PU/PVC vs PS Foam

[wolf.]

General build question, I have a half build Varieeze project, wings, canard are done, the fuselage is still missing the bottom and the outside glass. The Pu foam is sundamamged and I will substitute it. Now my question PU and PVC foam is cumbersome to get here in Italy, what speaks (apart from a possible fuell leak) against using Polystyrenfoam (the same density) for the fuselage bottom, sides, nose and canopyframe?? I guess the PU has been selected by Burt because of it's insolubility in Gasoline?

 

Wolf.

[Kent Ashton]

You don't want polystyrene/styrofoam anywhere where it might encounter a fuel leak. Generally, the sides and bottom of the airplane are fuel-resistant foams for that reason.

 

There are a few Italian Cozy builders and a large number of canards in Germany. I would try to make some contacts there and find out where to get the proper foams.

[wolf.]

Yes but apart from th efuel issue, from a structural point of view is there a difference?

 

wolf.

[Len Evansic]

Yes, there is a big difference.

 

You want to use PVC foams like Divinycell near fuel areas, actually just about anywhere. Polyurethane foams shouldn't be used for any structures, because they become frangible with UV exposure and have a tendency to become brittle. The long-term effects on PU foams were not well-understood when the Vari-eze and Long EZ plans were written. Polystyrene foams are OK for wing cores, but they have a tendency to delaminate from skins under dark coatings (like N-numbers or decals) or after extreme cold soaks. Oh, and there is that whole pesky melting in fuel problem.

 

-- Len

[rickh]

Len:

 

Would you care to elaborate on "cold soak"?

Would a recently built canard parked outside in Frostbite Falls, MN in Feb. have 'issues'?

 

Rick

[Len Evansic]

What I meant about cold soaks is that under most normal conditions, the blue foam will be OK. If you intend to spend most of your time flying above 18,000 feet, then you will have an extended cold soak that may lead to skin delamination from the foam, during aeroelastic loading. One thing to note is that the polystyrene in the blue extruded styrofoam billets is already in a glass state. Similar to glass, it becomes more brittle at lower temperatures.

 

This issue with polystyrene foam was discovered on a high-altitude plane at Scaled, that had these problems while flying very high for extended periods of time. The exact temperature that is required for the delamination was not measured. I have heard about this plane experiencing temperatures lower than -100°F, which is way colder than any Cozy will ever see. It was also implied that this type of failure was seen on other aircraft. I have seen extremely limited use of blue styrofoam, since I arrived at Scaled.

 

It was conveyed to me that PVC is a much more reliable foam for wings that experience cold soaking. I did not hear about a specific testing to verify this, but I trust the source of this information. Scaled does a lot of testing with a cold soak chamber for joint designs and material suitability. It should be noted that PVC isn't hot-wired, nor is it used as a solid billet like polystyrene. Instead it is used as a stiffening core for ribs or skins.

 

On a set of Long EZ wings, I saw extensive delamination between the skin and blue foam, under where dark decals were adhered. This type of delamination appears to be due to repeated localized melting of the foam. This is a good reason to not paint any horizontal surfaces with a dark color.

 

-- Len

[rickh]

Thanks a ton Len, very interesting read.

 

Rick