Yes, a simple scale down can be done rather easily, but there are a few items to keep track off, and pay attention to.

Simply scaling down to 48" span is basically linear scaling. Divide the wing span you want, by the wing span you got, (48/60=. The .8 is your lineal scale factor. Your copy place can scale to 80% (. and you're done, almost.

There are other considerations to work with when scaling a design. I prefer to consider the wing area when scaling. I believe it gives me a better size when I'm considering the engine I want to power the scaled plan with. Use the same idea as before, divide the area you want by the area you've got. Now, this is important. since we're dealing with squared numbers, our answer is also squared, so we must take the square root of our answer, to get a true scale factor.

For instance, I have a 700 sq inch, .60 and up, design that I want to shrink, and power with an OS .40LA.  The LA should be fine with a design having 620 sq. inches. this gives me enough area to keep the wing loading low. I'll have to do a few things to accomplish this, like remove weight. I can extend the scaled rib spacing back to the original spacing and get rid of a few ribs that won't be needed in the smaller sized wing. Removing weight by altering the structure is a good practise, though not often done, when scaling a design.

Here's an example using the above numbers.

620/700=.8857

The sq root =.94 as a scale factor.

This is a larger plane than it would be with simply scaling from 60 inch span to 48 inch span, but it fits the desired engine better.

... and using John's formula, if you scale a 60" wing to 48", that's 80% in span. Square root of 0.8 is 0.894, or 89.4%.

If you started with a 700 sq in area, this percentage would go to 602 sq in, in stead of John's 620 for his numbers... This is seemingly close, but it is better to work from wing areas. Check what works on the engine you want to use, and shoot for that area.

Enjoy! Stuff like this can get habit-forming, especially when it works well and you understand why.

Well I hate to say this but I was a straight "D" student in math.  That means my teachers passed me because they felt sorry for me, not because I understood what they were talking about! LOL

I think I understand what you guys are saying though.  Take the wing area I want, divide it by the large wing area and that gives me a percentage number to use to shrink the original large plan.  And if the large model was a good flier, the reduced model should also be a good flier.  Is this basically correct?

Well I hate to say this but I was a straight "D" student in math.  That means my teachers passed me because they felt sorry for me, not because I understood what they were talking about! LOL

I think I understand what you guys are saying though.  Take the wing area I want, divide it by the large wing area and that gives me a percentage number to use to shrink the original large plan.  And if the large model was a good flier, the reduced model should also be a good flier.  Is this basically correct?

Not quite.

When dealing with wing areas, you are dealing with squared numbers. 620 square inches divided by 700 square inches = .8857 squared. You have to remove the sq. from the answer, and you do that by using the sq. root of the number, which in this case is .9411239.

I have the availability to have my computer find the area of an object. Here's a practical example using this principle. I'll use at least 3 decimal space as the cad program is accurate to 17 decimal spaces, just to show how accurate it is.

Gordan's Tony has a wing area of 743.321 sq inches, and a span of 63.901 inches. It's designed for a .72 or larger engine.

I want to scale one for an OS .46LA. I've seen this engine doing a great job powering a plane with 640 sq inches.

So I want to scale the Tony down to 640 sq inches, I don't know what the span will be because this will not be a lineal scaling job.

I have to find the scaling factor that will give me the square inches I want, then I can measure the span.

640 sq inches divided by 743.321 sq inches = .861....inches squared.

I need to get rid of the inches squared portion of the answer, so I find the square root, which = .9279012. (I'm going to use all the decimal places here because I want the exact square inches after all is done.) This is my scale factor.

Now going back to my Cad program where I had the computer figure the wing area, I can use the scale command, and use .9279012 as the scale factor.

The computer now resizes the wing using this information.

I do another inquiry for area, and the computer measures the new wing at exactly 640 sq inches. The span now measures 59.294 inches. The new wing is now 4.607 inches shorter, but the area is a little over 103 sq inches less.

In my earlier example, I may not have been clear on one point. That 60 I referred to was not the wing span. it has no place in figuring using sq inches of area. it referred to the engine size, a .60 engine related to a .40 engine. They were only used to illustrate the size airplane normally utilizing the wing areas noted.

I use the example of the Tony showing that the math works, and the Cad program shows the figures to be correct.

So simply put.

Take the area you want, divide it by the area you have, find the square root of the answer, and you have your scale factor, using the area (Inches squared).

If all you are concerrned about is a lineal distance, such as wing span, then it's simpler, but may not reflect exactly what you think it does.

Take the span you want, and divide it by the span you've got. No squared numbers, so no square root to arrive at the scaling factor. A clean and simple linear scaling.  

Glenn, Serge and John...

My bad. I was in a bit of a rush when I wrote that and put it down backwards.

Of course 0.8 squared is 0.64.... I knew that, but wife standing in door waiting to go somewhere made me a bit careless...

(I don't do it the way I wrote, way above, when I'm doing it for real.)

Fortunately, almost any pocket calculator will do this. In the above example, you'd just punch in "540", the divide symbol, "720", "=", and the square-root symbol, in that order, and the scale factor appears in the window.

Not my calculator.

I hit 540 - Enter - 720 - Divide - Square Root.

RPN rules.

Randy Smith has plans for a smaller Dreadnaudht .35-40 size
Ed

Hmmmm,

Didn't we just go over this about a month ago in Scale Down?

I guess we didn't get it definitive enough huh?

Jim Pollock   

My concern would have been that the metallic brown wouldn't scale well, but it seems to have come out just fine.

Or you could just buy a Dreadnought 40  or  Tempest 40 plan  and build it as is     


Randy