Suction mounts on a canopy

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

Ed Winchester wrote:
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 should have spotted that: my bad. However my point, that the maximum
possible force that can be exerted is the pressure per unit area times
the area, is still valid.

This is the highest force that can be applied if you try to pull the cup
straight off the canopy without twisting it, sliding it, or lifting its
edge.

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.

Quite true unless you pull on it at right angles to the panel.

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.

I'd restate that a bit. The Axim is applying a downward force of 1/2 lb
at the end of a 6 inch lever, so that is, as you say, 1/4 foot-lb. But,
that is being resisted by a counterbalancing torque thats conventionally
represented as a point force acting at the center of the cup at the end
of a lever that is pivoted at the cup's lower edge: visualize the cup
peeling off the canopy: it cones unstuck and detached by swinging round
its lowest edge. This is also a 1/4 foot-lb, but the lever is only 2
inches, so the outward force on the cup is 1.5 pounds weight. Higher
than your estimate, but still tiny compared with the force needed to
detach the cup.

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.

Agreed.


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