I have a plans set for Reese's Cloud Dancer 120. What I want to do is build a 41" electric version but what I notices is that while reducing the overall size by 50% or so, I lost more than 70% of the wing area. I discovered this when trying to specify the dimensions in MotoCalc and the area seemed alarmingly small.
I am no engineer, it took a little bit of head scratching and re doing my calculations before some hazy reccolection of highschool geometry showed up with something about area reducing by the square of the hypoteneuse tangent to the radius divided by pi. Or some such thing.
What I am thinking of doing is enlarging the ribs and wing saddle but building the wing with the 41" spacing, in other words increasing the chord but not the span.
Is this even necessary? Should I also enlarge the tail group?
Other Ideas?

If you reduce the image size 1/2, the area reduces to 1/4. (1/2 x 1/2)
Build light.
The Cloud Dancer should handle the reduced area OK.

Paul is correct (As usual)

I run into this problem with clients and their pictures all the time

If you have a 2" x 2" picture, it has a total of 4 sq in

Reduce it by 50% and it becomes 1" x 1" for a total of 1 sq in

your original idea to slightly increase the chord -on the smaller one - is a good idea
on really small planes - lower aspect ratio is goood idea. also figure to have at least 25% of wing area fo stab - that is - increase the chord on the stab too.
bottom line
little electrics work better if they are a bit chubby looking.

If built faithfully to plans, reducing the plans usually results in an overbuilt plane that is needlessly heavy. Consider reducing the number of wing ribs and maybe using an open bay wing structure instead of a D-tube design with cap strips and shear webs.
Remember that not only is the plane lighter, but also it has a smaller wingspan, which reduces bending stress on the center of the wing by itself. The compounding of these two factors means that you can get away with flimsy wing structures that you would never get away with on the larger plane. Unsheeted foam wings are plenty strong for a typical 1/2A size plane.

Scaling -down- the structure as well as the shape would result in paper thin materials.
You'll have to adjust the sizes to be proper for the loads, which means a slightly beefier part.. but also prune the unneccessary stuff to compensate.

You want a good wingloading, right? Gather up all your equipment that's going to be in the airplane and weigh it. Make a guess at the airframe weight. Now, use that information to determine what area you need to get the wingloading you want, then scale it to the appropriate size.

If a linear distance is halved, the area is divided by 4 (1/2 x 1/2)

Don't worry though, the weight (which is related to the volume (1/2 x 1/2 x 1/2)) reduces by 8, so your half scale model will be only one eighth the weight.

Since the wing area was divided by 4 and the weight by 8 the wing loading has halved.

This 'scale factor' (which affects other factors too) is one of the main problems in designing models of anything.

-David C.

scaling down or up -- is a real trap for most modelers.
scaling down -- you usually can't stay light enough to get equivilant performance
scaling up - typically you quickly run out of power
for example: most 40% aerobatic types are never too heavy to fly well but power is never excessive
scaling down to really small stuff -like a 30" span - power remains ok but wing loading is almost always way too high for slow flying.
if only the air could be scaled to match the model.
from the first Wright Bros thing - getting enough power has alwas been a problem
ALL of the powered aircrft since then have only improved as better powerplants became available
all of the work on airfoils for them has been done in order to make the weight work with the power available.

Yes indeed!

Then perhaps the Reynolds Number could also be kept constant and the wing might act uniformally, whatever the scale!

-David C.

As I understand it, area goes up and down a square function of linear measurement, while acceptable wing loading goes up and down as a cube function. There is also the square-cube law. Strength is proportional to cross section. Weight is proportional to cross section times length. Suppose you have a 2 inch square stick 20 in long. It has 4 square inches cross section but weighs proportional to 80. Scale it down 1/2 to one inch square by ten inches long. Its strength is 1/4 that of the 2 inch square stick, but its weight is proportional to 10, 1/8 the weight of the larger stick.

Thanks everyone! This is really helpful. I do want it to fly slow so I think that increasing the chord is the first step, then the stab as well, just a smidge. I have no experience estimating the weight on this, being my second scratch project, and the point is well taken to build appropriate to the size rather than just include all pieces from the larger version only smaller.
I am selecting the wood carefully and using hard balsa ply for the light ply pieces . I am also going to use landing gear fixed to the fuselage and save the weight of the wing gear mounts. for wing hold down, good old fashioned dowels and rubber bands. I think that If I use stick built ailerons, wider that spec'ed that will also help the float factor.
Some of this is for weight and some as much for simplicity with hoped for weight savings.

good ideas - remember -it is physically impossible to make it too light -and still have acceptable strength.

Not trying to sound like a broken record, but it is possible to get the wrong wingloading. Either too light or too heavy is bad. You can't just scale something down without doing a little homework and expect it to fly like the original. Yeah, using TLAR it will probably fly, but not necessarily like you expect it to.
Try looking around at aircraft in the same class and see what their wingloadings are in the same power and size class. Use the information that's already available to eliminate having to experiment. Usually, you can get pretty close by scaling down to an area similar to another airplane in the same class(es) and it's not a terrible lot of work.

Thats a good Idea Ptulmer! I have the remains of a Spree Sport (Same as GP Super Sportster ep) here in the junk corner, I'll dig it out for reference.

Dick... for the life of me I can not figure out your comment
"-it is physically impossible to make it too light -and still have acceptable strength"
Does it mean that no matter how light I make it it will still be acceptably strong? or Is this a caution about building it strong enough.
I knew that D- in english would bite me someday...

English- my downfall.
On small models , especially powered small models, it is very difficult to make them as light as needed and still retain good flying characteristics.
Typically the novice builder makes them flimsy - rather than simply light.
The shining light in the darkness is the advent of Depron foam and li po battery electric motor powered models .
powerful smooth powerplants and a building medium that is light, strong and resilient. easy to work with and wing loadings of 4 ozs to the foot can be done by any one.
When I was a kid, the closest thing was light sheet balsa and the then new .020 and .049 glo engines. these broke easily.

Dick, are you saying that you were around for the beginnings of Cox .049s in 1945 and old enough to build models? Wow! There were earlier 1/2a engines, but Cox made them popular.
Slightly off topic, but there was some experimenting in the 1/2a forum awhile back with SWR's (sheet wing racers) and at 8oz they flew like real pylon racers and were nearly indestructible unless you nosed in. Thanks to these electric fliers, the equipment available for small glow is fantastic!

Think you meant 1954. The glow plug originated around 1948. The first airplane Cox 049 was the Space Bug which I had one of in 1954. then Space Bug Jr, Thermal Hopper, Babe Bee, Pee Wee, Space Hopper, RR-1, on and on. I think the two US made engines with the longest continuous production are the Fox stunt 35, and second the Babe Bee.

I once built a 66% scale version of Neil Allen's "Zenith 40", RCM plan #1212. This resulted in a 37 inch wingspan .15 powered pattern style plane that I could take to the field in my compact car without dissasembly. I used 1/16 sheet instead of 3/32 for the D-tube sheeting and ribs and 1/8 inch balsa instead of 3/16 for the fusilage sides. This resulted in a plane that weighed 33 ounces, a little heavy for a .15 but it flew.
If I do this project again, I'll probably power it with a 2100 mAh 3-cell li-po and maybe an e-flite 480 outrunner and strive to get the weight down to the 26 to 28 ounce range, or maybe I'll just buy me a Mini Edge instead.
In retrospect, the scaled down design was way overbuilt for a 37 inch wingspan plane in my opinion. I think 1/16 balsa sheet would be plenty strong for the fusilage sides and the 37 inch wing could have been a twin spar structure with alternating full and half ribs instead of a D-tube and still would have been plenty strong. Also, I could have built the fusilage and wing as one piece since I almost never took the plane apart for transport anyway.

I read in some scientific journal that the reason ants can lift many times their own weight is mostly due to scale effect. Scale an ant's design up to the size of a large mammal and it would be crushed by its own weight.

The Space Bug was new for '52, but Cox opened the doors in '45. To tell the truth, I wasn't aware that Cox didn't have their own engines at first! Here's all the info you could ever want on Cox engines: http://www.mh-aerotools.de/airfoils/cox_frameset.htm

I actually built my first control line model a Frank Hawks Time Flies -in 49- powered by a Atom 09 -but even whipped -it would not leave the ground
Next up was a Maverik- it did fly -for a moment
finally I built a Brave with a backport 23 O&R got me into the air on controline stuff-
I was made aware back then - what really made models work and a friend traded me flying in an Aeronca Champ and a Ercoupe for me helping him learn control line flying
a fair trade for a Jr High kid!
I learned "aeronautics" using the time honored OTJ approach.

The Space Bug was the first Cox aircraft engine. The Cameron and the early Cox engine were car engines.

I would still use shear webs with alternating half ribs correct? Initially, and maybe still, I was thinking of using every 3rd rib, there are 14 on the 98" version, and using 1/32 for the sheeting.

I would build something similar to the e-flite mini-Ultrastick wing which has a 37.75 inch wingspan. Full ribs are 4.375 inches apart with half ribs in between. The center section is fully sheeted and it is half sheeted (D-tube) out to the first full rib away from the fusilage. I'm not sure what the spar is made out of but I would bet that two 1/4 inch square hard balsa spars joined by sheer webbing would be adequate. I haven't folded this wing yet.

Fox, if you're still reading into this thread despite the attempts to hijack it off track then you will find that at around 41 inch span the wing loading to shoot for will be in the 7 to 8 oz/square foot range for spirited performance. It'll fly at up to 10 oz/sq ft but you won't get much fun out of it and the power off glide will suffer.

So with that sort of wing loading in mind you can find out what your target weight should be. Just divide the wing area in sq inches by 144 (that's 144 sq inches/sq foot) to find the number of sq feet of wing area and then multiply that by 8 oz/sq foot to find the target weight you should be building for. From there it's up to you to find out the fixed weight of the equipment and from there what is left for the structure.