Control System Failure

[Steve Fitton]

On its 699th flight, my Dreadnought was lost in the reverse wing over when I felt a big bang through the lines and handle and the plane instantly crashed and was demolished. It was obvious something broke, and examination of the wreck showed that the elevator pushrod was no longer attached to the flap horn.

All my planes to this point have used 3/32 wire with soldered brass washers as keepers and brass bushed control horns.  After a close look at what happened, I may have to consider going to ball link controls in newer planes.

   The first photo is a closeout photo of the pushrods/flap horn during construction.  Note there are washers on each side of the horn to limit side to side play.  The other photos are looking from below the flap horn assembly where the pipe mount was ripped out.  It looks like the inner washers are completely gone, and on the outer end of the elevator pushrod it looks like the part of the washer that was soldered broke off from the rest of the washer.  The bushing in the flap horn vanished somewhere during this process.  

I had been cautioned to solder the bushings in these double upright horns, and I told people I thought I didn't, but the closeout picture shows that I did in fact solder the bushings into the horn.  The closeout picture was taken in March, 2010 so my memory was wrong about not soldering.  Right before the fuselage was closed up I greased the bearing areas with Lubriplate grease from my M-1.

As I deconstruct more of the plane in order to salvage parts I will post a better view of the broken control parts...

[Brett Buck]

As I deconstruct more of the plane in order to salvage parts I will post a better view of the broken control parts...

 
   That is one bizarre failure.

    Brett

[Dave_Trible]

It sure is strange.  Sorry Steve.  Guess you got your money's worth from that old bird though.  Now you have an excuse to build a new one.  I'd be tempted to say soldering flux corrosion is at play. I use wheel collars here.  Waiting to see your replacement airplane.  At least it's now and not next spring.

Dave

[RandySmith]

Two things I would make mention of is 1- use a better grease, looks like from the wear I see on parts and all the bushings, it is gone missing
2- I see you soldered the horns on one plane, I really don't see it on the broke one?? the part at the bottom bushing that broke, you can see the bronze on the side in the first picture ?Was this one soldered on the bronze bushing?. The brass sleeves need grease too.

Ball links are OK too but will fail, I have seen many many ball link failures , Those exact Horns have been on many many planes with over 1500 to 2000 flights on them, Bill Rich has also put maybe 8 to 10 thousand flights on planes with those horns.
I have 4 planes with them in it, and have over 900 flights on one of them.
Be careful with Ball links and what type oil you use on them.

The deconstruction is good, It most always pays to look and see what you can gleam from this type stuff.

Randy

[Dan McEntee]

  On any linkage that is metal to metal I always put on a dan of grease before final assembly. Once it's built you can't get back in there, and like Brill Cream, a loittle dab will do ya in the long run.
  Sorry you lost the model,
   Dan McEntee

[RandySmith]

It sure is strange.  Sorry Steve.  Guess you got your money's worth from that old bird though.  Now you have an excuse to build a new one.  I'd be tempted to say soldering flux corrosion is at play. I use wheel collars here.  Waiting to see your replacement airplane.  At least it's now and not next spring.

Dave

I use rosin core solder on this type of job, but if I use acis or Silver solder I would "of course" clean neutralize and oil the parts I just soldered before putting them in the plane

[Fred Cronenwett]

My normal job is an engineer designing hardware that has to with stand flight loads. Any time we have a hole in a piece of metal we have 2D edge margin.

In other words if the hole in the piece of metal is .125 diameter the distance from the center of the hole to edge of the metal is .250. In the case of a .125 hole the metal horn would have to be .50 inch wide.

Measure the hole in the metal, then multiply the hole diameter by 4 and that will give you the width of the metal part.

What I see in your pictures is that the metal horn sheared. The pushrod may not have failed but the metal horn that held the pushrod failed possibly because the edge margin was not wide enough.

Anytime the edge margin is less than 1.25 you are at risk of shearing thru the metal horn.

Hope this helps,
Fred Cronenwett

[Steve Fitton]

Be careful with Ball links and what type oil you use on them.

...
Randy

I thought the general consensus was to use no lubrication whatsoever on ball links?

I know Gene uses Permatex anti seize compound on his. I was surprised to hear of a lube being used at all.

[Steve Fitton]

I use rosin core solder on this type of job, but if I use acis or Silver solder I would "of course" clean neutralize and oil the parts I just soldered before putting them in the plane

I always used Rubyfluid acid flux for everything except tanks.  On a job like this I wipe the joint with a kimwipe soaked in methanol after soldering.  I have not had any corrosion issues detected to date.  I will look at the other joints as I take apart the wreck and photograph them as well.

[RandySmith]

I thought the general consensus was to use no lubrication whatsoever on ball links?

I know Gene uses Permatex anti seize compound on his. I was surprised to hear of a lube being used at all.

I have seen wear and  locked up controls, plus controls that got very stiff from "no lube"  On the few ones I have used I have a RC Car Ball link lube that is made for plastic parts that I use on ball links. 
The plastic  grease is clear and synthetic.
I would Never use petro grease on them

Randy

[Howard Rush]

My normal job is an engineer designing hardware that has to with stand flight loads. Any time we have a hole in a piece of metal we have 2D edge margin.

2D + .06 when I was there. 

[don Burke]

2D + .06 when I was there. 

As the cops say when investigating a murder, "I'm sorry about your loss."

I think the bushing just allows more of a twisting moment on the horn due to pulling off center farther than if just through a regular horn.

Am I right that there are no washers still soldered to the pushrod end?

I think that the fore=aft width of the horn was too little, .030 edge distance not enough, IMO. plus the material even though in double shear configuration was too thin and "twisty" due to the pushrod pulling on only one side of the horn.  To prevent twist the horn should be a lot thicker.  The pushrod loads should go into the horn at or very near it's centerline, something that is accomplished with the pushrod captured BETWEEN the dual horns. 

IMO vibration caused the rod to move inboard and outboard, increasing the twisting moment on the horn assembly, eventually beating the bushing out and destroying the solder job on the washers. I usually use a piece of paper like the thin cardboard on small parts bags from the hobby shop.  I punch a hole the diameter of the wire, put the piece between the horn and washer, then after soldering I pull the cardboard out.

[55chevr]

I think there's was some misalignment ... The top hole has more moment than the lower but that doesn't seem enough wear comparing both when the lower shows no wear from the photos. What did the rear and bell rank look like.


Joe

[Howard Rush]

Bummer, Steve.  Better now than June, but still bad.

[Brett Buck]

Ball links are OK too but will fail, I have seen many many ball link failures , Those exact Horns have been on many many planes with over 1500 to 2000 flights on them, Bill Rich has also put maybe 8 to 10 thousand flights on planes with those horns.

  I have seen about 10 times as many soldered washer failures as ball link failures. This isn't link failure, anyway, it looks like the horn upright failed after the bushing started moving in the horn.

    Brett

[L0U CRANE]

Very much agree on the tear-out distances, although I don't always observe them on my lighter load models...

As to "neutralizing" solder joints - I like Sta-Brite (very low-silver) solder as easy to use and nicely strong. The flux is a thin acid liquid. After making a joint with this stuff, I go through a several step process.

Acid? Neutralize with baking soda/water paste. It WILL bubble up until All the acid residue is neutralized.

Rinse with water. Buff any available metal bright with reasonably fresh Scotch-Brite.

Swirl in alcohol - lacquer thinner, methanol, ethanol, isopropyl - to get the water out.

Oil, worked in so that the alcohol-bared metal is lubed. I have used Lubriplate w/o bad effects. Any oil heavier than 3-in-1, that isn't likely to evaporate quickly, should do. Lately I've used Rislone green, GMA's recommended longer-term engine storage lube, but as I'm flying less often lately, I don't have a track record on it.

The only clearly system failure I've had seemed on 'too-late' inspection of parts that would not be 'inspectable' ordinarily, involved a possible crystallized solder joint at the pushrod from bellcrank to flaps. I'd slit a brass tube, flattened the sides of the slit, and trapped a 6-32 nut between them. Drilled and thread-chased (A Model T Ford threaded kingpin arrangement, if you must...)

A dearly lamented departed friend had a habit of launching my models with a slight flip-up of the tail as he let go. Seemed only to happen with models for which I'd spent hours carving those early Zinger "prop-blanks" into good ones. One-blade, unbalanced props apply rather severe vibration to models. After 6 or maybe 8 such launches, something inside let go. Most of the bits looked as they were during assembly, but the joint to the 6-32 nut had cheesed-out...

Another important thing I have not seen addessed lately - stress. Applied stress is load divided by area, most simply put. Materials bearing stress can have different 'strengths.' Where 'strength' is not very large, using more area can allow the weaker material to bear the load.

E.g., if good 5-ply is twice as 'strong' as lite-3-ply, double the area to meet the same load, applied the same way. We use reinforcements, gussets, doublers, etc., to increase strength at the point of application AND along the supporting structure.

[Randy Cuberly]

Well, I'll add my two cents into this failure analysis.  Without close microscopic examination of the wear areas of the parts I can of course only make educated guesses from the pictures of the before and after photos and surmise what the major wear causing factors were.

The "before" photos show significant gaps at the ends of the pushrods between the soldered washers and the actual control horn bushings (which by the way are not very wide).  These narrow bushings lead to a higher unit loading on the bearing than would occur with a wider bushing, yeah I know that's obvious, however it also leads to another very significant load factor and that is a sliding motion side to side through the bearing surface combined with the oscillating rotational motion of the control function.  This sliding motion is the result of slight misalignment of the pushrod through the bushing and would slide in one direction during "up"
control and the other direction during "down" control, as the load forces pass from front to back on the bushing.  It is of course further agitated by vibration which could result in a fretting (or hammering) motion of the pushrod against the bushing.
This sliding motion is also detrimental to lubrication in the bearing surfaces over time as it tends to scrub the lubrication (grease in this case) out of the bearing to the sides, leaving metal to metal contact.
A bearing setup of this type should always strive to eliminate side to side motion as much as is possible without causing undue friction.
I would guess that the combined stress of the oscilating control motions, the sliding motions of the pushrod, and the fretting from vibration coupled with the loss of lubrication in the bearing from simply being pushed out of the bearing, were too much for the material combination and the relatively small surface area of the bushing.  Of these loads I would guess the oscillating normal control functions to be the lowest load and the least damaging.

A microscopic examination of the wear patterns on the parts would probably confirm what I've said...

PS:  Oh yeah...make the bushings wider and eliminate the sliding motion and this problem would likely disappear.

Randy Cuberly

PPS:  I think the tear-out distances are somewhat less important in relatively lightly loaded systems like ours.  I doubt that they contributed much to this failure...at least until the wear became extreme.

[Fred Cronenwett]

Edge Distance is sometimes 2D, 2D + .030, and in some cases 2D + .060 depending upon the conditions and tolerances.  you can reduce the edge margin but as it gets thinner the margin of safety goes down.

For our purposes 2D is a good starting point, and remember it is measured from the center of the hole to the edge of the part

Fred

[Doug Moon]

.....it looks like the horn upright failed after the bushing started moving in the horn.

    Brett

Bingo!!!  Once whatever bushing was used between the rod and the horn uprights was worn out or came loose it began to beat on the upright day in and day out. This enlarged the hole in the horn upright eventually it was too large and the daily banging on it was too much to handle.  Steve is a very accomplished flier and can put some serious pressure on the control system.

I had this very thing happen to me on an elevator horn. Opened up the hatch to see the bushing was long gone and the clevis was banging on the steel horn and the whole was about three times bigger than it should be.  YIKES!!!!  It was on it's way to this very same result.  I soldered in some bushings and off it went for a while longer until something else failed causing a total disaster.  

I am curious if you noticed if the plane became more difficult to groove or fly precisely over time, especially towards the end.  Did you feel like you just couldn't quite get it to stop right where you wanted it every time?  I would think the play in the system would at some point get so much so that the control system would lose some of its ability to be very precise.

[Doug Moon]

I thought the general consensus was to use no lubrication whatsoever on ball links?

I never use any kind of lube on ball links.  It's some kind of nylon/plastic over steel.  There is nothing to lube.  Doing so with the wrong material will only ruin the system.  The plastic will absorb the oil and they will grab the steel ball and your system will be sticky for the rest of its life.  

I had a side exhaust model that absorbed quite a bit of oil over its life and the links finally started showing some sticky signs and it started flying like crap.  I crashed it later in the heavy winds and close inspection showed the links were oily and tight as a drum on the ball.

[Doug Moon]

I use a Ball Link on the BC and that's it.  Using a Ball Link at the bell crank removes that tiny bit of pressure you get on the hole when you just use an L bend through the BC.  The rest are clevises.  They make the whole system linear. 

[Steve Fitton]

...I am curious if you noticed if the plane became more difficult to groove or fly precisely over time, especially towards the end.  Did you feel like you just couldn't quite get it to stop right where you wanted it every time?  I would think the play in the system would at some point get so much so that the control system would lose some of its ability to be very precise.

In hindsight there were plenty of signs something was wrong.  Derek even told me to cut into it last month but I had convinced myself the problems were all in my head and not with the plane.  For a long time, there had been black metal dust leaking out of the horn exits at the flap and elevator hingelines.

Controllability problems were subtle and intermittent, but there, I think, since Monday at the Nats.  At practice, I noticed the neutral on the plane felt a little off, and that maybe the lines were sticking.  I inspected them closely, and lo and behold found a popped strand on one line (that set had about 300 flights on them).  I changed lines, but never could get neutral to be perfect the rest of the week.  At least the sticky feel seemed to be gone.  When I got back home from Muncie I had to reset neutral because it felt rather far off from what I thought I had set during the Nats.

The plane felt ok through the Philly and Jim Coll contests, but at home, practicing at the turbulent home field it would sometimes act like the lines were sticking really bad, especially in inverted flight where it almost crashed once or twice in the inverted laps.  It was suffciently unnerving the first time it happened that I thought for sure the airplane had structurally broken, yet an examination after flight showed not so much as a stress crack anywhere.  I must have looked it over ten times in the last month.  The controls felt free and correct on the ground with no binds or hangups.

It really seemed like the crazy neutral and inverted flight problems were entirely from the wind at the practice field.  You could have one flight with several funnies, then the next be perfect.  

At the Huntersville contest I had to reset neutral again on the handle, then reset it when I got home.  I do remember fighting hard to hold it level on the inverted laps during the contest, but the wind was squirrely at Huntersville, too.  The final piece of information came the day before the crash, when I was flying with John Tate and he commented that I was holding noticeable down handle in level flight.  We went through several handle adjustments and the plane just didn't have a neutral anymore-it needed to be held against a tendency to dive or climb, and a 16th" change in the handle sent you either diving or climbing.  

Its easy to use hindsight here, but I just never considered failure a possibility as far as the control system was concerned, so I kept looking elsewhere.

[Doug Moon]

I am curious to see what the wear looks like on the elevator horn?

I am sorry to see the plane go.....losing a plane always sucks the big one!

[Doug Moon]

The flight characteristics you described above are the all the signs of slop at the flap horn.  

A couple of times a year I will grab the LOs hold them tight at the LO guide and grab the flap and try to move it.  It should be pretty darn tight.  Sure there will be a little play on older planes but I shouldn't be a noticeably more amount than I started with.

[Russell Shaffer]

Doug, do you make those clevises? 

[Steve Fitton]

I looked some more.  The elevator horn seems to have no detectable wear. 

From the side I can see that the bellcrank to flap horn connection is badly egged out with almost no bushing there.

[RandySmith]

Seems like the biggest problem was the lack of lube, when all the lube vaporized and went south you just had near dry metal to metal contact
You can even see the wear on the flap horn rod tube bearings

Randy

[Steve Helmick]

Doug, do you make those clevises? 

Those are UHP products, I'd wager...they look like a good bet to me. Click the link...a good distance down the page.    Steve http://www.ultrahobbyproducts.com/Accessories.html

[Brett Buck]

Doug, do you make those clevises? 

  Those are the UHP type. That's the kind of beef you need to make clevis work, even the strongest RC types are not nearly tough enough.

   The controls take an incredible beating on stunt planes, there's almost no way to make them too strong.

     Brett

[Douglas Ames]

2D + .06 when I was there. 

Before the mass exodus of Airline Engineers where I work, it was 1.5D min. with approval for a structural Faux Pas.
Now we just have College kids that have a hotline to Boeing.

[SteveMoon]

Sorry to see that Steve. Ok, time to get building and painting!!

As said before, NO lube on ball links. It's as simple as that. I also use
a system similar to Doug, but have used ball links at the flap horn and
UHP clevis at the elevator horn with no problems. I have done it both
ways, clevises at the flap horn and ball links at the flap horn without
trouble.

Steve

[john e. holliday]

Alright gentlemen,  I was finally going thru all my CD's and getting them in order and noticed I had not opened the July-Aug 2009 issue of Stunt News.   So I spent time reading it and it has articles about ball links and other stuff we need to consider.  Most I had forgotten during the years of flying.   Especially the article by our own Bob Whitely titled Tips, Hints and SWAGS starting on page 62.   You will have to read the article for what SWAG stands for.   But in my experience any lubrication attract dust and dirt.   Makes for lapping compound in the joints.   

[Steve Fitton]

Here are some other pictures I took this morning.

Of possible significance regarding wear is that I found that the elevator horn was well lubed, possibly because it seems that exhaust residue was entering that area from some damage around the tail wheel strut.  The wood in that area had some castor residue on the surface, although the wood was not oil soaked.

[Steve Helmick]

My thought is that the inner washer/solder joint failed first, being aggrivated by the offset of the pushrod(s) from the center of the levers. When the washer/solder failed, the bushings were battered loose, then eventually wore through the uprights and complete failure. Sorry for the loss of your plane. I think it's worthwhile looking through the wreckage to see how systems resisted wear & tear, but am not at all in favor of reusing old stuff, most especially control system hardware.  Steve 

[Norm Faith Jr.]

As best as I can tell, it was not mentioned...Access panels (inspection doors) guys! Full scales have them for this very reason. They are not overly complicated to engineer into "kit designs" or your own designs. 
Norm

[Brett Buck]

As best as I can tell, it was not mentioned...Access panels (inspection doors) guys! Full scales have them for this very reason. They are not overly complicated to engineer into "kit designs" or your own designs.  
Norm

   I have an access panel at the rear, but only to adjust the elevator WRT the flap. The problem with doing it at the flap horn or bellcrank is that this is the absolutely the worst place to put it from structural standpoint. Cut a hole right over the trailing edge of the wing and it has pretty dramatic effects on the rigidity of the aft fuselage, and cracks will tend to form right at the corners from the vibration and flight stress. I have been tempted when I was using soldered washers, but always figured the chance of it failing from lubrication issues was not worth the loss of robustness.

    Complete lubrication failure as shown here is very rare. Far more often the washer falls off, and that you can't check with an inspection hatch.

     Most people have had that happen. Mine was shortly after I moved to the Bay Area. I was flying out at the old Sewer Plant site in Redwood city, and had just replaced the stabilizer after it had folded the previous week. The pushrod was still attached, no problem. I had gotten through an entire flying session, and I noticed some guy in a station wagon had pulled up and was watching from the direct upwind position . I figured, hey, I am going to really drill this thing and show off. Got the entire flight in, 3 foot bottoms everywhere, and finished the flight with a great 4-leaf. I was going to really bomb the pullout, so I waited as late as I dared, and then - nothing. As perfect a vertical entry into the ground as possible, right up the wing leading edge, right at this guy's feet. It took a bit to recover and then I went over there to see what had happened. I walk up, say something like "well, I guess that was impressive, but don't ask me to do it again". He holds out his hand and says "Hi, I'm Ted Fancher"...

     Brett

[Balsa Butcher]

Great story Brett. It would have made a good episode on the late great Paul Harvey's "Now you know the rest of the story" radio broadcast...Good Day!