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Old Dec 29, 2010, 11:23 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Discussion
Skeletonized Foam Airframes Thread

There seems to be more interest lately in skeletonized foam planes. So, I thought it might be worth starting a thread where people can post:
  • their planes
  • their techniques
  • links to threads on their planes, etc.
  • people can ask questions, share info on covering materials, or whatever

Ok, let the posting begin ...

Gordon
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Last edited by Gordon Johnson; Dec 29, 2010 at 11:34 AM.
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Old Dec 29, 2010, 11:24 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Cutting by hand

First, I want to make the point that it does not require a laser cutter to make a skeletonized foam plane. All it takes is something to punch out corner holes, a sharp knife, and a bit of patience.

Here's how to cut out a wing. Although I did the wing for my Aileron Vapor (now called a VaporBender) on my laser cutter, it is not necessary to have a laser to make this wing. I've cut out similar stuff by hand for years before I had a laser.

All that's required are some sharpened brass tubes. I use K&S tubes from a hobby shop, purchased years ago. I sharpened the outside end of the tubes with a file. Then, I sharpened the inside with a hobby knife. I wrap mine with duct tape on the other end to keep them from cutting into my palm.

Print out the two wing halves from the plans. It helps to print on colored paper so it will show up better on white foam. Cut out these templates with a sharp hobby knife. Dust the back of each template with a minimal amount of 3M77 or similar spray adhesive. It only takes a little and you want to be able to remove the template afterwards. Apply the templates to the foam to be cut.

I demonstrate here for only one wing half. You would want to butt the two wing half templates against each other and cut out a one piece wing.

Using whatever diameter brass tube comes close to the radius of each inside corner, punch out the corners with a pressing twisting motion. The tube diameter does not have to match the corners on the plan exactly. Next, using a very sharp hobby knife cut out the sections between the punched out corners. Then, cut out the perimeter of the wing. And, your skeleton wing is done ready to be covered.

I spent a total of 10 minutes punching holes and cutting out one wing half. So, it really doesn't take that long to make your own skeletonized wing.
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Last edited by Gordon Johnson; Dec 29, 2010 at 01:54 PM.
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Old Dec 29, 2010, 11:25 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Covering Weights and Skeleton Ratios

Once you have a skeletonized airframe part, fuselage or wing, or tail, you need to cover it or skin it with something. The two main choices are some sort of mylar or film covering or thin foam. Here's a table with weights per square inch for various foams and covering materials. By combining these with a skeleton ratio of how much foam material can be removed in the skeletonizing process you can estimate your airframe parts weight and how much weight you have saved.

See Post #11 for an example of how to estimate the weight savings over solid foam from using the skeleton method.
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Last edited by Gordon Johnson; Jan 26, 2011 at 12:30 PM. Reason: expanded list of mylar coverings
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Old Dec 29, 2010, 11:26 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Skeletonized Toon Jet

Here are some pictures of my current work in progress skeletonized version of my Toon Jet. Span is 11 inches.
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Old Dec 29, 2010, 11:26 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
11-inch DR-1 Triplanes

in the Fall of 2010 I built these two 11-inch versions of my larger DR-1 tripe.

The clear mylar version used 3mm Depron for the fuselage, 1.1mm Zepron for the wings and tail, and 2um clear mylar covering. Propulsion was a 4mm gear drive using watch gears (built by Ralph Bradley), cell was either 30mAh or 60mAh, RX was ZTRON, and control was HingeAct. Weight was about 9 grams. Thread is here.

I built the second (blue) version of the 11-inch size in December. It uses 6mm Depron for the fuselage for more stiffness, a BSD 7mm gearbox, Vapor brick, and RA Microlite covering. Weight is 18.7g and thread is here. This tripe is intended to be a zippier version.
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Old Dec 29, 2010, 11:27 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
16 in DR-1 Tripe

I built my first version of a DR-1 Cartoon Tripe in the Summer of 2010. The goal was a really really low wing loading, which came in at about 0.65 oz/sq.ft.

Fuselage is 6mm Depron, wings are 3mm Depron, covering is RA Microlite, propulsion is BSD 7mm gearbox, battery is 60mAh or 90mAh, RX is ZTRON 900mhz, and servos are Falcon Femto. Wing span is 16 inches and final weight came in about 26g. Thread is here and also contains plans.
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Old Dec 29, 2010, 11:29 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Aileron Vapor (VaporBender)

In the Spring of 2010 I built this plane for my son, who wanted something like my AirBender to fly. It's based on a standard Vapor, but all that's left is the fuselage stick, wing pylon, and gearbox. Weight came in at 18g, 3g more than a standard Vapor. RX is AR6400 4-channel with external servo, Battery is 60mAh FR. Wing and tale shapes are adapted from the AirBender. Thread is here, which contains plans.
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Old Dec 29, 2010, 11:29 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Air Bender

I built the Air Bender in Winter 2010, after months of tweaking the design and specs. This plane was intended to be aerobatic but fly really really slow. So, it uses really really extreme equipment. RX is 900mhz HipHop from Nick Leichty as are the 0.65g each servos. Propulsion is Gasparin G10 brushless, and controller is Micro Invent. Battery is 90mAh FR. Span is 18 inches and weight is 19g. Wing Loading is 0.95 oz/sq/ft and thrust/weight is about 1.3 to 1. Thread is here, which also has plans.
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Old Dec 29, 2010, 11:31 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Tiny Dancer

I built the Tiny Dancer over the Spring/Summer 2009 period. I started the design before the AR6400 brick and external servo first became available in the PZ Sukhoi. With this plane I wanted to try a skeletonized aiframe skinned in very thin foam. I used Durobatics 025 foam for the skin. Essentially I lowered the airframe weight down to about what it would be if I had milled pockets in the Depron, about 30% lighter. Wing span was 18in, and final weight was 37g. Propulsion was Mighty Midget BL 1-cell. The thread is here.

This plane has been retired. It hit too many walls while I was learning to turn with my left thumb. I'll probably do a second version with clear mylar covering for my son in the next year or so. I offered that we could build him an Air Bender or Tiny Dancer and he chose this one.
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Last edited by Gordon Johnson; Feb 08, 2011 at 12:18 PM.
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Old Dec 29, 2010, 11:32 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Skeleton Sport

I built this plane in the Fall of 2008. It was my first plastic covered skeleton foam plane. It is based on BSD's Mega Sport plane, which I have and fly outdoors on a regular basis, and sometimes indoors. I was curious what I could do with the Vapor brick and lighter construction to get the wing loading down lower.

This plane uses 6mm Depron for the fuselage and 3mm Depron for the wings and elevator. On this plane I cut slots in the wing and fuselage to accept 1/32 ply strips to stiffen the struction. I've largely abandoned this technique in subsequent planes and use external CF rod bracing instead.

Covering was colored Reynolds Wrap. I have not used this covering since this plane since it is so much heavier than RA Microlite and WES Technik clear mylar.

Propulsion was MM single cell BL with a BSD converter for the Vapor brick to control a BL controller.

Its thread is here
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Old Dec 29, 2010, 11:33 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,429 Posts
Estimating Skeleton Weight

Ok, now an example to help estimate the weight of a part before building one. However, if you keep notes over time you'll develop your own measurements and rules of thumb.

Let's take my Airbender wing as an example. The wing span is 18 inches and wing area is 102 square inches. 3mm Depron weighs 0.08g per square inch, from the table in post #3. So, if the wing were made from solid 3mm foam it would weigh about 8.15g.

My skeleton wing uncovered weighed 2.25g. This works out to about 27% of the weight of the solid foam wing. This is what I refer to as the "skeleton ratio". For those cutting by hand I think a rule of thumb of about a 35% skeleton ratio is more realistic, and really very little different. Of course, the more foam you leave in your design the higher the Skeleton Ratio will be.

So, for a particular plane, take your wing area, times the skeleton ratio you think you will be able to achieve, and multiply it times the weight per square unit of the foam you will be using and this gives you an estimated weight for the skeleton.

Next, estimate the weight from the covering. In the case of my Air Bender I used the WES 2um film, which weighs 0.00142 g/sq.in, and multiplied by 102 square inches gives a weight for the covering of 0.15g.

Add the weight of the covering to your weight for the skeleton and you have the weight of your wing, or other part. There is, of course, the weight from the 3M77 Adhesive. But, it's marginal if you dust it on, which is all that's needed, and the adhesive weight can be ignored.

So, to recap for my Air Bender:

Skeleton Wt = Area x SkeletonRatio x FoamWt
= 102 x 0.27 x 0.08
= 2.2g

Covering Wt = Area x MylarWt
= 102 x 0.00142
= 0.15g

Total Est Wing Wt = Skeleton + Covering
= 2.35g

Use this same technique for tail surfaces and fuselage and related pieces such as side stiffeners and you can estimate how much you can save on the weight of your airframe. If CF rods are to be used to add strength back in then they should be factored in.
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Last edited by Gordon Johnson; Jan 01, 2011 at 09:59 PM.
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Old Dec 29, 2010, 11:36 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
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temp10

temp10
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Old Dec 29, 2010, 11:46 AM
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West of California Speedway, East of LaLa, North of Disneyland
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Here's my non-laser method for making templates:

I draw or print from a plan the wing, fuselage, tail pieces, etc. I cut around the outline, leaving about 1/2 inch outside the perimeter of the part. I then spray some 3M Spray Mount onto the backside, and place the template-to-be onto a piece of thin cardboard (I save empty cereal boxes for this, as they make nice firm templates). Now cut the template with scissors - and be sure to try out your sharpened tubes to make the corners rounded.

As far as sharpening tubing, I have used the method that Gordon suggested, but I use a round file and sharpen the inside diameter. However, I am going to purchase a tapered reamer this afternoon from the local Ace Hardware. Typically, a T-handled reamer will range from 1/8" at the tip to around 1/2" near the handle.
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Old Dec 29, 2010, 02:06 PM
Pile-it
Alien_Tech's Avatar
United States, CA, Palmdale
Joined Nov 2007
1,692 Posts
I'm spoiled...I have a laser I used Parklite covering, I know its heavy, but it's all I had. Looking forward to trying some mylar.
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Old Dec 29, 2010, 07:36 PM
TheyreComingToTakeMeAway!
derk's Avatar
USA, ID, Coeur D'Alene
Joined Dec 2003
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very cool thread Gordon! i'm sure it wont be long until i have one to post myself
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