Alan Tolle wrote:
I see a couple of recent items which need further clarification.
The only time when the Powersport RV-3 had any trouble cooling was on the
ground in a slow taxi. If I could taxi with some speed to keep the air
coming into the annular inlet I could taxi forever. A manually switched
electric fan on one of the water radiators would have solved this problem
easily. I think McGowan did this on his Long Eze.
The shape of that annular cowl inlet was very carefully crafted. We were
indeed that sophisticated. Extensive inflight testing with and without the
spinner had revealed no difference in either cooling or top speed. Yes, it
looked like a lot of drag, but it wasn't. Arnold discovered the same thing
on his AR-5. We were using wood propellers and changing often ( for speed
vs climb) and the spinner was in the way. So we left it off. It was only
there for looks anyway.
Our big interest at Sun and Fun that year was in taking Bruce Bohannon"s
money in the Shell three D speed dash. Using the nitrous oxide I could
better 4000 feet per minute climb, and I had climbed the 1000 meters in less
than the 50 seconds that Bruce required when he beat the Thunder Mustang in
the finals. When I posted the best qualifying time on the first day Bruce
knew that he had been beaten at his own game. But the horsepower boost
from the nitrous (up to 350 hp total) also started the destruction the rear
prop shaft bearing over the 20 or so minutes of dialing in we had done back
in California. The Sun 100 race finished the job. Damn. You can't win if
you break.
It is true that in the RV-3 the first 15 inches or so of the exhaust system
were in the cold air plenum and were totally unshielded, which obviously
decreased the cooling efficiency by adding heat to the cooling air stream.
But that is not the point. The point is that the system worked. The
installation could develop full rated horsepower all day long. The only
change mentioned that we might have made would have been to copy the cooling
core arrangement from the Powershort RV-4, which was simpler and more nearly
optimized. (The cooling core arrangement in the RV-3 was specifically
designed to fit around the rather bulky induction system of the Powersport
180 Superlite engine. That arrangement was not required by the Powersport
200 engine.)
I have the suspicion that the other rotary installations flying today of
which I have any knowledge are cooling limited. I get that idea from
statements made by several contributors to these very pages. In spite of
that several rotary powered aircraft have made it to Oshkosh, Sun and Fun,
Copperstate, etc, which proves that the installations can go the distance,
but says nothing for performance. Perhaps you don't realize how important
performance (and I don't mean fuel economy) is to the homebuilder. Faster
is better and second sucks are the names of the game.
Note that the all up installation weight (engine and gear box, prop,
coolers, and fluids) of the 13B conversion is going to be only very slightly
less than that of the Lycoming engine that it is going to replace.
Therefore for weight and balance purposes the rotary installation should
place all of these items inside the cowling where the Lycoming engine was
or would have been.
I realize that this last statement contradicts some of the opinions
expressed recently on these pages regarding cooling core location.
Certainly no criticism is intended. Simply realize that I enter this
forum through a different door. I am not a technical type working with a
library and a computer screen --- I am, however, a well experienced kit
aircraft builder who is quite handy with a wrench and a rivet gun. And I
fly what I build --- 4000 hours worth. Having been deeply involved in
homebuilding for 20 years I have a very good idea of what is going on out
there. I gave guys like Anders and Saylor their first RV-4 rides.
Most home builders do a good job on the airframe, only because the kits are
good and so are the instructions as to how to build them. The engine
installation is a completely different story, but the builder usually
manages to hang the Lycoming and wire and plumb it by looking at a Piper
Cherokee and reading a Tony Bengelis book. The poorest work on nearly every
homebuilt I look at is inside the cowling. The fact is that most of today's
builders do not know enough about engines to do any better, let alone to
know that they did poor work. The "learn as you go" approach yields some
very questionable results. Note that in accidents and incidents ralated to
mechanicals it is ususlly something about the engine which caused the
problem. The airframe is seldom at fault.
For the sake of simplicity alone the installation of the rotary package
should be all forward of the firewall. There is adequate room inside the
cowling for the entire rotary installation, radiators included, in nearly
all kit aircraft (One exception might be the tricycle retract with the very
minimal vertical dimension cowling, like the two place Lancair. I concede
that Kruizwyk has a special problem that requires a special solution. That
does not mean that everyone else has that problem or should use that
solution.)
There is adequate room for everything inside the cowl EXCEPT for one
important point, something which has been mentioned on these pages before
and which should be getting the attention now being given to trying to get a
large enough radiator in a belly scoop or a tail cone. It has made no
sense to me to saddle a beautifully compact engine like the 13B with a gear
box (you call it a PSRU) longer than the engine itself. The planetary gear
set only takes up 2 inches and 6 inches is plenty long enough for the prop
shaft. Perhaps the engineers in this forum can explain the necessity for
all the extra length. If there is no engineering explaination, why is the
extra length there? I would really like to know.
When the gear box is short enough (all of the Powersport boxes were) there
is adequate room between the engine and the firewall for all the cooling
radiators one will ever need. The radiators do work there (400 hours of
hard running test flight data says so) and the installation details (things
like cooler mounting, baffling and cowl flap) are simple enough and
inexpensive enough to be do-able by the average builder. This is the
easiest way to solve the aircraft rotary engine conversion cooling problem
and get everyone up to speed, so to speak. Solutions by those with lots of
money and facility will be a lot more exotic.
(Before someone reminds me that the cowls on the -3 and -4 were 6 inches
longer than the normal 36 inch length used for Lycomings, I will report that
there were 16 inches between the firewall and the engine crank pully. 10
inches is more than enough room for the cooling cores. The long cowls were
occasioned by the much lighter weight of the first two Powersport prototype
engines, which used home grown aluminum end (side) housings. dry sumps, and
other trick things. The light engines were 6 inches forward for reasons of
aorcraft center of gravity.)
When I was doing graduate research work at UCLA Medical Center I learned
that to get the right answer one had to first ask the right question.
Perhaps this forum has been addressing the wrong question. The real need
here may be for a shorter gear box, not a radiator in a belly scoop.
This is food for thought, anyway. There is no intention here to start any
arguments like the long running flap over intake systems. I strongly
defend the idea that every home builder has the right (but not necessarily
the knowledge or ability) to do anything he wants to do in his aircraft
construction. All of this is just an opinion from another point of view, an
opinion developed from having been there, done that.
Thanks Allen. I agree with you on the PSRU length and hopefully the New
Power
Sport and Tracy Crook are addressing this issue as we speak. BTW you can
move
the battery up to the cowl and shorten the cable needed for the starter
if you need more weight up front to balance a rear radiator system.
The main reason for a near vertical rad and a long in and out ducts is
an attempt to reduce cooling drag. As you well know speed is
directly proportional to drag while it is also proportional to
HP cubed. Climb on the other hand is proportional to HP as
you proved. Thats also why those Harmon Rockets climb so well
but are not that much faster than a regular RV.
I agree that the best place for a rad, if you have to
put it under the cowl, is behind the engine. Unfortunately you are
asking the air to make two sharp 90 degree bends and the exit duct is
not of ideal shape or location.
PL
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