Sandwich Rudder?

[Thomas Lukasczyk]

Hi all

Recently, I discovered cracks in the rudder-skin of my RV-4. The cracks occur at the forward most rivet of various stiffeners.
If you push the skin slightly and watch the surface move, you will know why.
I kept stop drilling them and patched them up with small pieces of 0.020 Alclad attached with high-performance metal adhesive.
None of the cracks ever grew, but when there were three patches on the rudder, it was time to do something about it. I have done "left-rudder-aerobatics" lately and I assume that has caused the cracks. The plane has 200 hours TT now and I feel certain maneuvers do accelerate this problem.
Although it is a small, rather cosmetic flaw, it still bites you and you wonder whats up next.
Long story short, I am going to build a new rudder.

As far as I know, these are the most common countermeasures against cracks. Are there others?

-Use 0.020 skins (already had them)
-Put RTV between stiffeners at trailing edge (done that, no cracks there)
-Use Pro-seal or Sikaflex to "wet-rivet" the stiffeners on
-Use small RV-10 type angles to attach the stiffeners to the spar (Rivets, Sikaflex or Pro-seal)

I am also considering a way to omit the stiffeners completely.
What about a blue-foam core that fills the inside of the rudder? I would glue it in using a special (aluminum to PU-foam) adhesive for aluminum-sandwich components for freeze trucks.
This stuff works pretty well and it will not come of ever again. It does not age and temperature is a non-issue. It is even suitable for structures of aluminum vehicles, like busses, railway-waggons and RVs (the bulky ones without wings).
Everything else is going to be per the plans.
The main concern so far is weight. It is probalbly going to be a little heavier than the stock-rudder (not necessarily). I do not worry about CG but I fear the flutter characteristics might change.
The RV-4 does not have a counterbalaced rudder so my plan is to not exceed the residual momentum of the old rudder. (I hope to have translated "residual momentum" correctly)
Has anybody done something similar? Is there anything you can think of why I should not be doing it this way? (other than "never change a running system").



Thomas, RV-4 http://www.rv-4.de

 

[Pilottonny]

PUR-foam

Hello Thomas,

What about putting the rudder between two pieces of heavy plywood (maybe reinforced with stiffeners) and injecting ordinary building PUR-foam. After curing you will have a completely foam-filled rudder. Because the foam will stick to the inside of the skins very well, it will be very stiff, no more cracking of the skins, that's for sure! You can measure the rudder and easily calculate the additional weight (minus the stiffeners you will leave out?).

Anyway, this is not advice, be carefull of what this will do to the complete system and handeling of your aircraft!

Have fun,

Regards, Tonny.

 

[Mel]

Most foams will absorb and retain water. Not a friend to aluminum. Not to mention the added weight of the moisture.

 

[N941WR]

Most foams will absorb and retain water. Not a friend to aluminum. Not to mention the added weight of the moisture.

What he said...

You didn't say what engine and prop you were running. I have heard that 180 HP w/ CS props can cause cracking on the thin skins used on the early RV's.

If you elect to rebuild your rudder, use the thicker skins and proseal the stiffners in place prior to riveting.

 

[Tom Martin]

I have been thinking about this issue. For those cracks to form the metal must be moving/flexing in and out. This might take a bit of work but consider the following.
position the stiffeners, side to side so that the flanges are both up, or both down. Now you could install a "strap" between the forward ends of the stiffeners. This would stop the skin from flexing at this point. Getting the straps the right length could be done by using some small pieces of wood the right width as the spar at each stiffener location to mimic the spar. When straps are riveted in place the wood blanks are removed and the spar installed and riveted.

 

[JonJay]

You are on the right track with the proseal...

...however, only as insurance. As covered in other threads, I am convinced that the largest factor in cracks developing is under bent trailing edge and improperly rolled leading edge. I have seen cracks on elevators and rudders as they are built the same way. However, there are many airplanes out there with many hundreds of hours with no cracks, big engines, and no special regard to construction other than following the plans.
I would resist using brackets to tie the stiffners to the spar. The spar was not designed for this and you might introduce unforseen problems.
The .020 skins seem to be more forgiving, but they are not necessary in my opinion. Now, if I was going to build over, they would be .020, just in case.

 

[Mel]

Esactly!

...however, only as insurance. As covered in other threads, I am convinced that the largest factor in cracks developing is under bent trailing edge and improperly rolled leading edge. I have seen cracks on elevators and rudders as they are built the same way. However, there are many airplanes out there with many hundreds of hours with no cracks, big engines, and no special regard to construction other than following the plans.
I would resist using brackets to tie the stiffners to the spar. The spar was not designed for this and you might introduce unforseen problems.
The .020 skins seem to be more forgiving, but they are not necessary in my opinion. Now, if I was going to build over, they would be .020, just in case.

My -6 has been flying since early 1993 with .016 skins and NO additional pro-seal, mini ribs, or anything else. No cracks to date. Just built per the plans with properly formed leading and trailing edges.

 

[JonJay]

Be careful

I have been thinking about this issue. For those cracks to form the metal must be moving/flexing in and out. This might take a bit of work but consider the following.
position the stiffeners, side to side so that the flanges are both up, or both down. Now you could install a "strap" between the forward ends of the stiffeners. This would stop the skin from flexing at this point. Getting the straps the right length could be done by using some small pieces of wood the right width as the spar at each stiffener location.

You might introduce stress into other parts, like the spar. Again, built correctly, cracking is minimized if not avoided entirely. Also, aligned in the way suggested you would not be able to bend the trailing edge properly as the stiffners can not clear each other unless they are offset.

 

[Tom Martin]

You might introduce stress into other parts, like the spar. Again, built correctly, cracking is minimized if not avoided entirely. Also, aligned in the way suggested you would not be able to bend the trailing edge properly as the stiffners can not clear each other unless they are offset

Yes the stiffeners can be installed that way, you just have to move one of them half a hole to make it work. I was not suggesting that the stiffeners be attached to the spar but to each other. And yes you will still be able to roll the leading edges properly. I agree that my suggestion is not meant to cover improperly bent leading and trailing edges. Some planes can go for years with no problems and yet others, for some reason, do have issues. I am not convinced that these issues are entirely due to surfaces that are somehow not built correctly. There are other factors such as engine vibration, prop pulses, aerobatics, rough strips etc etc that could also come into play.
At one time, years ago, and perhaps still, Van recommended a dab of proseal at the trailing edge to hold the stiffeners together. I am suggesting a method of holding the forward ends of the stiffeners together. Correct me if I am wrong but do we not want to have control surfaces that are stiff and do not flex?

 

[rjtjrt]

"improperly rolled leading edge"
What are the do's and don'ts of properly forming the rolled leading edge?
John

 

[JonJay]

No induced stress.

"improperly rolled leading edge"
What are the do's and don'ts of properly forming the rolled leading edge?
John

The skins should lay flat on the spar with little or no pressure and you should be able to insert cleco's in the leading edge holes with little or no pressure as well. If you have to force things together the induced stress will over time cause the cracks. That is the common theory anyway.

 

[Thomas Lukasczyk]

Thank you for your answers

I do not think that an improperly rolled leading or trailing has caused the cracks in my case. One of them starts right aft of one of the cutouts for the bearings. There is no bending or rolling involved there.
My trailing edge is crack free but I do not think that simply doing it right will cure the problem. One of the cracks is on the left and that side was crack-free until I started spining and snap-rolling (left) more often.
There is one 1200+ hours RV-4 over here that is used for competition aerobatics most of the time. Virtually every stiffener has a crack at the "first" and the "last" rivet.
Like mine, it is a 160hp FP.
I suspect that certain aeros are a mayor cause of cracks. Of course pre-loaded rivets and improperly bent skins add their part. Prop-pulses might also cause fatique over time.

@Tonny
Yep, thought about that, too. It would certainly be a fast and accurate way if the expansion of the foam was handeled properly. There are also Builder-foams that do only expand very little and stick to virtually every surface.
Unfortunately I did not find any specs about aging, strength, steam-tightness and pressure-tightness. What about the adhesive of the foam? I have no doubt that it will hold but I like to read about it in a spec-sheet.
The foam I am propably going to use ist steam-resistant and does not absorb water. It can handle the pressure and does not expand with altitude.

@Tom
Wanted to do it almost like you wrote. I doubt there will be a problem with the spar. Even if attach-angles were used instead of straps. The spar takes all the forces created by the rudder this way or the other.
I just fear it would be hard to assemble the rudder without pre-loading the stiffeners that way.


I cut out a test-core today. There are no lightening holes in there, yet. It is about the same weight as the stiffeners would be.

Thomas http://www.rv-4.de

 

[GAHco]

I would think....

My -6 has been flying since early 1993 with .016 skins and NO additional pro-seal, mini ribs, or anything else. No cracks to date. Just built per the plans with properly formed leading and trailing edges.

I would think that with the 3 bladed Cato prop, your harmonics are different and less likely to get a synchronous re-verb going with the even # of piston firings and the odd # of blades. Also each blade is carrying less load.

Jes my suspicions?

I am not an engineer, I can't even drive a train.

 

[Tom Martin]

Thomas
I am always interested in seeing how things can be done differently. Please send some pictures of how you do the foam insert.

 

[Mel]

I would think that with the 3 bladed Cato prop, your harmonics are different and less likely to get a synchronous re-verb going with the even # of piston firings and the odd # of blades. Also each blade is carrying less load.
Jes my suspicions?
I am not an engineer, I can't even drive a train.

I have to agree that the Catto 3-blade probably helps, but the first 11 years were flown with a 2-blade prop and some fairly aggressive acro.

 

[GAHco]

Pilotage

I have to agree that the Catto 3-blade probably helps, but the first 11 years were flown with a 2-blade prop and some fairly aggressive acro.

May be you were aggressive but smooth, and the prop just made things SMooother

 

[Dean Pichon]

I had similar cracks in my rudder, too

After about 3 years, I started getting cracks as you describe. I stop-drilled and RTV-sealed about 5 such cracks and decided it was time to build a new rudder. I considered a foam core, but decided the core geometry would be too difficult to cut accurately. The rudder is a tapered wedge and would require a "compound" cut. I considered foam ribs but decided I was straying too far from Van's design.

In the end, I used .040 skin, and both prosealed and riveted both stiffeners and spar and tried to "more carefully" bend the leading edge. I tried to get the bend so that little or no pressure was required to hold it in position. I'm not sure that this was the culprit, but thought is was worth the effort.

I noticed the other day that Cirrus rudders (aluminum) have an incompletely rolled leading edge. Itermittent strips across the sides hold the leading edge in position. It looks much easier to fabricate and apparently works. It would be necessary to leave only enough roll to prevent the edge from being exposed at full rudder deflection.

Good luck,

Dean Pichon

 

[Thomas Lukasczyk]

Hello

Since the test core turned out well, I am giving the foam insert a shot. The parts from Vans should be here any day now. I will post some pictures as soon as the parts "assemble themselves on the way down after they have been thrown into the air".
Can not wait to get my hands dirty on aluminum again.

Thomas http://www.rv-4.de