Hi Kevin,
So in trying to turn all this into something useful and understandable:
You say that "Static RPM for a fixed pitch prop depends on prop
pitch". To me that means static rpm at full throttle without
carburetor heat for a given engine with conditions corrected to
standard day temperature and humidity at sea level, wind calm, with a
given fuel on a one airplane with a specific fixed pitch prop will
always be the same.
Presumably the range of static rpm allowed according to the Type
Certificate Data Sheet (TCDS) or the Aircraft Type Certificate (Type
Certificate) is to accomodate the necessary range of allowable
propeller lengths and pitches. It would be nice to have a chart
showing the effect of temperature, humidity and pressure altitude on
rpm/horsepower so as to have some frame of reference for the difference
in static rpm these factors can cause.
Given the tendency of high pitch (climb) props to overspeed in cruise,
perhaps the "ideal" pitch for such props is the lower permissible
static rpm? Conversely, given the tendency of low pitch props to be
unable to develop sufficient rpm in cruise to achieve rated horsepower
that such props had best demonstrate highest permissible static rpm?
Since the "standard" props do not tend to overspeed in cruise, and
since higher rpm across the normal operating range directly correlates
with higher torque and higher horsepower it would seem that these, too,
would typically offer the best performance if they exhibited static rpm
at or near the upper permissible range.
I would think that with other than a "climb" pitch prop the "ideal"
fixed pitch prop would allow the the engine to spin up in cruise such
that maximum possible horsepower (not maximum rated horsepower) was
developed at a pressure altitude of 6,000' (or whatever your choice of
average pressure altitude for cruise flight would be) at the rpm you
would normally use for cruise. This may well description a "cruise"
prop that does not allow the engine to develop sufficient rpm for an
acceptable initial rate of climb, particularly for those aircraft based
at higher altitudes.
Let us presume, however, that the STC engine demonstrates adequate
climb performance with a prop pitched as described. If the
after-the-fact static rpm of such a fixed-pitch prop were outside of
the "allowable limits" for the Ercoupe (et al) it is, by definition,
illegal.
Let's look at what is legal. According to ATC-718, a 415-C or CD with
a 75hp Continental engine and the McCauley metal prop should fall
within a static rpm range of 1950-2100rpm. If the engine is upgraded
to the 85hp specification, the static rpm range becomes 2025-2225rpm.
Interestingly, per TCDS 787, the static rpm range for the 415-D, E and
G with the McCauley 1A90CF or 1B90CM is 2025-2225 (even if the D Model
has the 75hp it was built with). This conflicts with the
representation in the 415-D "Approved Flight Manual" which calls for a
single static rpm of 2060 for the McCauley prop without regard to
pitch, and would seem to suggest that if one did manage to install a
"standard" McCauley that demonstrated this static rpm, tweaking it to a
climb or cruise pitch would make it illegal because of the inevitable
change in static rpm.
The static rpm range McCauley 1A90CF or 1B90CM prop on a 90hp engined
airframe is 2250-2375, even though the C-90 develops rated hp at 100
rpm less than the C-85! This suggests that engines fitted with the
0-200 crank, rods and pistons (higher compression) will not develop
maximum horsepower in the air if the C-85 McCauley prop static rpm
range is not in the 2250-2375 range (which would, as a C-85, be
illegal).
Since such installation is acceptable by McCauley in terms of loading
and rotational stress considerations on the "real" Continental C-90,
there would seem to be no logical reason the FAA would not approve an
increase in the static rpm range for the C-85 engines with the
C-90/0-200 rotating parts installed via a 337 or STC which would
legalize use of the increased performance due to the higher octane of
today's fuel and the higher compression that makes additional power
available at altitude at less than rated power?
Regards,
William R. Bayne
.____|-(o)-|____.
(Copyright 2010)
--
On Jan 25, 2010, at 19:00, Kevin wrote:
It's not so much that it is moving more air but more inches through
the air. Even though the 48" is only moving the airframe 48" per
revolution it is able to do more revolutions per minute so the over
all movement is greater. It can do this because the prop is flater and
the engine can get a higher static or low speed RPM. But that then
hurts it in cruise because the engine can over rev so when you
throttle back for cruise you are moving less inches forward then the
50". This is the reasoning behind controllable props and why a
controllable prop can generate a static RPM at red line and a fixed
pitch can not. Static RPM for a fixed pitch prop depends on prop
pitch.
Kevin1
