Author Topic: LTS Gear Longeron Damage  (Read 1642 times)

wafer

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LTS Gear Longeron Damage
« on: Sep 13, 2017 »
I have been reluctant to post because I believe my situation is isolated.  After some discussions under the “Bent Bolts” thread recently, I reconsidered as it may help someone.

The attached subject damage was to my LTS, which uses a different method of attaching the gear to the fuselage than 1, 2, or 2L. If you have a stretch taildragger, and built to plans, you have the same set up too, as well as a 2L tri-gear, I believe.  The set-up is 2 bushings per side welded to the longeron with a .063 stiffener (attached plans). Each gear hole for the four mounting bolts have an upper and lower chamfer to allow the gear to flex. A few discussions here on the forum about the design, but no failures reported.

The damage mostly consists of a crack in the longeron by each bushing.  Additionally, there is a crack in the weld area between a bushing the .063 stiffener on the right forward section. Sickening is another description, as I have been in some form of shock since discovering in May.   

After cutting off the damaged areas, you can see the real force the longerons were under, with a heavy pulling force from the most forward bolts.

I have over 120 hours on the airframe, with a guess of ~200 landings. All landings were on a hard surface, as tri-gear Sonerais are not to operate off sod.  A few landings were “not perfect”, but nothing to warrant this damage. My typical gross weight is under 900 lbs.  And my main wheels rotated/spun well with little drag.  Brakes were not applied/locked during landings.

So what caused this damage?  In the past, both Pitts (with spring gear) and Christen Eagle owners suffered similar problems when operating off rough surfaces.  The fix were “radius plates,” like those offered through Grove.  But talking with Grove, this situation where the gear is actually mounted to the frame with bolts through the gear, radius plates will not help.

One possible cause is the mounting bolts were too tight and did not allowing flexing in the camber area.  I first discovered the situation because the bolts were actually loose in the front.  I replaced the bolts (no, they were not bent, and nor this damage present), and tightened to maybe 20 ft. lbs.  I am not aware of any torqueing limit, as I just made sure they were tight.  About 10 hours of flight later, I noted play in the gear.  I was close to annual time, so I started unbuttoning and… well…, you saw the pics.

Another possible cause is the use of round tubing.  The stretch plans call out for square ¾  .035 longerons in this area.  Back in 1993 when I obtained my plans and ordered my tubing, it was a common practice to substitute round tubing for the square tubing longerons due to costs.  So I have round tubing at the longerons, as an exception to the plans. Otherwise, my building of this area was done per plans.  The bushings would have more of an area to be welded to with a square longeron/stiffener, offering more rigidity and less flexing during loads.   

It was suggested in the “Bent Bolts” thread that extending the bushings past the bottom of the longeron/stiffener could translate too much loads on the longerons.  My stiffeners were rounded, but offered a flat surface for the gear to touch.  The bushing were filed to be flush with the stiffeners, so I do not think this was the root cause. 

One final cause could be the landing forces for a tri-gear.  The gear is mounted close to the CG, under the wing. I hold off letting the nose down as long as I can after touchdown, and eventually rotate to the nose gear -- typical for tri-gear operations.  But different than taildraggers, that land in an almost taxi ready configuration.  The momentum of the fuselage on landing wants to get the tri-gear nose on the ground, so holding off the flare puts some additional force on the most forward bolts.

So what really caused it?  Well, looking back I would have opted for square longerons as they provide more bushing rigidity.  My front bolts loosened, I suspect from the tri-gear forces slowly working the elastic nuts loose.  With the front bolt being loose, there was some “slapping” action upon landing that I suspect actually started some longeron and/or weld failures. After tightening, the loads got transformed to longerons even greater, and given the round tube situation it went downhill real fast after that.  So, some form of everything is my guess, that started with the bolts loosening.

I have been slowly licking my wounds to fix this.  Man, you forget how much assembly went on until you have to disassemble!  Since I want to use the existing gear, I need to mimic the previous longerons as to locate the bushings in the same locations.  Thus, I have spliced a new .035 longeron tube via rosette welding a 5/8 .049 thick tube within the longeron.  A side note….I am gas welding this area and was not sure how far the heat would transfer.  With the use of silicone “hot plates” and cutting away the fabric from the repair center, I have been able to keep the heat off the fabric.  .

After tacking the bushings in place, I plan to use a square 4130 7/8 tube .049 thick to “cap” around the longeron, with cuts around the bushings.  I will then bend the .063 stiffner per plans and weld it up.  And as noted, make sure the bushings are filed flat with the stiffener, and not extend into the gear.  This will get me pretty close to the plans.

Finally, I plan to use drilled bolts with cotter pins to mount the gear, and inspect more frequently than annually. 

Maybe this is some help to you; maybe not.  But thanks for listening!
Mike "Wafer" Then
SII LTS

wafer

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Re: LTS Gear Longeron Damage
« Reply #1 on: Sep 13, 2017 »
All the attachments follow....
Mike "Wafer" Then
SII LTS

wafer

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Re: LTS Gear Longeron Damage
« Reply #2 on: Sep 13, 2017 »
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Mike "Wafer" Then
SII LTS

wafer

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Re: LTS Gear Longeron Damage
« Reply #3 on: Sep 13, 2017 »
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Mike "Wafer" Then
SII LTS

wafer

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Re: LTS Gear Longeron Damage
« Reply #4 on: Sep 13, 2017 »
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Mike "Wafer" Then
SII LTS

wafer

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Re: LTS Gear Longeron Damage
« Reply #5 on: Sep 13, 2017 »
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SII LTS

Tom H

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Re: LTS Gear Longeron Damage
« Reply #6 on: Sep 15, 2017 »
That is good info, Wafer.  Since my project is still uncovered, I will check this area for weakness and consider reinforcement.

acrojohn

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Re: LTS Gear Longeron Damage
« Reply #7 on: Sep 15, 2017 »
Wafer,
    My Acrosport II has a Grove gear leg modification that included radius bars per the Eagle design notice but it did not have sufficient bracing forward and aft of the bolts to keep the longerons from bending and fatiguing from the loads placed on the tubes when braking - Not your issue.
    The flat stock welded to the side of your longerons goes a long way towards bracing and distributing those fore and aft loads, but does very little to arrest the rotational loads caused by the flexing of the gear spring as it naturally bends under load.  That was the original purpose of the radius bars.  The Pitts, Eagle and Acrosport sandwiches the gear spring between the radius bars and allows the gear to float.  There is no direct bolt attachment at this point fixing the gear spring rigidly to the longeron.  It must move independently from the longeron to avoid transferring the load.
     I have seen your configuration on another S-IILTS, but the gear spring bolt holes were significantly countersunk on the top and bottom to allow the gear to flex around the bolt and avoid transferring any binding and loads to the longerons.  I believe countersinking the holes will help your configuration.
     I have provided some pictures of my fix only to show how the gear on the Acro is sandwiched between the radius bars to allow the spring to rotate.  The Sonerai design seem prone to damage if the gear is not allowed to move independently.  I'll ask the builder of the S-1IILTS I tested to provide the specs on the countersink.  This plane has been flow heavy and has no sign of wear.

John
« Last Edit: Sep 15, 2017 by acrojohn »
AcroJohn

Tom H

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Re: LTS Gear Longeron Damage
« Reply #8 on: Sep 16, 2017 »
Acrojohn, that structure has both vertical and horizontal frame tubing meeting at all the gear attach bushing points - seems real sturdy.

Question - it looks like your gear is attached to an hour-glass shaped sheet metal part with a single bolt at the center, to keep the gear centered side-to-side relative to the fuselage.  Am I seeing this correctly?

wafer

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Re: LTS Gear Longeron Damage
« Reply #9 on: Sep 16, 2017 »
John,

Copy on the countersunk feature providing flex.  Yes, I am interested how far your LTS friend countersunk his.  I am glad to hear of positive reports with this bolt through config. 

Mine are countersunk; I believe the ~ 1/4" specified, but I will recheck.  I can make it larger.

I have tried envisioning many ways to fit a strap in this area.  I really like the strap configuration. Just not enough room with the trigear.

Wafer
Mike "Wafer" Then
SII LTS

acrojohn

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Re: LTS Gear Longeron Damage
« Reply #10 on: Sep 20, 2017 »
Sorry for the slow response.

Tom, Yes, the fuselage required extensive redesign for the Grove Gear.  We had to change the tube cluster from a single station cluster for the conventional gear to a twin cluster at the front and back of the bushing locations to distribute the loads rather than centralize them.  What is difficult to see is the heavy plate gussets that capture the bolt bushings and distribute them away from the longeron.  The lower gusset plate that supports the upper radius bar is .125 plate.  The upper gusset plate is .062. The gusset integrates the longeron, horizon diagonals, and the heavy wall bushings.  The forward longeron from the forward wing attach fitting to the firewall has also been upgraded to .058 wall thickness.
 
As you mentioned, there is a butterfly shaped sheet that fits span-wise across the floating gear spring that keeps the gear centered.  This is functional similar to the Sonerai I gear centering parts.  My S-1 will include modified radius bars. 

My plane typically flies at an acro GW around 1520 Lbs and the Grove Gear may look cool, but a single spring gear leg puts terrible stress on longerons.

Mike, I still need to talk to the owner, to get his specs on the countersink.  I would think the objectives of any design that integrates a spring gear must include isolating the spring movement away from the longeron and distributing the bolt bushing loads to the maximum extent practical.

John
AcroJohn

wafer

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Re: LTS Gear Longeron Damage
« Reply #11 on: Sep 20, 2017 »
John-

     I looked after I posted last, and I have at least the 1/8" chamfer the plans call for in the gear (not 1/4, like I thought).

     Others have been flying successfully with this set up, and I assume they followed the plans.

Wafer
Mike "Wafer" Then
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Thaddeus

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Re: LTS Gear Longeron Damage
« Reply #12 on: Oct 21, 2017 »
Wafer, I thought about this since you posted and came up with an idea of what is causing the issue. I've marked up one of your photos. The cracks are from rotation forces that cause the longeron to twist. Basically, the gear legs are fulcrums that attach to the longerons via the two mounting bolt flanges. When the plane lands, the weight of the aircraft divided by 2 is transmitted up each gear leg to the two bolt attachments, which are welded to the inside of the longerons.  Consider that the gear is about 3 feet long, so at least 300 plus pounds of torque is being applied to the longeron on each side in a twisting motion. The gear is trying to rotate up along the outside of the fuselage and all that prevents the rotation are these two little bolts attached to the longerons on the inside. After a few landings, the gear wins and the longerons twist and fracture. It's like a teeter totter with one side being 40 times longer than the other!

I did notice that the reinforcement plate was not fractured, yet. It was carrying the last of the rotational force and was flexing instead of fracturing.

So, the solution should transfer the gear rotational load to the diagonal or upright frame braces efficiently and not allow the longeron to twist. I think the gear attachment design should be modified to transmit that rotational force up the side of the structure instead of the focal twisting point on the longerons. The present bolt patterns are sufficient to prevent lateral gear movement, fore and aft, but is causing the failure by allowing the landing load to twist the longerons as if someone was using a pipe wrench.

It is difficult to make the longerons strong enough to resist the landing torque that is trying to twist them. Better to weld in gussets that go from the attachment bolt flanges to the upright bracing, which can then counter the landing gear torque.

The other way is to make the center section of the gear structure more rigid so it cannot flex with the gear load. The center has to be flexing down somewhere to allow the rotational force to be transferred to the longerons and twisting them.  In other words, we have to make the gear flex up around the outside of the longerons instead of pulling down on the longerons on the inside and twisting them.  The easiest way would be to weld a chrome moly U channel across the frame over the top of the gear so that it bridges all four mounting flanges (bolts go through it) and then weld to the longerons, the end reinforcement, and to the upright frame members. That would prevent the gear from flexing (rotating) in the center of the frame, would stop longerons from rotating, and it would transfer the rotational landing gear force to the upright frame members. I would also add a gear mounting bolt or two, in the middle of the reinforcement to prevent the center of the gear from down flexing, which it has to do to twist the longerons. Thaddeus
"It seems wisest to assume the worst from the beginning...and let anything better come as a surprise.” 
― Jules Verne, The Mysterious Island

Thaddeus

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Re: LTS Gear Longeron Damage
« Reply #13 on: Oct 21, 2017 »
It’s late, I forgot how to multiply! 1,000 lbs torque on the long side of the gear attachment. The Acro has a much more robust attachment that prevents the external rotation. Basically a gusseted attachment.
"It seems wisest to assume the worst from the beginning...and let anything better come as a surprise.” 
― Jules Verne, The Mysterious Island

Thaddeus

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Re: LTS Gear Longeron Damage
« Reply #14 on: Oct 21, 2017 »
Here’s a sketch, I’m sure someone can do better than this.
"It seems wisest to assume the worst from the beginning...and let anything better come as a surprise.” 
― Jules Verne, The Mysterious Island

 

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