I started working towards my commercial ticket. Naturally, every time I fly, I build hours
towards the 250 minimum. Like private
and instrument pursuits, the initial declaration of intent followed by a concrete
plan helps focus on the goal. I plan on completing all cross-country
requirements by the end of this year.
The critical elements of the commercial certification
include complete understanding of aircraft systems and proficiency in aircraft
control, maximizing performance. As part
of meeting first goal, I have am fortunate to be flying the Diamond DA 40. It may not have a retractable landing gear,
but there is plenty of opportunity to understand power and efficiency with
respect to controllable pitch prop and engine.
The second goal is met through a series of maneuvers, including
chandelles and the 180 degree power-off landing.
The rest of this entry focuses on performance. I put all this together from my first flight
applicable to the twenty hours of dual flight commercial requirements. The flight consisted of a two hour minimum
VFR cross country flight to an airport greater than 100 NM straight line
distance from the departing airport. I
chose to fly down to Lynchburg, then to Greenbriar (WV) for the local eatery’s
famous pulled-pork sandwich and then back to Leesburg. All
my long flights as of late have been IFR.
The VFR flight was a joy and I picked a great day to do it.
Although the flight discussions included review of flight
planning (NOTAMs, weather, runways, fuel management, weight and balance), much
of the time was spent discussing performance.
I was reminded how critical weight and fuel planning go hand in
hand. All airline pilots KNOW landing
weight, with consideration for fuel burn.
Landing weight allows the pilots to optimally choose the slowest
approach and landing speed. Recall that
higher weight raises the stall speed and requires a faster approach. A faster approach means a longer landing
roll. Comparing this landing roll to
runway distance requirements is critical to prevent over-runs and failure to
comply with LAHSOs. Talking with Mooney pilots, I began realize
how critical weight calculations are to their every-day operation. Mooneys have a bad reputation with newly
minted private pilots. Why? Basically, they are not as forgiving of
improper weight/speed management. Of the
two Mooney pilots I know, they both have handy little weight/approach speed
ratio charts. They watch and record fuel
flow regularly through the flight and ‘double’ check indicated fuel with
expected fuel (roughly fuel flow per time * time elapsed).
I am leaving out consideration of wind speed and temperature
since the calculation determines indicated air speed. Ground speed and true air speed factor in the
landing roll distance calculations, as provided in any POH. Higher temperatures result in higher true air
speed and ground speed (not indicated air speed). Wind does the reverse. As a private pilot, these calculations are
typically done in pre-flight. A private
pilot is not usually tested on calculation updates while flying. However, from what I gather from hanger talk,
air transport pilots use flight computers to due the math as part of the
landing checklist.
Engine Performance
The new engine in the Diamond presented a good opportunity
to learn about the systems. The first
50 hours of a new 180 HP engine is critical break-in period. Most of the running time is at 75% power (135
HP) for longer durations. It is not the
time to do pattern work! One of the
interesting challenges has been diagnosing a 60 degree (F) spread in cylinder head temperatures
between the four cylinders. This maybe more common with normally aspirated
carbureted engines. Prior to the engine
replacement, the spread was closer to 30 degrees in the fuel injected Lycoming
IO-360. Cylinder one has been running
hot. Running 100 degrees rich of peak
(ROP) has resulted in higher fuel burn and considerably cooler cylinders two,
three, and four. Even leaning and
enriching slowly to allow the temperatures to stabilize after each adjustment
has yielded the same consistent results.
A big spread indicates that the
cylinders are not supplying the same amount of power. Such an in-balance may result in
roughness.
What could be causing cylinder one to run hot. A couple of ideas include
(1) Fuel intake port nozzle issue resulting in
less fuel being injected into the cylinder one thus making the hot cylinder
leaner than the rest.
(2) An
air flow irregularity used the injector to regulate fuel force (the fuel
pressure differential)
(3) Baffling
and cowl airflow inconsistencies resulting inconsistent cooling.
Since the engine has been running smoothly and playing with
nozzle inspection did not yield any identifiable issues, idea number three
seems likely. The POH is the guide to
achieving 75% power. A standard atmospheric temperature, the
Diamond runs 75% power at 500 feet with 24.1 inches of manifold pressure and
2400 prop RPM. Over 6000, the 75% power is not achievable according to the
POH. For example, at 8000 feet with full
throttle (which is where I keep it), the manifold pressure is under 24 inches
(closer to 22) and the fuel flow is roughly 9.8 GPH.
Empirical studies demonstrate operations at either LOP or
ROP with the IO-360 results in lower cylinder head temperatures. In fact, the graph of CHT with respect to
Fuel Flow looks like a mountain with the peak at peak EGT (and CHT). Next
time I fly up, I may try a LOP operation of cylinder one. What I expect to see is that one of the other
three cylinders SHOULD get hotter, as it would be closer to the peak EGT. If this is the case, then perhaps ideas 1 or
2 are applicable.
As a commercial student, knowledge of turbo-charged engines
seems appropriate, although not required.
I will not be working with a
turbo-charged engine at anytime in my commercial training. Turbo-charged engines require close
monitoring of engine temperature.
Lycoming TIO-360 engine turbine inlet temperature (TIT) should not
exceed 1650 degrees Fahrenheit.
Turbochargers use exhaust gases to propel a turbine to compress and
pressurize air flowing into the manifold. In some engines, automatic control of
exhaust gas into the charger allows the pilot to following similar guidelines
to RPM and mixture control as with non-turbo charged engines. The only change is diverted focus on TIT
instead of EGT. Other systems use a manual
waste gate control to control to diversion of exhaust gas into turbocharger inlet. A closed waste gate at low altitude can
push manifold pressure to unsafe levels.
Thus, monitoring manifold pressure is a critical component of these
‘manual’ systems. Automatic waste gate systems
are typically controlled by oil pressure.
Oil pressure opposes a spring to close the waste gate, increasing intake
pressure. As expected, an increase in
throttle increases oil pressure, thus closing the gate, resulting in MORE
power. A pressure relief valve protects
the system of over boosting. With
automatic oil driven systems, the engines must be sufficiently warmed up prior
to full power application in order to guarantee adequate pressure for full
power. I have heard of other types of
pressure regulated chargers applying some sort differential pressure system
with manifold and exhaust pressures.
The two pressures run hand in hand.
Lower manifold pressures indirectly affects the exhaust pressures,
causing turbo charger adjustments. The
end result is a perpetual fluctuation, which is inefficient, causes frequent temperature
changes and occupies pilots attention.
My next flight is a 2 hour night VFR flight. I enjoy flying at night. I went out for a quick flight the other night
to work on holds and approaches. In the
fall, the cool air is smooth. I started
to work on the chandelle maneuver. My
first observation is that the speed drops off very quickly. However, optimal
altitude gain is not achieved unless continuous increase of back pressure is
applied. I stopped short before
continuing back pressure. When I completed the turn, I still had 10 to 15 knots
of useable speed.
