Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Tue, 03 Feb 2004 16:24:38 -0500
Message-ID: <redirect-2973668@logan.com>
From: Epijk@aol.com
Subject: Re: [LML] Propeller Heads, Help!
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Scott:

I can't comment on the aerodynamic efficacy of your cut-down prop, but the 
speed of your aircraft suggests that it's not too bad.

HOWEVER, it gives me the willies to consider a solid-crank IO-320 driving a 
CS-prop which had 7 inches chopped off the blades. Consider the following:
a) The IO-320 crankshaft does not have the torsional-absorbing counterweights 
which the higher-powered 360's have, and in certitfied form, most likely has 
a yellow band on the tach defining an RPM range best avoided due to harmful 
interwactions between the engine excitation and one or more natural frequencies 
of the prop blades;

b) The original 84-inch Hartzell (from which someone lopped off 7" per blade) 
was, in all probability, designed for and surveyed on a Lyc 540 or 
Continental 520 / 550, and found to have no potentially harmful interactions between the 
6-cylinder engine excitation (3rd order) and any blade natural frequency;

c)  The lopping of your blades changed one or more of the blade natural 
frequencies (most likely causing them to increase), but the magnitude of the change 
is unknown without a survey. The resulting change could easily cause a 
serious interaction between a blade frequency and the second-order excitataion of 
your 4-cylinder engine, which could be stressing your blades to a level they 
cannot sustain;

d) Metal props on direct-drive piston engines pose a daunting set of 
vibration problems, the solutions to which are non-trivial, but which include blade 
profiles, mass distributions, root thicknesses, and a host of other factors, in 
order to avoid the harmful interactions described above.  Consider the fact 
that a prop which is safe on a particular certified IO-360 (8.7 compression, 
counterweighted engine) becomes UNSAFE after the STC'd installation of 10:1 
compression pistons (more on that and other prop facts at 
http://www.epi-eng.com/Prop_TOC.htm)

e) If you're determined to keep on flying this prop, you might consider 
having Hartzell do a vibration survey on your engine/prop/airframe combination. 
It's not cheap, but it sure beats the prospect of trying to land after the loss 
of a couple of inches off one blade tip (probably followed soon thereafter by 
the entire engine).

Jack Kane

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<HTML><FONT FACE=3Darial,helvetica><HTML><FONT  SIZE=3D2 PTSIZE=3D9>Scott:<B=
R>
<BR>
I can't comment on the aerodynamic efficacy of your cut-down prop, but the s=
peed of your aircraft suggests that it's not too bad.<BR>
<BR>
HOWEVER, it gives me the willies to consider a solid-crank IO-320 driving a=20=
CS-prop which had 7 inches chopped off the blades. Consider the following:<B=
R>
a) The IO-320 crankshaft does not have the torsional-absorbing counterweight=
s which the higher-powered 360's have, and in certitfied form, most likely h=
as a yellow band on the tach defining an RPM range best avoided due to harmf=
ul interwactions between the engine excitation and one or more natural frequ=
encies of the prop blades;<BR>
<BR>
b) The original 84-inch Hartzell (from which someone lopped off 7" per blade=
) was, in all probability, designed for and surveyed on a Lyc 540 or Contine=
ntal 520 / 550, and found to have no potentially harmful interactions betwee=
n the 6-cylinder engine excitation (3rd order) and any blade natural frequen=
cy;<BR>
<BR>
c)&nbsp; The lopping of your blades changed one or more of the blade natural=
 frequencies (most likely causing them to increase), but the magnitude of th=
e change is unknown without a survey. The resulting change could easily caus=
e a serious interaction between a blade frequency and the second-order excit=
ataion of your 4-cylinder engine, which could be stressing your blades to a=20=
level they cannot sustain;<BR>
<BR>
d) Metal props on direct-drive piston engines pose a daunting set of vibrati=
on problems, the solutions to which are non-trivial, but which include blade=
 profiles, mass distributions, root thicknesses, and a host of other factors=
, in order to avoid the harmful interactions described above.&nbsp; Consider=
 the fact that a prop which is safe on a particular certified IO-360 (8.7 co=
mpression, counterweighted engine) becomes UNSAFE after the STC'd installati=
on of 10:1 compression pistons (more on that and other prop facts at <BR>
<A HREF=3D"http://www.epi-eng.com/Prop_TOC.htm">http://www.epi-eng.com/Prop_=
TOC.htm</A>)<BR>
<BR>
e) If you're determined to keep on flying this prop, you might consider havi=
ng Hartzell do a vibration survey on your engine/prop/airframe combination.=20=
It's not cheap, but it sure beats the prospect of trying to land after the l=
oss of a couple of inches off one blade tip (probably followed soon thereaft=
er by the entire engine).<BR>
<BR>
Jack Kane<BR>
</FONT></HTML>
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