Continual butyrate dope shrinkage?

[FR Drew]

Folks, this is a question for those of you who've been building model aircraft for a long time.

My work involves re-doing the fabric on vintage aircraft (the real thing 1:1 scale and all that!) and our experience and what we've heard seems to indicate that although frabric treated with cellulose nitrate dope will shrink initially, after that it's stable. Conversely, cellulose butyrate doped fabric seems to keep on shrinking.

The observed result is that we have aircraft from WW1 with nitrate doping where the fabric is fine and aircraft re- covered with fabric 30 years ago using butyrate where the stitching is being torn apart and the ribs are punching out through the trailing edges of the wings.

Any of you folks got experience with nitrate vs butyrate on cotton and can you back up what we're experiencing?

All input gratefully accepted. It'll make a huge difference to how we apply fabric to our future aircraft restoration/conservation projects. (when some of the aircraft are worth up to 6 million US this is pretty important to us)

please feel free to type me direct at work on:

andrew dot w dot pearce at awm dot gov dot au.

TIA

Drew

[Cryhavoc]

Drew,
You're correct in your assumptions. Nitrate Dopes are often refered to as non-shrinking where as Butyrate Dopes are notorious for it.

However, The Butyrate Dopes are more durable and resistant to chemicals and hot oils which makes them more popular. Also, they tend to be compatible as a base coat for other paints like lacquers and epoxies where as the Nitrates are not.

Because of the desirable qualities of the Butyrate there have been techniques used to mitigate the shrinking effects of the Butyrate. Medical grade castor oil can be added. For model use were talking about 2-3 drops per 3.5 oz of dope. There is a company called Brodak that has a line of Butyrate dopes. One of the products they sell is a Rejuvenator. I thought I read where you can add a small quantity to prevent shrinking. Supposedly, you can brush it on a doped finished that has started to crack and it will soften and "un-shrink" it???

As none of my models has ever lasted long :( , I have unfortunately never tried to prevent the shrinking so I couldn't give first hand knowledge of any techniques. But, I hear it mentioned so often that there must be some substance to them.

Here is a Control-line model airplane forum with plenty of old timers that should know more about this.

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi

Hope this helps,
Mike

[FR Drew]

Thanks Mike, much appreciated.

The more I read about the issue, the more consistent the responses and info becomes regarding shrinkage, flammability, rejuvenators and so forth.

At this point I'm beginning to get more than a little concerned that there must be a huge stack of museums around the world with planes just like our two that are happily sitting in storage and tearing themselves apart.

I'll add your response to my files.

Again, many thanks.

Drew