CLBetten

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In the past when I have changed the size of airplanes I have had to guess what powerplant I would use. I have even changed the size of a plane to put an engine I liked into service. I understand that flying weight of any airplane is affected by a lot of factors especially when building from scratch. Is there a fairly accurate formula for calculating weight when changing size? I.E. for every 50% of size change the weight will change by a factor of 400% etc.

limeybob

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Here you go.
http://www.mnbigbirds.com, its under Resources /tools
Calculator available here; http://pages.sbcglobal.net/limeybob/#software
bob

longdan

One way to work it out is to use a cube. ie - if you have a cube that is 1" x 1" x 1" and you increase it by 50%, then it becomes a cube that is 1.5" x 1.5" x1.5". Its weight (and volume) is therefore 3.375 times as great as the original cube. Or if you decrease the 1" cube to 75%, it is now a cube that is 3/4" x 3/4" x 3/4", so its weight is 0.422 times the weight of the 1" cube. Hope that makes sense.

John 38

As longdan says, use the cube of the scale factor to estimate the new weight - eg for an increase of 50% then new size is 1.5 and cube that to get approx volume/weight change.

Wing area changes by the square - ie for 50% , 1.5 x 1.5 = 2.25 so the wing loading will go up by 3.375 / 2.25 = 1.5.

For power I keep the power / weight ratio the same so if the original had a 0.10 engine then the scaled up will require a 0.33 sized engine.

These are approx as you will find that some parts - engine bulkhead and U/C may need to be stronger for the higher stresses.

Also, wood sizes may not scale up to standard sizes and you need to guesstimate which way to go - eg 1/16th wing sheeting may be OK on the scaled up.

hope this is of use

I scaled up Vic Smeeds Mamselle by X2 and then by X3 and the power went from 0.5 ccs diesel to .30 4 stroke and then 90 4 stroke which flew the mamselles vintage style and these engines fitted quite closely to the above calculation.

john

limeybob

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Acceleration a K^0 Acceleration Does Not Change With Scale
Dimensions L K^1
Areas S K^2
Volumes v K^3
Weights W K^3
Forces F K^3
Time t K^.5
Velocity L/t K^.5
Horsepower HP K^3.5
Thrust T K^3
Wing Loading by Area W/S K^1
Wing Loading by Volume W/v K^0 Acceleration Does Not Change With Scale

Bob

CLBetten

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Thanks I will be able to make good use of those numbers. If I had specified exactly what I was thinking about building you guys would have done the math for me since the first plane on my mind is a .40-.46 size that I am enlarging 150%. As luck would have it I guessed about right (hopefully) since I have been planning it around a 1.50. If I understand and use the cube formula then a .40 sized engine for the original plan would calculate as .40 X 3.375 = 1.35 so a 1.35 for a 150% version of the same plane right?

Thanks, Cliff

Lnewqban

Cliff,

For a solid body, the mass increases eight times when the dimensions increase twice.
For a model airplane, having so many cavities, that proportion should not apply, I believe.

Check this thread out to see some examples:

http://www.rcuniverse.com/forum/fb.asp?m=9538253

BMatthews

Lnewqban is dead on right.  Due to changes in all the wood sizes and the large number of cavities there is just no way to produce an equation that would be in any way accurate. 

But some rules can be applied in some cases.  For small shifts in size where the wood sizes do not change and you don't add any more parts the weight of the sheet parts will rise by the square of the scale factor the same as the wing area does.  We can say that because the thickness of those parts is remaining the same.  Blocks used for special shapes will change by some factor in between the square and cube based on how thick you leave them when hollowing out..  Strip stock weight will increase by the linear factor .  So if you fudge it then you can expect the weight of the only slightly scaled up or down new version THAT USES ALL THE SAME WOOD THICKNESSES to roughly change weight by the square of the scale factor.

Really big size changes will bring with them the need to increase or decrease sheeting used for many parts.  And often some areas need to be greatly re-engineered.  As soon as this happens you can't use the handy trick noted aobve for small size variations.  The need for size related re-design of the internal structure and bumping up the sizes of wood for the more extreme size changes make things so highly complex that the calculations are all but a pure guess.

John 38

strip wood if scaled up exactly will increase in volume and weight by the cube. A scale up of the size envisaged will mean all strip wood being scaled up - strategic parts probably rounded up and less strategic parts rounded down to match available stock sizes

Cliff, you didn't say what model you are scaling up - a 40 sized aerobatic ( 60" span ?? ) will go up to 90" span (??) and that will end up with more weight/power than say a vintage style model scaled up.

john

CLBetten

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I understand this is not exact science especially when you consider the design changes that become necessary. From experience I knew at least that the weight changes at a greater percentage than that used to change the physical size. The formulas will give give me a better idea of what I will be working with. In the past I have had it easy because I have only enlarged old timers and they are very forgiving.

The plane I am getting ready to start is a Sig LT 40 enlarged from a 70" to 96" wingspan. So I enlarged the plans just under 150%. I want to use a SuperTigre 2500 for power.

BMatthews

I think you may want to do a reality check on this one.

The stock Sig LTdesign is a rather lightly built model of 70 inch span that comes with a .46. In spirit of design, namely a structure that is half way between the light sticks and bones of an old timer and a solid all sheeted model, it shares a lot with the Graupner Senior Telemaster. The ST is a 94 inch span model that only uses a .60 two stroke or a .90 four stroke. So that's a 10cc 2S or 15cc 4S. And you want to basically make a Senior Telemaster sized Sig LT40 and power it with a 25cc two stroke? I'd suggest that this is rather excessive. It would also lead to nose weight issues having such a big engine on the front of a big but rather lightly built design.

It's also interesting to note the size to weight ratios between the LT and the ST. As I said both models share the same philosophy of fairly light open structure. The LT is listed as coming in at around 6 lbs for it's 70 inch span. The Senior Telemaster is 10.5 lbs for it's 94 inch span. So the ST is 1.35 time the span of the LT and 1.75 times the weight. This is a pretty good example of what I mean about how you can't scale the weight for something built up like a model airplane where there's lots of variations in the final "density"due to structural considerations. And certainly you can't say that if you make a model bigger by X then the weight will be X^3 because a model is not a solid object. And in fact in this case the weight is only raised by roughly 6^1.3. So in this example of two similarly built models but of differeing sizes we only see a rise in weight of roughly the LINEARscale factor. This is because much of the volume increase in scaling up a model is going to be filled with air.

Now obviously this is a special case. Both of these designs rely on largely open structures that are then covered. On a typical all sheeted warbird scale model that uses balsa over foam and gets a surface finish that typically starts with finishing resin and light glasscloth the actual weight scaling factor would be higher because there's more material and less air volume involved. Some collecting of comparable examples of the same plane done to differeing sizes would be needed to produce a some sort of 3 dimensional graph that compares size, weight and structural "density" to see how the trends go.

Anyway, given all this I'd suggest that if you're keen on using the S2500 in a Sig LT scale up that you want to be looking at more like a 120 inch span model. Built with the same sort of structural philosoply a 120 inch version should come in at around 16 to 18 lbs and be a lot more in line with the power level from the S2500. Usingthe same idea of scaling by the linear dimension which the LTto STcomparison suggests would be valid in this case blowing the LTup by 1.7 times to 120 inch span suggests that you should be able to design and build such a model in the same STRUCTURALstyleas the LTand STto around 21 lbs. I'd suggest that this is a more suitable size and weight for the ST2500. At that size and weight it should float around in a most trainer or Cub like manner. Add flaps for some short field fun and you'd have a really sweet fun model to fly. And of course there woudl be lots of room for an onboard camera.

CLBetten

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I originally thought the 2500 would be a good engine for an 8' version based on my experience with my 9' Kloud King and my Sig Spacewalker ll. with 2500's in them. The Kloud King is quite hollow but due to the short nose etc. I still had to add nose weight. It is not extremely overpowered. I would build another one just like it. When you remind me of my 8' Senior Telemaster that flew fine with a .61 and was overkill with a .90 I see your point however.

BMatthews

9 feet is 108 inches which would not be a totally overkill option for the 2500. But I'll bet that the model basically went uphill at only one or two clicks up from idle with that engine. And if you needed nose weight in addtion to the S2500 I would have to suggest that you built the tail section of the Kloud King far too strongly and heavy. Those models when originally built to their free flight specs would have only needed a little nose weight in addtion to the sparky .60, batteries and coil. And all that weighs MAYBE half what the 2500 weighs. And atlikely around6 to maybe 7lbs with a Super Cyclone or similaron the nose the KK would have gone uphill at a prop hanging but solidpace50 to 60 degree angle spiral climb. So that is what makes me suspect that you overbuilt the tail which lead to the added noseweight despite the overkill size of the engine.

It's also not so much the size but the weight. I fly an7 foot span old timer that is 5.5 lbs with an old OS loop scavenged .35. The climb is very much in keeping with the old Brown Jr models from the days of this design and I like it that way. It still gets up to around 500 to 600feet in a minute and a half and for me that's just fine. I could not see putting even a modern schneurle .60 in this model since it would turn it into a SAM missle for only a few added oz of weight.

Since you've flown a few models with your 2500 you should be in a good position to know how models of a given weight fly. The key is to then say to yourself "How big can I make a Sig LT copy and have it come in at X lbs?".

You may want to google for "scaled up telemaster" or "enlarged telemaster" to get some guidance and examples of what others have done. I suspect their experiences with making giant Telemasters will serve your project well.

CLBetten

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I still fly the heck out of my Kloud King. I spend most of my time around half throttle. I live at about 4500' altitude. The short kit I built it from was designed around a Ryobi 31cc. The tail feathers are all stick built but I did add some weight making them detachable. I have a couple of pictures of the prototype that shows the structure a little better than mine. The original model was a 63" span I believe? There is another thread on RCU with a 108" wing flying beautifully with a .91 four stroke. My hat is off to him.

longdan

I have an old trainer (GMS .47 engine) that I use for thrashing around when no other planes are flyable. As an experiment, I've tried to see how much weight I can put in it and still fly it. It lifted a 1 kg lead weight. It was a bit more docile and needed a bit more throttle than normal, but it still flew. The whole plane weighed 1.2 kg ready to fly (before the addition of the 1kg weight). This tells me that weight calculations when scaling up or down can be flawed big time. Think about it - this plane would fly regardless of whether it weighed 1.2 kgs or 2.2 kgs. Thats a huge range given the original weight of the plane. This needs to be taken into account when trying to scale the weight of a plane up or down. since many planes will still fly even when loaded up- with ballast weight.
I used to fly slope sorer gliders. Often I would load it up with lead to make it fly faster.