Composite skins (Balsa verses Foam)

[Konrad]

Composite skins (Balsa verses Foam) on molded composite models.

As I’m ambivalent (of two minds) on the subject, I’d like to ask what is your take on the subject of wing skin materials?

I like Foam skins such as Rohacell as they are man made they are very consistent in their properties. Being a plastic they closely match the thermal characteristic of the epoxy matrixes. I like that because they are uniform (monolithic) there is no fiber to split from impact. That is damage stays localized. I also like that foam is impervious to water. This is real nice when wet sanding during any repair attempt. The only real down side I see to the use of foam as a composite skin is that it is much more prone to dents (hanger rash).

Balsa it appears to be a bit more durable when looked at for day to day wear (less hanger rash). But I find it is not as stable to changes in heat and humidity. I find it much more difficult to repair. Impact damage tends to run away from the point of impact following the wood grain. Also I can’t sand the fiber epoxy lay ups using the wet sanding method as the balsa swells when exposed to water. Unfortunately it shrinks when it dries causing ripples in the shin surface.

So what do you look for in the skin material of your composite models?

All the best,
Konrad

 

[jvaliensi]

Mixing wood with carbon seems less than ideal to me, because of the differences in thermal expansion and water absorption. Isn't wood less dimensionally stable compared to carbon and resins?
But many are successful with the combination. Don't they fiberglass wooden boats?

 

[Konrad]

Mixing wood with carbon seems less than ideal to me, because of the differences in thermal expansion and water absorption. Isn't wood less dimensionally stable compared to carbon and resins?
But many are successful with the combination. Don't they fiberglass wooden boats?

Yes, to the first part.
And yes you can find fiberglassed wooden boats. I pity anybody needing to repair that structure if it is 10 years old or older. I think most of the fiberglassed wooden hulls I've worked with were polyester not epoxy.

That's another topic polyester verses epoxy. I will tell you that I will not buy a polyester lay up, for our airplanes!

 

[Wayne]

If given a choice I like foam as the filler. The density and properties can be controlled with foam. When it comes time to damage repair I find that the foam planes usually keep the damage to the area of impact and do not spread down the wing. (splinter) Also, I find they are lighter, yet still very strong.

With this said, there are MANY options for the foams. The fully molded gliders tend to use very similar foams, but when you start looking at full scale or other composite structures you will find MANY different foams exist for this same purpose. Some have not survived the test of time well. Typically not a concern with a model plane.

I will agree 100% about boats with wood.. Buyer beware!