Tracy Crook wrote:

Paul Lamar wrote:

The area of the water
outlet passage just below the upper most bolt boss is
even worse at about .66 square inches. IMHO this is one of the major

reasons

why the Mazda pump cavitates at only 8000 RPM and flows so little
water when it is not cavitating. See the picture below of the pump

with

the upper part cut off and the water passages ported as much as

possible.

It makes no sense to have a hole in the block with 3.25 square inches
and then have a restriction in the pump of .66 square inches.

The neck-down to .66 sq inches might be excessive (although I have seen

no

evidence of too little flow in my installation) but it will not cause
cavitation.  In fact, restriction on the outlet REDUCES the chances of
cavitation.  This is why car racers in the earlier days got more

overheating

problems when they removed their thermostats.  The drop in outlet
restriction caused the pump to cavitate.

Also note that the combined area of my two 5/8" cooling hoses is only

.61 sq

inches. and this is a much longer path than the 1 inch or so in the pump
outlet.

Tracy Crook
rws@altavista.net
www.rotaryaviation.com

I am not sure I agree with this Tracy. You have a closed loop system here.
A restriction anywhere in the loop will cause problems. Worst case
scenario of course is the loop plugged completely. I guarantee cavitaiton
under those conditions. Even in an open loop system if you plug
the output of the pump it WILL cavitate. Obviously there is some
minimum size of a hole required as a function of the RPM of the pump.

To clarify things it is ABSOLUTELY ESSENTIAL TO RUN THE STOCK MAZDA
THERMOSTAT UNLESS YOU PLUG THE BYPASS!!!! THE HAYNES MANUAL IS WRONG
ABOUT THIS!!!! REMOVING THE THERMOSTAT WITHOUT BLOCKING THE BYPASS
IS GUARANTEED TO CAUSE OVER HEATING in a Mazda rotary.

You live in flat country too Tracy :-) I want to make sure my 250 HP
turbo engine cools when climbing out of the 110 degree F desert at or
below sea level and over a 12,000 foot mountain against a 30 kt headwind.

Paul Lamar

Tracy Crook.
Yes, you will have to pay closer attention to cooling system at 250 HP but
basics never change.  Cavitation in a centrifugal pump (which is what we are
using) is more likely with less restriction on the outlet.  This has been
proven time and time again in open or closed systems.  If someone can prove
me wrong on this, please do so. Basics like this are important when trying
to diagnose  cooling system problems.

A turbo compressor map is a close analogy to a centrifugal pump
and restricting the flow causes cavitation at lower RPM. This is easy
to prove. Drive the pump from your variable speed lathe with a ball
valve on the output. PL

I may live in flat Florida but I've climed over 11,000 foot mountains
immediately after taking off from 6600 ft MSL airports where the density
altitude was over 9000 feet and the air temperature was over 90 degrees.
The local pilots warned me to circle and gain altitude before approaching
the mountains west of Sandia airport (NM) and their advice was mandatory for
most GA aircraft.   The RV-4 not being most GA aircraft, I set course
directly for the mountains and cleared the tops by 4000 feet and was still
climing at 500 fpm.  I'm telling this story to illustrate the point that in
spite of stories to the contrary, my cooling system is not all that
marginal.  I've always been honest about it's operational limitations (like
ground cooling) and many people have gotten the impression that it is barely
adequate.

On the subject of ground cooling, I've been talking to a lot of builders on
the verge of completing their rotary powered planes and many of them have
spent countless hours trying (and sometimes succeding) to get the engine to
cool at full throttle on the ground.  My question is, Why?   If you are
successful, it generally means that your cooling system is overweight and
generates enormous drag. (the exhaust augmentor system is an exception) Are
you willing to pay these penalties for the bragging rights to ground cooling
at 100% power?  In the course of normal aircraft operations, just when do
you do this?   My engine will overheat within 2 minutes under these
conditions but it is not a problem.  It should be noted that most all
factory (piston powered) aircraft will overheat on the ground at full
throttle.

Tracy Crook
rws@altavista.net
www.rotaryaviation.com

 
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