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/