inch step will give double the rotation as
long as the pitch is constant.
Measuring the cup is done the same
way with the smallest step. However the
cup pitch may not be the same as
measured by the Orlic gauge. This is
because we are looking at a small area of
rapidly changing pitch differently. Both
gauges measure the average pitch over
different areas. If we use the smallest
rotation for the Orlic gauge and the
smallest step for the Wood/Speedmaster
gauge, the cup reading would only be the
same if the Wood/Speedmaster gauge
reads five degrees and the Orlic gauge
reads .0625 over the cupped area. It
doesn’t really matter, except when
comparing readings that may be taken
using different gauges.
Another propeller parameter is the
blade rake as shown in the blade cross
section picture above. This can be
difficult to measure, especially if the
propeller doesn’t have a straight rake
that goes through the propeller axis.
Place a small straight edge along the
blade to find a position where it touches
from hub to tip. The angle this makes
with the axis is the rake. Most stock
model propellers have the same rake
from leading edge to trailing edge and
the rake line goes through the shaft axis.
That situation is what the blade cross
section picture above shows. A way to
generate this kind of rake will be
discussed later.
The final propeller parameter we will
discuss is the blade area. Part 1 showed
the various ways this could be measured.
It showed a cut using circular arcs to
shape the projected blade outline. The
area of these arcs was easy to calculate
(Continued on Page 6)
PROPWASH
April 2014
5
area, you should get the same .05
reading in most areas of the blade from
hub to the outside edge. As cast, I get
between .049 and .052 most places on
the blade with my pitch gage. If you pick
a ten degree rotation, the indicator
reading will be twice as big or .10 inch
in a constant pitch area. Props with
progressive pitch will usually have
higher readings (increasing pitch) as you
go from the leading edge to the trailing
edge. Many propeller modifiers add
progressive pitch to stock props.
Cup is measured in the same way.
Usually you will need the smallest
rotation stop to stay within the cupped
section. Even so, you are actually
measuring some average pitch in an area
of rapidly increasing pitch. Our example
has a fairly constant indicator reading
of .085 (2.16 mm) where the five degree
rotation is in the cupped area. That
makes the average pitch in the cupped
section .085 x 72 = 6.12 (155.4 mm).
The Wood/Speedmaster style gauge
is a little harder to understand, even
though it measures the same thing as the
Orlic gage. With the same ABC 2018
prop as above I will go through the
calculations. These gauges come with a
pitch chart, so you don’t need to do any
math. Here we use a fixed step set by the
black stepped wheel and measure the
rotation. This time we calculate the
fraction of a full 360 degree rotation the
step represents. For the lowest .0625
inch (1.59 mm) step the gauge should
read 6.25 degrees on our 3.6 inch pitch
prop. That is 1/57.6 (360 divided by 6.25
is 57.6) of a full circle so the pitch would
be 57.6 times .0625 (the step height) or
3.6 inches as above. Again, this rotation
should be constant over all but the
cupped area in the stock prop. A .125
There are many ways to measure
pitch. Today modelers usually use one
of two pitch gauges. The Orlic style
gauge measures the distance the blade
advances with a set rotation. There are
variations on this gauge, but the
principles are the same. The Wood/
Speedmaster style gauge measures how
many degrees a blade needs to rotate for
a fixed advance. The older Hughey
gauge uses the same principle but with
less precision.
As you can see in the picture above,
the Orlic gauge has a dial indicator and
a series of stops that define the amount
of rotation for the measurement. To find
the average pitch in the area you are
measuring, divide the indicator reading
by the fraction of the circle. Let’s use an
ABC 2018 prop as an example. The
nominal pitch is 1.8 times 2.0 inches or
3.6 inches (91.44 mm). If you pick a
five degree stop and measure some-
where in the middle of the blade, the
dial indicator should read .050 inches
(1.27 mm). Five degrees is 1/72 of a full
circle so the pitch will be .05 times 72 or
3.6. Because this prop has a constant
pitch everywhere except in the cupped
Cup Gauge
Orlic Pitch Gauge
Wood/Speedmaster Pitch Gauge
A 3-D view of a full propeller