Bubble canopy / solar heating

"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