Re-curving mylar on a glider

No fire, a cheer. You've pointed up an important issue. Many of us have
seen replacement seals come off in a few months, even when done
professionally by reputable shops. So, we keep the factory seals on as
long as possible, and sometimes just a bit longer. Really, life would
be so much better if we could trust that replacement mylar would stick.
What are the factories doing the first time around that we aren't doing
for replacements? Is it just a better job of sanding/cleaining the
surfaces?

John Cochrane BB

I wish I knew, John. One reputable repair shop with excellent
relationships with German manufacturers expressed amazement and, to be
fair, initial concern that my seals were so old. But when I described
my situation, he said, well, if they're still attached well, replace
the safety tape and just keep an eye on them because you'll never get
new seals to adhere as well as the factory does. He said the
performance of the Mylar itself (i.e., curvature or tension) wasn't
really a question; the issue was the adhesive.

I've seen several postings that describe sanding/cleaning the surface
of both the wing and the Mylar as well as how to apply the adhesive and
the seals, the proper temperature and pressure to use, etc. But when
more than one expert says the factory stuff is much better, I, too,
have to ask what it is that they are doing different. What's the secret
formula???

In the meantime, I acknowledge some risk in leaving the old seals in
place. But I find it difficult to prove (as opposed to intuitively
"knowing") that replacing the seals makes it any less likely that I
will have a failure.

Chip Bearden
ASW 24 "JB"

C'mon guys, break loose with the $120 and go buy some new mylar, pink
tape, and safety tape. Why hang your neck out for tape? Clean out all
that dust and crud while you have the old tape off. You'll feel better
every time you fly. It's cheap insurance! Snoop
wrote:

snoop wrote:
C'mon guys, break loose with the $120 and go buy some new mylar, pink
tape, and safety tape. Why hang your neck out for tape? Clean out all
that dust and crud while you have the old tape off. You'll feel better
every time you fly. It's cheap insurance! Snoop

Been there, done that, didn't work out so well. I had to replace the
mylar on the outer half of the ailerons when the winglets were
retrofitted to my glider. It took several attempts to get them to stay
on properly longer than a month or two at a time. Money isn't the issue
- it's safety and convenience. The factory mylar shows no signs of
losing it's adhesion: I can tug on it, I can see through the mylar to
the carbon fiber it's glued to, and the glued area is uniform, clear,
and black.

When I was having this problem, I talked to Martin Heide (the ASH 26 E
designer), and he said "I have that problem on my glider, too! I must go
down to the factory floor and find out what they are doing". Sadly, he
didn't get back to me, and in the meantime, I got the aileron pieces to
adhere adequately. Looking at the glue through mylar, the joint does not
look as uniform as the original factory portion.

And you will be interested to know the wing mylar never had safety tape
on it, but the elevator and rudder mylar did. Without the safety tape,
it's easy to inspect the glued area. I do replace the safety tape on the
top of the elevator every 2-3 years, but the tape on the bottom of the
elevator and on the rudder lasts much longer. I don't remember needing
to replace it.

--
Eric Greenwell - Washington State, USA
Change "netto" to "net" to email me directly

"Transponders in Sailplanes" on the Soaring Safety Foundation website
www.soaringsafety.org/prevention/articles.html

"A Guide to Self-launching Sailplane Operation" at www.motorglider.org

Bruce,

Ditto on my LS1-d.

However, this summer I did have the mylar on the lower
surface of one wing give some problem. The 'safety'
tape kept the mylar from leaving the ship, but the
mylar somehow broke away from the double-sided tape
and it rotated 90 degrees, more or less and was perpendicular
to the wing. Of course, this happened in flight.
So, I was flying around with a 30 mm high spoiler in
front of the aileron. The length of the 'spoiler'
must have been five feet (1.5 meters). This started
on the first leg of a 300 km FAI triangle. Since the
airplane flew fine, except for some 'into the draggy
wing' stick I had to hold during the flight, everything
went fine. So, how did the mylar rotate 'up' and create
the spoiler, even though the leading edge of the mylar
was taped to wing? Why didn't the airflow keep the
mylar pushed down against the wing? If you know, please
tell me!

I guess some airfoils are more susceptible to problems
with mylar than others.

Ray Lovinggood
Carrboro, North Carolina, USA
LS1-d

At 16:12 16 October 2006, Bruce Greef wrote:
BB wrote:
IThere have been several accidents resulting from
seals coming loose,
and causing a 'spoiler' effect on the control surface.
One is here

http://www.ntsb.gov/ntsb/brief.asp?e...07X04237&key=1

I think there was at least one Grob crash in Germany.
Some manuals
specifically state there has to be tape on the elevator.

So, yes, this is a serious issue. Since hearing of
the first crash,
'check condition of mylar and hold-down tape' has
been part of my
critical assembly check.

John Cochrane BB

Finally - one undisputable advantage of the all flying
tail on my Cirrus. ;-)

Does anyone have any actual knowledge (as opposed to conjecture) of
whether the incidents involving Mylar seal failures have occurred for
factory vs. replacement seals? I'm genuinely eager to do the right
thing. But having been warned by some experts that the factory seals
adhere much better, I'm reluctant to trade one unknown for another.

Any real data out there?

Chip Bearden
ASW 24 "JB"

Ray Lovinggood wrote:
Bruce,

Ditto on my LS1-d.

However, this summer I did have the mylar on the lower
surface of one wing give some problem. The 'safety'
tape kept the mylar from leaving the ship, but the
mylar somehow broke away from the double-sided tape
and it rotated 90 degrees, more or less and was perpendicular
to the wing. Of course, this happened in flight.
So, I was flying around with a 30 mm high spoiler in
front of the aileron. The length of the 'spoiler'
must have been five feet (1.5 meters). This started
on the first leg of a 300 km FAI triangle. Since the
airplane flew fine, except for some 'into the draggy
wing' stick I had to hold during the flight, everything
went fine. So, how did the mylar rotate 'up' and create
the spoiler, even though the leading edge of the mylar
was taped to wing? Why didn't the airflow keep the
mylar pushed down against the wing? If you know, please
tell me!

I guess some airfoils are more susceptible to problems
with mylar than others.

Ray Lovinggood
Carrboro, North Carolina, USA
LS1-d

At 16:12 16 October 2006, Bruce Greef wrote:
BB wrote:
IThere have been several accidents resulting from
seals coming loose,
and causing a 'spoiler' effect on the control surface.
One is here

http://www.ntsb.gov/ntsb/brief.asp?e...07X04237&key=1

I think there was at least one Grob crash in Germany.
Some manuals
specifically state there has to be tape on the elevator.

So, yes, this is a serious issue. Since hearing of
the first crash,
'check condition of mylar and hold-down tape' has
been part of my
critical assembly check.

John Cochrane BB

Finally - one undisputable advantage of the all flying
tail on my Cirrus. ;-)




A lot of us use 3M adhesive remover to get the sticky up after removing
the mylar and transfer tape. Don't apply new transfer tape until you
clean the 3M adhesive remover off or the new tape will come loose in a
short time. I like to clean up with Acyli-Clean from Dupont on the
surface prior to putting the transfer tape down. Available at auto
paint stores.

A friend bailed out of his Ventus A some years back because the mylar
came loose in front of the elevator. Don't cut corners here.

WM

However, this summer I did have the mylar on the lower
surface of one wing give some problem. The 'safety'
tape kept the mylar from leaving the ship, but the
mylar somehow broke away from the double-sided tape
and it rotated 90 degrees, more or less and was perpendicular
... So, how did the mylar rotate 'up' and create
the spoiler, even though the leading edge of the mylar
was taped to wing? Why didn't the airflow keep the
mylar pushed down against the wing?

Guessing he no internal seal of the aileron, or seals between
fueslage and control rods. Then, high pressure from the cockpit
ventilation escapes out the aileron hinge line. Add to this a
separation bubble forming just in front of the aileron hinge line so
the air isn't moving there anyway. If I'm right, seals and zz tape
would give a different result. But of course the mylar will be glued on
tight so we'll never know...

John Cochrane BB

Guessing he no internal seal of the aileron, or seals between
fueslage and control rods. Then, high pressure from the cockpit
ventilation escapes out the aileron hinge line. Add to this a
separation bubble forming just in front of the aileron hinge line so
the air isn't moving there anyway. If I'm right, seals and zz tape
would give a different result. But of course the mylar will be glued on
tight so we'll never know...

John Cochrane BB

If The bottom wing surface is sealed tight along the hinge line and
no high pressure is moving from the cockpit along that line
(a normal cockpit has always a slight negative pressure,
which should be controlled by the rear exhaust)
and providing the turbulent flow on top is attached at the hinge line,
the Mylar, even if only the leading edge is taped, should stay down.
If the boundary layer is thicker the Mylar may start flattering like
a
stiff flag and makes noise, but it is not lifting. Only when separated

flow exist could the Mylar stand up. With no seals on the bottom
all bets are off

On my modified HP18, in the flap region, I have a 13/4" Mylar tape
excluding the glue line, that just sits on top of the flap surface.
It does stay put and makes no noise through out the normal range,
from stall to 125kt

On an other note, the glue line should be cleaned and roughed with
# 240 grit and cleaned again. Only the best and most aggressive
3M double sided tape should be used. A 1" ball bearing with a1/4 "
wide race mounted to a handle is used to press the Mylar
down. The local area should be warmed with a hot water bottle
or similar, before pressing with a rolling motion. The point pressure
is very high and forces the glue into the smallest of surface
indentation
for that long term bound.

John,

You are correct: No seals at the butt rib for the
control rods (one of the things I've been meaning to
do. For the past ten years...)

No internal seals at the ailerons.

This old bird does have zz tape on the wing, but not
as the 'safety tape' for the mylar. That would be,
of course, way too far back for this fat old airfoil.
The zz tape was on it when I bought it.

Ray

At 22:06 16 October 2006, Bb wrote:
However, this summer I did have the mylar on the lower
surface of one wing give some problem. The 'safety'
tape kept the mylar from leaving the ship, but the
mylar somehow broke away from the double-sided tape
and it rotated 90 degrees, more or less and was perpendicular
... So, how did the mylar rotate 'up' and create
the spoiler, even though the leading edge of the mylar
was taped to wing? Why didn't the airflow keep the
mylar pushed down against the wing?

Guessing he no internal seal of the aileron, or
seals between
fueslage and control rods. Then, high pressure from
the cockpit
ventilation escapes out the aileron hinge line. Add
to this a
separation bubble forming just in front of the aileron
hinge line so
the air isn't moving there anyway. If I'm right, seals
and zz tape
would give a different result. But of course the mylar
will be glued on
tight so we'll never know...

John Cochrane BB

I use Acetone to clean out all of the old adhesive.
It doesn't leave any residue and completely evaporates
for good clean surface. Nasty stuff though use good
rubber gloves and good ventilation. Never had any tape
come off prematurely just after the AZ sun has dried
out the adhesive.