Cutting angle legs at 45 degrees to save weight

[plehrke]

Since they are designed to be unattached to anything, Trimming the stiffener at angle does not effect the “as designed” strength since the load in an unattached vertical leg of a stiffener goes to zero, it has to since no where to go. The load tapers down so no reason the geometry can’t taper down as well.

Big effect can be how you end the stiffener down where it is riveted to the skin. That can be a large stress riser if trimmed too tight to the attached flange or has a sharp internal corner in the flange’s radius, since load in that stiffener is driven down to the end rivets on the attached flange. Recent history on some very big aircraft has shown these to be trouble areas. At work our standard practice is to clip them all off to structure to avoid the stress being driven down into the skins.

I trimmed mine at 45 degrees and stayed 1/8 to 1/4 inch above the flange fillet. Then I put a generous radius on the corners but staying clear of the bend radius.

This is good attention to details but most likely will effect the weight less than if you drank a cup a coffee or not before you went flying.

[Marc Bourget]

Originally Posted by AndyWW

That's an interesting but cryptic comment! I would love it if you or someone else could elaborate...

I picked up an engineering textbook from Northrup Factory published in 1942. Detail Design and Engineering. Stiffener taper was explained in the best practices section.

The book is currently stored so I can't offer images.

[rockwoodrv9]

I really enjoy reading about the ways people come up with to shave small amounts of weight off their planes. I am sure if I built a second time, I would look for more ways I could have saves a pound here or there. First time, I just want it one!

My 9A, painted, full interior, with pants on came out at exactly 1100 pounds. It is higher than specs, but about what most builders end up with.

I have the panel and interior I want and a few comfort items I don't think I would be wiling to give up to save the weight.

My question is, how much better performance would I have if I stripped my plane down and saved 50 pounds to get to specs? I don't have enough RV experience to know if a pound or two would change the way the plane flys. I know when I was doing triathlons I cared about ounces on my bike when in reality it didn't make any difference since I was not an elite athlete. I can see it being fun and a challenge, but I wonder if a person would know if a 10 lb weight was hidden under a seat.

Thats the fun of experimental. You get to do it how YOU want to!

[PilotjohnS]

Why not add 50 lbs and see if there is any change? Then double the effects for a 50 lb weight lost.

[PilotjohnS]

By the way I think the ends need to be parabolic, not at a 45 degree. This reduces the weight savings but should yield no change in strength ( pun intended)

[rv8ch]

Just toss some designs in your favorite FEA tool and let us know what happens!

From the little I've read on this, reducing the strength of the stiffener at the end is actually a good thing to reduce the stress on the skin, so it's not just weight savings.

I'd have to guess that correctly sized and placed lightening holes in the stiffeners could also add up to a few grams of savings.

Considering the amount of weight in skins on our aluminum aircraft, I'd also guess that a honeycomb structure molded into the skins could save weight - so for example a typical 0.025" skin could be replaced with honeycomb pattern of 0.025" and the space between the honeycomb could be say half that. With the pre-punched skins we know where the edges are, they could be the full 0.025".

Probably not scalable to manufacture skin like this, but imagine a CNC machine shaving little hexagons about 0.010" deep after the holes have been punched. Or, just skip the fries.

[Marc Bourget]

Milling the skins would be lots of work.

John Thorp once described a method of reducing skin thickness for the Wind Derringer. The skins were "chemically milled" by dipping the aluminum in an acid tank. You could generate a taper in the thickness by suspending the skin from a timed motor that would slowly pull the skin out of the tank.

FWIW

[AndyWW]

Thanks for all the replies. I think I feel pretty confident to cut the normal stiffeners at 45 degrees. But what about the aft end of the lower longeron (RV7)? I'll email Van's but would be interested to hear if anyone has cut the horizontal leg of that at 45 degrees? Intuitively it seems like that little triangle can't possibly be adding any strength and is just along for the ride, but it is a meaty part of the aircraft carrying a lot of load, so getting out the band saw on that part makes me a little nervous!

(I know many of you think this is a pointless discussion about saving a few ounces, but building is about learning too. To say nothing of the fact that I don't like the idea paying to cart around any useless bits of metal for hopefully thousands of hours once I get this thing flying!)

[Marc Bourget]

It's not all about saving weight. Don't forget the stiffness factor.

Part of the design component and probably the major purpose is to avoid a sudden, significant, change in stiffness - generating a "stress concentration"

FWIW