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flap motor disconnect during landing

KayS

Well Known Member
don't know where this topic belongs to... i kindly ask the moderator to move if necessary.

today was my 10th solo flight with the RV7. turned from downwind (with 10° flaps) into base with 70 - 80 knots and wanted to set full flaps. suddenly there was a small bang and the airplane made a slight sag. "Ok, something's wrong with the flaps". thought about a second to abort the landing but i realized that the problem will not fix itself, if i stay in the air longer. made the landing without flaps, flare a little bit lower and faster.

i didn't touch anything before the picture was made. you can see that the thread of the rod end bearing was engaged maybe two turns or so. it comes apart by pure staring at it and i wonder how that one lasted for around 20 landings. if you lower your flaps the flap motor has to pull on the linkage, in this case the force was enough to pull the bearing out and to tear the safety wire.

what i learned so far:

1. practising no-flap-landings is a good idea. no drama in the RV-7 (i guess all RV`s) but you just should exercise it from time to time. luckily during transistion training my CFI strongly insisted to do some landings with 0° flaps and my response was "what should go wrong these?" well...

2. during construction i was a bit anal when it came to thread engagement of rod end bearings (best is more than half of thread length), but this one i just overlooked. i will check all of them on my airplane doesn't matter if they can turn or not. these thingys tend to be used mainly on flight controls and engine stuff, areas that are kinda important.

3. during critical phases of flight one should not fuss around with technical problems. just concentrate on the next opportunity to land.

4. i thought my 7 shows really great workmanship, but that doesn't have to mean a lot. :)

you guys take care...
Kay
 

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Good Post!..it's how we learn.

I think its a great post, as no matter how hard we try to make no mistakes building/maintaining, it happens. We LEARN from these findings, and shouldn't ever criticize, as it may save someone else's misfortune. Now fix it ,and move on!..also, excellent job of managing the situation and keeping a cool head!
 
Supposed to drill a hole in the actuator and safety wire to the rod end bearing to prevent it from spinning off like that.

Look at drawing 33, detail E
 
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yep, i know this SB and implemented it. on the pic you can see the safety wire. i do believe it was a thread engagement and not safety wire problem.

but what is also possible that maybe the safety wire broke and then the bearing turned itself out of flap actuator...?

@Steve: that's a good point as i did not use loctite or so on the jam nut. i remember discussions on this forum if loctite is necessary on jam nuts or not... it's like religion and politics. I did use loctite on the rest of the aircraft's jam nuts though but definitely not in this spot.
 
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Looks like the safety wire is secured to the threaded end of rod end. It should be around the bolt that passes thru the bearing.
 
Looks like the safety wire is secured to the threaded end of rod end. It should be around the bolt that passes thru the bearing.

Wow! Ok, seems i screwed up on that one. Looking at the SB (last time i saw that several years ago) it shows clearly that the wire has to go through the bolt head. don't know how i missed that...

now i think it was in fact a safety wire problem and the unsrewed bearing was just the symtom.

The SB talks about two ways to prevent the bearing from unscrewing: the jam nut AND the safety wire. if it was not an overlooked thread engagement, then both means failed in my case.
 
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safety wire

Seems the goal of the safety wire is to keep the black part from rotating, which could unscrew the it from the rod end bearing if the lock nut comes loose. This is very easy to inspect on the 8. Asymmetric flaps near the ground would probably not be fun, which is why I like the Van's design of the single flap arm.
 
looking at drawing 33 Detail E it asks for an AN4-10A, not AN4H or AN4-10. maybe that confused me. but anyway...

1. will get an AN4 bolt with an hole for safety wire in it and implement the SB correctly.
2. will torque the jam nut tight and with blue loctite
3. will make sure enough threads are engaged.

that should cure the problem. have no desire to experience something like that again. the event wasn't a true nightmare but a bit scary though.
 
Seems the goal of the safety wire is to keep the black part from rotating, which could unscrew the it from the rod end bearing if the lock nut comes loose. This is very easy to inspect on the 8. Asymmetric flaps near the ground would probably not be fun, which is why I like the Van's design of the single flap arm.

that's what i was thinking on my way back home... single actuation of both flaps is a good thing. i am really not sure if i would have written something on this forum today if the flaps would have been assymetric.
 
Looking at the SB (last time i saw that several years ago) it shows clearly that the wire has to go through the bolt head. don't know how i missed that...
The picture in the SB looks to me like the safety wire loops around the shaft of the bolt, not thru a hole in the head of the bolt. Do I have that wrong? I think a drilled head bolt would be better though, to keep the wire away from moving parts.
 
Dave: you're right, drawing shows the wire through the shaft and not the bolt head. But it shouldn't make any difference.
 
that's what i was thinking on my way back home... single actuation of both flaps is a good thing. i am really not sure if i would have written something on this forum today if the flaps would have been assymetric.

How would you get asymmetric flap failure with a failure of the flap motor (granted, I have a 7, not an 8, maybe they're different enough...?)
 
looking at drawing 33 Detail E it asks for an AN4-10A, not AN4H or AN4-10. maybe that confused me. but anyway...

1. will get an AN4 bolt with an hole for safety wire in it and implement the SB correctly.
2. will torque the jam nut tight and with blue loctite
3. will make sure enough threads are engaged.

that should cure the problem. have no desire to experience something like that again. the event wasn't a true nightmare but a bit scary though.

Look carefully at the SB drawing again.
The wire doesn't go through the bolt head, it shows it going around the shank of the bolt.
I admit that the drawing isn't done very clearly because the wire can't actually contact the bolt shank. It will actually be wrapped around the knob on the side of the rod end ball.
 
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All that is good info, but wasn't the cause of the failure the lack of sufficient thread engagement?
 
Scott: please check post #13. Should not make a difference, or?

The intended purpose of the safety wire is to prevent the shaft of the motor from rotating in the direction that would allow it to unthread from the rod end.

Any configuration of safety wire that will do this should provide the same level of safety.

What should be kept in mind is that there is a slight torque on the motor shaft (in opposing directions) when the motor is extending and retracting.
For this reason, the safety wire should be installed in such a way that movement of the motor shaft, regardless of which direction it attempts to rotate, is always putting tension on the safety wire.
If it is not, then the alternately reversing loads could be repeatedly bending the wire and eventually cause it to break.

With the statement above being true, the problem I see with wiring to the bolt head is that the bolt is fixed and does not rotate. That means there will be a slight angular change between the motor shaft and the hole in the bolt head every time motor goes from full extension to full retraction. This will be changing the distance between the two wire points slightly and possibly cause an eventual fatigue failure of the safety wire.
 
With the statement above being true, the problem I see with wiring to the bolt head is that the bolt is fixed and does not rotate. That means there will be a slight angular change between the motor shaft and the hole in the bolt head every time motor goes from full extension to full retraction. This will be changing the distance between the two wire points slightly and possibly cause an eventual fatigue failure of the safety wire.


Scott: thanks for your detailed Info! Will take an bolt with hole in the shank.

I don't think a bolt with a hole in the shank will address the rotation issue Scott is describing....
 
Kay,

From the photo it looks like the rod end bearing shank is too short. Perhaps this is the wrong rod end bearing.

I could be wrong as I’m not an RV-7 guy.

Carl
 
Safest bet is to do it EXACTLY like the plan/SB describe and show. If you don't understand it, Find someone local to you that does and ask them for help.
 
ok, now i think i got it... safety wire not going through any hole in any AN4 bolt. Instead it is looped around the bolt shank.

Thanks Dave, Scott, Sam!

missed the forest through the trees....
 
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Kay,

From the photo it looks like the rod end bearing shank is too short. Perhaps this is the wrong rod end bearing.

I could be wrong as I’m not an RV-7 guy.

Carl

i'm pretty sure that it is the correct rod end bearing but i will double check when i am at the airplane.
 
i'm pretty sure that it is the correct rod end bearing but i will double check when i am at the airplane.

If you have the correct rod end and it was only engaged a couple of threads, it is possible that the push/pull rod at each flap is too long.
 
If that's the case, then it would behoove the OP, IMO, to check ALL pushrods on the aircraft. You got lucky that it was that particular one that failed, and the flaps simply then drifted in trail. But a failure of an aileron pushrod is going to have very a bad ending; same with the elevator.

I'd have a good, hard look at every link in every flight control, including the rudder.

If that's what happened, that's a serious builder error...failure to ensure enough threads are engaged in a rod end.

There must be enough threads engaged on both ends of every pushrod so that even if it manages to unscrew itself all the way to one end or the other, the rod ends are *still* securely screwed into the tube.
 
RV7A Flyer:

like i wrote on this thread already: i am usually really anal when it comes to thread engagements. that is i have an hard time to explain to myself how this happened. so maybe the case was really that the bearing was sufficiently engaged but the jam nut failed. and my safety wire routing seems to be not making any sense at all.

just recently i checked all linkages of the primary flight and engine controls and they look good. the flap actuator bearing is special here as it is the only one that tends to unscrew itself by nature of the flap motor. all other rod end bearings on the aircraft don't have that problem so pronounced. and that is, i believe, the reason Vans calls for a "double protection" in this spot. but anyway... i will check all of them again.

Scott:

my thoughts too. if it was engaged only by 2 turns or so from the beginning and gets fixed now then the flaps should be out of rig and the flap pushrods need to be adjusted. hope to make it to the hangar today to check the whole thing. will report back.
 
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Don't trust only onto the safety wire! It will brake also after many fore/back movements. Maybe you recognize it at an annual ... maybe not.

Two things that help:
- Use Loctite on the eyeball-thread and on the counter nut.
- Make a disk out of POM/Delrin/Aluminium that prevents the eyeball "to bang" by each rotation change to the weldment. This "bang" is what start loosing up the nut. This disk is only on one side needed and should not press against the eyeball, but should reduce as much open space as possible. So the rotation is stopped when it begins.

And the safety-wire to finish the installation.
 
When Vic Syracuse performed an inspection of a friend's RV, he mandated using 0.041" (thick) safety wire for the flap motor push rod. At the time, I wondered why - although I still went back to my own airplane build and changed out the thinner safety wire for the thicker wire. Well, I no longer wonder why Vic wants to see the 0.041" wire on this part! Thanks for the informative post.

_Alex
 
Help, what am I missing?

I just completed this installation on my RV4. I dont understand how the actuator shaft can unscrew off the Heim joint stud. The Heim ball is constrained from rotating by the two flat linkage bars. My actuator drive shaft does not rotate as it advances, so it is not attempting to unscrew off off the Heim stud. There must be something inside the actuator tube that constrains the drive shaft from rotation as it advances. If that constraint broke it would be possible to unscrew the drive shaft from the Heim stud. What am I missing ?
 
I just completed this installation on my RV4. I dont understand how the actuator shaft can unscrew off the Heim joint stud. The Heim ball is constrained from rotating by the two flat linkage bars. My actuator drive shaft does not rotate as it advances, so it is not attempting to unscrew off off the Heim stud. There must be something inside the actuator tube that constrains the drive shaft from rotation as it advances. If that constraint broke it would be possible to unscrew the drive shaft from the Heim stud. What am I missing ?

The actuator is basically a jack screw. When the motor rotates the internal screw, friction causes the black pushrod to rotate with it, just like rotating a bolt with a nylok nut will cause the nut to turn if not held. You must hold the pushrod from rotating in order to force the jack screw to convert rotation to linear motion. A secured heim joint will transfer the rotation and the sides of the attachment point prevent the rotation. Once the heim is loose, the pushrod can easily rotate on the heim threads.

This type of actuator does not have in internal mechanism to constrain rotation of the pushrod. Many linear actuators do, but this specific one does not. It's design requires the end interface to do that. I would not be surprised that sometimes the pushrod doesn't rotate. However, that doesn't mean it can't or won't in the future.

Larry
 
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The force is with you

Seems like the force on an RV-7 is a pull, and for an RV-8 it's a push. If I have this right, wouldn't this change the likelihood of it unscrewing?
 
ok folks, just came back from the airport. fixed it and made a test flight, works so far. it was not major surgery, just had to remove the side cover of the flap actuator housing and it was even not necessary to remove the rod end bearing. you can screw the actuator onto the bearing with both being in place.

i applied blue loctite (the one that is removable) on the entire length of the bearing thread, so the inner thread of the actuator and the jam nut are secured with loctite. Made sure the bearing is almost all the way in and the jam nut is tight. i put in the safety wire, this time the way the SB tells you. in my original setup the safety wire was looped around the thread of the bearing and not the bolt shank. i think it's safe to call that a brain fart. anyway...

after the fix the flaps lined up perfectly with the trailing edges of the ailerons in neutral position and full down travel was still 42°. the geometry didn't change. that leads me to the conclusion that in fact the thread engagement was sufficient in first place and both means to secure the bearing onto the flap actuator failed in my case.

one observation i made: the safety wire will move with every change of the flap motors direction. so at some point it is likely to break due to fatique.

i am not here to blame Vans design for my own mistakes, but honestly speaking... the safety wire solution as depicted in the SB doesn't appear very nice to me. for sure there should be a way to improve it but have no idea how.

i would suggest to the others to check what their safety wire is doing and if the jam nut is still tight. i don't want to exaggerate but my flaps failed on base 600 ft above the ground. imagine that happens on your final or flare close to full flap stall speed. not so cool.

Cheers
Kay
 
Seems like the force on an RV-7 is a pull, and for an RV-8 it's a push. If I have this right, wouldn't this change the likelihood of it unscrewing?

i would guess it reduces the likelyhood. in the 7's setup the high load of lowering flaps is transfered into an pull between rod end bearing and flap actuator. it seems it's vice versa on the 8. but what do i know... maybe Scott could make a safer statement.
 
We try and prioritize data and service experience above how something looks (but we sometimes get pulled into that same trap as much as anyone else).

The safety wire SB goes all the way back to the earliest instances of applying it to the factory demonstrators.
This includes the original RV-6A (now 32 years old), 7A, 8A, & 9A prototypes and the RV-7 transition trainer.
They all have the original safety wire as far as I am aware (I do maint. and inspections on them). The RV-6A has north of 5800 hrs and I think the RV-7 is right around 6000.
 
Thanks LR172

This is more complex with a ShowPlanes FPS involved . If the SB safety wire failed, or even allowed shaft oscillation upon reversal, it would trash the notched rod that sets the various flap deflections in the micro switch carrier.

I think a more elegant solution than safety wire would be a shaft color type piece on the rod end that limits shaft rotation like outriggers on a boat. Unfortunatelly, the show planes notch rod carrier is already clamped on the end of the actuator rod. Hmmmm, more to do !
 
i would guess it reduces the likelyhood. in the 7's setup the high load of lowering flaps is transfered into an pull between rod end bearing and flap actuator. it seems it's vice versa on the 8. but what do i know... maybe Scott could make a safer statement.

The only thing that would make them different is if the actual tension or compression load is different between the two installations. I don't know what the actual loads are or if they are different.

The thing that helps keep the shaft from turning on the rod end if the jab nut is loose, is the relative friction between the threads on the rod end and the threads on the interior of the shaft. The higher the load (regardless of whether it is tension or compression) the higher the friction. As long as this friction is higher than the rotational torque induced on the motor shaft, the shaft should not be able to rotate.
I suspect that once the jamb nut is loose, any turning of the shaft on the rod end probably happens when there is very little load on the motor such as when raising or lowering the flaps when the airplane is on the ground.
 
This is more complex with a ShowPlanes FPS involved . If the SB safety wire failed, or even allowed shaft oscillation upon reversal, it would trash the notched rod that sets the various flap deflections in the micro switch carrier.

I think a more elegant solution than safety wire would be a shaft color type piece on the rod end that limits shaft rotation like outriggers on a boat. Unfortunatelly, the show planes notch rod carrier is already clamped on the end of the actuator rod. Hmmmm, more to do !

That FPS rod is relatively flexible. However, the risk is when the rod is short to it's attach point. On my 10. I set the rod attach bracket such that it is neutral as sits in the very center of the side to side or rotational pushrod travel in this position to minimize bending of that rod as the pushrod rotates through it's allowable range. You can rotate the pushrod side to side by hand to see what interference you are getting.

My saftey wire does not limit side to side rotational travel, so you should assume that over time it will be rotating back and forth from stop to stop. Fender washers instead of AN416's will help to limit the rotational travel if you think it is a problem.

Larry
 
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In the automotive world, they are fender washers.

In the aero world, they are large area washers. AN970.
 
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