Spar Sizing


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Well-Known Member
Apr 17, 2007
Also if you use two spars then the fraction carried by each spar is a function of the spar location, airfoil moment coefficient (cm), and airspeed- as speed increases the rear spar will carry a higher portion of the load.


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Staff member
Log Member
Oct 18, 2003
Saline Michigan
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.
Lift that the wing has to carry is roughly the weight of the airplane times the max g you will fly at plus the download on the tail under that g. I would check your calcs if you would tell me max g to be flown at.

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.
If you are counting on only having one-half of the lifting load in the forward spar, you will be disappointed. Most airfoils have coefficients of lift and drag and moment declared about the 25%c point - the pitching moment is pretty much flat when you measure about that spot. So, if you put one spar at 25%c and another at say 70%c, you react pitching moment by lifting against the aft spar and pushing down on the forward spar. The you react lift pretty much by lifting the forward one. Superimpose those two sets of loads and you know how the lift is distributed between the two spars. Tell you this, usually the forward one is a lot beefier than the aft one. Sounds to me like you ought to look at your max g pullup, get the Cl, Cd, and Cm and calculate lift, drag and moment, then get calculate the reactions in each spar before you size. The usual approach is to size the forward spar to carry all the bending moment... The drag spar becomes a much lighter spar by comparison.

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?
Round does not usually make an efficient spar. Beams usually look like channels or I-beams for a reason. A square is substantially more efficient a beam than a circle, but the best efficiency you can get in a solid is as tall as you can make it and still be inside, then only as wide as you need it. I for a rectangle is b*h^3/12 where b is the base or width of the rectangle and h is the height. Fit in as much height as you can, make it as wide as you need. This will likely make as light a spar set as you can get away with.