plan to pick it up next weekend and doubt I will be able to bring myself to remove any rivets on the skin to access the forward side of the spar.

The aircraft is almost 22 years old and has over 1100 hours on it, but the HS is brand new. Before sending it to the paint shop, I built new elevators with 0.020 skins and built a new HS to minimize the down time associated with fitting new elevators to an existing HS.

I've been evaluating an integrated doubler/hinge pair that I could locate on the aft side of the spar and rivet in place (w/Cherrymax rivets) without removing any skin rivets.

Here's the part I've been thinking about. It would be machined from a hunk of 4130 alloy steel (same material as the current hinge brackets).

Hinge-Bracket.jpg


I'd like to structurally bond it in addition to rivets to reduce the likelihood of cracking of the spar at the rivets holes.

This is still a "thought experiment" though it is gaining momentum within the confines of my mind.

Please share your thoughts, critiques, and ideas.
 
Your design does not include the nesting effect that puts load directly into the spar flange and skin. This is an important feature of the factory design.

Also, to machine that from billet would be mightly inefficient - starting with a block over 2" thick and taking most of it down to 0.06". But you could cut it out with wire-EDM pretty efficiently and if you nested them on the block of material there would be relatively little waste.

But as I said, you do want to have the nesting feature of the factory doubler.

I've had to put off inspecting the HS on my -4 as it's still in the paint shop. I plan to pick it up next weekend and doubt I will be able to bring myself to remove any rivets on the skin to access the forward side of the spar.

The aircraft is almost 22 years old and has over 1100 hours on it, but the HS is brand new. Before sending it to the paint shop, I built new elevators with 0.020 skins and built a new HS to minimize the down time associated with fitting new elevators to an existing HS.

I've been evaluating an integrated doubler/hinge pair that I could locate on the aft side of the spar and rivet in place (w/Cherrymax rivets) without removing any skin rivets.

Here's the part I've been thinking about. It would be machined from a hunk of 4130 alloy steel (same material as the current hinge brackets).

Hinge-Bracket.jpg


I'd like to structurally bond it in addition to rivets to reduce the likelihood of cracking of the spar at the rivets holes.

This is still a "thought experiment" though it is gaining momentum within the confines of my mind.

Please share your thoughts, critiques, and ideas.
 
Your design does not include the nesting effect that puts load directly into the spar flange and skin. This is an important feature of the factory design.

Also, to machine that from billet would be mightly inefficient - starting with a block over 2" thick and taking most of it down to 0.06". But you could cut it out with wire-EDM pretty efficiently and if you nested them on the block of material there would be relatively little waste.

But as I said, you do want to have the nesting feature of the factory doubler.

Hi Steve,

Thanks for the reply. I have been considering how to tie this part to the spar flanges in addition to the web. There are 2 issues that hamper this approach (for me):

  • Tolerance stack-up prevents my design from simultaneously aligning the part to the flanges and putting the pivot hole in the proper location.
  • My HS rear spar is dimpled so it would be challenging or impossible to get a flanged doubler to contact both flange faces simultaneously. How are others with dimpled spars approaching this issue?

If I pursue this approach, I will have to accept that possibility of finding cracks at some future inspection. The question is: is it enough better than the legacy design to be worth the cost and effort.

Regarding the block of material required to make this, I agree there will be a lot more chips than part when I am done. The finished part will weigh about 0.2 lbs while the stock will weigh around 7 pounds. Wire EDM and nesting is a good suggestion. I will explore that.

Thanks again,

Dean
 
Hi Steve,

Thanks for the reply. I have been considering how to tie this part to the spar flanges in addition to the web. There are 2 issues that hamper this approach (for me):

  • Tolerance stack-up prevents my design from simultaneously aligning the part to the flanges and putting the pivot hole in the proper location.
  • My HS rear spar is dimpled so it would be challenging or impossible to get a flanged doubler to contact both flange faces simultaneously. How are others with dimpled spars approaching this issue?

If I pursue this approach, I will have to accept that possibility of finding cracks at some future inspection. The question is: is it enough better than the legacy design to be worth the cost and effort.

Regarding the block of material required to make this, I agree there will be a lot more chips than part when I am done. The finished part will weigh about 0.2 lbs while the stock will weigh around 7 pounds. Wire EDM and nesting is a good suggestion. I will explore that.

Thanks again,

Dean

If you are determined to eliminate the flanges (something I would not do....) why not just cut the flanges off the Vans kit and use it? Sure would save a bunch of machining.

But I would use(ed) the Vans kit as is and invest(ed) in an artist brush for painting a few rivet heads. :)
 

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If you are determined to eliminate the flanges (something I would not do....) why not just cut the flanges off the Vans kit and use it? Sure would save a bunch of machining.

But I would use(ed) the Vans kit as is and invest(ed) in an artist brush for painting a few rivet heads. :)


Hi Sam,

Your installation looks great. As far as I can tell, your skin is machine countersunk so you were able to nest the flanged doubler tightly between the flanges of the spar. With my dimpled spar, I don't see a means to do this. (Finding a way to get around this issue could change the way I approach this.)

I did consider cutting the flanges from Van's retrofit kit, but key to my concept is to tie the hinges together with as few point loads (rivets) as possible. (In part this is to partially compensate for the fact that this concept does not tie to the spar flanges.) I thought about welding hinge "ears" to a flat plate to eliminate most of the machining, but am concerned about holding tolerances and confirming the quality of the weld.

Thanks,

Dean
 
Hi Sam,

Your installation looks great. As far as I can tell, your skin is machine countersunk so you were able to nest the flanged doubler tightly between the flanges of the spar. With my dimpled spar, I don't see a means to do this. (Finding a way to get around this issue could change the way I approach this.)


The skin and spar are dimpled. I countersunk the doubler flanges (the installation docs say to dimple it if I recall correctly) and pushed the doubler into place, it fits very nicely.
 
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I used the RV6 PRE PUNCH kit on my aircraft and the nested doubler fits perfectly and tightly. My skin and spar flange were dimpled and I dimpled the nested doubler as well. Once the six top and six bottom skin to spar rivets are removed you can push the doubler in place. The spar and skin have enough give in them to allow the doubler to be pushed into position past the spar dimples. It takes a little force to push it in, but mine "snapped" into place and fit perfectly (thanks Vans!). In fact it is a little tough to remove from the spar after test fit. I was also concerned with the "dimple interference" when I was pre gaming the job in my mind. In actual practice it all worked out without a hitch. Hope this helps and you have a similar experience.
 
My concern with the dimples as well. In my case on the 4 ( non prepunched) I have to drill the nested doubler using my spar as a template then dimple the doubler. Not sure how to do this without a loose fitting doubler?
Duff
 
My concern with the dimples as well. In my case on the 4 ( non prepunched) I have to drill the nested doubler using my spar as a template then dimple the doubler. Not sure how to do this without a loose fitting doubler?
Duff

There is enough flex in the spar and doubler flanges to fit the doubler in place for match drilling. During one of my test fits the countersinks in the doubler weren't matched with the dimples in the spar and I was able to move the doubler as needed. No need to over-think this. :)
 
The question is: is it enough better than the legacy design to be worth the cost and effort

Dean

Dean, if the bending loads from the hinge flange are not transferred to the spar flanges then the skins will not be carrying the load, they will just twist the spar web until the load is distributed via the spar web. This is the loading that caused the cracks.

I am not sure why Vans chose to redesign the fix and eliminated (what appeared to be an elegant fix) of the first hinge redesign . . . but knowing Vans, they did an FEA and looked at the actual loads and the tension/compression to the skins in the process. That could be the only reason (my brain) they extended the aft "nested" piece to include more rivets on top and bottom to transfer that load. Unless you take all this into account and consider the stiffness (modulus) of steel vs aluminum, a TLAR design will not likely equal the benefit of SB design.

Honestly, I have no cracks (250hr) and will likely just wait until one appears to do the modification.
 
Dean, if the bending loads from the hinge flange are not transferred to the spar flanges then the skins will not be carrying the load, they will just twist the spar web until the load is distributed via the spar web. This is the loading that caused the cracks.

I am not sure why Vans chose to redesign the fix and eliminated (what appeared to be an elegant fix) of the first hinge redesign . . . but knowing Vans, they did an FEA and looked at the actual loads and the tension/compression to the skins in the process. That could be the only reason (my brain) they extended the aft "nested" piece to include more rivets on top and bottom to transfer that load. Unless you take all this into account and consider the stiffness (modulus) of steel vs aluminum, a TLAR design will not likely equal the benefit of SB design.

Honestly, I have no cracks (250hr) and will likely just wait until one appears to do the modification.

Hi Bill,

I missed Van's first (elegant) re-design and am only familiar with the current SB. (I recall hearing about the change, but missed it before it was replaced.)

The fact that so many aircraft have flown so long (and undoubtedly some flew with cracks for a while) without structural failure indicates the original is strong enough for the loads, but is cracking due to fatigue cycles. I agree tying the hinge to the flanges and skin is a good fix. I'm interested in potential fixes that don't involve the skin.

If I can accept the legacy design is almost good enough, is there a solution space that brings me to good enough using only the spar web. A one-piece, steel hinge assembly/doubler may distribute the load (even more so if bonded) over a greater portion of the web. If the ears are integral to the steel doubler, more of the bending loads will go into the entire steel doubler rather than the two small footprints of the legacy hinge brackets.

I don't know whether or not I have cracks, as I won't get my plane back from the painter until next weekend (if all goes well!). I'm not likely to have cracks as the HS has only 40 or 50 hours on it. Like you, I will have some time to keep an eye on it, consider options, and move ahead once I am comfortable with a plan.

For those that installed the flanged doubler, was in necessary to access the forward side of the spar to match drill the holes for the hinge rivets?

Thanks for helping me think thru this issue.

Dean
 
If I can accept the legacy design is almost good enough, is there a solution space that brings me to good enough using only the spar web. A one-piece, steel hinge assembly/doubler may distribute the load (even more so if bonded) over a greater portion of the web. If the ears are integral to the steel doubler, more of the bending loads will go into the entire steel doubler rather than the two small footprints of the legacy hinge brackets.

The issue is that the elevator hinge creates a point vertical load and its associated fore/aft moment needs to get reacted into the skin with a fore/aft couple into the skin. The spar web has flanges that attach to the skin and transfer the load into the skin forming a closed cell torque box. (With bigger loads this would be helped out by a rib backing up the fitting and spar web with multiple fasteners to get that couple into the skin.) It is not a “bending” load as you describe. It is a vertical load and moment couple between upper and lower fasteners that attach the fitting. “Thicker spar web” may help with the load at the rivets attaching the fitting but then that load is transferred in the spar web through the rivets and goes around the 90 deg bend in the spar web to get to the skin attach fasteners. Going around that bend is the material thickness of the spar web and therefore a prime location for crack. A doubler may solve the current crack location in the vertical web of spar but will probably cause new cracks at edge of the doubler in the corner of the 90 deg flange that attaches the skin. If you think this through it is pretty intuitive. In my opinion that corner is the fatigue detail that Vans second iteration in the repair SB solved by having the new doubler nest and pick up skin fasteners.

Putting a large steel plate on the spar web may have another negative consequence related to horizontal tail bending. Bending due to lift on the tail is a reacted into the skin in a inbd/outbd couple (not fore/aft couple like i explain above for the point hinge fitting load) The spar is aluminum of a particular web thickness and a particular stiffness. All of a sudden the bending inbd/outbd in the spar web hit a very very high increase in stiffness of a steal doubler. Rapid increases in stiffness is prim location for cracking. This is why spar always taper in thickness changes. Think of tree branches always break at intersection to thicker branches.

No good engineering is ever presented without its caveats.
-A freebody diagram would be helpful for both of these I explain above but I am on vacation with only an iPhone to write this response.
-I am a structural design engineer so some strength engineer my give a better explanation, since they always want to prove us designers wrong.
 
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