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<<What I want to say is, there is plenty of room for losses (and there will
be) in the induction system, if we are going for RAM boost, but will still
have a gain.
If anybody can tell me where my assumptions are wrong, I would love to be
enlighted. Please bear with my poor english.>>
Villi, your English is excellent - far better than my German! Just to make
it worse, I'll use English units..
Here is the way I calculate it:
First, constant density calculations are appropriate as the pressure change
is a small fraction of ambient. The volumetric flow (360 cu. in., 2700 rpm)
will be about 8100 cu. in./second. The volumetric efficiency of these
engines is quite good, so we'll use that number. The inlet diameter of the
carburetor is about 2.5 inches (?) so the average inlet velocity is 735
in/sec. In a 4 cylinder engine the peak velocity is quite a bit higher, as
much as twice the average, but to keep things simple we'll leave it at 735.
The aircraft velocity at 180 knots is 3600 in/sec. The ideal approach would
be to size the inlet for that speed, in this case it would be 8100/3600=2.25
sq. in., or 1.7 inches diameter. Then expand the area of the tube gradually
(less than 10-degree angle of the walls is the rule of thumb) until it gets
to the carburetor inlet area. Avoid any sharp bends and you have the
maximum ram air recovery. However, at any slower speed, the engine will
have to "pull" the air to a high velocity through the inlets only to
re-expand it in the tube, creating losses. One way around this is just to
increase size of the air inlet. A blunt object will do a reasonably good
job of slowing the air ahead of it, essentially creating a free-stream
diffuser. Then it can inhale this higher pressure, lower velocity air and
not have to slow it as much within the ducting. This is what, as I
understand it, the jet engine designers do. Look in the inlet of a large
fan jet during takeoff on a humid day and you will see fog where the engine
is accelerating the airstream as the inlet is too small for that speed. The
fog disappears pretty quickly as the plane picks up speed. At cruise you
will likely see (if you could see it) air "spilling" out around the nacelle
as the inlet is then too large.
All this ignores the requirement of an air filter, which requires the
velocity to be reduced to a very low number anyway. What to do? I would
think the logical approach would be to size the inlet for a velocity
somewhat lower than cruise speed, say half to 3/4 of that speed, and then
begin to expand the passage area as close to the inlet as possible
(gradually, of course). When the area is at least twice the inlet area (now
the air is down to maybe half the free-stream velocity and contains only 1/4
the energy) go ahead and do with it what you want - you will lose some of
the remaining energy, but not all. Increasing the diffuser much beyond that
is not very effective. As close to the carburetor inlet as possible,
provide a bell-mouth directly in line with the inlet. There is an ideal
shape for this, but look at a trumpet - the shape is very close to ideal -
and it doesn't matter much if it is non-ideal. Doing all this I suspect you
will recover 75% of the free stream kinetic energy and getting more than
that is probably not worth the effort unless you have lots of space under
the cowl.
I encourage comments/criticism
Gary Casey
ES
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