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Empennage Control Surface bend and RTV

HighSchoolBuilders

Well Known Member
Hi All,

I had a go at bending the right elevator and rudder on my RV8. I made a bender as suggested in the manual and read many posts and sites and photos before I gave mine a go. Now that I've bended it, I am still not sure if I've done it correctly. Am I suppose to bend it to make a "sharper" trailing edge? Or am I suppose to bend it so that the two sides will reach the spar easier for riveting?

Between the rows of stiffeners, it seems like the skin it's not completely flat and it's slightly convex, or bulging outward. I am not sure if I am missing something or if my bend is not enough. The leading edge looks okay without the bulge indicated in the manual.

Also, the manual talked about RTV application at the end of the stiffeners and I visited a few post regarding this issue. I sent Vans the following and perhaps any builders can give me some advice?

"The manual called for a dab of RTV at the stiffeners to avoid rubbing and cracking. Some builders said it's for the earlier model, and I had a look at my rudder and elevators, indeed when I close them up, they're still quite far apart and don't seem the two side will ever rub against each other. Will I be able to get away without the RTV?

Also, the brand RTV is not available in Hong Kong. The closest thing I have in hand is silicon but it's acid base. I am concern if the acid is going to damage the aluminum in the long run (I tried it on a piece of scrap, primed with epoxy primer, so far okay)."

Many Thanks!
Hank
 
Hi All,

I had a go at bending the right elevator and rudder on my RV8. I made a bender as suggested in the manual and read many posts and sites and photos before I gave mine a go. Now that I've bended it, I am still not sure if I've done it correctly. Am I suppose to bend it to make a "sharper" trailing edge? Or am I suppose to bend it so that the two sides will reach the spar easier for riveting?

Between the rows of stiffeners, it seems like the skin it's not completely flat and it's slightly convex, or bulging outward. I am not sure if I am missing something or if my bend is not enough. The leading edge looks okay without the bulge indicated in the manual.

Also, the manual talked about RTV application at the end of the stiffeners and I visited a few post regarding this issue. I sent Vans the following and perhaps any builders can give me some advice?

"The manual called for a dab of RTV at the stiffeners to avoid rubbing and cracking. Some builders said it's for the earlier model, and I had a look at my rudder and elevators, indeed when I close them up, they're still quite far apart and don't seem the two side will ever rub against each other. Will I be able to get away without the RTV?

Also, the brand RTV is not available in Hong Kong. The closest thing I have in hand is silicon but it's acid base. I am concern if the acid is going to damage the aluminum in the long run (I tried it on a piece of scrap, primed with epoxy primer, so far okay)."

Many Thanks!
Hank

They give a radius for the T/E's (1/8" I think). The skin should just touch the spar without having to flex the skin. The rtv is mostly to help prevent the aft most rivets holding the stiffs. from cracking the skin. The stuff here is Permatex, it should have a vinegary smell.
 
Alternative

Hank,
A better alternative to RTV would be Pro Seal (fuel tank sealant) if you have some handy.
 
The RTV with the vinegary (acetic acid) smell should be avoided if at all possible. There is a type that is non-acidic which is best.

Glenn Wilkinson
 
I believe it is the "sensor safe" RTV you are refering to - most autoparts stores have it (in the US at least).


The RTV with the vinegary (acetic acid) smell should be avoided if at all possible. There is a type that is non-acidic which is best.

Glenn Wilkinson
 
Thanks Thanks Thanks!

Thanks guys! Unfortunately, I bought a whole lot of acid base silicon. I would not hesitate to stop using them if it's not an option on my aircraft, but if there are places I can get away with them, I prefer not to waste them.
 
I'm at this point as well; I have heard a lot of very bad things about using silicone any where near these planes befor paint. :confused:
 
Thanks Bob!

I did it with the flat side of the wood instead of what you have in your log. I did it with the conventional way with the flat side of the wood, but sure will look into your method when the time comes for the left elev.

For my bend, I bought the 2 sides comfortable enough to rivet it onto the spar. The two sides were about 1' apart after bending and when relaxed. The trailing edge doesn't bulge but the skin between rows of stiffeners is bulging outward slightly. I tried to take a couple of photos, the first photos shows you the almost completed right elevator. The second photo, you may notice the skin between the stiffeners is bulging outward a bit, and you can tell by the reflection of the skin. (Still uploading, will post shortly)

As with the silicon, I did it for my rudder because the stiffeners seems like it's close enough for rubbing. The only place where I can imagine the rubbing occurs is near the center of the stiffeners, not at the trailing edge. But reading thru many others log and photos where they all had their RTV dab at the trailing edge, I start to have doubts whether if I added weight to the aircraft without actually gaining any benefit if I dabbed it at the wrong place with the wrong silicon.

I am the only homebuilder at my location, so sometimes I am having hard time convincing myself that I am doing it right when there is no one to compare to.

Calvin, what kind of bad thing did you hear with the silicon (I assume you're talking about the acid base type silicon)?
 
Photos

picture-226.jpg


picture-227.jpg
 
Most people are talking about the exterior paint job when they refer to "bad things about silicone and paint". Apparently, it is nearly impossible to remove the residue 100% and even a little silicone residue will prevent good paint adhesion.

FYI. Proseal is paintable.
 
It is my opinion that the pictures clearly show a surface that is not bent enough. The skin should lay flat against the rib and the trailing edge radius should be smaller. Also, the skin should be flat back to where the actual bend begins, not bulged as shown. Underbending leaves a strain that can be a contributor to cracking at the last stiffener rivet.
 
Can I bend it again? Is there a way to fix this?

I agree with Larry - it is underbent, and yes, you can bend it again - just put it back in your squeezing jig and give it a little more force. Do it a little at a time, and go easy until you get it right. The good thing is that you can always bend it a little more from where you have it - if it was overbent, it would be a bit tougher to undo.

Paul
 
Straight edge

Mine came out exactly the same so dont feel bad. I used a straight edge to lay across the trailing edge to easily see where it was bulging and then I took my hand held seamers (with duct tape on them to prevent damaging the skins) and tweaked it very gently and carefully to get straighten it out. Worked well for me to bring it into shape as my homemade 2x6 brake didnt quite get it there.
 
result

Hi All,

Just to leave a msg here regarding the result of my under bend trailing edge. I went for the handseamer because my wood bender is not giving me the result. With the handseamer, it worked....but it left dents at where the handseamer's edge was. The dent are very minor, not sure what it will look like when the aircraft is complete and painted.

Hope this will help the next guy running into the same problem.

Hank

2.jpg


3.jpg


4.jpg
 
"noncorrosive" RTV

I believe it is the "sensor safe" RTV you are refering to - most autoparts stores have it
No, "sensor safe" does not necessarily mean nonacidic. For example here's the data sheet on Permatex Sensor Safe Blue: http://www.devcon.com/prodfiles/pdfs/fam_tds_375.pdf It states it is an acetoxy cure silicone, and says "the curing process can cause corrosion on some surfaces".
Dow Corning 732 silicone is the one you need
No, that's not it either. Here's the data sheet: http://www.glueonline.co.uk/shop/dow_corning_732_data_aviation.pdf It is also an acetoxy cure product and "may cause corrosion on some metallic parts or substrates".

If you want a "noncorrosive" RTV, one that's commonly available is Permatex Ultra, which uses an air moisture cure. Data sheet: http://www.devcon.com/prodfiles/pdfs/fam_tds_371.pdf

I don't know if the acetic acid in the others is really a problem or not when used on RV aluminum. For all I know, it might just help etch the surface and improve adhesion. But I thought I would try to clear up some confusion about which products have it, if that's a concern in your application.

--Paul
 
Trailing edge bulges and underbends

Hi All,

Just to leave a msg here regarding the result of my under bend trailing edge. I went for the handseamer because my wood bender is not giving me the result. With the handseamer, it worked....but it left dents at where the handseamer's edge was. The dent are very minor, not sure what it will look like when the aircraft is complete and painted.

Hope this will help the next guy running into the same problem.

Hank

2.jpg


3.jpg


4.jpg


Hello!

I know it's been quite a while since this last post was written, or even updated, but three years later I'm running into this same issue here.
Now I believe this is a common thing to happen at any point when working with thin aluminum sheet.. Pretty much like aluminum foil: it'll get easily bent, bulged, or scratched.

So what was the moral of the story? How did the final product came to be? Whats was e result, if any, after priming, painting, or even flight testing?
Do these bulges, dimples, underbends cause any unpleasant/undesired control feel?
 
After bending mine, and they are harder to bend than you would think, especially the right elevator, my skins laid pretty flat down on the spars/ribs. If I remember correctly, the radius of the bend was around 3/32", i.e. 3/16" dia. Also, I used gobs of proseal at the ends of the stiffeners.
 
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