Upsizing Plans - Questions and Wonderings

[wedgetailaus]

Hi all,

The last few days I've been really inspired by the idea of building some FT designs upscaled to 200%, double size!

I've probably scratch built 10+ FT planes so I have some experience but they were all at 100% size.
I've seen a couple of threads and a bit of info about upsizing around the forum but having never done it before I still have questions! If it's okay I'll just number them off and if there are any someone can answer I would be greatly appreciative. I'll be sure to post my build photos and completed aircraft when they're done!

1. Does doubling size require more than double the number of sheets of foam? On the Monster Planes episode David describes the 200% Spitfire as using 11 sheets. Why does it require more than just double the 4 sheets used on the standard FT Spitfire? Is it just reinforcing parts or am I missing something? Or should it require 4x the sheets because it's twice as long and twice as wide? Why not quadruple the sheets?

2. Wing spars! Should I use foam spars or would ply/carbon be better? If I scaled up a simple cub, mig or something else would a foam spar be okay? How does a single spar work on a plane with dihedral?

3. Calculating thrust. How do I pick a motor and prop? Just use a calculator buy weight and wing area I guess? Get something similar to a plane of equivalent size? Mig3 should be like a Carbon Z T28?

4. servo, pushrod, control horn size. Same as above, should I use a plane of a similar size to guide my choices? I've never used anything beyond a 9g analog servo or FT control horns so there is a lot of new stuff for me to learn in this regard.

5. CG. I heard in an FT video somewhere that upsizing plans will make them more tail heavy. What do I need to be mindful of to reduce the issues here?

6. I understand that a/b folds and tabs will need to be adjusted when plans are resized. Anything else to think about in this area?

Sorry for all the questions. I'm expecting failures and challenges but I thought I'd look to the community for advice to minimize my hiccups!

Cheers!

 

[Hai-Lee]

Firstly I will answer the number of sheets question as the answer is the simplest.
When doubling the linear dimensions ie 200% then the area actually increases by a factor of 4 (2x2).

Most people use the linear measure when talking of doubling rather than the surface area of a plane.

Spars are normally made of stronger material like spruce, pine, Oak, Ply, CF, F/Glass, or what ever takes the builders fantasy or he has in stock, (laying around).

Motor and prop selection are a little complex depending upon your design goals. A larger plane pushes far more air but only around 4 times at the same speed as the original and wings perform more efficiently as their size increases. When scaling up it is often smart to reduce the wing thickness to keep the drag of the wing lower than the scaling would otherwise produce. So generally keeping the build relatively light and increasing the power to double or 3 times the original will give a similar speed. To go twice as fast any aircraft would require a 8 times increase in power for the same size so you see the roundabout effect in design selection. For a strong enough performance I like around 100 to 200 watts per pound, (500Grams), of flying weight unless Building a motor glider or a definite speed machine.

Servos will need to be upgraded to more powerful ones if the control surfaces become too large or heavy but for most "Doubles" a good 9 gram servo and a solidly fixed control horn would do.

CG location should be the same as it is on the original plans and as for the tail heavy condition it is true because the quadrupling of the surface of the material is more noticeable in areas where the surface area is greatest, (the tail). The trick here is to add fuselage doublers and even wing LE reinforcing by adding another internal layer of material beneath the wing LE skin. Normally the nose will require some reinforcing because the motor, ESC and battery will be of greater capacities and weights.

The A and B folds will definitely need to be adjusted unless you use doublers on the inside of the fuselage. Often the doublers are definitely used forward and the empty half of the rear channels can be filled with 5mm square balsa sticks which add strength and do so at about half the weight of Fb.

One thing you did not cover in your questions is that as size increases the relative strength of the FB decreases or in other words the larger the piece of FB the less resistant it is to side impact and distortion under load. Try bending a 10 mm piece of FB and then try using the same force upon a much longer piece. The longer piece will bend far easier! Long unsupported sections of FB have very little structural strength though the weight is greater.

I hope I answered the questions for you or at least gave you some insight into the issues. My wording does not always express exactly what I am trying to explain so if you have any further questions or require further clarification please advise.

Have Fun!

 

[Maxstudio]

Great Answers Hai-Lee! I also am considering a 200% build on the Fokker DR1 and had all the same questions, which you answered very well. I've got some planning to do!

 

[wedgetailaus]

Thanks Hai-Lee! Appreciate your very detailed response. If I get a build going on a giant plane I'll post and no doubt come with many more questions!