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There are a number of important considerations when choosing a prop
for a particular installation. First, type and diameter. these are
chosen based on vibration considerations. The resonance of the
engine's rotating assembly, the RPM range of operation, and the
resonance of the prop are all critical. This is why you sometimes
see an RPM range where operation is restricted.
The McCauley 1A/B 90 props are allowed on both the C90 ant the O200,
which have virtually identical rotating assemblies, but different
operating RPM ranges. The C90 is limited to 2475 RPM whereas the
O200 is limited to 2750 RPM. Consequently, the length of the prop
allowed for each engine differs. The O200 must be between 71" and
75" whereas the C90 is allowed to range from 66" to 78". This
information can be found in the propeller type certificate.
That said, the airframe type certificate usually further restricts
the choice of props. In the case of the O200 in a 'Coupe, the STC
sets the limits.
Rather than specifying a pitch, the TCDS generally specifies an
allowable static RPM range. Any prop listed on the TCDS that meets
the diameter restrictions and static RPM range is acceptable. The
upper range of the static RPM is climb territory and the lower end cruise.
The Skyport O200 STC specifies a particular prop and pitch. As I
understand it, that is because the originator of the STC was
experimenting with different pitches to determine flight
characteristics and static RPM range when he passed away. The 69-48
happens to be the one he was using at the time and the STC was issued
with that restriction. Urban legend? I don't know. Some day when I
have a lot of free time I may try to get that changed. There is also
another STC, which we do not own, to allow use of a Sensenich prop
with a coarser pitch on our O200 installation.
So, a McCauley 1A/B 90 71-52 could be used on an O200 if it's allowed
on the TCDS and if it meets the static requirements of the TCDS, but
if you hit a blade of grass with the tips of the prop it no longer
will meet the diameter restriction.
John Cooper
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865
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