Paul raises excellent questions.
"I'm still a little confused as to why the larger reservoir is needed
unless it is simply to assist slowing the flow and providing more even
coverage of the cylinders."
That is precisely why you want as large volume over the engine as
possible. Goal: to reduce free stream velocity much and efficiently
as possible so that static pressure is maximized and momentum of flowing
air is minimized on top of the engine. This creates the best, most
even distribution of air pressure and flow over the cylinders and
heads. It is hard to do well in the limited space available
with front entry flow.
And it is usually also helpful to lower velocity through the inlet
rings, like 50% or less of the free stream velocity. Otherwise you have to
slow high inlet velocity inside the cowl if it comes in fast through the
inlet rings.
The slow down in air speed can be outside (a somewhat blunt front end
with larger inlets) or inside ("pointy" front end with little
inlets). Little inlets need to be followed by a diffuser to
efficiently slow the air flow, hard to do in a short
space. That is why it is usually beneficial to have
larger inlet area and careful design to prevent external air flow
separation from air "spilling" out around the inlets. Inlet lip
radius and cowl contour around and behind the inlets become
important.
"A plenum is a smaller reservoir and I'm guessing the lower [leakage]
losses of a plenum are dramatic compared to [leakage] losses in a normal
cowl?"
Yes. A plenum should have as large a volume over the engine as
possible within the confines of the cowl for reasons noted
above. Too small and the velocity inside is too high and you
get friction losses. Bigger volume - better. But ... Why use
plenums? Because conventional baffling installations leak like
hell. It all depends on attention to detail and the ability to seal
a vibrating engine against a stationary cowl.
Here is the basis for concern: In the 80's Miley et al did
NASA-sponsored work on cooling. Their test plane was a Piper Turbo
Aztec with turbo 0-540 engines. They made lots of measurements
in flight and on the ground. They went so far as to connect the
engine in the airplane and inside the cowl to a dyno (no prop) with a long
shaft and hooked up a big blower to pump air into the cowl via tight
fitting ducts sealed into the cooling inlets. This permitted them to
make a careful measurement of the cooling air flow actually entering the
cowl inlets.
Findings: if the engine received 100% air flow over heads, cylinders,
and oil coolers as required to keep temperatures under control, the ACTUAL
air flow into the inlets was 150%. In short, 1/3 of the total air
flow entering the cowl inlets leaked AROUND the engine and ended up in the
lower cowl having done - nothing - except create additional drag from
momentum loss associated with the leaking air.
So the idea of plenums as verified in that report (8 megabyte file,
long complicated technical report) is to minimize leakage.
Factory baffle installations of that period, and in many cases still
persisting today, are simply awful. Leaks everywhere. The
Cirrus is best of class for current factory engine installations.
A good plenum with a good fit between plenum and inlets to
accommodate engine movement yields better use of cooling air and lower
leakage and thus lower cooling drag. It also produces greater
pressure drop across the engine itself and thus better cooling.
Why? Leakage around the engine helps to pressurize the lower
cowl since more air (engine cooling air plus leakage air) has to escape
out the lower cowl exhaust ports back to the atmosphere. More
pressure below engine for same pressure above the engine
equals lower pressure drop across the engine and thus lower air velocity
across the engine and thus less cooling.
If you can make your conventional baffles as tight as a good plenum
system, the result should be equally effective. Extreme attention to
detail including allowance for all that engine vibration and relative
movement relative to the cowl must be taken into account. I don't
know how to do this. Others may.
There is another way. I saw a photo of one racing aircraft that
used rubberized fabric above the engine attached as rubber baffling would
be around the perimeter of the top of the engine, but forming an complete
"balloon" or tent with some kind of coupling to the cowl
inlets. When pressurized by inlet flow, it blew up and was
resisted by the upper cowl, same pressure and loading as with conventional
baffling. But no baffling gaps from poor installation or
vibration. Think flexible, inflatable plenum. Unconventional,
but should be as effective as a well designed rigid plenum as it creates
maximum possible volume over the engine with absolute minimum leakage, IF
it is well executed.
"If a plenum is not used, the best method for measuring the seal of a
typical cowl would be differential pressure measurements from pitot at
inlet to static inside the cowl?" Er, um.... Short answer: No.
Let's be careful here. Which static inside the cowl? If you
measure ship's pitot to static above cowl, you get an idea of the amount
of total pressure recovery your inlets are delivering above the
engine. That is useful. One would like 80-90+%. But you
will NOT get a measure of leakage. Other than the way described
above, there is no easy way to measure leakage.
The cooling of the engine is dependent on the pressure drop between
the top of the cowl and the bottom of the cowl, that is, pressure
differential across the cooling fins, top to bottom. To measure
this, plumb a line to above the engine, and a second to below the cowl,
and put these lines on a water manometer held by your co-pilot. You
can make a manometer from a wooden yardstick with a clear tube forming a U
around the yard stick, held on with tape or plastic clamps screwed to the
yardstick. Measure the pressure difference as the difference in
water level on the two sides of the U. This value is what
engine cooling depends upon.
Usually in the real world at climb - cruise conditions pressure drop
from top to bottom should be 8-10 inches of water at the lower altitudes
(say 6000-8000 feet), more (possibly much more) at high altitudes (teen's
and twenties) due to thin air effects. If you are not getting
the required pressure drop across the engine, then your CHTs (and probably
oil temperature) are probably high. More power requires more
pressure drop to carry away the heat.
If you measure ship's STATIC to bottom of cowl, that gives the
pressure drop from inside the bottom cowl through the exhaust ports back
out to the ambient. If it is high it could be because the exhaust
port area is too small (unlikely in a Lancair IV, probably same in a
Legacy), OR it could be because you have huge leakage around the engine
due to poor baffling.
Check numbers.
1) You should have total pressure from ship's pitot tube to above the
cowl to see how well the inlets are working.
2) You should have pressure drop across the engine (top to bottom) to
see what actual engine pressure drop is.
3) You should have lower cowl to ship's static to get pressure drop
going out the cowl exhausts.
4) All three numbers should sum up, within experimental error, to
equal total pitot tube pressure which will be expressed as
IAS. If not, try again.
Good data is hard to collect.
I hope this helps.
Fred