Rudder oil canning.

azav8or · #1 10-25-2005, 07:56 PM

I know this has been discussed briefly in the posts, but I am concerned on my rudder it might be worse than others describe. It is fairly pronounced in the center 3 section of each side. I have drilled out the rivets and now have access to the inside. I have thought about prosealing some vertical stiffeners between the horizontal ones. Does anyone have any idea as to whether this might work or ideas of your own?

Thanks, Joe.

dan · #2 10-25-2005, 08:12 PM

I've seen people use Great Stuff spray/expanding foam (from the aviation aisle at Home Depot). Just be sure not to over-fill the rudder with it. And you might think about clamping the rudder in some plywood or other fixture to keep the panels from bulging under foam expansion. Just an idea. Haven't tried it myself, just know a few people who did.

)_( Dan
RV-7 N714D
http://www.rvproject.com

jcoloccia · #3 10-25-2005, 08:39 PM

Just remember that you never know what a designer had in mind when they did something. It's possible to add structure and actually lower the load bearing capacity of a part by concentrating loads places they weren't designed to go. Ask me how I know this

my $.02

Dave Cole · #4 10-26-2005, 06:36 AM

I would be very careful about adding any weight to a control surface. Not only can the change in mass balance affect flutter, but, in the case of the rudder, it will affect the airctaft CG as well. You may not like the results.

Dave Cole
RV-7 wings

FlyerJumper · #5 10-26-2005, 08:05 AM

Quote:

Originally Posted by jcoloccia

It's possible to add structure and actually lower the load bearing capacity of a part by concentrating loads places they weren't designed to go. Ask me how I know this

Ok, I'll bite... how?

jcoloccia · #6 10-26-2005, 09:03 AM

Okay. Let's take a really straightforward example. Lets say that you have a sheet metal main gear. You look at it and decide that you don't like how springy it is. You decide to make it "stronger" by sticking a rod between the left and right gears.

That WILL stiffen the gear, however, you've likely reduced the load bearing ability of the gear by concentrating the stress that WAS spread out over, say, a foot (the gear flexed), into a space of a couple of inches (the rod). On the first hard landing, the gear would most likely fail by ripping out the rod.

In the case of the rudder, vertical stiffeners between the regular stiffeners could concentrate stress at the ends of the vertical stiffeners. i.e. under load you might cause creases in the skins. I don't know. I didn't design the rudder and have no idea what loads it sees in flight. It seems like it'd be OK, but a phone call to vans would yield a definitive answer.

The other problem, of course, is anytime you change the weight and stiffeness of a control surface, you become a test pilot and need to worry about flutter.

-john

FlyerJumper · #7 10-26-2005, 09:31 AM

Quote:

Originally Posted by jcoloccia

Okay. Let's take a really straightforward example. Lets say that you have a sheet metal main gear. You look at it and decide that you don't like how springy it is. You decide to make it "stronger" by sticking a rod between the left and right gears.

And here I thouht you had a personal experience that you were begging to share

Good info none the less.

jcoloccia · #8 10-26-2005, 10:06 AM

LOL....you were looking for some dirt.

I was heavily into radio controlled modelling when I was in high school and college. Our school was competing in a maximum lifting contest w/radio controlled planes. i.e. they specify the engine and propellor, and you design a plane around that to lift the maximum amount of weight. The wings attached much the same way as they're attached on the Citabria....there were some posts that basically just slipped into the sides of the fuselage. When you pulled up on the wings, there was some definite flex in the fuselage but that was OK. We built it light and I EXCPECTED the flexing. It was spread out over a large area.

The other guys where uneasy with it and wanted to stiffen it up. They strung kevlar line from the bottom of the wings to the bottom of the fuselage to act as super light struts. I advised against it because it concentrated the forces at one point at the bottom of the fuselage instead of the allowing the entire fuselage to absorb the forces as I'd designed. Towards the end of the contest, the wings folded in flight. Failure mode: ripped the fuselage apart at the kevlar "reinforcement". By stiffening it up, we essentialy turned a benign compressive and shear force at the top of the fuselage (depends on the material properties for strength) into a very dangerous tension force (that depended on the glue joints for strength). If I had known they where going to do that, I would have designed the fuselage completely differently.

Lesson learned