Johannes,
You wrote:
In a message dated 3/20/2006 1:22:57 A.M. Central Standard Time,
jschredl@web.de writes:
1.
Calibration of probes: use 4 pc. EGT and CHT thermocouples with the
EPI800
system from Vision Micro. I checked one probe years ago with
confidential
results. But I agree, I have to re-check them all again to be
on the save
side.
Are you using bayonet type probes? Spark Plug Washer type CHT
sensors can yield a reading 50F higher than actual.
2. During a normal flight I have following results
(+-10°F):
2500 RPM; 23,3"Hg; OAT=72°F; at 4500ft with 185MPH
Indicated Airspeed;
Oil-Temp=193°F; Fuel = 7,5 gal/hour; (Temperatures
converted from °C to °F)
CHT
EGT
Cyl. 1 390°F 1400°F
Cyl.
2 410°F 1436°F
Cyl. 3 410°F
1418°F
Cyl. 4 390°F 1382°F
I
have a standard IO320 Lycoming (new when installed) with standard
Bendix
injector, Slick magnetos and standard pistons.
I notice Cyl. 2+3
have slight higher EGT and same to CHT.
Today I braved high winds and cold weather to try to collect similar
information. Although the temperature was somewhat cooler, these results
are consistent with past tests. Note that I have ECI Cermi-Nil cylinders,
9:1 compression ratio pistons, about 300 hours SMOH (15 hours on cyl #3 OH),
Light Speed Electronic Ignition and a small (7 Vane) Stewart-Warner oil cooler.
So....
23.5 MAP, 2500 RPM, 4500 feet MSL, 30.17"Hg (1020 Mb), Dalt 3600, 4C
(39F) OAT,
176 KIAS, 186 KTAS (these are Knots) and Oil 165F, Fuel 9.7 gph.
Note: #3 EGT peaked at 1430F, 8.4 gph (EI timing was 25 degrees BTDC).
After enrichening to 9.7 GPH:
CYL CHT EGT
1 340F 1290F
2 330 1320
*3 340 1320
4 340 1310
I then closed the oil cooler air intake door far enough to raise the oil
temp to 176F (no change in CHT) and
note that my EPI 800 spare temp probe is located at the
oil cooler air exit where it registered 69C (156F).
Next I opened the throttle fully (WOT), MAP of 27.4", 2500 RPM, 11.7 gph
(100F rich of peak), 192 KIAS (201 KTAS)
with all the CHTs dropping about 10F.
On return I climbed to 5500 MSL where the density altitude was 4600 feet
and the temp was 3C.
There I ran 23.5 MAP, 2500 RPM, 178 KIAS, 9.6 GPH, 176F oil with the
door partially closed and
the CHTs were even at 330F.
SO,
a. Do you really mean 185 MPH or 185 Knots? The difference would
be significant in gph and cooling differential pressures.
b. Is your oil cooler a Stewart-Warner (other brands are less efficient)?
How many vanes?
c. Is your Vernatherm correctly set to send all the oil to the cooler when
the temp exceeds 180F? Oil accounts for at least 1/3rd of the engine
cooling. The vernatherm can be tested in hot water to see if it fully
shrinks (opens) at 180F.
d. Has your throttle body been checked to see if it is delivering all the
fuel it should? Fuel consumption seems a bit low unless your HP is much less
than mine or;
e. Are you running at peak or lean of peak EGT? (I tend to see lower EGTs
because of the electronic ignition, but not that much lower.)
3. Measuring the pressure conditions was my first attempt to
get more
information about this problem.
I have to add I used the static
pressure of the cockpit, not from the static
ports. Now you tell me there
might be a considerable differenc I have to
repeat measurement using the
exact static pressure!
The reason why I used a airspeed indicator was I can
compare it better:
Aircraft speed v: thats the maximum of pressure I can
ever get (knowing
p-dynamic = 1/2 * rho * v * v) -> max ram air
The
"air-speed" at the upper cowling never can be higher than v! In
reality
less than v because the "upper cowling" is not perfect sealed and
air is
going down through the cylinder fins. My feeling 75% of v (100%) is
a good
value.
While there is a relationship between aircraft airspeed and possible upper
cowl pressure, using a % airspeed relationship is not
meaningful. It is the upper/lower cowl pressure difference that is
significant regardless of airspeed. Thus, the pitot/static use of an
airspeed indicator is only useful to convert to pressure, such pressure reported
in the literature as inches of water, not airspeed% or psi. My friend
with an E-Racer (rear engined) is concerned with the pressure differential even
when he is idling his engine and the aircraft is at zero
airspeed.
Very
supprising to me is a pressure of about 40% below the cylinder. My
first
thought was: this is much to high to have a good pressure
difference
between upper and lower plenum! Here is the biggest potential ot
improve
cooling by reducing this pressure.
Reading all your mail show me
that´s "normal"...
By the way: the airspeed indicator (I use for
"pressure-measurement") needle
is vibrating about +-3knots and gives a not
too bad reading. However using
some kind of damping should improve this
instrument
Since this lower cowl "airspeed" was computed from cockpit
"static" pressure, it is not understandable except to point out that the engine
compartment has a positive air pressure with respect to the cockpit (see my
separate e-mail) and that is important for other reasons.
If I take the speeds in your original e-mail graph (say, aircraft at
180) and convert them to pressure (upper 135, lower 65) and subtract the
difference, that should be the upper/lower cowl pressure difference since the
static source is the same:
In Knots, 135=11.9" H2O, 65=2.8", difference = 9.1" H2O certainly enough
for cooling. However, if we are using MPH:
in MPH 135= 9" H2O, 65=2", the difference is 7" H2O which should be
enough for cooling although it is at the lower end and certainly won't be as
high in a Vx or Vy climb.
4. On the backside of my baffles I have holes for the oil
cooler (as
described in lancair manual) and cabin air. Both holes are about
2"
diameter. Variating the hole for the oil cooler do not have a
signigicant
change to CHT but of course to the oil temperature (I use
different cut out
sizes for summer /winter time)
Interesting. Note that if you are using the Lancair cabin heat valve
(1.5"), when the heat is turned off the hot air is merely directed into the
lower cowling rather than the cabin. Also, you are dumping the oil cooler
exit air into the lower cowling and it is possible that these air exits may
affect the engine cooling air's smooth exit as well as creating a low pressure
across the top of the cylinders. I found it interesting that your very
nice cowling nose gear door made no difference as I was just considering one to
improve cooling efficiency and thus reduce cooling drag.
5. I building a pressure cowling which seals the baffles
perfect. At the air
inlet section I tied to have only very small gaps
(about 1/8"). Right now I
did not consider to have a perfect air flow
design with smout ramps less
than 7° angle (will be
improved).
The short and sharp ending of the Lancair cooling air inlets tend to have a
turbulence which makes the opening appear smaller to the air flow than it
actually is.
Conclusion:
I see the aim to get a lower CHT is not that
simple (as I already know
before). But when measuring a "40%-Pressure"
below the cylinder I thought
that´s the reason! But when the air outlet
area from the lower cowling is
big enoth and is acting like a venturi
system there should be a much lower
pressure...
Maybe there are
turbulences or other reasons (like Gary Casey mentioned )
for this high
pressure!
I also liked the slight flare (away from the fuselage) at the air exit
point of the bottom cowl as this should create a slight low pressure and assist
the exiting air rather than have the airstream curl back over the sharp cowl end
and slightly close up the exit area in a virtual sense.
Verrrrry Interrrrrresting!
Scott Krueger
AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)