REFLECTOR: Cruise Mystery

[Al Gietzen]

Chuck;

 

Those were some very strong wind conditions you were flying in.  Back when I
lived on the north coast of CA I occasionally fly the Skyhawk inland just
for the fun of flying along the windward side of the so-called 'Trinity
Alps'.  Even with upper level wind of about 30-40 kts I could pull the power
back to near idle and maintain airspeed and altitude while flying 'downhill'
in the rising air currents. Sail-planing in a 172.

 

Al

 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Chuck Jensen
Sent: Sunday, February 15, 2009 5:48 AM
To: Velocity Aircraft Owners and Builders list
Subject: REFLECTOR: Cruise Mystery

 

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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