Calculating wood required for a plan build

[Tony Harrison 2]

I took plans off the Web for a twin-engine twin-boom aircraft and had them printed out to a smaller scale, resulting in a model of approximately 60" wingspan. It's a project for the future, though I'm half tempted to start this winter. Being lazy, I wonder if there's a simple way to estimate how much balsa etc I need, other than laboriously going over the plans with a ruler and calculator in hand... Might be an easy answer, or I'll just have to do lots of guessing. What's my best approach? How do others tackle this?

rgds Tony

[kevin b]

Piece of paper, ruler and a pencil.

Then you have to figure out what lengths to buy for economy.

[Nigel R]

Experience.

Guesswork.

The odd bit of measuring.

Construction style makes a huge difference.

Are you buying a foam wing?

Will you cut strip wood yourself or pay up to get pre-cut stuff?

 

I normally overbuy. It's cheaper to get too much and then use it a year later, than it is to buy too little and have to get a single sheet of wood posted, or worse yet, buy some of the 5x normal price guff that you find in generic hobby shops. I also tend to ignore plans a bit, and make parts from whatever I have lying around.

 

Hobbycraft foamboard makes good ribs and formers in a pinch. Replaces most 3/32 or 1/8 balsa internal structure parts. B&Q 4mm ply can be used almost anywhere you see 1/8 or 1/4 ply called for (one thickness, or double it up). 

 

BTW 60" twin engine twin boom, I'd expect the wood bill to be well over £100 these days, possibly nearer £150 even.

 

What plan is it?

Edited November 2, 2021 by Nigel R

[Ernie]

Hi Tony, I don't know of an easy way.  It doesn't take too long, and if you are not very accurate, and add a wee bit on, you will allow for wasteage. 

In addition, its a good way to get an understanding of the drawings.  Also, don't forget to think about grades of balsa. But of course, there is very much written about that. 

 

ernie

[David Ovenden]

I start with the wing and add up the number of sheets of balsa , strip wood for spars  and leading edge and trailing edge wood ( if special section matetial. Make a list onpaper

Then I do the same for fuselage and tail. (Plus tail booms in your case)

Total up and add a bit extra I  case you miss something. For wing ribs you need to estimate how many you can cut from a 36x4 " sheet. Don't forget to tessellate the shapes to minimise waste!

So in short, no really easy answer. But it's not that difficult either Tony! It's a nice project for a winter 's evening.

[Tony Harrison 2]

53 minutes ago, Nigel R said:

Experience.

Guesswork.

The odd bit of measuring.

Construction style makes a huge difference.

Are you buying a foam wing?

Will you cut strip wood yourself or pay up to get pre-cut stuff?

 

I normally overbuy. It's cheaper to get too much and then use it a year later, than it is to buy too little and have to get a single sheet of wood posted, or worse yet, buy some of the 5x normal price guff that you find in generic hobby shops. I also tend to ignore plans a bit, and make parts from whatever I have lying around.

 

Hobbycraft foamboard makes good ribs and formers in a pinch. Replaces most 3/32 or 1/8 balsa internal structure parts. B&Q 4mm ply can be used almost anywhere you see 1/8 or 1/4 ply called for (one thickness, or double it up). 

 

BTW 60" twin engine twin boom, I'd expect the wood bill to be well over £100 these days, possibly nearer £150 even.

 

What plan is it?

Thanks Nigel (and others of course). No plans to use foam, don't like the stuff. But I ought to look into Hobbycraft foamboard, whatever that is. Never tried cutting strip wood though I should, can't be that hard - I've built two traditional balsa kits in recent years but this will be my first attempt at DIY from a plan. Your estimate of wood costs shocked me! Phew, that much? Plane is a Cessna Skymaster (USAF O-2A), been interested for years in Vietnam-era FAC aircraft from the Bird Dog on. Twin engines, push-pull, doubt that two powered (electric) motors are necessary so the rear one will just be a freewheeling prop...

rgds Tony

[Tony Harrison 2]

21 minutes ago, David Ovenden said:

I start with the wing and add up the number of sheets of balsa , strip wood for spars  and leading edge and trailing edge wood ( if special section matetial. Make a list onpaper

Then I do the same for fuselage and tail. (Plus tail booms in your case)

Total up and add a bit extra I  case you miss something. For wing ribs you need to estimate how many you can cut from a 36x4 " sheet. Don't forget to tessellate the shapes to minimise waste!

So in short, no really easy answer. But it's not that difficult either Tony! It's a nice project for a winter 's evening.

Hi David, thanks for the tips. A while ago I mentioned this project to you, and I believe you pointed me toward the plans site in question - downloaded them and had Stan Yeo print them out at a slight reduction in scale. I have a wide vocabulary but had to look up "tessellate"! Yes, my natural meanness will compel me to squeeze as many ribs as possible out of a sheet... But I hate cutting out ribs.

BTW I've pretty well completed my electrification of the Arising Star, looking forward to getting my local chums to help get it into the air. Old OS engine is still seized, but I'll continue efforts to soak/free it up.

Best wishes, Tony

[David Ovenden]

9 minutes ago, Tony Harrison 2 said:

Plane is a Cessna Skymaster (USAF O-2A), been interested for years in Vietnam-era FAC aircraft from the Bird Dog on. 

rgds Tony

 

So you will like my current project! Mohawk OV 1

[David Ovenden]

5 minutes ago, Tony Harrison 2 said:

 

BTW I've pretty well completed my electrification of the Arising Star, looking forward to getting my local chums to help get it into the air. Old OS engine is still seized, but I'll continue efforts to soak/free it up.

Best wishes, Tony

Maybe it is really properly seized but I imagine it's just completely gummed up with congealed castor oil

[Tony Harrison 2]

1 hour ago, David Ovenden said:

So you will like my current project! Mohawk OV 1

Did I know about that? Don't remember you mentioning it, but yes indeed, very interesting - I wonder if this is a kit (never seen one mentioned anywhere) or more likely a plan build...? I look forward to hearing more, maybe seeing it next year.

And re the OS engine, yes, I'm sure you're right - I have a bit of 2-stroke mix handy, will dunk it in that for a few days.

Best, Tony

[Nigel R]

1 hour ago, Tony Harrison 2 said:

But I ought to look into Hobbycraft foamboard, whatever that is.

 

Thick(ish) paper, on both sides of a 5mm foam core. About as heavy as 3/32 balsa but fraction of cost. You get four A2 sheets for a tenner - dirt cheap, given that a single sheet of 1/16 balsa is now about £2.50.

 

I did a build thread last year for a model that used a fair amount of the stuff...

 

 

 

1 hour ago, Tony Harrison 2 said:

Your estimate of wood costs shocked me! Phew, that much?

 

Two years ago I'd have guessed at £50-£75. Then world events happened and have somewhat affected balsa supplies ☹️

 

 

 

 

Edited November 2, 2021 by Nigel R

[Tony Harrison 2]

9 minutes ago, Nigel R said:

 

Thick(ish) paper, on both sides of a 5mm foam core. About as heavy as 3/32 balsa but fraction of cost. You get four A2 sheets for a tenner - dirt cheap.

 

I did a build thread last year for a model that used a fair amount of the stuff...

 

Thanks Nigel - I'm familiar with this stuff after all, artist's foamcore, bought from art supply places. Good idea to substitute this for balsa. I looked at your previous build article, have bookmarked it.

rgds Tony

[David Ovenden]

1 hour ago, Tony Harrison 2 said:

Did I know about that? Don't remember you mentioning it, but yes indeed, very interesting - I wonder if this is a kit (never seen one mentioned anywhere) or more likely a plan build...? I look forward to hearing more, maybe seeing it next year.

And re the OS engine, yes, I'm sure you're right - I have a bit of 2-stroke mix handy, will dunk it in that for a few days.

Best, Tony

It's an ARTF kit from Bkack Horse (discontinued model for some time now) I picked it up from someone selling up.

[kc]

My way of estimating balsa is to take a 4 inch by 36 sheet of balsa and lay it on the full size plan and see how it fits and how much waste is left.  Then count up the number of times ( sheets)  needed allowing for both fuselage sides plus top and bottom.  Note quantity of sheets and thickness plus grade needed.  Of course use a 3 inch sheet if that seems better, and you could also work from a small scale plan and have a scaled down sheet of balsa if needed.

Much the same with stripwood hold a strip against all strip parts and count.  Where there is part lengths move your thumb down the strip as you place on each part.  Judge whether the main strips or sheets have any unused part which might make a smaller item again by holding a thumb over the used part.   ( Rule of Thumb perhaps?)

Be sure you have allowed for both upper and lower wing sheeting etc.

A couple of wing profiles placed on a sheet of balsa can show how many could be fitted on one sheet and therefore how many sheets needed.  Some ingenuity used to interleave tails of ribs might save balsa.

Then allow a spare sheet of each size for safety and perhaps errors in cutting.  If buying by post it will be cheaper to buy plenty extra than pay postage for another shipment!

Before ordering go over the plan and see if there is any thickness that's been overlooked, and also see if any have special light or firm requirements.   Order piano wire with balsa shipment and also any fittings to save postage.  Balsa Cabin and also SLEC etc do a wide range of fittings as well as wood.

Most of the Peter Miller free plans of a couple of years ago cost around 25 pounds worth of balsa and ply then so perhaps no more than double that now.  I would guess any 60 inch model would take far less than £100 if careful layout and cutting is used.   I will revise my guess if you state exactly which plan so i can examine it on the computer screen.

[Nigel R]

Balsa is now around 2.5x what is was about two years ago, kc. The Skymaster's fully sheeted 60" wing will need around £30 of wood, just for the skin. If you can bring the whole thing in at 'far less' than £100, you are a better man than I.

 

For the record, I have very similar size build on the go. Sport model. Nothing too fancy. Target weight 6lb-7lb, 650sq in wing area... comparable to the proposed Skymaster... the wood bill (and only the wood, no fittings or hardware) was £150.

 

 

[Tony Harrison 2]

20 hours ago, kc said:

My way of estimating balsa .................I will revise my guess if you state exactly which plan so i can examine it on the computer screen.

Many thanks kc - detailed and very useful advice. I'll try to follow it. On the Outerzone site there are a few plans for the Cessna Skymaster, and this one isn't what I originally downloaded but it's described as 60" wingspan, so about the same as mine: Cessna 336 Skymaster (oz8180)

rgds Tony

Edited November 4, 2021 by Tony Harrison 2

[kc]

Well I had a glance at that plan on Outerzone and even though it's difficult to see exactly what sizes are used ( it seems to be a kit plan ) my estimate is that about 89 pounds worth of balsa would suffice.  Plus a sheet of 3mm lite ply at £3.65 to make the formers.   That is assuming one uses the balsa economically - eg using offcuts from wing sheeting to produce some ribs, laminating spare 1//4 to produce the shaped tank hatch  and spats etc etc

I made the assumption that fuselage would be from 1/8th and wing sheeting & ribs from 1/16th, the formers from 3mm liteply.     One might save some balsa by making the fuselage sides out of 2mm Liteply, maybe make the booms from laminations of liteply and balsa to gain strength.

 

It is surprising how much the balsa costs to make a scale model with traditional methods.   Making flat & ugly sports models from foamboard etc seems quite attractive!    Looking for secondhand scale kits might be worthwhile as the owners might not realise how much the balsa is really worth now!

 

One thought on the 'windmilling' rear prop.   Years ago in RCME Peter Russell in Straight and Level answered my letter to him about a windmilling prop creating drag - he said it does!  In fact he said the best way of slowing the model down for landing is a slow running prop.    I cannot dispute his expert answer so you might consider making the rear prop work with a motor or else allow for the extra drag of a non powered prop.  

Finally, Tony  if you state the plan you actually downloaded I would be interested to view that too if it's online now.

[Nigel R]

21 minutes ago, kc said:

Making flat & ugly sports models from foamboard etc seems quite attractive! 

 

I think an amount can be done to reduce the wood bill. Some ideas to use less wood on sport builds.

 

Open frame 1/4" truss to replace some/most/all of the sheet sides.

Planking for front decks (not thick blocks). Or even stringers.

Block used only where unavoidable such as cowls.

Rear deckings and underside to use stringers for the shape (maybe on top of a truss if needed for strength).

Foamboard for internals stuff like formers.

Foamboard as a core for tail surfaces with some 1/16 stringers on top to make it look good.

 

D Box wings instead of fully sheeted (although that saves less than you might think).

Foamboard ribs.

Obechi (or similar) veneered foam wings instead of built-up?

 

Glass cowls laid up over foam plugs? Glass & resin hasn't gone up in cost like balsa.

[kevin b]

Re Cessna rear motor.

Just put a cone at the back end to look like a spinner.

You wouldn't see the prop in flight anyway !

Edited November 4, 2021 by kevin b

[kc]

I think Kevin is right - just a cone instead of a motor and prop.  This would help with CG as putting a motor & prop behind CG would only make a tricky CG problem worse.   It appears that the twin rudders would need some fancy setup like 2 servos at the tail, long pushrods/ snakes  through the booms or complex closed loop wires if the rudders need to work.   Of course much same for elevator - rear servo or snake or pushrod through the boom.

All in all a challenging model considering the need for wing joiners and  lightweight but rigid booms.   There are plenty of easier models for a first build from plans!  

[Tony Harrison 2]

5 hours ago, kc said:

I think Kevin is right - just a cone instead of a motor and prop.  This would help with CG as putting a motor & prop behind CG would only make a tricky CG problem worse.   It appears that the twin rudders would need some fancy setup like 2 servos at the tail, long pushrods/ snakes  through the booms or complex closed loop wires if the rudders need to work.   Of course much same for elevator - rear servo or snake or pushrod through the boom.

All in all a challenging model considering the need for wing joiners and  lightweight but rigid booms.   There are plenty of easier models for a first build from plans!  

Good points, kc, about the servo complexity. I might (should?) go for something simpler...

rgds Tony

[kc]

Tony, I think you said you had built from kits but not from plans.   In that case I suggest building from a good plan with instructions - not from an ex kit plan without all the kit parts or from a scaled up or scaled down plan where you have to work out new wood sizes etc.     Changing a model from ic to electric presents enough challenges!

I suggest you look at either a purpose designed electric plan such as recent RCME plans especially Peter Miller ones like Ohmen etc.   Or at ones from the US RC Modeler range on Outerzone etc.   RCM plans have very complete instructions and photos in the articles.   They are mostly ic so some changes would be needed to get battery access etc, but the plans are very detailed and don't usually have any drawing errors.

Scale models take a lot of extra work and there is always the compulsion to produce the perfect scale plane which often results in giving up before finishing!    Far better to make a good sports model and get some flying in.    It might be that a sports model finished in a scale colour scheme might suit - one that came to mind was the Strikemaster from RCM.  Another similar style is Striker by Peter Russell an RCME plan thats on Outerzone.    There's hundreds of good sports plans to work from that would produce a good plane with less hassle than a scale job.  

[Andy Gates]

There is an easy way.

 

Just buy plenty of balsa, any excess can be used on the next build.?

[Don Fry]

But, and I admit I have more wood than a model shop used to stock, it’s a bit of an investment today. 

[Andy Gates]

Me too!

I topped up just before the first lockdown kicked in.

I probably have around £150 (at old value) of balsa left in my shed at the moment after 2 builds.

Most of mine is thin sheet, 1/16", 3/32" & 1/8" as that is what I use most