andrew phillips

andrew phillips

Well Known Member
Am I imagining things? I thought I read about this on Saturday on VAF then the problem happened to me on Sunday. Now I can't find the post that mentioned it.

The inside wheel pant brackets looked fine all of last winter and up to yesterday (wheel pants off for winter). I do admit that during walk around I did not check them out thoroughly so I don't know how long this problem existed. I flew for over 2 hrs yesterday including some 4g acro. When I got back there were 2 vertical cracks visible to me as I pulled the plane into the hangar. One in the first 1/3 and one in the back 1/3 of the bracket. The flexing of the bracket had actually caused the front to be able to rub on the brake rotor. I was able to tear the flapping sections off with my hands in order to remove them from the plane. The cracks occurred exactly where you would imagine they would if you think about the offset sections that connect to the pants acting as airfoils and vibrating from side to side.

I looked on VAF on Sunday night, to find that thread that I read on Saturday. No luck so far. It had directed me to the fairings etc. site for the stainless replacements which I went to and looked at the product. Did I imagine that thread? Was it a premonition? I will keep looking.
 
i had the same thing. i was getting a vibration on the ground. after about 10 hours flight time i saw that one of the brackets had the same cracks. I removed them both and i dont think i will fly with the brackets on wheel pants off.
 
brackets

We tried making the brackets out of SS -- no dice. While that material is ductile and rustproof, it will not stand up to the continuing flexing of that application. We switched to .090 4130 and have had zero failures since. I would suggest copying the standard bracket and design, but changing the material call-out for any future replacement parts. If you are worried about weight, I'm sure you could cut some lightening holes in 'em, and retain the strength.

Sod field ops make this change virtually mandatory.

Robbie Attaway had SS brackets on his website -- I am not sure of he still carries those or not? www.attawayair.com

BTW a C150/152 has .125 4130 brackets...that shows what the factory boys came up with!

Carry on!
Mark
 
I am of the opinion that some wheel pant bracket failures can be traced to dubious installation technique. When I first attempted to install the bracket, I noticed the fiberglass wheel pant noticably deformed when the rivets were set. How many builders just leave it at that? Any builder is inviting problems if he rivets the flat portion of the bracket to the curved inner surface of the wheel pant without compensating for the natural mismatch that occurs between the two parts. This will almost certainly build a permanent preload condition into the bracket. Depending upon operating conditions, ultimate failure could be immediate, postponed indefinitely...or not at all. Years ago and following my own advice, what I did was to to remove the rivets and then "liquid shim" all that natural mismatch out before resetting new rivets. I continue to believe that without an undesirable and totally unnecessary built-in preload constantly exerting force on the wheel pant bracket, failure of the bracket seems less likely to occur.

http://www.vansairforce.com/community/showthread.php?t=6750&highlight=Liquid+shim

a5c8px.jpg
 
not that bracket

Rick, it is the bracket on the other side (inside) of the wheel. It is bolted to the axle structure then the pant is screwed to the bracket. It failed because it was blowing in the wind while the pants are off during the winter.

Andrew
 
I have a pair of outboard brackets that broke in a circle around the circumference of the large AN4 washer used under the bolt head attaching it to the axle. So what Mark said...make 'em out of 4130. I did the same and haven't had any issues since.
 
WHAT??!!

andrew phillips said:
Rick, it is the bracket on the other side (inside) of the wheel. It is bolted to the axle structure then the pant is screwed to the bracket. It failed because it was blowing in the wind while the pants are off during the winter.

Andrew
Click to expand...

Hey Andrew:

You are saying that the bracket cannot take the airloads of normal flight? Please confirm.

Carry on!
Mark
 
Rick6a said:
I am of the opinion that some wheel pant bracket failures can be traced to dubious installation technique. When I first attempted to install the bracket, I noticed the fiberglass wheel pant noticably deformed when the rivets were set. How many builders just leave it at that? Any builder is inviting problems if he rivets the flat portion of the bracket to the curved inner surface of the wheel pant without compensating for the natural mismatch that occurs between the two parts. This will almost certainly build a permanent preload condition into the bracket. Depending upon operating conditions, ultimate failure could be immediate, postponed indefinitely...or not at all. Years ago and following my own advice, what I did was to to remove the rivets and then "liquid shim" all that natural mismatch out before resetting new rivets. I continue to believe that without an undesirable and totally unnecessary built-in preload constantly exerting force on the wheel pant bracket, failure of the bracket seems less likely to occur.

http://www.vansairforce.com/community/showthread.php?t=6750&highlight=Liquid+shim
Click to expand...

Rick:

I think you are hitting the nail on the head. Your comments apply to both the inside and outside bracket. Some of us have over 2,000 hours over 12-years without any bracket failures.
 
Rick6a said:
I continue to believe that without an undesirable and totally unnecessary built-in preload constantly exerting force on the wheel pant bracket, failure of the bracket seems less likely to occur.
Click to expand...

Mounting bracket preload is certainly lousy craftsmanship, but....

Fatigue failure requires load cycles; load, release, load, release. Preload is often used to counter cyclical load; if the preload stress exceeds the expected cyclical load stress, there is no fatigue. The best example in our world is a Lycoming connecting rod bolt, but the principal extends elsewhere.

Just polishing the pins old bean ;)
 
reply for Mark

I removed the pants in early december when the first snow came. If there were cracks in the bracket at that time already then I failed to notice them. I was assuming that the cracks happened as a result of airflow over the exposed bracket. Perhaps the crack stated with the pants on then just became worse over time, I will never know.
 
RV6_flyer said:
......Your comments apply to both the inside and outside bracket. Some of us have over 2,000 hours over 12-years without any bracket failures.
Click to expand...
Gary,

Agreed. It comes down to quality control. Unfortunately, at the moment I don't have any good detail photographs of the wedge shaped area of "liquid shim" that I applied to the inside of the wheel pants at all four contact points. As this photo of an RV-8 wheel pant bracket illustrates, that flat area with the slash marks cannot hope to perfectely mate up the curved inner surface of the wheel pant unless a shim of some sort is fabricated to make up for the natural mismatch between the two parts. The inside of the wheel pant this bracket mates to reveals just how much build-up of liquid shim was required to work out all the gaps. Every builder is different. Lots of variables. Some may over-torque the attach screws to pull out gap thereby generating a preload on the bracket whether the aircraft is operating or not. Also, I'm guessing routine rough field operations can be much tougher on brackets than smooth paved surface operations would. Bottom line: I kinda doubt the designer planned on built-in bracket preload for this area. Good shop practice suggests to me that eliminating any preload causing gaps in the first place is a reasonable step in the right direction.

2em28u1.jpg
 
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