Building from enlarged plans. - RC Groups
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Oct 14, 2017, 09:00 AM
Bellanca Kruesair
epoxyearl's Avatar
Discussion

Building from enlarged plans.


There's been a lot of discussion about enlarging a small rubber powered aircraft into a larger one, and the suitability of the materials involved.

I'm starting with a Comet 54" Rubber powered Taylorcraft at 1-1/2"scale, and doubling it (200%) to build a 1/4 scale floater at 108 inches.
Now, forget about 'Scale'- this is going to have all the mistakes included in the original model kit. This is only for fun and enjoyable hours putt-putting about on those lazy days...It won't handle a lot of wind, and it'll certainly bust all apart if you run it into an immovable object.

I'll be back later.
Latest blog entry: Rut-roh !
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Oct 14, 2017, 09:13 AM
Registered User
Lookin' good ! If your going to that size, I see on the photo there could be room to make a hatch on the bottom of the fuse at the CG and drop a little 'cheer' for the kids. Just a thought.
Race 66
Oct 14, 2017, 09:17 AM
Bellanca Kruesair
epoxyearl's Avatar
I'm starting with the wings..Carl Goldberg used Balsa Spars in his Senior Falcon kits with a 69"wingspan. They were 1/4" hard Balsa, with a 1/8" X 1/4" spruce spar cap, top and bottom.
The main spar will be using his design, because we can laminate the 3 pieces together with our favorite glue, and clamp them perfectly straight while the glue sets.-
In this case the original spars ended up at 3/8" thick after the enlargment process. Perfect !-We'll use 3/8" X 1/2" Balsa spars, with 1/8" by 3/8" Spruce or White Pine or Bass Spar caps- your choice...I'll bet you could use hard Balsa, since they're laminated. Now we WANT to laminate them, as there's a lot of strength gained with those inner glue joints.....and you can keep them straight.

Wing ribs will be 1/8" solid balsa, or 1/8" lite ply, if you want to cut some lightening holes into them. One thing I dislike about these ribs is a 'thru-hole' for the spars..But they don't show in the finished wing. Now, there's no reason you can't just use notches like the original model did.....
Latest blog entry: Rut-roh !
Oct 14, 2017, 10:11 AM
Bellanca Kruesair
epoxyearl's Avatar
Quote:
Originally Posted by RACE 66
Lookin' good ! If your going to that size, I see on the photo there could be room to make a hatch on the bottom of the fuse at the CG and drop a little 'cheer' for the kids. Just a thought.
Race 66
yeppers- just might do that...It'll certainly go slow enough to be accurate.... Ha- candy for 'ballast' on heavy days.
Latest blog entry: Rut-roh !
Oct 14, 2017, 10:19 AM
Bellanca Kruesair
epoxyearl's Avatar
I just got back from the shop on the hill, after making the wing ribs...A Bandsaw made quick work of cutting 24 ribs, and the belt sander whipped them into shape in no time at all

I stack saw the ribs (all at once) and a 36 grit belt grinds an accurate airfoil. Changing to 80 grit finishes them off smartly.
Latest blog entry: Rut-roh !
Oct 14, 2017, 10:51 AM
Registered User
Looking good Earl. Can't wait to see how this goes.
Oct 14, 2017, 12:09 PM
Registered User
AA5BY's Avatar
subscribed.... looking forward to installments of the build.

Have you picked a power plant yet?
Oct 14, 2017, 12:21 PM
Registered User
AA5BY's Avatar
I laminated the hem fir joiner spars on a 90" biplane build that uses three piece wings. They are 1/2 x 21/16ths using a typical odd number of laminates (five). They are probably way stronger than needed but the end product still only had 17 oz sq ft loading... so no worry.

Lamination's can do amazing things. They carry high loads and resist warping.
Oct 14, 2017, 12:55 PM
Bellanca Kruesair
epoxyearl's Avatar
The engine will be an OS .90 four cycle (out of the 'Lawnmower'- don't need to 'cut grass' in the winter)

I started laminating spars just now. I used a 24" (or there abouts) 3/8" X 1/2" white pine piece on the inner half that plugs into the fuselage, and scarf joined the outer Balsa piece to it about 1/3 the way out the wing. near the strut mounts.. The upper and lower spar caps will be glued in place on both main spars at the same time, being sure they are straight and true..

Titebond 2 is my favorite glue, and it'll be used throughout the build.
Latest blog entry: Rut-roh !
Oct 14, 2017, 01:04 PM
Bellanca Kruesair
epoxyearl's Avatar
Quote:
Originally Posted by AA5BY
I laminated the hem fir joiner spars on a 90" biplane build that uses three piece wings. They are 1/2 x 21/16ths using a typical odd number of laminates (five). They are probably way stronger than needed but the end product still only had 17 oz sq ft loading... so no worry.

Lamination's can do amazing things. They carry high loads and resist warping.
Yes- thank you..In my opinion too many people are unaware of the tremendous strength in laminating a relatively weak piece of wood (or several pieces, actually) into a spar. Those glue layers add a heaping helping of strength.
Un-locatable at the moment, I have Piper Cub main spars that were laminated from two pieces of 3/8"spruce into a 3/4" X 6" Spar, from the Factory.
Latest blog entry: Rut-roh !
Oct 14, 2017, 04:02 PM
Bellanca Kruesair
epoxyearl's Avatar
Let's do some Math, for the larger Model.

1. the original model used 1/16" wing ribs, and the double sized one has 1/8" thick ribs..okay.
2. The small model used 3/16" X 3/8" wing spars, and doubling them resulted in 3/8" X 3/4" spars...We wanted them stronger, so we added 1/8" White Pine spar caps, after cutting the 3/4" spar down to 1/2". The 'capped' spars are MUCH stronger, and rigid..They will not warp easily, because the wood grains cancel any warping tendencies.
3. The 1/4" square leading edges became 1/2 " inch squares, (Duh !), and that's adequate, as well.(We are using hard stock there, not considering weight , in that area.)
4. The scaled up rib spacing became too wide for good covering support, so we're adding a couple of extra ribs in between the scaled up ones. The larger model was showing 6" spacing, and we reduced that to 4" by adding ribs. We are not interested in SCALE spacing,- just good covering support.
5. (snicker) I have some wooden wheels that R/C Robbie made for me, for another project. They'll be put into service here.
6. The 1/8" Stabilizer will be thickened to 5/16" hard Balsa Outlines, as will the Rudder and Fin.
-So far, doubling the original sizes has proven satisfactory in most cases.- and that was automatic, with the enlarging process..
Last edited by epoxyearl; Oct 14, 2017 at 07:35 PM.
Oct 15, 2017, 02:51 AM
B for Bruce
BMatthews's Avatar
The one thing I would worry about in all this is the rib spacing. I see it has half ribs which will help a lot. But I think I'd simply have made the half ribs into full ribs.

This isn't only a case of support for the covering. It is also to provide more support to the trailing edge.

Other than this one niggle I'm liking all your options for a super light slow flyer. In fact I might even have gone with 1/2x3/8 for the leading edge. Or possibly made the leading edge a "V" shape that then gets sanded to shape to give you about a 1" wide leading edge but which is hollow inside from the "V" shape laid on its side. Mostly this is about making for a cleaner line of the covering coming off the leading edge.
Oct 15, 2017, 06:25 AM
Bellanca Kruesair
epoxyearl's Avatar
The rib spacing caused some concern, and 4" seems to be a better 'fit' for this construction. I probably won't install false ribs, but just add a few 1/4" spars spaced about an inch apart, back to the spar....The Telemaster is a good example of this type of construction, and we all know how well they fly.
I'll address the trailing edge reliability with large gussets on each side of the ribs.
I'm trying to avoid sheeting anywhere, to keep the 'feel' of the rubber powered model.
I really enjoy the enlarged 25c Guillows jet-like Gliders that are becoming common, built as R/C planes.
I intend to get a glass cowl, and make formers to fit inside, so a Balsa cowl can be fabricated. I only have plans, (no parts) so there are no formers shown to enlarge. If anyone else wants to build one of these, that difficult problem will be solved, with glass...ME?- I'm all over a Balsa cowl.

I have saved the slate from a Pool Table, and one section is being used as a building board. I'm not qualified to build a straight wing panel, but spot gluing the ribs to the table may rescue me.
Latest blog entry: Rut-roh !
Oct 15, 2017, 01:38 PM
Bellanca Kruesair
epoxyearl's Avatar
I modified some of the ribs for the Aileron openings, allowing for a 3/16" sheet to close the ends of the ribs in that area...The Ailerons showed at 24-5/8 ", and I didn't want to cut extra wood up, so I shortened them to 24" overall. That allowed me to get the pieces I needed out of 1 sheet, 48" X 4".

I'll use Klett pinned hinges with a back up block in three places.
Last edited by epoxyearl; Oct 15, 2017 at 01:47 PM.
Oct 15, 2017, 04:12 PM
AndyKunz's Avatar
Well, I have a 72" Aeronca here scaled up from Comet plans that tells me you want a stronger spar - I'd go with top and bottom spruce with a web between them.

It fell from the sky when the spar failed at the strut.

Andy


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