Building Foam Wings

At the beginning of May of this year I bought and skinned a foam wing. As with all my projects I set it aside to work on the house and other fun things. Today I came back to it and found a gap between almost all of the skin joints. Do foam wings expand in the heat? I live in very high humidity and the average temp is around 90 plus the last couple of weeks, plus my shop is a metal building (OVEN). I filled the gaps and covered the wing but wonder if I will have problems in the fall and winter? I have always made built up wings and thought this would be quick and easy.

Thanks,

>At the beginning of May of this year I bought and skinned a
>foam wing. As with all my projects I set it aside to work
>on the house and other fun things. Today I came back to it
>and found a gap between almost all of the skin joints. Do
>foam wings expand in the heat? I live in very high humidity
>and the average temp is around 90 plus the last couple of
>weeks, plus my shop is a metal building (OVEN). I filled
>the gaps and covered the wing but wonder if I will have
>problems in the fall and winter? I have always made built
>up wings and thought this would be quick and easy.
>
>Thanks,

Ron B.
F4Fguy

Zuriel:

It sounds as though the skins were not properly joined before adhering them to the wing.There's not much you can do at this point but fill the gaps and hope.

To answer your specific question,yes, the cores do expand and contract,but so do the skins.In any case, the differential expansion is easily accommodated by properly applied skins.The first requirement in skinning a foam wing is to ensure that there are no internal stresses in the skins.This means that you absolutely must true the edges of all the sheets so that they fit together without any gaps with NO force applied to the sheets.Then ,and only then,they should be glued with minimal glue in the joint ,then sanded while still flat on the bench.The cores also need to be sanded free of all ridges ,bumps ,etc., and vacuumed free of dust before applying the adhesive of choice and bonding the skins in place.The August issue of Flying Models has an excellent article on preparing foam wing skins.Personally I don't care for CA in this application(I prefer aliphatic),but it does work.

Hi Zuriel,

Foam does not expand, but wet and dry balsa skin changes the dimmensions. I think your problem is, that your skin is not prepared properly. You must do joints well as Ron wrote you, but you must also dope the skin BEFORE vacuing to prevent humidity problems and you must also well attach the skin to the foam. If foam is well attached on whole area to the core, forces in joins are minimal. The gap can apear only if skin does not stick on foam. Well applied skin on foam can not make gap, it must open also gap in foam.

>Hi Zuriel,
>
>Foam does not expand, but wet and dry balsa skin changes the
>dimmensions. I think your problem is, that your skin is not
>prepared properly. You must do joints well as Ron wrote you,
>but you must also dope the skin BEFORE vacuing to prevent
>humidity problems and you must also well attach the skin to
>the foam. If foam is well attached on whole area to the
>core, forces in joins are minimal. The gap can apear only if
>skin does not stick on foam. Well applied skin on foam can
>not make gap, it must open also gap in foam.

Ron B.
F4Fguy

The coefficient of thermal expansion of polystyrene polymers varies (depending on the specific polymer) from 50 -85 in./in./deg C x 10-6,which is about 5-8 times that of aluminum.The problem ,however is not uniform stresses such as these, but localized stress caused by uneven skin joints,non-flat skins and irregular core and skin surfaces.These stresses can be much higher than the strength in either the skin to skin or skin to core joints and can cause vertical as well as horizontal separation.I have never found atmospheric humidity to be a problem in skinning foam wings.I lived for many years in Richmond VA where a typical summer day(and sometimes night) was 95-98Deg.F and 98% humidity.The real solution to the problem is to avoid any built-in stress in the skinned wing.