Double covering fabric covered wings

wrote:

On Thu, 06 May 2004 19:57:24 GMT, Richard Lamb
wrote:

250 degrees is only the _starting_ point.

Last pass is at 350 degrees!

If the first layer has been coated with Poly Brush, the heat from the
iron will quickly soften the coating. PB starts melting at 200 degrees.

By the time the iron is up to full temp, it would be hard not to melt
into the previous PB layer - possibly causing fabric pulls and gouges.

Maybe the outter thin fabric doesn't have to be tightened that much?

As long as the first layer is strong enough, the second might only need
to be snug enough to pull out the wrinkles. Call it non-structural?
Second pass at 250 on light fabric _might_ do it?

Bottom line, Corky, I don't really know much, I've never tried it.
It seems like there may be some potential pitfalls in the doing part,
but it sure did look pretty on that Pacer.

Right, 350 is the final pass for the first layer. But the way this
was explained to me, you definately do not want to go to 350 on this
outer layer.

I don't know whether you iron the pinked tapes or not, but if you do,
do you go to the full 350 degrees? I thought not but I haven't
actually covered a wing myself, I've just helped out here and there.

Corky Scott


I pre-shrunk my tapes at 225 - 250.
Otherwise, the warm Texas sun could shrink them - after the fact...
and it would probably show after a while - big time.

Once in place (with PB), I did iron the _edges_ of the tapes to get
them down super flat. But it was generally a short quick swipe at 250.

If the tapes have not been pre-shrunk, they will shrink some at 250 -
usually unevenly - depending on how hot it gets where. Makes for
wiggly tape lines - not considered a Good Thing(tm).

If that happens, you _may_ be able to straighten the edges out by
adding 25 degrees (at a time) and ironing only the unshrunk areas.

Another tip is that slick backing paper from self adhesive sanding
disks, and such. With the slick glossy side down, I found I could
work a small model iron at 250+ and not make gouges - IF - I let the
surface cool down before trying to remove the paper.


Since my wing design doesn't allow rib stitching (foam ribs) I taped
every rib with 2" medium weight pinked tape (false ribe too).

Besides local reinforcement over the rib contact areas, they also should
act as a rip stop for puncture/rip type skin failures.

It's a little extra weight, but it makes me feel much mo betta.

Richard Lamb


http://home.earthlink.net/~n6228l/happy.htm

Corky ad Richard,

I have experience with this double covering method. I have done it a few
times. It does not match the polyfiber manual but it worked very well on high
power and acro airplanes. Feel free to email me directly if you would like
more info on how it is done. If you look at the cover of the new June Issue
of Kitplanes, 2 of the 4 model 12s on the cover are finished in this method.

Kevin Kimball

KJKimball wrote:

Corky ad Richard,

I have experience with this double covering method. I have done it a few
times. It does not match the polyfiber manual but it worked very well on high
power and acro airplanes. Feel free to email me directly if you would like
more info on how it is done. If you look at the cover of the new June Issue
of Kitplanes, 2 of the 4 model 12s on the cover are finished in this method.

Kevin Kimball


Thanks Kevin.

Were we even close?


Richard

Any idea of what the weight penalty would be in dope and fabric vs. reduced
drag? And is this method more or less expensive than pinked tape?

Just wondering,
Harry

Shellac dope would have added 76 lbs. to the weight of a Wright 1902 glider,
and almost 155lbs. to the 1903 "Flyer." ...Based on tests made with my 1899
kite, August 2001

Corky, Richard and Harry,

Corky was pretty close to having the method down that we used. I have heard of
those who have used polybrush for the bonding of the 2 layers. We don't. The
example of the tri pacer mentioned is interesting in that if the airplane is
still in standard catagory, the recoverer must have gotten a special field
approval for using the method. We have not used it on certified airplanes.
Only experimental. Jim Younkin did use the double cover method on 3
staggerwings he converted from D models to G models a few years ago. He got
the covering included in the long list of field approvals for those 3 airplanes
only. No STC for it. Double cover is not standard on staggerwings from the
factory.

The double covering we have done is based on the techniques used on these 3
airplanes. We made some changes in how the coatings are applied and have had
great luck thus far. I would be more than happy to discuss the way we have
done it with you directly. It is a bit too invloved to fully list here. If
you are wanting a bare minumum finish and gloss or slickness is not important,
don't double cover. If you have a low powered non acro airplane, don't use
this method. This is a method that is not only intended for a slick finish but
is more valuable as a tougher, stiffer, higher durability system for big
engines, big props, and hard acro.

The Staggerwing and pitts model 12 are good examples of airplanes that can
benefit from such a covering system. It is not approved of by Polyfiber nor is
covering wings and not using some form of mechanical fastener like stitches or
screws. There are airplanes out there that simply glue the fabric to the ribs
and have success because of the type of airplane it is ad the design of the
structure. Double covering is not the norm but rather is a special use fix for
a problem of losing tapes in the prop wash on big engine acro airplanes.

The weight difference is always a question. I can only give example from the
airplanes we have done which are pitts model 12s, 4 of them and the
McCullocoupe. We have a standard finish we like to see which is good enough to
place well at flyins. We use urethane top coat colors on all the airplanes we
build. So, if we want the same finish on a model 12, we have to fill the 3.4oz
heavy duty fabric with a few additional coats of polyspray, sand more, and
possibly sand the color to get a satisfactory gloss out of the course fabric.
This is by no menas a bury the tapes finish. The tapes will still be fully
proud above the base fabric. Still, with all that effort, the weave is still
strong through the paint and we have to use 4" tapes on the ribs to keep them
from blowing off the wings.

When we double cover, we end up with no tapes to blow off, 5.1 oz per square
yard of fabric weight, a fine weave fabric to fill that takes far less coatings
to do so and a finished fabric/coatings weight that is within 1 or 2 pounds of
single cover on a model 12.

A model 12 and a staggerwing have ribs that are very close together. When you
use 4" wide tapes over the ribs, you have double covered the wing 70-80% with
just very small strips of base fabric being exposed.

The process is not for the first time cover job. You need to know your way
around the fabric system before trying it. We advise our builders to go the
standard way unless they have experience and are willing to go it alone without
tech support from the covering supplier.

Labor time is about equal to the standard method. Base fabric install is the
same labor either way. Second layer can tape more time than taping would.
But, sanding is faster.

In terms of material cost, you can price 3.4 oz and 1.7 oz fabric for the size
airplane you are covering. Then price tapes. Coatings cost will be basically
the same.

Kevin