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Author Topic: Hummingbird Goon  (Read 1279 times)
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aerotrope
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« on: January 09, 2021, 06:36:43 PM »

 . . . not a small gangster bird, but the 24" span short kit classic air racer, from Hummingbird Model Products.  Thought I'd get at it while we're still avoiding the pestilence.

A fair amount of instructions and details on the plan.  Ribs take full advantage of laser cutting - quite intricate.
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Russ Lister
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« Reply #1 on: January 09, 2021, 08:00:14 PM »

As you say, intricate ... but I like it.
I suppose the glue:balsa ratio goes up, but overall it must be lighter and of comparable strength to a normal full balsa rib.
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aerotrope
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« Reply #2 on: January 10, 2021, 01:20:24 AM »

I'm trying to be careful with glue.  I use a little plastic micro-tip dispenser on the tube, the opening is about .020".  Parts fit so far is just snug, so that helps minimum gluing.
The kit balsa is fairly light, maybe 6 or 8 lb? I've stripped wing spars from a sheet of 7 lb and it seems comparable.
Laser cutting is good.  I'm needing to do a little knife work to get clean parts releases, but the lines are very thin and almost free of char, only a pale tan.  I prefer cutting to sanding anyway, and this cleans up with a swipe or two of 240 grit. 
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BG
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« Reply #3 on: January 10, 2021, 12:32:05 PM »

Thrilled to see someone doing a build/review of one of my kits. I will be watching with interest and will hope to make improvements or adjustments if you find issues.
A note on wood: I try to buy wood in the high 5# to high 6# density range. In a pinch I will use low 7# wood for parts where I think a bit of extra strength would be good. As a general rule the wood in my kits falls in this range.
A note on laser cutting: I try to minimize kerf (cut line thickness) and scorching as much as possible and generally prioritize this over perfect and uniform penetration. For very consistent sheets penetration is uniform and complete. For wood that had a lot of density variation across the sheet the penetration will be more variable and require a bit of work with the blade. Personally I am not bothered by this (I guess I enjoy the process) but I understand that customers buy laser-cut kits to minimize the need for cutting parts out (especially complex parts like those ribs). I am always battling this issue because more power solves the penetration problem but tends to result in more scorch and a wider kerf. The scorch is cosmetic a wider kerf effects parts fit and so I prioritize this aspect.

Final note: I finally got the tissue done for this design I will provide you with a tissue set when the times comes. Also the new 3D printer in my life might be handy for the spinner and some scale details. 

BG
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« Reply #4 on: January 10, 2021, 03:24:17 PM »

BG: You must be a patient man to design ribs like that. I wish you'd been involved with the design of the kit I'm currently building, but you would have needed a time machine.
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« Reply #5 on: January 10, 2021, 07:06:42 PM »

I am glad some of you like the intricate built up ribs. I should add that ordinary sheet ribs are also an option for those interested in purchasing my kits.

BG

 
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aerotrope
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« Reply #6 on: January 11, 2021, 12:53:41 AM »

Centre section and left wing almost framed - just need to trim and glue spars to the wingtip.
When starting the centre section I found that I'd used one of its 1/16" thick ribs for the right wing root, so I continued the theme for the left side. I'll add some 1/16" sq diagonals to brace the root ribs as a tissue wrinkle preventive, something I usually do anyway.
The right panel came out at 3.6 gm before and 3.3 gm after sanding
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aerotrope
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« Reply #7 on: January 13, 2021, 11:41:30 PM »

Here is the 1/16" sq anti-tissue-wrinkle bracing on the root rib. Total weight added is .08g

Also a pic of the stab coming together.
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DavidJP
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« Reply #8 on: January 14, 2021, 03:22:56 AM »

My, that does look a lovely kit.  Even I could be tempted but doubt I would cover it.  One of those presented to me in my formative years would have been marvellous.  Well done Bernard and aerotrope.
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OZPAF
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« Reply #9 on: January 14, 2021, 04:53:46 AM »

I agree - it looks a really enjoyable kit to put together.

John
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aerotrope
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« Reply #10 on: January 21, 2021, 12:21:36 AM »

Tail feathers.  1.85 grams for sanded stab/elevators .

I'm going with r/c DT, so am hinging the elevators rather than making the full stab pivot shown on the plans.
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aerotrope
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« Reply #11 on: January 21, 2021, 07:31:31 PM »

Fin and rudder .6 gm ready to cover.
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aerotrope
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« Reply #12 on: January 26, 2021, 12:19:51 AM »

First fuse side going together.  I used some firm 8 1/2# wood for the longerons and uprights 3 through 6.  The other uprights are all 6#.   Matched uprights for the second side are in the line above.
The 4th and 5th uprights will be partly cut away after the lateral stringers and wing saddles are in place.
The plan indicates a target max weight of 65 g - I'm hoping to keep it down to 50 or 55.
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aerotrope
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« Reply #13 on: February 09, 2021, 12:11:17 AM »

Both sides done and boxing up, centre section first then tapered nose and tail sections.  I cracked the longerons at the bends, then hardened the breaks with thin CA after assembly.
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aerotrope
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« Reply #14 on: February 09, 2021, 12:45:41 AM »

Two-part formers to orient the grain for strength all the way across.  Bottom formers being glued up and attached to the fuse. 
At this point I realized that all the uprights and crosspieces are lasered on this sheet, so I needn't have cut my own. First short kit I've seen with any sticks at all. (note to self - pay more attention)
My only change to the design so far is the addition of 12 gussets for a bit of extra torsional stiffness. Total weight cost was .2 g.
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BG
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« Reply #15 on: February 10, 2021, 02:38:27 PM »

Looks good Julian. I was wondering why you were using your own cross-pieces. I should have mentioned something. Most of my kids now have much of the strip wood precept to length.

B
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aerotrope
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« Reply #16 on: February 10, 2021, 08:23:50 PM »

I'm still wondering too.  Oh well . . . onwards.

Photo is of engine bay being prepped with temporary cross bracing to maintain squareness during the rough-and-tumble of sheeting.  I'm intending to sheet the bottom (currently up) first, then remove the fuselage from the board, attach the top formers and stringers, then sheet the top cowl.

Also, something I meant to mention in my last post is that the stringer notches in the formers are all cut half-depth, thus saving all that tedious sanding of former edges so they don't show through the eventual covering.
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aerotrope
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« Reply #17 on: February 22, 2021, 12:43:55 AM »

Back to the airplane factory.
I molded top and bottom cowlings out of 1/20" 6 pound sheet - soaked overnight then taped to a Corona beer bottle to dry - just the right curvature.
Bottom cowl is two sheets divided by centre stringer.  Top cowl is one piece, fitted by sanding 1/20" off upper centre stringer, a fairly minor variance from the plans.
Pic on building board shows half of bottom cowl, pic on floor shows full top cowl.
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aerotrope
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« Reply #18 on: February 23, 2021, 12:55:49 AM »

Two views of the left wing saddle and associated stringers.  I had a nice sheet of 5 1/2 pound wood to strip all the top and side stringers from.  Bottom stringers are 7 or 8 pound as that's (hopefully) the first part to be caressed by the welcoming earth.  For the same reason, I'll be going with tissue over mylar on the bottom, for the added puncture resistance.
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dosco
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« Reply #19 on: February 23, 2021, 08:02:48 AM »

Very nice!
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aerotrope
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« Reply #20 on: Today at 01:22:20 AM »

Nasal work.

Nose block, prop block and spinner stack.  In a moment of brain fade I failed to photo the nose block before carving, so this shows the rough sanded state, then a bit better sanded, then internally carved.  Started out at 4.6 gm, now down to 2.7 gm.  Spinner turning tomorrow.
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