Mike Noin

I have a 8' DeHavailland Beaver that is covered in Sig Koverall fabric and dope. There are bellcranks in the wings for the flaps and the ailerons, I would like to replace these with servos. To do that I would have to cut into the fabric and build hatches for the servos....etc. What I don't know how to do is finish the edges of the holes I would have to cut and how to match the new hatches with the older finish on the plane? The plane is about 10 years old. I have just bought it from an old timer. A word of warning to anyone buying a used plane. If the seller says its in flying condition and you are paying for a plane that is in flying condition, don't buy it until you actually see it flying. Otherwise buy it thinking that it is not in flying condition to avoid dissapointment. Mine had a stripped glow plug thread in the O.S. 90 FS and the radio stopped working 2 days after I brought it home, in the shop fortunately and not in the air. Anyhow enough whining. Are there any other hints I should know before I cut into the finish?

Jim_McIntyre

Repairing dope and fabric is very easy but requires a little skill. I prefer to align the seams with underlying structure to aid in hiding the seams but for holes, I just cut away extra fabric until I have a round hole. I then cut a patch to overlap ~1/8" all around. Next, clean off the area with thinners and apply dope to soften the existing dope. Use butyrate dope not nitrate as nitrate doesn't work well over butyrate (the reverse works and is common) and final coats are always butyrate.
Now, lay the patch over the area and dope through, pulling any wrinkles out but don't try to make it too taut, the fabric will shrink when you apply later coats. Get a wrinkle or off centre? no worry, re-apply dope until it all softens and re-position. Let it dry overnight, apply filling coats to fill the weave as desired (I add talc {baby powder} to help fill), sanding lightly between coats and leaving plenty of time to dry. hit it with some color and you're done.

HTH, Jim.

Mike Noin

Thanks Jim.

Does dope fade much over its life? I am afraid of having a bright white patch over a faded white finish. How can I test to see if the colours have faded? What grit of sand paper do I sand with and how much talc do I mix with how much dope? How can I prevent fraying on the patch piece?

Thanks again.

Jim_McIntyre

THese skills are easier demonstrated than described ... maybe someone else knows of a website with pics? ... meanwhile, I'll see what I can do.

Does dope fade much over its life?
If left in the sun, all things fade.

I am afraid of having a bright white patch over a faded white finish. How can I test to see if the colours have faded?

lay up a piece on some scrap balsa and check it when it dries. If I was concerned, I'd priobably give the entire plane a fresh coat of paint (same as what the big boys do).

What grit of sand paper do I sand with...
It depends what quality of finish you want and how much effort/patience you're willing to expend. Generally, I don;t use anything coarser than 600 and sand lightly without cutting the fabric. The last coat of color gets treated with rubbing compound.

and how much talc do I mix with how much dope?
I mix up a slurry of 50/50 thinned butyrate until it begins to take on a pasty consistency. This is the part that is best demonstrated, err on the side of thin unless you really enjoy sanding.... the idea here is to fill the weave, depending on what type of finish you want (baby smooth to obvious cloth weave).


How can I prevent fraying on the patch piece?
Cut with good quality sharp scissors and dab the first coat of dope more than brush.

HTH, Jim.

Mike Noin

Thanks again Jim.

If I had a digital camera that took halfway decent photos I would post the results but I don't so I'll just write about them. Might be a while before i do this modification, I am flying this plane as it is configured now and really enjoy the flight characteristics. Just would like the ability to control each surface with its own servo.

Jim_McIntyre

Glad to be of help.

I had to get in to the top wing of my triplane to replace a cross threaded blind nut.

I've challenged many people to locate the patch... none have so far.