Hey  Paul,

What do you think of designing the coolant and oil heat exchanger  systems
to
be somewhat self contained and modular?  I realize that  cooling issues are
getting pretty well licked on the rotary, but I can't  help but think
things
could be even better..

What I envision is  building the inlet scoop and diffuser to attach to one
side of the  exchanger, and the outlet plenum and restictor flap attached
to
the other  side of the exchanger.  Attach this whole assembly directly to
 the
motor mount.  Where exposed to heat from the exhaust, the  construction
would
be of metal, but otherwise composite.  The rest of  the cowl would be
designed to attach to the firewall, and ot the exchanger  plenums.  Louvers
would be used to provide direct ventilation for the  remaining engine
components, and to reduce problems related to heat soaking  after shutdown.

A couple of the benefits are:

-  Direct/idealized control of airflow to/through/from the heat
 exchangers,
allowing smaller, lighter, and more efficient  installations

- Elimination of the "pressure cowl."  A  pressure cowl mandates relatively
heavy, stiff sections in order to  maintain effective sealing around the
heat
exchangers.  The pressure  cowl must contain (withstand) the dynamic
pressure
associated with ram  inlet air at flight speed times the total containing
area - significant  forces involved.  Additionally, since the pressurized
section involves  many penetrations for controls, structure, and
instrumentation, the sealing  becomes piecemeal and complex.  If the
critical
pressurized sections  are contained around the heat exchangers, the rest of
the cowl becomes  essentially a lightweight fairing around the engine.

Another idea I had  was to design the exhaust system so that much of it is
external to the  cowl..  Some people would think this is ugly, but I
maintain
that  proper design would make it adequately aerodynamic, lower the heat
load
in  the cowl, make inspections much easier, and thus represent an
 improvement
in efficiency and safety.  I have seen quite a number of  artfully designed
and constructed motorcycle exhausts that i don't think  would be the least
ugly were they mounted on an  airplane..


Matt-

Good idea Matt. Good write up  too.

Paul Lamar

Great Idea, but not new. In the home built world, some are using plenum
cooling, on air cooled engines. In these, air is contained above the
cylinders  in
a plenum which is continuous with the baffles. The top of the cowl has no
specific air pressure. With these, the lower chamber is, of course vented
out
the bottom, or top. Even with these, it is a good idea to have some airflow
around the engine, to help eliminate radiated heat.

Now, there is a welding company who manufacturers a liquid oil cooler which
is plumbed into the radiator. Thus there is no need for an air cooled oil
cooler. This unit may be mounted anywhere under the cowl, or, I suppose
anywhere
else. The radiator will probably have to be enlarged.
*                      *                *
*                     *                      *          *

With a liquid cooled affair, the heat exchanger does not have to be mounted
on the engine mount (with it's increased stress due to arm and weight,) but
can  be mounted on the firewall (easier in a pusher) since the coolant
tubes are
 flexible and all you need to do is duct the cooling air to the high
pressure
 side and exit it blow or above in a lower pressure area.

The Baffling does not have to be made of metal. It can be made of
 fiberglass
and epoxy (better curvature and less eddies in the airflow.) If you  are
anal, you can even use high temp resin.

Rich
--
The problem with oil/water heat exchangers is; your putting all your
cooling eggs in one basket. One third of rotary cooling is the oil.
If you separate them it prolongs the time the engine will
run with a water cooling failure. Since one must now increase
the volume of the water core by 1/3 to handle the oil cooling
load there is no savings.

Also the rate of heat transfer is a strong function the difference
in temperature between the coolant and the air :) You want to
maximize that. If you have a liquid coolant temperature problem you
will also have an oil cooling problem. Needless to say I don't
recommend oil/water heat exchangers for aircraft use.

I covered all this in my book How To Cool Your Wankel including
the correct shape of the baffling AKA diffusers.
Also see http://www.rotaryeng.net/cooling.html
        http://www.rotaryeng.net/Oblique-flow-diffusers.pdf
        http://www.rotaryeng.net/Voyager-Eng.pdf

* Aerodynamics of Propulsion. D. Kuchemann & J. Weber McGraw-Hill Book
Company
Inc. 1953 Lib. Of Cong. Card # 52-6541 The bible of the aerodynamic design
of scoops and ducts. Also covers ducted propellers.

* Compact Heat Exchangers. W.M. Kays and A. L. London Kreiger Publishing
Company. Malabar Florida. 1998 The bible of heat exchangers.
Some work on ducts (called headers in this book). Very comprehensive.

Paul Lamar


Only some airframes work with the exchanger(s) between firewall and the
engine.  Some motor mounts are too short for this to work very well, so the
exchanger(s) must be mounted below or beside the engine.  The weight of the
exchanger(s) is easy to deal with on the motor mount.

I am proposing exactly _not_ mounting the heat exchanger(s) where they
either ingest or exhaust air to or from an open area within the cowl.  I am
looking for predictably good cooling performance, transferable from one
airframe to another.  If the inlet and exhaust air paths are idealized in
the form of nice ducting, and then other components are fitted around these
ducts, the results will be good.

This is like that old thing of the pickle jar, the stones, the gravel, and
the sand.  If you make sure you get the stones (engine, heat exchangers) in
the pickle jar first, the system will work okay (because you have satisfied
functional fundamentals).  After that, you fit the accessories in the
remaining available space being confident that the big things are going to
work well.

Once the duct routing is figured out, it's certainly possible that cutouts
could be made to accommodate control cables, wiring harness, and plumbing,
while maintaining a good seal for the air flow.

The duct(s) should be metal where both sides of it are exposed to heat - by
the exhaust on one side and the heat exchanger on the other.  This would be
the case where the heat exchanger is mounted near the exhaust system.  The
metal blocks radiant heating of the exchanger from the exhaust (which can
dramatically lower the exchanger's efficiency).  Plus, I don't think the
normal resins used in light aircraft construction are up to operating in
close proximity to a rotary exhaust system.  Such regions would likely be
relatively small and might instead be served by heat shields between the
exhaust and the exchanger duct.  This is probably especially critical on
turbocharged engines, and 13b's (peripheral port exhaust).  The 13REW with
it's side exhaust and inherently lower exhaust temperature may not require
quite as much care around the exhaust.


Regards,

Matt-