The I-Section Spar - How-To

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The I-Section Spar

By Charlie Fite

Being a retired engineer, scratch building balsa and foam airframes satisfies my need to keep both my mind and my hands busy. I love finding prototype, never-built, one-offs, or experimental aircraft from the early days of flight through the 1960s and turn them into standoff scale radio controlled models. Consequently, theres usually very little to go by except 3-views and or sketches. That appeals to me. Wikipedia and RC Groups are my best friends. Given the lack of detailed information available, I have to design all the parts, pieces, and details in order to build the model. This article is about how I design and build the main spar of the wing.

In his book A Brief History of Time, Stephen Hawking said that for every formula he included, hed lose half his readership. I promise not to bore you with the math.

There are a number of ways to design and build a wing spar. Solid square, solid rectangular, boxed girder, C-shaped, and I-shaped spars are the most popular. Ive settled on the I-section because it is the most economical and most efficient shape. It is also the strongest for its weight and its the reason the structural steel members that support the loads of buildings and bridges are used. Hence, it is my natural choice.

As a result, Ive never had a wing fail by folding in the middle.

So here goes.

I-Section Spar Design

The main spar has to carry the loads imposed on a cantilever wing. Those loads include both vertical and horizontal shear, along with bending moment. The torsional (twisting load) on the wing is another one to consider. Of those loads, vertical shear imparts the least amount of stress on the spar, followed by lateral shear. Bending stress is usually an order of magnitude greater than either of those. The addition of wing struts or flying wires reduce those stresses considerably by transferring the wing loads to the fuselage by either tension or compression, but can significantly increase the drag on the airframe.

An I-section spar consists of top and bottom flanges with a web that is located down the centerline of the flanges. The top flange is in compression, the bottom flange is in tension and carries the bending load of the wing. The web resists both the lateral and vertical shear loads imparted by the weight of the aircraft and the moment couple caused by the difference in the flange loading. There is also a torsional load on the wing, but that is primarily carried by the skin. Ill get to that in a minute.

The picture above illustrates the strength of an I-section spar.Thats a 10kg lead bar placed at mid span of a 54 wing, without its sheeting. Deflection is less than 0.1. Suffice it to say that far exceeds any G-stresses imparted by flight loading.

Almost all my wing spans are between 36 and 72, simply because I cant get wings any larger than that in the car. Hence, the main spar components dont change much. The difference is the depth of the spar. The greater the span, the deeper the spar. And remember, the deeper the spar, the more load it will carry.

The flanges (the spar caps) usually consist of 1/8 x 1/2 bass wood resting in notches in the ribs. The web is usually 1/8 balsa. The wing joiner is always aircraft grade plywood, the same thickness as the width of the flanges. The shear webs completely fill the area between the flanges and the ribs. In extreme cases (high wing loads) Ill make the caps out of 1/8 ply and the webs out of 1/16 ply. There has always been a controversy with regard to the direction the grain of the shear webs should run. Some say horizontally, some say vertically. Im in the vertical camp because balsa is stronger in shear across the grain rather than with the grain. I dont know how many articles and technical papers Ive read regarding the strength of balsa, both in tension and compression. The overwhelming majority of them indicate that the grain of the shear webs should run vertically. I believe them.

The torsional or twisting load on a wing is mostly carried by the sheeting. It can also be carried by X or K bracing, though that is a more tedious solution. I always D-box the front of the wing between the leading edge and the spar, both top and bottom, usually with 1/32 bass or 3/32 balsa. Additionally, Ill add a narrow strip of bass or balsa on the top and bottom of the trailing edge (V-boxing). Lastly, Ill add rib caps on each exposed rib, effectively turning each rib into an I-section. Secondary spars or stringers add to the rigidity of the wing. The picture of the cross section of the Japanese Zero wing illustrates that.

That pretty much covers how I design and build my wings. I hope this little treatise is of interest, and helps provide an insight into wing design and construction.

Yall keep your nose up in the turns.

Any negative posts or posts that are an attack against the intelligence, character will be redacted.

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Old 08-02-2021, 12:08 PM
ghoffman is offline
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Mike Patey's recent videos on how he built Scrappy, has exactly this design.
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Old 08-27-2021, 01:01 PM
Mithrandir is offline
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if anyone is actually interested, I have an excel spreadsheet that inputs all the wing parameters, span, section thickness etc etc etc... and velocities, weights and "G's" to be encountered and outputs the parameters for the spar... (Also inputs Cm data and outputs skin thicknesses for wing skins... but this is highly tuned to composite skins)

I can not give away the spreadshet, as it is company property... but shoot me yer data and I run it through....
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