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Cutting angle legs at 45 degrees to save weight

A

AndyWW

Well Known Member
I searched old threads but couldn't find anything on this. My untrained engineering instinct tells me, and I have a vague recollection of reading about this somewhere, that if you cut off the end of a stiffener angle at 45 degrees (on the leg that isn't riveted or bearing against another part obviously) there is no reduction in the strength of the structure. In some places in my RV-7 plans, for example the control surface stiffeners and the brake pedals, cutting this way is suggested. But in other places, for example the forward fuselage stiffeners, the lower longerons and the armrests, it isn't. Three questions emerge from these observations:

1) Is my untrained instinct correct?
2) Why do the plans not call this out consistently?
3) Is 45 degrees the magic number, or are higher angles such as 60 degrees also acceptable?

I'm very grateful for any help, and think this might be an interesting discussion. I'm trying to build as light as possible so this might be a way to save a few more ounces!
 
R u a racer

While there is probabaly some benefit to this, I think the easier thing to do would be to lay off the breakfast burrito instead. My point is that this may be true and save some weight, but I think the analysis to show it is not detrimental will be heavier than the savings; something only racers will do. Just remember, “why do bridges fall down? Because We professors give you partial credit.”
 
Not going to comment on the specifics of the 45 degree cutoff, but I do complement the dedication to such detail. Weight is the enemy in airplanes, and every chunk of unneeded aluminum that finds your scrap bin will payoff forever.
 
Recall that the rudder stiffeners are cut at an angle far exceeding 60*. I wouldn't think it detrimental to cut at a 45 or 60* angle.
When I built, I rounded my corners aggressively.
 
It was the mantra during my standard build, along with other metal that mysteriously went missing.
 
Not a real structures engineer, but if you look at the load distribution across a beam (the angle), the loads decrease linearly towards the ends. Therefore the strength of the beam can decrease near the ends. Depending on the fastener distribution along the flat, the angle can vary. Assuming that the flat is riveted, the end rivets represent the end of the beam, anything past the end rivets, save the necessary edge distance is a waste. If the flat is glued down, then loads distribution goes to the end of the adhesive.

If you google ?load distribution in a beam?, there are some good diagrams.
 
Begs the question: could you trim to the center of the full length?
(taper to zero at both ends and only full width at center of stiffener)
Probably could if there were only three rivets -- one at center and at ends. So the answer to the question on how much of an angle (30, 45, 60...) to trim to, the answer is probably: how close is the next rivet. So you can probably trim from zero (rounded) at end to full width over next rivet from the one near the end.

Edit: However, there is another factor: the twisting moment when load is being applied. As for longerons, I think that one of the loads is tension or stress (push), so there I'd be more weary of trimming to an angle -- probably because of the twisting moment. I suspect you'll get few or no answers from real structural engineers because it's not that simple and depends on the specific instance where the member is located.

Why spend time and effort cutting lightening holes in ribs, flap spars, flap brace and so on? Because it all adds up to a significant number of pounds.

RV-3 and 4 and possibly 6 plans do not mention drilling holes in the brake pedals, does not show trimming angles, and so on. I've noticed that in newer kits a lot of attention has been given to minimal material. Blessings of design software and CNC machines, I suspect.

Finn
 
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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.
 
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AndyWW said:
That's an interesting but cryptic comment! I would love it if you or someone else could elaborate...
Click to expand...
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.
 
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!
 
45 degre

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)
 
FEA

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

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.
 
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
 
Lower longeron

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!)
 
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
 
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