Back in April, I and my buddy bought a couple of ARFs from ARF MFG (same as GoldWing) named 20cc Sbach 340. At that time the model was undocumented even on GoldWing's website so we took a risk. The model has subsequently been released in the West through Troy etc.

When I did an unboxing, I was pretty appalled by the quality or rather the lack of it. The fuse was ultrathin ply, the wing didnt seem to have a spar etc. However, having bought it we had no choice. In the meantime, I developed back problems and stopped flying. My buddy however, went forward with reinforcing it- he birch ply laminated the fuse upto the wing tube, rest of it he balsa sheeted almost all the way to the tail on the fuse sides. The bottom of the fuse was just stringers. Since we have a rough and ready field and transportation, that has to be reinforced. Initially, I thought it was soft balsa stringers but it turns our to be much stiffer birch or something. He glued CF strips on the sides of the stringers and then pasted further birch stock on top, but lets the structure as stringers. He went ahead and flew his model with a 120ax and ended up adding a lot of nose weight (he says its 300-400g but I feel it may not be that much). It turned out to be an excellent 3D model and fairly sturdy- he has had a number of deadstick plop downs and hasnt broken too much.

Now its my turn to start building mine. I made templates for the reinforcement sheeting so I can sut them easily. Typically I would use 1/8 midwest ply to reinforce but that adds a lot of weight. My buddy suggested to balsa sheet most of the model and restrict ply only to the fuse upto the wingtube which includes the motor box and sides of the model (where the original ply bends out when only trying to install the wing).. I am not comfortable with leaving the bottom as stringers and will sheet that too between the stringers. Now all of this will add a lot of weight if done in ply (his is already 5KG). Secondly, I intend to go gas on the model and he is suggesting to put in a dle30. My question is, how much stiffness and strength will balsa sheeting add? Alternatively, what should I do? Should I consider CF plates instead of ply / balsa?

Ameyam

Balsa sheeting can add a lot of stiffness. You might also consider thinner ply.

Air = air in weight so open areas of construction are ideal and I'd be hesitant to fill them with something that is heavier than air when a need does not exist. In the past 40 years we've moved away from solid sheeting and I see that as a good thing. The covering material serves as the skin and strengthener and provides resistance to twist and torsional flex of many open surface areas and does so without the weight of an underlying balsa surface.

That the under surface might seem vulnerable when lifting the plane is a good trade for the weight savings of not sheeting over the stringers.

One good example of construction differences is the Ultra Stick 120 by Hanger 9. The second iteration (Ultra Stick Lite 120) was the exact same planform as the first but 2.5 lbs lighter than the first. That is 20% weight savings with a resulting much better flying plane needing a less powerful engine, the first needing a 1.6 engine and the second very happy with a 1.2.

So... I'd say, don't go sheeting happy. Second, aircraft ply should be restricted to firewall and gear support. Third, lite ply should be lightened with generous holes wherever possible. Fourth... trust the covering to add the final degree of strength needed for integrity.

I agree here. I see nothing wrong here. All I would do is go over all the visable glue joints with medium CA and call it a day.

Its not that easy, this is a 'chinese' ARF. You can understand how 'lightly' its built from the fact that the sheeting on the side of the fuse into which the anti rotation dowels go, simply bends out while fitting the wing. The wing itself is a tight tolerance fit. My AW Extra 30cc is similarly built but much stiffer. So some strengthening is going to be required whether sheeting or CF stock.

With the under belly, we do carry our models in small cars with the models stacked one on top of the other. Plus our field is rocky. If I leave it as is, it may not survive the ride to the field, let alone being picked up.

The ply I would normally use to sheet from inside is 3/32 birch ply. I also have 1/16 birch ply. Both are quite heavy and difficult to cut. Recently I reinforced the LG Block on my Reactor bipe and it took me all evening to sand a piece of 4"x2" into shape so it would fit. A small hack took a lot of effort (I bit the bullet and got a jig saw since then). My buddy has used similar stuff on from the engine box all the way upto the TE of the wing, subsequently he used balsa upto the tail section. The model doesnt have too many stringers in the structure, its just ply formers, so stiffening is definately required. Should I consider balsa + ply construction similar to what he has done or should I go for CF plates. If so, I will need to order from Tower so CF will be expensive

Ameyam

Personally I reinforce in the following manner :
1) carbon fiber rods - stiffest in its class light weight very little flex
2) carbon fiber planks - flat and will flex somewhat on its widest flat side
3) flite skin , I used this on inside structures added to balsa or even over open weak structures .
To me this is the biggest bang for your buck vs wood options of ply , bass ect

I am not very clear though. Which parts do I use where?
I think I should use birch ply (in several layers and cross grain) to strengthen the LG Block.
Then further ply on the engine box that extends all the way upto the wing tube. In an impact (God forbid), this part will take and transfer the force. If I use CF on it and it shatters, it will be difficult to repair.
Wing tube is cardboard, so I should fibreglass that.

From there on I am blank

What should I use to sheet the inside of the fuse behind the wing- balsa or CF? I am thinking balsa because that much CF will be just too

What would be typically size for CF stringers and CF sheeting?

Ameyam

Yup, agreed.

He is not used to this type of plane construction.

I see many people do the same without considering WHY the plane might have been built that way to begin with.

3D planes often use this type of light ply construction and the relatively inexperienced pilots immediately rip off the main gear on hard landings.

ameyam: If you still have hard landings, consider only beefing up the gear mounting areas and engine box.

Otherwise try not to overthink this, as you are usually prone to do. A light plane is a happy plane.

A bridge made of girders and thin cross members is much stronger then one made of solid steel.

Hi Jose,

I ma used to this type of construction. Like I said, my AW Extra 300 30cc is similarly built and my Phoenix Topstar is mostly frame only. But both these have strong stringers additionally to stiffen. There is none of that here.

Wait out a bit, I will post some more pics, that will make it a bit more clear

Ameyam

Additional images attached.

You will note that there are no stringers anywhere. This is just interlocking ply construction.

See the video how the wood flexes with just a little pressure. As stock, this model was meant to fly of tarmac and can take no abuse. The model is 67" span and has come with a 20cc badge. But with a 120ax its tail heavy and I am considering putting a DLE30on it

http://youtu.be/VltvmDt2qwI

Ameyam

You could put in some 6mm square cross braces to stiffen up the fuse. Basically you would be making a truss structure. This would add alot of strength without much weight. Be careful about adding a bunch of weight and then strapping on a bigger and heavier engine. That is a good recipe for ruining an otherwise good airplane. I think you have done this in the past with some of your other airplanes.

I see nothing wrong nor that needs "fixing", nor anything "wrong" with the quality. It is a 3D plane.

Yes the model is best flown off tarmac, if you want to fly off grass just pull off the pants and add bigger wheels.

Lite ply bends when you exert sideways pressure on it. That's the way it is.

Again you're overthinking it.

As someone once told me, "improve the pilot not the plane" when it comes to adding strength for landings, etc...

This appears to be built to be a light and agile flyer.

A 20cc is right for this plane, a 120AX is likely to be VERY overpowered.

If it is tail heavy don't change the engine, instead put the battery packs on the nose, beam mounts, etc... move the rudder servo forward and move the engine as far forward on the mount as you can.

Check out this Youtube video of the plane flying with a 20cc DLE on it... it is MORE than enough... the guy is doing 3D stuff with this plane...

[link=http://www.youtube.com/watch?v=If0i6Xa8HGw]Click Me![/link]!

its a bit too light for my reckoning. I want a light airplane but not so light it comes apart in a few flights. In the past I have over strengthened some airplanes but they were testbeds for various things so needed to be that way. Here I have a tested model already- my buddy's airplane is around 5KG and his 120ax doesnt have the pulse power (I think he means he doesnt have the pullout) with his reinforcement.

I wont sheet everything as I was thinking when I posted first. Sheeting would be only for the engine box and that extends all the way upto the wing tube outer. Plus I will ply the fuse side where the wing attaches so that it becomes stiffer. My buddy used 1/8" birch ply (I think). Like speedracerntrixie said, I will put in a truss structure. I will be likely using 1/16 ply with CF stringers. For the fuse part behind the canopy, I will add thin balsa (whatever thinnest I get) simply because I wont be able to add in stringers accurately (you have to put your entire arm into the fuse to install them). Then I will CA wherever I can get to. Also, I found the wing to be a bit delicate, so I plan to run some epoxy along the wing tube side with a brush. Thats about it

With the engine, I will do a temporary install with the 20cc and then take it from there

Ameyam

The Thin CA and the CF re-inforcements do not add appreciable or noticible weight so you're OK there.

Don't go overboard though. Remember that ply is stronger than balsa structures. Sheeting is not going to add any appreciable structural strength to what you have.
The truss is a better idea IMHO and easier to do. If you want to go further you could lay thin CF rods along the existing wood ones.

You should see my Addiction-X.... there seems to be NO supporting structures in it and looks like it will fall apart if you breath on it, yet the plane pulls "G"'s like it is no ones business, thanks to the CF bonded to the wood.


Try to stay with the 20cc and resort to moving things around instead of going to a larger engine.

That will help keep the wing loading low, which will be crucial.

While I don't favor this myself, I've seen a lot of guys strap batteries directly to an engine beam mount to help get the C.G. right.

The balsa sheeting was to add stiffness. Also, it could serve as a base for the truss member. A long unsupported stringer (truss link) will be prone to buckling, the balsa sheeting will prevent that. Plus it will prevent the link from seperating at the ends by giving more area for adhesion. So let me see what can be done

I want to keep the engine 20cc, lets see what happens. A temporary install using cable ties to check the balance is in order after I finish the remaining assembly

Ameyam