landing gear/gearbox information

How to keep the Cessna 170 flying and airworthy.

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n2582d
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Re: landing gear/gearbox information

Post by n2582d »

I find it interesting that Cessna went from an NAS147-34 bolt at the inboard end of the gear on the '48 to using a weaker AN7-20A on the later models.
Gary
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ghostflyer
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Re: landing gear/gearbox information

Post by ghostflyer »

I change my undercarriage bolts every 500 hrs or after a very harsh landing . I always torque my bolts up and have found one once very loose . After a very close inspection [dye penetrate used] of the gear box section the nut on one the attach bolts appeared to have its threads compromised . New NAS 147 bolts and An 363 are now always used. P plonk is not used in my aircraft .
Reason being if P plonk is used the area around the gearbox where the spring attaches is beefed up and if after a heavy or harsh landing and if there is any damage to be done the forces are transmitted then on to the weaker areas of the fuselage. In board of the rudder pedals attach points and the tunnel where the flap handle resides are is where cracking of the sheet metal have been found in the past in other aircraft mostly 180,s . Hard to prove it’s because of Pplonk but it’s nearly always because of this mod .
marathonrunner
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Re: landing gear/gearbox information

Post by marathonrunner »

Guess I posted this reply in the incorrect forum. I copied and pasted it. Cheers

I agree with George with how hard do you want to hit a brick wall. However these STC/mods need to be an analyzed a bit better before being incorporated.

Unfortunately, I work on a lot of Cessna 180/185 aircraft...far inferior to the 170 due to the ever failing trim system and the stinger tailwheel. I can write volumes but will keep this to the Reader Digest version as opposed to the Michners version.

I think we can all agree that energy cannot be created or destroyed. It just manifests its way to the next weakest point. I do not have pictures to share now as I am not home, but convalescing after a major accident. There are tailwheel conversions for the 180/185 and I will not say the STC holders as I have already had lengthy conversations with individuals at the ACO responsible for the STC. They have larger wheels and tires which make it "better" for off airport landings without ripping off the tailwheel. Unfortunately the forces manifest themselves into the airframe and end up making the holes larger to the point of failure for the rudder bellcranks where the tailwheel springs attach and also wow out the tail stinger attach points. The tailwheel also becomes loose on the stinger.

It becomes a point of what do you want to fail more, the tailwheel or the rudder bellcranks which will then not allow steering? I have sent many examples to the FAA and written many useless MDF reports.

The solution for the individual responsible for the STC holder is to make a statement in their documentation to inspect these areas. The bellcranks are difficult to access even with panels in the fuselage.

Not a 170 issue but again when you reinvent the wheel and make it better there is almost always a consequence.
It's not done till it's overdone
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ghostflyer
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Re: landing gear/gearbox information

Post by ghostflyer »

I totally agree with you marathonman , the Cessna 180 is similar to the 170 but not the same . Many years ago I was invited into the army [australian] [conscription] and due to my previous back ground I was posted to a aviation unit . In this unit we had about 24 cessna 180,s all different [Plus a heap of rotary wing and other fixed wing ] due to the nature of different missions many aircraft came back in all states of dis repair. PS. We also had a couple of bird dogs also .
The tail wheel,area was always giving us problems plus the main attach point for the main gear was always in for repair. One area of modification was the top plate of the gear box section where is was increased thickness [can’t remember how thick] and made of 7075 T3 . The gear box was beefed up plus a strengthing bracket in the tunnel . This was before Pponk existed. Plus the engine mount area of attachment to the fuselage was cracking problem also .
Cessna were of no help as they expected us to come up with the fixes and then they would incorporate it into their manuals . The Cessna 170 a/b models are a great design but fitted with the wrong engine . Buts that’s history .
Some of the Thai pilots that we training at the time really challenged the design of the c 180.
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DaveF
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Re: landing gear/gearbox information

Post by DaveF »

n2582d wrote:I find it interesting that Cessna went from an NAS147-34 bolt at the inboard end of the gear on the '48 to using a weaker AN7-20A on the later models.
Do you think the NAS147 bolt is strong enough to perform the same function as a Pponk, that is, to cause the gearbox to tear apart in a groundloop? That's the only reason I can think of that they'd change the bolt out just to have a weaker one. But with only one year of production in the books, could they really have gotten enough field experience? Or that they'd even care? More likely the AN7 was cheaper and more available.

I've never accepted the idea that I'd rather have a weaker attachment method, choosing to have more frequent gear collapses. I'll take my chances with a less-likely catastrophic one, thanks! But I understand the argument, and now I think that if you could use a bolt that's a tiny amount less strong than the gearbox structure, there'd be a reason not to install the Pponk. *Just before* the gearbox structure would suffer damage, let the bolt fail. So you get the full strength and the lesser damage. But if the bolt is much weaker, I'll keep my Pponk. Good luck calculating what bolt to use!
Metal Master
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Re: landing gear/gearbox information

Post by Metal Master »

Just add to the conversation:
The link below is and earlier post concerning landing gear box repair and damage. It has multiple pictures of the damage caused to my airplane when the previous owner had a relatively minor ground loop and tripped over the left main gear which ripped out the door post gear box, strut attach and caused heavy damage to the under belly of the aircraft.
The point being is the gear was attached with just the older AN style bolt. It had no P-Ponk mod and yet the bolt was strong enough to carry the mounting structure all of the fasteners attaching it to the bulkheads and still rip the entire fuselage out. This would have occurred P-Ponk mod or not. I have repaired multiple Cessna tail draggers from ground loops. I have installed The P-Ponk mod in 185's. Not in my 170A. But if someone gave it to me I would install it. But it is not insurance that the damage will not happen.
Regards,
Jim
Metal Master

http://www.cessna170.org/forums/viewtop ... =+gear+box
A&P, IA, New owner C170A N1208D, Have rebuilt some 50 aircraft. So many airplanes, So little time!
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n2582d
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Re: landing gear/gearbox information

Post by n2582d »

I see that the NAS 147 bolt is a carryover from the C-140. There the landing gear attach structure is welded steel rather than aluminum castings. I have no idea but perhaps the yield strength of the NAS bolt was more compatible with that steel attach structure and later, on the the 170A, Cessna decided it was overkill (or needlessly expensive) to continue the use a NAS bolt there??? It really would be interesting to test what yields first with the various combinations of bolts and nuts at the inboard gear attach point.

Edit: Jim, were were typing at the same time. You kind of answered my question with your response. Do you know what style nut - AN363 or AN365 - was on the AN7 bolt?
Gary
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GAHorn
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Re: landing gear/gearbox information

Post by GAHorn »

There seems to be a question whether Cessna made the change from NAS to AN bolt in order to provide a "less damaging" failure mode...??
I wonder if design engineers actually consider that prospect when doing their design-work.

WD Thompson, Cessna test-pilot and author of "Cessna, Wings for the World", tells a story of stress-testing (I seem to recall that it was a cantilever-wing design) in which the wing was designed and built... then placed in a test-rig and repeatedly run through cycles to see if it would withstand the design-life expected. As the number of design-cycles were approached, everyone observing began to breathe sighs-of-relief that the design-life was being met.
Then, as the desired number of cycles were exceeded ...(they continued the test to find the failure-point)... the design engineers lost their smiles. When the wing design went far past the number of cycles... the engineers began to frown. Finally they were unhappy that the wing far-exceeded it's expected failure-point.

To a pilot, this might be a reassuring thing, to know your airplane is stronger than necessary...the more the better. :P

But to the design-engineers.... they'd overbuilt it...and that mean it was heavier than necessary and, to them demonstrated a type of failure on their part. :cry:

I have a difficult time accepting the idea that Cessna deliberately designed the landing gear attach to fail in a "less damaging fashion". I suspect the reason they switched from the NAS to the AN bolt was a decision based on availability and cost considerations.
And I also believe the reason they did not make that gear-attach stronger (such as what a Pponk accomplishes) ... was in order to avoid unnecessary weight for what, at the time, was believed to be a rare type of accident. Ground-loops are not commonly thought to cause a gear to "fold under".... they tend to be thought of as outside-manuevers which drag wingtips. (We all know, of course that inside-ground-loops can also occur, but they are less common?)
The cross-wind landing that is too gusty or might otherwise cause loss of control is probably considered to be within the judgment/decision-making capabilities of a pilot... who would either reject the landing or select a different runway. I wonder if the engineers simply did not take into consideration that sometimes a pilot is "just damn determined" to try to "save this landing"... and end up placing a side-load that folds the gear under the airplane.... And since that inboard-end of the gearleg is usually loaded in a "downward" manner to support aircraft weight...and only "lightly loaded" when in flight (supporting only the weight of the wheel/brake-assy)... there was no desire to add unnecessary weight, mfr'g complexity, etc.. to that gearbox.

In summary, I have a difficult time believing they deliberately under-designed the gear attach to make repairs less-costly... or to "save" airplanes so Cessna would not have to sell many replacements.
'53 B-model N146YS SN:25713
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Metal Master
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Re: landing gear/gearbox information

Post by Metal Master »

n2582d wrote:I see that the NAS 147 bolt is a carryover from the C-140. There the landing gear attach structure is welded steel rather than aluminum castings. I have no idea but perhaps the yield strength of the NAS bolt was more compatible with that steel attach structure and later, on the the 170A, Cessna decided it was overkill (or needlessly expensive) to continue the use a NAS bolt there??? It really would be interesting to test what yields first with the various combinations of bolts and nuts at the inboard gear attach point.

Edit: Jim, were were typing at the same time. You kind of answered my question with your response. Do you know what style nut - AN363 or AN365 - was on the AN7 bolt?
It was an AN 365. Real crusty looking
A&P, IA, New owner C170A N1208D, Have rebuilt some 50 aircraft. So many airplanes, So little time!
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