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Building Tips and technical articles. => Building techniques => Topic started by: Matt Piatkowski on October 10, 2019, 12:24:33 PM
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Hello,
Please see the attachment. I have used 3/8" light-medium balsa (6.7 lbs./ ft^3) for the portion of the profile fuselage having the length of ~32".
The ruler shows this length.
I would like to increase the torsional stiffness of this portion of the fuselage and the choices are:
1. Laminate 1/64" plywood to both fuselage sides and then dope it with medium Silkspan.
2. The same as 1. but remove about 40% of balsa by cutting circular holes in it.
3. Dope Polyspan at 45 deg. angle to one side and at (-) 45 deg. to the other - no lightening holes
4. Epoxy the carbon tape at 45 and (-) 45 deg. to create the truss-like surface structure and then cover the fuselage with Polyspan as in 2. - no lightening holes
5. Laminate carbon cloth directly to bare balsa - no lightening holes.
The entire fuselage will be painted with one of the fuel proof paints.
Each option adds some weight and has advantages and disadvantages.
The model is 60 size, 675 sq. in wing with the Max Bee II, rather sizeable, horizontal tail.
Your opinions and comments regarding the above mentioned choices are appreciated.
Thank you,
Matt
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Matt,
If you are after stiffness, whether in bending or in torsion, you have greatly handicapped yourself by using 3/8" core material. That means you only have 3/16" from the neutral axis to the outer fiber in bending, in the terminology of a structural analyst. For a 60-sized plane?
That means that to regain the stiffness that you would have had just from 1/2" balsa you're adding weight. And for a 60-sized plane, most guys complain that the fuse isn't really as stiff as they'd like it.
You're going to need more than polyspan to get that back. You may find that even carbon fiber strip will buckle on you if you get a lateral shock load. Think hanger rash, or....
Cutting away core material in a plywood skinned construction may reduce the weight some, since the plywood is going to carry all of the lateral bending loads due to higher modulus and location, but the issue then becomes how much can be cut away without allowing the plywood skin to buckle and separate. And, you have more than just the lateral bending load case. For torsion you need a closed tube to be efficient. A tube with a horrible section aspect ratio of 4 or 5 inches tall and 3/8" wide....is just not efficient. To carry the side skin stresses from one side to the other, the top and bottom balsa sections will need to be pretty deep.
If you want an example of a profile fuse on a 60-sized plane, go look at Frank William's Bearcat. Plans were in Model Aviation. I don't think the core cutouts were completely optimal, but they worked.
Dave
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First, I'd go with a 1/2" thick fuselage. 3/8 is kinda crazy. If your local rules allow it, go thicker yet.
Second, it may work best to embed a carbon fiber tube down the center of the fuse, ala Ken Culbertson. Pultruded tube (all the fibers lined up) if you can't do better, but try to find some that uses woven fibers. It'll be way more expensive, but also way stiffer.
Using a round tube avoids the aspect ratio problem that Dave alludes to, and basically lets you put the weight of the tube to good use.
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A cheap source for the carbon tube can be found at your local archery club. Ask for discarded women's arrows :-).
Regards,
Wolfgang
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In the PAMPA archives there are articles by Larry Cunningham "Building fuselages for stiffness". It's all you need.
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There's also the carbon tube fuselage option.
Consider that a 14x13x1000mm tube only weighs around 32g(1.14oz), and a larger diameter 16x15x1000mm tube only weighs 38g(1.34oz), and is 51% stronger.
(https://stunthanger.com/smf/index.php?action=dlattach;topic=22554.0;attach=86713;image)
(https://stunthanger.com/smf/index.php?action=dlattach;topic=22554.0;attach=86717;image)
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Hi Brent,
The vibration spectrum for the electric motors is more forgiving.
What is the size of this model? Nobler like?
Everybody,
Thank you for your opinions and suggestions. I will most likely use the combination of carbon tubes and carbon tape to stiffen the rear portion of the fuselage that will be covered with Polyspan at + and - 45 deg. and painted.
Happy Flying,
M
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Matt, that plane was built by Bob Branch. He used a Brodak P-40 ARF wing/tail. 560 sq, 53.5" span. Fuselage measurements are stretched to around 17.5 hinge line, 10.5"-ish nose.
https://stunthanger.com/smf/gettin-all-amp'ed-up!/c-14-carbon-fuse-electric-profile/
I built a Pathfinder sized test fuselage using 12x10mm tubes and it is astonishingly stiff. My next carbon tube fuselage will utilize even larger diameter, thinner walled tubing. Either 14x13mm tubes or 16mmx15mm. 16x15 is 30% lighter and 40% stronger than the 12x10, which is already crazy stiff.
(Possibly even using up to 18mmx17mm, as these large diameter, thin wall carbon tubes are quite light.)
https://design215.com/dcal/toolbox/tubing-calculator
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You can build a box fuselage lighter, stronger, and less expensive than a profile, at least in the larger sizes.
I have noticed that, for example, profile Cardinals noticeably wag their tails in flight.
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Matt,
Continue making your fuselage out of 3/8 balsa but skip the ply and use 1/16 balsa put on vertically and at 30 or 40 degree angle. You put it on right to left on one side and reverse it on the other. cover with 1/2 oz cloth with water based urethane and prep for finish.
I've done all of my 60 sized and other also with this method and they are as rigid as a ball bat with absolutely no flex.
Example put the one fuselage side sheeting on with pieces angled this way /////// and then put it on the opposite way on the other side.
Dennis
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I have made fuselages very torsionally stiff by starting with a 1/2" center core with trussed openings behind the wing and 1/16" balsa (yes) sheeting, followed by 45-degree biased fiberglass attached with very thin epoxy. It is no heavier that glass with dope and a lot stiffer. It is very difficult to twist these fuselages. Time required for the 1/8" x 1/2" balsa truss work is modest.