Does scaling up work?

[wisconsin_flyer]

Hello! I have recently built the Versa wing, and it flies great. I am looking to build the Spear, as I like the payload capabilities. However, I would like to modify the design so that I can essentially double the size, and of course, increase the power behind it as well. I was planning on using a 6S lipo with a single motor at around 480kV, with a 2 blade 12x10 prop. Based on my calcs (In MotoCalc), that would give me approximately a 23 mph level flight at about 45% throttle (in theory of course). The question is - can you double the size of planes like this and expect similar characteristics? I get that it will generally fly slower, but does this seem like a reasonable setup? Are there other things I need to consider here? thanks!

 

[DamoRC]

One thing to be aware of is that if you exactly double the size of the current plans the foamboard used will weigh 4 times as much as the original. Large wing area is good for flying slow but only if you can keep the weight down.

Still, I think it's worth a go with that amount of power.

DamoRC

 

[Merv]

The question is - can you double the size of planes like this and expect similar characteristics?

I’ve not tried what you want to do but I don’t think it works that way. I know wing loading changes as you increase size. That is an indoor plane will have a wing loading in the range of 1-2 oz/sq ft, whereas a plane the size of a Bushwhacker will be 10-12 oz/sq ft. Your only changing size by 2x so I may be mistaken.

You might look at a plane similar in size to the one you want to build and see what they are using for power, wing loading, ect.

 

[jajefan]

One thing to be aware of is that if you exactly double the size of the current plans the foamboard used will weigh 4 times as much as the original. Large wing area is good for flying slow but only if you can keep the weight down.

Still, I think it's worth a go with that amount of power.

DamoRC

Actually, the foam board weight will be 8 times as heavy, and the wing surface area will be 4 times as large, thereby making the wing loading twice as heavy

 

[DamoRC]

Actually, the foam board weight will be 8 times as heavy, and the wing surface area will be 4 times as large, thereby making the wing loading twice as heavy

On area - yep agree. But I don't see where you are getting 8 times as heavy, unless you assume that we are also doubling the thickness of foamboard used. If not then the weight multiplier is the same as the area multiplier.

Thoughts?

DamoRC

 

[d8veh]

On area - yep agree. But I don't see where you are getting 8 times as heavy, unless you assume that we are also doubling the thickness of foamboard used. If not then the weight multiplier is the same as the area multiplier.

Thoughts?

DamoRC

Double the length doubles the area, then you need double thickness for strength, so every sheet would be doubled. That's if you do everything double, but with more thought and adjustment to the structure, you might be able to save some weight/thickness in some areas.
if you started with a 1ft square, which is 1 sq ft, double would make it a 2ft square, which is 4 sq ft, then you need two for double thickness = 8 sq ft.

Basically, wing area follows a square function, and mass follows a cube function, so wing loading will always go up as you increase in size.

 

[DamoRC]

Double the length doubles the area, then you need double thickness for strength, so every sheet would be doubled. That's if you do everything double, but with more thought and adjustment to the structure, you might be able to save some weight/thickness in some areas.
if you started with a 1ft square, which is 1 sq ft, double would make it a 2ft square, which is 4 sq ft, then you need two for double thickness = 8 sq ft.

Basically, wing area follows a square function, and mass follows a cube function, so wing loading will always go up as you increase in size.

Mathematically makes sense. But in reality I don't believe there is a need to double the thickness of foam for the entire bird. Wings can still be single layer with some additional sparring, sidewalls of the fuse might need some doubling etc. So the weight of the final plane should not be 8x.

Additionally, although the wing loading (oz/sqft) will increase, the cubic loading (oz/sqft^1.5) is the same if you double the size of wing with double thickness foam and my understating is that the cubic loading is a better estimate of the similarity between flying characteristics of differently sized planes.

DamoRC

 

[Hai-Lee]

I concur with the 4 times as heavy argument as there is no need to use double thickness FB everywhere Actually there should be a slight bit of extra weight from additional wing bracing/spars to keep the rigidity of structure. This minor additional weight increase is normally compensated for by the higher Reynolds number associated with the larger wing, (The air doesn't scale and the bigger wing is just more efficient).

Most FB models can be scaled moderately with ease and success if increasing in scale but reductions are extremely difficult without special lightening techniques. to keep the wing loading in line with the less efficient wings.

Just a few thoughts!

have fun!

 

[Merv]

The question is - can you double the size of planes like this and expect similar characteristics?

Bigger planes are generally easier to fly, they are more stable. The down side, they suffer more damage in a crash and are more difficult to store & transport. When you double the size you will change how it flys. If you double the size, do you double power requirement? I think you’d be better off looking at a plane of similar size. Copy what they use.