Building obsession... Let's "spar..." - RC Groups
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Apr 12, 2012, 06:57 PM
CPA# 39
EscapeFlyer's Avatar

Building obsession... Let's "spar..."

Okay.... Not very funny. But I get an E for effort, don't I?

I posted on another forum and thought I'd put this out here too....

I enjoy the challenge of a constructed wing. For most, who cares, right? Put the stupid thing together and call it good!

Nooooo.... NOT ME!!!

Help me digress into my obsession if you would.

A wing is a wing is a wing. I disagree completely. If you are into classic pattern aircraft, the wing IS THE airplane. It has to be perfect or it will ruin everything. Then you will constantly fight yourself, not just the characteristics of the design!

This is the major problem I have been fighting: the square spar.

I think a person actualy loses the airfoil by using a square spar.... am I wrong? Sure, you can always sand the airfoil into the skin, but you lose something in that.

I have learned in plans building, you cannot trust the outline of the ribs given to you. So always use the 1st and last ribs as the template, and cut all the other ribs large enough to sand them to shape- on the build- with a long flat- perfectly straight sanding bar (these are expensive btw). You sand it down to the airfoils of the two tip ribs. This will guarantee a straight wing- IF you let the equipment do the work and you don't force it.

I say all that to suggest an idea. Why not cut the spar slightly tall and sand it to the airfoil while the wing is being built and jigged into place? Wouldn't this fix the flat topped wing over the spar and actually allow the cap strips to meet the forward sheeting without the fight?

Then there is the C-beam shear webbing!

Wouldn't an I-beam spar be stronger? Would it be stronger to cut a groove to fit the webbing and glue the webbing "into" the spars?

Where would I find the right tool to cut a 1/16' - 3/32" groove down the center of the spar? Would it be a router tool or a dremel router tool?

I have doxilia to thank for learning how to utilize the 1/4" sq truss method.... but isn't that still, in essence- C-beam technology?

This is all about the hobby of building. There are proven techniques such as traditional square spars.... But again. This is a hobby. It doesn't have to be easy, or common. It does need to work though.

It's my obsession and I'm stickin' to it! Ha.

Opinions on this?

Last edited by EscapeFlyer; Apr 12, 2012 at 07:20 PM.
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Apr 12, 2012, 08:05 PM
Hope to get out of life alive
kenh3497's Avatar
OK........ You asked for it!!

On the subject of spars. Instead of using a 1/4 square spare use a 1/8 by 3/8 laid flat. So the 3/8 side is the top/bottom of the wing. Now add shear webbing to both sides so you have a box beam spar. You should be able to drive a truck over it and not break it. Maybe make the spar 3/16 thick so you can sand the airfoil into it or cap it with super soft balsa to make sanding the airfoil into the spar easier????

Personally, I prefer a fully sheeted wing. If you want an accurate airfoil, it has to be fully sheeted to maintain what you or the original designer wanted.

Apr 12, 2012, 08:10 PM
Registered User

You could always have the ribs be deeper than the spar so that the flat surface is not exposed, or as you said, just have a very deep spar and then sand it down.

Regarding the I-beam spar and cutting a groove into the spars so that the shear web fits into it may not be the best way of doing it since it could cause the spar caps to split. In full scale airplanes, I believe that th I-beam is built by gluing 1/2 of the spar caps to the front and the other 1/2 to the rear face of the shear web. In that type of construction, you would probably have a few uprights between the spar caps to provide stability to the shear web in compression. In our case, our shear webs are pretty thick (say 1/16") so maybe it does not matter that much. Then, there is also the question of the grain orientation on the shear web...

Apr 12, 2012, 08:53 PM
KE your cub.
Curare's Avatar
If you really want to talk strength in spars, go talk to the glider guiders.

They've developed some insanely strong spars, which are way over designed for what we need. Contrary to popular opinion, we don't have that much load on the spar, and especially for classic pattern, we have thick wing sections which also help us keep the strength up.

In the past, on smaller sized models, I've used the egg crate method with great success. It requres a little more forethought but creates a strong, straight wing. It's been used for years in control line stunt, where their flight loads also exceed pattern aircraft.

Also, just a quick one on airfoils. I personally don't beleive it's that important. When you have a look at what people were using in full size aircraft, it's getting far removed from the expected NACA 00 airfoils. In my opinion, the determining factors for a pattern airfoil are: symmetery, leading edge radius, and percentage thickness. The rest is just joining the dots.
Apr 12, 2012, 09:11 PM
Hope to get out of life alive
kenh3497's Avatar
IMO, it's awfully hard to beat a sheeted foam wing. It's plenty strong, can be a light as a built up, relatively fast to build and an accurate airfoil.

I know for some, foam goes against the grain.

I must admit I'm not real big into pattern as there was never much activity in this area. Am I correct, some or maybe quite a few of the modern (2M) pattern plane are using built up wings? Maybe it's time for me to change my view

Apr 12, 2012, 09:58 PM
Registered User

One of the guys in my club is an extremely good builder who also happens to fly IMAC. His current plane, an Extra 260 (built from a kit) has a 112" wing. The wing is of traditional foam core construction covered with 1/16" balsa. By the way, when I asked him about it, he also mentioned that the wing has no spar. The wing tube is supported by a ply base rib and a balsa block embedded within the foam at the tip of the tube. Nothing else! If this works on a 40 lb aircraft, it should certainly work on a 10 lb plane. If anything, it tells you how much strength the monocoque structure of the balsa skin has.

Now, if you are building a traditional wing with ribs, you do need a spar, but if it is fully sheeted, I suspect that the main function of the spar is to locate and hold the ribs in place. For the most part, I feel that models are way over designed when it comes to strength needed to actually fly.

Last edited by viva_peru; Apr 13, 2012 at 11:22 PM.
Apr 12, 2012, 10:55 PM
Registered User
doxilia's Avatar

I think your dilemma is a fractal question. The flat spot presented by a spar with 90 degree angles is foil altering to the extent that it presents a deviation from the true continuous airfoil. How much it is altered depends on the width of the spar. In other words, what percentage of the wing chord does that "flat spot" present to the airfoil?

If you take a smallish 25 size model with a 9" chord for example, a 1/4" wide spar represents less than 3% of the wing chord. With a bigger model, the deviation typically is smaller as we rarely use spars wider than 3/8" on classics. So the question is, how much does a 3% deviation (or less) from true airfoil affect the performance and characteristics of the wing?

If you are like me, you'd likely be hard pressed to tell the difference. It's kinda like the "good speaker wire" argument. Having said that, one way to reduce that "airfoil alteration" is to use a thinner but deeper spar as suggested below (well, above for some depending on how you have RCG settings configured).

However, when building with 1/16" balsa sheeting, that deviation can easily be "sanded out". If you have what you might call a flat spot where the sheeting goes over the spar, sanding the sheeting on the wing can re-produce a continuous original airfoil. At the end of the day, as Greg says, it does not matter.

As to the shear web/truss structure, an interesting and very clever approach is to build the wing with an I-beam structure. Bur rather than worrying about grooves in the spars (after all it makes little difference whether the web is glued "in to" rather than "on to" the spar), one can "notch" the webs into the ribs. An alternating 1-2 prong to the webs allows them to be inserted into and glued to the ribs and spars. This technique is used by the C/L stunt guys. Because there is full contact between ribs, webs and spars, the entire structure is very strong. It's not hugely different from a properly trussed or webbed approach behind the spars but it is more elegant and it also results in a wing that is "self aligned" at the spar juncture. The Aurora 30 and Skymaster 25 that we are designing has a wing structure as described above - very cool!

Apr 13, 2012, 12:46 AM
CPA# 39
EscapeFlyer's Avatar
This is great!


So I will maintain strength if I maintain mass, but reshape geometry?

Not as wide as the standard (in context to Joe Bridi designs), but deeper to compensate?

Ex. Most Bridi built up D-tube wing designs use 3/8" X 1/2" spar- the length is horizontal. (UFO or Dirty Birdy maybe?)

I would help my dilemma to use 1/4" X 5/8" spar - the length being vertical? (To the rear of the rib notch on existing horizontal spar designs?)

The wing would work the same, but I would lose most of the distortion on top of the spar?

I am sure that, for most, it seems as though I am trying to re-invent the wheel here. But this is where I enjoy this part of the hobby.

Last edited by EscapeFlyer; Apr 13, 2012 at 02:18 AM. Reason: A more thought out response.
Apr 13, 2012, 07:38 AM
Registered User
Originally Posted by viva_peru
Now, if you are building a traditional wing with ribs, you do need a spar, but if it is fully sheeted, I suspect that the main function of the spar is to locate and hold the ribs in place. For the most part, I feel that models are way over designed when it comes to strength needed to actually fly.

In a fully sheeted wing I think that the main job of the spar (apart from holding the ribs in place) is stopping the skins collapsing between the ribs when under load. Isn't that why we use webs?
As for overbuilding, I think my models get stressed more when carrying and storing them, but maybe that's just me being clumsy
Apr 13, 2012, 09:00 AM
Registered User

I would say that you are correct on both accounts. The webs add quite a bit of rigidity to the wing by taking up the resulting shear load when a bending load is placed on the wing, but they also provide "crush" resistance to the structure since their grain is normally aligned vertically.

Regarding abuse, I feel that my models suffer the most while being taken to the field and back. The flying is easy on them, well, maybe except for the landing part... I cannot count how many times a tail has hit a door or a wall.

Apr 13, 2012, 10:20 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
Thinking or guessing isn't needed, the best way is to look at the construction of real wooden airplanes because all is known and for a long time.
An example, one picture, to show about dimensions of spars and of course there are many pages more with calcualtions, material choices, construction.

Apr 13, 2012, 07:32 PM
Hope to get out of life alive
kenh3497's Avatar
For years Spruce has been considered the go to wood for spars and "stressed" members in model and full scale aircraft. It seems of late, spruce cannot be found and is being replaced by bass wood. Well You can find spruce but it's in hunks of wood to large for model use.

Are there any alternatives to spruce? Would very hard balsa replace a spruce spar in a fully sheeted wing?

Apr 13, 2012, 08:51 PM
CPA# 39
EscapeFlyer's Avatar
Interesting question... and I would like to hear what others say as well.

I wouldn't know where the dividing line would be (where one ought use hardwood or composites instead of or over balsa) as I have not played with anything over a 60 sized airplane as far as a build goes.

I have always been told that spruce or basswood would always be better than balsa.... but Joe Bridi has proven that wrong in my world.

I would suppose size and wingloading would play a large part to that. I wouldn't be surprised if Cees can show us exactly where the engineer's perspective rests from within in the books.

Good question!

Apr 13, 2012, 08:55 PM
frequentflyer1's Avatar
Last edited by frequentflyer1; Apr 13, 2012 at 09:00 PM.
Apr 14, 2012, 11:59 AM
I bail out, anywhere, anytime
Taurus Flyer's Avatar
How they did:

First post about wood, used for several parts.
When you recognize the sorts written in Italic I can translate what you want to know.

For our models we have to remember a lot of strength is of the covering, that's why I use silk ans dope for example.
So the spars can be made of balsa in these cases IMO. Plastic film is much more flexible so ?????
My wings are also 19 a 20 % thick, also important for strength as result of the material of covering.
Gliders, large span, the spars are much more important in my opinion.
In the book is also written about the webbing and that takes a part of forces too but depending of the direction of the grain for example. Can make pictures of that too if people are interested.

Last note, wood can withstand more pulling forces than pressure, that's why we see different spars in top and bottom position of the wing, see the other picture of post 11.
Sometimes we see damage near the root, in topside of the wing sheeting as result of too much pressure.

Last edited by Taurus Flyer; Apr 14, 2012 at 12:50 PM.

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