The following observations may be of interest
in the light of recent discussions.
On the flight to the resent Rotorfest, I
thought the exhaust noise level had increased at one point. This was just
chalked up to fatigue and the longer than normal time in flight since this was
the longest XC attempted to date. My usual procedure for adjusting
throttle position is to advance it until no further increase in rpm results
even though there is normally about ¼ of the total travel still
available. My throttle body is a cut down Mazda one with two openings as
described in Tracy’s
guide. Since I use the stock oil metering pump, the throttle position adjusts
the metering pump stroke and advancing the throttle more than necessary results
in accumulation of oil in the sump. At one point when checking the
throttle position, the RPM actually dropped slightly when advancing the
throttle further. I thought to myself “this is strange”
and returned the throttle to its original setting since things were running
smoothly.
At the Rotorfest, I listened with interest
to Mark’s presentation on muffler lifetime or lack thereof, wondering why
my first attempt at a muffler had survived so long. I attributed this to
my normal operation at high density altitudes which results in reduced max
power levels. I also described the construction of my “bomb”
to several people who were interested in it. I now realize that I
didn’t know what was in it myself. During the preflight before
leaving the Rotorfest, I noticed a rattle in the muffler when checking its
security to the belly of the plane. After returning to Laramie, I investigated the rattle further
and the results are shown in the attached pictures. The fact that the
center baffle broke is not surprising since it was simply a push fit into the
shell and was vulnerable to flexing since it was flat. The repair uses a
conical shaped baffle with a solid ring around its outside circumference to
make it even stiffer. We’ll see how long that lasts.
The recent discussion on not developing
full power, especially Ed’s information on seeing almost 20 gph at max
power settings at seal level got me thinking about my performance. I have
seen up to 16 gal/hr at 4500 ft DA on a rare excursion to that low an
altitude. That didn’t seem unreasonable considering the
sophistication of my setup compared to Ed’s. Looking back at the
data log from the flight home from the Rotorfest, there were some interesting
observations concerning the departure from 40XS. That segment of some of
the data from the log is shown in the attached plots.
A couple of things in the data seem to me
to stand out. One is that I am recovering nearly full manifold pressure
in the plenum at wide open throttle compared to ambient as shown in the data of
the manifold pressure before start-up and during runup and take off. This
has always been the case. The second thing that stands out is that I was
only using between 10 and 11 gal/hr at wide open throttle with the mixture
adjusted for max power. I didn’t look at the fuel flow reading during
runup or take off and only noticed these low values when examining the data log
recently. I am confident that the fuel flow readings are quite accurate
since the fuel actually used for this trip matches the instrument readout very
well. While some of you may have experienced some apprehension at such a
pathetic power production level, it seemed normal to me since I usually operate
at density altitude from 7000-10000 ft. Another thing I noticed in the
data is the significant rpm drop when the prop unstalled just prior to
liftoff. The rpm usually drops a little at that point but not nearly this
much. I remember noticing this on departure, but was otherwise occupied
and didn’t give much further thought at the time. Not shown in the
plots is the coolant temperature which reached 225 degrees at the stock
location in the flywheel end iron at 8 minutes into the log. This is
about 20 degrees higher than normal even at the reduced fuel consumption level
during this time. Oil temperature showed a similar response.
After removing the muffler to investigate
the rattle, standing it on end would allow the broken segment to move to the
area of the outlet and block some of the exhaust exit area. Laying it
horizontally again would allow the broken segment to slide down the conical end
where it could not be seen in the exit. I suspect that the exhaust gas
carried the broken segment up the slope where it could block part of the exit
when operating at power levels above idle. Luckily, the blockage was not
enough to prevent generating enough power to sustain flight.
In any case, the data log is a record of
the effect of increased back pressure on the performance of my NA 13B.
Just as has been described in the recent discussions, the result was as
expected: decreased max fuel consumption and decreased max power
production. In addition, I saw increased coolant and oil
temperatures. Maybe the increased back pressure was responsible for the
decrease in rpm upon fully opening the throttle in stabilized flight, but
I’m not sure why such an effect would arise.
For what it is worth…
Steve Boese