The flood gates are open again….
In response to Tracy’s question
about trying the B controller, It was my experience that the B controller
always worked better than the A. By programming the B controller
directly, the staging glitch could be essentially eliminated. After
programming the A controller as best as I could and copying those parameters to
B, the glitch would appear in the B controller but not nearly as pronounced as when
the same parameters were used with the A controller. I could never get
the glitch to persist with the B controller. I think this makes sense
considering the information provided by Tracy and Ed. Enriching the
mixture upon staging may have minimized the problem that was happening with the
A controller, but mixture may still have been too rich during staging when used
with the stable B controller.
I have attached an image of the data
logged during a prolonged staging glitch. Also included is an image of the
staging region of the mixture map used by my modified system where no staging
glitch occurs. The map is unique to my system, of course, since many
variables are involved in developing a good mixture map. My system may be
simpler than some in that all four injectors are the same part number and well
matched. From previous investigations, I know the limits of the linear
range of the injector fuel delivery vs pulse width and I have increased the MAP
where staging occurs so the injectors are operating within this linear range.
With my setup, it has been possible to compensate for operating the injectors at
shorter pulse widths than the linear range to some extent by programming the
mixture table, but this seems to be operating on the ragged edge of injector
reliability and the effectiveness of this compensation seems variable from day
to day. In my opinion, there have been no problems associated with having
the injector staging point at MAP values closer to where long term operation of
the engine occurs. I’ve collected data where the controller was
changing rapidly between unstaged and staged operation and the only way of
knowing this was to examine the data after the fact. The engine ran
smoothly the whole time.
Based on the data collected and shown in
the attached image, I’m not even sure that the glitch has anything to do
with staging or even the mixture delivered to the engine. My fuel flow
indicator integrates the primary injector pulse time and doubles that when the secondaries
are being used. With 4 well matched injectors all operating in the linear
region, the fuel flow indication should be reliable. There is no fuel
flow anomaly seen in the data. The only deviations from expected behavior
are the lack of rpm increase with MAP and the O2 sensors going lean (which are
an indication of what is coming out of the engine- not what’s going in).
Is the whole fuel delivery system capable of following an unstable MAP
indication so quickly that individual injector pulses are so lean or rich from
one chamber to the next that misfiring occurs but the average fuel delivery
rate is still correct? The bottom line is that the main flaw remaining
in my system is my curiosity. Why should I care what the problem was as
long as it’s gone and stays away? I’m working on that.
In response to Tracy’s comments on cooling
drag and airspeeds:
During the last month, I changed part of
the fuel system and eliminated the fuel transfer system with its associated
switches, timer and electric fuel pump. That was replaced by a duplex
fuel valve that I made (I couldn’t handle the price of Andair’s
duplex valve). Now I can just operate the valve and take fuel from either
tank and return it to the same tank with no electrical requirement. The
change in operational procedure is minimal: change the valve position
periodically instead of pushing a button, but decreased parts count and failure
modes are an improvement, I hope. Ed’s recent experience reinforces
my inclination toward two independent fuel sources. At least it’s
no longer possible to transfer fuel out the right tank’s vent, not that
I’ve ever done that, of course.
I exercised the fuel valve in the shop
(the screen, pumps and filters all easily come out of the plane by taking out 4
bolts and disconnecting the fuel lines and electrical wires) and verified its
operation even under conditions that should never happen in the plane. The
system passed all the tests I could think of. After reinstalling the fuel
module, I tied the plane down and made a ground run for 40 minutes at low cruise
power settings (7gal/hr fuel flow), changing the valve position every 5
minutes. Again everything worked fine.
The ground run was also a test of
alternator out flight endurance. With the alternators off, the battery
voltage was 10 volts after 40 minutes and the engine was still running
fine. This was with only the engine essential electrical systems powered
up. I should have made that test a long time ago instead of worrying every
time I flew, not knowing how long I could maintain altitude in the event of
alternator failure. The recent discovery of Steve Fossett’s final
destination, and realizing that on our return trip from CA we flew within a few
miles of that place if not directly over it, finally prompted me do this
endurance test. I’m quite sure that the time, effort, and expense
of the search for Steve Fossett would not have been expended looking for Steve
Boese.
What does this have to do cooling drag and
with airspeed? I’ve really never had problems from
insufficient cooling. If I can operate my plane tied down on the ground
for 40 minutes with an OAT of close to 70 deg F while burning 7 gal/hr of fuel
and not overheat, it seems quite possible that the cooling system still may not
be optimized. I’ve always had a thermostat installed in the water
pump housing which prevents the coolant temperatures from going too low.
In flight at higher power settings and
moderate temperatures, the thermostat is fully open, however, so I am not sure
the airflow through the cowling is excessive. The inlets to the coolant
radiators are unmodified from the original cowling configuration as supplied by
Van’s. My instrumentation shows that I am recovering over 90% of pitot
tube pressure at the coolant heat exchanger faces. The inlet to the oil
cooler is the same area as one of the original Van’s inlets and the new
duct to the oil cooler is long enough and shaped such that it seems likely that
internal pressure recovery is occurring in it. The coolant radiators are
so close to the air inlets that the duct shapes are definitely not desirable
and internal pressure recovery in them is unlikely. Since I have good
pressure recovery, this must be happening in front of the inlet opening.
Is this what is meant by external diffusion? Is a bubble of high pressure
air sitting at the front of the cowling while in flight? If so, then the
sensation of coming up against a wall while accelerating in flight is more
accurate than I thought. The wall is just transparent.
The change in the cowling shape and
reduction of the outlet area resulted in maybe over 5 knots increase in IAS but
not more than 10. That increase might be substantial at higher airspeeds,
but where I am stuck- probably not. Also, the fact that I can remove the huge
“bomb” and only gain 3 knots at most seems to indicate a high
degree of drag elsewhere and not just in the region of the cooling air outlet.
Other than having an oil cooler inlet slightly larger than the original Van’s
snorkel, the increased outlet area, and the ‘bomb”, the external
configuration of my plane is standard RV-6A and I have been told that my
construction is as good as many others. The concept of reaching a
critical point in drag reduction is a new one to me. In that context, my
efforts and lack of positive results start to make some sense.
Tracy, your analysis is more than likely
correct (as usual) and I am thankful for your input.
Steve
-----Original Message-----
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Tracy Crook
Sent: Thursday, October 23, 2008
10:00 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: N613BX
update
Here are some more comments about the MAP sensors for
those who are comfortable checking their own electronics hardware.
Steve's advice about output filters on MAP
sensors is good. I had noticed some instability on some (but not
all) Motorola pressure sensors so I have added a small capacitor (.1
uF) to their outputs on later versions of the EC2 and on the
EC3. I also add these on all updates if the unit does not already have
them. There is also another factor that affects this.
The first two versions of the EC2 board were layed out by an outside
source. The ground path for the map sensor was so long and
convoluted that it aggravated the tendency for instability on the
sensor, especially on controller A. This was also the cause of
mixture going crazy on controller A when transmitting on the radio in some
installations. Adding a short ground jumper from A controller
MAP sensor ground to the ground plane on the board fixed this problem. This
jumper is also added whenever I do updates. I was wondering if
Steve had ever tried switching to B and seeing if it had the same staging
glitch. If the problem was on A only, it may have been due
to the ground problem. Let me know if you tried
that Steve. I'd recommend adding the jumper in any case.
This same type of ground layout problem
caused the radio receiver noise (a hiss heard in the aircraft radio) on
the single part EM2. All affected EM2 customers were
notified to have them updated and I think all of them have been at this
point. This was the reason I quite having my PCB board layouts done by
outside sources. Just what I needed, another job :>)
Very surprised by your airspeed numbers
Steve. They are significantly lower than I would
expect. Your plane looks pretty clean except for the cooling
outlet. Purely speculation but I'm guessing that cooling drag is still
slowing you down. Both my -4 and -8 have only 50 sq in of cooling outlet
which means even after you reduced it by half you still have about
60% more outlet area than I do. Cooling problems can sometimes be
helped by enlarging the outlet but this is always a high drag
solution. I can't remember what kind of inlet ducting you have but
that is always the area where cooling improvements and drag reduction have been
found on my plane. BTW, did you notice much change in top or cruise
speed before vs after the outlet change? Until you get to the
critical point in reduction there is usually very little change.