Bonding aluminum to fiberglass

[foamhead]

I am building aluminum fuel tanks for my KR2 and was thinking about adding a layer of fiberglass over the outside of the alum. Would the difference in the expansion rates cause any problems. The tanks are about 5 ft. long and the epoxy is vinyl ester if that has any bearing.

 

[jbknudsen]

Greetings,

I am not sure what the reasoning is for laminating the tanks with fiberglass, I guess for protection or maybe some abrasion prevention? I don’t know, haven’t built any planes lately, I do have an instrument rating however.

Anyway, lets see if this help you out. My thought on the matter is that one would need to know the thermal coefficient of expansion for all of the materials to be used.

Alum: 2.5 x 10^-6 in/in -F°
Fiberglass: 11.1 x 10^-6 in/in -F°
Epoxy: unknown variable

Then determine a realistic temp variation and calculate the two material expansion.

For this example I will assume the outside temp is 40°F and the raised temp is 80°F giving us a temp range of 40°F. Additionally I will use your 5 ft. tanks.

Aluminum tank length in inches at 40°F = 60.000 inches
Aluminum tank length in inches at 80°F = 60.00003 inches

Fiberglass sheet length in inches at 40°F = 60.000 inches
Fiberglass sheet length in inches at 40°F = 60.00011 inches

The thermal expansion for the two materials is not much (difference of 8/one hundred-thousands of an inch). The epoxy thermal expansion rate should be available from the supplier. If the coefficient is between 2.5 x 10^-6 in/in -F° and 11.1 x 10^-6 in/in -F° than I would not be too concerned. However, I have not built a plane am not exactly sure what the normal process is.


If you get a chance, check out this website

http://www.navpen.com

best regards,

Jay

 

[foamhead]

The fuel tanks will be leading edge tanks and will be the shape of the airfoil. The wing skin will then be over the tank but has nothing to do with the tanks except that. It would be no trouble to cover the tank with a parting agent and let the skin float or to bond it to the tank. Any opinions? Thanks for the replys!

 

[DanT]

http://www.dow.com/derakane/lit/tds.htm

no thermal expansion specs there, but shrinkage on the order of 7% appears fairly typical. I wonder if the residual strain from that much shrinkage might be more important.

 

[lafdot]

Coating aluminum with polyurea will give you a highly bonded polymer that can handle temperature range from -50 deg F up to + 350 deg F. The coating, when applied with special equipment, will cure in under 15 seconds, can be applied at thicknesses over 1/4 inch in a single step. The polyurea polymer demonstrates tensile values of over 3500 psi while exhibiting elongation value of over 300%. So the expansion and contraction of the aluminum will not factor into the coating's ability to handle stress. Prep includes walnut shell blast, wash with TSP, then applying a vinyl wash primer first then spray application.
see =

http://www.rapidcoating.com

 

[harrisj]

Jay:

I think your numbers are wrong - using your data the 60 inch aluminium structure would expand to 60.006 not 60.00003.

Don't think it matters anyway in this case - I'm just picking nits!