Are we putting too much balsa into models?

[Simon Chaddock]

If the D box is solely to provide torsion resistance then a non grain surface material is preferred.

Of course a well designed D box also contributes significantly to the bending resistance but the extra forces generated under bending may not simply require a thicker skin but just additional support to resist the skin buckling under compression and drawing in under tension. Typically achieved with closer spaced ribs. 

This brings on the question of what the ribs and skin are made of. To make matters worse a rib inside a D box is largely a support item whereas a 'free' rib has to resist bending forces as well. This suggests they should have different forms of construction.

 

jd1 is quite right that the strength of the material at the outer surface of the wing has the greatest impact on its rigidity. However applying the same material all over the wing surface is not structurally ideal either.

 

I do not claim to have a magic solution but this sort of analysis does indicate that using the same material throughout such a complex load bearing structure is not likely to give the best result.

It quite reasonable to follow "It is proven to work so leave it be" but it is also wise to ask "Can it be done better?" particularly with the availability new materials.   

 

    

Edited January 30 by Simon Chaddock
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[Richard Acland]

This evening I have been pricing up balsa,  spruce and a few odds and ends for my latest project. I had already purchased a laser cut cut kit of components but need sheet and spruce for spars. When one sheet of 3/8"x4"x48" balsa costs £9 I can see the attraction of using less balsa, but what is the alternative.

[Christopher Wolfe]

One thing that bemuses me is that some people go to much effort to build a great lightweight airframe and then hang great slabs of balsa on to the flying surfaces for control purposes.

 

I am referring to;- rudder, ailerons and elevators.

 

Just take a look at many classical aircraft (WWII fighters especially) and whilst the airframe may have been 'stressed skin' the control surfaces were often a lightweight structure and fabric covered.

 

As they were not load bearing structural members (as opposed to a wing-spar) they were quite adequate for their intended purpose.

 

I suspect that the original designers knew their stuff and maybe wore out a few slide rules to reach this conclusion.

 

'Hangar rash' is not an issue for a model as a well designed and braced laminated outline might flex and absorb the odd knock better than a solid lump of wood, but it's best not to bash your models into doors in the first place.

 

* Chris *

 

 

[Nigel R]

2 hours ago, Christopher Wolfe said:

the control surfaces were often a lightweight structure and fabric covered.

 

Just pulling a random WW2 example, Hurricane, the elevator was an appropriately sized metal torque tube - able by itself to resist the twisting forces it would experience when the elevator was moved, and tapered metal ribs, which I believe were capped (which would then form an I beam structure, and also aid in resisting racking and warping) were attached. I'd imagine this had some degree of analysis applied. Note, there are loads on an elevator, just like every other part of an airframe, your statement that it is not a load bearing structure is not true.

 

A slab of balsa is an easy route to making something that works for us. It is, "good enough". Is there better for a particular application? Sure.

[kc]

15 hours ago, Richard Acland said:

I can see the attraction of using less balsa, but what is the alternative.

Balsa is such a nice material to use that we don't really want an alternaive except to reduce cost.   As the wing sheeting seems to use the most sheets that could be the best area for economy.     Thin white card might tbe a good substitute but seems difficult to obtain the really lightweight stuff in small quantities.

 

Laminating film seems very cheap and I have noticed how much stiffer an A4 piece of paper becomes when laminated both sides and much stiffer still when laminated twice both sides.    There seems possibilities that several layers of laminating film applied over paper or tissue etc might produce a suitable material for wing D boxes.   Maybe the paper could be decorated first.   I would envisage laminating flat with the paper lowest then the paper side might be glued to the ribs when it was bent around the wing.    Maybe just a few carbon tows could be applied diagonally to a multiple layers of laminating film.     Has anybody tried anything like this?

[Nigel R]

You could switch to veneer for wing sheeting... it might need some sort of semi-solid substrate... maybe lightweight bead foam...

 

(reinventing the wheel department)

 

balsa cabin are listing poplar veneer for under £8 a sheet of decent size - 15" x 60"... that's heaps cheaper than balsa

 

Edited January 31 by Nigel R

[stu knowles]

I did some trials with white polystyrene fom cores covered with brown paper and PVA. Some local glasscloth reinforcement, all of which worked to a greater or lessor degree.

One had 1/4 square balsa top and bottom spars let in before covering, another had carbon tow 'spars' top and bottom and another had the BP covering overlaid with 50GSM glasscloth & epoxy.  They all flew well.  From memory, the lightest had the carbon spars and just BP/PVA stressed skin.  The only balsa was the leading edge and a hinge spar.  No veneer at all

[Rich Griff]

Thanks for the heads up about the veneer Nigel, thanks.

 

Nice one stu...

 

A shaped white foam core, recessed spars, balsa leading and trailing edge and covered with brown paper or even patterned gift wrap works well.

 

Cereal packet ( wheatabix packet seems the best, family pack for size ) or banana box cardboard sheet minus the holes is good too. A bit heavy but great, free and reusing at the same time. Reusing white foam that is found in skips etc., Re purpose, re use, re cycle.

 

Turns out aeromodellers have been doing this sort of stuff for years, some of at at least.

 

So annoying to find that most fruit and veg comes in cardboard boxes these days. A free source of raw material that had dried up sadly, the "wooden" boxes that is, that ended up in landfill in years gone by.

 

Can corrugated cardboard be used to make s successfully flying model ?

Edited February 1 by Rich Griff

[leccyflyer]

21 hours ago, kc said:

Balsa is such a nice material to use that we don't really want an alternaive except to reduce cost.   As the wing sheeting seems to use the most sheets that could be the best area for economy.     Thin white card might tbe a good substitute but seems difficult to obtain the really lightweight stuff in small quantities.

 

Laminating film seems very cheap and I have noticed how much stiffer an A4 piece of paper becomes when laminated both sides and much stiffer still when laminated twice both sides.    There seems possibilities that several layers of laminating film applied over paper or tissue etc might produce a suitable material for wing D boxes.   Maybe the paper could be decorated first.   I would envisage laminating flat with the paper lowest then the paper side might be glued to the ribs when it was bent around the wing.    Maybe just a few carbon tows could be applied diagonally to a multiple layers of laminating film.     Has anybody tried anything like this?

Not for a wing structure, but a folded sheet of laminating film -the thicker 125um type, having been put through the laminator - can be used to make a very lightweight and completely effective rear turtle deck on a funfighter.

[Tom Gaskin 1]

@Rich Griff last century there were a number of plans using corrugated cardboard for the primary structure with a bit of ply for high-stress areas.

 

Tom

[Rich Griff]

Yeah, the me109 fuz, but they bottled out using the material for the wing..

 

[Tom Gaskin 1]

Check out the FUN-TU plan on OZ, it's an all-cardboard twin designed for two .20's

 

Tom

[Simon Chaddock]

I an sure you know what I am going to suggest as an alternative - thin sheet foam!

Not a substitute for all balsa applications as it can range from hard to soft but thin sheet foam is a great deal cheaper.

As it is several times lighter than the balsa typically used for wing skins it can be quite a bit thicker yet still bends well. This can have advantages in the building process.

Now if you can combine that with "printed" wing ribs there can be no need for any balsa at all.

 

My own rather extreme example of foam "construction". A 2.2m Glasflugel 201 Libelle.

Made entirely from 3mm thick Depron sheet foam with a hard balsa/foam/hard balsa 'composite' spar. No carbon used anywhere and no 'skinning' material either just a light coat of acrylic paint

Flew well but after several flights I lost sight of it in a thermal flying just that bit too far down wind! Never saw it again.    

 

 

 

[Don Fry]

When I was a little sprog, 10, 11?, I looked at the ads, back of aeromodler. And drooled over a KK Falcon.

I will have to sort it one day. I have a plan to build, and am sure it can be built to a stupid weight with 3 mm depron, 29 micron doculam, a bit of balsa, and a battery up front to give an hour of flight time. And I will need a beer, and a deck chair. 

[Futura57]

5 hours ago, Simon Chaddock said:

I an sure you know what I am going to suggest as an alternative - thin sheet foam!

Not a substitute for all balsa applications as it can range from hard to soft but thin sheet foam is a great deal cheaper.

As it is several times lighter than the balsa typically used for wing skins it can be quite a bit thicker yet still bends well. This can have advantages in the building process.

Now if you can combine that with "printed" wing ribs there can be no need for any balsa at all.

 

My own rather extreme example of foam "construction". A 2.2m Glasflugel 201 Libelle.

Made entirely from 3mm thick Depron sheet foam with a hard balsa/foam/hard balsa 'composite' spar. No carbon used anywhere and no 'skinning' material either just a light coat of acrylic paint

Flew well but after several flights I lost sight of it in a thermal flying just that bit too far down wind! Never saw it again.    

 

 

 

Sorry you lost your model Simon. I was wondering if you have a photo or drawing / sketch of the wing section and spars.

[Robin Colbourne]

The issue with a lot of kit and plan models is that they are designed to be simple to construct, thus giving the builder maximum liklihood of getting them finished and flying.
Ideally material should be where it is needed to give strength and taper away to nothing where it isn't.  As an example, an Airbus A340 has wing skins over an inch thick at the wing root, tapering away to about 1/8" thick or less at the tip.
Full-size gliders often only have a root rib, tip rib and a hefty mainspar at the deepest part of the wing chord.  Wing skins are a sandwich of glass fibre cloth laters either side of a upper and lower skin foam cores.  These cloth layers are at 45° to the spar and at right angles to each other.  The primary objective is to resist torsional,wing twisting loads.  The WW1 Albatros DVa did something similar for its fuselage but without the core, and the WW2 De Havilland Mosquito for its whole airframe, with balsa instead of the foam core.
The main reason modellers don't copy this construction is that it needs complex jigs to construct the curved surfaces and that lengthens the build process unless several of the same design are to be built.

Edited February 3 by Robin Colbourne

[Jonathan M]

12 hours ago, Robin Colbourne said:

The issue with a lot of kit and plan models is that they are designed to be simple to construct, thus giving the builder maximum likelihood of getting them finished and flying.

 

👍

[Dickw]

13 hours ago, Robin Colbourne said:

...........
Full-size gliders often only have a root rib, tip rib and a hefty mainspar at the deepest part of the wing chord.  Wing skins are a sandwich of glass fibre cloth layers either side of a upper and lower skin foam cores.  ....................................

The main reason modellers don't copy this construction is that it needs complex jigs to construct the curved surfaces and that lengthens the build process unless several of the same design are to be built.

Many of the commercially available competitive glider (and other types) kits from specialist manufacturers do actually use this method of construction and variations on it. As you say, it does require complex jigs and moulding techniques so tends to be expensive and often made to order only.

 

I have several such models, some without any balsa in them and some where balsa is used between glass or carbon cloth layers to form the upper and lower skins.

 

Dick

[Simon Chaddock]

Furura57

My Libelle wing section was a actually a compromise and variable along the span.

The scale wing section was remarkably thick particularly at the root. Note the very fine trailing edges!

 

Such thick sections do not work well at small model sizes. however I had to use the scale section at the root as the cockpit canopy fitted around it and was a significant part of the Libelle's "look".

My  solution was to reduce the section quite quickly from the root to what I felt gave a better thickness chord ratio but retained the general proportions of the original.

I actually built a "test" centre section to investigate the unusual thin balsa "surface" top and bottom wing spar flange tnat gave the maximum possible depth to the spar.

The wing was built in 3 pieces although was a single piece when all glued together.

The root section resting on the fuselage. Over this length the thick root changed to a more reasonable one. 

This section then remained to the tip but was reduced in proportion to the wing chord.

The RH outer wing panel still under construction being tested for bending strength.

Note the balsa spar flange tapers to nothing towards the tip. The final few centimetres of the wing simply relied on the foam skins for strength.

Sorry there is not much detail but much was done 'by eye' although all the foam and balsa bits were always cut in left and right pairs so the final wing stood a chance of being aerodynamically the same L & R.

The fuselage was entirely built using narrow planks as a 'half shell' over the plan.

As with the wing the L & R former halves were cut together. Of course each plank had to cut/sanded to the correct shape to match its already in place neighbour before being glued in itself. With 20 odd planks per side a slow and labour intensive process but when carefully sanded it gave a good imitation of a glass fibre moulded structure.

Worth all the effort? No way!  

I simply wanted to convince myself it could be done particularly as I had flown a full size Libelle!

Maybe when I have got tired of making foam EDFs I might have another go. 

 

[Andy Gates]

On 29/01/2024 at 19:57, kc said:

The recent RCME articles on Ivan Pettigrew's designs prompted the thought that we are all building too heavily and using lots of balsa.  One of my clubmates has built several Ivan designs and the result is very impressive - large but light scale electric models flown every weekend on quite small lipos.   A look inside the fuselage shows they are ' full of emptiness'  very few bulkheads!   The wings mostly have some LE sheeting though.    

I am the culprit here and as stated I have a few "Ivans" machines and I am a member of the "BIMBO's" - see RCG for clarification.

My machines can be seen here.

 

Ivan plans come from a time of electric flight powered by NiCads and geared brushed buggy motors, not the lipo and brushless stuff we use today.

This means the airframes had to be light and resilliant not unlike free flight machines.

In other words they are "built to fly, not to survive crashes".

With them being so light they fly well and quite slowly so they do not carry much energy into a crash if they have one.

 

I have carried Ivans building principles into other machines.

A David Platt Miles Magister flies quite nicely for me as it is about half the weight of the original.

I have almost finished a TN FW190 made in the same fashion, aiming to be at 60% of the originals weight.

 

So next time you are building, look at what you are doing. How many times have we seen errors in plans?

Think, does that piece really need beefing up or strength (in my view weight) added?

If you add strength are you not moving stress elsewhere?

That means more mods to strengthen the next bit. All that adds weight and increases flying speeds that needs more power / bigger batteries.

 

I stick with Ivan and "build in lightness"

[kc]

Andy could you state the size of the Lipo used in those models such as the 75 inch Chipmunk and the 74 inch Mosquito on your website?   I think the small size will suprise most of us...... 

 

I

[Andy Gates]

Of course KC.

 

My Chipmunk started off using 3S 1000mAH Kokam packs as that was what I had at the time - shows how long ago it was. I was flying from a football field in most weather conditions. Winter meant a puddle covered field so I used to fly between the puddles. Chipmunk performance was such that I could take off by the time it went from the touch line to the 5 yard line.

These got replaced with a by 3S 2200mAh packs for quite some time. I used the Chipmunk for my A test too.

 

Later I started training people using the Chipmunk and added a second 2200 pack in parallel to give a easy 20 min flying time. During its life the motor had a change of bearings as I wore then out.  She sadly died when the ESC burned out in flight.

 

The Mosquito flies off of 2 x 3S 2200mAH packs in parallel although I now use 2700mAH packs as that is what I have.

Same applies for the Beaufighter, Mini Catalina and Twin Otter.

 

Sorry to go slightly off topic

Edited February 4 by Andy Gates

[kc]

Thanks Andy, I think most people will be suprised at the small size of those Lipos for such large models.   It seems it's all down to lightweight building and plenty of wing area.     These models are literally ' every weekend ' models - I have seen Andy fly them most weekends and the Chipmunk really was used for training new pilots.

It would seem that design of sports models could follow this trend and have larger balsa models that are lighter than the current popular models..........