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gmcjetpilot(at)yahoo.com
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 Posted: Thu Feb 09, 2006 8:52 am Post subject: WW 200RV Prop RPM Restriction? Hartzell WW MT |
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First I'll say I the Hartzell Blended Airfoil is the best value and the
fastest constant speed prop you can put on. I think WW and MT
make good products and will be lighter than the Hartzell. MT claims
no restriction but they are VERY vague about it. I have seen nothing
in writing or that they have tested it on modified engines. However
with a wood core blade I suspect that they are OK at least with a
stock engine since wood is such a good dampener.
Alex you are right the stress level for Al fatigue damage can be lower
than steel, but steel can be horrible, depending on type of steel and
heat treat.
Yes fatigue life is not really infinite for an aluminum prop, but I meant
it's so long that for practical service it represents such a long
calendar period it is considered infinite. Large jets are good for 20
years of economical service with 24/7 severe service with a factor of
2, or 40 years. Boeing's stand is with proper maintenance and
inspection an airframe can go forever. However the FAA and aging
fleet initiative makes inspections so expensive and intrusive (x-ray,
eddy current, ultra sound and mass dis assembly) that it becomes too
expensive to continue to fly it commercial. However a used B707 is
built like a tank, and if only operated with low use private purposes
makes an awesome large business Jet (John Travolta). These B707
are over 40 year old and built for stout. The freight companies are
thrashing 40 year old DCV-8's nightly. They just keep replacing
parts. On the other hand an old fire bomber (C130A) lost a wing and
recently in the news the Chalks Airlines Grumman lost a wing,
which is a dramatic and tragic reminder of what metal fatigue is.
In the past people modified metal props off of slow factory planes for
their fast homebuilt. They would cut them down, re-pitch them. The
vibratory stresses where so great that they broke in a few 100 hours
(I think at the 2/3 span).
Alex, yes I do think the interaction of crank and prop vibration is the
issue, but it's a prop restriction not a crank restriction. If Lycoming
wanted a life limited crank they could save some lbs, but they have
such an overbuilt piece of steel that stresses are below that critical
level, where damage is nil. As it's now you can rebuild a Lycoming
almost infinitely. They mandatory replacement parts are the highly
loaded rod nuts and studs, for example. However you can weld up a
7000 hr cracked cylinder or use a 10,000 crank and go fly.
They have S-N charts, stress-cycle charts. Higher the stress the less
the cycles. Makes sense. Than there is stress concentration (sharp
corners) and how the stress cycles, such as is it from zero to positive
or tension-compression and back, which does far grater damage. If
you keep a part in compression you can improve the fatigue life. I
made a living doing fatigue life analysis on large aluminum transport
jet airframes. I can tell you a sharp nick on a prop is critical. The prop
mid to tip area is critical, where the cross section is thinner. The root
has details that are critical, where the fillet radiuses are in the mount
flange area in the hub. To counter act stress they add material (lower
stress), use good details (smooth surfaces, big radius lower stress
concentration) and treat the material to shoot penning in some cases
to add residual compression to the material.
My point is props are designed for a very very long fatigue life time.
Although not really infinite, it's so long, for all practical purposes the
prop will be dimensionally and physically worn down or is scrap for
other reasons, like corrosion, before the fatigue life is reached. When
you bring your Hartzell in and the P-shop says you are .001 under
the min thickness at the second station from the tip, you are cooked.
I have one of those "scraped" props on my RV. I called and the
reason they specify a min dimension is not fatigue or stress but just a
somewhat arbitrary dimension as a cutoff based more on loss of
performance than fatigue. Some big props that start life at 6 inch
chord can be trimmed down to say 5 inches they just say that is
enough for performance reasons. Some times they do find cracks;
micro cracks that if left could cause a failure. That is why there is
limited time between overhauls recommend. Crack growth rate is
another part of fatigue analysis. You assume a crack and calculate if
it will grow to be critical before the next inspection cycle. The time
limited aspect is for corrosion, which is the killer of most old props
left out in the weather.
Hartzell has recognized after a period of time some parts of
old designs where not unlimited life and need to be restricted, thus
the dreaded AD. In the past they made hubs and clamps with steel.
We are talking 1940's, 50's design that have not been made in a 1/2
century. They have limited life, repeated inspections and some
stress relieving process that must be done every overhaul (which
period is now shorter), such a shot penning. Shot penning puts a
residual compressive stress in the outer surface which can help
fatigue life.
The compact hub like the HC-C2YK and the later BA airfoil are
derivatives of a design that has millions of hours and been improved
since the 60's. Even a HC-C2YK has been improved and a new
Hartzell with the same part number is nothing like the one make in
1969. They continually improve them so it's a mature design and well
tested. That is why you find the NEW BA airfoil has NO RPM
restrictions!
"SUPERIOR XP-360 RESTRICTIONS:
Hartzell Propeller Model HC-C2YR-1BFP/F7496 (BA prop) is
vibrationally approved when mounted on Superior Air Parts model O-
360-B1A2 and IO-360-B1A2 engines rated at 180HP at 2700 RPM
with magneto ignition and installed in Van's Model RV-6A and similar
single engine tractor aircraft. There are no operating restrictions."
http://www.hartzellprop.com/kitplane/index_kitplane.htm
With HC pistons, EI and FADEC there are restrictions but they single
power point restriction (one MP/RPM) for the new Hartzell BA prop,
but they are not a range of RPM's as with the classic HC-
C2YK/F7666.
George
From: "Alex Peterson" <alexpeterson(at)earthlink.net>
George,
Good post regarding the prop restrictions. A couple additional
thoughts: Regarding fatigue life, what you said is true for alloys of
steel, but
not for aluminum. Even at low stresses, the fatigue life is not
practically infinite.
Alex Peterson RV6-A N66AP 712 hours
Maple Grove, MN
From: Ron Lee <ronlee(at)pcisys.net>
Supposedly MT makes a prop without an RPM restriction.
Ron Lee
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Michael Wynn
Joined: 10 Jan 2006
Posts: 148
Location: San Ramon, CA
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| Posted: Thu Feb 09, 2006 6:41 pm Post subject: WW 200RV Prop RPM Restriction? Hartzell WW MT |
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Thanks, George
That was both helpful and educational.
Regards,
Michael Wynn
RV 8 Wings
San Ramon
Do Not Archive
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Michael Wynn
RV 8
San Ramon, CA |
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