Guys,
The main problem with playing with pitch on the propeller is one never wants to
overload or lug the engine on takeoff.
See Rotax SL 912-16R1 which states to set WOT for takeoff no lower than 5200 on
the 912 and 912S/914 with specified fuel octanes.
Reason is detonation. If you run high quality gas 5200 Static WOT pretty much
assures you that detonation is unlikely.
Since detonation is destructive and at 5000 RPM can't be detected by ear easily
and the engine has no knock sensor, it is probably a good idea not to lug the
engine down below that.
Aircraft manufacturers are interesting.
To tout the top speed of an aircraft a fixed pitch is set to 4000 for WOT
takeoff.
Needless to say one can check the Rotax Operational charts and that is at
the limit of the propellers capability and frankly beyond the edge of possible
detonation, but at 7500 feet MSL the cruise speed is decent so it is put in
the brochure.
For takeoff distance, the aircraft manufacturer sets 5650 and gets spectacularly
short takeoffs. Of course, immediately after liftoff the engine is at 5800
RPM and the throttle is ripped back to prevent an overspeed.
Manufacturers are interested in airplane sales. You will not get the exact
performance
of the book normally.
It is prudent to set 5200 for auto gas operations especially where octane
ratings
may be suspect.
In test flying to get the best from your aircraft the test pilot must fly boring
short flights at differing prop settings to get the best all around prop
setting.
For me that was 5000 RPM on AvGas WOT Static. I got 5500 at 90 Kts climb, 5000
at cruise at 134Kts. In a 900 pound APS or empty wt. trigear.
Clean composite aircraft will run out of prop with a fixed pitch almost all the
time. You will have to live with reducing the power to maintain 5000 RPM for
cruise or run around like the LSAs at higher RPMs trying to get somewhere.
Performance testing requires repetitive tests at specific RPM, MP, FF, Altitude
and weight. Make a spreadsheet and note the prop pitch for the test, fuel type,
WOT static indications, TO roll, Climb speed to get 5500 exactly, and log
the RPM, Throttle position, MP, FF, at each cruise altitude. Land and tweak the
prop and go fly again compensating for the altitude change as the temp changes
to maintain roughly the same density. SPREADSHEETS TELL THE STORY!
Don't go cheap on fuel.
I have tested an 80 HP KR at 4000 RPM WOT on AvGas and the takeoff was less than
spectacular. Over a 1000 foot takeoff roll. Float in the landing was so bad
shutting the engine off is preferred on a hot day. 10,000 foot speed was quite
impressive. If I recall that little KR2 went nearly 138 Knots on a fixed Warp
Drive tapered blade. (Cold day!) Never did that again. Most of that planes
life was done at 5000 RPM WOT Static. Speed dropped only about 10 knots but
you could get off the ground in 800 feet and land on a 3000 foot runway and
turn off at mid field without shutting off the engine.
I prefer to spend the bucks and install a constant speed prop on composite
aircraft.
You get the best takeoff distance, climb rate and best cruise your engine
can provide.
I still don't do Max Continuous at altitude with the 914. I stick with my
charts
and look for the best range, max endurance, or time to destination I can get.
My little Europa responds. On 912S equipped aircraft, fuel flow does not
decrease with altitude above roughly 3500 MSL. So, either a leaning kit is
needed
or at WOT your gas milage tends to sag, so setting the throttle slightly
back and or adding a leaning kit increases performance and range slightly.
On a 912S ram air is not normally a good idea. It tends to lean the engine
excessively
rather than add MP. The typical Bing is designed to suck air, that is
why the Bing 64 on the 914 has all those gimmicks to keep the mixture right.
Go get some data on your plane. But don't blow the engine doing it. It's
expensive
and time consuming.
Best Regards,
Bud Yerly
-----Original Message-----
From: owner-europa-list-server@matronics.com
<owner-europa-list-server@matronics.com>
On Behalf Of n7188u
Sent: Friday, April 23, 2021 11:38 AM
Subject: Europa-List: Re: Cruise speed data for Monowheel with 912ULS
First let me apologize to the group for trying to dig so deep into this subject.
I am by no means an expert but I have been around a bit. However, my goal is
to learn and as an Engineer I do tend to overthink stuff a little (good thing
when you design life critical medical devices :) ).
Griffo, this subject is so complex, and open to so much debate that yes, what
you
mention is valid, but a few items beg to be clarified as to ensure we are
comparing
apples to apples.
First, RPM alone is not enough to determine the power output of an engine
(although
you mention speed which could be used as the additional parameter but I prefer
not to due to variability). You need a way to determine that the pitch in
your prop is set correctly to start with (and hence the engine is developing
the desired power). A common way to "communicate" this to aircraft builders is
through the "static RPM" parameter. But, although a good starting point for
first flight, the final outcome is so dependent on the prop design that the
parameter
alone is not enough to properly set a ground adjustable prop.
And yes, the "art" of adjusting a fixed pitch prop will always depend on the
preference
of the user so is there really a "right" answer?, certainly not. Keep
in mind though that the Europa, as well as my LongEZ, ire aircrafts designed
for speed so it is my goal to maximize speed and still retain acceptable TO
performance.
Also, the engine operates most of the time in cruise regime, so to
achieve descent efficiency it is desirable to operate at power/RPM settings that
avoids "chocking" the engine with low throttle settings.
The consensus I found in the Rotax-Owners forums that made most sense to me is
to adjust the prop pitch so that you get 5800 RPM WOT in the air at level flight
(of course this would be altitude and temp dependent). Then check RPM on takeoff
to make sure it will still provide acceptable takeoff and climb performance.
If you set your prop with that specific baseline then yes the MAP info is
unnecessary for comparison purposes but only if everyone does it that way
(unlikely).
BTW, I have spoken to CTLS owners in my field and they agree that 4900 to 5000
RPM on initial climb is customary. As soon as you go to cruise climb speeds that
number goes up significantly. If you don't do that the cruise performance will
be dismal. They still get impressive TO performance and keep in mind that
you are operating at that lower RPM high torque regime for a very short period
of time. My LongEZ gives me only 2200 RPM on initial rollout (totally
unacceptable
per Rutan) but goes up to 2500 by the time I am rotating. On cruise I have
to live with 2700 with my throttle lever at around 1/4 open (100 RPM less than
redline) which is not great. So is the nature of the beast. In the LongEZ
I play with altitude to achieve optimal RPM. Yes, altitude is my VP control :)
But that works on a prop that is slightly overpitched.
Right now , on my Europa, I am getting around 5300 RPM/80 knts @ WOT on climb
but
WOT at level flight still exceeds 5800 RPM. At cruise, I checked today, I am
at 5200 RPM/22" MAP at 3000' but only truing 115 ktas. I think based on this
I still need more pitch. BTW, Bud Yerly told me my airplane will be slow since
I still don't have my final shiny coat of paint. I believe him.
Of course I can keep tweaking the blade angle until I reach a good compromise
(and
ultimately that will be the process) but with just a little feedback from
the group I can then compare the performance of my airplane to other folks
flying
the Europa out there.
Best Regards,
Chris
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