Sam said re his Sterba 56 x 56:
"(I could only get 3000 RPM with wide open throttle)
which saves a lot of wear on the engine internals . . ."
There ya' go gentlemen. The fastest prop of the three he tried was the 56
x 56, a prop most KR people would almost unanimously presume would be too
much for our VW engines. With my 52 x 56 Sterba I get 3100+ WOT so this
experience of Sam's matches perfectly my own observations. Lower RPM's
mean lower engine temperatures which means less stress and wear and
liklihood of failure over time. Plus, less fuel burned means more range.
Sam's surprise at how little difference the coarse prop made on his
takeoff and climb also agrees with my experience. The coarse pitch really
comes into its own when you get up into the thinner air at altitude. Down
low, there's very little penalty. I've been claiming this for years so
its good to have Sam's confirmation. The prop may only turn 2800 static
but those coarse blades are chewing more air with each revolution than a
flatter-pitched prop and up high, in the thinner air, they get a better
bite that translates into speed and efficiency.
Jeff or any engineer will surely have apoplexy over what I'm about to try
and explain, but anyway . . . . no one has ever confused me with Stephen
Hawking. There's two ways of extracting power from a given engine. RPM or
torque (twisting force). Of course, any power setting is going to use the
two together but one can choose to emphasize one or the other through
one's choice of propeller. The coarse prop increases manifold pressure,
perhaps better thought of as cylinder pressure or, better yet, BMEP.
Through a series of gears and levers and explosions, this power is
channeled into a twisting force that bites into the air and shoves it
rearward. A flat pitched prop does the same thing without any particular
emphasis on BMEP. Flat props shove air rearward by flailing wildly,
generating heat and noise that certain types of pilots consider "sporty".
Aviation is for everybody so I'm not being judgmental here . . . it's
simply that there is something very _wrong_ with these people. Smiley
face.
Engines. They run better when they have no obstructions (like butterfly
valves) obstructing the passage of air through the engine. Running WOT,
the ideal configuration for any engine, a flat-pitched prop lets the
engine spin without restriction, resulting in lots of heat from all the
friction being generated inside the engine, and noise from those same
sources in addition to the prop tips exceeding the speed of sound
creating the screech of tortured air. The sound of a T-6 taking off is
the textbook example. Paul Lipps, the brilliant engineer behind Phantom's
many wins at Reno, used to say it hurt his ears since that sound is the
very essence of wasted power. To run a VW at such speeds for even
moderate periods will soon cause something to melt, bend or break. How
much better it is to make the engine work hard by forcing it to turn
coarsely-pitched blades. Flat pitched props allow the engine to fight
itself internally, a battle which has no winner. With the throttle fully
open - no artificial resistance impeding the passage of air through the
engine - the air resistance against the blades provides the natural
limitation to the engine's desire to fly apart. Without this constraint,
this limiting factor of the engine's ability to turn . . . there is no
end to the troublesome possibilities. In Sam's case, 3000 RPM is as fast
as it will go. In mine, it's 3100+. Assuming we both have the same
engine, his prop turning at 3000 WOT is producing the same thrust as my
engine turning 3100-3200 WOT. The difference is, I'm generating more
internal friction which must be overcome with fuel consumption. In both
cases, the engines are working at their maximum WOT capacity and
producing the same amount of thrust - but one is more efficient than the
other. In both cases the same mass of air is being moved. In an engine
running WOT with a prop insufficient to limit the RPM to appropriate
levels, a large percentage of power being generated is being wasted due
to engine parts furiously fighting each other and by the noise of a prop
creating drag with the tips in persistent stall. The resultant heat is
not only damaging various components, it's using the engine's own power
to overcome itself. Instead of generating thrust, it's generating heat. .
. a total waste. Wasted as well is the energy lost as the stalled prop
tips generate noisy turbulance instead of thrust. The slower turning
engine, its prop limited by the natural resistance of the medium within
which it moves, producing thrust and wasting little in the form of heat
and noise, is in harmony with the natural world. Jonathan Livingston
Seagull.
The emphasis with the slower turning engine, in my case, is inclined
toward torque. Cylinder pressures will be higher with this engine, but it
will run cooler. It is easy to imagine how much more sense it makes to
use a coarsely pitched prop, one using torque instead of RPM to move air
rearward. This engine runs cooler, lasts longer, uses less fuel for the
same amount of work and is utilzing an all-around better method of
extracting power.
I've read that prop efficiency, for a given power source, can be better
gained by going to a greater diameter prop than by increasing pitch or
adding blades. If I were to go to a more efficient prop than my 52 x 56,
I would choose a 54 x 56 rather than going to a 52 x 58. I'm estimating a
54 x 56 would give me a WOT RPM of 3000+, which would be nice. Steve
thought the ideal RPM for the VW engine in aircraft application is 2900.
I suppose if I went to a 56 x 56 I would get full WOT power turning only
2900+ once leveled at cruise up where I normally fly. Theoretically, if
the throttle is fully open and the engine isn't being bogged down by
having too much prop - that is, if it isn't "lugging", I should be
getting the same thrust and thus the same speed as I do with my current
Sterba, but using less fuel. Currently my average cross-country speed at
altitude is, averaging numerous trips in my log, is 149 MPH when at my
typical cruising altitudes of 11K to 14K. At 13.5 I'm using 2.5 - 3 GPH
and pulling 45 to 50% power. If I could get the same speed while turning
2900+ instead of 3100+, I would gain another hour (or more) of cruise and
a minimum of 150 additional miles of range. Unless I decide to throw some
money at this theoretical efficiency boost, I'm not likely to try it . .
. but it would be an interesting experiment. More than just an experiment
though, doing this might give me the margin I need to make it from
Bellingham to Ketchikan non-stop, thus avoiding a stop in Canada. A trip
to Merrill Field in Anchorage has been on my bucket list for years and
with my current range that segment from Bellingham to Ketchikan is doable
only under ideal conditions and in that stretch of territory conditions
are very seldom ideal or predictable. I could really use another 150
miles of range for that trip.
The point I'm taking from Sam's prop reports is one I've been making all
along. The rabbit might be sporty, but it's the turtle that gets there.
Mike
KSEE
_______________________________________________
Search the KRnet Archives at https://www.mail-archive.com/krnet@list.krnet.org/.
Please see LIST RULES and KRnet info at http://www.krnet.org/info.html.
see http://list.krnet.org/mailman/listinfo/krnet_list.krnet.org to change
options.
To UNsubscribe from KRnet, send a message to krnet-le...@list.krnet.org
