Spar Breaking Test!! Beam Strength Designs - RC Groups
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Apr 28, 2011, 11:43 PM
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Spar Breaking Test!! Beam Strength Designs

How strong is my spar? Whether we are building, replacing, modifying, or fixing our wings this question always comes up. So rather than the tried and true method of spending hours and hours building a plane, and then watch that work of art meet a bitter end to learn how strong it was, I decided it would be worth it to build and break as many spar designs as possible before starting another project.

The set-up.
-My set-up is upside down. On a flying plane the fuselage is pulling down on the spar and on my tester I use a fishing scale that pulls up. I hacked off part of a C-clamp and welded a brace to it so I can slowly control the pulling force and even measure the flex distance at the breaking point.
-These spars would much rather twist and break sideways instead of stay strait and break in their strongest direction. Therefore I used white particle board as walls to sandwich the spars to keep them from twisting that unfortunately hide us from seeing most of what is going on. I am also using different quantities of washers to space walls just slightly wider that each spar. I know different spars do resist rotational forces better than others, but I only want to test vertical load strength. Plus in most models, using D-box construction or Monocote are far better ways to add rotational resistance to your wing than with your spar.
-Spars are held 30” apart with the scale in the center. If you envision the anchors as the locations for the center of lift for each wing then this very loosely represents a plane with a 60” span (although I’m sure engineers would hate that I allow such close-enoughs since real wigs experience lift at every point of the spar)
Last edited by hasselblad7; Apr 28, 2011 at 11:57 PM.
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Apr 28, 2011, 11:47 PM
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- I had to decide what the “average” spar design would be as something to compare to. Considering both sailplanes and parkflyers I decided to use a ¾” C-beam using 1/8”x1/4” spruce as the “starting point” (see image)
- Some spars are easier to build than others. I am a decent builder and when I built the sample spars was trying to make sure the spars would not break because of faulty building techniques. However if any spars do break because of bad workmanship, we can assume 90% of the builders out there will have the same problem, especially considering my advantage of not having to build around any ribs or sheeting.
- It is important to notice HOW the spar breaks. If only PART of the spar breaks, “Weakest Link” rule says that the other components could have been smaller creating a lighter spar of the same strength, or if the spar delaminates from the webbing that means the spar components could potentially hold more load with a better spar design or better building technique.
Apr 28, 2011, 11:48 PM
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I’ve read enough misconception on RCG that I feel I need to first clarify some basic understanding about beams. When a beam is under a load, the top of the beam is under horizontal compression, the bottom is under horizontal tension, and the center is under vertical compression and horizontal shear. I found a some diagrams off the internet that show this fairly well. So when we design our spars we are trying to keep the tops from squishing, the bottom from pulling apart, and the center from squashing vertically and shifting horizontally.
Don’t worry, all this happens within a the average 2x4, and the simple way to do this is to keep as much of your beam materials as far apart from each other at the top and bottom while keeping enough material between them to hold them together. This is why most structures around us are built with I-beams, and the simpler to build (for modelers) C-beams.
Apr 28, 2011, 11:52 PM
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Lets do this!
How many times have we asked questions here and gotten an engineer give us a long winded explanation only to conclude in the end that he doesn’t have enough information to give you your answer! Well lets get some answers! Enjoy:
Apr 29, 2011, 12:21 AM
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I think this is going to be interesting.

Seems like the details of how you are applying the load to the spar are going to be critical. Might help to glue a block in between the spar caps in the middle and apply the load to it. Also, if the spar is twisty, it may create a fair amount of friction with those walls. That would make them appear stronger then they are. Not sure whether the good approach is to use something really slippery on the walls (UHMW polyethylene?) or some other technique. Maybe make spars wider than they are thick and they'll be stable? Could then look at the strength per unit width. Or perhaps you could glue some plywood to the caps to make the spars deeper vertically and prevent twisting, except for that open area in the middle. Of course that will make your stiffness testing a little harder to do accurately. Or you could make two spars and have crosspieces between them at intervals to keep them from twisting.
Apr 29, 2011, 01:32 AM
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The walls do let the spars slide fairly freely but I'm sure there is "some" friction. I think the best way to look at this would be comparative. I plan on doing everything exactly the same so at least we will get accurate data on what is stronger than the other and by how much. I may not know exactly how strong one of my planes is but I usually have a good idea of how to build the next one stronger.
Apr 29, 2011, 01:39 AM
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Test 1

I’ve read that designers sometimes use thicker airfoils just so they can utilize thicker spars that give more strength. So how much stronger is a thicker spar than a thinner one? And are the increments large enough for us modelers to pay attention?

Since most models I’ve built use a C-beam design I built five different C-beam spars, all using 1/8x1/4 spruce. All are made using thick CA glue and 1/16 balsa webs. 1st spar is ½” thick with each spar growing at 1/8” increments with the fifth spar at 1” thick.
Apr 29, 2011, 01:58 AM
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- So it looks like the relationship strength has to thickness is linear as apposed to exponential.
- I would have never thought a little 1/2" spar could hold 9 pounds!
- By thickening the spar by just 1/8” more you can increase the strength of your wing by about 25%, or by about 3 pounds. When you consider that’s about the weight of the park planes/gliders we fly that means that 1/8” is about 1g more strength!

Application: This gives a good argument for using a tapered wing platform for very strong wings without having to reinforce your spar. Also 1/8” just happens to be the same width of the two 1/16” balsa sheets we often use to cover the spar in a D-box construction. If we don’t cover the spar we could thicken it by 1/8” and about 1g to our strength.

FYI: When the spars reached their maximum, instead of breaking with a “bang”, the scale would increase to their maximum load and then slowly begin to decrease about 10% before the anticipated quick snap would happen! It gives hope that if you overload your wing for a brief moment if you quickly release your elevator you may be able to milk it home without folding your wings.
Last edited by hasselblad7; Apr 29, 2011 at 02:36 AM.
Apr 29, 2011, 02:27 AM
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Instead of the particle board, use thick plexiglass, then you can see the whole spar.
Apr 29, 2011, 07:59 AM
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Would you please build one "I" beam spar of comparable dimensions (i.e thickness and width of spars, distance between spars and thickness of webbing) to one of your C beams and compare the breaking force.

Tnks, Alan

P.S. Did the spar and webbing joint separate in 2nd picture in post 8?
Last edited by Al Offt; Apr 29, 2011 at 08:05 AM.
Apr 29, 2011, 08:13 AM
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Most reported failures of folded wings with "I" beams indicate that the top spar failed in compression. In your experiment, that spar would be the lower one so I would have expected your lower spar to fracture rather than the upper one.

Is an upper spar failure a characteristic of a C-shaped beam?


P.S I have no problem with your method of applying a "point" force with the round bold (simulates the end of a joiner rod) but I do wonder what would happen if you were to place a piece of 1/8 inch spar material between the bold and beam to be tested.
Last edited by Al Offt; Apr 29, 2011 at 08:22 AM.
Apr 29, 2011, 11:28 AM
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I need to clarify, since I am so used to working upside down, when I say "top" I actually mean what would be the top of the spar if in a wing. So the member in compression consistently breaks first usually leaving the member in tension in tact. The 1/2" C-beam is the rare case where everything seemed to break together.

At first I was putting a piece of cardstock between the bolt and the spar to even out the load a bit but it didn't seem to do much. I imagine a piece of wood would even out the load more causing breaks to happen at a slightly heavier level.
Apr 29, 2011, 11:37 AM
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Test 2

How much stronger does adding a second top layer of spruce add to the spar and is it worth the added weight? Many materials are twice as strong in tension than they are in compression. Therefore to create the strongest and lightest spar it seems natural to have twice as much material where the compression is than where the tension is.

In this test I used two spars both 7/8” tall. I used identical construction of C-beam as in test 1, with one having an added extra laminated stick of spruce.
Apr 29, 2011, 11:43 AM
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Conclusion: A significant gain in strength. Slightly improved strength to weight ratio as well. A very good simple way to add strength to a spar. When it broke the bottom stick delaminated from the webbing but the sticks stayed in tact so perhaps a glue fillet or creating a box by adding webbing on the other side would increase strength even further.

Note: If you look closely in the photo even though the spruce has ripped away from the webbing there is still balsa from the webbing attached to it. So it wasn't a matter of not using enough glue but the strength of the balsa.

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