Author Topic: GEAR FAILURES AND HOW TO AVOID THEM  (Read 5261 times)

Paul Rule

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GEAR FAILURES AND HOW TO AVOID THEM
« on: January 19, 2012, 10:50:25 PM »
The Cadet was one of the very first production airplanes with retractable landing gear and that has clearly been a problem area.  I do not know of any Cadet that has not been on it's belly at least once. Maybe someone will post the exception here but I doubt it.  This thread is to share some of the reasons for gear problems and maybe prevent a few gear-up landings in the future. 

I think gear problems can be catigorized, at minimum, as follows (maybe you will want to add to the list):

1.  Forgetting to put it down.  (The warning system should prevent this ... if its working and we are paying attention.)
2.  Having some mechanical failure with no warning.
3.  Putting it down and thinking it is locked when it's not.  (Not understanding how it works...?)
4.  Having the gear unlock after putting it down and locking it.  (Lever slips out and gets pushed to right by your leg.)
5.  Inadvertently bending something by letting the gear fall when extending (and then the pins don't go in).
6.  Something (push rod system) gets bent inflight due to high "Gs" and the pins won't go in when you extend.

As I look at the list, I believe that it is in REVERSE ORDER of most likely to happen !!  My experience and conversations with several others is that the threaded ends of the push rods (can and will) bend a little and (if you are lucky) you notice that the pins have become harder to lock in.  Being a prudent (and suspicious) sort you then inspect and find the culprit before it bends more on the next flight and the pins refuse to lock.  Culver service memorandum no. 1  dated 10-24-41 still applies and is GOOD info.  On page 2 it says; "To neglect the warning implied by stiff operation is to invite trouble."

There have been a number of ideas on how to make the push rods more rigid and less likely to cause problems including making new ends with 7/16" threads in place of the standard 3/8".   The factory part is made from 4130 thick wall tubeing.  It is interesting to note that a tube is more rigid then the same sized round solid part.  Since many have been replaced with fabricated parts this may not have been noticed.

This ought to start some good discussion.
« Last Edit: January 19, 2012, 11:00:25 PM by Paul Rule »

Paul Rule

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #1 on: January 20, 2012, 09:00:57 AM »
I'll add a drawing I did years ago for the "Culvers Going Places" mag:

Woody

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #2 on: January 20, 2012, 04:26:47 PM »
I have logs from the day N37823 (373) rolled out of the factory.  There is nothing that I can find of any damage or accident of any kind on this plane.  Maybe I don't know where to look but it appears to have been recovered only once in 1954 with a change in color from red to green. Since 1964 it was hid away in a hanger in Fl. needing to restore.

Paul Rule

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #3 on: January 20, 2012, 06:22:42 PM »
Woody, I hope you have "The One" !!   Sometimes it's hard to find... do the pushrod ends look original and identical?? Are they made from tube stock?  Any log entry where the prop and/or exhaust stacks (or pitot tube or carb air box)  were replaced with no reason given?  Some Cadets settled onto their bellys very gently....   If you have the expander brakes are the light weight metal covers over the brakes bent, damaged or repaired on the bottom?  If a gear folds out toward the wing tip it gets the plywood nose rib next to it and it's often repaired not replaced.

Hope you find none of this!!!  P.R.

Woody

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #4 on: January 20, 2012, 07:06:07 PM »
Breaks and such were in great shape with NO rust.  It had expander brakes but I did not feel safe with them.  Push rods are solid with I think 3/8" ends.  Everything still had the green primer on it even though it was removed and repainted.  There was no repair work on the spar etc. Some trailing edges had to be replaced from dry rot only.  Dry rot was on one the right wing spar at the wing root which was cut out (1 inch) and replaced which is in my pictures on web site.  This plane only has 530 hours on airframe which I guess helps.
From reading the comments, I sure want to make sure the pins on the ratchet system gets enough bite and seats right.  I'm learning a lot from everyone.

Bill Poynter

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #5 on: January 20, 2012, 09:34:39 PM »
Letís go back to the original subject of this thread.  I believe that Paul's reason number 3 is a very common cause of gear related mishaps.  If left at the 50% engaged point, side loads on the taper on the locking pin can send it to the completely disengaged position.  Without visually inspecting the tab on the lock lever in the cockpit, you can't be sure that it's fully engaged in the slot.  Since that's really your only indication that the locking pin is fully seated, you need to be really sure that everything is properly adjusted. 

When trying to lock the gear down, if you allow the locking lever to return to the center position and you aren't holding the wheel against the down stop, the landing gear can then move out of alignment for locking it down.  At this point it's impossible to move the gear back into alignment without moving the locking lever back to the free-fall position. 

After experiencing an incident of the gear folding on landing rollout, I now make a point of dealing with the gear long before nearing the traffic pattern.   
« Last Edit: January 22, 2012, 09:57:51 AM by Bill Poynter »

Paul Rule

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #6 on: January 22, 2012, 06:03:32 PM »
I got the drawing for the link end converted to a .pdf for anyone who needs it for reference.

It is interesting to note that the factory changed it twice, both times adding strength.  Kind of indicates a problem area.....

Neal LaFrance

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #7 on: February 07, 2012, 05:07:43 PM »
Hi Cadet Pilots, Neal agian, asking questions about landing gear adjustment. Has there ever been a document writen on the procedures to adjust the gear after it has been compleaty removed from the plane?? I have read all the published factory procedures and never found one. I have the factory parts manual. Does any one have an overhaul manual for the airplane
Showing assembly of the landing, and adjustment?

Paul Rule

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #8 on: February 08, 2012, 10:38:11 PM »
Neal,  To the best of my knowledge Culver never produced any detailed maintenance instructions other then the 5-6 pages in the back of the parts book and the Service Memos. Neither gives information on rigging the gear retract system. 

Here is my procedure for what it's worth... (this assumes that everything is assembled and not too far from correct function).  Maybe others will add their ideas below:

1) Take time to get a good secure method of lifting and supporting the airplane even when you are climbing in and out of it.  Also take time to read and understand Service Memorandum #1 - Care and Operation of Retracting L.G.
2) Make completely sure that there is NO EXISTING DAMAGE to the system.  No bent push rods, no bent or partly sheared bolts and everything secured correctly.  It is not possible to get the rigging right if any of the pushrods have been bent in service.  NOTE: Both main gear push tubes were made with slight bends to clear other things in the wings.
3)  With the gears 1/2 way up and the lever in the "RAISE" position insure that the locking pins are just clear of (but very close to) the 'half moon' plates that move with the gear.  See my gear pin drawing to understand why.
4)Disconnect the LH gear (both push rod and locking pin rod) and block it in the down position for now.  Rig the RH gear in the up position such that with the ratchet's last click, it is within 1" of as far as it can go up.  Adjust the outer link in 1/2 turns to do this.
5) Lower the gear and adjust the 'down stop' screw (lower LH end of the rack & gear box) so the lock pin locks the RH gear. The pin movement into the gear should be 'smooth as silk'.
6) With the RH gear locked down rig the LH outer link in 1/2 turns until that lock pin also slides in 'smooth as silk'.
7) Re-connect everything and retract the gear.  If it is possible to retract the RH gear back to the same 'ratchet's last click' and the LH gear is not hanging out of the well... be happy, that is about as good as you can do.
8) If the LH gear is ahead of the RH and prevents getting back to that 'ratchet's last click' you will have to start over and rig the RH gear a little lower, say 2 inches from hitting. 

One gear seems to always lead the other into the wheel well.  This is in part because one gear 'rack' is above and one is below the center gear so the gears are 'pushed' from different points.  It is real hard to get them fully retracted, together. 

From a safety standpoint it is most important that the lock pins move into the lock position as smoothly as possible and as completely as possible AND do it only when aginst the down stop screw.  This is the "standard" by which you must judge the 'health' of your system as time goes on so get a feel for it now and remember what it's suposed to work like.  If some day it does not work as well that is your "red flag" to fix before further flight or fly gear down only... until you can.

JoeB

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #9 on: February 21, 2012, 09:41:26 PM »
I've found that the gear retraction and lowering procedure is much easier to complete than I expected, but please explain this, exactly how much effort should the gear 'Locked/Raise/Lower' lever require to move from each of the 3 positions? 

The retract system in #246 is very smooth in operation when raising and lowering the gear, but I do find that the locking lever itself is bit heavier in operation than I anticipated (like the design has little mechanical advantage/leverage).




Paul Rule

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #10 on: February 22, 2012, 12:22:31 PM »
Joe,

The mechanical advantage is not a problem but what you describe causes me concern.

I recomend strongly that you find and fix what ever is causing the lever to drag or not move freely.  A correctly rigged and un-dammaged gear lever SHOULD MOVE AS "SMOOTH AS SILK".  If it is hard to move this indicates either mis-rigged or damaged!!!

The one exception is that you should relieve the "UP" pressure on the ratchet (by pulling on the wheel) in order to move the lever fron RAISE to LOWER.  This is a two handed operation (therefore- trim it up and/or hold the stick with your knees).  Then keep your right hand sliding/gripping the wheel to slow the "fall" of the gear as it extends.

Sometimes (in the air) a little down pressure on the wheel is needed to hold the gears aginst the stop in order to slide the pins in freely.  This seems to vary with individual airplanes.  Gear fairings (or 500 x 5 wheels) may cause an air load that stops the gear before it reaches the down stop.

If it is difficult or drags moving into the lock position that is most critical and indicates that the down stop is not rigged correctly or that the two gear are not in 'sync' and do not arrive at the exact down position together. 

 Put it up on jacks and see if it is as hard to move on the ground as it is in the air.  If  all the above does not help and it is it is hard to move on the jacks ... fix it before flying again!  Far too many Culvers have been on ther bellys!  Review my rigging post.
« Last Edit: February 25, 2012, 12:54:47 AM by Paul Rule »

JoeB

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #11 on: February 22, 2012, 01:06:05 PM »
Hi Paul,
Thanks for this feedback, I will take some time to do a close inspection as soon as possible. 
'Smooth as silk' will be the goal
-Joe

Brett Lovett

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Re: GEAR FAILURES AND HOW TO AVOID THEM
« Reply #12 on: February 23, 2012, 10:22:21 AM »
Joe,

My "locked/raise/lower" lever (I usually just call it the gear selector), has a strong spring holding it down into the detents.  It does take a fair amount of effort to pull it up out of the detent (considerably more so than the typical detent on an electric gear selector), but then should move smoothly and easily between the detents as long as you are holding it all the way up against the spring pressure (and relieving the gear weight on the ratchet mechanism when appropriate as Paul discussed above).

Brett