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Author Topic: Rolled Balsa Fuselage - Polecat X  (Read 1071 times)
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ScienceGuy
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« on: November 18, 2016, 01:22:05 PM »

One of the planes I am currently building is the Polecat X p30, wing and stab are built, next is the fuselage. I have never built a rolled balsa fuselage, so any advice might be helpful.

http://scienceguyorg.blogspot.com/2016/11/model-airplane-building-in-full-swing.html  My blog post about what I am building.

Bill Kuhl
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frash
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« Reply #1 on: November 18, 2016, 01:58:15 PM »

One of the recent NFFS Digests has a good article on rolling tubes by Chuck Markos. Also on the Bradley (Ralph and Paul) site there is an article on rolling tubes for No-Cals. You may be between these two articles is size of your plane.

Fred Rash
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ScienceGuy
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« Reply #2 on: November 18, 2016, 02:03:24 PM »

Thanks Fred, I will look for the article.

Bill

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frash
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« Reply #3 on: November 18, 2016, 02:07:21 PM »

There is also a good one somewhere by Jim O'Reilly. He rolled the wet expanded balsa blank around a form that was slightly too large so that the edges would not meet even when wet. When dry and the wood had shrunk back to size, he put the blank on a form that was slightly too small, HELD IT VERTICALLY, slid the wood slightly off the bottom of the form, carefully tacked the bottom seam with instant glue, slid the balsa down, and repeated the process. The only danger is that you glue the balsa almost tube to the form and have to start over.

There are many other instructions for rolling tubes, but these three seemed to be the best for me.

Fred Rash
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USch
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« Reply #4 on: November 18, 2016, 02:24:19 PM »

I normally cover the inside of the fuselage sheet, cut grossly oversize, with Esaki and dope before rolling the fuselage. Then I put the sheet, cut slightly oversize, in the bathtub or any container with warm water and leave it for some hour. At this point I roll the sheet around a brass tube of the right diameter and spiral wrap a cloth ribbon on it. Cloth ribbon and not plastic tape because cloth let transpire the evaporating water during the drying. The whole thing is placed in an made up carton or polistirene box with one or two lights of 30-40W inside, controlled by a dimmer. Temperature about 40-50°C. Leave overnight to dry completely. Next morning I take off the ribbon, cut the blank to size and glue the seam. Before starting the gluing session put some candle wax to the mandrel, otherwise you may glue the fuselage to the brass tube !!! Do not glue outside of the mandrel if you like straight fuselages. This is particularly true if you build conical tail booms.

Urs
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calgoddard
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« Reply #5 on: November 18, 2016, 03:56:42 PM »

Cut a rectangular blank of balsa wood sheet with the appropriate length and width.  The plan should give the length of the rolled fuselage, which will be longer than the length of the typical P-30 rubber motor (19 inches).  I recall that the length of the Polecat fuselage is quite a bit longer since Don DeLoach flies with a 6 x 3/32 inch rubber motor for a very long motor run.

Some people suggest A grain balsa, but it’s hard to find and I have had very good results with C grain. Either way, the grain should extend parallel to the length of the forming mandrel (discussed below).  I feel that 1/32 inch thick balsa sheet is a little on the skimpy side for a P-30 fuselage, so I suggest 1/20 inch sheet thickness.  7 - 8# density sheet balsa is good.  If you are keen on making the 40 gram minimum, you might need to use carefully selected 1/32 inch wood with a 6-7# density for the rolled fuselage.  

The ID of the rolled fuselage needs to be large enough to accommodate a blast tube that can comfortably surround a wound 6 x 1/8 inch rubber motor.  For a P-30 I would probably use a ¾ inch OD Aluminum pipe or PVC pipe as a forming mandrel.

I used to cut the balsa wood sheet blank so that its width was larger than the circumference of the mandrel. The edges of the sheet overlapped when rolled, and after drying, I had to carefully perform a straight cut to get perfectly abutting joined edges.  This required a jig.  Now I just make the width of the blank slightly smaller than the OD of the mandrel so that there is a tiny gap between the opposing side edges of the blank.  

Consider spray painting the inside of the sheet with lightweight flora spray paint to resist the absorption of motor lube.

Heat the balsa sheet in boiling water for 10 minutes.  I make a temporary water tray out of Aluminum foil. I don’t add ammonia to the water (hate the smell and ammonia is poisonous).

 Wrap the mandrel with parchment paper - this is silicone coated paper used for baking cookies.

Use old cotton bed sheet to firmly wrap and hold the balsa sheet around the mandrel.  I use strips of masking tape to hold the sheet firmly around the mandrel.  Let this combination dry overnight.  You can put it in an oven at 200 degrees F. for 10 - 20 minutes or use a hair drier to speed up the process if you are in a hurry.  

Remove the tape and bed sheet.  Line up the abutting edges of the tube and wick CA into the joint as you squeeze the abutting edges of the sheet together. Be sure to reinforce the motor peg holes with 1/64 plywood sheet. It’s a good idea to glue a ring of 1/64 plywood around the front end of the tube.  

Some folks cover the balsa tube with Esaki tissue and dope the same for added strength but this adds weight.  

 

 

 

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Hepcat
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« Reply #6 on: November 18, 2016, 06:44:35 PM »

What puzzles me is why anyone should want to make a circular fuselage when a proper rectangular one is so much superior.
<smile>
John
 
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USch
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« Reply #7 on: November 19, 2016, 05:02:13 AM »

What puzzles me is why anyone should want to make a circular fuselage when a proper rectangular one is so much superior.
<smile>
John

Because round is beautiful  Kiss
and a round section is the geometric shape with the shortest circumference for a given area =  less material/weight with equal strength

Urs
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hastf1b
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« Reply #8 on: November 19, 2016, 05:17:57 AM »

Less weight o.k. But you have to create a support pad / pylon which also brings back weight. And the tailboom is still missing.

Heinz
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DavidJP
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« Reply #9 on: November 19, 2016, 06:39:55 AM »

If it is of any help I made my first and so far only "balsa tube" short while ago using much the same technique as described above but perhaps not so elaborately - for example I sprayed the wood once it was on the former with water and let it dry naturally.  On removal it simply curled up almost ready to be joined. In fact it was quite easy to be successful first time!!

The pictures show first the situation after doping on the tissue and the second after removal from the former when the water damping had dried audit has ben trimmed.  By the way I used the former again to "cyno" the two edges together remembering first to wrap the former in a type of cling film I had. (it was little thicker than the normal stuff and whine I spread over plans when building.  My instructor (Spencer Willis) suggested that when cutting the edges to be joined it is an idea to angle the blade so the two eyes are bevelled and give a little more surface area.
Attached files Thumbnail(s):
Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
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Hepcat
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« Reply #10 on: November 19, 2016, 07:49:11 AM »

Urs my good friend,
I am afraid that you are the 'Engineer hoist by his own petard'.  Round is beautiful? Model aeroplanes used to be beautiful when modellers gave their creations curvaceous bodies and one could be differentiated from another.  Like boats, aeroplanes are female.  Have you ever seen a beautiful female with a body like a broomstick?  Just think of Sophia Loren and tell me what similarity she has to a balsa tube.
John
 
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DavidJP
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« Reply #11 on: November 19, 2016, 08:44:33 AM »

May I join you there John?  Everything in our society today is so uniform.  I wonder if original thought is evaporating rapidly.  Thankfully being the age I am I doubt I will witness the final outcome.  Mind you a lot of competitionmodels are bought now and so probably come from the same stable.  And if that is the way to be competitive than that is it I suppose?

Incidentally the tube I showed above was for a D/F model. 
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USch
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« Reply #12 on: November 19, 2016, 08:53:26 AM »

Just think of Sophia Loren and tell me what similarity she has to a balsa tube.
John

Dear John,

of course I would not recognize Sophia Loren looking like a broomstick, but the same applies to a Sophia Loren looking like a crane  Grin

Urs
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Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
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DavidJP
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« Reply #13 on: November 19, 2016, 10:30:40 AM »

Amusing Urs,  and quite flattering that crane engineers should copy from model aircraft structural design.
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ScienceGuy
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« Reply #14 on: November 19, 2016, 12:42:20 PM »

There is an article in the latest Aeromodeller magazine about building strong fuselages. The author said the round tube fuselage squashes easily.

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calgoddard
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« Reply #15 on: November 19, 2016, 01:14:39 PM »

Hepcat -

As usual, there is much merit in your comment (Reply #6).

I have built P-30 models with slab, stick and rolled fuselages.  See the attached pictures.

In my personal experience a rolled balsa wood fuselage did not help me get closer to the 40 gram minimum weight. A rolled fuselage requires the extra structure of either a wing saddle or a wing pylon.  Any purported strength advantage of a rolled fuselage does not seem to improve flight performance.

On the other hand, the P-30 models designed by a number of world class fliers, including Bob White, Stan Buddenbohm, Don DeLoach and Clint Brooks, have rolled balsa wood fuselages.    
Attached files Thumbnail(s):
Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
« Last Edit: November 19, 2016, 02:19:31 PM by calgoddard » Logged
Hepcat
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« Reply #16 on: November 19, 2016, 11:22:11 PM »

Urs,
I have to acknowledge a great riposte in #12.  You may have been 'blown up by your own bomb' but you had a parachute ready to get down again; and to do it with my own fuselage design was a masterstroke.  I shall probably sulk for a month or so before venturing any more opinions.
John
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USch
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« Reply #17 on: November 20, 2016, 05:27:07 AM »

.......I shall probably sulk for a month or so before venturing any more opinions......
John


 Grin Grin Grin Grin Grin

John,
amici come prima!
as a penalty I let you translate this one. You already challenged me with 'Engineer hoist by his own petard'

Urs

PS for Bill ScienceGuy, sorry for disguising your original argument
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TRuss
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« Reply #18 on: November 20, 2016, 10:44:23 AM »

There is an article in the latest Aeromodeller magazine about building strong fuselages. The author said the round tube fuselage squashes easily.



Things can be quite strong but still crush easily, especially thin walled tubes.  An empty 12oz beer or soft drink can support an adult man, (a thin adult man with good balance)vertically, as long as there are no dings in the can.  As soon as he touches the sides of the cans with a quick poke it will perfectly collapse upon itself.  It's a great party trick if you can pull it off.  My point is that measuring the rolled tube fuselage's resistance against crushing forces in no way invalidates it's strength under the loads and forces that it will face in regular use and flight, which are for more likely the loads that is was designed to withstand rather than being designed to withstand crushing forces.  
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Maxout
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« Reply #19 on: November 21, 2016, 07:57:55 AM »

In my personal experience a rolled balsa wood fuselage did not help me get closer to the 40 gram minimum weight. A rolled fuselage requires the extra structure of either a wing saddle or a wing pylon.  Any purported strength advantage of a rolled fuselage does not seem to improve flight performance.

On the other hand, the P-30 models designed by a number of world class fliers, including Bob White, Stan Buddenbohm, Don DeLoach and Clint Brooks, have rolled balsa wood fuselages.    

Durability is king in classes like P-30. Not to be disparaging, but my experience with a variety of fuselage types has shown pretty conclusively that people who think a stick built fuselage (or event a rectangular sheet fuselage) is stronger than a rolled fuse of the same weight are grasping at straws to avoid having to build broomsticks. It just ain't so.

Rolled fuselages can also be built lighter if appropriate wood selection and sizes are utilized. If you're not flying in wet conditions, tissue covering on rolled tubes is unnecessary. It is helpful if you have some weight margin, since extra strength never hurts.

For further evidence on this matter, take a look at the trend in indoor cabin toward rolled tube fuselages over stick built. There's no minimum weight in that class, by the way--he who builds lightest has the advantage.
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calgoddard
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« Reply #20 on: November 21, 2016, 10:24:25 AM »

Maxout -

Thanks for "weighing in" on this topic.

The torsional strength of a rolled balsa fuselage seems quite superior to that of a rectangular balsa fuselage of the same weight and length and where the OD of the rolled fuselage is the same, or even slightly smaller than, the width of the rectangular fuselage.

As is so often the case in our hobby, optimizing the rolled balsa fuselage depends on selecting the proper quality and size of balsa wood.

When I painted the exterior of my P-30 rolled fuselage with flat black floral spray paint, it was mistaken for carbon fiber composite from a distance Smiley

Here are a few more pictures to help ScienceGuy. The wing pylon in the third picture is set up for a pop-off wing DT.
Attached files Thumbnail(s):
Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
Re: Rolled Balsa Fuselage - Polecat X
« Last Edit: November 21, 2016, 10:44:37 AM by calgoddard » Logged
USch
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« Reply #21 on: November 21, 2016, 01:07:29 PM »

Speaking of outdoor models I would always cover at least the inside of any rubber model fuselage with paper and dope. Especially today with silicon rubber lube you have to protect the balsa with a non porous material otherwise the balsa absorbs the lube. Personally I also cover the outside because it adds a lot of durability to the fuse and the weight penalty is so small that it can be overcome by adequate wood selection.

About pro's and con's of round or square fuselages just some scrap math.
If you like to build a fuselage with the same wood density and thickness (example is with 1mm wood), the same length and the same wetted area outside you get either

a round tube with 32mm outside- and 30mm inside diameter
a square tube with 25mm side outside and 23mm inside (not considering the weight of the triangles to place in the corners to strengthen the weak joints)
That means you have 7mm less on a square fuse to fit a decent blast tube during winding.

If you like the same "space" in your square fuselage and build it with 32mm sides you will add 27,5% more wood (and weight) and increase the wetted area by the same amount.

I did consider only fuselages with fully sheeted sides.

my 2-pence worth,

Urs
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bentodd
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« Reply #22 on: November 28, 2017, 11:39:14 PM »

I carve  wing pylons from blue foam.  I find it a lot easier than two formers with balsa sheet wrapped around the formers  They are plenty strong

One other thing I find helps is to use a slightly tapered former.  I make the former by rolling on tapered paper to create the taper.

I glue the seam on the former with some nonstick surface underneath.  A couple of taps and the balsa tube pops off.
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flydean1
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« Reply #23 on: November 29, 2017, 09:09:54 PM »

Interesting observations all round.  However, MAXOUT knows whereof he speaks being a PHD Engineer.  I'm sure there are others with equal credentials.

I, however am a Redneck Flight Instructor from Dothan, Alabama, whose eyes glaze over when faced with anything that resembles math or science.

I have built a couple rolled tube P30 fuselages.  Fairly straight forward but it took two tries to come up with a good one.
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