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Old Feb 10, 2013, 09:07 PM
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Mark,
I am not visualizing "...chordwise" 1/8" square basswood doublers to the top sparcap...". Would you mind adding a bit more description?
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Old Feb 10, 2013, 10:39 PM
MrE
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Same question here. Im not able to picture how that would be done.
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Old Feb 11, 2013, 10:40 AM
Dragons, Windmills. All Same.
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Well, this is what I see. Sorry for the rough drawing as I just whipped this up with MS Paint and my very limited talents.

Jared
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Old Feb 11, 2013, 11:03 AM
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That is what I had envisioned when I read the suggestion. It is a great idea and does not add much weight.

My original intent was to do less work on the spar by just buying direct carbon replacements. I love to build but have limited time so I like to stay with laser kits that slot together without too much customization. I find these small tasks take considerable time for me to implement (five hours thinking about it before the 30 minutes of actual labor). All fun but I need to stay on track to get the job done.
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Old Feb 11, 2013, 11:09 AM
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Based on Marks suggestion of adding strength to the top spar only -

Is it correct to assume that the top spar cap is the weaker component due to compression vs. tension and therefore, if I wanted to do less work but add some strength to the wing, I could just add the carbon tape to the top spar and skip on the bottom?

Or do you really need to do both, even though the bottom cap is using lighter carbon tape? Not sure how much the disparity between the two creates problems.

Or use the solid carbon rod on the top and use bass wood on the bottom? Then, the extra weight is only maybe an 1 once. Does this bring the structure back into balance in some way?
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Old Feb 11, 2013, 11:48 AM
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Quote:
Originally Posted by TOSSPilot View Post
Based on Marks suggestion of adding strength to the top spar only -

Is it correct to assume that the top spar cap is the weaker component due to compression vs. tension and therefore, if I wanted to do less work but add some strength to the wing, I could just add the carbon tape to the top spar and skip on the bottom?

Or do you really need to do both, even though the bottom cap is using lighter carbon tape? Not sure how much the disparity between the two creates problems.

Or use the solid carbon rod on the top and use bass wood on the bottom? Then, the extra weight is only maybe an 1 once. Does this bring the structure back into balance in some way?

If your goal is to save time and money, and the way to do it is by adding carbon to only one side, then most definately add it to the top.
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Old Feb 11, 2013, 03:45 PM
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Because fibrous materials have roughly twice the tensile strength that they have in compression, I modified a lot of my models in the '70s by doubling the top spar cap in thickness... They were a lot stronger and the weight penalty, especially when tapered toward the tip... wasn't much.

My biggest problem of late is not spar strength, but flutter resistance... and my heavy winch-pedal foot...

Darth Carbon Lover...
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Old Feb 11, 2013, 04:05 PM
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Originally Posted by TOSSPilot View Post
That is what I had envisioned when I read the suggestion. It is a great idea and does not add much weight.
My original intent was to do less work on the spar by just buying direct carbon replacements. I love to build but have limited time so I like to stay with laser kits that slot together without too much customization. I find these small tasks take considerable time for me to implement (five hours thinking about it before the 30 minutes of actual labor). All fun but I need to stay on track to get the job done.
One 1/8 x 3/8 x 48 bass sparcap weighs about 18g. The same size in carbon will weigh 60g, which is a lot of added weight and also insane overkill. The Mirage wing would shred in many other ways before that sparcap would fail.
You can add a 0.014" carbon strip instead, but that's prone to delaminations, and will require making the spar notches deeper.

Adding the 1/8" square doublers will double the wing's bending strength with a minimum of fuss, and you can still use wood glue for everything. The 1/4" shear web on the Isthmus Models kit should be just barely adequate for the doubled load, since the original Mirage had a 1/8" shear web.

Extending the sparcap notches in a handful of ribs is pretty easy, and it doesn't have to be precise -- two quick cuts with an X-acto will do it. Easier than deepening all the notches for the alternative carbon strip.
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Old Feb 11, 2013, 04:09 PM
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Originally Posted by TOSSPilot View Post
Based on Marks suggestion of adding strength to the top spar only -
Is it correct to assume that the top spar cap is the weaker component due to compression vs. tension and therefore, if I wanted to do less work but add some strength to the wing, I could just add the carbon tape to the top spar and skip on the bottom?
Roughly doubling the sparcaps' bending strength is all that's reasonable IMO, because the rest of the wing won't be able to take much more than that. So making the spar even stronger is in effect adding dead weight.
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Old Feb 11, 2013, 04:16 PM
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Originally Posted by schrederman View Post
My biggest problem of late is not spar strength, but flutter resistance... and my heavy winch-pedal foot...
On the Mirage, flutter resistance is obtained by using a stiff covering like regular Monokote on the center panel to give it torsional stiffness.
It's OK to use Ultracote-Lite on the tips, but be sure to scallop the trailing edges if you do so. Light trailing edges = increased flutter resistance.

Lighter tips also reduce yaw inertia which improves handling. On a RES glider you cannot make the tips light enough.
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Old Feb 11, 2013, 10:02 PM
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Originally Posted by markdrela View Post
Lighter tips also reduce yaw inertia which improves handling. On a RES glider you cannot make the tips light enough.
I agree that tip weight is a definite handling problem. I read once that you recommend glassing balsa D-tubes on a bias on the Bubble Dancer, for flutter resistance. Just the center panel or the entire wing? Also what weight glass. cf, or Kevlar would you recommend?

Sorry for somewhat hijacking the thread...

Darth Carbonator...
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Old Feb 12, 2013, 12:22 AM
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Mark:

Thank you for the follow up and bringing reaffirming the ease of installing the bass spar cap doublers. After considering the overall design and the intented use of the plane, it makes sense to be modest with this modification. So, I will give it a try.

I had debated about doing the scallop trailing edge and whether it added much weight savings but had not considered other benefits regarding flutter. Good to know this step is worth the effort.

Craig
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Old Feb 12, 2013, 12:36 AM
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Originally Posted by schrederman View Post
Sorry for somewhat hijacking the thread...

Darth Carbonator...
Good next topic since it came up. Hijack away

On the same topic, what about diagonal cross braces in the bays behind the spar? How much benefit does this provide vs. the added weight?
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Old Feb 12, 2013, 09:31 AM
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Good next topic since it came up. Hijack away
On the same topic, what about diagonal cross braces in the bays behind the spar? How much benefit does this provide vs. the added weight?
Mostly pointless.

One way to make the Mirage wing mostly flutter-proof is to add a D-tube to the center panel out of light 1/16" balsa. The upper part needs to be faceted to preserve the turbulation effect of the stringers. The bottom can be curved as usual. Most of the added work is to remove 1/16" from all the center rib outlines ahead of the spar to make room for the sheeting. But all the ribs are the same, so all the ribs can be modified in one stack. The 1/8" square bass stringers are replaced by 1/16" x 1/8" very hard balsa stringers. The faceted D-tube is made by installing flat balsa "planks" which are butted over the stringers. You start at the spar and go forward. Then trim the overhang from the LE, and finally install the LE strip. This is actually easier than installing one curved D-tube sheet in one piece.

This mod adds essentially no weight, because the basswood stringers go away, and you can also now use a very light covering for the center panel rather than Monokote.
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Old Feb 12, 2013, 09:37 AM
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Originally Posted by schrederman View Post
I agree that tip weight is a definite handling problem. I read once that you recommend glassing balsa D-tubes on a bias on the Bubble Dancer, for flutter resistance. Just the center panel or the entire wing? Also what weight glass. cf, or Kevlar would you recommend?
Sorry for somewhat hijacking the thread...
Darth Carbonator...
Glassing just the center panel D-tube with 0.75oz bias glass should be sufficient for most RES ships which have 1/16" sheeting. This will make a better match to a very strong spar with carbon.

But the BD has 3/32" 8-10lb (firm) balsa sheeting on the center panel, and really doesn't need this glass for flutter resistance. It may be good for insurance to suppress splits in the D-tube sheeting from landing dorks. Such splits do degrade the torsional stiffness quite a bit.
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