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Loose Main Gear Legs

In fact with the shorter arm to the fulcrum point they probably see more stress than the inboard bolts.

Even if there was a fulcrum point outboard of the outer bolts that resulted in the outer bolts taking some tension in a heavy vertical impact, the inner bolts would always see more (roughly in proportion to the relative distances of the bolts from the fulcrum point).
 
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Open to calculation

This would depend on the section modules of the material between the inter bolts and the outer bolts.

The inner bolt takes some of the load applied to the outer bolts and this section would have to bend to make the outer bolts to go into tension. ( I believe the inter bolt would take most of the load ? )

The difference of force on the front bolt and back bolt would depend on the sin of the angle between the bottom of the channel and the axle of landing gear and the angle of force ( tail high or tail low )

Also the force it talks to rotate the mass of the tire wheel assembly to the speed of the aircraft and the drag of the gear assembly.

guessing without real numbers is guessing

Don, actually the outboard bolts are subject to tensile stress on a hard landing because the gear legs splay out from the fuselage causing a fulcrum point at the point of contact with the bulkhead. In fact with the shorter arm to the fulcrum point they probably see more stress than th inboard bolts.
 
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Data Point

Unfinished RV-12 that has gear installed and torqued:

lgboltbefore.jpg


Now with another 1/2 turn on the bolt without torque wrench - just tightened as much as possible with small open end wrench:

lgbolt2.jpg


I was also able to get a half turn on the outboard bolts as well.
 
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Dave, the bolts in question are the two outboard bolts. Certainly, the inboard bolts needs to be tight also, but it doesn't seem to be causing any problems.

I have 265 hours on my -12 and tightened the bolts yesterday. The outboard rear bolt was tight, the outboard forward bolt needed a full turn to tighten it up to specs. A 1/2" crow's foot on a 6" extention. No need for a second wrench on the bolt head underneath, in my case anyway.

IMHO it is hard braking that is causing the cracking.
 
Dave, the bolts in question are the two outboard bolts. Certainly, the inboard bolts needs to be tight also, but it doesn't seem to be causing any problems.

I have 265 hours on my -12 and tightened the bolts yesterday. The outboard rear bolt was tight, the outboard forward bolt needed a full turn to tighten it up to specs. A 1/2" crow's foot on a 6" extention. No need for a second wrench on the bolt head underneath, in my case anyway.

IMHO it is hard braking that is causing the cracking.
Good point, and I should have mentioned that I was also able to further tighten outboard as well.

I only have three landings in an RV-12, but one of the things I commented on at the time was how much more effective the brakes were on the 12 as compared to my 6, for whatever it's worth.
 
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Let's put a drag chute on and forget brakes. Then it will resemble the SR-71 with a nose pitot and landing chute. :)
 
Inspected the landing gear bolting today. With 150 hours, found all 4 of the outboard bracket bolts loose and had to tighten them 3/4 to 1 1/2 turns till they were tight. The 3 inboard bracket bolts on the left and right side were OK, checking them with the torque wrench. Tightened them to the upper end of the recommended range in the plans. As shown in a previous photo, the large center bolt in the inboard bracket, does not grip very well.

No evidence of channel cracking.

I have created a copy of the RV-12 Safety Alert Form for your use. It can be found here:

https://dl.dropbox.com/u/28599072/RV-12%20Safety%20Alert%20Form.doc
 
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What about the good old Loctite?

Just thinking that the number of responses on this thread already overtook the ones about the Engine Mounting Cap Screws...

Thought it doesn't resolve the strenght issue, consensus there was to seal the bolts with blue Loctite. I'm considering the same for these outboard nuts. (Or should I use the hard loctite instead?) :confused:

Godo
 
Loctite

Keep in mind, Loctite changes the torque characteristics. You have locknuts on there...most likely they haven't come loose. Most likely the bolt was not set originally and the nut was not capable of pulling it thru to set. Many hours later, many landings and bolt has set...leaving a locknut in space and a loose bolt. Making sure the bolt is set completely to begin with and then applying torque will solve that problem. The nuts themselves will not come off, they are locknuts. Loctite is used for screws or bolts into a threaded space so they don't back out.
 
Powder Coating!!!!!

The mention of powder coating above finally turned on a light bulb. As I found out when I had my panel and boarding steps powder coated, the process also coats the inside of the holes. Most of us had to ream out the holes on the brake assemblies to get bolts through them, right? I, for one, did not ream out the holes in the gear legs, just drove the bolts through with a block of wood and a hammer. I think the extra thickness of the PC created enough friction to meet the torque specs without seating the bolts, much less putting them in tension. As the coating wears, the bolts get loose! I doubt the prototype's legs (and maybe not Mitch's New Blue?) were coated, which is why Van's isn't seeing that problem. You guys still building run a tap through the bolt holes; the rest of us will have to keep tightening them until the coating is all gone.

That said, I CANNOT see how having the outboard bolts even 1/16 inch loose could cause the structural damage we're seeing in photos. My gear legs would have to move aft at least 3/8 inch to crimp the brake line and there's no way that could happen as long as the gear remained in the channel (assuming the channel doesn't deflect!). I can see a couple of possibilities.
1) the outboard bolts are broken, due to over-tensioning because of the PC friction drag and/or hard landing but still look OK because they're stuck on the holes or

2) I noticed that the front of the outboard mounting blocks (don't remember the P/N but that's what the outboard bolts go through) is snug against the factory-manufactured shop head of a squeezed rivet in the center bulkhead.
I remember having to file the front of those brackets to get them past the rivets and flush against the bottom of the channel. Is it possible that in the damaged airplanes the brackets were riding against the bottom of those rivets and either wore or broke the heads off during landings, allowing the brackets to move?

Wayne 120241/143WM
 
Checked N1212K!

Just checked mine and found no cracks or wrinkled skin. Both outside front bolts were tighten 1/2 turn.;)
 
Notes

RV-12 Gear Leg Attachment Cracks

After studying this thread, I made a few notes for myself.

Symptoms:

Cracks radiating from forward outboard landing gear attach bolts in the center section aluminum “C” channel web

Cracks more likely on right side.

Skin deformations about the landing gear

Crushed brake lines from contact with aft skin where gear leg passes through skin

*

Possible causes/contributing factors:

Hard landings

Heavy braking

Excessive touchdown speeds

Grass runways

Loose gear attach bolts

Powder Coat

Gear legs that are longitudinally stiff

*

Failure Mode Thoughts:

Two different mechanisms, one for skin damage another for crack formation.

Skin damage, crushed brake lines:

Hard landings, a torque stress? *Mechanism(s) is possibly the tail cone rotating down due to momentum and/or gear legs rotating aft due to impact.

*

Possible solution to skin damage:

Land better

Cracks:

Repeated impacts could be the mechanism of these failures. *Each individual impact could be less that that required to deform the skin, but could over time work harden (and thus more brittle) and eventually crack the “C” section web. *The source of these impacts is probably pulsating longitudinal forces on the wheels. Loose attachment bolts (facilitate impact transfer of energy), higher landing speeds (more energy), heavy braking (that causes shutter), not smooth runway surfaces, and gear legs that do not absorb longitudinal forces, would all contribute to impact forces being applied to the “C” section web.

Possible solution to Cracks:

Redesign landing gear to deal with longitudinal impact forces. Out of the builders hands.

Add reinforcements to “C” section web to mitigate impact forces.

Fully seat gear attachment bolts. *Ream holes, lube bolt shafts, seat (hammer?) bolt heads fully, remove powder coat (insure metal to metal contact). Add in non-thread resistance to torque values and torque to high end of range. Check bolts regularly.

Land slower.

Operate only on smooth surfaces.

Use minimal braking and insure that braking does not induce wheel shutter.

Ultimate Plan:

Follow Van’s recommendations when offered.


-Dave
 
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Dave,

Good shot at summarizing the reports. I'm not sure the rigor of reporting allows a complete analysis yet, but you have summarized the reports well.

I did a check on my RV today. After 29 Hours and 26 landings I found no cracks in the bulkhead channel. My inboard bolts were all tigt. I got a little less than one turn on each outboard bolt.

I am thinking about two mods I might make:

1. Fashion an anti-rotation device that keeps the nut from turning. The bolt might still creep out, but at least it would make it easier to retighten if the nut couldn't rotate.

2. Drill a 5/8" diameter hole under each outboard bolt head so I can get a straight shot with an extension on a socket to retighten the bolts.
 
N412BR. 40 Hours. About 70 landings.
No skin wrinkles, brake lines fine, no cracks.
Inboard bolts all tight.
Outboard bolts took about a half turn to get up to the high end of the torque spec, but note that THE TORQUE PAINT HAD NOT MOVED!

As others are saying, looks like the parts are settling in. I'm going to check those every 10 landings or so for awhile.

I did have to get a set of crows feet to torque the outboard nuts from above. It was not necessary to hold the bolt from below. 70 inch-pounds uses the smaller (1/4 drive) of my 2 torque wrenches, and the crows feet were 3/8 drive not 1/4 drive. But I had previously purchased a 1/4 FEMALE to 3/8 MALE adapter (an unusual tool) in order to use the small torque wrench on something where I needed a 3/8 drive socket.
 
Access to bolts

I really like the idea of access holes to make tightening the bolts a one-man (sorry, person) job but I don't think I'd want to drill more holes in an area of the fuselage that already may be having structural problems. Let's wait and hope Van's chimes in on this idea quickly. I'd thought about drilling out the rivets that attach the covers above the landing gear and making them into access panels with nutplates and screws but there's a LOT of rivets there and I couldn't get that many screws in there. We really DO need access to the attach bolts!

Wayne 120241/143WM
 
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Bolt stretch?

Has anyone actually removed any of the loose bolts and checked for stretched bolt, particularly in the threaded area? Just wondering.

AJSWA.
 
Have you got any ideas about what to look for on the bolt threads? I'm not sure evidence of stretching this small amount would be obvious, but I don't have any experience evaluating bolt threads except for gross damage.
 
Torque the bolts

The Bolts are not stretching and the nut is not turning loose .
The parts are all settling in. This connection needs to be re-torqued two or three times until it is totality seated.
My View
 
Joe Sounds Logical

Loose bolts: What Joe is saying is logical.

Channel Cracks: I'm wondering if the radial cracking around the bold head has been caused by overtightning the bolts. The lower bracket that supports the landing gear does not mate with the bottom of the channel leaving a gap between the bracket and the channel that may be enabling the bolt head to pull through the channel if overtightned. Tremendous compression forces exceeding the strength of the channel's web may be occurring.

Another scenario: If the aggregate force of the excessive bolt compression and rough landing exceeds strength of the channel ????

Just throwing a thought out there.

Hoping you'll be kind......:eek:


Gary Eldridge
 
I too had puzzled over that gap between the lower bracket and the channel. I suppose the possibility exists that the end of the bracket is bending under unusual forces, creating the loose bolts, or as stated, the channel is being deformed by proper bolt torque, or both.
 
I too had puzzled over that gap between the lower bracket and the channel. I suppose the possibility exists that the end of the bracket is bending under unusual forces, creating the loose bolts, or as stated, the channel is being deformed by improper bolt torque, or both.

It is not likely that the U-1202PC bracket bends causing loose bolts. The bosses on the ends (where the bolts pass through the holes), fit tightly against the edges of the gear leg. The gear leg would have to deform to allowing the brackets to bend.

I am assuming you meant to write improper bolt torque causing deformation of the channel. Properly torqued bolts would never have the tension load required to cause the cracking seen in the channel. I am doubtful that over torquing could do it either. I am pretty sure the threads would fail long before you caused any damage to the channel...
 
Excessive Breaking

It takes a 100HP and 700 feet to get the aircraft flying.

Trying to stop it in 200 feet takes a lot more energy

The kinetic energy required to stop an aircraft is a function of the weight (mass) of the aircraft multiplied by the square of its speed.

If the bolts are tight and the breaks are applied after the aircrafts drag slows the aircraft a little there would be no problem.
.
If vans adds reinforcement to this connection it will be only to help with excessive hard breaking. (Tradeoff strength for useful load )

Remember this is a Light sport Aircraft Not a Army Tank

My View
 
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RV-12 Gear Leg Attachment Cracks

After studying this thread, I made a few notes for myself.

Symptoms:

Cracks radiating from forward outboard landing gear attach bolts in the center section aluminum ?C? channel web

Cracks more likely on right side.

Skin deformations about the landing gear

Crushed brake lines from contact with aft skin where gear leg passes through skin


*

Possible causes/contributing factors:

Hard landings

Heavy braking

Excessive touchdown speeds

Grass runways

Loose gear attach bolts

Powder Coat

Gear legs that are longitudinally stiff

*

Failure Mode Thoughts:

Two different mechanisms, one for skin damage another for crack formation.

Skin damage, crushed brake lines:

Hard landings, a torque stress? *Mechanism(s) is possibly the tail cone rotating down due to momentum and/or gear legs rotating aft due to impact.

*

Possible solution to skin damage:

Land better

Cracks:

Repeated impacts could be the mechanism of these failures. *Each individual impact could be less that that required to deform the skin, but could over time work harden (and thus more brittle) and eventually crack the ?C? section web. *The source of these impacts is probably pulsating longitudinal forces on the wheels. Loose attachment bolts (facilitate impact transfer of energy), higher landing speeds (more energy), heavy braking (that causes shutter), not smooth runway surfaces, and gear legs that do not absorb longitudinal forces, would all contribute to impact forces being applied to the ?C? section web.

Possible solution to Cracks:

Redesign landing gear to deal with longitudinal impact forces. Out of the builders hands.

Add reinforcements to ?C? section web to mitigate impact forces.

Fully seat gear attachment bolts. *Ream holes, lube bolt shafts, seat (hammer?) bolt heads fully, remove powder coat (insure metal to metal contact). Add in non-thread resistance to torque values and torque to high end of range. Check bolts regularly.

Land slower.

Operate only on smooth surfaces.

Use minimal braking and insure that braking does not induce wheel shutter.

Ultimate Plan:

Follow Van?s recommendations when offered.


-Dave

Hi Dave,

This is a good summary of the situation so far. The only thing I can see that is missing at this stage, is the number of aircraft with the damage that the pilots feel they were operating within the normal range.

Does anyone have a count on this?

Cheers

Julian 120316
 
Properly torqued bolts would never have the tension load required to cause the cracking seen in the channel. I am doubtful that over torquing could do it either. I am pretty sure the threads would fail long before you caused any damage to the channel...

I have not installed my gear legs yet, but am almost at that point in the build. What concerns me is that, on my channel at least, the flange cutouts for the outer bolts do not finish flush with the top surface of the channel web. There is a small lip probably not much more than 1/32" high which passes through the edge of the bolt hole. Clearly, this means the nut and washer will not have a full bearing surface, and there will be a non-uniform stress concentration around the bolt hole at this point. The lip is visible in this photo.

7986933874_9335a7496e_z.jpg


I certainly have some concerns that this could potentially contribute to cracking of the channel web, so I'm considering using longer bolts and putting a 1/8" packer under the washer to spread the load better.
 
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I'd like a little reinforcement designed by Van's

In the fuselage area of concern IF it wouldn't lead to other yet unseen problems.

In LSA and the RV-12 in particular we have a maximum weight that we can legally fly at. Let's say that the reinforcement that is designed by Van's weighs 10 pounds. Though that is probably high, I'll incorporate it into my 12 build as a 'just in case' beef up.

It is beginning to look like these structural problems are caused by hard braking and??? No one wants to stomp on the brakes but if I must, as in the case of an emergency, I don't want to have to be weighing the cost of repairs as I try to avoid whatever it is that is ahead of me on the runway.

I'm not shooting at anyone here but in my opinion, the best way to reduce the 'gross weight' that you fly at in the 12 is to go on a diet and exercise. There are few of us over the age of 50 who couldn't stand to loose some weight. I'll speak for myself and my wife who have in the last two years lost a bunch of weight and now, with both of us in an airplane, we have a total combined weight of 325 pounds. All you have to do is to change the way that you eat - again, not shooting at anyone in particular. Shoot at me if you want, I'm a man.
 
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For some reason, I guess since I started this thread, owners have been reluctant to share their damage with others on the VAF, so they confided in me, in some cases only after I am sworn to secrecy. I am personally aware of three cracked channels (photos of all three are in this thread) and a total of 9 wrinkled side skins. That in itself is meaningless, since I have no way of knowing how many more (if any) are out there, nor how many have been reported to Vans.

.
Hi Dave,

This is a good summary of the situation so far. The only thing I can see that is missing at this stage, is the number of aircraft with the damage that the pilots feel they were operating within the normal range.

Does anyone have a count on this?

Cheers

Julian 120316
 
"I'm not shooting at anyone here but in my opinion, the best way to reduce the 'gross weight' that you fly at in the 12 is to go on a diet and exercise. There are few of us over the age of 50 who couldn't stand to loose some weight. I'll speak for myself and my wife who have in the last two years lost a bunch of weigt and now, with both of us in an airplane, we have a total combined weight of 325 pounds. All you have to do is to change the way that you eat - again, not shooting at anyone in particular. Shoot at me if you want, I'm a man."

Consider yourself "SHOT". BANG!

Bob Bogash
N737G
 
OK, no plane is expected to fall 20 feet to the runway and not sustain damage. But ANY plane must be expected to be able to use maximum braking after touchdown and not damage itself in the process! I have trust in Van's engineering that they got that right and this problem has to do with something other than the use of maximum braking. I mean, that would be Structures 101.
 
For some reason, I guess since I started this thread, owners have been reluctant to share their damage with others on the VAF, so they confided in me, in some cases only after I am sworn to secrecy. I am personally aware of three cracked channels (photos of all three are in this thread) and a total of 9 wrinkled side skins. That in itself is meaningless, since I have no way of knowing how many more (if any) are out there, nor how many have been reported to Vans.

.

It also seems too that not many have reported their damage through the proper channels to Vans. Which leads me to believe that pilot error may have been to some extent a lot of the problem.
 
Speed squared = kinetic energy

Bill
I believe your statement is about 90% correct.
However The kinetic energy required to stop an aircraft is a function of the weight (mass) of the aircraft multiplied by the square of its speed.

Land the aircraft faster than the design speed with loose bolts to many times by jumping on the breaks and getting tire skip things start to go wrong.

If information that I am getting from phone calls is truthful this is not from hard landings.

From talking with vans there demo aircraft has had its share of hard landings and no damage

Also we have heard that the 12 keeps flying and flying when they try to land.
This would be consistent with too fast of a landing and hard breaking.
( Pilot Error ) ( Learn your aircraft )

My view




OK, no plane is expected to fall 20 feet to the runway and not sustain damage. But ANY plane must be expected to be able to use maximum braking after touchdown and not damage itself in the process! I have trust in Van's engineering that they got that right and this problem has to do with something other than the use of maximum braking. I mean, that would be Structures 101.
 
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Joe, I get your point. And I have been learning to approach really slow because I will eventually be operating off of a 1260 foot strip. But consider - The break sheds kinetic energy by gripping the spinning disk and heating it and the pads up. Is that rate of shedding HIGHER when the disk is initially turning faster? Or are the brakes actually less effective initially in that case? I dunno.

Also consider this. Say you have a nice smooth "landing" at 70 kts. The plane is not ready to land. There is not necessarily much weight on the wheels. The brakes are applied firmly. They could lock the wheels or one wheel. Skipping along in that condition might put a lot more backward force on one side compared to the other, although the amount of force on either side should be within the design limits (?). Actually doesn't a locked up wheel provide less braking action than a turning one? Also in such a case it seems to me that the plane would head for the edge of the runway real fast.
 
As come on now Bob, admit it, that seems to be aimed directly at me:D
"I'm not shooting at anyone here but in my opinion, the best way to reduce the 'gross weight' that you fly at in the 12 is to go on a diet and exercise. There are few of us over the age of 50 who couldn't stand to loose some weight. I'll speak for myself and my wife who have in the last two years lost a bunch of weigt and now, with both of us in an airplane, we have a total combined weight of 325 pounds. All you have to do is to change the way that you eat - again, not shooting at anyone in particular. Shoot at me if you want, I'm a man."

Consider yourself "SHOT". BANG!

Bob Bogash
N737G
 
Bill
I believe your statement is about 90% correct.
However The kinetic energy required to stop an aircraft is a function of the weight (mass) of the aircraft multiplied by the square of its speed.

Land the aircraft faster than the design speed with loose bolts to many times by jumping on the breaks and getting tire skip things start to go wrong.

If information that I am getting from phone calls is truthful this is not from hard landings.

From talking with vans there demo aircraft has had its share of hard landings and no damage

Also we have heard that the 12 keeps flying and flying when they try to land.
This would be consistent with too fast of a landing and hard breaking.
( Pilot Error ) ( Learn your aircraft )

My view

Joe, though not an official notification, this is pretty much my view also (as an explanation for the cracked C channels anyway).

As a (personal opinion) response to some of the comments that have been bouncing around the forum......

The ASTM's that the airplane was certificated under, have a huge # of tests that need to be complied with. Quite a few of them are related to the landing gear. Most of those are physical drop tests where a fuselage is loaded to a specific weight and dropped from specific heights in a variety of conditions meant to simulate hard touch downs at high alpha angle of attack, very flat (improper) touch downs (all three wheels at once), and even a test for nose wheel first (very improper touch down).
One of the gear tests is an aft pull load applied to the main gear legs. This is to simulate the potential load caused by heavy braking at gross weight, with the maximum friction coefficient that could ever be expected between the tires and the ground. An additional amount of load is designed into this test for a design safety factor just as is done for wing tests, etc.
I do not believe that the speed which brakes are applied, has a major effect on what load the gear leg feels. The load will peak at the moment that the tire is just about to begin to skid/slide. This load point will always be relatively constant regardless of speed. It is primarily, only effected by variables of brake pad/disk condition, tire condition and traction/grip rating, surface condition (pavement/grass, dry/wet, etc.), and to some degree, the airplanes weight. In theory, it could be argued that a higher load could be induced at a slightly lower speed, because the higher the speed, the more lift that is being produced by the wing (even once rolling on the wheels), thus reducing the amount of weight that is actually induced into the contact footprint between the tire and ground. Less weight on the tire will lower the possible load before the tire begins to skid/slide.


The RV-12 test fuselage passed all of the tests (BTW, technically a new fuselage could be used for each test, but in the RV-12's case, the same one was used for every test).
An RV-12, if built as designed, and flown properly, within the design gross weight, should perform the same way that the drop test fuselage did.

So, why have some airplanes gotten damaged? That is still not fully clear at this point. As of early this week, only 3 reports had come in to Van's. I believe more have come in during the week, but not as many as would be expected considering the # that Don has reported receiving information about. As Mike S. already mentioned, is the unwillingness for people to notify Van's an indicator of what caused their damage? We will probably never know.

Summery....
I think damage to the channel has been caused by excessively loose gear legs. At least one of the photos posted here by Don (as far as I know, it has never been sent to Van's by the airplanes owner) looks to have evidence of plastic deformation in the surface of the channel from the washer pounding into the surface (I,E, there appears to be a depression slightly bigger than the diam. of the washer, actually pounded into the surface of the channel).
Another owner here on the forum was recently petitioning for help in solving a nose wheel shimmy problem. From the description he gave, I posted that it didn't sound like nose wheel shimmy was a likely cause. A little while later he posted in this thread that he found the outboard bolts for his main gear legs loose. He has never stated so, but it is my guess that the loose bolts were the cause of what he thought was nose wheel shimmy.

Considering the tests that the design originally passed, and the operational experience that exists within the fleet, I don't feel that RV-12 owners need to be worried about going easy on their brakes. Particularly on grass runways (pavement should always induce the highest loads). Builders do need to be sure their legs are tight (there will likely be recommendations issued regarding this).

Finally... It is my opinion that the damage to side skins is a different issue, and not directly related to loose gear legs and/or damage to the channel. I believe it is pure overload from a very hard landing (possible even with some level of brake application at touchdown).
It is always possible that some owners have experienced a very hard landing, with loose gear legs, so that the overall problem would be amplified, but I think they are two individual issues, that in a few cases might at first look like the are linked, but are probably not. I don't think enough evidence has come into Van's to in any way substantiate they are directly related.
This issue is also being looked at, though it would be helpful if the owners that have experienced damage to side skins, would submit reports that included a download from their D-180.
 
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Have just checked our landing gear and found no cracks, wrinkled skin or any visible signs of anything amiss. We were however able to tighten all the outboard bolts between ? and ? turn.

For information we bought our 12 from the builder with 10 hours on the clock so have no knowledge of how tight the bolts were at construction. There are two of us flying the aircraft and we now have about 100 hours total operating from Manchester Barton (EGCB) in the UK which is not the smoothest of grass airfields! A rough estimate of the number of landings on grass would be in the order of 130.


Alan
 
For what it is worth, people that have communicated with me support your two different causes/effects theory. Of the three cracked channels, one of them had no skin wrinkles. Of the 9 reported skin wrinkles, two of those had cracked channels.
All but two of the wrinkled side skins deny any hard landings, two believe that a particular hard landing caused the damage, however the damage was not noted until quite some time after the event. I believe all the wrinkled skins reported loose bolts as well as the wrinkles. I have made no attempt to keep track of all the loose bolts reports.

Joe, though not an official notification, this is pretty much my view also (as an explanation for the cracked C channels anyway).

As a (personal opinion) response to some of the comments that have been bouncing around the forum......

The ASTM's that the airplane was certificated under, have a huge # of tests that need to be complied with. Quite a few of them are related to the landing gear. Most of those are physical drop tests where a fuselage is loaded to a specific weight and dropped from specific heights in a variety of conditions meant to simulate hard touch downs at high alpha angle of attack, very flat (improper) touch downs (all three wheels at once, and even a test for nose wheel first (very improper touch down).
One of the gear tests is an aft pull load applied to the main gear legs. This is to simulate the potential load caused by heavy braking at gross weight, with the maximum friction coefficient that could ever be expected between the tires and the ground. An additional amount of load is designed into this test for a design safety factor just as is done for wing tests, etc.
I do not believe that the speed which brakes are applied, has a major effect on what load the gear leg feels. The load will peak at the moment that the tire is just about to begin to skid/slide. This load point will always be relatively constant regardless of speed. It is primarily, only effected by variables of brake pad/disk condition, tire condition and traction/grip rating, surface condition (pavement/grass, dry/wet, etc.), and to some degree, the airplanes weight. In theory, it could be argued that a higher load could be induced at a slightly lower speed, because the higher the speed, the more lift that is being produced by the wing (even once rolling on the wheels), thus reducing the amount of weight that is actually induced into the contact footprint between the tire and ground. Less weight on the tire will lower the possible load before the tire begins to skid/slide.


The RV-12 test fuselage passed all of the tests (BTW, technically a new fuselage could be used for each test, but in the rV-12's case. the same one was used for every test).
An RV-12, if built as designed, and flown properly, within the design gross weight, should perform the same way that the drop test fuselage did.

So, why have some airplanes gotten damaged? That is still not fully clear at this point. As of early this week, only 3 reports had come in to Van's. I believe more have come in during the week, but not as many as would be expected considering the # that Don has reported receiving information about. As Mike S. already mentioned, is the unwillingness for people to notify Van's and indicator of what caused their damage? We will probably never know.

Summery....
I think damage to the channel has been caused by excessively loose gear legs. At least one of the photos posted here by Don (as far as I know, it has never been sent to Van's by the airplanes owner) looks to have evidence of plastic deformation in the surface of the channel from the washer pounding into the surface (I,E, there appears to be a depression slightly bigger than the diam. of the washer, actually pounded into the surface of the channel).
Another owner here on the forum was recently petitioning for help in solving a nose wheel shimmy problem. From the description he gave, I posted that it didn't sound like nose wheel shimmy was a likely cause. A little while later he posted in this thread that he found the outboard bolts for his main gear legs loose. He has never stated so, but it is my guess that the loose bolts were the cause of what he thought was nose wheel shimmy.

Considering the tests that the design originally passed, and the operational experience that exists within the fleet, I don't reel that RV-12 owners need to be worried about going easy on their brakes. Particularly on grass runways (pavement should always induce the highest loads). Builders do need to be sure their legs are tight (there will likely be recommendations issued regarding this.

Finally... It is my opinion that the damage to side skins is a different issue, and not directly related to loose gear legs and/or damage to the channel. I believe it is pure overload from a very hard landing (possible even with some level of brake application).
It is always possible that some owners have experienced a very hard landing, with loose gear legs, so that the overall problem would be amplified, but I think they are two individual issues, that in a few cases might at first look like the are linked, but are probably not. I don't think enough evidence has come into Van's to in any way substantiate they are directly related.
This issue is also being looked at, though it would be helpful if the owners that have experienced damage to side skins, would submit reports that included a download from their D-180.
 
Bumpy Barton EGCB

Have just checked our landing gear and found no cracks, wrinkled skin or any visible signs of anything amiss. We were however able to tighten all the outboard bolts between ? and ? turn.

For information we bought our 12 from the builder with 10 hours on the clock so have no knowledge of how tight the bolts were at construction. There are two of us flying the aircraft and we now have about 100 hours total operating from Manchester Barton (EGCB) in the UK which is not the smoothest of grass airfields! A rough estimate of the number of landings on grass would be in the order of 130.


Alan

As Alan says, Barton ... is not the smoothest of Grass Airfields.:D

http://www.youtube.com/watch?v=IERK-JVieM0&feature=relmfu

John
 
drop tests where a fuselage is loaded to a specific weight

so besides the CG issue, what if the aircraft was loaded "over-gross?" would that fall in the category of hard-landing, side-skin damage?
 
drop tests where a fuselage is loaded to a specific weight

so besides the CG issue, what if the aircraft was loaded "over-gross?" would that fall in the category of hard-landing, side-skin damage?

Definitely possible..... The structures are designed and rested with the expectation that the airplane will not ever weight more than 1320 lbs.

BTW, as an industry standard, when this type of testing is done, the fuselage is not loaded to equal the total gross weight value (1320 lbs). The reason for this is that when a wing stalls, the lift it is producing doesn't drop to zero. So an airplane that drops in fully stalled still does not hit on the gear with a force equal to it weight at the time.
 
Yust askin'

Does the astm testing include longitudinal impulse loads? Like locked brakes 'skipping / shuttering' or rough irregular surfaces that would cause the wheels to oscillate longitudinally. This type off load would be entirely different from what is tested by a 'drop test.'

FYI: Yust is southern for just.

-Dave
 
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One of the gear tests is an aft pull load applied to the main gear legs. This is to simulate the potential load caused by heavy braking at gross weight, with the maximum friction coefficient that could ever be expected between the tires and the ground. An additional amount of load is designed into this test for a design safety factor just as is done for wing tests, etc.

Does the astm testing include longitudinal impulse loads? Like locked brakes 'skipping / shuttering' or rough irregular surfaces that would cause the wheels to oscillate longitudinally. This type off load would be entirely different from what is tested by a 'drop test.'

Refer above for a portion of my post the described the longitudinal load meant to simulate the highest load braking would be expected to produce.
 
Thanks Scott

As I said earlier, I'm an aero engineer, not a structures guy but I agree with your summary completely-- there are two different issues that have cropped up simaltaneously. There doesn't appear to be any way the loose bolts could contribute to the skin wrinkle UNLESS that center bulkhead is twisting. Even so, I can't see how things could move enough (almost a half inch in my airplane) for the brake line to contact the skin was long as the gear legs are inside the channel. There would have to be some additional damage someplace else on the skins.
That said, I REALLY would like for Van's to let us know if drilling access holes under the belly to allow checking the bolt torque on a regular basis is acceptable. If that section of skin is already being stressed I don't think I want to put more holes there.

PS There's no way you can land this airplane at 70 knots-- if you're showing more than than 50 knots before the flare it just ain't going to land!

THANKS

Wayne 120241/ 143WM
 
Scott,

Thanks for your reply.

Am I corect that, by your reply, the gear is NOT tested for repeated longitudinal impact loads, but rather a slowly applied longitudanal load (in addition to the drop tests).

Thanks, Dave
 
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Am I corect that, by your reply, the gear is NOT tested for repeated longitudinal impact loads, but rather a slowly applied longitudanal load (in addition to the drop tests).

That depends on your defenition of tested.

During the ASTM compliance testing, the gear on the RV-12 was not tested using a test rig that would repeatedly cycle a longitudinal load on the gear legs. The reality is, I don't think any company has ever produced a single engine aircraft (including Piper, Cessna, etc.) where this type of a test was done in a simulated situation.

The RV-12 gear has now been dynamically tested to some level on 180+ flying airplanes. The red prototype (N412RV) had right at 800 hrs T.T. when it completed its session of transition trainiing last week. It received a good looking over... no evidence of damage to the spar channel was noted.

As mentioned previously, I have a pretty high level of confidence that RV-12 owners don't need to worry about damaging their spar channel, if they confirm that their gear leg bolts are at the proper torque value.
 
RV-12 Safety Alert e-mailed to Van's 9.13.12

N712BK Landing Gear History(9/14/12)
4/19/10-0.2 hrs tach-1st flight
5/5/10-12.7 hrs tach-30 total landings-During my biennial flight review I was high and performed a left slip with full flaps. I came out of the slip with a higher sink rate than I wanted and the aircraft bounced as I was applying full power for the go-around. There was no second bounce and we did another pattern and landed. The bounce was uncomfortable but neither I nor my Instructor felt that anything other than an exterior inspection was necessary so the wings were not removed. Nothing was noted from the exterior inspection.
5/18/11-71.7 hrs tach-139 total landings-1st Condition Inspection and the wings were removed for the 1st time. The aircraft was not painted at the time and a slight skin ripple was noted on the right side behind the gear leg and both sides had a slight skin deflection above the forward edge of the gear leg. I have not been able to find the photos that I thought I took. There was also a visiting pilot that I thought took some photos but he cannot find his photos either.


4/11/12-109.8 hrs tach-225 total landings-started 2nd condition inspection.
4/11/12-N712BK left side showing very minor skin ripple aft of 2nd row of rivets. Note small skin deflection above forward edge of landing gear and at rear edge of wing spar cutout. This is the same appearance as I remember from the 1st Condition Inspection other than the airplane had been painted.

4023ce9c.jpg


4/11/12-N712BK right side showing wrinkle behind 2nd row of rivets. Slight deflection of skin above forward edge of gear and broken skin at rivet below spar cut out. Again this is what I remember it looked like at the 1st Condition Inspection.

a6020bef.jpg


5/29/12- completed 2nd condition inspection.


8/27/12-145.0 hrs tach-258 total landings-This was the last flight prior to checking landing gear outer bolts.
9/8/12-With an open end wrench all 4 outer landing gear nuts turned with the bolt.
9/11/12- Removed wings and found additional damage to the right and left side around the landing gear legs.
9/11/12-N712BK left side wrinkle has become more pronounced and there is a skin crack out of the bottom rear corner of the spar cut out. It appears that the brake line may have touched the rear skin but there is no dent.

071cd8e9.jpg


9/11/12-N712BK right side wrinkle aft of 2nd row of rivets is more pronounced. No change in the crack through the rivet hole below the spar.

a8d49951.jpg


9/11/12-N712BK right side looking forward showing dent in brake line.

134b3f3d.jpg


9/14/12- Put fuselage on saw horses and tried to wiggle gear legs fore and aft prior to torquing the 4 outer bolts. By sticking my finger between the brake line and the aft skin I could feel some slight motion. Torqued bolts to 132 inlbs. The torque on the inboard bolts was checked and they were OK. There were no cracks around the top or bottom bolt areas.

Bob Kibby, Van?s customer # 120105
 
Just so there is no confusion, Bob Kibby's plane is the first damaged one I actually saw, and is included in the counts I have provided, this is not an addition to those already mentioned.
 
Comment

I think it's pretty amazing how similar some of this damage looks compared to others. I just hope Van's has a team working on this for the sake of them and the builders. Transparency would benefit everyone involved, and the sooner the better.

No dog in the fight, just sayin.

.
 
I think it's pretty amazing how similar some of this damage looks compared to others.

Actually, when considering some of the common understandings of engineering, it is not surprising at all.

Every product ever designed and mass produced by man, has potential failure points. The designers goal is to try and make those points fall outside of the expected use perimeters of the product. If multiple users exceed a particular expected perimeter in the same way, the same failure is likely to occur every time.
This is not specific to just RV's when it comes to failures on airplanes.
Walk through an aircraft wrecking yard. You will repeatedly see the same damage to the nose gear attachment of failed nose gear on a particular model of Cessna. Totally expected.
 
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Actually, when considering some of the common understandings of engineering, it is not surprising at all.

Every product ever designed and mass produced by man, has potential failure points. The designers goal is to try and make those points fall outside of the expected use perimeters of the product. If multiple users exceed a particular expected perimeter in the same way, the same failure is likely to occur every time.
This is not specific to just RV's when it comes to failures on airplanes.
Walk through an aircraft wrecking yard. You will repeatedly see the same damage to the nose gear attachment of failed nose gear on a particular model of Cessna. Totally expected.

So with all this being said, I'll ask the tough question: is Van's actively looking into this issue, or is it something that is thought to now be acceptable (from Van's point of view) given the folks who have admitted to a harder than normal landing? I'm hoping more of the report forms are coming in to Van's, which should help to escalate the investigation?

I understand that this is a Light Sport aircraft. I understand it's not a Cessna trainer. I understand it's not a tank, etc. But what I do NOT understand is how one harder than normal landing could cause havok on the side(s) of a new airframe/fuselage? I see students beating the heck out of a C-162 LSA at our airport, with no visible damages to the side skins. This thing bounces all over the runway on a daily basis. Literally. I realize it is a production aircraft, but wouldn't it have been subjected to the same tests/load tests that the RV-12 was put through, before LSA approval? Flame away, but something just doesn't seem right here. I am just hoping that someone comes up with a solution which may eliminate this issue, given that everyone will most likely have a harder than normal landing at some point in their flying career.
 
So with all this being said, I'll ask the tough question: is Van's actively looking into this issue, or is it something that is thought to now be acceptable (from Van's point of view) given the folks who have admitted to a harder than normal landing? I'm hoping more of the report forms are coming in to Van's, which should help to escalate the investigation?

Gary, I have already answered this question (to you personally during a phone conversation). Yes it is being looked at. It would be more productive if more people would provide the data download from their D-180's.

I don't think I said anything in my post to imply otherwise. I was simply commenting on the "it sure is surprising" comment.
 
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