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Please, if possible, I humbly ask that we keep the posts in this particular thread pointed in the direction of solving the technical problem and NOT allow it to degrade into a conversation debating customer service at Infinity or whether or not retractable main landing gear is a good idea or not. Let's try to use this thread as a way to share important information with guys that have already gone down the Infinity Gear path.

 

Considering the recent collapse of Waiter's gear in mind...

 

http://www.canardzone.com/forum/showthread.php?t=38241

 

I am seriously concerned about the failure mode. I have been flying this gear an awful lot over the past year. I have made HUNDREDS and HUNDREDS of landings in all kinds of conditions. I don't know for sure if I have more landings than most on this gear but I have to be pretty high on that list.

 

To recap:

 

1) The airplane was recently returned to flight status after a 4 year rebuild.

 

2) The airplane had flown roughly 20 hours on this gear system prior to the incident

 

3) Waiter (in his posts) mentioned the first few landings as hard ("carrier") landings.

 

4) Following a test flight, after a normal landing, during taxi back to parking, the right side lower drag brace pin (The only mechanical device in place to secure the gear leg down and locked) failed allowing the right main landing gear leg to fold under the weight of the airplane.

 

5) The lower drag brace pin failed at the location of the drilled vertical hole which is intended to accept the insertion of a roll pin for the purpose of main pin retention.

 

6) As already stated by Waiter, the time of failure occurred while engine was at idle and the airplane was moving at taxi speed and not (Thank God!) during takeoff or landing. It can be assumed that failure of this pin during high speed ground handling and subsequent collapse of a gear leg has the potential of causing a catastrophic situation where the potential loss of the airplane and loss of the crew are greatly increased.

 

Questions I would like to answer:

 

1) When is the pin loaded with stress and how much stress.

 

a) loads during retraction?

b) loads during extension?

 

I am assuming these loads are based on the weight of the landing gear only plus the resistance of the airflow. Aircraft weight not a factor.

 

c) What are the potential loads during ground handling?

 

I am assuming these shear loads are much greater considering the load on the pin must now support the weight of the airplane especially during cross wind operations.

 

2) What direction is the stress applied in relation to the direction of the roll pin. My concern here is based on the fact that (I'm guessing a little) the main pin is drilled 90 degrees to the shear forces being applied at the point of greatest stress (high speed ground transition - cross wind landing)

 

In my mind, the retaining hole in the main pin is oriented in the worst possible direction or would this matter?

 

3) I'm not familiar with the math. How can I calculate the potential load on the main pin based on the weight of the airplane vs the arm location of that pin. In other words.. A fully loaded airplane at 1500 LBS would create X lbs of force on that pin during a side load.

 

4) Would the removal (for inspection) of that pin be an issue if I re installed it with a cotter pin and not the current style roll pin? Would the cotter pin cause any more or less stress on that retaining hole? I would like to assume that the failure of the main pin was detectable and not without warning if the pin could have been inspected for stress cracks.

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Please, if possible, I humbly ask that we keep the posts in this particular thread pointed in the direction of solving the technical problem and NOT allow it to degrade into a conversation debating customer service at Infinity or whether or not retractable main landing gear is a good idea or not. Let's try to use this thread as a way to share important information with guys that have already gone down the Infinity Gear path.

 

Considering the recent collapse of Waiter's gear in mind...

 

http://www.canardzone.com/forum/showthread.php?t=38241

 

I am seriously concerned about the failure mode. I have been flying this gear an awful lot over the past year. I have made HUNDREDS and HUNDREDS of landings in all kinds of conditions. I don't know for sure if I have more landings than most on this gear but I have to be pretty high on that list.

 

To recap:

 

1) The airplane was recently returned to flight status after a 4 year rebuild.

 

2) The airplane had flown roughly 20 hours on this gear system prior to the incident

 

3) Waiter (in his posts) mentioned the first few landings as hard ("carrier") landings.

 

4) Following a test flight, after a normal landing, during taxi back to parking, the right side lower drag brace pin (The only mechanical device in place to secure the gear leg down and locked) failed allowing the right main landing gear leg to fold under the weight of the airplane.

 

5) The lower drag brace pin failed at the location of the drilled vertical hole which is intended to accept the insertion of a roll pin for the purpose of main pin retention.

 

6) As already stated by Waiter, the time of failure occurred while engine was at idle and the airplane was moving at taxi speed and not (Thank God!) during takeoff or landing. It can be assumed that failure of this pin during high speed ground handling and subsequent collapse of a gear leg has the potential of causing a catastrophic situation where the potential loss of the airplane and loss of the crew are greatly increased.

 

Questions I would like to answer:

 

1) When is the pin loaded with stress and how much stress.

 

a) loads during retraction?

b) loads during extension?

 

I am assuming these loads are based on the weight of the landing gear only plus the resistance of the airflow. Aircraft weight not a factor.

 

c) What are the potential loads during ground handling?

 

I am assuming these shear loads are much greater considering the load on the pin must now support the weight of the airplane especially during cross wind operations.

 

2) What direction is the stress applied in relation to the direction of the roll pin. My concern here is based on the fact that (I'm guessing a little) the main pin is drilled 90 degrees to the shear forces being applied at the point of greatest stress (high speed ground transition - cross wind landing)

 

In my mind, the retaining hole in the main pin is oriented in the worst possible direction or would this matter?

 

3) I'm not familiar with the math. How can I calculate the potential load on the main pin based on the weight of the airplane vs the arm location of that pin. In other words.. A fully loaded airplane at 1500 LBS would create X lbs of force on that pin during a side load.

 

4) Would the removal (for inspection) of that pin be an issue if I re installed it with a cotter pin and not the current style roll pin? Would the cotter pin cause any more or less stress on that retaining hole? I would like to assume that the failure of the main pin was detectable and not without warning if the pin could have been inspected for stress cracks.

Flight hours have almost nothing to do with landing gear. you can not rate the success or failure of the system this way. the numbers of landing cycles needs to be considered in the evaluation process.

One factor that also needs to be considered is the flexing of any or all the components and air frame to cause a load on the pin that is not in direct shear. we have seen failures of the internal components that were suppose to keep the outer gear tube aligned with the inner tube. This fact would suggest that there is a twisting load on the inner alignment tube that is caused by the leverage of the wheel axle. This twisting would put a bending load on the pin. this twisting force in both directions could be a cause for the stress failure of the pin since the roll pin hole is aligned with the direction of the bending load on the pin.

Calculating loads on landing gear system would be simple if the loads were all static. the impact and vibrational loads are much more difficult to calculate and the cause of most landing gear component failures. doing a drop test or two is not going to show the same results as many landing cycles. The gear for a certified aircraft would go through thousands of test cycles both simulated and real but in the homebuilt world all the test cycles are real and every test might be the one just before a failure will occurs. I doubt that if you add up the number of landing cycles done on this model of gear you would find that it is much less than a thousand cycles to date.

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If you are going to publish opinions in a public forum, I would sincerely hope that the manufacturer would be included to avoid a one-sided and possibly biased view of a product.

 

I hope that the previous statement is not viewed as too political.

I have nothing further to say on the subject.

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... Calculating loads on landing gear system would be simple if the loads were all static. the impact and vibrational loads are much more difficult to calculate and the cause of most landing gear component failures...

One of the major issues in landing gear design is spin-up loads, for which an estimation process is given in FAR 23, Appendix D, section 23.1. These may be a large factor in applying loads to the system.

 

However, since the gear collapsed under a side load, it's likely that there had been fatigue of the pin at the drilled hole, and the side load imposed by going around the turn was just the straw that broke the camel's back.

 

Without knowing more about the exact design of the drag brace/downlock system, it's impossible to speculate on how the loads are imposed and what the design issues might be.

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Shear loads on the pin (side to side) Side loads during landing (crosswind)

 

Also, the gear leg is literally lifted into the well by this pin.

 

Although there aren't a lot of samples to collect data from, Bill Theringers is probably the high time with the most gear cycles.

 

I'll be talking to my Machinest this weekend, I think Phil had a retrofit using a shoulder screw. If so, I would like to see a photo of this!

 

 

Here are a couple hi resolution photos.

 

I can see what looks like a stress line(???) that runs about 45 degrees to the roll pin!

 

Waiter

post-706-141090172469_thumb.jpg

post-706-141090172513_thumb.jpg

post-706-141090172556_thumb.jpg

post-706-141090172597_thumb.jpg

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Shear loads on the pin (side to side) Side loads during landing (crosswind)

These will be the highest loads that this pin sees, and they can be VERY large. You've got a moment arm multiplier too, from the force at the wheel (think about landing in a 5 degree slip, and the forces created at the wheel by that, and then multiply by 3-5 or so) to the force at the pin.

 

Also, the gear leg is literally lifted into the well by this pin.

That force should be far lower than the landing forces described above.

 

I can see what looks like a stress line(???) that runs about 45 degrees to the roll pin!

The pictures, though a bit blurry, show a very clear brittle fracture of the metal right through the stress concentration (factor of three) caused by the roll pin drilled hole. The 45 degree "divot" running across the fracture face could be one of a number of things - inclusions in the metal (what's the metal? High strength steel? Low carbon steel?), crack initiation points, manufacturing defect, etc.

 

Here's what I surmise is going on. The main pin that broke is installed in the gear leg and retained with the roll pin. _IF_ the main pin (don't know the official name) was a perfect, zero clearance fit into the gear leg hole, then the pin would be in single shear as force was put on it in BOTH directions (since the force direction changes with left or right turning forces from the gear). The roll pin area would never see any stress.

 

However, if there's clearance in the hole (and more clearance is worse), the pin would be in an oscillatory bending mode as the forces went one way and then the other - think about the case where the pin was 1/8" smaller than the hole - it would have to bend to resist the forces on it. Even a few thousandths of clearance MIGHT be enough to cause large bending stresses. Turns and bumps while taxiing and taking off/landing would put hundreds/thousands of cycles on this pin, at possibly very high force levels (depending upon runway/taxiway condition, crosswinds and manufacturing tolerances). The stress on the pin from bending will create tensile stress, which leads to stress cracks at the stress concentration (roll pin hole). Eventually, the crack grows, and then bang - one more cycle of stress while turning and hitting a small bump - the material fractures at the stress concentration and stress crack.

 

The side of the pin inside the drag link sees similar issues, but since there's a spherical bearing, no roll pin hole, and a retaining clip on the end, the stresses in that side of the pin will be far lower than in the fixed side of the pin.

 

I would suggest a design fix, since this seems to be clearly a design and manufacturing issue, but I"m just speculating here and won't suggest anything unless I could examine all the parts myself and take a bunch of measurements.

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As some of you know, the Infinity Gear was originally designed using an axel that centered the wheel with the strut. However for some reason it was manufactured using a different axel, the MATCO A1-A, which moved the wheel centerline out approximaltly 1/2" or so (not sure the exact distance) off centerline.

 

This places twisting loads on the slotted AL guide tube, possibly leading to failure.

 

I spoke with MATCO at Oshkosh 2008, and they have fixed the problem by redesigning the A-1A axil by moving the location of the brake mounting plate to properly align the wheel centerline with the strut centerline.

 

The CORRECT, modified Axel is the A-1H model. Currently it is not listed on Matco's website, I intend to querry them next week and order a set (about $55 each).

 

This should be a mandatory change. I can't understand why JD hasn't made this information widely known.

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My tire lines up perfectly centered under the strut tube.

 

Waiter

This may seem like a good thing but it may also be the cause of the problem. with the tire centered it means that there is no constant load on the pin but a oscillating load. as the wheel is rolling the load is constantly switching from a left steering load to a right steering load. this would be known as zero scrub radius. in automobiles having zero scrub radius is a no no and will cause wheel shimmy. many other factors can cause this condition of oscillating loads, strut angle forward and aft, wheel toe in or tow out, amount of caster and camber.

a good resource for this information is in a book called

Light gear design for light aircraft by Ladislao Pazmany

http://www.aircraftspruce.com/catalog/bvpages/pazmanybooksManuals.php

this is a good book and does explain why most of the design features of this landing gear were not designed to create the most optimum landing gear system.

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I tried to remove my pins (Lower side brace) for inspection.. After a frustrating few hours I finally found a way to remove the roll pins but once they (retaining roll pins) were out and the side brace was removed, I found the main pin to be fit into the hole with no tolerance whatsoever. In other words, the pin seems to be pressed into place or perhaps even assisted in retention by an adhesive.

 

Is anyone smart on how the pin is held in place?

 

My don't want to do damage to the area just because I am trying to inspect the parts for damage. It reminds me of pulling the wing mounting pins on my old varieze. Those pins had threads in them so creating a pin puller was possible. In this case, the only thing I can think of is to force the pin out of the hole from the forward point where it protrudes through the angular mount which is attached to the gear leg.

 

Waiter, were you able to get the pin out of the hole yet?

 

:irked:

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Is anyone smart on how the pin is held in place?

Yes. I spoke with the manufacturer and he said they were pressed in place. He was convinced that the press in fit was sufficient but was persuaded to add the rollpin later on "To prevent rotation." It was not installed for retention purposes as previously stated.

 

I did not ask how many sets were manufactured that way.

 

If you want his phone number, it's on his site.

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I rigged up a pin puller and was able to "mostly" remove the lower pivot pin. Pressed in is an understatement! Even with the pin puller, it required an awful lot of force to move the pin. I was able extract the pin until it was flush on the forward portion where it protrudes inside the side brace Mount's angle bracket.

 

After seeing how tight and exacting that fit is, I can't imagine how Waiter's pin failed in the way it did AND if even if it had, I'm surprised it came out on its own considering how much force is required to pull that pin versus the type of retention hardware is installed on the aft end. It's nothing but a simple retaining ring and a washer.

 

That method tells me that it's virtually all shear loads and almost no tension loads.

 

I'm curious about elongation of the hole where Waiter's lower pivot pin was.

 

Elongation and a shimmy.. yea... I can see the failure in that case.

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Spent the afternoon at the machine shop with landing gear parts.

 

After beating many ideas around we came up with what seems like a nice simple, retrofitable fix.

 

We're going to make new pins that are identical to the originals, with one major exception: The end of the pin that protudes into the cavity of the side brace (the piece that say Infinity Aerospace) will be machined down and threaded (5/16x24)

 

We'll make a matching block that will slide up into the hollow cavity, It won't fill the entire cavity, just about 1/2 inch. This block will have a hole and be threaded to accept the pin. Kind of like a custom made nut.

 

ALSO - We're going to make a steel guide tube to replace the original aluminum tube. (.090 thick chromalloy tube, with lower and upper donuts welded to it. Holes and slot then machined into tube and donuts)

 

The pins should be available in a couple weeks, the guide tubes will be ready after Christmas.

 

 

 

Waiter

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We're going to make new pins that are identical to the originals, with one major exception: The end of the pin that protudes into the cavity of the side brace (the piece that say Infinity Aerospace) will be machined down and threaded (5/16x24)

 

We'll make a matching block that will slide up into the hollow cavity, It won't fill the entire cavity, just about 1/2 inch. This block will have a hole and be threaded to accept the pin. Kind of like a custom made nut.

How does that fix the fatigue problem? Yes, it removes a stress concentration at the hole, but if the pin is actually pressed into the hole correctly, there's no stress at that area (obviously not the case in your part, at least). But having a threaded portion at the end requires NO press fit on the lower part of the pin and also ensures another stress concentration at the base of the threaded portion. Without a press fit and with enough clearance to allow the pin to be threaded in, you're still in danger of relative motion and fatigue, and there's nothing to prevent the pin from unscrewing.

 

Unless I'm misunderstanding something pretty fierce, I like the "fix" less than the original design.

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Without a press fit and with enough clearance to allow the pin to be threaded in, you're still in danger of relative motion and fatigue, and there's nothing to prevent the pin from unscrewing.

 

 

We talked about this. This will be an ongoing inspection item for now. Part of this decision was, can this retrofit be performed without removing the gear or special tools (machine shop work).

 

HOWEVER

 

If we say, no, the gear really needs to come off and go to the machine shop, then this opens up other possibilities;

 

We also talked about tapering the pin where it goes into the hole. The only problem with tapering is, it requires machine work on the strut arm to cut the matching taper in the existing hole. (Not a real big deal, if it really needs to be done, might also be doable on location with the correct reamer)

 

If we do a taper, the nut must be tightened (snugged) to pull the taper pin in snug. This might be possible, a custom made washer that fits up inside the cavity opening (similiar in shape as our custom made nut) and possibly an AN4 or AN5 small area nut (MS21042). There should be enough room to get a wrench up there to tighten the nut. (A sloted nut with cotter pin would be preferable if using taper pin)

 

Looking at the photo

 

http://www.canardzone.com/forum/attachment.php?attachmentid=3101&d=1258743161

 

There isn't a lot of room, but it might be do-able.

 

May be possible to buy a pre-made taper pin (i.e. 3 inch long AN386-4), then machine it for this application. i.e. the taper portion would go inside the reamed taper in the side brace. The large end of the taper would be machined down to 3/8 diameter and have a grove cut for the retaining ring. This would allow all the original hardware, spherical bearing, etc to be used for the side brace/drag link.

 

Heres a little blurb on taper pins:

 

http://quid.us/hummel/taperpin.html

 

Marc, Whats your thoughts on this?

 

Waiter

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We talked about this. This will be an ongoing inspection item for now.

Since we're not talking about a wear item here, but something that breaks, I wouldn't be comfortable thinking that I'd be able to tell when something's going wrong, inside a hole I can't see into, with "inspections".

 

Marc, Whats your thoughts on this?

I think that an engineer should be hired to do some analysis on the gear and redesign areas of it that seem to have had issues in the past. I think that having non-mechanical engineers hunt and peck at attempting to fix design deficiencies, if that's what they are, is just pushing problems from one area to another, or creating one problem from a different one. I think it's extremely unlikely, that after 15 years of production and usage, having end users attempt to come up with fixes for design or mfg related failures is going to solve any problems.

 

It's not possible to redesign this gear (or anything, for that matter) via committee on the internet.

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Waiter,

 

Now that you've removed the pin, can you take pictures of the hole?

 

I'm very interested the possible wheel shimmy angle. If you say your wheel centerline is centered on the strut centerline, that suggests you're use the modified A1-H axles.

 

Perhaps the stock A1-A axle is a better choice, despite the torsion load it places on the internal slotted guide tube?

 

In either case, the Aluminum slotted guide tube should be replaced with a steel version.

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I'm using the original axels, this is a first generation gear set.

 

We havn't removed the broken pin yet. Wanted the machinest to look at it.

 

The portion that broke, the hole is slightly elongated. the pin wiggles slightly when placed in the hole.

 

Waiter

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I'm using the original axels, this is a first generation gear set.

Waiter

When I spoke w/JD, he claimed MATCO changed the specs on the axle, causing the wheel to be offset from centerline. So it's possible that the 1st Generation gear sets have the right axle, and later versions require the A1-H retrofit.

 

I'll measure mine (2nd Gen) in a couple weeks. Meanwhile I ordered the A1-H axles from MATCO.

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Mine are the origional set and they do not have any offset either.

 

Phillip Johnson

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I just talked to the machinest;

 

 

After the existing hole is cleaned up and reamed (it is slightly elongated now), we'll make the new pin to press fit it into the new oversized hole. We estimate it will be about .005 larger than it is now.

 

 

The new pin will be exactly like the old one, except for oversize and also, no roll pin hole in the middle of the press fit. Instead, we'll put a roll pin at the very end where the pin protrudes out the back. This will be a last ditch effort to retain the pin incase it ever elongates again. (This will be a preflight item, make sure the pin is OK and still tight in the hole)

 

The press fit will be responsible to hold the pin in, not the roll pin. The roll pin is there simple as a last ditch effort to keep the pin from sliding out in case it ever becomes loose.

 

***********************************

 

I took the other strut off the plane. The pin on this one looks OK, but we're going to press it out and replace it with a new pin so there won't be a roll pin hole in this one either.

 

 

While disassembling the good strut, I noticed two items:

 

1) The foot on the outer guide tube was loose.

 

and

 

2) The inner guide tube had a small amount of twist in it. The groove has a slight twist and was binding slightly in the outer tube.

 

THis isn't a big deal as I'm replacing the inner guide tube with steel tubes that we are making.

 

 

AS FOR THE FOOT - As per JDs instructions (a couple years ago), I did drill out the three retaining allen screws and installed the AN3 bolts. These bolts were the only thing holding the foot onto the tube.

 

The foot appears to be glued in place. WHen I cleaned all the adhesive, the foot was a very loose fit on the tube

 

I showed this to the achinest, he's going to weld the foot directly on the tube. (They're both made of steel). He'll cut a small grove in the bottom of the foot and also a groove in the tube, he'll then weld the tube to the foot and the groove will be filled in with the weld. This will happed on the bottom of the foot, not the top.

 

 

 

Waiter

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Larry;

 

Nice artwork, Looks like the real thing (except for the three holes)

 

Waiter

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Bolting the flange to the mounting pad would arrest the forces flexing the pin and deforming the bore. A bolt circle of 4 or more counterbored holes would be better than the 3 I drew.

 

Larry

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Bolting the flange to the mounting pad would arrest the forces flexing the pin and deforming the bore.

 

I am NOT an engineer.

I would question the timing of the deformation of the bore that the pin is pressed into.

Was it the deformation of the bore that caused the pin to fail?

Or, was it the failure of the pin that caused the deformation of the bore?

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