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Old Feb 12, 2013, 04:26 PM
aeajr is offline
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Craig,

If I read this right, you are working on a new build, a kit. Right?

As far as I know, from my limited building experience, adding carbon to the spar caps is not a lot of work. Sand down the spar the thickness of the carbon and epoxy it on under a little pressure. Whatever thickness you use you add strength, be it compressive or tensile, and you don't need a lot if you just want to add some insurance. Probably .021 or less on the top and nothing to .007 on the bottom. Again, whatever you add is stronger than what you remove.

Wrapping should not take long. It is just there to reduce the chance of delamination, or so I am told. If you are not building an Uber launch machine you could even wrap the spar with unwaxed dental floss which costs next to nothing. Not as strong as kevlar but that strength is probalby not needed anyway.

Also, if I read this correctly you are just trying to add a little extra insurance in case you over pedal the winch. Is that correct? Or are you looking to make it full pedal launch capable?

The other people in this thread are the super builders. They will correct my silly assumptions, but, I don't think this is a lot of work. As you said, 5 hours of thinking and 30 minutes work. I do the same thing and wear myself out overthinking things.

Very interesting discussion. Going to follow along.
Last edited by aeajr; Feb 12, 2013 at 04:51 PM.
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Old Feb 12, 2013, 04:45 PM
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Quote:
Originally Posted by markdrela View Post
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.
Gee... but it does look cool...aka Sagitta... Got to love the look of a Sagitta w/ transparent covering.... Interesting though...

Thanks Mark.
Old Feb 13, 2013, 12:16 AM
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Quote:
Originally Posted by aeajr View Post
Also, if I read this correctly you are just trying to add a little extra insurance in case you over pedal the winch. Is that correct? Or are you looking to make it full pedal launch capable?.
That is mostly correct. My underlying goal is to build more retro planes but I want to learn about opportunities to improve the strength of these models using modern materials without signficantly altering the original appearance. I want the added strength as a hedge against wing failure on a "normal launch".

For example, you are at a contest away from home. The line has no stretch, the head wind is strong and a boomer thermal is moving through the launch zone. I think they called it the 'THE WALK OF SHAME" as I returned from digging my missle out of the lawn.

I was seriously considering just dropping in solid carbon spars because it was easy. With all things, I suspected that this "lazy" approach had draw backs, which was highlighted in this discussion.

Thanks again for all the feed back and help.
Old Feb 13, 2013, 07:34 PM
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Quote:
Originally Posted by aeajr View Post
Craig,

Sand down the spar the thickness of the carbon and epoxy it on under a little pressure.
You don't want to sand the spar. You are much better to adjust the rib notch to suit the added carbon fiber.
Old Feb 13, 2013, 08:05 PM
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Quote:
Originally Posted by TOSSPilot View Post
That is mostly correct. My underlying goal is to build more retro planes but I want to learn about opportunities to improve the strength of these models using modern materials without signficantly altering the original appearance. I want the added strength as a hedge against wing failure on a "normal launch".

For example, you are at a contest away from home. The line has no stretch, the head wind is strong and a boomer thermal is moving through the launch zone. I think they called it the 'THE WALK OF SHAME" as I returned from digging my missle out of the lawn.

I was seriously considering just dropping in solid carbon spars because it was easy. With all things, I suspected that this "lazy" approach had draw backs, which was highlighted in this discussion.

Thanks again for all the feed back and help.

Ah, the memories.....
Old Feb 13, 2013, 09:53 PM
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Quote:
Originally Posted by markdrela View Post
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.
I would love to build a second Mirage wing using these ideas. I have managed to flutter mine more than once but got lucky with little damage.

I am a little unclear on exactly where the 1/16x1/8 hard balsa strips go in relation to the sheeting.

The first image shows a Mirage rib. The red lines show where to trim 1/16" off for the sheeting. The lower cuts would follow the airfoil shape and the top cuts would be straight? Or should they also follow the airfoil?

Then do the 1/16x1/8 balsa strips go back into the slots where the old 1/8x1/8 basswood was or??

Thanks for all your contributions to the hobby!!



Old Feb 13, 2013, 10:57 PM
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Quote:
Originally Posted by MrE View Post
The first image shows a Mirage rib. The red lines show where to trim 1/16" off for the sheeting. The lower cuts would follow the airfoil shape and the top cuts would be straight? Or should they also follow the airfoil?
The top cuts are straight. Ideally the bottom cuts follow the airfoil, but this isn't critical.

Quote:
Then do the 1/16x1/8 balsa strips go back into the slots where the old 1/8x1/8 basswood was or??
After you trim the ribs, the new slots are 1/16x1/8. That's where the balsa strips go. The top D-tube plank edges join together over the strips. The bottom D-tube sheeting ideally is curved, and is installed in one piece.

Since the center is glassed, you can put a butt joint there. So the D-tube can be made from 24" long sheets which are easier to edge-fit together.
Old Feb 14, 2013, 12:02 AM
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Thanks Mark!!!

That should be a very simple and valuable mod
Old Feb 15, 2013, 10:32 AM
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middle of the road method


Quote:
Originally Posted by TOSSPilot View Post
That is mostly correct. My underlying goal is to build more retro planes but I want to learn about opportunities to improve the strength of these models using modern materials without signficantly altering the original appearance. I want the added strength as a hedge against wing failure on a "normal launch"..
Although the Skybench BigBird/Oly3 woody's have a d-box wing and very strong as is, Ray added this "how-to" and this one to his website for those who wanted a bit more spar strength insurance. This solution may work for you as a middling choice between strength and appearance.
best wishes, js

ps - Mark Millers tap-tap-tap winch method is the easiest solution of all

Following picture linked from Skybench.com website
Last edited by jswain; Feb 15, 2013 at 10:38 AM.
Old Feb 15, 2013, 10:59 AM
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I notice in that photo there are no thread wraps. Is that a finished spar or is there something to be added?

It also looks like the carbon appears to be below the spar cap rather than on top. Is that the reason there is no wrapping?
Old Feb 16, 2013, 01:02 AM
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I wouldn't use full cf spars , just add .030 thick x 3/8 wide cf , full width shear webs , and wrap the whole thing with Kevlar thread ...
Old Feb 17, 2013, 07:07 PM
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And then strengthen the tips, and the joiners, and the wing hold system, and the fuse bottom (so the hook doesn't pull out), and the fuse sides (so they don't separate on tow), and the stab (so it does't blow off in a zoom)...
Old Feb 17, 2013, 07:27 PM
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Let's keep in mind here that the model in question is a Mirage. A woodie RES ship. As I understand, the goal of the OP is to strengthen the wing a bit, making it more able to take a moderate winch launch.

We could debate all day on how to build the strongest possible spar system, but in the end we need to keep in mind the intent and expected use. I rather doubt anyone is planning on launching a Mirage full-pedal with a wicked zoom at the top. But I've been wrong before....
Old Feb 18, 2013, 09:37 AM
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Quote:
Originally Posted by TOSSPilot View Post
That is mostly correct. My underlying goal is to build more retro planes but I want to learn about opportunities to improve the strength of these models using modern materials without signficantly altering the original appearance. I want the added strength as a hedge against wing failure on a "normal launch".

For example, you are at a contest away from home. The line has no stretch, the head wind is strong and a boomer thermal is moving through the launch zone. I think they called it the 'THE WALK OF SHAME" as I returned from digging my missle out of the lawn.

I was seriously considering just dropping in solid carbon spars because it was easy. With all things, I suspected that this "lazy" approach had draw backs, which was highlighted in this discussion.

Thanks again for all the feed back and help.
Quote:
Originally Posted by LVsoaring View Post
Let's keep in mind here that the model in question is a Mirage. A woodie RES ship. As I understand, the goal of the OP is to strengthen the wing a bit, making it more able to take a moderate winch launch.

We could debate all day on how to build the strongest possible spar system, but in the end we need to keep in mind the intent and expected use. I rather doubt anyone is planning on launching a Mirage full-pedal with a wicked zoom at the top. But I've been wrong before....
Yes. let's always keep in mind the goal of the discussion. Essentially making the wing a little stronger, not build a Supra.
Old Feb 18, 2013, 04:49 PM
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Quote:
Originally Posted by aeajr View Post
Yes. let's always keep in mind the goal of the discussion. Essentially making the wing a little stronger, not build a Supra.
I was kidding. My point was, don't add 0.030 CF to the spar. There is no point over build one component of a balanced structure.


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