How Much Do The Holes Weigh ?

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wsimpso1

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Ok, I'll bite at the bait...

So, a "lightening" hole, cut and flanged, will weaken the rib significantly, but a riveted aluminum "space frame" rib is not significantly weakened as designed?
The issue in each and every case is: Is the resulting modified part strong enough and stiff enough over the long run?

I am always concerned when I find that a builder has taken upon themselves to remove material that the designer had included. When the designer finished the design, it was built and flown, and thought to be better than some reasonable level of strength, stiffness, and durability. Once material has been removed, is it still known to be good - or is the builder guessing that the designer was less than fully weight conscious or worse, incompetent? Heck, if I thought the designer was less than excellent, why would I want to build and fly the airplane anyway?

In the best of all worlds, the designer of, say, a wing, considered spar sizes, more ribs, less ribs, full ribs, skeleton ribs, cross bracing, etc, and chose the lightest design that does everything for this airplane and will be durable in service.
  • If the wing in question was indeed optimized or even approached the optimal design, taking material out is likely to make the wing weaker than needed and/or softer than needed and/or less durable than needed;
  • If instead, the designer was at the other end of the scale, the ribs are closer together than they need to be, and simple plywood ribs were used to speed construction, and were thicker than they need to be, then maybe you can remove a bunch of material and still be OK. Maybe;
  • Or maybe a less than fully careful and competent designer ended up with some pieces that are borderline (or worse) by themselves, but give no trouble because the nearby structure is overly sturdy so the borderline part is not overstressed. Then when someone starts lowering the strength/stiffness of parts and the borderline one becomes understrength, cracks in service, and, if they are lucky, catch it and have to build a new set of wings.
If you are not qualified to design the example wing, how will you know if the "after" wing or other structure is OK? You do not. You are guessing. Guessing is BAD!

Then there is the issue that many designs out there were intended for one weight and Vne that were published in the plans and other materials. Then the fleet has builders who just have to install the next bigger engine, constant speed prop, IFR panel, carpeting and side panels, spend lots of time on perfect looking paint, etc. Then the bird is heavy and cruise speed turns out to be darned close to Vne, so the airplane gets flown to higher speeds and at higher loads than the prototype was ever flown to. It might even need some more strength and/or stiffness in a bunch of places.

I prefer to start with the assumption that most folks designing and marketing an airplane successfully probably had a pretty good idea of how much material is needed on each part and the design reflects that. Removing material that the designer put in may be a risk a builder is willing to take, but I recommend against it unless a significant number of that exact airplane with that exact modification have been flown for a while.

This is all explanation of a simple philosophy for most of us: Stick to the plans unless there is a known good change, then stick to the plans for the known good change. You will know what you will get then. Removing material that the designer put in? Not unless some other smart folks have done it too, then only after those folks have demonstrated it is still a good airplane.

Billski
 

Rataplan

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The issue in each and every case is: Is the resulting modified part strong enough and stiff enough over the long run?

I am always concerned when I find that a builder has taken upon themselves to remove material that the designer had included. When the designer finished the design, it was built and flown, and thought to be better than some reasonable level of strength, stiffness, and durability. Once material has been removed, is it still known to be good - or is the builder guessing that the designer was less than fully weight conscious or worse, incompetent? Heck, if I thought the designer was less than excellent, why would I want to build and fly the airplane anyway?

In the best of all worlds, the designer of, say, a wing, considered spar sizes, more ribs, less ribs, full ribs, skeleton ribs, cross bracing, etc, and chose the lightest design that does everything for this airplane and will be durable in service.
  • If the wing in question was indeed optimized or even approached the optimal design, taking material out is likely to make the wing weaker than needed and/or softer than needed and/or less durable than needed;
  • If instead, the designer was at the other end of the scale, the ribs are closer together than they need to be, and simple plywood ribs were used to speed construction, and were thicker than they need to be, then maybe you can remove a bunch of material and still be OK. Maybe;
  • Or maybe a less than fully careful and competent designer ended up with some pieces that are borderline (or worse) by themselves, but give no trouble because the nearby structure is overly sturdy so the borderline part is not overstressed. Then when someone starts lowering the strength/stiffness of parts and the borderline one becomes understrength, cracks in service, and, if they are lucky, catch it and have to build a new set of wings.
If you are not qualified to design the example wing, how will you know if the "after" wing or other structure is OK? You do not. You are guessing. Guessing is BAD!

Then there is the issue that many designs out there were intended for one weight and Vne that were published in the plans and other materials. Then the fleet has builders who just have to install the next bigger engine, constant speed prop, IFR panel, carpeting and side panels, spend lots of time on perfect looking paint, etc. Then the bird is heavy and cruise speed turns out to be darned close to Vne, so the airplane gets flown to higher speeds and at higher loads than the prototype was ever flown to. It might even need some more strength and/or stiffness in a bunch of places.

I prefer to start with the assumption that most folks designing and marketing an airplane successfully probably had a pretty good idea of how much material is needed on each part and the design reflects that. Removing material that the designer put in may be a risk a builder is willing to take, but I recommend against it unless a significant number of that exact airplane with that exact modification have been flown for a while.

This is all explanation of a simple philosophy for most of us: Stick to the plans unless there is a known good change, then stick to the plans for the known good change. You will know what you will get then. Removing material that the designer put in? Not unless some other smart folks have done it too, then only after those folks have demonstrated it is still a good airplane.

Billski
seems logic to me.
best is to have the design plans and structural analysis and calculations of the plane, so you can see what forces acts on a part. Now you can calculate if the part with holes still is strong enough.
but mind the stiffness is as important, as changing the stiffness of one part can effect the forces on an other part.
example, simplified example:
A weight hangs on two coper cables.A and B Each cable has a strength of 50 kg. So i can hang maximum 100 kg on them.
Now i replace copercable A with a carbon fibre cable of the same strength . So my weight hangs on A a carbonfibre cable which can hold 50kg and B copercable that can hold also 50 kg.
But now the maximum weigth I can hang on the system will be far lower than 100 kg.
( the carbonfibre cable will break first)20210822_190307.jpg
 
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challenger_II

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You missed my point, entirely.

The issue in each and every case is: Is the resulting modified part strong enough and stiff enough over the long run?

I am always concerned when I find that a builder has taken upon themselves to remove material that the designer had included. When the designer finished the design, it was built and flown, and thought to be better than some reasonable level of strength, stiffness, and durability. Once material has been removed, is it still known to be good - or is the builder guessing that the designer was less than fully weight conscious or worse, incompetent? Heck, if I thought the designer was less than excellent, why would I want to build and fly the airplane anyway?

In the best of all worlds, the designer of, say, a wing, considered spar sizes, more ribs, less ribs, full ribs, skeleton ribs, cross bracing, etc, and chose the lightest design that does everything for this airplane and will be durable in service.
  • If the wing in question was indeed optimized or even approached the optimal design, taking material out is likely to make the wing weaker than needed and/or softer than needed and/or less durable than needed;
  • If instead, the designer was at the other end of the scale, the ribs are closer together than they need to be, and simple plywood ribs were used to speed construction, and were thicker than they need to be, then maybe you can remove a bunch of material and still be OK. Maybe;
  • Or maybe a less than fully careful and competent designer ended up with some pieces that are borderline (or worse) by themselves, but give no trouble because the nearby structure is overly sturdy so the borderline part is not overstressed. Then when someone starts lowering the strength/stiffness of parts and the borderline one becomes understrength, cracks in service, and, if they are lucky, catch it and have to build a new set of wings.
If you are not qualified to design the example wing, how will you know if the "after" wing or other structure is OK? You do not. You are guessing. Guessing is BAD!

Then there is the issue that many designs out there were intended for one weight and Vne that were published in the plans and other materials. Then the fleet has builders who just have to install the next bigger engine, constant speed prop, IFR panel, carpeting and side panels, spend lots of time on perfect looking paint, etc. Then the bird is heavy and cruise speed turns out to be darned close to Vne, so the airplane gets flown to higher speeds and at higher loads than the prototype was ever flown to. It might even need some more strength and/or stiffness in a bunch of places.

I prefer to start with the assumption that most folks designing and marketing an airplane successfully probably had a pretty good idea of how much material is needed on each part and the design reflects that. Removing material that the designer put in may be a risk a builder is willing to take, but I recommend against it unless a significant number of that exact airplane with that exact modification have been flown for a while.

This is all explanation of a simple philosophy for most of us: Stick to the plans unless there is a known good change, then stick to the plans for the known good change. You will know what you will get then. Removing material that the designer put in? Not unless some other smart folks have done it too, then only after those folks have demonstrated it is still a good airplane.

Billski
 

PMD

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Is that supposed to be a test or trick question? In my grease monkey ignorance, I would say your statement "weight I can hang on system will be FAR lower" is a great exaggeration. IMHO the change will be cos of the angle of the A/B/Kg triangle resulting from differential in elastic deformation A & B under the actual load. You also need to define "can hold". Can hold to ultimate strength or to design load limit with DSF calculated for that purpose to generate the spec for the members in tension? Of course, I am assuming the lower extremities of A and B as well as the link to the weight are free to pivot. Come to think of it: if the top of A & B are also free to swing, the rotation of the triangle will also move the bottom of A&B laterally until the exact same load in tension is on A and B.
 

Rataplan

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Is that supposed to be a test or trick question? In my grease monkey ignorance, I would say your statement "weight I can hang on system will be FAR lower" is a great exaggeration. IMHO the change will be cos of the angle resulting from differential in elastic deformation x the actual load. You also need to define "can hold". Can hold to ultimate strength or to design load limit with DSF calculated for that purpose to generate the spec for the members in tension? Of course, I am assuming the lower extremities of A and B as well as the link to the weight are free to pivot.
well it is ment to create a feeling for basic mechanics flow of Forces end relation to elasticity. making it kind of imaginable.
"can hold" is with purpose chosen not to distract .
I should have added the horizontal distance between the cables nears zero compared to their length.
immagine instead the weight I pulldown the system till the force in the carboncable is 50 kg
the prolongation of the copercable and carboncable is the same so the force in the copercable will be less. if I pull it more
down the force in the carbon cable will exceed 50 kg.

Ergo it works also the other way around: by making a part less stiff other parts can be overstressed and not the part you made less stiff.
 

wsimpso1

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seems logic to me.
best is to have the design plans and structural analysis and calculations of the plane, so you can see what forces acts on a part. Now you can calculate if the part with holes still is strong enough.
but mind the stiffness is as important, as changing the stiffness of one part can effect the forces on an other part.
example, simplified example:
A weight hangs on two coper cables.A and B Each cable has a strength of 50 kg. So i can hang maximum 100 kg on them.
Now i replace copercable A with a carbon fibre cable of the same strength . So my weight hangs on A a carbonfibre cable which can hold 50kg and B copercable that can hold also 50 kg.
But now the maximum weigth I can hang on the system will be far lower than 100 kg.
( the carbonfibre cable will break first)
Let's start by my saying that this looks like thread drift. I think I know what Rataplan was trying to get at. I do not know why it was brought up here.

If we figured out out how much typical copper wire is needed to just hold a 490N (the force to lift a 50 kg mass here on earth), and did the same with unidirectional graphite epoxy, I found that you need about 7.5 times as much copper as uni carbon composite. The copper would yield at strain somewhere around 0.15% while the graphite-epoxy would get to first fiber failure at more like 0.8 to 1.0%.

Now if you somehow constrained the copper and carbon to move together, maybe make a cable of them woven together and glued with more epoxy, the combined cable will strain as a single unit. Here is the interesting part. Copper is about 73% as stiff as an equal cross section of unidirectional carbon composite. But since you have so little carbon here, the part will behave pretty much like a plain copper cable. Effective E of the combined cable is about 105% of copper alone.

Load it up, and it will elongate just a bit less than if only the copper were there, and the copper will yield when strain gets to 0.15%. The load the copper can develop after yield does not change a whole lot more in the strains we are working with here. So, the copper carries its yield force of 490 N at strains of 0.15% and all the way up to where graphite fibers start to break. All of the load increase from strain of 0.15 to 0.80 is due to the load carrying capability of the carbon - until you get to a load of about 965 N, when the first carbon fibers start breaking. Now this is a little less than the 980 N that the two cables could carry, but not a huge amount less. And once you break all of the graphite fibers, you go back down to the yield load in the copper alone, or about 490 N.

So, the peak load this assembly can carry is not hugely lower than the total of the parts separately, but the copper will still be carrying load after the carbon in this case has already broken. So what is the moral to our story? I do not know and Rataplan did not say yet.

How would you actually get dissimilar materials to strain together in the real world? Classic case is concrete reinforced with rebar. The rebar and concrete strain together at their combined stiffness. Pretension the steel and put it on the bottom of a beam loaded from above, and you can carry far higher loads than the concrete could by itself. This whole topic is presented as Composite Beam Theory in whatever Mechanics of Materials book you favor. Another way is to laminate graphite-epoxy on a light core or with fibers oriented in different directions and thus having different stiffnesses in any one direction.

Billski
 

mcrae0104

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I understood Rataplan’s point to be that modifying a design, even in ways that seem to make common sense, but without some knowledge of engineering, can yield unexpected and unfavorable results.
 

Rataplan

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Let's start by my saying that this looks like thread drift. I think I know what Rataplan was trying to get at. I do not know why it was brought up here.

If we figured out out how much typical copper wire is needed to just hold a 490N (the force to lift a 50 kg mass here on earth), and did the same with unidirectional graphite epoxy, I found that you need about 7.5 times as much copper as uni carbon composite. The copper would yield at strain somewhere around 0.15% while the graphite-epoxy would get to first fiber failure at more like 0.8 to 1.0%.

Now if you somehow constrained the copper and carbon to move together, maybe make a cable of them woven together and glued with more epoxy, the combined cable will strain as a single unit. Here is the interesting part. Copper is about 73% as stiff as an equal cross section of unidirectional carbon composite. But since you have so little carbon here, the part will behave pretty much like a plain copper cable. Effective E of the combined cable is about 105% of copper alone.

Load it up, and it will elongate just a bit less than if only the copper were there, and the copper will yield when strain gets to 0.15%. The load the copper can develop after yield does not change a whole lot more in the strains we are working with here. So, the copper carries its yield force of 490 N at strains of 0.15% and all the way up to where graphite fibers start to break. All of the load increase from strain of 0.15 to 0.80 is due to the load carrying capability of the carbon - until you get to a load of about 965 N, when the first carbon fibers start breaking. Now this is a little less than the 980 N that the two cables could carry, but not a huge amount less. And once you break all of the graphite fibers, you go back down to the yield load in the copper alone, or about 490 N.

So, the peak load this assembly can carry is not hugely lower than the total of the parts separately, but the copper will still be carrying load after the carbon in this case has already broken. So what is the moral to our story? I do not know and Rataplan did not say yet.

How would you actually get dissimilar materials to strain together in the real world? Classic case is concrete reinforced with rebar. The rebar and concrete strain together at their combined stiffness. Pretension the steel and put it on the bottom of a beam loaded from above, and you can carry far higher loads than the concrete could by itself. This whole topic is presented as Composite Beam Theory in whatever Mechanics of Materials book you favor. Another way is to laminate graphite-epoxy on a light core or with fibers oriented in different directions and thus having different stiffnesses in any one direction.

Billski
was just an extreme hypothetical model,
LOL made a fool out of myself not to calculate it . I should have taken glassfibre cable instead of coper or better nameless just saying one cable is stiffer ,would fit more in a hypothetical model... also the cables are not connected to each other.. the point is, changing a part can influence other parts, making a part stronger,stiffer can even cause that part to fail against what you would expect.
Its just a warning not just making holes in your plane or making a part "stronger" for those with less background knowledge and or ,cq experience.

About composites like glas carbon kevlar and raisin, well my knowledge of composites is restricted to analysis of concrete constructions in all its forms and all kind of reinforcements. (nice way to say i have zero knowledge of carbon glass or kevlar composites, but Im not stupid although the choice of copper was stupid.LOL)










about steel bars in concrete, the main reason steel and concrete goes well together is they have the same thermal expansion coefficient. And in case of a concrete beam the concrete on the tension side will partly fail and form tears you can't avoid. anyway. way off topic.
 
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mcrae0104

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about steel bars in concrete, the main reason steel and concrete goes well together is they have the same thermal expansion coefficient. And in case of a concrete beam the concrete on the tension side will partly fail and form tears you can't avoid. anyway. way off topic.
I guess we are a bit OT here, but ideally that shouldn't happen if the tension bars have adequate development length to grip the concrete and adequate stirrups are provided to control shear cracking. In the real world, though...
 

Rataplan

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I guess we are a bit OT here, but ideally that shouldn't happen if the tension bars have adequate development length to grip the concrete and adequate stirrups are provided to control shear cracking. In the real world, though...
it happens by definition because the very low tension strength of concrete compared to the pressure strength. you also calculate them and check if they are within limits
 

Aesquire

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The "make it stronger" mistake I often quote is the WW1 Fokker D.VIII monoplane parasol fighter.

The Army thought the wing design looked weak. So they ordered the rear spar on the cantilever wing made stronger. This changed the twisting of the wing under load, so instead of twisting trailing edge up to dump lift, it twisted leading edge up and increased the load. And tore the wing off. Only when you pulled Gs, maybe.

There was a!so a serious Quality Control issue, with unskilled workers asked to do Cabinet/fine furniture level work, under time pressure, and poor supervision/inspection.

Either problem was a killer. Together a disaster. Fixing the skill and QC problem was harder than just building to original plan. They even rebranded the new planes to try and spin the problem. "Oh, this is a D.VIII, not a EV!"
 

wktaylor

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"Know the rules well, so you can break them effectively." --Dalai Lama XIV

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"The laws and rules made by God [Physics, chemistry, basic behavior, etc] can only be violated at great risk to mind, limb and body. The laws and rules made by man are guidelines for daily living in a civilized society and may have-to-be broken occasionally for the greater good [but know the rules to break the rules!].” – WKT

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Boscovius

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Didn't get any response to this question earlier, and I understand this discussion may be more intended for those looking to engineer lightening holes in aluminum, but what I'm trying to understand is (and I know I may just have to fabricate a test piece and destroy it in order to get my answer) can I put lightening holes in a material such as foam core wing rib and then insert with epoxy resin a short section of carbon fiber or fiberglass tube that is just a tad wider than the material it's inserted into, then fillet the edges of the tube to the sides of the rib. Let's say for argument's sake that the foam core is sandwiched between carbon fiber sheet, so there is no issue with material bonding. Is it likely that I could save weight and not sacrifice strength?
 

wsimpso1

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So, a "lightening" hole, cut and flanged, will weaken the rib significantly, but a riveted aluminum "space frame" rib is not significantly weakened as designed?
Taking a piece and cutting a hole out of it makes it softer and weaker:
  • If it is still stiff enough and still strong enough within the bigger structure, then it is OK. That means it was over strength to start with, and the modification is OK;
  • If it was just barely stiff enough and strong enough before cutting the hole, then it is now understiff and understrength, and the design now is weak.
And most of the time, you do not know which case you have at the beginning, unless you either do all the analysis of the wing or get a definitive answer from someone who has.

Billski
 

AeroER

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Here's the catch with flanged holes - once the web buckles across the flange or bead, the flange cripples and remains crippled, and can no longer stabilize the web to the same load.

This leads to shear resistant webs at limit loads. Probably a good idea in a homebuilt anyway.
 

OhAnElBirds

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Ah I read the news today, oh boy
Four thousand holes in Blackburn, Lancashire
And though the holes were rather small
They had to count them all
Now they know how many holes it takes to fill the Albert Hall
-- Lennon & McCartney
 
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