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Propeller
 | 2010.11.19:
Early in the project I decided that it will have a constant speed
prop. Soon it will be time to decide on the specific make and
model. There are various trade-offs:
Metal vs. Composite
Metal props have been around for decades and are still the most
common. Propeller blades made of aluminum alloys are fairly
durable, and when they sustain minor damage (normal wear and tear,
nicks and scratches) they can generally be repaired by filing down the
damaged area. They are a known quantity, and any professional
prop shop is familiar with inspecting and servicing them. Their
primary disadvantages are weight (compared to the lighter composites)
and resonance issues. Resonance issues can becomes a problem
when engine vibrations excite resonant frequencies in the prop, which
over time can accelerate its wear and ultimate failure. The
natural resonant frequencies of metal props often fall right in the
range of the engine's vibrations, and therefore many metal props have
operating restrictions for certain RPM ranges, cautions against engine
configurations that haven't undergone vibration testing, etc.
Composite props practically eliminate the resonance problems, since
their natural resonant frequencies tend to be much higher than engine
vibrations and can therefore not be excited in any normal operating
regime. Composite propellers also tend to be significantly
lighter than their metal counterparts, which is generally a good thing
(as long as the airplane's CG isn't too far aft). The dark side
of composite props is that they are relatively new on the scene, vary
significantly from one make/model to the next in construction and
materials, and so are just less of a known quantity. They are
definitely the future, but the question is whether or not they are the
present. Some early composite props have also been know to chip
easily under normal use, and even from flying in rain. And while
it's harder to make blanket statements due to the broad variety, it is
typically the case that composite prop blades can often not be
repaired when similar damage on a metal blade could. Having said
that, composite props are getting better all the time, and these
weaknesses have been adequately addressed on newer models.
And one more consideration is cost. At this time composite props
are still more expensive than comparable metal props (some
representative numbers would be $7K for a metal prop, $9K for a
composite prop). Composite props are likely to come down in
price and eventually replace metal props altogether, but that will
likely take a few more years.
2-blade vs. 3 or more
For this class of airplane, a 2-blade prop is the lightest, most
efficient, and least expensive option. Some folks opt for a
3-blade prop because it can be smoother and more quiet, can have a
smaller diameter, and looks cool on the ramp. But the more
practical choice is 2-blade, and that's the way I'm going.
Make & Model
At this point I've narrowed it down to two candidates:
1. Hartzell "blended airfoil" metal prop (p/n
C2YR-1BFP/F7497)
2. Whirl Wind 200RV composite prop
The major tradeoffs between the two are pretty much the tradeoffs
between metal and composite as described above. But a few others
beyond that:
Hartzell is a long-established veteran of the certified prop market,
whereas Whirl Wind is a relative newcomer to the aviation market (they
also make props for airboats) and offers props for experimental
aircraft only. This is the classic trade-off. Hartzell has
a lot of service history behind it, but Whirl Wind is out ahead in
innovation.
It's worth mentioning that Hartzell is now also offering composite
props. But these are even more expensive than Whirl Wind, and
are less of a known quantity. While Hartzell has been
making props for far longer than Whirl Wind, Whirl Wind has been
making composite props for far longer than Hartzell. For those
reasons I've eliminated the Hartzell composite prop from the
competition.
The Hartzell prop is sold by Van's through an OEM agreement, whereas
the Whirl Wind prop must be purchased through one of their dealers.
The Whirl Wind also comes with the spinner assembly included, whereas
the Hartzell does not. Not a big deal, but noteworthy.
Although no-one to my knowledge has done a direct comparison of the
two props on an otherwise identical aircraft, comparisons have been
made between the two props on similar aircraft (same RV model,
similarly but not identically equipped). The data I've seen (on
the VAF forums) suggests that the Hartzell prop achieves a slightly
faster top speed than the Whirl Wind, a difference of about 2
ktas. But this apparent difference is likely to be under the
margin of error of this data due to other variables, so I'm assuming
the props are essentially equal in performance.
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