I am very uncomfortable with the idea of mounting the plastic front lower bearing block to the fuselage mounting bracket by tapping threads into the plastic blocks and using AN4 bolts.  

I looked up engineering standards for steel bolts in threaded plastic, nylon, TPFE, ABS, etc. and the smallest recommended thread pitch for a 1/4" bolt is 20 tpi.  The plan calls for safety wired AN4's which are 28 tpi.  Even though they are safety wired, they thread in upside down through the bracket into those plastic blocks.  

With the potential beating this plastic block can take from grass strip use or bumpy taxiways, etc. - plus whatever twisting friction exists from constant rudder use in flight - the only direct metal on metal structural bolt is the one long AN3 that goes through the 2 sides of the center structural strut channel at the rear of the block.  The front AN3 is only through the plastic.  That strut channel is riveted to the firewall with A4 rivets.  Even taking into consideration the countering tension of rudder cables - that tension is not fully there when the strut is partially compressed while on the ground. I've seen other builders' modifications like spring-loading the steering push-rods, etc.

There is constant movement being imparted to that plastic block, one way or the other.  Lateral, up/down (impact), rotation, friction...  and safety wire will do nothing to keep a bolt in when there are no threads left to hold the bolt... other than keeping it from hitting someone on the ground in the head.

Am I just being paranoid?  Has anyone used a different process of securing this plastic bearing?  I have a solution that I want to use but I thought I'd see if anyone else has done something else with theirs.

Thanks in advance!

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And your points are all good points, too!   ;>)  But actually, your Remos example makes my initial argument ... most if not all the models of the Zenith series use this bearing design and they do have a considerable service history including landing on surfaces no one would think of taking a Remos into (unimproved turf, sand, gravel bars, etc) - and apparently no problems with the bearing block!

However, I do concede my concerns are more theoretical than practical ... any event that generates enough force to tear out the bearing block is probably going to do a lot of damage even if the bearing block is still bolted to the mount!

 

Innovations and improvements such as yours keep us thinking outside the box!

 

Thanks,

John

I still think the Remos example is applicable.  The ZAC's landing on river beds are (mostly) 701's and later, 750's, with much better downward visibility.  Any pilot who is doing that is extremely skilled and capable and probably keeps his nose off the ground as long as possible (as with grass...) also.  I have never heard of a 601/650 put in service as a daily trainer, with inexperienced pilots just beginning to learn how to do short/soft field ops, or just plain landing the airplane.  I know I had many landings in the Remos that were pretty darn hard, not by intent... :>O  The plane flew many hours almost every single day, so the total hours and constant work occurred much faster than a pleasure bird.  

 Plus - you never know.  Look for collapsed nose wheel strut, not the bearing block... I'll bet you find some action.

A few hits:

http://www.aaib.gov.uk/cms_resources.cfm?file=/Zenair%20CH250%20Zen...

http://www.zenith.aero/forum/topics/750-front-gear-tubing

http://forums.matronics.com/viewtopic.php?p=237160

http://copilotco.com/mail-archives/matronics.2004/msg29552.html

http://airsafety.info/wp-content/uploads/xas111201.pdf

Talks about his "modified landing gear" : http://www.stol-adventures.com/bluff-landing.html

Just a few hits... again, it's totally realistic to say MY nose gear would collapse in any of these scenarios, if they were indeed beyond the structural capacity of the air frame.  Also - I have nothing but the highest regard for Chris Heinz' design.  He purposely kept many facets as simple as possible, keeping in mind amateur builders, simple tools, etc.  Maybe if you asked him to design a 650 TRAINER his nose gear would be beefed up a little bit... ya never know.

Those are mostly good examples of nose gear collapse, but none of them address the bearing block as having anything to do with their problem. Here's a more relevant comment (Mike had a STOL 750):


Comment by Mike Schlichtman on May 20, 2011 at 3:04pmDelete Comment

Paul,  The only thing that had any damage was the gear.  The cowl had a little damage when the nose dropped to the ground, but I am going to go ahead and replace it with new due to the fact that I did a lot of experimentation with ducts and airflow until I got the cooling figured out.  The fuselage, firewall and everything else held solid.  I did replace the lower bearing due to when the nose gear over twisted, it cut into the nylon just a little.  In reality, I could have reused it, but decided to replace it with new.  So to answer your question, even though the tube failed, everything else pretty much held its integrity.
John

P.S. - I discovered this perceived weakness due to my own stupidity... I had my front strut completely installed, with bungee, steering rods, etc. I had everything done except for the 4 bolts going into the bottom of the plastic block. I had both AN342A bolts torqued.  Without the bungee, it was totally smooth and turned almost effortlessly, and slid up and down with no binding.  

After I got the bungee installed, in my impatient haste to play pilot and experience my rudder pedals for the first time, I got into the cockpit and was stepping right and left.  The last time I stepped, the entire bottom third of the strut support channel separated from the firewall, popping the heads right off the A4 rivets.  It was then that I realized that the only thing countering that from happening was the 4 bolts, going into a plastic block.  It was after that realization, and re-riveting the channel with A5's,  that I decided to use a more positive method of fastening that block.    

NOW, I understand your concern! Lol!

 

I hope you at least got to make some airplane noises in the cockpit before the channel popped!

 

Just Kidding!!!

John

My guess is that if more builders did what I did and saw the immediate result... they'd think the exact same thing I did. It wasn't done out of guessing or speculation... seeing is believing, as they say.

Brian, you inspired me to do this mod on my Cruzer.  It was fairly straightforward...I used a 5/8" Forstener bit to drill the holes in the bearing block, Inserted the AN4 bolts, filled with JB Weld per your instruction and also added side brackets and also (probably total overkill) a bracket on the front of the block (the rivets on the front corner brackets are countersunk so the bracket sits flat and tight against the front of the bearing block).  Solid as the proverbial brick outhouse and negligible weight gain.  Thanks for the inspiration...

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