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Old Dec 29, 2009, 07:42 AM
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Albuquerque, NM
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Don
When I did my first Yardbird build, I built the mid panel according to the plans from Mountain Models--spruce on top, hard balsa on the bottom, shear web in between. Unfortunately, my hard balsa was not very mean--in fact, in retrospect, I'd say it was quite wimpy. The wing then failed right at the end of the joiner box. The joiner box itself stayed together quite well. The break occurred right at a stress riser.

If I understand what is meant by a 'double D' shear web, I didn't do that (shear webs on either side of the spar?). My spar in the mid panel is just spruce laminated withe carbon and a shear web in between.

Jack, So I'm not sure just what plan Mountain Models is selling now. Maybe I built a Yardbird I, even though the website said Yardbird II? Maybe it is time just to publish a single update which shows all of the improvements the Yardbird needs to avoid incidents like mine.

BTW, my Yardbird, which I think I'll dub the YardPHX (as I was able to salvage the outer wing panels and the fuse and tail) is ready to rise again (maybe over Phoenix in February?). It's not quite as nice looking as the first one, as I used some old MonoKote someone gave me, and whether it was the age of the stuff, or I just got used to working with UltraCoat, it didn't cover as nice. Well, from 200 meters, who will notice?
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Old Dec 29, 2009, 02:09 PM
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USA, FL, Winter Haven
Joined Nov 2004
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Double D Shear Webs

Richard: The Double D box section is not shown in my plans either, however, they are talked about in the Yardbird Build thread written by Jack. There are excellent photos of the Double D shear webs on Post #10, 11 and 32 of this thread. Please take a look at it and let me know if you did any of those things.
Don
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Old Dec 29, 2009, 02:59 PM
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United States, TX, Weatherford
Joined Nov 2002
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Don,

I only did one model with the double D box because I found that they were not really needed. With proper building techniques, and making sure the upper sheeting is glued to the ribs properly, there will be no flutter problems. That's what the double D was designed to prevent. The were effective in stiffening this wing in torsion, but it is stiff enough already...

Jack
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Old Jan 01, 2010, 11:43 AM
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Double D

Jack: I understand the purpose of the Double D as I read it in your previous post. Not being an engineer, however, it does seem reasonable to me that the addition of the 1/16 shear webs and carbon forward of the spar would add some strength to the wings span-wise. Am I correct? I'm just trying to figure out how the failed wings were built. Are there a number of Yardbirds successfully flying without failure that were built according to the original plans?
Happy New Year - Don
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Old Jan 01, 2010, 12:01 PM
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Don,

Absolutely, the double D will add some span-wise strength. That isn't transferred between panels because there's no joiner to carry this through. Yes, there are Yardbirds successfully flying built per original plans. However, those with a heavy foot on the winch are encouraged to add some carbon to the spars of the intermediate panels. If you want to really increase the strength, add some additional thickness to the cf in the main panel, and substitute carbon for wood in the intermediate panels. There WILL be a weight penalty for all of this.

Without having been there and watched the builders... it's impossible for me to say how they were built and why they failed. Some of the early Hawk builders complained about flutter. Mine never fluttered so I began inquiring as to how the upper sheeting was attached to the ribs... only to find out that they were using contact cement and other less-than-desirable methods. On some of the failed Yardbirds, I suspect that the spar material was not hard balsa. It really has to be the meanest, hardest stuff possible, or just substitute bass or spruce and let it go at that. Some people don't know how to pick wood, either, so it's hard to say why they failed.

Jack
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Old Jan 01, 2010, 03:09 PM
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Double D

Jack: Thanks very much for the thoughtful answer. At this point, I feel pretty confident that my Yardbird will do just fine, if I don't get too heavy on the peddle. All of my upper skins are attached with epoxy. I found the hardest 1/8 x 1/2 balsa that I could. I found it in a close-out lot at the LHS. I used a Bubble Dancer pod, boom & V mount obtained from Bud Elder, with a modified Bubble Dancer vertical. My stab is a foam Supra that only weighs one ounce. All up flying weight is 49.5 ozs. I haven't put it on the winch yet, but from hand tosses, it seems to fly very nicely. Again, thanks for all your input.
Regards, Don
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Old Jan 01, 2010, 03:11 PM
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Thanks isn't good enough... we need a PICTURE!!!

Jack
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Old Jan 01, 2010, 03:50 PM
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Frankly, I haven't figured out how to attach a picture to these posts. If you will send me your email address, I know how to forward a pic that way!
Regards, Don
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Old Jan 02, 2010, 08:15 AM
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Don,

After you finish typing your reply, there is an additional section BELOW the Submit Reply button called Additional Options. In that box is a button called Manage Attachments. Click on that, and you will get a popup window with the ability to Browse (find the picture on your computer) and Upload (post it into your reply). Don't worry about the Advanced Upload unless you are attaching a number of pictures at once. After you see your picture come into the popup window, go ahead and close the window and submit your reply.


Pics are a requirement on RCG.

Dion
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Old Jan 02, 2010, 04:39 PM
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Yardbird

My newly completed bird.
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Old Jan 02, 2010, 05:23 PM
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Looking GOOD!
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Old Jan 02, 2010, 06:06 PM
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USA, TX, Houston
Joined Mar 2004
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The Womack Air Wing

......was spotted in the Houston Area today tearing up the sky. The Number 2 Houston Hawk skied out after a pop-off and proceeded to attract Turkey Vultures in its air space. And an electrified Yard Bird showed us all how to cruise the field.

Here's an old crew chief showing off the planes. Great flying today, AJ!
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Old Jan 02, 2010, 06:29 PM
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Great to see AJ out flying!
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Old Jan 16, 2010, 03:11 PM
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Albuquerque, NM
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YardPHX Flies!!!

My rebuilt YardPHX has risen from the ashes. Got a couple of 8 minute flights after our club Hiss&Boink today. Flew real sweet-rode up the winch line without a hint of flex. Since I built about 1/8 inch washout into the mid panel, there was no hint of tip stall like the previous incarnation. And my landings with spoilers were almost as good as a flapped bird. (Well I do need to put a nose tooth on it). So the YardPHX is ready for PH(oeni)X
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Old Jan 16, 2010, 03:12 PM
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That's fantastic!
Congrats.

Target
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