Scaling

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StRaNgEdAyS

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Oct 20, 2003
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Northern NSW Australia
In the process of becoming familiar with the operations of my new CAD program, working within the already familiar realm of Large scale R/C models,I recently did an experiment on the issue of scaling (albeit in the other direction) just to examine the results for myself...
I took one of my large scale model designs (this Baby has a wingspan of almost 3M) and reduced it in scale, right down to the engine thrust...
now I should mention, these particular engines get disproprtionately larger as the thrust factor decreases, so when scaling down the engine therefore appears much larger in relation to the airframe...
this model was scaled down to 1/4 of the original size and even before I add the empennage, I can see it Ain't gonna fly, not without some substantial changes. Most noticably the engines, but also notice the comparative size of the wing foil...
This was just an experiment I did to check out the effects scaling has on these things...
Just to think years of experimenting and toying withg R/C models of varying scales, and I never actually did this before. (probably because The idea of drawing up a plane that looked so much out of proportion and had no chance of getting off the ground just didn't occur to me)
Now there are probably a couple of factors which I have blatanty (and purposefully) ignored, such as, as I scale down the weight of the plane also comes down at a disproportionate rate, so while my size is coming down to 25% of the original model, the weight will end up being less than 25% of the original, therfore I could use smaller engines etc, but this was an exercise in direct scaling, and I treated it as such.
 

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orion

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The thing to keep in mind is that scaling is not a simply proportional exercise. The linear dimensions may change in proportion to the scale, but the areas change as a square of the ratio and the volumes as a cube.

And the mass of the airframe and its power requirements end up scaling differently from those linear and powered factors. Also, you must take into account the perfomance requirements, the effect of Reynold's Number, as well as the effects of scaling on the powerplant itself (power density, prop efficiency, etc.)

This is why I tend to caution folks who are trying to work up or down from a given design - it's not a simple linear design process and usually the best procedure is to approach the new design as a completely new airplane. Only that way will you assure yourself that it will perform and behave in a predictable manner.
 

Holden

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Jan 18, 2003
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A guy named "Stan Hall" Wrote an article in sport aviation, 30 july 1987 that explains scaling well. I have used it and it seems to work rather well. Table one gives the factors. It is quite an undertaking to truly scale an airplane. When you do it correctly it will fly like the full size airplane, except on the low end.

For example, the typical 1/4 scale model of a 1600 lb airplane (like an RV6, for example) would weigh 25 lbs. Most store bought craft weight at most 10 lbs and most are 5 lbs.

One point that he does not cover is the Re # effects. At Re below 500,000 the lift will go in half and the drag will double. The problem in scale models arises when the main wing chord is larger than the tail chord, resulting in the tail dropping into the sub 500,000 range while the main wing is above. You can see this as a pitch down that appears as a stall, but may in fact not be a stall. Knowing what the Re number is by knowing the speed of the airplane will help, realizing what is happening. Also the chord size difference will cause some pitch differences depending on the rate of change of Cl as a function of the Re.

Hope that helped
 
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