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Old Jul 11, 2008, 06:03 PM
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Build Log
Bird of Time Build

I am getting ready to build a Bird of Time kit from Dynaflite. Kit was bought about two years ago (3/28/06) from Tower Hobbies for $64.99. I plan on using an E-Flite 480 (910 Kv) brushless motor on the front for power. If it works out right I will be looking at the 1100mAh A123 cells to power the motor. One thing that I am worried about is the reports of the 1100mAh A123s failing open. I know the 2300s fail shorted which may push me towards using the 2300s. I have a CC PHX45 laying around but might get a PHX35 for this. I am thinking I will use Hitec HS82s for the elevator and rudder. For the spoilers I am thinking of an HS55 in each wing. I will be replacing the spars for CF and balsa spars. The wing will be bolt-on instead of rubber banded. For a radio I will use XPS 8 channel receiver and CC-BEC to power the receiver and servos. Or at least thatís the plan as of right now.

I have open the kit up and started by labeling the die-crunched parts since there is no markings on them. The balsa seems fairly dense. Some of the parts may get remade with better balsa. The precut balsa blocks seem to be pretty good. The 1/4" ID Arrow Shafts will be replaced with 1/4" ID brass tube since the Arrow Shafts fit is way to loose. I will probably not be following the manual since I do plan on using CF for the spars and wrapping the joiner box with thread. Also I will use ply shear webbing for the joiner box that runs the length of the joiner box instead of inner and outer web shears. I have thought about CF for the outer wing panels spars but I don't think it would be worth it. I have ordered some tapered CF from CST for the inner spars. The box states 41 oz (no motor) but from looking at the balsa in the kit I think 50 oz (no motor) is closer. So with a motor and battery 60 oz should be achievable which puts the wing loading at about 7 oz/sq ft.

Alan
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Old Jul 13, 2008, 12:44 PM
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Today I punched out some of the ribs for the inner wing panels. The ones that are nearest to the joiner (AA) are heavy and fairly rock hard. With several over 4 grams per rib. So I found some medium balsa from the scarp bucket and cut afew ribs out to replace the AA ribs. I have only cut three out so far but all of them are under 1.5 grams. After seeing this I think I will cut all the ribs out which should save a good bit of weight in the wing.
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Old Jul 13, 2008, 08:46 PM
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Fairlie, New Zealand
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I'd vote for the 2300 cells rather than the 1100. I don't think the BoT would notice the weight. 3s or 4s?

Don't you just hate the wrong balsa being supplied in kits? That's why I don't build them, with the exception of the DJA Chrysalis i'm doing at the moment, but they handpick their stock for those.

BTW unwaxed dental floss around your joiner boxes will lay flatter and cause less of a bump in the covering.
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Old Jul 13, 2008, 09:20 PM
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I think I will go with 4s. For joiner box I will wrap the box then the sheeting will go over it on top and bottom.

Today I have been cutting out the ribs for the inner panels and so far all the ribs in the kit are 73 grams and the ones I cutout are 28 grams. Big savings and none of the balsa I used was marked light or very-light. So If you wanted it even lighter it would be possible. Tomorrow I will work on cutting out the ribs for the outer panels and wing tips.

The balsa for the fuselage sides is very heavy, but is that an item that reducing the strength for a weight savings is worth doing? Is the heavier balsa actual any stronger?

Alan
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Old Jul 13, 2008, 09:29 PM
I eat only vegetarian beef
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Fairlie, New Zealand
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Hard to say without seeing it, but usually wings are underengineered and fuselages over engineered. I always build to fly, not to crash, so the only area I use hard balsa is stick wood like longerons.

Other than just lighter flying weight, less weight at the extremities means a lower moment of inertia (the barbell effect) so better response to lift, faster response to control input and less drag in turns.

Nick
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Old Jul 20, 2008, 08:40 PM
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Ok, so far I have just been cutting parts out and making the spars. For the Spars I used a piece of 1/8 balsa sheet and cut 3/8" wide strips, fairly stiff balsa. Then I epoxied CF to the spars and sanded the balsa done so that the spars are 1/8" thick. This is the procedure that was recommended by Skycruiser:
http://www.rcgroups.com/forums/showp...38&postcount=6

I used CF from CST that is 48" long and starts at .060" and tapers to .014. I cut this in have and used the .060" to .037" for the top spar and the .037" to .014" half for the bottom spar.

Attached is a spreadsheet that I have started to keep track of the difference in weight of parts so that I can compare it to the final weight of the BoT.

Alan
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Old Jul 20, 2008, 09:18 PM
I eat only vegetarian beef
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Fairlie, New Zealand
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Rule of thumb is that pultruded cf is 30 times stronger than spruce of the same cross section. So your 0.060 on top is equivalent to a spruce spar 1.8 inches thick! Assuming of course that it doesn't buckle, which is why it's recommended to wrap the spar/web/spar with kevlar thread.
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Old Jul 21, 2008, 07:50 AM
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Skycruiser,
I do plan on wrapping the joiner box with thread. I just have not put that together yet.

Alan
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Old Jul 21, 2008, 03:05 PM
I eat only vegetarian beef
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Fairlie, New Zealand
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Unwaxed dental floss will be stronger and lay flatter.

Nick
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Old Aug 01, 2008, 10:51 PM
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I have started the left wing inner panel. I am about ready to wrap the joiner box with thread. While building this up I did replace the 3/8" web shears in the kit with some 3/8" light balsa and also ran the joiner box from the root out instead of the several pieces in the kit.
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Old Aug 01, 2008, 11:28 PM
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I'll throw a vote in the hat for the 2300ma A123 M1 cells. I have several gliders using 4S M1 packs I made up myself, and they're quite good.

Bulletproof is the word I'm looking for.

I also have a smaller foamie nite-flyer that I use a 4S A123/VPX pack (1100ma) on, and that's fine. They're good cells for a 125W set up, but beyond that I think you'd be disappointed. Just not enough capacity.

The BOT can easily handle the M1 cells. I'd go with those.
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Old Aug 02, 2008, 08:49 AM
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I have been running a 4s1p A123 2300 mAh pack in my Spirit 100 at about 550 watts and it is doing great. The only reason I was thinking about the 1100s was for weight savings, but if they don't do well above 125 watts then the 1100s are not worth thinking about. Thanks for the information.

Alan
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Old Oct 04, 2009, 07:53 PM
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It has been awhile. Work and all. Here are some pictures of an almost completed wing. I have just started on the stab.

Alan
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Old Oct 26, 2009, 05:24 PM
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Pictures of the stab.
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Old Nov 03, 2009, 07:24 AM
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looks great so far
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