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Hello Greg,
Thank you for posting the pictures of the 20B aircraft installation you
are fabricating. Your pictures and my personal past experience compels
me to offer some words of caution. First, let me say that I have no
idea as the the degree and quality of the engineering that has gone
into this project to date. I am operating on a worst case assumption so
please discard comments concerning issues that you have already
addressed. I hope to present these in a self evident fashion. Know that
I have been involved in rotary engine development since the late '70s,
that I was involved in developing and building a 12A engine that
competed and won it's class in the 24 hours of Daytona endurance race,
that I have previously developed high output rotary engines for
aerospace applications, that I worked with Charlie Jones at Curtis
Wright to help market their SCORE aircraft rotary engine to Cessna and
Piper and that I seriously evaluated the Mazda R26B engine (4 rotor
winning Le Mans in '91, see SAE paper #920309) for my Lancair IV-P but
opted to adapt a Lycoming TIO540. I flew that airplane, and won, the
Denver to Oshkosh race in 1996. I now have over 1,000 hours on the
original engine, including cylinders, pistons, rings and valves.
The Wankel rotary engine we know is actually a compromise from Felix
Wankel's original concept. The progenitor engine, designated DKM-54,
featured a rotating rotor and housing arraigned such that each had a
pure rotary motion. It was NSU's engineer Walter Froede that came up
with the internally geared epicyclic design we are familiar with
today. While this design is attractive due to its high power density
and minimal parts count, it is not without its problems. One of the
greatest problems is the high aspect ratio of the combustion volume.
This leads to high percentage of charge quench and therefore poor brake specific fuel economy,
high hydrocarbon emissions and high exhaust gas temperature. The CW
SCORE engines attempted to overcome these shortcomings with a heavy
fuel direct injection with spark assist. They were moderately
successful.
Another problem is the high power density with respect to heat
rejection. Nominally 20 percent of the total combustion energy must be
rejected to the cooling system. For a 300 Hp engine this heat flux is
more than 100 kW, enough to run 20+ average single family homes. This
is divided between the oil and coolant so the oil is at risk of
chemical breakdown above 240F (natural oils). I solved this problem on
the race engines by adding a tube shell intercooler between the oil and
water cooling systems that limited the oil temperature to 230F by
allowing the coolant to boil in the heat exchanger, dramatically
raising the heat transfer rates. In short, your three biggest problems
will be heat, heat and heat.
Comments referencing your photos and subsequent post:
1) Belly mounted cooling system. The engine is VERY sensitive to air in
the coolant. You MUST have a de-aeration system mounted above the
highest point of the engine. This is a convenient point to place a
"radiator cap".
2) Oil sump. It appears that you are using a stock oil sump
configuration. You will find this insufficient for the anticipated
continuos power levels. You need a high volume (12 Qt) remote tank dry
sump system. The oil tank should have a centrifugal de-aeration design
with multiple baffles. You should use a synthetic oil that is more
tolerant to high temperature operation.
3) PSRU. It appears that you are using a planetary gear reduction,
likely adapted from an automotive automatic transmission. Be advised
that this configuration will generate between 1% and 2% of the
transmitted power in heat, or about 5Kw at 300 Hp. You need a separate
oil circulation and cooling system for this reduction. Prior attempts
to use engine oil have been marginally successful, at best. I am also
assuming that you have sufficient torsion compliance between the engine
and gears to prevent cyclical load fatigue failure.
4) Induction Plenum. It appears you are planning single port injection
based on mass flow into the intake plenum. It is important that the
flow distribution of the plenum be tested, otherwise you may have poor
charge and mixture uniformity across the three rotors. Flow testing can
be accomplished with a shop vac and carburetor flow gage (balance).
5) Trochoid Cooling. Stock rotor housings have poor cooling
distribution for high continuos power outputs. You need to have
modified rotor housings and a high output water pump that is driven at
the optimal speed.
6) High EGT. While having individual exhaust manifolds to the turbo,
to utilize the kinetic energy of the exhaust, is good, these should be
fabricated from Inconel to survive the high EGTs without failure.
Likewise, the turbo needs to be a high temperature version.
7) Engine Mount. It is not apparent why the engine mounts are vertical.
Thrust loads from the propeller can approach 1,000 lbs. The mounts, as
designed, will not respond well to these loads.
8) PSRU mount. While I can appreciate that a heavy flat plate is easy
to fabricate, it is poorly suited to react the gyroscopic precession
loads and moments generated by the propeller. You would be wise to
perform an independent engineering analysis of this design.
9) Transverse Engine Mount Triangulation. It is not clear from the
photos but it seems like there is insufficient bracing to react
transverse nose gear loads. Ask Skip Slater about this one.
10) Reversed slip couplings. The exhaust slip couplings seem to be
reversed. Upstream should be the interior tube.
The above list is by no means comprehensive. It is simply a free
association exercise based on the posted photos. Engine installation design is serious business. There is a long list of designers that have died behind
their engines. Success is in no small part dependent on the ability to
understand and allow for the errors of those who have gone before.
Remember that almost any engine can power an aircraft. The quality of
the engineering and implementation of the installation determines the
longevity of the application and, sadly, frequently the pilot.
Fly safe.
Brent Regan
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