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]

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]

Originally Posted by Mel

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]

...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]

Originally Posted by JonJay

...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]

Originally Posted by 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.

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