<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=Content-Type content="text/html; charset=iso-8859-1">
<META content="MSHTML 6.00.2800.1458" name=GENERATOR>
<STYLE></STYLE>
</HEAD>
<BODY id=role_body style="FONT-SIZE: 10pt; COLOR: #000000; FONT-FAMILY: Arial"
bottomMargin=7 bgColor=#ffffff leftMargin=7 topMargin=7 rightMargin=7>
<DIV>
<P align=left>Some of you may have already seen this safety warning on the APC
web site, but I thought it was worth posting given the recent discussion on prop
failure. Many of the references are to racing applications, but I feel it's
still important information in our application.</P>
<P align=center>PROPELLER SAFETY CONCERNS</P>
<P align=left>All propellers are inherently dangerous. Model airplane propellers
are especially dangerous. Model airplane propellers used in high performance
racing are extremely dangerous. Model airplane engines designed and modified to
achieve maximum operating capabilities create unpredictable and potentially
severe loads, leading to various forms of potential propeller failure. Ignoring
reasonable safeguards may likely be catastrophic. This concern is the motivation
for the following discussion.</P>
<P align=left>Warnings included with propellers are intended to protect
consumers. They also protect manufactures against claims resulting from misuse
of the product. Most products with potential for causing injury contain
ample warnings about misuse. Some advertisements for products now contain
warnings, even before the product is sold! There is a strong proliferation of
warnings in most products having potential for creating injury or damage. This
inundation of warnings may cause consumers to become inured to product
warnings.</P>
<P align=left>The warnings about propeller use must be taken seriously,
especially for racing applications. It is very risky to assume that a racing
propeller blade will not fail, especially when used with state-of-the-art racing
engines. Yet, nevertheless, occasionally model aircraft operators are observed
standing in the plane of propeller rotation of high performance racing engines
running at full power. This is very frightening. The following information
reinforces the assertion that dangers of misuse are very real.</P>
<P align=left>Ideally, a product can be designed with credible knowledge of the
environment (loads acting on the product) and capabilities of the product to
withstand that environment (not fail). There is nothing ideal about
designing a model airplane propeller because some major components of propeller
loads are very uncertain. The principle load components acting on a propeller
are:</P>
<UL>
<LI style="COLOR: #ffffff"><FONT color=#000000>Centrifugal (from circular
motion causing radial load) </FONT>
<LI style="COLOR: #ffffff"><FONT color=#000000>Thrust/drag (from lift and drag
acting on blade sections) </FONT>
<LI style="COLOR: #ffffff"><FONT color=#000000>Torsional acceleration ( from
engine combustion and/or pre-ignition) </FONT>
<LI style="COLOR: #ffffff"><FONT color=#000000>Vibration (from resonant
frequencies or forced excitation) </FONT></LI></UL>
<P align=left>Another potential source of loading is aero elastic tip flutter.
This may be caused by self exciting aerodynamic loads at a resonant
frequency.</P>
<P align=left>These loads are discussed next in order.</P>
<P align=left>Centrifugal loads are very predictable, given rotational speed and
mass density distribution of a blade. Their contribution to total stress is
relatively small. </P>
<P align=left>Thrust/drag loads are somewhat uncertain due to complexities of
aerodynamic environments. The relative axial speed at the prop (at any radial
station) is aircraft speed plus the amount the air in front of the blade
is accelerated by the mechanics creating thrust. The latter may be approximated
using first order classical theory. Much empirical lift/drag data (from wind
tunnel tests) exists to quantify lift/drag loads, once relative velocity and
angle of attack distributions are established. </P>
<P align=left>Torsional acceleration loads are generally not known. Analytical
estimating technique used by Landing Products to quantify torsional acceleration
loads suggests that they can become dominant when pre-ignition or detonation
occurs. These analytical observations are supported by test experience with very
high performance engines running at elevated temperatures. The latter causes a
high torsional load (about the engine shaft) which creates high bending
stresses, adding to those from centrifugal force and lift/drag effects. These
torsional acceleration loads depend on unique conditions for specific
engines. Engines "hopped up" for racing appear to be especially prone to
create high torsional loads when lean mixtures lead to high cylinder
temperatures and pre-ignition/detonation.</P>
<P align=left>Vibration causes additional loads from cyclic motions. These
motions occur when resonant frequencies are excited or when cyclic load
variations exist on the blade. The magnitude of these variations depends on how
close the driving frequency is to the resonant frequency and the level of
damping in the propeller material. Engine combustion frequency is an obvious
excitation. Obstructions in front of or behind the blade can cause cyclic
variations in thrust load. Once a blade starts to flutter, those motions alter
the flow, causing variations in loading. High performance engines have caused
propeller tips to break, presumably due to fatigue failure from vibration.
</P>
<P align=left>Aero-elastic flutter is speculated to be a dominant mechanism
causing rapid fatigue failure near a tip when insufficient or destabilizing tip
stiffness exists. The interaction between variable loading and deflection
induces a high frequency vibration with unpredictable magnitude. </P>
<P align=left>Efficient propeller design practice utilizes
analytical/computational models to predict propeller performance and stresses.
However, the uncertainty in impressed and inertial loading from complex
phenomena requires testing to assure safe performance. Unfortunately, it is not
possible to assure testing that convincingly replicates worst case conditions.
The large combinations of engines, fuels, temperature, humidity, propeller
selection, aircraft performance and pilot practices creates an endless variety
of conditions. If the origins of severe loads were well understood, quantified,
and measurable, structured testing might be feasible that focuses on worst
case stack up of adverse conditions. However, since the origins of severe loads
are really not well understood, it is essential to provide sufficient
margins in material properties and design to assure safe performance. Propellers
that are used in fairly routine and widespread applications (sport and pattern)
lend themselves reasonably well to test procedures that provide reasonable
confidence. In time, a sufficient data base develops that can be used to
empirically quantify performance and "anchor" or "tune" assumptions used in
analytical models.</P>
<P align=left>However, propellers that are used for increasingly extreme
performance applications do not benefit from the large empirical data base sport
and pattern propellers enjoy. Assumptions and design practices developed for
current generations of engines may not be valid for emerging engines whose
technologies continue to push engine performance to greater extremes.
Consequently, propellers that are used in applications where performance is
already relatively high (and expanding) must be used with great caution.</P>
<P align=left>An adverse cascading effect occurs when propellers are permitted
to absorb moisture in high humidity environments. Composite strength, stiffness
and fatigue endurance all reduce with increased moisture content.
Reduction in stiffness typically causes resonant frequencies to move toward the
driving frequency (increasing torsional loads) and, the reduction in strength
reduces fatigue endurance. Composite propellers should be kept dry.</P>
<P align=left>In summary, please abide by the safety practices recommended by
propeller manufactures. This is especially important for high performance
propellers. Assume that propellers can fail at any time, especially during full
power adjustments on the ground. Never stand in or expose others to the plane of
the propeller arc. </P></DIV>
<BLOCKQUOTE dir=ltr
style="PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial"><FONT id=role_document
color=#000000> </DIV></BLOCKQUOTE></FONT></BODY></HTML>