Hi all!


I would like to have a go at cutting some foam wings.


The foam blanks I have are 23.5” x 15.5”.


I was thinking of using a NACA0018 aerofoil but I'm open to alternatives.


36" span or so for .15/.19 size models.


47” for .25-.40 size models.


For my first attempt I may leave the wings solid (unhollowed) until I
get a bit more practice with the hot wire.


I would like a tapered wing mainly for aesthetic reasons.


My main question is what root and tip chord should I go for on both sizes?


Thanks in advance.


Annette

I would just copy some existing plane for sizes and airfoil. Hey wasn't I just talking to you a minute ago?

Hi, Annette,

Not sure if you've cut foam before... If not, it would be a good idea to locate and work with a friend who has. You could waste a lot of foam getting the hang of it.

Aspeed has the right idea; the wheel has already been invented.

"Coring" a foam wing panel isn't particularly difficult if you can handle cutting the outer form. I use 'drop rod' gizmo. A brass tube guides an iron rod to drop vertically. The tube mounts to a strip of what we call 1"x2" wood. When all is lined up - heated rod is to drop through spanwise and emerge into the 'hole' in the template at the far end - the rod is heated (I use a propane torch to heat the entry 5 or six inches.)

The same templates serve both outer and core cuts, to keep them in the proper relationship. Once the rod drops through, the cutting bow can be disassembled to allow one end of the wire to pass through the hole melted in the core's interior, then restrung. The template's inner edge finishes cutting the cores free, restringing the bow as needed

Take a good look at how cored foam wings are cut. Most often there's a vertical foam web left between top and bottom outer surfaces - practically a main spar at the deepest point of the airfoil....

Consider also what wood structure you need. Recently, some are using molded leading edge surface sheeting. Formerly, and still popular, I'd guess, a strip of balsa is glued on to be formed as the leading edge, and the sheathing over the surfaces is either trimmed to edge-glue to it or to go over it after it has been shaped to the core.

Mounting the leadouts, bellcrank and pushrods needs thought as well, as does joining the left and right panels true square and strongly.

The trailing edge probably will need a strip of balsa for stiffness and support for flaps and hinges, if you use them, and is simplest when that strip is straight from tip to tip. The flaps, operating or not, can be formed of balsa to match your TE taper. The tip shape, also, is best added in balsa.

For a 47" foam span, and the engine range you mention, figure from 450 to 500 square inches area. The span times tip and root chords, averaged, provide the area. Ratio of chords should be about 10 to 7 or 8. Simple arithmetic? Remember that trailing edge balsa flaps are part of the total area. Example: 500 square inches would need an average chord of about 10 inches.

A root chord of 11 inches and a tip chord of 9 inches would serve, as would a 12" root and 8" tip, etc.

A wing's aerodynamic center chord-wise is straight when the 'quarter chord line' is straight - e.g., for the 12" and 8" tips, the root "ac" is 3" behind the LE, and the tip "ac" is 2" aft of the LE there. The LE "sweep" is 1" back, and the TE sweep is 3" forward.

Remember that straight "flap" hingeline, which suggests that the foam core area should have a 9" root chord, with the remainder added in balsa.

You may already have all these thoughts dealt with, but it is just me, explaining the blatantly obvious again... Relying on the assistance of an experienced friend can acquaint you with the numerous small steps none of us remember to mention when we try to explain how to do something.

Luck and enjoyment! Tell us more as it develops...

I was thinking more of a ukey 35/gotcha streak/extra chubby type model.

A pic of the chubby is attached.

Annette

Deleted

Looks very good! Nice cutting...

Are the panels passed through the fuselage? Is there a 'joiner spar' between them?

I see tape about 2/3 back from the LE - purpose? (I have used fibreglas reinforced tape on the underside of glider wings to improve stiffness against flexing under launch loads...)

How do you plan to cover the foam surfaces?

Like the Ukeys and a few Aussie designs, the bellcrank can be mounted externally with only some effect on the flying. Nice and simple.

Keep us updated?

It's not one of mine - from a Pete Tindal kit.

It's not tape - can't remember what it was - it has an internal spar.

Whole wing doesn't pass through the fuse just the spar the wing is butt glued to fuselage.

Covered with blue wrapping paper.

Has an OS Max 25 for power.
Annette

Looks good!

Waiting to hear how it flies for you - N.B.: Where I am it is near 80°F (25°C), calm and clear...

Most of the rest of the US is being slammed by a very nasty winter...

We pay it back in mid-Summer - mid - to upper 30s C.

A tip ~50% of root chord is most efficient. A tip ~70% of the root chord is most practical. Making narrower tips and deltas requires special techiniques.