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Posted for "Christopher Zavatson" <Christopher.Zavatson@udlp.com>:
Villi,
If the air intake were sized to keep area x cruise speed equal to
engine volume demand, you would not be able to recover any ram effect. You could still expand the cross sectional area and slow down the flow
to reduce losses through a filter etc, but you would have no net gain in
MP. To gain the most ram, you really need your induction system to act
like a pitot tube and not allow any flow into the engine. Once you
start allowing air into the engine, you begin losing pressure. In our
case the amount lost is, however, very small. The volume of air
ingested by the engine is less than that which could be taken in by our
3 inch diameter intake. The result is a reduction in velocity and a
pressure rise even if you never expand beyond the 3 inch diameter cross
section. This ratio of actual inlet cross section to engine volume
demand/cruise speed sets the maximum ram pressure that can be recovered.
Flow losses in the induction system must now be subtracted from this
maximum theoretically possible value. Since these losses are a function
of velocity, slowing the flow in the induction system down helps
minimize these losses.
Here is another way to look at this. Close up the throttle completely
and fly at 200 kts. The induction system is acting like a giant pitot
tube and is seeing 100% of the stagnation pressure. If you start to
open the throttle, letting some air to pass out the back of your
induction system, the pressure inside will begin to drop. Now since you
are trying to supply it faster in one end than it can be consumed at the
other, you still have a net gain of pressure. I calculate the ratio to
be about 3.5 in this case which leads to about 90% of the stagnation
pressure being theoretically recoverable(before induction losses are
included). Chris Zavatson
N91CZ
360std
BTW it looks like you got off by an order of magnitude in calculating
area x speed (7 x 218740) s/b 1.4 mil in^3. not 14 mil in^3. Excess
available volume is close to 3.5x instead of 64x.
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