Hi, maths not being my strong point and model materials being less than cheap if I got it wrong and the plane is too heavy to fly legally (20kg being the maximum). I’d appreciate someone much better at maths to give me a quick calculation to double check what I’m getting.

Assuming a direct relationship between size and weight (which there isn’t in reality due to material thickness not being scaled up so large models tend to get lighter in proportion to their weight)

If a 1800mm wingspan plane weights 3kg and a 2400mm wingspan plane weights 6.8kg then what would a 4000mm wingspan plane be expected to weight? Assuming also that the model design is virtually identical for the same plane design.

I’ve done my calcs of this very simple scale up but interested in what others opinions would be.

Thanks

You state that bigger planes don't get incrementally heavier due to the way materials are used, however, the larger plane is only 1.3 times wider, but more than twice as heavy?

true, though a large part of the weight is the engine and batteries which don't scale up very well, in fact the engine for the 2.4m version is almost the same as the 4m one but larger one has vastly more power. Model planes are damn tricky things to scale up on weights.

For it's size the 4m one will have the lightest weight for its scale as the airframe design is identical but material thickness is identical to the two other models apart from the main spars which are scaled up. I did build one of the 4m wing halves a few years ago and it was amazingly light for it size, amazingly too it seems to have been lost somewhere! it's a lite ply and balsa constructed model, I have both the 1.8 & 2.4m models but want to build the 4m one as a huge float plane as it will handle better on the water.

Given that you need to scale all dimensions, then the simplistic rule is that volume (and hence mass) increases as the cube of the scale factor. For your supplied example, 2400:1800 is 4:3 or 1.333. The cube of that is about 2.37 and 2.37 x 3kg is 7.1kg. As you say, is not an easy linear scale but it's close.

On that basis 4000:2400 is 1.667, cube of that is 4.63 and 4.63 x 6.8kg is 31.48kg. Might be a bit less due to material factors as above but unlikely to be under 20kg.

Edit to add: your info about batteries was added while i was typing. What are the airframe weights without batteries and motors? Those will scale more easily.


[Edited on 12/12/2016 by TheGecko]

In your example the larger plane is 33.% bigger than the smaller one and fro most thing weight goes up as a cube of the size i.e. 1.33 cubed is 2.37 so:

You would expect the bigger but identical in all respects plane to weigh
2.37 X 3kg = 7.11 kg

But it actually weighed 6.8 kg so difference is 6.8/7.111 = 95.64%

So 4000 mm plane should weigh:

4000/2400 = 1.6667
6.8 x 1.667 x 1.667 x 1.667= 31.48
31.48 x 0.956 = 30.1 kg approximately (Very approximately given the small amount of data provided)

He's right - the volume (approximating to the weight) goes up by the cube of the linear change. So if you double the size (assuming double the dimensions in all 3 planes) you increase the volume/weight by 8 times (2x2x2).

1800mm to 4000mm is a bit over double (actually x2.222), so the result will be more than 8 times the weight

1800mm = 3kg
2400mm = 7kg
4000mm = 33kg

Timing!

your weight predictions are much higher than mine but based on better maths and I see how that works.Certainly looks like I've avoided an expensive mistake.

Very good helpful answers, thanks very much

Interesting so really the largest I can go is a 3.4m wing, will keep an eye on the weight and see if I can keep it down compared to the smaller models. Thanks.

[Edited on 12/12/16 by Mr Whippy]

Easy. If everything was scaled up using identical materials then it will be as above (2x2x2) 8 times heavier.

But this rarely happens.

Once built a plan based Spitfire which had a designed weight of 5lbs, I managed to get it to 8lbs, flew beautifully.

Had a 80" Stomp (Tiger moth like), so light for it's size it often blew over just sitting on the runway.

So I think it's more likely to be the "How long is a piece of string " answer.

this will be a scaled up Precedent T240, been flying these for over 20 years (on my third 240 now). Got a 2m Spitfire too, nice plane apart from the realistic scary landings, more than once the wheels have been ripped off or the cowl or canopy broken. Tiger moth is like a paper bag in the wind, low winds only...

Do you have a shed or a Hanger for these?

Large spare bedroom 4m x 5m (with a lock on the door to keep the kids out) with a 3m x 1m 1" thick ply work bench for building very flat wings etc and lots and lots of loft storage...

Been building balsa rc planes since I was 11, now 43 so kind have built a few over the years, best hobby every

Most of the models can be broken down to fit in a car and their stored like that to save space and stop wing warping

With Kids, How?

My last proper kit is a Top-flite Spitfire.



Bought it along with a Saito 90 and Air retractable U/C ( before Kid's )

Still on top of the wardrobe un made, my eldest becomes a teenager this month.

Just a few Depron cheapies since.

yeah built that spit a very long time ago, ran it on the os120 sadly totally blootered it into the ground loosing it while chasing a wot4 (pulled too hard in the turn and big time stalled span and second later it was a crater) do still have the tail somewhere also bent the engine pushrod tubes and after that my other spits always had grp fuselages which were a lot easier to build.

Yikes! The 240 is big to begin with. I'd worry about building it too light- did you see the video of that massive (but light) turbine jet plane (? griffen). Didn't end well.

quote:

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Originally posted by Mr Whippy
Tiger moth is like a paper bag in the wind, low winds only...

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quote:

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Originally posted by BenB
Yikes! The 240 is big to begin with. I'd worry about building it too light- did you see the video of that massive (but light) turbine jet plane (? griffen). Didn't end well.

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I learnt to fly on a "Precedent T240" sure it was rudder and elevator only, and as said it was BIG to start with,
a 4 meter version I could sit inside of, comfortably

However, I progressed to semi scale, and had a Mick reeves spit, lovley to fly, landings were better with the wheels up
as our strip was not very big, plus a load of other warbirds, Hurricane, Seafury etc, all in the 2 mtr plus size,

I loved the hobby, but didn't like the expense, so no longer do it, infact only sold my Tranny, a few weeks ago, along with everything else R/c related,

steve

quote:

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Originally posted by BenB
Yikes! The 240 is big to begin with. I'd worry about building it too light- did you see the video of that massive (but light) turbine jet plane (? griffen). Didn't end well.

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Think the Red baron, pilot got into the wrong acraft, thought it was a pitts

quote:

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Originally posted by AndyW

quote:

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Originally posted by BenB
Yikes! The 240 is big to begin with. I'd worry about building it too light- did you see the video of that massive (but light) turbine jet plane (? griffen). Didn't end well.

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For me it would be almost impossible to calculate, it's not so simple as scaling of volumes , lots of difficult to estimate variables
I guess the first thing would be to estimate the weight of the power package, it'll be chunky with lots of extra strengthening to support it

In this sort of situation I tend to look for something similar that's already out there, eg

This 12' Telemaster has a flying weight of about 15KG (not sure if that's nitro or electric)
http://www.hobbyexpress.com/12_ft._telemaster_laser_cut_kit_1039257_prd1.htm

but a quick look around suggests that some have built them lighter

I would be more concerned of the all up weight . once your over the 7kg mark all the rules and regs change and i cant rember if its 15 or 20 kg and it has be properly checked . its been years since i flew but still own a hand full of helicopters and planes which i hope to put up for sale to fund the shell for my fury . but just be careful i stopped flying a few years back club was getting too dangerous with new flyers turning up with models that were not safe or incapable of flying . that and the rise of gps guided multicopters . seems any fool can fire one of them in the air nowadays. which i feel has simply spoiled the hobby for the true modellers as am pretty sure heavy regulations will be inforce soon when some one causes a big accident flying there phanton or other of the shelf multicopter or drone some where they shouldnt .

lots of nice helis for sale if your interested