Pete, James and everyone else on our Matronics list,
The pdf is attached showing some charts from the Rotax manual.
Short Answer:
Personally, we are getting worked up over nothing here for most of the Europa
owners
and it may bring up concern that the well maintained engine/prop combo are
causing harmonic issues. We know it is not an issue for most of us. For those
who wish to rebuild their engine, modify existing engines, produce or use
auto converted engines, these are informational and thought-provoking
conversations
on gearboxes and engines. Lets face it, Eggenfeller is still modifying
his gearboxes on all his engines (Honda and others). But for an established
aircraft with aviation engine/props that dealers recommend, these conversations
are not productive except with your feet up and having a beer.
I agree, that geared engines do have harmonic issues if operated outside the
prescribed
and tested manufacturers limitations. Where are those limits? In the
operators manual. Fly within the prop curve limits and the Rotax 9 series with
any reputable prop will perform properly. If an engine manufacturer has no
performance limitations, stay away from that engine as testing is incomplete.
There are no issues with flying the Airmaster, Whirlwind, MT, Hoffman, Woodcomp
(properly maintained), on the Rotax 9 series engines. Fixed pitch props made
by reputable companies (the non flexible kind) have no issues either (fixed
pitch
Warp Drive, Whirlwind, Sensenich, Woodcomp have no issues, but Kiev and some
other thin flexible props are potentially a longevity problem.) Unlike the
long stroke, over bored Lycoming 0-360/390 these engines have bore, stroke,
and compression issues which do in fact cause unfavorable harmonics so some
combinations
of particular props and engine dash numbers to have restricted operation
envelopes. The Rotax does not. Rotax does have recommended RPM/MP settings
for various power settings and charts giving adequate information on operating
envelopes.
Long answer for those bored or snowbound:
As for Rotax operational limits:
Most Europa Owners fly with the Airmaster Constant Speed propeller on the 914
and
frankly from 4000 to 5800 RPM there are no issues from 20-40 inches of manifold
pressure at a constant airspeed. In other words, there are no
engine/gearbox/prop
harmonic problems which will occur but you can lug the engine down and
cause issues. Go back in the archives and look at our conversations on engine
vibration. If you have vibration, follow those techniques to repair your
prop balance, suspension or carb issues. I have found that the properly carb
balanced Rotax engine with a properly static balanced prop and then a proper
dynamic
balance is all we need to have a smooth running Rotax. Many get by with
a simple static balance but a dynamic balance is always a good idea.
In flight using a portable dynamic balancer (such as a Dynavibe) has been used
in flight and found to be a better balance as the Europa bounces around quite
a bit on a ground run making the dynamic balance results rather chaotic. Many
times, I have had to limit the RPM in the ground dynamic balance to 4000 RPM
to make it flyable.
Back to power and RPM settings.
>From your Rotax Operators Manual:
For the 914:
For the 912S:
My Notes:
Who flies at only those power settings. Most of us, but not always.
You can fly a Rotax 9 series at any power setting and RPM up to 5500 (remember
the 5 minute limit above 5500) at virtually any throttle/MP setting but listen
to the engine for clues on how you are operating.
Downwind airspeed is 4000 RPM and 20-24 inches depending on aircraft weight
trigear
or mono.
Trigear is higher than a clean mono. That is about 25-30 Horsepower. Note in
stabilized flight the prop is loaded so the gearbox does not clatter because the
prop is pulling. You can loiter forever.
In flight where are the gearbox rattle areas?
Any throttle settings (power) that allow the prop to unload to where the prop
begins
to windmill.
Also, any power setting or throttle above that prop line RPM in the operations
charts as you are lugging the engine, in other words the prop is demanding more
power due to excessive pitch for the manifold pressure (throttle setting) you
have set.
First: Pull your headphones off or turn off the ANR and listen to the engine
and
prop. Pete and Area 51 and many others know what I mean. Frankly everyone
should listen to their airplane, it will talk to you. If you descend rapidly
with no regard to the sound of the engine you are doing it wrong. At 140-160
KTS with about throttle, even with a constant speed prop, the prop is being
turned by the airspeed windmilling the prop (RPM increases as speed increases
and decreases as speed is reduced is your visual clue) and once you begin to
slow
down, you will hit an area where the forward speed puts the prop at zero thrust,
where the prop cant decide whether it is windmilling or pulling. A vibration
or clatter is heard or felt. The noise of the prop changes also. Get
out of that range by pulling off more throttle or adding power and pulling the
nose up to slow down. This action of pulling off the power more to force the
prop out of the windmilling speed, or reducing the speed and keeping the power
up enough to keep the prop pulling is common to most aircraft, even direct drive
engine such as the Lycoming.
It isnt that hard, but it does require listening and understanding that there is
slop in the gears (which you should note on every preflight) hence it is
intuitively
obvious to the most casual of observers that there will be areas where
if I am flying at a particular airspeed and note that the airspeed is driving
the engine RPM more than the throttle, the gearbox will rattle once the two
power/airspeeds become equal, therefore one should think ahead. Once you slow
down, you need to pull the power or that 50% throttle you have in there will
go through a speed where the unloaded prop will almost go to a loaded prop (i.e.
the engine will be driving the prop rather than the wind or equal to it).
LEARN FROM IT.
When I and many other salesmen fly with customers, we are in tune to the
demonstrator
aircraft/engine/and prop, and we move the throttle as soon as we hear the
prop start to windmill. Yes, you can hear it. That way the potential customer
never feels this. Were not lying or sprinkling fair dust on the issue, we
just understand and avoid the clack. Once the buyer closes the deal, we
demonstrate
that area.
Second: Regarding Heavy Props.
There are no REPUTABLE prop manufacturers that recommend props that DO NOT meet
the rotational inertia requirements of the engine manufacturer. There are of
course some buyers who believe that the longer the prop the faster and insist
on too large of a prop for the engine. WRONG! See my paper on selecting the
right prop for your Rotax powered aircraft. The inertia limit for the Airmaster
with the wide chord Warp Drive blades is 67-67.5 inches. Lighter blades and
hubs can be longer in blade length but the Rotax wont turn it and actually
the plane will slow down. At 75 inches the Rotax powered aircraft wont exceed
95-100 Knots.
Third issue: What about over pitch or lugging the engine, can I hurt the
engine?
Car analogy, it is like starting out in 4th gear. Can you do it, yes, but
listen
to the engine lug and the clutch slip. (Your mechanic will love you for making
his car payment.) Or perhaps trying to pass in 6th gear with the pedal to
the floor without downshifting the gearbox. My favorite is pulling a trailer
where at full throttle in 4th gear it is causing the engine and car to vibrate,
knock, and rattle as it is trying to pull the load and accelerate.
If you fly routinely at RPMs (prop pitch settings) with manifold pressures that
are higher than the prop curve (recommended settings) you are potentially
lugging
the Rotax engine. The throttle plate is open trying to hold the RPM, the
engine is running richer trying to produce the power needed and your plugs are
fowling. Worse, is the engine may begin detonating (aka ping) due to the fixed
ignition, low fuel octane rating or similar issue.
This lugging can be heard in the aircraft also. The prop at a higher pitch than
optimum for the airspeed is stalling part of the prop. You can hear it
thrashing,
the engine begins to sound different. Rather than normal, like a sewing
machine, it is making a deeper hum and some minor vibration can be felt in the
calf muscles of the legs. Note that the carbureted engines which have fixed
timing, detonation is a possibility if operating with low octane fuel in these
conditions. As an example, I have to do this particular exercise in prop
testing
to check go around capability in high-speed aircraft (Europa, Fascination
and similar) where I simulate a prop stuck in full coarse pitch and the pilot
must execute a go around. The Rotax 9 Series will do this 4000 RPM full
throttle
drill using 100LL fuel, but it sounds like the description above. Does
the engine take it, YES, does it like it, NO.
Bottom line, the Europa has a wide speed range so we can fly in areas where the
prop may be driven rather than the engine driving the prop. This can cause the
gearbox to rattle. Those who have too much prop pitch for the power setting
(constant speed commanding lower RPM than the prop power curve in the manual)
the engine is potentially lugging and you are wasting gas and running
inefficiently
at a minimum.
As a pilot and student of aviation learn your Manifold/RPM settings until you
get
experience.
In the 912 or 912S equipped with a fixed pitch prop, set the prop pitch to
achieve
5200 static at full throttle. Go fly with reckless abandon without over
speeding
the engine in a dive. Note in a dive to pull the throttle or slow down
to keep the gearbox from rattling or over speeding.
I do not recommend setting a fixed pitch prop to below 5000 RPM at full throttle
static any longer. Those using lower octane auto fuel may find the engine
begins
to detonate in some conditions. (Note: for sales testing our cruise numbers
were done at 4000 static and full throttle to get the top speed and 10,000
foot cruise speed numbers up. Not a good rule with auto fuel, but we didnt
notice detonation using 100LL.) 5200 at full static power precludes these
detonation
issues, especially with MOGAS.
With a Constant Speed prop and a 912 or 912S Set Climb RPM (5500) nearly any
throttle
setting will do.
Cruise at 5000 and above sea level just about any throttle setting wont lug the
engine.
Personally, Takeoff setting and full throttle for takeoff, then once safely
airborne,
simply click or set 5500 for climb at full throttle, climb to altitude
and click or set 5000 at full throttle and you will be fine as Ive never seen
a manifold pressure on the 912 series go above 26 inches in a Europa at full
throttle
above sea level. It is as close to a FADEC as one can get. Just slap
the throttle up to full and click for the phase of flight.
The 914 power settings are not as simple but easy to remember.
These numbers are for efficiency and are not limitations:
Takeoff prop setting and full throttle (5700 RPM / 38-40MP) until safely
airborne.
Climb: Click or set 5500/34-35 (a 100% stop is really nice to have)
Cruise Normal click or cruise at 5000/31, for better milage 4800/28-29 is nice
and comfortable and gas milage is about 30 MPG.
Loiter with the prop at 4300-4500 / 22-28MP. You will be airborne forever. But
going slow.
Downwind to landing, set prop to Climb or Takeoff, the prop will be stopped at
the fine pitch limit, and normally you will be 4000 RPM / 20-22 or so at 1000
MSL.
No issues will occur at these settings and the engine runs superbly. Setting
Manifold
Pressures below the prop curve is not an issue in operating the 912 or
914 series.
Anytime you have a constant speed prop on a Rotax, set the fine pitch stop to
limit
full power max RPM to about 5650 RPM to prevent an overspeed due to rapid
throttle advancement in an emergency go around. Set the coarse limit stop to
about 4000 for the Europa static on the ground at full power. If you have a
914 please install a 100% stop as you will reduce the potential for overspeed
or lugging on a go around or touch and goes. Should you have a prop stuck at
the coarse stop (4000 or so) the climb out at 100% or 115% is quite comfortable
with no issues for short term operations. The engine operates fine up to the
100% stop limit in the 914 and even the 912S but you can tell it is lugging
---From the sound and vibration.
I prefer to have my clients keep their head in the books, know the recommended
pitch and power settings for flight and the RPM/MP limits of the 914/912S.
Running
cheap gas and excessive pitch can cause detonation, avoid it. Keeping the
plane light and simple increases speed and reduces wear and tear.
As far as changing the gearbox design, getting more power or speed by adding
components
and then doing reliability testing is fine, but my comment is changing
the gearbox/fuel system/ignition may fix one thing but it will affect many
others
(like weight size and complexity). Not worth my time. For me, if the engine
isnt broke, dont fix it. But performance increases is an interesting topic
over a beer or two.
Have a happy and productive new year to all.
Bud Yerly
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