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
Hey Paul,
I have engineering degrees in HVAC and Structure. I've spent most of my
professional time building ships and hacking computers. At the moment
I'm working as a database developer. I've been a Linux user since
before optical drives were invented. Remember those stacks of floppy
disks ;-) I've designed and built several K-8 size computer networks.
At the moment I'm fascinated by the mathematics of geodesic structures
and their relationship to the value Phi = (1+sqrt(5))/2. My plan is to
build a geodesic greenhouse roof. Probably a 3rd frequency Type 1
structure.
You may recall my previous post on counter flow cooling systems. Both
that post and this post draw heavily on my experience in HVAC design and
engineering. Engineers have the best fun!
One of the issues with measuring the flow velocity in ducts is dealing
with stagnation pressure at the sensing site. Careful consideration has
to be used to be sure that your pitot tube is measuring the velocity of
the flow, not the static pressure.
Flow velocities in ducts are used to calculate air balance across HVAC
systems where measurements at the diffuser are impractical. The flow
velocity in an A/C duct is so low that there in no need to consider the
compressive heating effects of the sensor in the gas stream.
Mark LaPierre
For measuring static pressure I use porous kitchen foam. Put a static tap as well as
a pito tube in the duct. I hot glue it to the end of drip irrigation tubing :)
We started with Magnehelic in the old days 1960's :)
Paul Lamar
That's a great solution for measuring the pressure in an open area such
as you have in the pictures. You would have to hole saw a good size
hole in a duct to be able to slip a gadget like that into it.
I figured out the math for the geodesic dome I was talking about. Now I
can calculate the coordinates of all the vertices and the strut lengths
of all three of the different strut lengths required to construct it.
It pretty cool.
Mark LaPierre
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