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Sbach 300 - an attempt at a lightweight build out of heavy 5mm european foam board

#1
Updates

Update #1 - the Wings
Update #2 - Fuselage (part 1) | #2a | #2b



Original post:



I guess the title says it all ;). Here's the concept drawing that I'll be using to design the parts:

concept.png

The Goals

While I can fly basic aerobatics, I could use some inexpensive trainer to try to start transitioning into 3D flying. While the obvious route would be to build something like the FT 3D, I'd rather design something myself, while trying to find a way to make use of the heavy foam board that is the only kind available to me locally.

Still, building an FT 3D using the techiques described here should be interesting for comparng the weight with the FT board version, so i may just end up doing it at some time anyway.

So: the major goal is to try and keep the weight of the plane at 600 grams or under. I have weighed the motor and the electronics and I can already tell it's going to be a major challenge - they gross 274 grams (w/o servo wire extensions, hopefully I won't need those) meaning the complete airframe must weigh 325 grams or less. We'll see if that's even possible to achieve with the heavy board ;).

IMG_20181105_100323.jpg

Here's the equipment: KEDA A22-15M (tried and tested, up to ~1000g of thrust with 9x6 to 10x5 prop), Turnigy Plush 25A ESC, Turnigy 1300mAH / 3s LiPo pack, Corona 8ch 35MHz receiver (I am still using my trusty 35MHz Optic 6), 4x Turnigy 8 gram servos (also tried and tested).

I'll be designing and drawing all the parts in Inkscape, so that if the plane flies well enough (and there's no reason to believe it won't :p) I will be able release the plans for anyone else interested in building it (it's a beautiful airplane and has perfect proportions for an RC model!). Even though the plans will be made for 5mm board, applying a slight adjustment while cutting should make them compatible with the 4.8mm Adams/FT/DT board as well.


Building technique

For the most part, I will be following standard FT building techniques. I will use the new, short version of the power pod (extracted from FT Simple Scout plans), FT control horns etc. The plane will be compatible with FT power packs - Pack B should be sufficient for regular flying, while Pack C should provide ample power for high performance aerobatics and possibly 3D.

However, there will be two major and important twists:

1.) The wing and the fuselage will be be built with the inside paper layer completely peeled off. Which means I will also be able to build a wing with a smoothly curved airfoil, using a relatively small amount of glue. For a few parts (like fuselage formers) the paper will be stripped on both sides and the foam reinforced with packing tape instead, but I don't want to overdo this technique (otherwise one might just as well go with depron instead).

The make of foam board I am using is basically equivalent to the Westfoam board which is 63% heavier than Adams/ FT board. Peeling off one layer of paper reduces the difference to about 20-22%, which is much more acceptable.

The tail surfaces are the difficult part, can't see a way to make them lighter other than stripping the paper off on both sides and replacing it with lighter one. This may be more viable once the plans are ready - it then might be an appealing idea to first color and then print out the plans and then glue them onto foam (i.e saving the weight on paint)

2.) As for the glue itself, I will be using UHU Por rather then Hot Glue - other than in a few places (like hinge reinforcement). I used it quite a lot in my Balrog build and I am very happy with it. The bond is very strong, clean and sparse application is easy, and the bonding time (10-15min) short enough not to be a significant problem. It doesn't have quite the strenghtening/stiffening qualities of hot glue, but the savings in weight are more important IMO.

IMG_20181105_100559.jpg

That's it for the initial post I guess ;), will try and update as regularly as I can.
 
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rockyboy

Skill Collector
Mentor
#2
As an idea that might help with the tail weight, I've wondered if peeling paper on one side and cutting holes out of the foamboard to leave just the outer frame and some built in spars, and then adding back paper or tape if necessary (i.e. if the holes end up going all the way through) would be beneficial. The only time I tried building with heavy foam board it was a brand where the paper just wouldn't peel off at all, but it sounds like you might have a material that try this.

You've got some great Inkscape skillz too! Very nice looking plan :D
 

Merv

Well-known member
#3
Good point about the hot glue, it's heavy. I try to use 1 stick per plane on my builds. I've build several of the FT3D's, the wing on the plans is far too small. I have better results with a larger wing. To me your wing looks small, a larger wing will reduce the wing loading. Might want to consider a wider wing cord.
 
#5
As an idea that might help with the tail weight, I've wondered if peeling paper on one side and cutting holes out of the foamboard to leave just the outer frame and some built in spars, and then adding back paper or tape if necessary (i.e. if the holes end up going all the way through) would be beneficial.
Interesting idea especially as it might help with balancing as well . I want to know the weight of the wings and fuselage fist though, and if at risk of running significantly overweight I will consider this very seriously. Cheers.
 
#6
I've build several of the FT3D's, the wing on the plans is far too small. I have better results with a larger wing. To me your wing looks small, a larger wing will reduce the wing loading. Might want to consider a wider wing cord.
Yeah I actually have similar intuition, but I wanted to keep the scale look.

Then again, the cube loading is a bit on the high side already, so I I will try enlarging the wing by anout 10% (i.e. 1.21 times the area) and see where it takes me. Shouldn't make it look too terrible ;).
 
#8
All right. The updated plan has been reuploaded.

Enlarged the wing as much as I felt I could without compromising stability and running into potential balancing issues. Increased the wingspan by 50mm and mean chord by 18mm. Moved the CoG slightly forward (to 25% MAC, at least for the first flights - so as not to enlarge the hstab). Also moved the landing gear forward, and elongated the nose by 10 mm.

WCL actually looks pretty good now... assuming I can make the weight.

Dinner time :).
 
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Bricks

Well-known member
#9
Not sure if you have it available but for tail surfaces could use EPP, Depron could use lightening holes in the rear of the fuselage with out effecting rigidity. I know when using Bordens foam board which is probably similar to yours keeping tail weight down is crucial.
 
#10
I can get both Depron and EPP no problems, though the latter is fairly expensive. But I want to see this one through with 100% foam board, invested enough time already into learning to peel off the paper properly, etc.

Plus I like working with foam board, and the planes I've built so far, though on the heavier side, actually fly great. I don't even mind the greater wing loading as I often fly in windy conditions. But I understand that 3D flying is a different beast, you really want low wing loading and great power/weight ratios so perhaps using Depron (like printing out plans on paper then glueing them onto depron with 3M 77 or sth similar) is the way to go in the future, if I continue building FT style models.

Can Depron, when laminated be folded and shaped into curves like foam board, or does it break?
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#11
Can Depron, when laminated be folded and shaped into curves like foam board, or does it break?
When you glue paper to Depron the properties of the complete result is very different. Depron is more rigid than the foam in FB. The paper will restrict expansion forces and so a bend with the paper on the outside will tend to crush the Depron and folds will appear easily. Thanks to the glue compression forces are also resisted so a bend with the paper on the inside can cause the Depron to fracture or split.

A better approach is to build the shape in Depron first and then apply the strengthening coating of glued paper or tape. I started building in Depron before I found a reliable supplier of FB.

Just what worked for me!

have fun!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#12
Just a heads up! I started out using the Por-UHU but it was far too expensive to use in quantity. In looking for an alternative I found a range of craft glues, (foam safe), that perform equally as well as the Por-UHU but at a fraction of the cost.

See:- http://www.sullivans.net/proddetail.asp?ProdCode=49900

I use this exact glue when available but any craft glue with the same or a similar solvent will do as a substitute. it saves not only weight as you mentioned but it also saves money as well. I even use in for those occasional retail foamie repairs and get quite impressive results, (when done properly).

Have fun!
 
#13
Update #1 - the Wings


Ok, I finally got some progress to show ;)

I got bogged down a bit with a couple of issues. Firstly, I am drawing the plans as I am building, and that requires frequent revisions and updates, also it's quite time consuming - although I keep getting better at it.

In the end, if the prototype flies well I intend to build another example straight from the plans (a friend with little FB experience has expressed interest in helping out, which would be perfect - to see how he manages and if the plans need further improvement).

The second issue was shaping the wing. I intended to make the airfoil completely smooth, but that proved far more difficult than expected. Especially in the tightly-bent area around the leading edge (I only creased it and didn't bevel, as I wanted to have a thick, high-radius LE for good low speed handling). I still feel it can be done with a great deal of patience, and perhaps some trick - like warming the foam with hot air, or maybe using a heavy roller of some sort on a soft surface underneath. Maybe someone here can help out with this, not sure what idea might work.

Anyway, after two unsatisfactory attempts I decided not to waste any more time experimenting and simply decided to make a number of FT-style creases on the inner (de-papered) wing surface. Since they are not "cuts" but only depressions in foam, I didn't put any glue in them, to save on weight.

That worked out quite ok and I managed to attain a decent shape, approximating the intended symmetrical airfoil with a thick leading edge pretty well.

Some other observations/comments:

1. I now have a strong suspicion that the chemical which facilitates paper removal from FB in the fluid I am using is actually isopropyl alcohol (I think also known as "rubbing alcohol"). Going to buy some next time I visit the chemist and test this theory out.

2. It seems I am actually close to being on course for making the weight. The wings - as shown in the photos - weigh 61 grams each. Allowing ~24-30g for servos and control rods and horns, complete wings should weigh ~150g or slightly less. That leaves ~175g for fuselage + tail, which may be difficult, but not impossible to achieve ;).

At any rate, it's a significant improvement over similar wings I've built in the past , which typically weighed ~190-200 g.

3. Dang those foam scraps keep accumulating at an unmanageable rate... ;) any buyers? A dollar a pound is all I ask... :p


Finally, the photos:

IMG_20181112_121324.jpg IMG_20181112_130548.jpg IMG_20181112_125611.jpg IMG_20181112_134433.jpg IMG_20181112_143830.jpg IMG_20181112_172915.jpg IMG_20181112_175408.jpg IMG_20181112_181053.jpg IMG_20181112_181146.jpg
 
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#17
Those wings look really good. I just built a 30" glider wing with a 4.25" chord. I too pulled the paper from the inside to make it easier to curve. I had to bevel the insides to get the foam to not collapse.
 
#18
I've never used Depron myself, its too expensive in the USA. Andrew Newton has had great success bending laminated Depron
Something to try one day. Here 6mm Depron costs pretty much the same as FB does, it's commonly used in construction.

Though FB is actually more expensive than in the US ;)
 
#19
Those wings look really good. I just built a 30" glider wing with a 4.25" chord. I too pulled the paper from the inside to make it easier to curve. I had to bevel the insides to get the foam to not collapse.
Yeah, that's the standard way to treat FB, at least at FliteTest ;)... minus the paper pulling. There are other techinques, like folding it over a thin FB spar for a curved top and flat bottom airfoil, but the FT way is more flexible.

And TBH the airfoil shape doesn't matter all that much with those small airplanes, unless you build for some sort of competition or pther special purpose. As long as you get it roughly right,you should be fine. And contrary to real airplanes, turbulent airflow is often desired at these small scales (things like L/D ratio and fuel consumption don't play much of a role, most of the time).
 
#20
Yeah, that's the standard way to treat FB, at least at FliteTest ;)... minus the paper pulling. There are other techinques, like folding it over a thin FB spar for a curved top and flat bottom airfoil, but the FT way is more flexible.

And TBH the airfoil shape doesn't matter all that much with those small airplanes, unless you build for some sort of competition or pther special purpose. As long as you get it roughly right,you should be fine. And contrary to real airplanes, turbulent airflow is often desired at these small scales (things like L/D ratio and fuel consumption don't play much of a role, most of the time).
Yeah. I was really hoping to get a nice fat curve like you're going for since it's for an unpowered glider. But no such luck.