CARDFAN

My Feedback: (64)

I'm going to scale up my 4 Star(SIG) plans from a 40 size to a 120 size; just love that airplane! My question is, in scaling the size up 33-1/3% what effect does that have on the wing airfoil??? I realize that increasing size does not have a 1:1 relationship with the various stress factors which can increase exponetially (I can deal with that) but what I'm not sure about is if the shape of the airfoil can be increased linearly without having -/+ issues. Fortunately the 4* has a constant cord and root thickness wing so blowing up the size and construction should be easy although, I will probably convert to barndoor ailerons from the designed strip style. I realize wing kits are available, but part of the fun with this hobby is the journey as well as the destination. Thanks, Ed[ul][*] [*] [*]
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Jim Thomerson

I don't think you have a problem. Can't see why the airfoil shape would change if you do a copy machine type enlargement. .

Campgems

The 4*120 has a much different wing construction that the 4*40 or4*60. While all three share the same airfoil, the spars and webbing are different on all three. I personally like the 4*60 design best and I have scalled it down for trainer wings in the 60" WS size. I eliminated the rear spar and the result was a light, strong and very effective wing. My concern in scaling up the 4*40 wing is that the extra loads of that big a plane might exceed the design of the 4*40 even scalled up.

I replaced the stock ailerons with 2" tapered ailerons stock on my 4*60 kits. When I scalled the wing down, I did the same. If you are going to use barn door ailerons, the tapered TE would be advisable.


Just some food for thought.


Don

BMatthews

Using the same shape of airfoil just scaled up will work just fine. It's a good airfoil with kindly charactaristics.

As for strength the spar increases it's load bearing ability with the square of the difference in spacing between the upper and lower caps. So if you use spruce spars and web them you'll be fine. If you prefer to add the webbing by just gluing it to side faces of the spars so they look like an ] instead of fitting them between so they look like an I then for the first half out from the center do both the front and back so they form a full box section. This is because the balsa webbing itself is the weakest part when you glue balsa. A heavy load on the spars and thus the webbing could tear the wood away from the glue line. Doubling up the webbing in the higher stressed inner half of the wing will do much to avoid any surprises. I'm not usually one for beefing up structures because it all adds weight. But doubling the webbing is a small amount of weight well spent.

A rear spar isn't a bad idea if you're using conventional ailerons instead of strips. The rear spar should be right at the hinge line so it can be used to accept the wing side of the hinges. And if you wish this would be a fine place to hinge some functional flaps to provide more fun.

Wedgetail

Scaling the design up by 33 1/3 sounds ok. Keep in mind thatdrag is proportional as the square of the speed. This means if you double the speed, the drag increases four times. The airframe would need extra stiffening and strength of the parts to be able to accommodate the extra loads imposed. If you double the size of the plane, you will need four times the power to maintain the original speed of the original in level flight. To double the size of the plane you don't multiply all the dimensions by 2. Instead, you multiply all the dimensions by 1.413. The most common error that I have seen builders make is multiplying by 2 when they want to double the size of their plane. What is missing in their calculations is that the resultant must be calculated as a volume. The plane is stated as length x width x height. So to actually build a model twice as big, the constant is not 2 but the square root of 2 which is 1.413
The airfoil can be directly scaled up to suit the wing chord you need. I highly recomend that you build a "box" spar not an "I" beam. For max strength, the webs for the spar must be glued to the sides of the two spars and glued to both sides. The webs run from the top corner of the spar down to the bottom corner of the lower spar.
It sounds like a great flyer.
Wedge