Long term build along: Beech 18 at 20% - RC Groups
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Dec 26, 2002, 07:49 PM
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SheldonYoung's Avatar

Long term build along: Beech 18 at 20%


For a long time I've wanted to build a Beech 18. It's a classy, elegant twin that deserves to be a large scale model. I am about to commit myself to the long process of designing and building a 20% (120 inche winspan) electrically powered model of the Beech.

I am now at the point with my modelling where I feel comfortable doing something this large and long term. I'm not in a hurry and I fully expect it to take a year or two before the first model is ready to fly. I'll have to do some learning along the way, but that's a good portion of the fun.

I have been collection information for a year or two on the Beech 18 and there's a full scale version at my local aviation museum (http://www.canadianflight.org) that I can use for reference. More information is always good, but both information and airframes seem readily available for this airplane.

I intend to use this thread as a sounding board, somewhere I can post my current thoughts about the design every few days and ask for help when I get stuck. If you think I'm heading in the wrong direction, speak up please. You could save me hours, weeks or even months worth of effort.

Thanks, and enjoy the ride.
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Dec 26, 2002, 08:10 PM
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SheldonYoung's Avatar

Why 1/5 scale?


Why did I chose to model the Beech 18 twin at 1/5 scale?

The biggest reason is that the Nick Ziroli Beech 18 is also 1/5 scale. Thanks to Ziroli there are a wealth of parts readily available, such as fiberglass cowls, windshield, retracts, dummy radial engines, cockpit kits and more. If the Ziroli plane didn't exist I may have done the model at 1/6 scale instead.

So why don't I just build the Ziroli plane you ask? I'll tell you anyway. It's built for 9 pounds of thumping gas engine and it's obvious. While I believe it could be converted to electric, the model wouldn't handle like it was design to. I want to compete with this model and it would just be too big of a compromise.
Last edited by SheldonYoung; Dec 26, 2002 at 08:12 PM.
Dec 26, 2002, 09:12 PM
H2OFLY

GO FOR IT !


This is one of my favorites, too.

And, wasn't it on floats, too?



Two motors, two floats, 2x 'normal' size...can't beat that!

Terry
Dec 26, 2002, 09:34 PM
"Experienced-Crasher"
Demon-Leather's Avatar

I think You can do it...


I think You can shed a LOT of useless weight from that bird... pounds & pounds of it. What kind of power are you going to use? Can You use some of those big LiPolys?.. that will shed You a few pounds too! Could You use a Ziroli fiberglass cowl as a mold & plug You up a thinner carbon fiber cowl? It doesn't have to take all that heavy vibration...there's a pound there!Snip & tuck,snip & tuck,.. You'll be in the air with a little luck! Bob
Dec 26, 2002, 09:52 PM
Registered User
SheldonYoung's Avatar
I debated re-engineering the Ziroli bird but it seemed almost as much work as designing another from scratch. You can see a bit of the structure here. It seems like it has more ply the full scale airplane.

Right now I'm estimating 26-30 cells a side, perhaps driving Mega 22/45/3s geared to turn a scale diameter prop (20") with a static current in the 45-50 amp range. Other than verifying it's possible and having an idea of the cell count, I'm not going to get too deep into the ideal power system because it's likely going to change several times between know and then.

One thing is clear, reducing weight will be paramount. There's a huge difference between a 30 pound model and a 40 pound model and if I'm not careful the weight could spiral out of control.
Dec 26, 2002, 10:02 PM
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SheldonYoung's Avatar
Terry, correct, many Beech 18s came equiped for a day at the beach. I'll have to make a point to make mounting points for floats an option in the design. Mine won't be on floats to keep the risks down.

http://ebushpilot.com/images/b18-2_6.jpg

http://www.vancouverislandair.com/ai...or%20sale.html

http://users.argonet.co.uk/users/vicsmith/B18f.html
Dec 26, 2002, 10:27 PM
"Experienced-Crasher"
Demon-Leather's Avatar

You can make Your Own "ply"


with balsa and carbon fiber laminate... Very light yet Very strong!
You could replace almost all the plywood that way, leaving only actual ply for key-stress areas like battery compartment, dihedral bracing, etc. OR.. a light box structure of spruce with carbon fiber laminate in between and "flesh" it out with balsa formers. Say You were going to build a internal box structure for the inside of the fuselage out of 1/2" x 1/2" spruce. You could laminate .014 x 1/2" carbon fiber in between 2 pieces of 1/2" x 1/4" spruce.It makes a very strong yet very light 1/2" x 1/2" stock. You can carry the re-enforcement to the extreme and add the laminate on the sides of that 1/2 x 1/2" laminated piece to form an "I" bean structure... You could beat someone to death with that stick and it would never break! The same principle is used in some full-scale ultralight aircraft construction. just a thought...
Last edited by Demon-Leather; Dec 26, 2002 at 10:30 PM.
Dec 26, 2002, 11:34 PM
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SheldonYoung's Avatar
Carbon fiber is a handy tool to have but I believe the key to lightness is in the structure itself. The best way to reinforce something is to remove the need for it.

Probably the biggest decision I'm going to have to make in this project is the internal structure. If I get it right it'll be strong and light and fly well. If I get it wrong it'll be heavy and be unpleasant to fly.

Most, if not all, of the model will be sheeted to accurately replicate the metal skin. I will probably take advantage of this fact and view the internal structure as a way to keep the sheeting in place stressed-skin style.
Dec 27, 2002, 11:06 AM
H2OFLY
Sheldon,

It is becoming obvious that you have done a lot of homework!

And - Thanks for the Screensaver(s)

Terry
Dec 27, 2002, 01:59 PM
E-Scale & Sailplane Guy
Bruce DeVisser's Avatar

plywood substitute


Demon-Leather mentioned balsa and CF - there is a lighter/cheaper way to go that is well-suited to emodels - end-grain balsa sandwiched between two layers of fiberglass cloth. This is good for stress areas (motor mounts, etc) as it will flex slightly, whereas CF tends to be stiffer.

You can make your own by finding some 4-6 lb contest balsa in 4 x 4 logs. (Not cheap wood, but lightweight glass cloth is cheap compared to CF sheet.) Slightly heavier wood is okay especially for skeleton shapes like open fuselage formers.

My preferred method is to use two sheets of 24 x 12 x 1/4-inch glass as the layup and press surfaces for a smooth surface finish - you will need a flat sturdy surface to place them on.
You can substitute other stiff materials, but you need to be able to load it with 25 - 50 pounds of sandbags during cure. I use slow-cure epoxies that have a slight amount of flexibility when cured - I want the sandwich resilient, not brittle. I also warm the epoxy slightly with a lamp to make it flow out easier.

Cut end-grain slices of balsa to desired thickness, minus allowance for the glass cloth thickness. It is critical to use a stable setup on your saw to maintain consistent thickness. (or you can ignore the cloth, and measure the thickness from your first batch, and adjust your build dimensions)

Gather a quantity of 4 x 4 squares and make sure they all fit together well, then glue/tape down wax paper (use light tack artist's spray glue to adhere the wax paper to the glass) or a coating of release agent on the bottom glass, lay out the bottom cloth (slightly oversize) and wet it out with epoxy, squeegee out all excess. Place the balsa squares in position, being careful to locate them horizontally as close as possible before seeting them - you don't want to slide them against the glass cloth as it may wrinkle. Prepare the top cloth (also slightly oversize) on the other glass, squeegee, invert the glass and position on top of the balsa. Weight it down evenly and go away for a day or two.

Remove the weight and carefully separate the top glass - a wide putty knife helps to break the seal. Repeat to remove the sandwich from the bottom glass. Trim excess cloth, wipe down with a cloth dampened with MEK or Acetone to remove any traces of wax or release agent, and you are finished.

You can also buy this material already made up in sheets from http://www.acp-composites.com/ and perhaps other sources, but it is not cheap (lots of labor!). You can also get further material ideas from their web site.

Good luck!
Dec 27, 2002, 03:20 PM
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SheldonYoung's Avatar
Thanks for the materials idea Bruce. I would like to kit this airplane if things go well so the less labour to build it the better. With any luck there won't be a need for laminates.
Dec 27, 2002, 04:02 PM
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SheldonYoung's Avatar

Wing Removal Location


Obviously with an airplane with a wing span of 115 inches the wings need to be removable. I can see three possible options:

Option 1: Split wings

The classic method of wing removable is to split the wing into halves in the middle. The main disadvantage is ensuring sufficient strength with the motors, batteries and retracts out on the wing. It may be a significant problem and will be barrier to keeping the airplane light. A obvious wing-fuselage joint is okay because the full scale has the same feature.

I'm not sure how I would access hold-down bolts, for some reason it's hard to find pictures of the underside of airplanes.

Option 2: Trips removable outside cowls

There is a joint on the full scale just outside the nacelles where the tips are removable. Access to the bolts might be through the nacelles or gear doors. The biggest problem with this method is the flaps extend past this joint.

Option 3: Trips removable outside flaps

Making the outer panels removable at the aileron-flap joint simplify the control surfaces over option 2.

The main disadvantages are the fuselage would be 67" wide, and there are metal panels overlapping the joint on the full scale. I'm not sure where the bolts would be access, there may be some hatches on the full scale.

I thought about making the rear of the fuselage removable instead, but I'd still be left hauling a 9.5 foot wing around .

Thoughts?
Last edited by SheldonYoung; Dec 27, 2002 at 04:09 PM.
Dec 27, 2002, 04:10 PM
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SheldonYoung's Avatar

Wing removal locations


Here's a graphic to illustrate the above post.
Dec 27, 2002, 04:43 PM
H2OFLY
Thinking out loud: Doesn't make sense to split the wing at the edge of the nacelles, as that would leave a (fragile) flap piece sticking out into Murphy's Law area.

I don't usually like to split a large, heavy wing in the middle...and that makes the most sense. You could use joiner tubes, have the bolt holes close enough to the middle that the fuselage would cover them.

BATTERY PACK LOCATION: I don't remember seeing anything about pack location, but after reading Bernard Cawley's latest Controlling Interest column, you absolutely want those packs in the nacelles, to avoid voltage spike problems. http://www.ezonemag.com/articles/200.../control.shtml

Terry
Dec 27, 2002, 05:14 PM
EDF Head
Haldor's Avatar
Cool project - a friend of mine has this plan and I've always desired to do it. On my own I have the Ziroli B25 plan that I've considered several times for electric. Lack of suitable workshop, funds and runway have steered me away for now.

As far as building the thing - making it light, there are several things to do. A lot of the Ply can be omitted, use only contest grade balsa. A few of the wingribs can be omitted aswell, especially towards the tip. thinner balsa skins too.

IIRC there is, atleast on the B25, a big hunky plywood spar that could be buildt very light using a composite material or balsa/foam sandwich. Or redesigned totally. Making new lighter molds of the nose, cowls is also viable options, thinner lay-ups with carbon. A lot of the weight on the glow/gas models of this size comes from the added fiberglass/epoxy surface and paint to an extent. WBPU and thinner,less cloth might be much lighter

Powerwise I was looking at ~25cells/1KW pr side.
I've seen the B25 having an AUW range from 27-41pounds and its 101" in span.
Regarding the B25 wing I intended to make it 3piece, all removable. Center section and wingtips nacelles out. Fuse 2piece.

Soo many planes - so little time....sigh


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