I went out on a cross country from Everett, Washington to Walla Walla and
back the
last few days. Cruising at WOT at 11, 500 feet I had a MAP of 19-20 and
was making
about 140 mph true rpm was around 6200 with a catto 3 blade 74 x 88
pitch. I need
some aerodynamic cleanup on my cooling scoop, something I intend to work
on this
winter. But I think that I'm probably only making about 150 hp or so- not
bad, but
not what I could make. I think that the NA 13BREW with four ports and
9.1/1
compression would not be a match for a 13 b with p ports and 9.7/1
compression. I'm
thinking that I might be building such and engine for an engine swap in
the future.
Oh, and by the way, it was a great day for flying.
Brian Trubee
Build another engine with a p-port Brian. It will really help up high.
A 4 port will never match a p-port. A good 4 port manifold is hard to
make as well.
I assume you have a 2.17:1 gear box.
Paul Lamar
I don't know if I would get an appreciable increase in power if I were to
use
premium, I am using regular mogas at the moment.
Brian Trubee
BTW power in a rotary is directly related to RPM in that range.
The best pay off is work on the cooling drag first.
Pressure on any surface facing forward is drag. Use a water manometer.
Get a Gopro camera to record the results.
Paul Lamar
Right now I'm happy to just be flying, I'll wait until the weather turns
crappy out
here before I start doing aerodynamic cleanups and other refinements. Oh,
and my
temps were pretty good- highest was 201 one the coolant, 208 on the oil,
that was at
10500 feet at 21 inches MAP and 39 OAT and 6000 rpm. . A little less
throttle and the
temps dropped down to the 180s-190s. Exhaust is running high- about 1950
-2000- but
my EGT probes are 1 1/2 " from the exhaust flange. The lowest that the
EGTs run
regularly are around 1800. I think that these alarmingly high egts are a
function of
the close proximity of the probes to the exhaust ports. Must keep an eye
on the
headers when doing maintainance/checks.
Brian Trubee
Hey Brian,
A good part of the temperature that you are reading is due to the
stagnation pressure of the high velocity exhaust gasses impinging on
your temperature sensor elements. The exhaust gases are compressed
against the leading edge of the sensor element. The effect can be quite
impressive at the supersonic velocities found in the exhaust flow of
rotary engines. The same effect causes triple digit temperatures on the
leading edges of high velocity aircraft, like the SR-71, when the actual
OAT is in the double digit negative degree range.
Boyle's Law explains the relationship of temperature with compression.
This effect is also explained by the the law of conservation of energy.
The kinetic energy contained in the supersonic velocity gas is
converted to thermal energy when the gas slams into the stationary
temperature probe and comes to a full stop.
This is the same energy that is recovered by the turbine blades in a
turbocharger. It also contributes significantly to the heating of the
turbocharger blades. It also makes a significant contribution to the
heating of the exhaust valves in boing boing engines.
If you want a more accurate reading of the temperature of the gas stream
the reading must be taken behind some obstruction that will slow the gas
stream, absorbing the heat resulting from flow compression, similar to
the way combustors are designed in jet engines, to slow the gas flow so
the flame will not be blown out by the air stream.
Mark LaPierre
Very interesting Mark. Thanks. You obviously have some experience with
this. What is your occupation?
Paul Lamar
Brian,
Can you explain your exhaust system? If I saw 1900* EGT's I would be very
concerned that it would damage the coolant o-rings. I managed to damage
the coolant o-rings on my 20B with 1800*+ EGT's. After changing to a less
restrictive exhaust the EGT's dropped down into the 1600* range. With
reduced exhaust back-pressure your EGT's will drop and your engine will
produce more power, and you will pick up some free airspeed. And yes, it
will be louder too.
Mark S.
20B N/A P-port
Were they Teflon Mark?
Thanks for revealing that problem.
Do you have any pictures of the damaged O-rings?
BTW some things can be done to cool the O-rings in the exhaust p-port area.
Here are some pictures of Brian's exhaust.
I doubt back pressure is Brian's problem as other people have used the same muffler
with out reporting overly high EGT readings.
The answer to the noise to back pressure ratio is a longer 3 or 4 inch diameter
muffler under the fuse. This adds little to the frontal area. I know you tried to
keep the muffler inside the cowl.
For the new guys the RX8 engines with side exhaust ports have EGT's up to 200F lower.
It's time we started measuring back pressure.
Paul Lamar
I think that the proximity to the exhaust ports and the velocity of the exhaust gases
would account for the high temps more so than anything else. My coolant and oil temps
never exceed 205 or so, and are generally around the high 180's in cruise. The first
six inches of exhaust is 1.75" id with the transition to 2" ID about 10 inches
downstream. So, I imagine the exhaust cools quite a bit when it gets to that point.
I'm doing an inspection on my exhaust system first thing when I get out the the
airport again.
Brian Trubee
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