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
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=501478#501478
|