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rich_aero

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I am designing a model airplane and plan to use a spruce spar. I used the schrenk approximation to calculate the the moment and shear stress along the semi-span. I found the moment at the wing root to be 2681 lb-in, which seems about right considering review of other design reports. For simplicity I modeled the wing spar as a simple cantilever beam and considered this moment of 2681 lb-in. Using a circular cross sectional area (.75 inch diameter spruce wood dowel) I found the stress at the surfaces of the dowel to be 65,661 psi. This far exceeds the compressive or tensile yield strength of any wood spar materials. At this point I am kind of stuck and am not sure how to accurately size the cross section of the wing spar. Any help would be appreciated.
 

BJC

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How big is the airplane?

The root moment sounds high for a light weight, limited span, model.


BJC
 

wsimpso1

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Moment estimate rule of thumb to check your bending moment calc. Max g intended times one half airplane weight times 45% of the semi-span. Do it in pounds and inches and you get in-lb. 10 g, 20 pound airplane, 60" semi-span is 2700 in-lb. Sound about right? Check your moment against this quick check...

Checked your spar stress math, and it appears about right. Circular spars are way inefficient...

What makes you think a 3/4 spruce dowel ought to be big enough? If the moment checks out and it has to fit in a 3/4" space, you will be investigating Graphlite rods for caps and graphite cloth for webs.

Billski
 

BBerson

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Aluminum tube spars are frequently used about to half span. More efficient than solid wood.
 

mcrae0104

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What is the wing root thickness? For example, a 6:1 aspect ratio Hershey bar wing with 60" span has a 10" chord; with a 12% thick airfoil, that's 1.2" available for your spar depth. Depth is your friend. For comparison, a rectangular spruce spar would need to be about 1.2" wide at 1.2" depth to carry your moment. Either I'm way underestimating your available depth or this is one heavy model. An increase to 2" of depth gets your spar width down to 0.2" (70% weight savings over the shallower one)

Also, how much factor of safety do you want to build in, and is it already baked into your root moment?
 

TFF

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Hmmm. A 3/4” dowel is a pretty big chunk of wood for a model. Is the intent to make plug in wing panels? A more conventional model spar would cut a good bit of weight.
 

Pops

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I have one spar fail with models. It was a TLAR wing design on a 65" span Semi-Scale F-86 with the swept wings. Spruce spars in a foam wing wrapped with 1/16" sheet balsa and 1 foot of the center section wrapped in fiberglass. Powered by a Super Tiger- G60 racing engine on high nitro.
In a vertical dive at WOT of about 800' and pulling out inverted with a low pass down the runway of about 4' and straight up to do it again. My RC flight buddy standing beside me said, that wing is going to fail. Next dive pulled to about 45 degs down when it was like an explosion when the wing folded and crushed the rear half of the fuselage and tails. Engine stuck in the ground and parts came floating down.
TLAR designing just doesn't get it.
 

Rockiedog2

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Pops it sounds like you were determined to tear the thing up...vert dive, WOT, 800 feet...a real F86 wudda come apart too. Were you a teenager when you did that Pops?
I tore up a buncha stuff too. Great fun. But the most fun of all is dynamite.
 

Pops

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Pops it sounds like you were determined to tear the thing up...vert dive, WOT, 800 feet...a real F86 wudda come apart too. Were you a teenager when you did that Pops?
I tore up a buncha stuff too. Great fun. But the most fun of all is dynamite.
I just thought that it would hold up . It didn't. I was about 34 years old. At that time I was flying RC in completion and practicing many hrs a week . Mixed my own fuel in 5-10 gal lots.
But that was just a fun airplane.

Chapter in my book about playing with dynamite. Father bought a case of dynamite and a bag of caps and told me ( at 12 years old) and my older first cousin that lived with us ( he was 15 years old) and a neighbor boy ( 16 years old) to blast out a water cistern in solid rock and he would be back home in about 3 weeks and wanted it finished.
For some reason Grandfather stayed out in his shop beside the barn working on building a new wagon he was building most of the time we were blasting.
 
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Rockiedog2

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What book Pops? Send a link. Hope you wrote about all those broken bones. I know guys who died in combat that didn't get hurt as bad as you have.
 

Pops

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What book Pops? Send a link. Hope you wrote about all those broken bones. I know guys who died in combat that didn't get hurt as bad as you have.
Just a few ( 15) chapter written and I need my granddaughter with an English major to correct it for me.
 
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ragflyer

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The bending stresses vary by the cube of the diameter for a circular rod spar. To bring the stresses down from 65,661PSI for a 0.75" spar to 9600psi which is max stress for spruce (a difference of 6.8 times) you would need to increase the diameter of the spar by a factor of 1.9 (cube root of 6.8) , in other words the spar would need to be 1.42" (0.75"*1.9).
 

rich_aero

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Thank you all very much for the advice. With your advice I made some new calculations. Here's some more info on my plane design. It's being designed for a school competition, that requires a plane to carry a very large cargo weight. Basically, a plane that just barely flies. The smaller the wingspan and the more weight carried, the higher the score in the competition. The plane is to build a 70" wingspan plane with a 14 inch constant cord length and a selig 1223 airfoil. Takeoff weight goal is 33 pounds. Using the schrenk approximation technique I was able to determine a bending moment of 2714 lb.-in at the root of the wing. Considering this moment and the modulus of rupture of spruce (approximately 10,000 psi) and a single solid round spar I found the need for a 1.6 in diameter spar (safety factor of 1.5). If using two spars, I assume I can divide the moment in half, which led to a 1.2750 inch diameter spar. I plan to use two spars one placed at 25% chord length and one further back on the airfoil rib. For calculations I am using the bending stress equation: sigma=Mc/I. My calcs for the round dowel style spar require quite a large dowel. Does anyone have any recommendations on using a round dowel spar vs a solid square or rectangular spar?
 

ragflyer

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If smallest span and greatest weight is what wins per your competition rules then biplanes or triplanes/multi planes makes the most sense.
 
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