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Feb 21, 2013, 11:28 AM
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sroge's Avatar

Rib thickness at 1/4 scale?

My scratch build is going on. The fuselage is finished, the DLE-30 runs quite well and today I bought wood for the spars and doubled them from root to 40%. I also designed the wing (Clark-Y) using Profili and now I have to decide which wing rib thickness might be appropriate. As in Germany everything is metric, there's 2mm balsa or 3mm balsa.

The plane is going to be of a size comparable to a 1/4 Clipped Wing Cub, having a wingspan of 89" and a chord of appx. 15 3/4".

The wing will be a d-box design, sheeted from LE to 30% and will have a strong spar at 75%, because here the wing will end as I am going to use the whole wingspan for ailerons and flaps. All ribs will have rib caps.

The wing will be covered with Koverall.

So what do you think? Will 2mm of rib thickness be enough? I found one guy scratch building a 1/3 (non-clipped) PA-18 using 3/32 ribs, which is 2.4mmóbut 1/3 is BIG compared to mine.

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Feb 21, 2013, 12:24 PM
B for Bruce
BMatthews's Avatar
The D box and cap strips will certainly help. But on such a model the few extra grams for the thicker wood for the ribs will most certainly go unnoticed. I'd suggest you go with the thicker option.

The only way the thinner ribs would be desireable would be if the selection of the 2mm wood over 2.5 was the final step in a whole long list of weight reduction tradeoffs. If you haven't been doing this throughout the design process then there's simply no point in going thin for this one item and risk the wing being crush sensitive due to the thinner rib wood.

Another point is that with all that sheeting and cap stripping again the choice of the thinner ribs is moot unless you've bought near Contest wood density stock for ALL the sheeting and cap stripping to hold the weight down. And if you have done this then there may be a case for using the thinner rib wood again.
Feb 21, 2013, 12:50 PM
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sroge's Avatar
No, there are no such considerations about weight.. I designed this plane to be simple, reliable (two separate elevator servos and power supply for each single servo) and sturdy enough for usage out side of club areas and runways. Strong fiber landing gear and big wheels.. trying to save 1oz of weight would be ridiculous on such an airplane.

So I'd rather take 2.5 or even 3 mm to make sure that the ribs are strong enough..

Feb 21, 2013, 01:43 PM
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Rib strength of real aircraft ribs compared to model ribs has always facinated me. Quite often real aircraft ribs are noticably/comaratively weaker than their model counterparts. Part of this disparity, I believe, and would lik to hear other's thought on this, comes from the fact that model ribs work in conjuction with covering material to act as a large part of model wing's torsional structure. Real aircraft wings have drag/anti-drag bracing between the front and rear spars. Models usually dispence with this and let the covering provide the torsional resistance. A real aircraft wing's covering , except for air loads, provides no structural ingrity whatsoever, unless we are talking about plywood or metal.

A typical truss wooden rib for Ryan M-1/Spirit St. Louis has chord of 84" , of clark Y section, and has the top and bottom rib caps made of 1/4"Wx1/2". On a !/4 scale model of same scales to 1/16 wide by 1/8 tall. Would a 1/16" sheet rib be considered sufficient for the 1/4 scale model? would a scale truss rib be adequate with say Koveral as fabric choice?

Strange things start to happen when modeling in the larger scales, Balsa "in-scale" lumber sizes start to look inadequate from more traditional modeling norms. But do weight problems accrue if one used scale sized spruce parts, etc? Is consideration of more scale like internal bracing a viable answer to covering concerns?

I've got a couple of 1/4 and 1/2 scale projects coming up and plan to explore both options for optimum strength/economy etc.
Feb 22, 2013, 09:22 AM
Designing something...
All depends on things like rib spacing, the hardness of the balsa and such. With that said I've got a ~3 meter Turbo Porter that, for the most part, uses 3/32" balsa ribs. Some of the more critical ribs are sandwiched between .4mm birch ply.

So for that span...if everything else is correct 2mm balsa ribs will probably work. Though the root ribs and some others should probably either be made thicker or otherwise reinforced (e.g. with thin plywood). Also, make sure that you don't over-shrink the Koverall.

Feb 22, 2013, 11:48 AM
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sroge's Avatar
Thank you guys..

today I launched Profili and finally printed the airfoil. I decided to place the spar at 25% and the wing joiner somewhere around 30%. As I did not want to order wood a second time I decided to make 36 ribs (1.5) instead of 18 and glue 2 together. There will also be 3 plywood ribs of each wing side: root, rib 2 and tip. The spacing will be around 4".

Feb 22, 2013, 02:13 PM
B for Bruce
BMatthews's Avatar
I'd reconsider your spar and joiner locations. It's always better in every way possible if the joiner sits between the upper and lower spar caps so the loads are directed into the caps in the most direct manner. It may mean that the joiner's slip tube has to do double duty as the spar's web at the wing root but that's OK as the joiner rod itself acts as part of the webbing at the same time.

If it's a blade style joiner then it's even easier to incorporate the joiner's box receiver into the spar's root area.

It also reduces the need for lots of plywood ribs at the wing root. And by doing it that way it's also stronger overall rather then relying on the plywood ribs to withstand the flight loads. It also puts less importance on building the rear locating joiner to be adequietly strong since the spar is right there to take the loads instead of sharing it through the plywood ribs with the rear locating joiner.

Typically the spar is placed at the point of maximum airfoil thickness. This has two simple advantages over putting them at some other point on the airfoil. First it puts the top and bottom caps at the greatest possible spacing which provides the best strength. Secondly sets the caps where they can be parallel to each other in the chordwise direction which makes fitting the webbing an easier job..
Feb 22, 2013, 10:43 PM
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sroge's Avatar
Hi Bruce,

I had just the same thoughts as you when I decided where to put the spar and the joiner. But even at maximum thickness (at a 400 mm Clark-Y total height is 46.8 mm) I found no way to keep the ribs from falling apart but by taking the joiner joiner out from between the spars.

sheeting top: 1.5
sheeting bottom: 1.5
top spar (doubled from root to rib 4): 10
top spar (also doubled): 10

now there's 23.8 mm left between the spars and my joiner will 19.5mm with a sheath of 1mm thickness around. As I am quite a newbie to wing building I did not want to separate the d-box part from the rest of the rib. With my next design I will consider this.

As for aligning the top and bottom spars at the 25 % location I made a few test prints with Profili. The default is to alilgn them at 90 degrees with the wing chord but then the spar outsides don't flush with the sheting perfectly. When you let Profili flush the outsides you will get the not-flushing at the webbings. I also tried to move the top spar a little around the 25% postion, but all this didn't satisfy me. So in the end I decided to take the default and align both spars 90 degrees @25% and forget ignore this little gap on the top side. As the d-box sheeting will meet the rib caps here no one will see it in the end..

Last edited by sroge; Feb 22, 2013 at 10:51 PM.
Feb 23, 2013, 11:29 AM
B for Bruce
BMatthews's Avatar
Stefan, you need to change your mental gear box and think differently.

First off put the spar caps on the outside and bring the D tube sheeting up to the spars. With this trick the spar caps become part of the D tube sheeting as well as work as spar caps. On the bottom the flat shape of the Clark Y makes this a cinch. On the top you can live with the caps being under the sheeting. Or you can use the same idea and have the caps on the outside surface.

Either way it would be nice to feather in the to shape of the caps to match the airfoil. A nice way to do this is to add on a little lightweight filler to the top of the cap and then sand it to the airfoil profile. You can typically find this sort of filler in the hardware stores. You'll know you found the right one when you pick it up from the shelf and you're sure the container is empty.

For the spar doublers in the root area don't double the THICKNESS. Instead use half width additions on the EDGES to make the caps wider for the first few rib bays. This not only makes your spar caps wider AND stronger but it opens up the area between the caps to allow fitting the slip tube between the spar caps.

You also need to achieve an excellent bond between the slip tube and spar caps. Merely gluing the slip tube to the ribs or along a thin line to the caps isn't enough.

An excellent and easy way to do this is to mix up some epoxy with a LOT of wood dust from a saw. You're looking to create a strong but light weight putty that bonds the spar caps to the slip tube. This putty becomes the all important webbing as well as the loading conductor between the spar caps and the joiner tube. You'll need to use a slow cure epoxy to get enough time to mix in the wood dust and to let it have enough time to wet out the spar wood. And even thickened a lot with the sawdust it tends to flow. Some thin plywood webbing on front and back will act as dams to hold the filler in place until it cures.
Feb 23, 2013, 01:02 PM
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sroge's Avatar
But bringing the flanges to the outside and let the sheeting coming from the LE meet the flangesóthis will result in a very small area of only 1.5 or 2mm height (depending on sheeting) where sheeting and flange meet (fig. 1)

This is an area where up to 2/3 of the of the plane's lift is generatedóat least this is what I thought about when I decided to let the flanges be located under the whole sheeting (fig. 2).

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Feb 23, 2013, 06:12 PM
B for Bruce
BMatthews's Avatar
Ah, but the spar caps do not need to withstand bending loads. If you look at how an I beam works the top and bottom caps only bend a little. The vast majority of the force in the top and bottom caps are compression on the top and tension on the bottom. So adding to the width of the caps instead of the depth is actually putting the material where it does the most good.

A sketch to go with the explanation below is shown at the end of this post.

In the top view is a partial ribs and spar sort of setup showing the spar doublers joined to the front and rear edges of the main cap. The main cap is brown and the doublers are green.

The first rib shows this in section. The blue is the epoxy and sawdust filler around the slip tube for the wing joiner tube or rod. The pinky brown lines are caps to avoid the filler from flowing out until it cures. The same colour lines are also used to indicate the wing sheeting or capstrips.

The epoxy putty provides the caps with a full fill webbing to solidly join the upper and lower spar caps. The doublers used on the edges both widen the main spar caps to allow a larger slip tube as well as extend out and taper to end at around the 25% span point. By extending out like this they also increase the strength of the spars in near the root area where the spar loads are the greatest.

The second rib section is the outer area showing the upper and lower spar caps joined by face webs front and rear. This isn't quite as good as a thick single vertical balsa center web between the two spar caps but it's a heck of a lot easier to build it this way.

By joining the top and bottom caps with the boxing in webs front and rear you create a far stronger setup.

And for a model of this size you most certainly want to use front and back webs. And the webs should be VERY hard balsa or even 1/16 ply. The reason for this isn't what you might think. Medium balsa would take the load but the glue joint would likely fail. You want to use a harder wood so you get better glue joints to the spruce or pine spars.

I also show the upper spar sitting about 0.5 mm below the sheeting as allowance for some light weight filler to be used instead of shaving the spar itself down to match the airfoil. Alternately a cap of thinner balsa than used for the sheeting could be glued on and that shaved down to match the D tube and rear cap strips.

The D tube concept is still with us. Glueing the leading edge D tube sheet to the spars still locks the upper and lower sheet together to form a closed and rigid "D" shape.

This method, or something very like it, is also the standard sort of method used for joining together wings of competition sailplanes. The ones with thin wings that need to withstand the tremendous line tensions during contest style winch launches that manage to break 200 lb test line with an alarming regularity. So it's a proven method.
Feb 23, 2013, 11:37 PM
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sroge's Avatar
Right, there are no bending loads. The decision to double the flanges vertically was based on other considerations. Doubling the webbing by using the front and back side of the flanges is one thing I will definitely think about.


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