Sagging Fabric suddenly (Superflight)

[wadecalvin]

My Wag areo cubby (new to me) has superflight covering and looked real good untill the other day after I flew it early in the cool of the morning. After the flight I noticed the fabric on top of one of the wings was sagging and kind of loose between three or four of the ribs, next to the wing tank. Went back in the heat of the day today and it was all tight again except for one wrinkle- It had been tight until now since May when I had a pre-buy done - and bought it. Any ideas? or similar experiences? Thanks

 

[jgerard]

Sometimes sagging fabric when cold is a sign that the fabric was over shrunk (past 400+ deg)

Jason

 

[Longwinglover]

Or not shrunk enough, initially.

Remember that with any process not using nitro cellulose dope, the only fabric shrinkage you will ever get is with the iron. Sometimes people that are used to covering with dope and switch to something like Polyfiber don't shrink the fabric enough with the iron.

John Scott

 

[Champdriver]

WadeCalvin, sounds like the Cover job was not heated to the magical 350 Degrees for the proper tension prior to processing. This is a very common problem with guys that refuse to believe the engineers and scientists that have Invented/Developed the use of Dacron in our Covering Systems. I have acquaintances that still use a heat gun to shrink their fabric instead of a high quality iron set at 350 degrees. One guy even asked me to do the paper work for his cover job, then complained that I wouldn't do the return to service, because he refuses to shrink the fabric per the STC.
The real bummer with your airplane is that once the "Paint/Primer" is applied, you can't "Re-shrink" the Fabric.
If it pulls back tight in the heat of day, I'd fly the airplane, and live with the slack fabric when it cools... Good Luck, Champdriver

 

[Hardtailjohn]

Yep...sounds like an "expert" has done the shrinking....and YOU get to live with the results. The other thing I've seen is that alot of guys wont bring it up evenly or to different stages. Had a guy that was ready to fistfight when I told him that once.... and he still has the sagging fabric every time it cools.....and all of his cover jobs have the same problem...but he's good at blaming the fabric. Champdriver and the others are right.... and there's a damned good reason that those engineers specify using an iron on fabric!
John

 

[NimpoCub]

How about checking for a structural reason your fabric isn't tight? If it WAS tight but now sags sometimes, perhaps some of the structure it was stretched over has changed shape.

I understand what everyone has said so far, but in my limited experience, the only sags I've seen were on bent planes. Just a thought.

 

[mike mcs repair]

my take/observations....

like other said too hot... but...

but usually from people not fully shrinking it slowly, in the 3 shrinking stages... those first cool ones are the most important, or else your fabric will "lock", kinda Chinese fingertrap itself, not allowing the threads to slip, and never really shrink & develop the tension it could & should.....

also with some of the thick/urethane top coat processes, they seem to go through a temporary looser phase if they change temperature a bunch...