Russell Kent wrote:

Paul wrote:

Spoke to a pilot at dinner tonight that has experience with
Continental water cooled aircraft engines in a Cessna twin.
Actually the airplane is owned by Patrick Swasey.

Two sets of engines have been gone through so far. According to
an old racing friend of mine the mechanic that works on them,
steam pockets in the water jacket is the cause. You need to
vent the highest part of the water jacket when the airplane
is in a climb attitude.

For a  tractor rotary this is no problem. The inlet and outlets
are in the right place. For a pusher rotary it could be
a problem without added vents.

"When they are running right they are great. Very smooth
and you can pull the power off and decend without worry about
alledged shock cooling."

Funny you should mention this.  Last night I was wondering why, now that
we're talking under wing position, the engines were still pusher
configured.  It seems that the pusher configuration makes you work harder
to give an undisturbed airflow to the fan, whereas a tractor
configuration always provides undisturbed air to the fan.  Are there some
drawbacks to a tractor configuration that I don't know about?

Russell

Mostly theory. Theory has it that you speed up the air with the
prop or fan and it gets slowed down and loses energy if it
has to rub against a lot of cowling or fuselage skin with a tractor.
Really bad if the fan has to blow through a long duct.

So pushers are more efficient. On the other hand with a long
cowling in front of the fan the pusher fan gets to injest a lot
of turbulent boundry layer. Not good. Its probably a wash.
The pusher is really bad if there are big bumps in front of
the fan. Same thing goes for big bumps behind a tractor
fan. One advantage of a larger diameter slower turning prop.

In that case it is better to enclose the engine in what is
called a core cowling and make the hub of the fan the same
diameter. The smaller the diameter of the  engine the better
because the OD of the fan is limited by the tips going
supersonic.

That is one of the reasons the rotary makes a better ducted
fan engine. One can get them into an 18 inch diameter tube. 

BTW  P&W is considering (as we speak) gearing the fan
down with a planetary gear box so a larger diameter fan
can be used on airliners. Larger even than the fan on 
a 777 that Robin was sitting in.
 
Imagine a gear box that can transmit 40,000 HP.

Paul Lamar
 
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