Scaling plans up or down? - RC Groups
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Mar 10, 2002, 12:32 AM
Registered User

Scaling plans up or down?

A lot of the best planes and plans are too big for my Endoplasma power system, but would work fine scaled down 20-30%.

Question is, when you scale down planes can I just take the plans to the copy shop and copy them at the reduced scale and build it. I realize I'd have to account for material thickness somewhat, but do wings, airfoils, fuse, and tail feathers all stay in the exact same proportions as the original?

Thanks, Balin
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Mar 10, 2002, 07:29 AM
Speed Demon
GregG's Avatar
Yes this should work very well. What plane are you planning on building?
Mar 10, 2002, 10:12 AM
Visitor from Reality
As long as the original is nothing too radical - tiny wingtips with a thin section tend to work worse as the size drops - this should work fine.

Figure out what span will give you whatever area you need, use that against the original span to give you a linear reduction factor first. Then apply that to a few critical areas - getting a battery pack in is one. If the fuselage over or under the wing won't take your packs, you are in for some grief. Deepening the fus off starts to look like a redesign, for one. Radio gear can get moved around - I always stick the fuselage servos aft of the wing aperture, for one idea.

Once you have a reduction factor and a fair chance of fitting the hardware off to the copyshop you go. The structure redesign should be simple, just do it over the plan using wood sizes off other designs. All my models between 6 and 20 cells have wing spars based on 1/4 x 1/8" spruce, for a hint...

Have heard from folk who've done it around here that costs on getting prints done of these big copier machines vary immensely and Kinko's is out the roof on costing on their 36" roll paper copiers. You might want to do a little phoning around some engineering drawing shops and the like.
Mar 10, 2002, 10:47 AM
Registered User

Your comment, probably simple and obvious to you, isn't to me

Figure out what span will give you whatever area you need
Ie. how do I know what area I need? Must vary by type of plane? I'm thinking aeroatic, like a 4*40 (though I don't have the plans to that, but it is a "type".) I've been trying to think this through for a few days and don't know the fundamentals well enough. Is there a rule of thumb or a formula (Orme's law) or the like that works?

Thanks Balin
Mar 10, 2002, 06:24 PM
I fly, therefore I crash
This is where the challenge to building the planes comes in, and why they can be so fun when you fly them - the fact that sometimes they actually work can be quite an accomplishment!

The Technical section on the ezone site has a wing loading chart by Dennis Weatherly:
and he gives a rough estimate of 23 oz/square foot as a good place to begin. Of course, it varies by plane, and micro models are way under that, and 3-D planes different than warbirds, etc. The most important factor I've found is just don't make it too heavy. Before I learned about wing loading, my little Guillow's Hurricane did a nice glide from my hand into the dirt, engines revving!

Once you've figured out what plane you want to build, see if you can find something similar and take a look at their weight, wing area, etc and compare it. Or, you could just build the plans at regular scale first, then build the smaller one later. . .
Mar 10, 2002, 06:36 PM
Visitor from Reality
Hi Balin
More rules than you got thumbs! Didn't realise you needed to go that far back - sorry

Okay, Orme's - Believe it's in the FAQ's. I like 35 square inches a cell, but amazingly enough, one square inch per watt comes out close too, looking at my fleet. You have to fudge this to how you care to fly, the speed your chosen type looks capable of, and how light you can build. 400 squares sounds a good ballpark for a model like a 4*10cell would perform, though you'd have to widen the fus from 'scale' to get a battery in with sub-Cs laid across the fuselage.

Measuring my beat up old 4*40 wing, I get 59" span x 10" average chord - my ailerons are a tad wider than kit spec, IIRC. Let's call it 60 x 10, and 600 sqins as that's easier!

Say 40" span - 67%. makes the chord 6.7" = 268 sq ins. Not good! 49" span is 82% for 8.2" chord and 402 squares - sounds good to me. 10 cells at 400W = 1 watt / sq inch, or 40 watts a cell for Orme's. Believe me, after 3-odd years, a 4*40 won't go much better on over 70W/lb.

Shaw's Law suggests that 70W/lb allows you 3.1lb of model - just about do-able if you build it like a real model, not a BARF. I'd even reckon that a decent prospect if you prop to 300W full out - my Putz-E flies pretty well on 512 sqares at 59oz (the #2 version will be much lighter!).

My new AXI powered ten cell will be around 320 squares, but is aimed at flying with CP1700 / 2000 nimh. Even with a parallel chord wing, it should be pretty nippy.

Hope that helps some

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