Bubble canopy / solar heating

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Thanks--

Steve.

Steve S. wrote:

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Thanks--

Steve.


Well, for what it's worth, you get better results pullingon air than
pushing. So instead of a whopping big intake scoop, look for where to
take the air OUT of the cockpit.

eg:
Instead of trying to pressure it in, looks for how to suck it out.

Because no matter how bigh the inlet is - if the air inside doesn't
leave - the air outside won't come in.

It just blows around your inlet and goes it's lazy merry way...

"Steve S." wrote:
So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Why not perform a test by building a mockup canopy using plastic wrap (e.g.
Saran wrap, Glad wrap and Stretch-Tite)? You don't need to worry about
aerodynamics, so the support frame you lay it over can be anything you can
cobble together. I presume all you need to do is match the exposed
insolation area and cockpit volume of the production canopy. (Maybe use
cardboard or aluminum foil wrapped around some wood posts to mimic the
volume of the shaded lower portion of the cockpit.) Hopefully the optical
properties are close enough to the final canopy material to not throw
things off. Throw in some thermometers, fans, stuff, and a sunny day and
you'll have a full-fledged experiment!

Also, the article "A Futuristic Anachronism" by Bob Fritz in the September
issue of Kitplanes magazine discusses a mechanism used by _experimenter_
Paul Lipps to deliver cool air to his cockpit.

"Steve S." wrote in message
ups.com...
I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?



Probably not much real help, but perhaps a little food for thought. When
studying data a few year ago for evaporative cooling an industrial shop, 2
to 3 air changes per minute was recommended for areas that generated a good
bit of heat of heat. You might check out the recommendations on sizing
evaporative coolers, or perhaps air conditioners, for given work or living
areas.

On Thu, 30 Aug 2007 00:23:06 -0500, cavelamb himself
wrote:

Steve S. wrote:

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Thanks--

Steve.


Well, for what it's worth, you get better results pullingon air than
pushing. So instead of a whopping big intake scoop, look for where to
take the air OUT of the cockpit.

eg:
Instead of trying to pressure it in, looks for how to suck it out.

Because no matter how bigh the inlet is - if the air inside doesn't
leave - the air outside won't come in.

It just blows around your inlet and goes it's lazy merry way...

the boy has got it in one.
my aircraft has doors and had lotsa ventilation.
I replaced the windscreen and put in all the screws and did a nice job
I thought. The aeroplane picked up 11 knots in cruise but the doors
kept popping open. There were lotsa screws missing before.
I now have a 1" x 2" ramp air exit down the back of the empennage
under the aircraft. I can have two vents full open and the doors dont
pop open. nil reduction in cruise speed.

the way your mates were going you were about to lose your canopy in
flight.
ymmv
Stealth Pilot

"cavelamb himself" wrote in message
...
Steve S. wrote:

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

This is one of those situations in which I have two little devils
sitting on my shoulders. One (an engineer, natch) is telling me that
three NACA's of a traditional size--say, 2 inchers, is enough for the
Van's guys and therefor good enough for us. The other guy on the
other shoulder (physicist) points out that the Van's guys are in fact
pretty hot most of the time and would like more air. He wants to put
in a pretty big scoop that is going to be hard to physically mount.

My physicist has run a batch of numbers and has got a scheme in which
he calculates that we can do 3 full air changeovers per minute. Now I
think that will keep the cockpit plenty cool, but again his scoop is
very hard to deal with and my gut says his plan is overkill. I think
we want something better than the sweaty Van's guys but less than the
monster mass flow my physicist wants. But I need a number to work
toward.

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?

Thanks--

Steve.


Well, for what it's worth, you get better results pullingon air than
pushing. So instead of a whopping big intake scoop, look for where to
take the air OUT of the cockpit.

eg:
Instead of trying to pressure it in, looks for how to suck it out.

Because no matter how bigh the inlet is - if the air inside doesn't
leave - the air outside won't come in.

It just blows around your inlet and goes it's lazy merry way...

This is from a sailplane pilots perspective. I fly under a very large
bubble canopy. Since I almost always fly on hot afternoons, fresh cockpit
air is a very big deal. I think you'll want to completely replace the
cockpit air several times a minute.

The advice to concentrate on the air exit is very sound. It's easy to scoop
in ram air but it has to eventually go back outside to achieve cockpit
ventilation. Try to think of an exit in a low pressure area. When you
think about it, this problem is just like cooling airflow for the engine on
a smaller scale.

Also worry about scooping up rain with the air. A bad vent design can soak
cockpit electronics in seconds. Water droplets have a harder time turning
tight corners than air so an elbow or two can act like a separator.

Finally, worry about noise. It's amazing how much noise an air scoop can
make. A scoop behind the prop will transfer pressure pulses into the cabin.
Maybe think about an inlet scoop under the wing outside the propller arc.

Bill Daniels

This is from a sailplane pilots perspective. I fly under a very large
bubble canopy. Since I almost always fly on hot afternoons, fresh cockpit
air is a very big deal. I think you'll want to completely replace the
cockpit air several times a minute.

The advice to concentrate on the air exit is very sound. It's easy to
scoop in ram air but it has to eventually go back outside to achieve
cockpit ventilation. Try to think of an exit in a low pressure area.
When you think about it, this problem is just like cooling airflow for the
engine on a smaller scale.

Also worry about scooping up rain with the air. A bad vent design can
soak cockpit electronics in seconds. Water droplets have a harder time
turning tight corners than air so an elbow or two can act like a
separator.

Finally, worry about noise. It's amazing how much noise an air scoop can
make. A scoop behind the prop will transfer pressure pulses into the
cabin. Maybe think about an inlet scoop under the wing outside the
propller arc.

Bill Daniels

The ventilation in my HP-14 (http://tinyurl.com/yvrghx) is far from optimum.
It gets very warm on a hot summer day at lower altitudes. However, with bit
of altitude (10,000+) I am comfortable.

Your point on rain is well taken. This summer while scooting along under a
cumulus cloud I encountered. a bit of verga. It turned out to be a
combination of snow and hail. It came through the vent and hit me directly
in the face. I am sure it also was soaking the instruments. The solution
was to close the vent.

This is a bit off subject; however, while flying in an aircraft with a
bubble canopy you should take precautions to protect yourself from UV
radiation exposure.

Wayne
HP-14 "6F"
http://www.soaridaho.com/

In article . com,
"Steve S." wrote:

I'm looking for input regarding the amount of air to blast through a
cockpit to offset the effects of solar heating under a large canopy.

...............

So, the crux of the question: Based on your gut, personal observations
or, in a perfect world, empirical data--how often do we need to have
an air changeover to keep a cockpit at a reasonable temp in hot sun?


Remember, too, that air can travel forward on the fuselage, from rear
stagnation points. If the canopy is well-streamlined, a port on the aft
end of the canopy will provide ventilation to the back of your head,
because pressure there is higher than on other parts of the plane.

"Orval Fairbairn" wrote in message
news
Remember, too, that air can travel forward on the fuselage, from rear
stagnation points. If the canopy is well-streamlined, a port on the aft
end of the canopy will provide ventilation to the back of your head,
because pressure there is higher than on other parts of the plane.

This is very true on my Emeraude. Air - and sometimes rain - will enter at
the rear of the canopy where it meets the fuselage. I had to carefully
weather-strip that area. Air also blasts in through any holes between the
cockpit and the interior of the rear fuselage, i.e. the flap actuator rod. I
think that air is entering near the tail and coming forward toward the
low-pressure cockpit. I tried to establish a good seal with the carpeting
using Velcro at the edges to keep that air out - especially in wintertime.

I have two NACA style vents on the sides of the fuselage located below the
windshield supports. They pressurize a "bay" of the fuselage sidewall and
the air then enters the cockpit through an eyeball fitting elsewhere in the
bay. The air has to turn a couple of corners, filtering out water and/bugs.
The vents are about 4" top to bottom. They really give a nice blast of air -
enough to stay cool. Air leakage at the rear corners of the canopy help with
the back of the neck, but I lay a couple of rags on them in the cool wx.

The *best thing* I did was to install the interior canopy shade sold for
RV-6's by Cleaveland Aircraft Tools. It is absolutely invaluable in any sort
of sun.

Rich S.

This discussion prompts a question.

The nose vent in my Nmbus 2C glider is very noisy - at least noiser than I
think it should be. With the vent closed, the glider is almost totally
silent. With the nose vent open, I have to turn up the radio volume.

The design is a ~2" hole in the very tip of the nose with a ~10" straight
pipe to which a corregated flexible tube (think scat tubing) takes the air
through an "S" turn to a vent door that is controlled by a pull knob on the
instrument panel. Nothing in the inlet tube or control door suggests
anything is making noise. The flex tubing with the circumferential ribs is
the lead suspect.

Is there a quiet replacement for the flex tube I could try? Maybe some sort
of muffler?

Bill Daniels


"Wayne Paul" wrote in message
...

This is from a sailplane pilots perspective. I fly under a very large
bubble canopy. Since I almost always fly on hot afternoons, fresh
cockpit air is a very big deal. I think you'll want to completely
replace the cockpit air several times a minute.

The advice to concentrate on the air exit is very sound. It's easy to
scoop in ram air but it has to eventually go back outside to achieve
cockpit ventilation. Try to think of an exit in a low pressure area.
When you think about it, this problem is just like cooling airflow for
the engine on a smaller scale.

Also worry about scooping up rain with the air. A bad vent design can
soak cockpit electronics in seconds. Water droplets have a harder time
turning tight corners than air so an elbow or two can act like a
separator.

Finally, worry about noise. It's amazing how much noise an air scoop can
make. A scoop behind the prop will transfer pressure pulses into the
cabin. Maybe think about an inlet scoop under the wing outside the
propller arc.

Bill Daniels

The ventilation in my HP-14 (http://tinyurl.com/yvrghx) is far from
optimum. It gets very warm on a hot summer day at lower altitudes.
However, with bit of altitude (10,000+) I am comfortable.

Your point on rain is well taken. This summer while scooting along under
a cumulus cloud I encountered. a bit of verga. It turned out to be a
combination of snow and hail. It came through the vent and hit me
directly in the face. I am sure it also was soaking the instruments. The
solution was to close the vent.

This is a bit off subject; however, while flying in an aircraft with a
bubble canopy you should take precautions to protect yourself from UV
radiation exposure.

Wayne
HP-14 "6F"
http://www.soaridaho.com/