REFLECTOR: Cruise Mystery
Terry Miles
terrence_miles at hotmail.com
Sun Feb 15 16:46:19 CST 2009
Chuck,
The NOAA expanded website has a nice set of educational articles on weather.
AOPA might have something too. I looked around, but I couldn't find the
particular one that sticks in my head now. It sounds to me like you were
just riding a pressure wave up and down. It is just like a wave on the
ocean. In fact fluid dynamics and aerodynamics have much in common until
you get to high speeds and compressibility. I could have been that you were
losing airspeed because you were indeed climbing relative to ground level,
but constant regarding "altitude" as pilots use it in everyday operations-in
other words you altimeter needle never changed. Were you over the Great
Lakes you could watch your GPS altitude change and your altimeter remain
unchanged.over the mountains of course that method is no help.
The TT likely is not using 11,000 feet as if it were an altimeter dial
reading or a distance above the terrain. It is attempting to fly a constant
pressure. Pressure lapse rates, just like temperature lapse rates are not
constant. The local surface pressure (QNH) reading you get from ATC does
not mean that you are truly at 11,000 feet. It only means that all the
other airplanes in cruise flight around you are flying with that same
pressure reference. In fact the weather man uses milibar charts for the
wind and temp-not altitudes. I hope that adds some to the ideas others
have suggested.
I am sure you know this, but turning off an ignition system also turns off a
set of plugs so the established chemistry of combustion as well as the flame
pattern within each cyl is going to go nuts and roughness is not necessarily
cause for alarm. I drop an ig system in the local flying now and then and
only on good days just to give myself a good butt and ear feeling/experience
for what that sounds like to provide myself some sense of reference. In the
weeks ahead I will try to find a better explanation than this.
Terry
_____
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of nmflyer1 at aol.com
Sent: Sunday, February 15, 2009 1:56 PM
To: reflector at tvbf.org
Subject: Re: REFLECTOR: Cruise Mystery
Well, that makes sense. I don't run LOP, mostly because I don't have the
equipment to do that. Scott & I both are fairly "used" to mountain waves and
such and consequently used to odd speed events.
One thing that would be interesting to try: If you find such an event
again, make a 180 and see if anything changes (other than the expected speed
flux). It might get you more data to use in solving the puzzle.
Good luck.
Kurt
-----Original Message-----
From: Chuck Jensen <cjensen at dts9000.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Sent: Sun, 15 Feb 2009 11:16 am
Subject: Re: REFLECTOR: Cruise Mystery
Kurt,
I monitored the OAT and it was about 41F the entire trip. If cooler air was
encountered and the mixture leaned out, it wouldn't account for the increase
in EGT, because I was already running LOP, so it would cause a drop in EGT
from further leaning. And normally, an increase in EGT will cause a drop in
CHT as less energy is being expended in the cylinder and more it going out
the exhaust where some of it burns, thus the elevated EGTs. Still, more
good ideas to consider.
Chuck Jensen
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org
<mailto:reflector-bounces at tvbf.org?> ]On Behalf Of nmflyer1 at aol.com
Sent: Sunday, February 15, 2009 11:17 AM
To: reflector at tvbf.org
Subject: Re: REFLECTOR: Cruise Mystery
Chuck
I fly through the rockies very often, altitudes cruising in my old Cherokee
235 average 12.5-14.5 altitudes. I have noticed, on occasion, peculiar
changes in speeds while transitioning areas with lots of wind. I do not have
the instrumentation that you have, but I have seen small fluctuations in EGT
temps on the 2 cylinders that have probes.
I would be interested to see if there were outside temp changes as you were
experiencing this event. With your much more precise instruments & ignition
system, a temp change (along with the wind) could explain an uncomanded
change in mixture. Add those two together and you might be able to explain
most of it.
Just Thursday, I was flying back from Gunnison (GUC) to Mid Valley Airpark
(E98) and had ground speed fluctuations from 158 down to 87 kts. Smooth as
glass. I didn't specifically notice any EGT changes, but I'll look closer
next time.
Let me know what you figure out.
Kurt
-----Original Message-----
From: Chuck Jensen <cjensen at dts9000.com>
To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
Sent: Sun, 15 Feb 2009 8:50 am
Subject: Re: REFLECTOR: Cruise Mystery
John,
It was certainly a strong weather system and most of the route was over the
mountains, but the ride was smooth, though turbulent below. I've had a few
instances of air mass vertical movements, but this seemed larger than that
and the differential between GPS GS and IAS stayed consistent. I would have
to think about that a little, but aircraft performance would seem to vary
with the lifting or dropping of the air mass and would seem likely to affect
IAS and GS at different rates.
And of course, I'm not sure how either would account for the broad swings in
EGT and less broad, but pronounced movement in CHT. Nonetheless, its an
idea and better than anything I have to date.
Chuck Jensen
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org
<mailto:reflector-bounces at tvbf.org?> ]On Behalf Of aminetech at bluefrog.com
Sent: Sunday, February 15, 2009 9:12 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Cruise Mystery
Chuck,
Maybe the high wind had something to do with it. I often notice mini
versions of the speed changes you observed. And they are more pronounced
the higher the wind speed. I have the IVO with manual pitch adjustment, so
I continuously check airspeed and rpm in the process of keeping the rpm
constant. My theory is that there are pockets of updraft and downdraft in
the transition area between the smooth air above and the gusty air below.
Were you experiencing occasional turbulence? With the A/P operating you
tend not to "feel" the up and downdrafts, except for the changes in speed.
John
--- cjensen at dts9000.com wrote:
From: "Chuck Jensen" <cjensen at dts9000.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Subject: REFLECTOR: Cruise Mystery
Date: Sun, 15 Feb 2009 08:48:05 -0500
Okay, here's one that hasn't given up its answer easily. Last week, I make
a quick trip over to 2W6 (St. Mary's Regions, just south of DCA) from
Knoxville, TN TYS in the Velo XLRG. The front of the wx system that tore up
Oklahoma on Wednesday had just passed the night before and the low pressure
was parked over NY (wx that brought down the Continental flight). At cruise
at 11,000' my ground speed was 254kts while my IAS was 161 kts. I was
cruising on the TruTrack III GSGV A/P, tracking the GPSS route with Alt-Hold
engaged. MAP was 20.1, RPM 2400 and fuel flow 11.7 gph.
Over the TRI area, I noticed the IAS start to slowly drop. I checked all
instruments and everything was nominal and there was no pitch or tone change
from the engine. The speed continued to bleed off until IAS was 113 kts.
The A/P had indicated it needed more nose-up trim to keep it in range. The
slow speed was confirmed by a pitch up about 5-6 degrees. I thought it
might be the Grand Rapids dual EFIS indication that was screwed up but I
confirmed the airspeed on the backup mechanical ASI and the pitch was
confirmed on the independent backup MCI LifeSaver AI.
Observable measurements. The oil temperature had increased from 198F to
225F, the CHT had increased 50F-70F and the EGT were all much higher...where
normally they run in the mid to upper 1400F, they had increased to low to
mid 1500F.
I had been running WOT because of altitude as well as running LOP. I
enriched the mixture and could hear just a minor increase in engine output,
even though the RPM and MAP didn't change. The increase was the same as
when you've been running LOP and have descended a few thousand feet and then
enrich the mixture.
Though I don't know that the slow but steady resumption speed coincided with
the enrichment, the speed did increase after a few minutes and climbed to
184 kts IAS and stayed there a few minutes before settling back to the 160
range. (Note, this could not have been speed gain from altitude drop because
I was steady 11K. One would have expected the speed to increase and slowly
achieve the original 160 kts, but it overshoot and actually went 10-15 kts
higher than normal cruise). 5-10 minutes later, this cycle repeated
itself, though the drop was to about 144 kts IAS, and then it would pick up
again to an IAS above my normal cruise and then settle back again---all
without change in MAP, RPM or fuel flow. Each time, there was a
corresponding change in CHT and EGT, though not as pronounced as the first
episode.
While I wanted to check the L/R ignition, I was over rough terrain and with
ground wind in most areas G40, I didn't want to have to try to make an
emergency landing if turning either ignition systems on/off (Rose
electronic on L, mag on R). On the return trip that afternoon, the winds at
altitude were >90 kts. Fortunately, they were quartering from the WNW and I
was headed 250. Still, I had to turn 20-27 degrees into the wind to hold my
heading---a new record. I couldn't go lower than 10K as the mountain
turbulence would not have been bearable...at least for the weak stomached,
myself included. There were several speed excursions on trip home, but none
as extreme as the first one on the way over.
Over the TriCities airport on the return trip, most of it at 115 kts GS, I
cycled the two ignition systems. The mag was quite rough but the plane
would have kept flying adequately. When only the Rose ignition was running,
it was quite smooth, but not perfectly so. In short, both ignition systems
were operating, though whether they were optimal I don't know. So, I'm
trying to sort out how these unaccountable operating changes could occur
with no apparent changes in power setting or plane configuration.
Impressions and first thoughts. There was no configuration change in the
plane during all of these excursions, though there was the obvious pitch up
as the speed slowed and a pitch down attitude as the new higher than normal
cruise speed was experienced, but no changes to the power settings. It
seems there was a distinct change in power output from the IO-540 300, even
though none was apparent from any audible change in the engine. My
suspicions center around the ignition system, primarily because of the
changes in EGT (the high oil temp may have been reduced cooling as speed
bled off and pitch up reduced cooling) that tracked the power changes. With
the apparent drop in power output, the EGT would steadily climb. Since
mixture is not being changes, I assume this would indicate a retardation of
the ignition spark...leaving more fuel to burn post-cylinder, resulting in
less power produced in the cylinder and more of the fuel! ! burned as it
left the cylinder, showing up as higher EGT temps---all while fuel flows
remained essentially the same.
Since I'm not an authority on ignition systems (nor much of anything else),
I'm not sure that I understand the mechanism that would cause the timing to
shift back and forth in this manner. I would love to have the increased
cruise speed that I would see for a few minutes at a time, but I'm not down
with the power drop on the other side of the excursions. Of course, perhaps
it's not the timing at all, but I'm at a lost to see what else could cycle
power and EGT in this manner.
So, I'm stumped as to where to look and what to address.
One fortuitous happening....I just happened to be recording the GRT EFIS
data on a USB stick, so I have the event captured on the stick. I later
captured 2 or 3 other excursions as well, so anyone that is able to look at
the GRT demo recordings, I'd be happy to email them.
Chuck Jensen
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