Woodie Fuselage material choice - Now Sagitta Fuselage Mods - RC Groups
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Nov 04, 2017, 08:29 PM
Sagitta Fanboy
Discussion

Woodie Fuselage material choice - Now Sagitta Fuselage Mods


So, I've decided I'm going to build 1-2 Sagitta 900 fuselage's this winter. I want an electric-launch one to fly at my electric club, and a lighter weight line-launch one for those light wind days (My current Sagitta is 56oz RTF, it flies nice, but doesn't like really light days)

Looking at the plans, I think 1/8" air ply is probably overkill for the fuselage sides (and is also a pain to source locally in the lengths required). I'm thinking I may be able to get away with 1/8" Balsa for the sides, with a 1/32" doubler from F7 forward, and maybe a strip of carbon tow running along each side from mid-boom to the rudder post to prevent breaks ahead of the vertical stab. I'll keep the ply forward floor for durability. I'm also likely to remove formers F4 and F6, as I don't see a need for a ballast compartment. If I decide to ballast, I'll use a lock-down system instead.

I'm also wondering if the spruce longerons actually add much durability to the fuselage. I suspect they'd be more valuable on the balse fuselage, but they seem like overkill for a 100" RES ship.

the Electric fuselage will get a 28mm geared inrunner up front, to maintain as much of the nose profile as I can. I'd like to narrow the fuselage on the line-launch ship, but that would require a new solution for pull-string spoilers on my current wing set (I do plan on building at least one new wing set as well, but that's for later, I've not decided what I want to alter on the wings aside from the joiner box)

Any suggestions on whether or not my alterations to the fuselage materials make sense?
Last edited by mawz; Nov 07, 2017 at 09:51 PM.
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Nov 04, 2017, 08:46 PM
aka : SteveBB..
Stevepilot's Avatar
I wouldn't eliminate the formers you're thinking of leaving out. They add strength to the fuz even if they're not holding ballast and make very little difference to the weight.
Nov 04, 2017, 08:49 PM
Sagitta Fanboy
Quote:
Originally Posted by Stevepilot
I wouldn't eliminate the formers you're thinking of leaving out. They add strength to the fuz even if they're not holding ballast and make very little difference to the weight.
The main reason I'm looking at eliminating them is that they're in locations where they will add little useful strength (middle of the wing box, and at a straight-sided nose section before it starts curving inwards) and by eliminating them I get a lot more flexibility in equipment location. The Sagitta nose section is rather overbuilt IMHO, although I'd leave them in if I was going to dork the ship on a regular basis.

F6 doesn't seem to do much at all unless you are installing the Airtronics adjustable hook, where it becomes part of the rear anchor. F4 may add some strength in the nose, but it looks to be more of a case where it's primary purpose is to prevent the ballast from impacting the receiver in a crash.
Nov 04, 2017, 08:58 PM
Sonoran Laser Art
For the sides 1/8 balsa and 1/64 ply/wing skin is strong and lite. You don't need 1/32, just unneeded weight. I doubt longerons are needed unless you want to sand into that area for shaping, then you would remove the added weight. Kind of depends on the design though.
Nov 04, 2017, 09:38 PM
Registered User
AMBeck's Avatar
I'll second the 1/8 balsa and 1/64 ply for the fuselage sides. You can glue in the 1/64 with a thin coat of laminating resin. It's light and doesn't warp (unlike titebond).
Nov 04, 2017, 09:53 PM
Sagitta Fanboy
Quote:
Originally Posted by CloudSniffer
For the sides 1/8 balsa and 1/64 ply/wing skin is strong and lite. You don't need 1/32, just unneeded weight. I doubt longerons are needed unless you want to sand into that area for shaping, then you would remove the added weight. Kind of depends on the design though.
The rear end of the Sagitta is essentially an oval wood boom. Stock is 1/8" lite ply sides, a 1/4" balsa top, 1/8" balsa bottom (which I probably would sheet cross-grain for strength. 1/8"x1/4" Spruce longerons and the corners are rounded down to the longerons. At the rudder base, the fuselage is about 1" high by 1/2" across. The forward base of the rudder is a known weak spot, which is why I was going to add a tow strip for reinforcement, but I suspect that will also eliminate the need for spruce longerons in the rear fuselage.

BTW, if anyone is unfamiliar with the fuselage design in question, the plans are on Outerzone.
Last edited by mawz; Nov 05, 2017 at 01:12 PM. Reason: Corrected miscalculated cross-section
Nov 04, 2017, 10:28 PM
Registered User
I'll lend a third endorsement to 1/8 balsa sides and 1/64 ply doubler that extends back to wing trailing edge . I personally wouldn't rule out the spruce longerons .
Nov 05, 2017, 12:36 AM
Registered User
If you could get 1/64 veneer, that would probably be even better.

I don't think that 1/8" lite ply is overkill. That fuselage gets very narrow at the back.
Nov 05, 2017, 07:23 AM
Sonoran Laser Art
Quote:
Originally Posted by mawz
The rear end of the Sagitta is essentially an oval wood boom. Stock is 1/8" lite ply sides, a 1/4" balsa top, 1/8" balsa bottom (which I probably would sheet cross-grain for strength. 1/8"x1/4" Spruce longerons and the corners are rounded down to the longerons. At the rudder base, the fuselage is about 1.5" high by 3/4" across. The forward base of the rudder is a known weak spot, which is why I was going to add a tow strip for reinforcement, but I suspect that will also eliminate the need for spruce longerons in the rear fuselage.

BTW, if anyone is unfamiliar with the fuselage design in question, the plans are on Outerzone.
With this being the case I would surely keep the longerons. Keep in mind though I have not build a Sagitta 900. I do hand one in the box I bought new in the early 80's still on hand.
Nov 05, 2017, 07:57 AM
Registered User
whacker's Avatar

Bending, Torsion


Lots of that going on from the end of the wing back

and depending on how you fly and land, even more action there

Sure balsa and ply would work,

I am willing to bet though that getting nice ply and balsa is just as easy as 1/8 Lite Ply your not on an Island there Adam

That area in front of the fin depending on how you sand can be suspect, the sheet offers something and so does the longerons

Is the section 1/2 x 1 perhaps

whacker
in Sunny NS
Nov 05, 2017, 11:03 AM
B for Bruce
BMatthews's Avatar
Lincoln beat me to the issue of the slender fuselage. But I'd like to reinforce this issue. With wood the strength and stiffness depends a LOT on the width. Slim them down too much and you had better use more and harder woods.

You've got two directions to go if you don't want to use the stock materials and dimensions. First is to use wood and carbon to build the strength into the structure. The second is to build up a light core from balsa and liteply and then fiberglass over the outside to gain the strength and stiffness you need. Otherwise the sort of plan you're considering will have all the stiffness of a wet noodle.

As mentioned the formers don't seem like they do much. But that's not the case. The formers support the sides from bowing in and collapsing just as certainly as does webbing between the upper and lower spar caps in the wing. If ANYTHING a skinny fuselage should use MORE formers rather than less.

Finally don't think that they didn't understand the gains from skinny fuselages back in the day. Models like the Sagitta and Bird of Time were about as skinny as they could be and still be strong and stiff enough to support the flying surfaces with a low enough flexing in flight and withstand the landings used in contest sailplanes. If you try to go too skinny you'll rapidly find the limits of wood as a construction material. The spruce corner longerons is another example of this. Spruce is a lot stiffer in tension than balsa. So the spruce longerons actually do a lot for increasing the strength of the model.

Even if/when you glass the model the preference is for the weave of the cloth to be on a 45* bias for better torsional stiffness.
Nov 05, 2017, 11:13 AM
aka : SteveBB..
Stevepilot's Avatar
Quote:
Originally Posted by BMatthews
Lincoln beat me to the issue of the slender fuselage. But I'd like to reinforce this issue. With wood the strength and stiffness depends a LOT on the width. Slim them down too much and you had better use more and harder woods.

You've got two directions to go if you don't want to use the stock materials and dimensions. First is to use wood and carbon to build the strength into the structure. The second is to build up a light core from balsa and liteply and then fiberglass over the outside to gain the strength and stiffness you need. Otherwise the sort of plan you're considering will have all the stiffness of a wet noodle.

As mentioned the formers don't seem like they do much. But that's not the case. The formers support the sides from bowing in and collapsing just as certainly as does webbing between the upper and lower spar caps in the wing. If ANYTHING a skinny fuselage should use MORE formers rather than less.

Finally don't think that they didn't understand the gains from skinny fuselages back in the day. Models like the Sagitta and Bird of Time were about as skinny as they could be and still be strong and stiff enough to support the flying surfaces with a low enough flexing in flight and withstand the landings used in contest sailplanes. If you try to go too skinny you'll rapidly find the limits of wood as a construction material. The spruce corner longerons is another example of this. Spruce is a lot stiffer in tension than balsa. So the spruce longerons actually do a lot for increasing the strength of the model.

Even if/when you glass the model the preference is for the weave of the cloth to be on a 45* bias for better torsional stiffness.
^^^^^ This...All of this.
Nov 05, 2017, 01:10 PM
Sagitta Fanboy
Quote:
Originally Posted by whacker
Lots of that going on from the end of the wing back

and depending on how you fly and land, even more action there

Sure balsa and ply would work,

I am willing to bet though that getting nice ply and balsa is just as easy as 1/8 Lite Ply your not on an Island there Adam

That area in front of the fin depending on how you sand can be suspect, the sheet offers something and so does the longerons

Is the section 1/2 x 1 perhaps

whacker
in Sunny NS
Whacker,

You're right on the section, I misread the plans when I was going over them last night and scaled everything up by 50%. It is indeed a 1/2"x1" section at the tail.

Getting good balsa around here isn't actually an issue, but getting good quality ply locally in sheets long enough for fuselage sides is surprisingly difficult. I can get it readily in shorter length sheets though. Might simply have to bite the bullet and order from Aircraft Spruce.
Nov 05, 2017, 01:17 PM
Registered User
whacker's Avatar
Mawz



Balsa USA ; Ply and Balsa

They will have it to you in a week.

National Balsa

Good source for Spruce

Get them to fill a 48 x 12 x 6 box with some good wood, your still young , surely you'll be able to use it all

Order 48 Inch lengths to start with, and you'll have enough length to build something bigger better later


There's kits that come up on occasion as well, and then you won't be searching for stuff on our Island here, and just can go ahead and build to your satisfaction

whacker
in Sny NS
Nov 05, 2017, 01:19 PM
Sagitta Fanboy
Note I was not planning on glassing these fuselages (I do have a passing idea of doing a glass fuselage at some point, but am not setup to do more than small areas of glassing right now).

On further consideration, I'm thinking if I do go with the balsa sides, the carbon tow reinforcement likely will be a must. This would probably be two longer sections on the sides, extending from the rudder post to mid-boom, and a smaller section on the underside from mid-fin forward with rear end of the top stringers as the mid-point.

For now, the Spruce longerons are definitely staying, and possibly F-4 and F-6. I know the Electric will probably need some former adjustment, but I'll probably stay stock in the nose for the line-launch fuselage at this time.

These skinny fuselages are gorgeous, and perform well, but it's clear that optimizing the construction is not simple.

I do thank everybody for their input, and appreciate any further input. I'm looking to get some more Sagitta's flying, rather than just validation for my choices.


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