Drain Holes in Box Spars and Box Tailplane Spars

[Clarke Tate]

I am building a 1930's design that was originally built with no drain holes in the wing box spars, or the box spars for the tail or elevator. A number of builders are closing the spars without any drain holes. A Canadian builder was told by his inspector that he had to have drain holes. The holes will not alter the strength enough I assume to make any difference and better than condensation buildup inside unvented spars. I have tried using a soldering iron using a pointed tip made from 1/4" drill rod and this burns a nice 1/4" drain hole in scrap trials. Any thoughts from tech counselors or others as to vented or not?

[martymayes]

I would follow standard practices and guidance and install vent holes where appropriate, if appropriate.

[WLIU]

I will suggest that if the inside surfaces of the box spars are sealed with appropriate varnish before the final glue up, no drain holes should be needed. The varnish should prevent the wood from absorbing what moisture might be trapped within the small sealed compartments. So the extra work of adding holes is wasted. That said, its just time and if it makes you feel good, have at it. The one caution that I will offer is that the edges of the new holes will need to be sealed with varnish or you are creating a path for the wood to absorb the very moisture that you want to protect against.

So I will conclude that it is a waste of time and can cause more harm than good if not done properly.

Best of luck,

Wes
N78PS

[colinc]

Hi Clarke,

same aircraft!! I have just finished off the spars and in accordance with my Inspector's instructions, they have vent holes in each sealed section, roughly in the middle, simply to allow for venting.

The insides are coated with Rhodius Varnish - still trying to find out why Rhodius is the 'traditional' varnish for the job. It has been suggested that it is resistant to mould etc.

regards,

Colin

[WLIU]

For what its worth, wood aircraft structures are finished with "spar" varnish, which is more flexible than varnishes used on furniture.

As for mould, you can mix in a powder (anti-microbial?) that kills mould. But I don't know of anyone who does it for aircraft. Modern two-part varnishes are pretty good.

Best of luck,

Wes
N78PS

[martymayes]

The insides are coated with Rhodius Varnish - still trying to find out why Rhodius is the 'traditional' varnish for the job.

??? Can you even get Rhodius varnish in the US? I know it's the 'traditional' aircraft varnish in the UK - but this is the US....slap some good ole fashioned epoxy varnish on it!!

[Clarke Tate]

Thanks for all of the responses. I placed drain holes in what are generally low areas if one wingtip is low, near the base of intercostals. I used System Three Clear Coat epoxy, as this is a penetrating epoxy, as a coating. This was originally to be followed by a System Three urethane marine varnish. I chose not to use the urethane marine varnish as I haven't decided on what system I will use to finish the aircraft. On areas where there will be UV exposure I plan to use System Three Clear Coat epoxy followed by System Three SB-112 that has UV inhibitors. All of the coats must go on within 72 hours for the layers to bond. I had very tedious sanding on one of my spars as I didn't get back soon enough for the final layer!

[Clarke Tate]

Thanks Colin!
It is good to know your inspector thought vent holes were a good idea.
I asked this general question on our builders group last year. I was finally getting around to actually
making the holes and wanted some last opinions. The vent holes are now done so it is correct now!

Hi Clarke,

same aircraft!! I have just finished off the spars and in accordance with my Inspector's instructions, they have vent holes in each sealed section, roughly in the middle, simply to allow for venting.

The insides are coated with Rhodius Varnish - still trying to find out why Rhodius is the 'traditional' varnish for the job. It has been suggested that it is resistant to mould etc.

regards,

Colin