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Old Dec 28, 2014, 10:56 AM
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Balsa spar configuration

For sake of arguement lets assume that I'm going to put a full span spar into a foam wing. I slice the wing in two along the 25% line say, so that I now have two pieces, the leading edge and the trailing edge. The kerf on the saw blade for this cut relieved 1/8" from the foam. I then notch the foam at the cut to fit in a 1/4" square strip top and bottom of both the front and rear foam. This series of cuts gives me the volume occupied by an "I" beam spar which I will glue the foam onto recreating the original airfoil with the spar in place. The main portion of the spar is cut from 1/8" balsa. Here's my question: do I want the grain going up and down just like in stress webbing, or do I run spanwise? I'm leaning toward the spanwise config, but not sure. Any thoughts?

Opps! Forgot to add this in. The wing will be 1/16" balsa sheeted. The spar will be glued to the sheeting as well as the foam.
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Old Dec 28, 2014, 11:36 AM
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Vertical grain.. Always. If going horizontal grain then fitting caps to it is close to being redundant.
Only purpose of a web in an I beam is to ensure the stringers do not buckle.
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Old Dec 28, 2014, 11:40 AM
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Gotcha!
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Old Dec 28, 2014, 01:00 PM
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Span wise.

And use 1/4" balsa
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Old Dec 28, 2014, 01:24 PM
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I'm going to suggest that "sometimes vertical" may be appropriate. Only you the designer builder can say what you are trying to accomplish.

In your stated case I would bet that running the web grain spanwise is more than adequate for all practical purposes. And within that general configuration there is no obvious benefit for one practice over the other. Sometimes we tend to think we "must" be/do things a certain way, when they are are not, in reality, required.

I've repaired and flown a number of full size aircraft with I beam spars. Most seen are routed timbers with the web grain having to run spanwise. Ply webbed box and I beam spars are, as a general rule, stiffer than their timber counterparts . And yet, some of those have their ply faces aligned vertically, while others have them span wise.

In general, the purpose of the web is two fold. First and formost is to keep the upper and lower flanges of the I beam from moving past one another, span wise. The second is to keep the two flanges from deforming in their relation to each other. In most typical wing situations the upper flange wants to bend inward, toward the lower flange, while carrying flight loads. The web only needs be strong enough to resist crushing. So, 1/32" sheet web, with vertical grain orientation, may be quite adequate for your needs, yet the practicalities/ease of construction might warrant thicker material and or an alternate grain orientation.

All in all, general principles and practices of engineering are a good thing good to keep in mind, but they are not all encompassing rules. Quite often practical considerations out weigh rote thinking.
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Old Dec 28, 2014, 07:12 PM
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In a sheeted foam core wing, a significant portion of the structural strength comes from the sheeting, assuming it is properly, and evenly adhered to the foam substrate. So, in this case, spanwise spar webs should be adequate.

As a comparison, in a woodie sailplane wing with typical spar construction of uppern and lower caps, and a web inbetween them, vertical grain is essential in order to provide the strength needed.
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Old Dec 29, 2014, 10:18 AM
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The "in flight" stress in a spar web is always 45 degrees to the vertical or horizontal axis. As a result, it makes no difference if you are worried about flight stress. However, vertical webbing is more immune to hanger rash as that stress is usually perpendicular to the wing; i.e. dropping stuff on the wing, squeezing it when working on it etc. There have been lots of articles written on constructing home built planes and all suggest that the spar webbing be with the grain of the web be at a 45 degree angle to the span. Realize that this stress also switches 180 degrees depending on whether the stress is positive (normal flight, positive G load) or negative (landing loads or inverted flight, negative G load).
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Old Dec 29, 2014, 12:10 PM
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Regarding the grain of the shear web...hears something I posted a while back: http://www.rcgroups.com/forums/showp...6&postcount=31

Cheers,
Brian
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Old Dec 29, 2014, 12:17 PM
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If its a foam core fully sheeted wing, no sheer webbing is my way.

Ive done everything from 25size sport models to 40% 3D models with only a foam core with 1/4" sq. balsa spars and 1/16" sheeting on the wing.
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Old Dec 29, 2014, 12:55 PM
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I agree... just cut the 1/4 spar notches in the top and bottom... forget the webbing. My answer is somewhat predicated by the use of balsa spars, which indicated that not a lot of additional strength is anticipated. Easier yet.... a strip of carbon fiber top and bottom under the sheeting if you've got it.
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Old Dec 30, 2014, 12:31 AM
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Most folks just sheet the wing and use the skin as the spar. If you're expecting serious G loads then splice the sheeting together with a middle sheet that is on the hard side to act as more of a spar. But even then I doubt anyone bothers. If you want to gild the lily this way then make the center portion of the spliced sheet about 20% chord wide and centered over the area of maximum thickness. The foam core will be your webbing.

The only real reason to do a spar the way you're describing is for a thin section airfoil. Like less than 10% and with a relatively high aspect ratio.
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Old Dec 31, 2014, 09:43 AM
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What Bruce said.

A foam core model aircraft wing that has been completely sheeted is immensely strong for its weight and size. A spar is almost never needed.

More reasonable recommendations can be made if you give more information... span, chord, weight of finished model, etc.

Tom
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Old Dec 31, 2014, 10:56 AM
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Tom, the plane is a Kaos variant. it has a span of about 64" weight is anticipated to come in at 7 1/2 lbs, +/- . I'd guess velocity at times of 100 mph, pull both pos and neg G's. As I intend on flying pattern manoveurs with the plane, and judges are rather fussy, I'd like the plane to be as consistent as possible, so that I can blame only myself (or the judges) for my low scores. This means stiff throughout, but for now I concern myself with wing flex only. Ever see the range of motion on the B-52 wing tips? Don't want that for this one.

After reading over these posts, I am leaning toward a pair of partial span joiner spars to help pick up the gear loads and keep the wing in 1 piece. I figure on using full span cf strips top and bottom that tie to the spars, foam, and sheeting. 1/16" sheeting glued with urethane, vacuum bagged.
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Old Dec 31, 2014, 11:27 AM
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Your last idea of a sub spar is just what folks have done for many years of foam wing use on similar models. The carbon strip is just "gilding the lily". But if you want to use some it won't hurt other than making things a bit more complicated to make.
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Old Dec 31, 2014, 11:39 AM
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Quote:
Originally Posted by sdad View Post
Tom, the plane is a Kaos variant. it has a span of about 64" weight is anticipated to come in at 7 1/2 lbs.
Ok, the original Kaos had a built up D-box wing and that wing is NOWHERE NEAR as stiff as a sheeted foam core.

And they never had a problem with wings folding up.


Quote:
This means stiff throughout, but for now I concern myself with wing flex only. Ever see the range of motion on the B-52 wing tips? Don't want that for this one.
Comparing one of our small model's wings with the flex that is intentionally built into B-52 (and most airliners) wings is comparing apples to antelopes.

You would have to go to great lengths to intentionally introduce that flex. Really great lengths. Really, really great lengths.

Quote:
After reading over these posts, I am leaning toward a pair of partial span joiner spars to help pick up the gear loads and keep the wing in 1 piece. I figure on using full span cf strips top and bottom that tie to the spars, foam, and sheeting. 1/16" sheeting glued with urethane, vacuum bagged
Those spars are primarily there for transmitting landing loads to the fuselage, which is a different thing than flight load induced wing flex. I would have those there myself for that purpose.

Most modelers never build a test sample of anything to determine destruction limits. I am not an exceptional builder, and would never claim to be, but I have built MANY test wings (and other parts) to determine load carrying ability and overall stiffness and strength.

I have used balsa sheeting and bagged fiberglass (which is what I use exclusively now, no need to sheet with balsa with a proper glass layup, but that is another issue). What I found was that most foam cored model aircraft wings are grossly over-engineered (and hence way heavier than they need to be) for the loads to which they are subjected.

For example your cored, sheeted, Kaos wing will never "flex" as usually meant by the word "flex" very much (and your B-52 example). It may indeed fail, but the failure will be instantaneous with very little deflection and the failure will be in compression of the top surface (opposite the applied load). But that failure with the wing sheeted as you said you were sheeting it is almost guaranteed to never happen, even without any full span spar at all.

Tom
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