I may be a minority of one, but it appears to me we are giving
paperwork undue influence over an owner's
right to secure and enjoy maximum safe airframe/engine performance.
Airframe drag is what it is on a
given Alon, Forney or Ercoupe. Engine health is what it is so long as
the engine is "airworthy" and the
owner does not contemplate overhaul in the near future.
If performance is a priority to the Ercoupe or Forney owner, he likely
only occasionally flies around with an
open canopy. If the pilot gives priority to preserving number and
quality of choices in event of power loss,
he likely does not fly cross country at low altitudes. If performance
isn't a priority, most any prop will do.
The idea is curious, at best, that some FAA drone or DER would purport
to"de-rate" an 0-200 from 100 HP
to 90 HP output because the Alon engine mount had not been "proven"
adequate for 100 HP operations.
There should be an option for the owner to operate in the Experimental
category for such evaluation as the
FAA deems necessary to such proof (and I seem to recall an STC "out
there" that should be referable as
approved engineering data for 100 HP on an Ercoupe mount).
In the above context I consulted the Operator's Manual for Continental
A & C Series & 0-200, Form X30012,
1980. The "Introduction" states that "Recommendations, cautions and
warnings regarding operation of this
engine are NOT (emphasis added) intended to impose undue restrictions
on operation of aircraft, but are
inserted to enable the pilot to obtain maximum performance from the
engine commensurate with safety and
efficiency." (refer back to my first sentence) I would suggest that
the FAA has identical responsibility.
The 0-200 Sea Level Performance curve is on page 30. At any other
altitude, performance will be less; and
under "non-standard" conditions other than winter such should also be
presumed. Note that no
propeller or airframe information is given because engine performance
is unrelated-see below.
Fred Weick "wrote the book" on aircraft propeller design. I have a
copy. At the 2750 RPM that the 0-200
achieves rated horsepower, and for any airplane speed between 100 and
140 MPH. propellers under
80" diameter tip speed has no effect on efficiency. Also, maximum
revolutions available at 10,000' MSL with
an unsupercharged engine are only about 3% lower than maximum
revolutions available at sea level.
He states that while the thrust, torque, and power of a propeller vary
directly with the density of the air,
propeller efficiency is independent of density, remaining the same for
a given RPM regardless of altitude.
Thus, the "cruise" propeller reaching greatest efficiency at maximum
sea level speed will also reach greatest
efficiency at maximum horizontal speed at 10,000' MSL, and the "climb"
propeller providing best rate of climb
will do so all the way up from sea level to maximum possible ceiling
for an engine/airframe combination.
Contemporary practice ignores the fact that the "ideal" propeller for
maximum speed performance is of smaller
diameter than the "ideal" propeller for climb or cruise efficiency. We
have discussed static RPM at times on
this list, usually with reference to ATC 718 and 787 limits.
ATC 718 purports to require a static RPM range for the "C-75 or C-85"
74FC (74" original) Sensenich wood
prop at "maximum permissible throttle" (which I would presume to be
identical to "wide open when no one is
looking): not over 2100, not under 1850 (RPM). Ercoupe Service
Memorandum No. 37 shows excerpts from
ERCO engineering report testing of an Ercoupe at 1260 lb. with the C-75.
The pitch of the selected 74" production wood Sensenich allowed the
engine to turn up 2600 RPM in level
flight to achieve a maximum speed of 121 MPH. The pitch of the
selected 73" optional metal McCauley
allowed the engine to turn up 2500 RPM in level flight to achieve a
maximum speed of 122 MPH. Note in
each case that ERCO was not in the least concerned with the original
arbitrary and excessively conservative
"maximum continuous horsepower" rating Continental assigned the C-75
before it utilized the exact same
same mechanical parts in the later C-85 and Continental confirmed to
ERCO it was aware and unconcerned.
In every example in the Continental Operator's Manual described above,
the (desired?) Prop Load rises to
intersect the Full Throttle horsepower curve at maximum "rated
horsepower". It should be obvious that any
propeller selected must allow "rated horsepower" to be achieved in that
part of the aircraft operating
envelope the owner gives priority.
Since the C-90 in the Forney and Alon has a cam that develops rated
power at 2475 (100 RPM less than the
C-85) I would expect that the most efficient prop choice would be a
McCauley 74" (or more) in diameter with
a maximum pitch allowing 2475 RPM in level flight at 1400 (F-1) or 1450
(F-1A) lbs. gross ("Cruise" prop). A
good "Climb" prop should be three or four pitches less. The best
possible fixed-pitch compromise should be
somewhere in between.
For an 0-200 in an Alon, I would start with the 69.5 to 71" McCauley
pitched to achieve 2750 RPM in level
flight at 1450 lbs. and, say, a true airspeed of 127 MPH ("Cruise"
prop). If I were buying the prop new, I'd
want the full 71". Again, a good "Climb" prop should be three or four
pitches less; and the best possible fixed-
pitch compromise somewhere in between. This is the long, hard way to
agree with Ed's suggestion that a
McCauley 1A105/SCM7153 might be as good a starting point as a 1-A-90CF
or 1-B-90CM.
Static RPM, in each case should be the range exhibited by the tested
range of diameters and pitch thus
derived. There are no longer any published "critical ranges" for
McCauley props listed in ATC 718 and 787,
therefore unless a selection might emerge from the above that had
characteristics of concern to McCauley,
it should be performance and not paperwork that determines propeller
selection.
Just my opinion (and I've never owned an Alon nor experimented with
Sensenich metal props).
William R. Bayne
.____|-(o)-|____.
(Copyright 2009)
--
On Jan 21, 2009, at 11:56, Ed Burkhead wrote:
Wayne wrote:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
The Alon with the O-200 in it might have been caught in the 1990
contaminated fuel crisis wherein there were several O-200's installed
by the fuel companies when there fuel inventories were contaminated by
jet fuel. The reason the O-200's were installed was that there were
not enough C-90 cases for Continental to rebuild at the time. The oil
company had to replace about 300 engines nationwide. Sandy Beliwitz
got an O-200 and she hated it, her plane was slower and used more fuel
with the short propeller.
Before I went swapping propellers I would like to know if I was making
power with my engine with an accurate Tachometer, and static power
reading with the propeller I already had legal paperwork for.
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Hum, that’s an interesting thought about the engine replacement.
The propeller standard on the Cessna C-150 that was specified for the
O-200 engine installation on the Ercoupes makes a climb propeller on
the Ercoupes. And, that can be good because a lot of the Ercoupe
owners who install an O-200 are people who really need maximum climb
performance due to high altitude airports or terrain.
The Alon airframe is somewhat more efficient than the Ercoupe because,
I think, the improved canopy shape. Alons seem to cruise about 5-10
mph faster than Forney Aircoupes which have the same engine. The C-90
equipped Alons climb nicely, too, compared to C-85 equipped Ercoupes.
(I haven’t compared their climb to C-90 equpped Forney Aircoupes.)
Putting a prop on an O-200 equipped Alon that would be a climb prop on
an O-200 equipped Ercoupe just doesn’t seem right. It’d be too much
of a climb prop and cruise would stink.
Within the legalities, I think it would be a good experiment to mount
the 1A105/SCM7153 prop from a C-90 Alon and record the climb and
cruise performance, static rpm and in cruise redline rpm you get.
Don Baker has been working with an AI and DER, I think. I hope he can
get this resolved properly without too much expense.
Ed,_._,___
