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Bent flap actuator rods (purposely) and other flap questions...

moespeeds

Well Known Member
Friend
During the pre-buy on my RV8 we noticed the flap actuator rods were bent for clearance. I figured I'd just correct the issue later.

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Today I'm installing the PH Flap Motor and decided to put the straight rods in while I'm at it. I now see why they were bent, the angle a straight arm needs to follow is nowhere near where it needs to be. It's much worse on the left side.

Opening up the through-hole is not an option as I'd need to cut up higher into the fuselage side, and I'm already at the max without cutting into the flap stop.

These photos show the issue:
pecgBWO.jpg

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So I see my options as:
#1 Leave the bent rods in, they've been working just fine for 380 hours

#2 Space the rod outboard down at the flap, but how far out could I go down there? I could also weld or modify the big white actuator that moves both flaps. I'm a very competent welder it would be no big deal to fab and weld new arms or spacers on the inboard side of the existing arms, but then how much will the arm protrude into the cockpit with the new angle?
WmMjVzC.jpg



Next question, specifically regarding the PH Flap Actuator. How much play should there be in the whole system? The builder left all the rod ends with no spacers so they could float back and forth along the bolts holding them to the actuator arm. Same with the motor to arm connection. If I slide washers in there to remove all the side-play, I get a lot of drag on the whole system which I'm sure isn't good for the motor.

Also, should I rig the flaps to pull tight right up against the stops on the fuse, or leave a small amount of play in there? When you grab a retracted flap, should it have any slop in it at all?

Thanks in advance guys
 
This one is puzzling, and a bit troubling Moe - I have never heard of anyone bending those pushrods to make them work on an -8, and I would be fairly nervous about doing so, since they are tubes in compression. Granted, the hexagonal tubs are a lot stronger than the aluminum cylinder tubes used originally, but there can be a lot of compressive force on them from deployed flaps, and having one decide to buckle could make for an interesting ride if you were down low.

The puzzling part is, as I said earlier, I have never seen a case where sooner had to bend one to get the clearance to work out in an -8. See if you can find another -8 nearby to look at and see how the holes fit. Most people have to open them up a bit in one direction or another - but they always seem to fit within the fairing.

More investigation is warranted.

Paul
 
The puzzling part is, as I said earlier, I have never seen a case where sooner had to bend one to get the clearance to work out in an -8. See if you can find another -8 nearby to look at and see how the holes fit. Most people have to open them up a bit in one direction or another - but they always seem to fit within the fairing.

More investigation is warranted.

Paul

Where do the holes normally exit the fuselage on the -8? On the -6, they exit right at the junction of the bottom and side skins. I absolutely hated cutting that opening.
 
Where do the holes normally exit the fuselage on the -8? On the -6, they exit right at the junction of the bottom and side skins. I absolutely hated cutting that opening.
Same with a 4,,
And my left & right are not exactly the same.
 
Another option would be to just swap the aluminum rods out with steel and bend them the same way. Other then getting into spacers or surgery on the Vans supplied actuator weldment I don't see another way to do this.
 
Maybe look over the plans, make repairs to the fuselage and install the pushrods I.A.W. the plans.
 
Does anybody know if the RV-7 flaps maybe shorter than the -8? Perhaps these are shorter flaps and that’s why they have that doubler on the inboard end? I’ll take some pictures tomorrow and try and post them.
 
bent flap rods

haveing been in 7 rv8 builds, all have bent flap flap rods, out of 4130 rod.the reason is to keep from in my opinion from unnecessary cutting on the fuselage.
 
Not totally clear

I think from the image that you show that the rod is bent more forward on the bottom portion, but it isn’t totally clear…. Is it bent more outward on the bottom portion instead? If so, I do remember the plans allowing for a washer to be used when threading the tie rod end into the flap to space it more towards the fuselage. I don’t have the plans in front of me, but check them if the latter is the case. If the former is the case, then sorry, I haven’t a clue how to proceed.

Good luck,
Sam
 
the inbrd rib404 and 406c angle are probably not in the correct location forcing the bad angle on the Rods....
 
Can one of you guys with a completed 8 show me a picture of the inboard end of your flap, where the rod connects?

The issue must be there, as the actuator location and angles are fixed.
 
Looking at section A-A on drawing 14B, it appears that the FL-704-L end rib has been inserted with the flanges outboard instead of inboard. This alters the entire structure, function and load transfer capability of the F-706B-L plate as it is supposed to be securely riveted to the end rib. I can also see that the F-706B-L plate has been stood off with some angle to permit the plate to sit in what would appear to be the correct point in space but it's obviously not quite right. That would account for the ugly surgery, geometry and bending of the rods to make it work. The path from the flap actuator to the flap should only require a very small amount of trimming as the correct path is quite linear. To make this work correctly, the end rib and plate really need to be replaced and installed as per the plans. While it might work OK at the moment, it simply hasn't been put together correctly and the result is some workarounds to obtain functionality. Additionally, the load path from the rod end goes straight to the lower skin and not to the underlying structure through the end rib creating a twisting moment on the forward spar, particularly on flap extension. I'm not surprised that this setup doesn't have a great deal of stiffness.
 
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Can one of you guys with a completed 8 show me a picture of the inboard end of your flap, where the rod connects?

The issue must be there, as the actuator location and angles are fixed.

Here you go. I don’t recall anything in the plans that called for bending the flap rods. It does call for enlarging the fuselage openings but if installed correctly there should be minimal removal.
 

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Looking at section A-A on drawing 14B, it appears that the FL-704-L end rib has been inserted with the flanges outboard instead of inboard. This alters the entire structure, function and load transfer capability of the F-706B-L plate as it is supposed to be securely riveted to the end rib. I can also see that the F-706B-L plate has been stood off with some angle to permit the plate to sit in what would appear to be the correct point in space but it's obviously not quite right. That would account for the ugly surgery, geometry and bending of the rods to make it work. The path from the flap actuator to the flap should only require a very small amount of trimming as the correct path is quite linear. To make this work correctly, the end rib and plate really need to be replaced and installed as per the plans. While it might work OK at the moment, it simply hasn't been put together correctly and the result is some workarounds to obtain functionality. Additionally, the load path from the rod end goes straight to the lower skin and not to the underlying structure through the end rib creating a twisting moment on the forward spar, particularly on flap extension. I'm not surprised that this setup doesn't have a great deal of stiffness.
That setup, with the flanges facing out, looks fine for an RV-4. That's exactly what the 4 plans call for (less the bent actuator rod).

Never having built an 8, the photo of the flap attachment looked fine to me. After a little google research I can see how that attachment is different for the other models and how that rib position was reversed from the 4.

Interesting,,
I wonder if that part of the flap design had changed over the years.
 
Looking at section A-A on drawing 14B, it appears that the FL-704-L end rib has been inserted with the flanges outboard instead of inboard. ...

Good eye! Here's an extract from drawing 14B:

RV-8 Flap FL-704-L.png

The drawing contains other views of the flap that confirms what you wrote.
 
Here you go. I don’t recall anything in the plans that called for bending the flap rods. It does call for enlarging the fuselage openings but if installed correctly there should be minimal removal.

I posted this yesterday but wasn't sure on an 8, so deleted it. The pics above look just like my 6 flaps and the OP's pics show a 3/4" angle installed that moves the support plate away from the rib and moves all of the weight carrying to the wrong areas and pushes the heim out 3/4". Due to the forces going to the wrong places, I would consider fixing it, as that 1/8" plate is supposed to be riveted to the end rib.

Larry
 
Really good observations by Chris Oborn in post #13.

My goodness, that is just disgraceful workmanship. I recommend that the OP pursue the following actions.

1. Refer this matter to Vans Aircraft for their corrective recommendation. I’d be reasonably confident that they will recommend that this detail be reconstructed according to their plans.

2. Check out the whole aircraft very carefully. This could be just the tip of the iceberg.
 
Both configurations of the inboard flap rib/bracket have been used on different models over the years.
Either one is technically acceptable and if built correctly should result in the same lateral position for the point where the flap actuator rod attaches.

The way the OP's are built as shown in his photos are actually per the plans for the RV-8.

The photos posted by Karl in post # 14 show how the fuselage opening should look if everything is built and working correctly.

To recap what has already been mentioned - bending a pushrod that is loaded in compression is a bad thing. It greatly reduces the load they are capable of taking in compression. In this case, since they are made from the solid hex material, it may not be a factor but since we haven't tested a pushrod in that configuration we can not endorse it.

BTW, the standard pushrod (the one that a lot of people change out because they say it is not strong enough) has been tested.
It was done many years ago when someone claimed that one failed in flight (it later was discovered that they had bent the push-rod, and it had nicks in it from contact with the fuselage opening).

The testing done at that time showed that the pushrod was capable of taking a compression load that was way above the calculated load induced by the flap on any of the RV models it was used on. The actual flap loads hadn't been measured on any RV model at that time, but the margin was so large, even accounting for a modest overspeed beyond Vfe, that no design change was seen to be necessary.
 
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Thanks for all the input guys!

My head is spinning because I can't keep track of who is posting about the actual RV-8 flap design firsthand, and who is speculating based on other models that they own.

I will replace the rods with 1018 steel hex tube, which is readily available locally. I did build an RV8 back in the 90's, and do not remember having this issue, but there's a lot of other things I've forgotten since then as well!

The workmanship on the aircraft overall is immaculate. I had a very good first impression when I initially looked it over, and this was confirmed during the pre-buy from the very experienced A&P I used.
 
Late to the party but I installed "altered" flap push rods. I recall doing this for clearance concerns, and not wanting to cut a bigger hole in the side of the fuselage. I didn't feel comfortable using the standard push rod material, so upgraded to a larger/heavier section of solid aluminum rod as I recall. At the time, I felt it was an adjustment required for a new model as I was a pretty early builder. Has worked perfectly for 21.3 years, ~2300 hrs.
 
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The photos posted by Karl in post # 14 show how the fuselage opening should look if everything is built and working correctly, even though the inboard end of his flaps are not built per plans for an RV-8 (as already mentioned, it shouldn't matter).

Scott, I respectfully disagree that my "flaps are not built per the plans for an RV-8".

They were built EXACTLY per the plans that I received from Vans (drawing 14B dated 2/06/02). The instructions clearly state “Rivet FL706B and the platenut to the inboard FL-704 rib…”. If those changed along the way, that's a different story. The plans I have show clearly that the end support plate FL-706B fits flush with the inboard flap rib FL-704. There is no space between the support plate and the inboard flap rib on an RV-8 (at least not mine).
 

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Scott, I respectfully disagree that my "flaps are not built per the plans for an RV-8".

They were built EXACTLY per the plans that I received from Vans (drawing 14B dated 2/06/02). The instructions clearly state “Rivet FL706B and the platenut to the inboard FL-704 rib…”. If those changed along the way, that's a different story. The plans I have show clearly that the end support plate FL-706B fits flush with the inboard flap rib FL-704. There is no space between the support plate and the inboard flap rib on an RV-8 (at least not mine).

Sorry, My mistake
When I looked in the plans folders for the RV-8 and RV-7 this morning, I for some reason ended up looking at a drawing that showed the style of flap inboard end as used on the RV-6 when looking for the RV-8. I thought it odd at the time but didn't question it.

I have edited my post with the correct info (that the RV-7 and 8 are the same, and only the RV-4 and 6 used the angle).
 
Thanks Scott. You had me wondering about it.

So the statement “The way the OP's are built as shown in his photos are actually per the plans for the RV-8” would also not be correct.
 
Sorry, My mistake
When I looked in the plans folders for the RV-8 and RV-7 this morning, I for some reason ended up looking at a drawing that showed the style of flap inboard end as used on the RV-6 when looking for the RV-8. I thought it odd at the time but didn't question it.

I have edited my post with the correct info (that the RV-7 and 8 are the same, and only the RV-4 and 6 used the angle).

Well now this is curious Scott, or maybe I am just wrapped up in to many posts with double negatives….but my RV-8 flaps are built exactly the way to OP’s are built - with the angle standing the actuating rib away from the sheet metal rib….and my flaps were Quick Builds - done by the factory. So….either that is acceptable, or we’ve got a whole lotta airplanes pout there with the wrong configuration.

That still doesn’t explain why the OP has bent pushrods - mine are straight, and the holes in the fuselage are actually quite small (by comparison with the size of the holes on our -3) so something still isn’t right about how his airplane went together. Or its just a serial number specific thing? My serial number is 82133, kit delivered 2004(ish).

Paul
 
Thanks Scott. You had me wondering about it.

So the statement “The way the OP's are built as shown in his photos are actually per the plans for the RV-8” would also not be correct.

Well now this is curious Scott, or maybe I am just wrapped up in to many posts with double negatives….but my RV-8 flaps are built exactly the way to OP’s are built - with the angle standing the actuating rib away from the sheet metal rib….and my flaps were Quick Builds - done by the factory. So….either that is acceptable, or we’ve got a whole lotta airplanes pout there with the wrong configuration.

That still doesn’t explain why the OP has bent pushrods - mine are straight, and the holes in the fuselage are actually quite small (by comparison with the size of the holes on our -3) so something still isn’t right about how his airplane went together. Or its just a serial number specific thing? My serial number is 82133, kit delivered 2004(ish).

Paul

I don't have the time to dig into the details at the moment but I think it has possibly been both ways (was changed at some point to simplify the build), which may be why I some how stumbled onto an alternate drawing this morning.
As (I think) I posted earlier (Yes, it is in post # 22), they both would be considered equal from a structural and dimensional standpoint. I do not think the OP's version has anything to do with his alignment issues.
My first guess would be that the sweep of the wings was not properly aligned when the rear spar attach holes were drilled. This could have left the aft end of the wing too far outboard.
 
Don't bend your flap rods

A couple of years ago at condition inspection I decided to upgrade to the newer style flap rods. I had access to a calibrated load frame and wanted to see how much force the old round tube rods would take.
At 680 lbF the rod started to bend, after that it took <300 lbF to continue deforming it.

I also have QB flaps, delivered in 1999 and I have the pushrod attach rib away from the root rib requiring a locknut between the two ribs.
 

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My first guess would be that the sweep of the wings was not properly aligned when the rear spar attach holes were drilled. This could have left the aft end of the wing too far outboard.

I'm thinking that if that is the problem then the wing sweep angles may be way off. And they may vary between the individual wings. The OP should check this out.

The bent flap rods are a sure sign that something is wrong with this aircraft. Putting in beefier bent rods does not address the real problem....it just ignores it.
 
If the rods are bent outboard as they exit the fuselage, that tells me that there should be a straight line path from the torque arm inside the fuselage to the hard point on the flap. If the OP installed straight rods with the appropriate heim bearing at the top, is there anythign that would prevent it from working, or does the top heim bearing reach its limit before the flap rod can be connected at both ends?

Perhaps the builder didn't want to cut such a big hole in his/her fuselage?
 
I'm thinking that if that is the problem then the wing sweep angles may be way off. And they may vary between the individual wings. The OP should check this out.

The bent flap rods are a sure sign that something is wrong with this aircraft. Putting in beefier bent rods does not address the real problem....it just ignores it.

Totally agree. Easy enough to check that rear spar bolt location just by removing both wing root fairings. The dreaded “snowman” drill pattern or lack of proper edge distance on that rear spar.
 
This one is puzzling, and a bit troubling Moe - I have never heard of anyone bending those pushrods to make them work on an -8, and I would be fairly nervous about doing so, since they are tubes in compression. Granted, the hexagonal tubs are a lot stronger than the aluminum cylinder tubes used originally, but there can be a lot of compressive force on them from deployed flaps, and having one decide to buckle could make for an interesting ride if you were down low.

Paul

An interesting wrinkle to this point is that, believe it or not, the critical compressive load for buckling of a curved (bent) tube is the SAME as a straight tube! Strange but true.

However, with the bend, there is more chance that the material will reach the yield point from the secondary bending moment and bend, prior to the critical load for buckling.

In any case, I really recommend to everyone that these tubes be replaced with 4130 steel tubes.

I can see how an appropriately bent tube might facilitate a smaller hole in the fuselage to provide full flap deflection without the tube contacting the fuselage skin. But since the hole is completely covered by the flap when retracted, it doesn't really matter if a bigger oblong hole is required for clearance.

I also recall that before I replaced the kit-supplied rod-ends with proper captive rod-ends, there was a very close interference problem with something in the fuselage inside the skin - I don't quite remember what. This became moot when I replaced the supplied part with an AN3 bolt, an AN970 washer to keep the rod-end captive (as is normal practice), a proper rod-end bearing, and a spacer. The spacer did the trick to shift the angle of the actuating rod enough to avoid the contact I had been worried about.
 
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Steve, how big of a spacer did you add? This is the only realistic solution to this that I can see. My MK1 eyeball says I'd need to go inboard 3/8" to get the rod straight.

I also recall that before I replaced the kit-supplied rod-ends with proper captive rod-ends, there was a very close interference problem with something in the fuselage inside the skin - I don't quite remember what. This became moot when I replaced the supplied part with an AN3 bolt, an AN970 washer to keep the rod-end captive (as is normal practice), a proper rod-end bearing, and a spacer. The spacer did the trick to shift the angle of the actuating rod enough to avoid the contact I had been worried about.
 
Steve, how big of a spacer did you add? This is the only realistic solution to this that I can see. My MK1 eyeball says I'd need to go inboard 3/8" to get the rod straight.

We have made recommendations to use spacers over the years when someone’s build has a slight interference that could be solved that way. But by spacers I mean up to a couple of thick washers.
3/8” would cause a huge jump in bending moment of the push rod attach point, and not a good idea in my opinion.
 
Sorry to be off on a tangent . . . .but

An interesting wrinkle to this point is that, believe it or not, the critical compressive load for buckling of a curved (bent) tube is the SAME as a straight tube! Strange but true.

Having leaned to the solid mechanics side for my MSME, I am going to have to check this out, Steve. The loading case just does not seem support this conclusion, but won't say so without a source and first principles to back it up. A reference to the supporting equations and assumptions would be appreciated.

On the other hand buckling is actually a perturbed solution so that could yield the same force for less than a one diameter displacement. I'll have to go look in my Timoshenko. The loading case, as a practical matter, must also include moments at the ends (as a slender column) due to the friction of the loaded spherical joints. This all assumes we are talking theoretical actual loading case and not a handbook guideline for design with integrated safety factors including acceleration, vibration etc. Design practice can include a host of considerations.
 
Steve, how big of a spacer did you add? This is the only realistic solution to this that I can see. My MK1 eyeball says I'd need to go inboard 3/8" to get the rod straight.

My spacers ended up something like 3/16". Going from memory here. I trial fit with washers, then when I had the thickness right, I turned a piece of steel tube to the right length to slip over the bolt.
 
Having leaned to the solid mechanics side for my MSME, I am going to have to check this out, Steve. The loading case just does not seem support this conclusion, but won't say so without a source and first principles to back it up. A reference to the supporting equations and assumptions would be appreciated.

On the other hand buckling is actually a perturbed solution so that could yield the same force for less than a one diameter displacement. I'll have to go look in my Timoshenko. The loading case, as a practical matter, must also include moments at the ends (as a slender column) due to the friction of the loaded spherical joints. This all assumes we are talking theoretical actual loading case and not a handbook guideline for design with integrated safety factors including acceleration, vibration etc. Design practice can include a host of considerations.

The behavior of curved beams is much more complicated than what is predicted from simple linear stability analysis. So, while the deflection still goes to infinity at the same value of P_cr, the deflections become 'large' at loads much less than P_cr, with a deflection vs. load curve that is asymptotic to the value of P_cr for the straight column. A non-linear analysis is presented in "Theory and Analysis of Flight Structures" by Robert Rivello.
There is also a nice approximate solution presented in https://www.efunda.com/formulae/solid_mechanics/columns/curved.cfm
which gives:

w=w_o[P/P_cr/(1-P/P_cr)]

note that w still goes to infinity at P_cr, the critical load for bucking of a straight beam.

Because the deflections can become large prior to the load predicted by the linear stability theory, what typically happens is that the material reaches yield stress and the column fails prior to reaching P_cr.

Years ago I wrote a fully non-linear FEA code that would correctly model very large deflections. It was an iterative method where the local stiffness matrix of each element is re-computed based on the deformed geometry.
There are interesting cases where the stiffness is so low, and the material yield stress is high enough, that 'buckling' doesn't really have any meaning. Something like a thin Willow stick will simply bend over and find an equilibrium solution with the weight hanging below the apex of the arc of the stick. This would not really be considered a column anymore of course. To get the iterative non-linear FEA to converge on some cases took a great deal of "under-relaxation" and it was very slow and expensive for complex structures.
 
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My spacers ended up something like 3/16". Going from memory here. I trial fit with washers, then when I had the thickness right, I turned a piece of steel tube to the right length to slip over the bolt.

That would be a spacer that would very closely mimic the plans spec'ed rod end with the integral stud, when used with a plain rod end like you installed.

Adding an additional 3/8" long spacer to the stud length of the rod end that the OP already has installed, would be a very long stand-off (higher bending moment) that I personally wouldn't feel good about without some detailed testing.
 
That rod end with the integral stud is 3/16" at both ends right? I want to replace them and I have the ACS page open right now.

When I check these wings to make sure I'm not flying an X-29 what should I be looking at? I've got the wing root fairings off already.
 
I would think the best way to check the wing sweep is to drop plumb bobs from the leading edge of the wing. I think theres a section in the builders manual about this.
 
When I check these wings to make sure I'm not flying an X-29 what should I be looking at? I've got the wing root fairings off already.

A simple check would be to measure from the edge of the fuselage belly skin, outboard to the row of rivets on the next outboard wing rib.

Measurement should be pretty close to the same near the main spar and aft spar.
 
2.5" all four points. I'm disappointed, I always thought these were cool airplanes.
 

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