Re-curving mylar on a glider

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

A friend of mine tried to make his own curving machine, using
a length of steel pipe and a propane torch. We had great fun
watching as he burned his fingers and leaped about yelling
"Ouch ! Ouch !" and made all manner of wavy plastic, but it did
get a bit scary when he almost set his shop afire. After that he
ordered new pre-curved mylar from Germany.

See ya, Dave

PS: I'm sure Bumper could do better ! But not as entertaining.

PPS: Pipe is the wrong idea; as you pass the myler through
it heats the edges and not the area needing the curve...

PPPS: Bumper already knew that.

bumper wrote:
The Mylar is originally curved by passing the strip through a set of curved,
heated rollers. Complicated enough, all by itself (g) . . .

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.

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.

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.

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

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

Follow the acetone with denatured alcohol as a final cleaner. McMaster-Carr
does sell an acrylic adhesive "primer". I tested some, though not a very
good test, and could not discern any improvement over tape adhesion without
the primer. In both cases the tape stuck well.

I is important to apply the tape when it is warm or use a heat gun to
pre-heat the surfaces. As others have mentioned, sand all surfaces to be
bonded, including that portion of the Mylar. I use a 1/2" wide bearing
mounted in a wood handle to apply pressure to the mylar/tape bond.

The Mylar curing machine was one that Larry Mansberger had. I think he said
that Rick Wright either built it or helped. No pictures, but it looked a bit
like an old film projector, though the Mylar had a more direct path. Mylar
passed over a heating element and then through a curved roller, probably
aluminum, to impart the set and remove the heat.

all the best,

--
bumper ZZ (reverse all after @)
"Dare to be different . . . circle in sink."
Quiet Vent kit and MKII yaw string

"Cliff Hilty" wrote in message
...
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.

bumper wrote:
Follow the acetone with denatured alcohol as a final cleaner. McMaster-Carr
does sell an acrylic adhesive "primer". I tested some, though not a very
good test, and could not discern any improvement over tape adhesion without
the primer. In both cases the tape stuck well.

I is important to apply the tape when it is warm or use a heat gun to
pre-heat the surfaces. As others have mentioned, sand all surfaces to be
bonded, including that portion of the Mylar. I use a 1/2" wide bearing
mounted in a wood handle to apply pressure to the mylar/tape bond.

all the best,

--
bumper ZZ (reverse all after @)
"Dare to be different . . . circle in sink."
Quiet Vent kit and MKII yaw string

Agree with all of Bumper's notes above. When I re-did the mylar on my
LS4 wings after doing some gelcoat work, I followed all of the above
procedures. Since the adhesive we are using is a pressure sensitive
adhesive (PSA), it's important to make sure that you apply a uniform
and comprehensive pressure on the joint. I wasn't smart enough to use
a roller, but I found that a plastic putty spreader worked pretty well.
I did notice a distinct improvement in performance when both the
wing and the tape were pre-warmed to about 70-75F.

P3

wrote:
A friend of mine tried to make his own curving machine, using
a length of steel pipe and a propane torch. We had great fun
watching as he burned his fingers and leaped about yelling
"Ouch ! Ouch !" and made all manner of wavy plastic, but it did
get a bit scary when he almost set his shop afire.
See ya, Dave

Dave,
Here is what I do.I buy the 7 mil sheets (I figure it provides less
friction than the 10 mil) and roll it up inside of a peice of carpet
roll tube.Then I bake it at about 160 in the oven for 30 minutes.Then
cut it into strips and use scotch 665 double sided tape to adhear it to
the wings and 3M 850 mylar tape for the safety strip.I can do top and
bottom of my flaps ailerons and tail a couple times over for about 50
bucks.I replace it every 3 to 5 years and I have never had a
problem.This does seem a little less entertaning than your buddys
method, but it works fine.
K Urban