An I-beam for a tail?

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Grimace

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Some material testing may be required here, but follow along with me on the basics.

Primary gliders just have a truss with two dimensional "spar caps" and some reinforcing at the joints. These tails tend to be rather short, which I assume reduces twisting forces. They may also use some wire bracing, I've seen.

So why not an I-beam for the tail? Specifically, a composite I-beam with preformed carbon fiber rods, where the flanges of the I-beam get wider at they approach the cg in order to provide more lateral stiffness. I think you could even increase the radius of the fillet on the inside of the flanges (as you go towards the cg of the plane) to increase resistance to rotational forces and also stiffness to resist vertical tail / rudder forces. From the back of my napkin, it looks like something like a 3 inch wide flange top and bottom which widens out to about 6 inches (3 inches per side) at the base of the tail ought to be capable of doing what a tail needs to do.

I know someone will ask why, so here's the reasoning: the idea is to build a simple part 103 glider with nothing but 2 dimensional molds. Basically, nothing more complex than aluminum or steel sheet, either bent or curved, serving as a mold (with some obvious exceptions for things like wingtips, nose bowl, etc).

And I think that on a low-speed glider, with a Vne of something around 85mph, such a simple way of extending the tail rearward ought to be possible without too much of a weight penalty over, say, the Carbon Dragons split mold (Formica/Plexi) design which is elegant, but more time consuming.

The big issue is that I haven't seen this done before, so I wonder what I'm missing.
 

Matt G.

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The big issue is that I haven't seen this done before, so I wonder what I'm missing.
I-beams and other open sections have poor torsional stiffness compared to a tube. So, all else remaining constant, an I-beam used for a tail boom will be considerably heavier than a tube, as it will require more material or bracing to obtain the required stiffness.
 

Grimace

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In this use case, a little weight could be sacrificed for simplicity of construction, given that speeds are not a primary goal. A foam core within the shear web could help provide some torsional stiffness.
 

cluttonfred

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I don't see that an I-beam is actually that much easier to build than a box beam, especially a caps outside box beam which makes for easier clamping/gluing/riveting. You do sacrifice a little torsional stiffness but not as much as an I-beam. This is what I mean by "caps outside" and the orange sketches represent glued wood and plywood and the blue ones riveted aluminum. Plywood and carbon could work, too, with the webs set up like the caps-outside aluminum sketch and the aluminum angle caps replaced with carbon tape and resin.

HBA concept sketches.jpg
 
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Grimace

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I don't see that an I-beam is actually that much easier to build than a box beam, especially a caps outside box beam which makes for easier clamping/gluing/riveting. You do sacrifice a little torsional stiffness but not as much as an I-beam. This is what I mean by "caps outside".

View attachment 113598
You're flat out right. I was thinking of an I beam with a foam core between shear webs, and the caps would be joined, but that can basically be viewed as a box beam. The right-most picture is the closest to what I was thinking of.

One of the ideas is to use the "cove" in the outside box beam to mount the pulleys, cables, etc in a way that is accessible without removing a single panel. Mount the pulleys through the web/core, no obstructions. It wouldn't be the best aerodynamically, but I think it would be adequate for a slow glider and greatly improve inspections and the ease of maintenance.
 
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Jimstix

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Short version: I-beams suck at resisting torsion. Longer version: To make an I-beam stiff enough in torsion to prevent flutter (even at low speeds) it will be way too heavy. Easy access to the control cables and pulleys would come at a very high price. I suggest you use a round tube or a square tube if you want an off-the-shelf solution. The RS-15 tail boom is round, Strojnik motor glider uses a square tube tail boom, both extruded aluminum. You could also make a tail boom from graphite or glass per Marske's method. Stick with closed sections.
 

Riggerrob

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Cluttonfred's fourth iteration makes it much easier to rivet and install external parts (ribs, pullies, etc.).
 

Grimace

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Short version: I-beams suck at resisting torsion. Longer version: To make an I-beam stiff enough in torsion to prevent flutter (even at low speeds) it will be way too heavy. Easy access to the control cables and pulleys would come at a very high price. I suggest you use a round tube or a square tube if you want an off-the-shelf solution. The RS-15 tail boom is round, Strojnik motor glider uses a square tube tail boom, both extruded aluminum. You could also make a tail boom from graphite or glass per Marske's method. Stick with closed sections.
What do you think of cluttonfred's 4th image? An outside box beam? I'd never heard that term before, but that's exactly what I was thinking of. I was figuring a foam core of between 1/4" and 4" depending on what was needed for torsional loads.

I was only calling it an I-beam for lack of a better term. But outside box describes it perfectly.
 

Jimstix

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What do you think of cluttonfred's 4th image? An outside box beam? I'd never heard that term before, but that's exactly what I was thinking of. I was figuring a foam core of between 1/4" and 4" depending on what was needed for torsional loads.

I was only calling it an I-beam for lack of a better term. But outside box describes it perfectly.
The 4th section is better. The 4th section is typical of a wing spar. They will resist some torsion but are really meant to react in-plane bending. Ideally, if you must use a box, for most resistance to torsion, the box would be square. However, if easy access to control runs is a must, use a circular tube of glass or carbon, and put the controls outside in a fairing.
 

cluttonfred

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Hmmm...would the "caps outside" box beam or even an I-beam make sense as booms for a twin-boom type? Joining the ends of the booms to the wing center section on one end and to a horizontal tail surface at the other ought to reduce the torsional loads on the booms themselves.

11456.jpg
 

mcrae0104

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Joining the ends of the booms to the wing center section on one end and to a horizontal tail surface at the other ought to reduce the torsional loads on the booms themselves.
Not unless the vertical-to-horizontal stab connection is a rigid “moment” connection, which is difficult due to the slenderness of the intersection. Without this, the two booms can twist in unison as the h-stab sways side to side. A closed section for the boom is preferable to an I beam.
 

WonderousMountain

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You might be able to reduce torsion considerably with a symmetric or low profile tail. The large vertical is there to prevent ground strikes, and is not really necissary for usual flight training.

R (3).jpeg
 
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