OK, I'm not sure how I feel about this. It is hard to look
at this objectively when I
have spent my entire adult life considering intake flow velocity
and how to keep it UP due to the demads of carburation. I THINK
that I interprete Taylor to mean bigger is better, but only up
to a certain point. AH, the devil is in the details and the only
way I see to solve this is to experiment - on my list of things
to do when I get the airplane flying.
However, no one has addressed exhaust gas dilution and how
the large ports
affect low speed/rpm operation - do not underestimate the
importance of this if you are running a fixed pitch prop. The
engine has got to idle in order for the landing task to be
performed safely.
At low rpm the exhaust still contains enough high pressure to
backflow into the
intake since both the inake and the exhaust are open at the same
time (overlap). This causes all kinds of problems such as lack
of oxygen in the intake charge, high levels of CO(??) and NOX which tend
to put the fire out, decreases the low pressure of the intake,
and increases intake temp. I've probably forgotten something
important here, but you can see the problem.
As stated above, large intake pipes means low velocity at
these low RPM's.
Small intake pipes mean higher intake pipe velocities and
pressures - these higher velocites create inertia in the intake
charge which blow past the exhaust leakage and maintain the low
pressure of the intake port - these actions help to decrease the
negative affects of exhaust dilution.
Ken Powell
The overlap of the periphery port and the bridge ported rotaries is
not a factor in very low speed performance. The driver has a habit
of letting the clutch all the way out just as soon as the car starts
moving. This should be the worst case for an overlap problem. There
is no problem. The car just drives away without protest, with a 2:1
first gear and a 411 rear gear. It shouldn't, but it does. Under 500
RPM is my guess. It will only take a hint of throttle at that speed
but it does work just fine. The bridge port is not quit as nice as
the periphery port because the port closing is actually later than
the periphery port, but they both run fine very slowly with light
loads. Idle (no load) for both is 2000 RPMs but even that can be
slower, it just sounds like hell.
Come in high. Slip it in and land. Who cares if the engine quits
:-)>
How many people still do slips? Hold up your hands.
Lynn E. Hanover
I do. It is excellent training for emergency landings.
Paul Lamar
I have a couple of things to cover here.
I went flying today and played my usual game in the pattern - Pull the power
completely at the numbers on downwind, crank in 20 degrees of flaps (or whatever
is appropriate), then set it down on the numbers without adding power. If you
misjudge the approach (or wind) and have to add power, you loose the game. If
you don't set it down within 50 yards of the numbers (land long), you loose the
game. My CFI taught this technique because you will always make the field if you
are in the pattern if you should loose an engine. Also great practice for short fields.
I have only had my ticket a couple of years and know that you guys can teach me
alot so suggestions are welcome, but be nice to a low time pilot who is still learning.
I practice this a lot and almost never 'loose the game' anymore. And yes, I do
forward slips but I am not as precise as I need to be - still working on the technique.
Item 2 is an addition to Lynn's comment above. Hopefully, the prop load at idle
speed will provide enough load that my PP engine will idle as Lynn describes
above. A SMOOTH idle is important because of the redrive. Seems that I
remember that Tracy idles at 1200 rpm (Tracy?) which is a worthwhile goal. I really
want my PP engine to idle smoothly at this speed or lower. The less thrust that is
generated while getting into short fields, the better (and safer). Remember, this
engine is for a RV-4 that will be flown mostly in rural areas and a lot of short dirt
strips.
Ken Powell
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