Suction mounts on a canopy

A small amount of spit for a seal and do it while the air is at low
pressure eg while as high up as possible.

Well, seems this works fine, I have just tried this technique on a 3.5 hours
flight up to 7000 feet.
Suction mount fell off after 30 minutes of flight (after being attached on
the ground), but it didn't fell off after being attached on 7000 feet!
I'll try water instead of spit next time.

I use one of Paul's suction cups to hold my iPAQ. I use a little water
from my Camelback to wet the cup before attaching. I try to make sure I
press down all around the cup and squeeze out as much air as possible. I'm
also attaching to the lower edge of a 1-26 canopy, so there is not a lot of
curvature at the point of attachment. I've installed it at 7000' and flown
to 18,000 and back without it coming off, including flights of 5+ hours. I
generally have to pull on the edge of the cup and peel it off to remove it.

John Scott


wrote in message
ups.com...
I read somewhere that water will work better than spit, and my own
experience reinforces this. I can't explain why, but try water
instead of spit. Fred

Hokey science alert:

I'd reason that the most pressure the atmosphere can
exert on the outside of the canopy, even with a true
vacuum in the suction cup, is atmospheric pressure
- about 1kg/cm^2. A fingertip has about 1cm^2 of area,
so to gain a feel for the scale of what we're talking
about, try pressing down on kitchen scales with one
finger until the scale reads 1kg ( 2.2lb / 35oz / 5.47
standard Hungarian apples). Not inconsiderable, but
(I would have thought) not enough to risk damage of
any kind.

To test this theory more thoroughly, you could try
wandering over to a glider on the flight line and exerting
the same pressure with a finger on the canopy. I'd
wager no localised distortions will be inflicted -
at least, not on the canopy.

Simon

PS As a disclaimer: My entire knowledge of physics
is based upon observations I made watching hollywood
movies.

PPS Steven Seagal movies.

At 01:48 12 April 2007, Paul Remde wrote:
Hi Bernie,

I doubt it would crack a canopy. My fear (founded
or unfounded) is that the
localized deformation of the canopy would lead to internal
crazing -
especially in extremely hot or cold conditions.

I just wouldn't want to risk a $2000 to $5000 glider
canopy.

Paul Remde

'Bernie' wrote in message
oups.com...
Has anyone ever seen a canopy cracked by a 'lever-action'
suction
device? You've got me worried now ..........

Hi Simon,

I don't think the issue is how much pressure the outside air pressure can
put upon the outside of the canopy. The issue is that the suction cup is
shaped like a ... well... a cup. It has enough force to pull the canopy to
form to the shape of the inside of the cup. That causes local stresses on
the canopy. It may or may not be a visible deformation.

Paul Remde

"Simon Taylor" wrote in message
...
Hokey science alert:

I'd reason that the most pressure the atmosphere can
exert on the outside of the canopy, even with a true
vacuum in the suction cup, is atmospheric pressure
- about 1kg/cm^2. A fingertip has about 1cm^2 of area,
so to gain a feel for the scale of what we're talking
about, try pressing down on kitchen scales with one
finger until the scale reads 1kg ( 2.2lb / 35oz / 5.47
standard Hungarian apples). Not inconsiderable, but
(I would have thought) not enough to risk damage of
any kind.

To test this theory more thoroughly, you could try
wandering over to a glider on the flight line and exerting
the same pressure with a finger on the canopy. I'd
wager no localised distortions will be inflicted -
at least, not on the canopy.

Simon

PS As a disclaimer: My entire knowledge of physics
is based upon observations I made watching hollywood
movies.

PPS Steven Seagal movies.

At 01:48 12 April 2007, Paul Remde wrote:
Hi Bernie,

I doubt it would crack a canopy. My fear (founded
or unfounded) is that the
localized deformation of the canopy would lead to internal
crazing -
especially in extremely hot or cold conditions.

I just wouldn't want to risk a $2000 to $5000 glider
canopy.

Paul Remde

'Bernie' wrote in message
roups.com...
Has anyone ever seen a canopy cracked by a 'lever-action'
suction
device? You've got me worried now ..........

Another thing to consider is that when parked in direct sun, the plexi over
the suction cup will get a lot hotter than the surrounding canopy. This will
cause thermal stress as well.

bumper

"Paul Remde" wrote in message
news:MqxTh.50146$oV.12792@attbi_s21...
Hi Simon,

I don't think the issue is how much pressure the outside air pressure can
put upon the outside of the canopy. The issue is that the suction cup is
shaped like a ... well... a cup. It has enough force to pull the canopy
to form to the shape of the inside of the cup. That causes local stresses
on the canopy. It may or may not be a visible deformation.

Paul Remde

"Simon Taylor" wrote in
message ...
Hokey science alert:

I'd reason that the most pressure the atmosphere can
exert on the outside of the canopy, even with a true
vacuum in the suction cup, is atmospheric pressure
- about 1kg/cm^2. A fingertip has about 1cm^2 of area,
so to gain a feel for the scale of what we're talking
about, try pressing down on kitchen scales with one
finger until the scale reads 1kg ( 2.2lb / 35oz / 5.47
standard Hungarian apples). Not inconsiderable, but
(I would have thought) not enough to risk damage of
any kind.

To test this theory more thoroughly, you could try
wandering over to a glider on the flight line and exerting
the same pressure with a finger on the canopy. I'd
wager no localised distortions will be inflicted -
at least, not on the canopy.

Simon

PS As a disclaimer: My entire knowledge of physics
is based upon observations I made watching hollywood
movies.

PPS Steven Seagal movies.

At 01:48 12 April 2007, Paul Remde wrote:
Hi Bernie,

I doubt it would crack a canopy. My fear (founded
or unfounded) is that the
localized deformation of the canopy would lead to internal
crazing -
especially in extremely hot or cold conditions.

I just wouldn't want to risk a $2000 to $5000 glider
canopy.

Paul Remde

'Bernie' wrote in message
groups.com...
Has anyone ever seen a canopy cracked by a 'lever-action'
suction
device? You've got me worried now ..........

Hi Paul,

As I understand it there will be no pulling force acting
on the inside of the canopy. This is admittedly counter-intuitive
to anyone who tried the following bored-in-science-lab
experiment; if we place a pipette against our skin
and release the nipple, our skin feels as if it is
being pulled towards the low pressure. In fact the
higher pressure inside our flesh is pushing our flexible
skin towards the low pressure area.

Back to gliding, which always tends to distance itself
from the world of skin and nipples, this does imply
that any minute pockets of trapped air in the canopy
might pull- sorry, might cause the very insidemost
parts of the canopy to be PUSHED in towards the low
pressure within the suction cup, potentially damaging
the canopy. However, I presume such pockets don't exist;
there would be visible depressions in the canopy where
these pockets had cooled after the forming of the canopy,
and any such pockets would be just/almost as prone
to deforming the canopy during a high wave flight.

Without the existance of air pockets, I reckon the
situation would be just as I described before - complete
with disclaimer..

Simon

At 21:24 12 April 2007, Paul Remde wrote:
Hi Simon,

I don't think the issue is how much pressure the outside
air pressure can
put upon the outside of the canopy. The issue is that
the suction cup is
shaped like a ... well... a cup. It has enough force
to pull the canopy to
form to the shape of the inside of the cup. That causes
local stresses on
the canopy. It may or may not be a visible deformation.

Paul Remde

Just so we all agree on the laws of physics, any force exerted on the
canopy by a suction cup is due to the differential in pressure on one
side of the plex vs. the other. trapped air bubbles in the plex would
not have an effect. air pressure at sea level is about 14.7 PSI.
Assuming the suction cup pulled a perfect vacuum (unlikely) and it had
a diameter of 2", then the maximum possible force would be:
Pi*D*14.7=92.4 lbs of force. Not insignificant. Another way to look
at this is that it would require 95 lbs of force to pull the suction
cup off the canopy.

At 6000' atmospheric pressure drops to about 12 psi, yielding 75lbs of
force. Still pretty high. Note that deflection of the canopy in this
area would be pretty small, but stress internal to the material would
be high. Low temperatures, UV exposure, etc would exacerbate the
issue.

In reality, a suction cup probably doesn't come anywhere close to
pulling a perfect vacuum, so the numbers would be much lower, but I
couldn't guess how much. I can't offer any analysis on skin/nipple
distances.

Matt (jr)

On Apr 13, 4:03 am, Simon Taylor
wrote:

Back to gliding, which always tends to distance itself
from the world of skin and nipples, this does imply
that any minute pockets of trapped air in the canopy
might pull- sorry, might cause the very insidemost
parts of the canopy to be PUSHED in towards the low
pressure within the suction cup, potentially damaging
the canopy. However, I presume such pockets don't exist;
there would be visible depressions in the canopy where
these pockets had cooled after the forming of the canopy,
and any such pockets would be just/almost as prone
to deforming the canopy during a high wave flight.

Without the existance of air pockets, I reckon the
situation would be just as I described before - complete
with disclaimer..

the canopy. It may or may not be a visible deformation.

Why is the maximum possible force Pi*D*14.7 ?



"Matt Herron Jr." wrote in message
ups.com...
Just so we all agree on the laws of physics, any force exerted on the
canopy by a suction cup is due to the differential in pressure on one
side of the plex vs. the other. trapped air bubbles in the plex would
not have an effect. air pressure at sea level is about 14.7 PSI.
Assuming the suction cup pulled a perfect vacuum (unlikely) and it had
a diameter of 2", then the maximum possible force would be:
Pi*D*14.7=92.4 lbs of force. Not insignificant. Another way to look
at this is that it would require 95 lbs of force to pull the suction
cup off the canopy.

At 6000' atmospheric pressure drops to about 12 psi, yielding 75lbs of
force. Still pretty high. Note that deflection of the canopy in this
area would be pretty small, but stress internal to the material would
be high. Low temperatures, UV exposure, etc would exacerbate the
issue.

In reality, a suction cup probably doesn't come anywhere close to
pulling a perfect vacuum, so the numbers would be much lower, but I
couldn't guess how much. I can't offer any analysis on skin/nipple
distances.

Matt (jr)

On Apr 13, 4:03 am, Simon Taylor
wrote:

Back to gliding, which always tends to distance itself
from the world of skin and nipples, this does imply
that any minute pockets of trapped air in the canopy
might pull- sorry, might cause the very insidemost
parts of the canopy to be PUSHED in towards the low
pressure within the suction cup, potentially damaging
the canopy. However, I presume such pockets don't exist;
there would be visible depressions in the canopy where
these pockets had cooled after the forming of the canopy,
and any such pockets would be just/almost as prone
to deforming the canopy during a high wave flight.

Without the existance of air pockets, I reckon the
situation would be just as I described before - complete
with disclaimer..

the canopy. It may or may not be a visible deformation.

John Wilton wrote:
Why is the maximum possible force Pi*D*14.7 ?

Physics 101. Area of the suction cup in sq. in. multiplied by
atmospheric pressure at sea level.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

Martin Gregorie wrote:
John Wilton wrote:

Why is the maximum possible force Pi*D*14.7 ?

Physics 101. Area of the suction cup in sq. in. multiplied by
atmospheric pressure at sea level.


John, the formula for area is Pi*r*r. Pi*d is circumference.

I'm not a physicist, but I think you're looking at this problem all
wrong. Which is rigid, the canopy or the suction cup? The suction cup
which Paul was recommending earlier is soft rubber. So the deformation
is there. The only force on the canopy is that caused by the
deformation of the rubber, plus the torque or weight applied to the mount.

I happen to have that suction cup in my hand right now, and I estimate
that it takes about 10 pounds of force to compress it. Then, after
compression, that same 10 pounds is trying to spring back, but is being
prevented by the vacuum. This load is continuously applied as a bending
load trying to deform a disk of plexiglass the size of the cup.
Plexiglass is pretty stiff, I've never seen any deformation of the
canopy when the cup is on. I'm certain that at no time is the stress
greater than when you apply the cup. If one were to put their hand on
the outside of the canopy at the same time to provide counter pressure,
that stress could be reduced to almost nil. That cup is 4 inches in
diameter, or 2 inches radius. The release force would be 2*2*3.14*14.7,
or about 185 pounds, assuming the suction was perfect. Now, my Dell
Axim which I use this way weighs about 1/2 pound, and when hooked to the
cup, it has a moment arm of about 6 inches with the mount I bought from
Paul. That's 1/2 pound * 1/2 foot, or 1/4 foot pound of torque being
applied to the mount. And, I don't think that is increasing the overall
force exerted, it's just redistributing the 10 pounds of compression
force over the disk area.

And if you put the suction cup on properly, there is almost no air
inside there, certainly less than 10%, so it is probably 90% of a
perfect vacuum. Now, it you tried to pull the suction cup straight off,
that would apply the maximum force to the canopy, and as the cup started
to pull off, the volume inside would try to increase, making the vacuum
even greater. But if you remove it by pulling the little tab, then it's
released with almost no force because the vacuum is broken.

The bottom line for me in this is that I think I'd be most careful when
installing and removing the cup. I won't worry too much about the loads
it applies in use.

Ed