S

sjhurlbut

Well Known Member
During a recent trip we encountered very gusty winds on the ground (30G50 kts). With the airplane tied down (no hanger space) we had some damage happen to the rudder. I use a common home made rudder lock that others employ as well that bolts the rudder horn to the rudder stop using 0.125 alum and AN3 bolts. I wanted to share this with others because I believe in high winds, this type of rudder lock is not sufficient. The rudder is held at the hinge but is allowed to "twist" at the trailing edge causing compression on one side of the rudder and tension on the other. The effect was the split rivets at the bottom of the rudder. It also just so happened that with the wind shift that day, the airplane ended up in the worst 90 degrees to wind position.

So if you encouter any strong winds, consider a full rudder lock that entends from VS leading edge to rudder trailing edge and move the airplane to face into wind.

Steve
RV7A
rudderdamagebx9.jpg
 
Wow! That's amazing Steve. Sorry for your damage. I'm still trying to figure out how a "full rudder lock" you mention would prevent the damage though.

Larry
 
Wow is right! I am really having trouble imagining just how this happened, with only that damage shown. Just to be clear, you locked the rudder with a fore/aft piece of aluminum, from the stop to the bar that the rudder control cables connect? Were additional rivets above what is shown in the picture sheared? Is the damage confined to what we see in the picture?
 
I use a 1/4" U shaped pin in the rudder horn and stop, which is basically the same. I don't see how this would cause damage as shown, either. That part of the rudder can not hit the elevators, at least not on a 7.

I think that damage was done by something other than wind, unless the wind blew something into the rudder.

Roberta
 
Sorry to be the voice of disagreement, but to me, it sure looks like the rudder failed exactly where/how I would have expected it to.

The rudder lock holds the bottom of the rudder in a fixed position, and the balance of the rudder is trying to move with the wind. This will induce a torsional force into the rudder, as the majority of the surface tries to pivot on the hinge line, and weathervane.

The locked bottom is resisting this, and the farther away from the hinge line, the greater moment arm. Thus, the failure at the trailing edge, at the bottom where there is not only a stiffener, but the bottom rib also.

But, it could also have been flying debris, from the wind, not just the wind. However, the forces involved from either debris, or just wind, still reach max force and max resistance right where the failure occurred.
 
I agree with Mike which. Van locks the rudder using the same system by the counterweight, put some remove before flags on it and you're good to go... the only reason I know of their rudder lock is that it was not removed during preflight on my demo flight, luckily the control check caught it.
 
I guess I was mistaken. The damage, at first, looked like impact. But the twisting-tearing could be from the sideload force of the wind. Must have been quite a wind storm, though. These same forces are encountered during low speed flight and rudder operation.

I arrange for a hangar wherever I go, or I go somewhere else. I don't need to see that kind of damage anywhere. Ruins the day.

Roberta
 
robertahegy said:
...Must have been quite a wind storm, though. These same forces are encountered during low speed flight and rudder operation....
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I agree, Roberta, it is hard to understand. I know I have applied full rudder at 80 knots (snap roll) and that would seem to be the same kind of force. I once had a rider apply full rudder at a far higher speed, which I think may have been the cause of a skin crack, but nothing like the damage in this photo.

One difference might be that the wind came directly from the side, but that would seem to be a lower force than flight loads, as the surface would be stalled.

Can anyone help me understand how a ground wind would be different and stronger than the flight loads?
 
It is relevant to me that the rivets have only failed on the "shop" head side (all the factory heads appear intact and are probably still holding the extruded trailing wedge). This is the rudder with the thin .016 skins and double flush rivets.

Is it possible that the rivets have not sheared under load but rather they have simply pulled through the thin skin on the "shop" head side because they were insufficiently formed in the first instance.

In reality I have seen a number of RV7(A) rudders with inadequately formed double-flush shop heads at the trailing edge.

So that is the question......did the rivets actually shear under load....or did they simply pull through under load on the shop head side due to insufficient formation.
 
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hydroguy2 said:
Steve, did you get your plane home ok? or do you need pieces/parts?
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To answer a few questions. I don't think anything actually hit the rudder. We were called to the airport to inspect "a rip in your plane". You could imagine the nerves in my stomach after that phone call. The wind was crazy and the airplane looked like a bucking bronco even with all 3 tie downs tight. There were no hangers available. We untied the airplane under great stress and somehow managed to back it into a 16' wide garage door tail first. Tied it down again with cement blocks and tried to block the wind using a massive snowplow truck.

The rivets actually pulled through the skin and did not shear. I believe they were fully set and the parts were bonded with fuel sealant. The only other damage occured forward of the rivets along the rudder skin itself. There was apparant buckling of the skin which can be seen a bit in the picture. I believe the difference between flight loads and wind gusts loads was due to the gusty nature of the winds. We were in the nearby town during the storm where the wind wasn't actually that bad. By the time we got to the airport the airplane has been subjected to the gusty winds for over 12 hours. The winds were so strong that the local commuter aircraft cancelled flights that day.

Our bad day continued as we backed the airplane into the garage and removed the rudder lock. The rudder lock was not as tight as it usually is and I noticed a slight elongation on the bolts holes were the lock attached. This may have contributed to the loads on the rudder as it was actually able to swing 10-20 degree back and forth. So with the rudder now free swinging I went back inside the building to borrow a drill. The wind caught the rudder (still inside the garage) and slamed it against the elevator. The rudder stop provides about 1 inch of clearance between the rudder and elevator and the swing in the rudder was violent enough to actual cause the rudder to bend and hit the elevator. When I got back to the garage there was about a 1 inch by 1/2 inch triangle shaped hole in each side of the rudder. By now I was getting really frustrated because you can imagine that with the gusty winds the airplane was still bucking around. I had images of the whole thing just flipping over.

So we put the rudder lock back on, drove into town to get some alum pieces. Came back, cut out the damage and riveted a patch on top into the bottom rib and a stiffener (both sides). The aft edge was drilled out to #30 and pop riveted back together. Total repair time was about 5 hours. We went for lunch and tried to calm down. Headed back to the airport to check the weather for return flight home (about 1 hr 20 min flight time). It was IFR for take-off but destination was almost clear. Winds were still gusting 45 so sat around a bit longer. Eventually loaded up the airplane. Winds were straight down the runway gusting to 35 and made the decision to go. Time was about 6 pm. Pulled onto the runway with IFR clearance and added power. Airplane lifted off before throttle was forward.

On the way home we got a nice push but the airplane was out of trim. Ball was about 3/4 to the right.

Got home, had a few drinks, ordered a new rudder. She is back to flying straight as an arrow and no residual damage from the storm.

I have a few more pics that I can share if interested. Besides this last day we had a wonderful vacation.

Steve
RV7A
 
rudder lock idea

We use a simple lock here on some of our planes that might help. It is 2 pieces of PVC tube (3/4" I think) that is tied together at one end and covered with foam pipe insulation. After being slipped over the whole rudder/stab assy the open end it secured with a rubber band (bungee cord). This holds everything secure, is light, and would probably absorb some of the stress that the wind applied to the rudder.
 
sjhurlbut said:
The rivets actually pulled through the skin and did not shear. Steve
RV7A
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I think that what you experienced was a rivet failure under load followed by subsequent adjacent rivet failures in the classic structural "unbuttoning" effect. And the fact that the rivets all pulled through on the shop head side indicates quite clearly that the heads on that side were not as structurally sound as on the factory head side. This points to insufficient forming of the rivet shop heads.

I can see from your photo that the shop heads have not been formed sufficiently....the holes where the shop heads have pulled through are considerably smaller than the dimple size.

I see great variations in how some builders form those double-flush shop head rivets. Some of them look virtually like factory heads...others are just little knobby things.

When you remake your rudder I recommend you get more formation of the trailing edge rivets on the shop head side.

I ground a pair of squeezing sets to the angle of the trailing edge wedge specifically to do that task. And then I squeezed them pneumatically in a two step operation.

In the first step I used an angled set on the factory head side and a standard orthogonal set on the shop head side. This brought the rivet up to a 50% formation. In the first step the standard set on the shop head side stopped the rivet tail from bending over. Then in the second step I used an angled set top and bottom for the final forming of the rivet. My shop heads look very similar to my factory heads....they almost fill up the dimple. Here are some photos of my rudder trailing edge rivets in the area in which yours failed. You can left click on the pix for larger size and higher resolution.

Firstly my factory heads:


And my shop heads:
 
I like alternating the rivets with every other rivet with the shop head on the right side. This will prevent the unbuttoning effect.
 
Norman CYYJ said:
I like alternating the rivets with every other rivet with the shop head on the right side. This will prevent the unbuttoning effect.
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I think you are suggesting progressively reversing the orientation of the rivets (correct me if I'm wrong). Some builders do this in an attempt to prevent building a bow into their trailing edge. However I do not think it will prevent progressive rivet failure in the unbuttoning mode if the shop heads are insufficiently formed. What will happen is that each rivet will just fail on the shop head side (ie the side of failure will alternate).

The real answer to the problem is to properly form the shop heads in the first place.
 
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Captain Avgas said:
The real answer to the problem is to properly form the shop heads in the first place.
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Yikes...Section 5: Construction Materials, Processes, and Useful Information...not so useful regarding double flush rivets?

Trailing edges are riveted with ?double-flush? rivets. These are standard rivets, but instead of setting the shop head on a flat surface, it is set in a dimple and ends up flush with the skin surface. However, a double flush rivet will not look the same on both sides. The factory flush head will set almost perfectly flat. The finished shop head will be flush with the skin, but it will not fill the dimple completely...it?s been described as ?an acorn sitting in a dimple.? Do not fall in the trap of trying to use a longer rivet and ?fill the hole.? The rivet will bend over instead of setting properly.
 
DrillBit said:
Yikes...Section 5: Construction Materials, Processes, and Useful Information...not so useful regarding double flush rivets?
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Anyone who has got as far as the canopy will understand that quite often Vans do not provide the clearest or best instructions. :)

But jokes aside, if you refer to Figure 5-9 that is associated with the above instructions you discover that the description of the shop head as being like "an acorn sitting in a dimple" might in fact be very poor phraseology on Vans part. Figure 5-9 shows the shop head as actually being quite well formed. It does not fully fill the dimple (nor do mine) but it is NOT an "acorn sitting in a dimple" to my way of thinking.

I would suggest that this poor use of language by Vans has probably encouraged many builders to feel good about under-forming their shop heads in this location. ;)
 
First off, alternating rivet sides will help because of the v-extrusion in the middle, which adds thickness to the skin on the side that the manufactured head is on. If you alternate that side, it effectively thickens the skin a bit. I was sorely temped when making my rudder to use universal head and not double countersunk, because it is easy to fall into that trap of underformed heads.

I think My solution right now will be to use -4 rivets in the bottom 6 to make it a little stiffer. Any thoughts?
 
RV6 rudder gust lock tested to 70 kts

Man, I have a hard time imagining that much damage done by a 50 kt gust.

True story. I was flying home to Tulsa from Houston once and the weather forecast was so good that I was listening to tunes instead of flight following. Approaching Oklmugee, it seemed was odd that the haze was so bad that I couldn't see Tulsa. When I called up approach I found out that what looked like haze was actually a solid wall of rain. Approach ordered me to land immediately because the winds at Tulsa were 70 gusting to 90! Well, I dove for Okmlugee, landing downwind, and had a heck of a time holding the runway as I taxied to the tie downs, in what must have been a 30 knot quartering tail wind. I barely got tied-down when the storm hit. The FBO was closed, and all I could do was huddle outside the door in a crook in the hangar to get out of the wind. But I could see the anemometer through the window. The straight winds averaged 50-70 knots for the next half an hour.

And then it was gone as quickly as a came. I carefully inspected for damage, found none, and then continued on to Tulsa. My rudder lock was the standard u-shaped wire though the rudder horn, just like what you described.

Maybe the riveted trailing edge or the bigger size makes the 7 rudder more susceptable to gusts, but I tend to think it must have been something else. I can tell you for certain that a '97 vintage RV6 rudder, with the older .015 skins, held up to 50-70 knots for half an hour with no problem.
 
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osxuser said:
First off, alternating rivet sides will help because of the v-extrusion in the middle, which adds thickness to the skin on the side that the manufactured head is on. If you alternate that side, it effectively thickens the skin a bit.
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I think we've now transcended into the realm of what I would call speculative and "intuitive" structural assessment.
 
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I never liked the idea of the rivets on the trailing edge of the rudder. I really feel the original rudder did it's intended job. Needless to say... the new style rudder is staying in the "bench" and the old style is going on the plane.
 
It happened to my 9A too!

I experienced exactly the same damage. The airplane was parked on the ramp for 1 hr. The gust bend my U-shaped horn lock completely. There were impacts between the horn and the stop. The lower half of the trailing edge popped open (skin torn). The torsional force even bend the wedge. The reported gust 9 miles away was 33 kts. I suspect the gust at the field was stronger hitting from the side. The big rudder of 9 needs stronger lock!!!

trailingedge.jpg


trailingdamage.jpg
 
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tc1234c said:
I experienced exactly the same damage. The airplane was parked on the ramp for 1 hr. The gust bend my U-shaped horn lock completely. There were impacts between the horn and the stop. The lower half of the trailing edge popped open (skin torn). The torsional force even bend the wedge. The reported gust 9 miles away was 33 kts. I suspect the gust at the field was stronger hitting from the side. The big rudder of 9 needs stronger lock!!!
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Ted, it looks like the rivets have pulled through the rudder skin on one side. I am presuming that the skin that the rivets have pulled through is the side that has the rivet shop heads. And therefore that the factory rivet heads on the other skin have NOT pulled through the skin. Is that correct.

If so it may well indicate once again that there has been insufficient forming of the shop heads.
 
insufficient formed rivet head

You are right. Only shop heads got pulled through the skin. Fortunately, after drilling out the rivets and bending back the wedge the trailing edge looks straight. No other damages noticed. This time I will be careful to set the rivets properly. Of course, I need to make a stronger rudder lock too. The failure of the lock lead to the failure of rivets.
 
Not alone.

tc1234c said:
You are right. Only shop heads got pulled through the skin. .
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Never mind Ted, you are not alone. A lot of the RVs I see have markedly underformed rivet shop heads in that area. It's Vans fault....in the instructions they talk about the trailing edge rivet shop heads looking like "an acorn in a dimple". That mental picture encourages builders to under-form the shop heads. However the skin is so thin on the rudders that underformed shop heads can easily pull through.
 
tc1234c said:
You are right. Only shop heads got pulled through the skin. Fortunately, after drilling out the rivets and bending back the wedge the trailing edge looks straight. No other damages noticed. This time I will be careful to set the rivets properly. Of course, I need to make a stronger rudder lock too. The failure of the lock lead to the failure of rivets.
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Hmmm. The lock failure might have prevented worse damage, depending on just which way the wind was blowing. The wind force is probably large compared to the inertia of the aluminum skins as the rudder hit the stops. Wow.
 
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