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.
Johannes: The probes I
use came with the Vision Micro EPI800 system and are screwed into the
cylinder.
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
Scott, when looking
to your values I noticed an error in my values while
converting fuel consumption (used Imperial Galons ;-(( my right
fuel consumption is: 8,7 US-gal/h
I also see your EGT is
much lower than mine: my EGT peak at 4500ft is about 1510°F! I lean always to
the rich side at least -100°F as reccomended in the lycoming manual. Is this
correct? My probes are all 3" down from the tube
flange:
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. Johannes:I
mean 185 MPH = 161 knots (IAS) and 8,7 gal/h
b. Is your oil cooler a Stewart-Warner (other brands are less efficient)?
How many vanes? Johannes: Stewart-Warner 6 vanes
(note: blue silicone
seals are removed and changed against pressure cowling)
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.
Johannes: I´m not shure. But on a hot summer day
cruising at 75% oil temp does not exceed 205°F. I think is o.k. Is
it?
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; (Johannes: Right! Correction see
above)
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.) Johannes: rich
side -100°F
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.
Johannes: you are
right, but I thought the "air speed" gives me an idea about
the pressure situation...
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.
Johannes: Your are
right: I´ve learned to use not the cockpit static
pressure...
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.
Johannes: The unit
indicated in the graph is Knots (IAS)
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.
Johannes: To be
honest: I did only a short flight under "bad weather conditions" and did not
notice a huge drop of EGT (as expected ;-). As soon I´ve time I will repeat
this tests using a static port and get more precisely
data!
Johannes: (note: blue
sealings are removed); oil cooler exit air: a good point! This could have
a negative effect to the cowling outlet
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.
Johannes: I totally
agree with you! But until now I refused to touch this cowling part
because of paint design...;-)
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.
Johannes: Thank you
very much for your detailed answers! Step by step I get a better understanding
about the situation and some realy good new ideas! It is encouriging me to
procede testing!
We had a long and cold
winter but now spring is coming gradually! It´s time to sart flying
season!
Johannes
Verrrrry Interrrrrresting!
Scott Krueger AKA
Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)