Are we putting too much balsa into models?

[kc]

Are we putting too much balsa into models?   Could we economise on expensive balsa?

 

A lot of  designs show wing sheeting on LE top and bottom to form a 'D' box also TE which comes to lots of sheets of expensive balsa.   Is it all necessary?

 

Looking at some designs shows LE sheeting is almost eliminated.   Consider the Ugly Stik a very well proven 60 inch  design which has no LE sheeting  just an extra 3/16 sq spar top and bottom and some riblets.   There is some TE sheeting though and very deep main spars.  No capstrips and no spar webs to fit.  Must be much quicker and cheaper to build than similar size wings with extensive sheeting.  
The Ugly Stik has been around for almost 60 years and often imitated yet nobody seems to have complained about the wings not being strong enough for the job.  

 

[J D 8]

One thing the sheeted D box or part sheeted leading edge does is resist warping/twisting well. Not just for flight but for storage over time. The wing on my Fun Gus 3 channel fun fly. Wing was built for another OD [ only sheeted on top 1/32] over forty years ago and has seen service on Fun Gus for many years and is still good.

[Futura57]

Yes, the sheeting forming a D-box in conjunction with spare-to-spare inter-rib webbing creates a structure much more resistant to torque/twist/warping. Take a cardboard tube like that from a toilet roll. Gently twist each end in opposite directions to gauge its reaction to this torque. Now cut a slit along the length of the tube and repeat. See what happens. Not an exact analogue but a D-sheeting wing can be thinner and fly much faster without flutter than just spars and ribs. It's horses for courses. You probably wouldn't balsa sheet the wings of an indoor rubber powered model. I do think there is a place for cheaper alternative sheet materials such as foam board, Depron, cardboard in the right place. Have you seen what @Simon Chaddock does with foam.

[Martin Harris - Moderator]

Full size aircraft have almost universally adopted the D box and stressed skin as the lightest way to achieve adequate strength.  It's a long time since external bracing wires were used to resist torsional loads!

[Nigel R]

Ugly stik was originally designed in times of doped silk?

 

This provides a hefty dose of resistance to twist, when finished. Shrink film plastic, less so.

 

But if you want to build open frame wings, go ahead. They'll simply be less strong than fully sheeted.

 

Ps gliders guys are probably interested in accuracy of wing section and would probably prefer a fully sheeted wing of some description...?

[kc]

Of course the Ugly Stik has a very thick wing section and the plan specifies 1/4 by 1/2 spruce spars which are 'deeper' than most models (1/2 inch is set vertically ) but it seems to work.  Anybody had an Ugly Stik that warped?   

I have to say that when I built a 'Stick' type model something half way between a Liddle Stik and an Ugly Stik in size I used a fully sheeted D box, capstrips and webs between the hardwood spars.  As it's a D box it hasnt warped but I wonder if it would have been just as good with simpler construction.

 

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.    

[Martin Harris - Moderator]

Full of emptiness...haven't heard anyone else use that expression since my old chemistry teacher at school...

[Tim Hooper]

Didn't the late Dereck Woodwood describe the best models as' fresh air - lightly trimmed in balsa'?

[Futura57]

Geodesic is the way to go 🙃

[Geoff S]

I added spar webs to my scratch, plan built Das Liiddle Stik back in 2020.  It's been one of my 'go to' 3S models ever since.  I also slightly modified the ribs so that the top and bottom spars were at the same angle to allow easy fitting of the spar webs.

 

 

As the drawing. The spars aren't in line.

 

 

My ribs.

 

 

The wing with spar webs but no sheeting to complete the 'D' section or cap strips on the ribs

 

 

... and the complete model.  It's pretty rugged (so far!)

 

[Brian Cooper]

A "D" box makes the wing very sturdy and resistant to warping. 

Be aware the warping can happen when the ailerons are deployed. 

 

Rigid wing: good

Wobbly wing: bad. 

[David Ovenden]

For scale models with a sheet (ali or ply) covered wing then an "open" structure would look very wrong. 

[GrumpyGnome]

With the seemingly ever-escalating trend towards over powering planes, any lessening in structural integrity needs careful thought.....

 

Feels like if it won't climb vertically for 400 feet, or can't do 399 foot loops, it's underpowered.

Edited January 30 by GrumpyGnome

[Nigel R]

Required twist resistance will be linked to, I think -

 

* Maximum speed (where flutter might be an issue)

* Aileron deflection (3d? regular sport aerobat? stooging-around-scale-like-performance?)

 

Horses for courses, as usual. Some models want lots of torsional rigidity, some not so much.

[Futura57]

I love the smell of balsa dust in the morning!

 

 

Without going Full On a good place to start replacing balsa is for wing ribs and fuselage formers. Simple flat parts that are relatively easy to cut out. However, you might still need to resort to balsa or light ply for selected items for; strength around undercarriage fixings, wing mounts and engine mountings. But you could easily save a sheet or two of balsa, even on a smallish light electric model.

 

Gain experience in using alternative materials then try, say, foam board on the underside rear fuselage and single curvature turtle decks. Planking with foam is for the brave.

 

I built my new Strange Quark model almost entirely from Depron and XPS foam. 1/16 balsa wing core sandwiched in 6mm foam skins top and bottom and lite ply for the fuselage EDF mounting former. Admittedly it's predominantly formed from flat faces with a sprinkling of single curvature thrown in. The whole airframe structure, less covering and paint, cost about £10-£15 when you pro-rata the amount of material cut from the large foam sheets. Call it £30 all in finished with laminating film ironed directly on to the foam and DIY match pots to finish. Not bad for a striking looking 800mm span futuristic and practical EDF model. Still in development of course.

 

Anyway, I digress a little from balsa, but I guess I'm saying, fill your boots.

 

Ah, am I missing the point? Is this topic more about using less balsa rather than looking for alternatives. Sorry. 🤔

Edited January 30 by Futura57
Apologies

[Simon Chaddock]

There is an argument that using balsa for an entire airframe is a somewhat an inappropriate use of a single material.

Balsa like any wood has a grain. Works well in simple tension and compression as in a spar but it is not so good when used as a thin skin to resist torsion. You need a grain free homogenous material for that to achieve the most efficient use.

 

Once material cost comes into play the need to use the suitable material for the forces involved becomes ever more important. This of course does mean you have to know what the forces are and the properties of the material to be used.

 

As a hobby I am willing to sacrifice my time and follow the principle "Simplicity of fabrication may have to take second place to component functionality" in order to reduce material cost Easier to say then do but then I am retired!

[Chris Freeman 3]

Aircraft are built to perform in a certain way for certain flight performance and appearance. When building a aircraft a often look at what I want from the airframe and build it accordingly. I love building oldtimer like structures with strip balsa and around 15 years ago found a Stick plan using this construction and have built many examples in 4 different sizes, the largest had a Laser 300 on for a while when it was being run in! The construction is light and very strong but we do limit the speed flown as I am sure they could flutter. The first stick built is still on the floats it was test flown 15 years ago. This is a 10 year old example. 

[J D 8]

  When I first started in Radio Control I built a Rojair Cadet 62 but quickly found it was a bit much for me and decided to put it to one side for a bit and make something slower to have more thinking time. I had built a two channel glider light weight free flight style with open  structure wing and my first use of film covering.

 As a glider it flew fine but now I decided to fix a motor on the nose [ DC Saber] Had a good few flights until I got a bit cocky and started to chuck it about more. Started a spiral dive for speed to do a loop but with rudder straighten nothing happened, same with full opposite rudder.  As it spiraled down Saber flat out I could see the wing twisting holding in the turn until splat! Film and light structure not so good then.

  Rojair went on to teach me more until I let it get to far away in poor light, had to dump it before it reached the sea. Another lesson learned.

[Bruce Collinson]

2 hours ago, Simon Chaddock said:

There is an argument that using balsa for an entire airframe is a somewhat an inappropriate use of a single material.

Balsa like any wood has a grain. Works well in simple tension and compression as in a spar but it is not so good when used as a thin skin to resist torsion. You need a grain free homogenous material for that to achieve the most efficient use.

 

Once material cost comes into play the need to use the suitable material for the forces involved becomes ever more important. This of course does mean you have to know what the forces are and the properties of the material to be used.

 

As a hobby I am willing to sacrifice my time and follow the principle "Simplicity of fabrication may have to take second place to component functionality" in order to reduce material cost Easier to say then do but then I am retired!

I'm slightly hesitant to intervene here given the seniority of some of the contributors, but something about "a thin skin to resist torsion" just rang a faint bell and started me conjecturing a D-box wing with shear webs and which structural element might be in torsion.   Could it be the webs and therefore the spars?   Thin balsa would have little compressive strength but would add tensile strength if adequately restrained, i.e. glued down to the l/e, ribs and spars.   Maybe it is torsion, restrained over short lengths by being attached to ribs at relatively narrow centres.   Maybe its little compressive strength becomes adequate when so restrained too.

 

I suppose hypothetically I could build a rig and twist it to failure but the workshop contains higher priorities.   I did spend time one Xmas with several yards of 1/4" square building king post trusses to see whether the king post was in compression (everyone's assumption) or tension, as I suspected.   It was inconclusive and I now suspect that the upper part above the diagonal purlin braces is in tension but the part below is compressed.   Equally hypothetical.

 

For clarity, this is designed to stimulate debate, not a criticism.

 

BTC

[kc]

One way to minimise balsa in wing ribs is the Mini Waste wings method -  worthwhile on large models perhaps.

 

 

[kc]

Bruce - the thread was started to provoke discussion and to get the best ideas known to us all.   Good debate so far.

 

To finish with the Ugly Stik -  lack of D box just seems to work for that type of model with really thick wing section.   The designer - Phil Kraft - said " The open frame construction of the wing has given no problem with warping"   

 

Perhaps we could now discuss the Ivan Pettigrew designs and methods of building.  Not much discussion yet on Modelflying it's mostly on another forum.   So I was surprised when I first saw an Ivan design at my club - a 75 inch Chipmunk weighing about 5 pounds and powered by a 3S Lipo.    Very impressive in the air.  The construction is shown on this website.    Interesting!   and the Derek Woodward quote of  " fresh air - lightly trimmed with balsa " describes this well.

 

Should we build more in that style? 

 

 

[Bruce Collinson]

KC,

Thanks.

Extending my amateur structural engineering home-spun philosophy, is the difference with a thick wing section that because the spars are further apart, braced by the ribs to the l/e and t/e, they are better in torsion than if they are closer together?   If there was a single spar mid depth in the ribs, would there be significantly less torsional stiffness?

 

Loads can be counter intuitive.   Early water wheels were over-shot with thick spokes, presumably to accommodate the weight of the water at the top of the wheel.   The perfected version was under-shot over a variable sluice and with skinny spokes, like a bike wheel.   Someone twigged that all spokes are in tension.   They could equally easily be string.

 

BTC

[Nick Cripps]

The best shape to resist torsion has a circular cross-section. Think of things whose sole purpose is to transmit a twisting force (torsion) such as a car propshaft, it is circular in cross-section. To keep the weight manageable, the shaft is actually a tube whose thin skins are sufficiently strong to transmit the load.

 

The D-section on our models formed by the curved sheeting from the leading edge to the spar (top and bottom) combine with the shear webs on the spar to create an approximation to a thin-walled tube and hence can resist the twisting in the wing due to control and aerodynamic forces.

 

The purpose of spars is to resist the bending load in the wing, created by the weight of the model and any additional g-loads in loops etc. The spar contributes very little to torsional strength which you can easily demonstrate to yourself by taking a metal ruler and seeing how easy it is to twist along its length. If you then put the ruler on its edge and try to bend it, you will see how well it resists bending, just like a spar.

 

The optimum structure for a spar is to have 2 flanges separated by a shear web (think I-beams). The further apart the flanges are, the more bending load they can resist (up to a point when the shear web fails due to buckling) so having a flange close to the upper and lower surfaces of the wing section is ideal. Even better, the shear web between the flanges (holding them together) also forms part of the D-box mentioned above and contributes to the torsional resistance of the wing - bonus!

 

The wooden structures we use in model aircraft have been optimised in both the model and full-size aircraft world for decades and, where properly designed, provide the necessary strength with the minimum weight. People like Ivan Pettigrew clearly understand the principles involved and have created some impressive models.

[john davidson 1]

The strongest wings I have built were folded depron  , depron spar and covered with vinyl from a sign shop, the strength is on the outer surface where it does the most good. Almost indistructable

[Nigel R]

6 hours ago, Simon Chaddock said:

Balsa like any wood has a grain. Works well in simple tension and compression as in a spar but it is not so good when used as a thin skin to resist torsion. You need a grain free homogenous material for that to achieve the most efficient use.

 

Wings are quite long and thin, so maybe the skin having a grain oriented in the direction of the longest measurement is quite useful. We wouldn't run the grain fore and aft, for instance, as it would leave all the spanwise strength to the spar structure. With the skin grain spanwise, it can contribute more usefully to the whole structure.