Forming
Props With Your Computer |
by Bill Parmley
August 20, 2005
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some help from your computer you can form
prop blades with very precise diameter, pitch,
and pitch/diameter ratios. Bill developed
a spreadsheet application that does the calculations
for you, and then wrote this article to explain
the concepts involved. He's also provided
a step-by-step tutorial that takes you from
concept to finished prop! |
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I got
interested in forming propellers when I decided
I wanted to duplicate the shape and performance
of the Peck-Polymers
four-inch plastic prop. It seemed to be the right
size for the small, 8" wingspan models that
I was working with, but was too heavy, even with
extensive scraping. So I decided to form my own
prop from balsa, and began the quest to learn.
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I read
and reread the chapters on propellers in Don Ross's
two books: Rubber Powered Model Airplanes
and Flying Models, and
made close examinations of a number of plastic
propellers.Several days were spent thinking about
the subject, and in the course of that time I
put together several Excel spreadsheets
to calculate and plot diameter, pitch, pitch-to-diameter
ratio, and blade angle in different ways. As a
result I came to a much better understanding of
propeller shape and function, and I also developed
a very useful
spreadsheet. The spreadsheet can be used to
calculate the shape of a propeller forming
block, given prop diameter and pitch-to-diameter
(P/D) ratio as input variables.
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| To use
this tool, you must first decide the diameter and
P/D ratio you want your prop to have. Don Ross’s
books discuss how to select these values based on
model size and type. The diameter will most likely
be dictated by the size of the model. Suggested
typical pitch-to-diameter ratios are 1.2, 1.3, and
1.4 for endurance, sport, and scale models, respectively. |
The table below shows what my spreadsheet looks
like. The red boxes contain the input variables,
which are used to calculate pitch, blade angle,
and the shape of the forming block.
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When you enter numbers for prop diameter
and P/D ratio, the spreadsheet calculates
the pitch and blade angles automatically.
Once the prop blades are formed and ready
to mount to the hub, you'll set them to
the proper pitch by measuring the distance
from the center of the propeller to a specific
angle. As shown on the spreadsheet, convenient
angles are 45° and 30° degrees,
which correspond to basic drafting triangles.
The angle you choose to measure to - whether
45° or 30° - should be the one that
falls nearest the center of the prop blade.
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I've found the tip angle to be the easiest to measure,
since the triangle isn’t hidden under the
blade and you don’t have to measure two things
(radius and angle) at the same time. Note that if
the tip angle is around 22.5° then a measuring
device for this angle is easily fabricated by folding
a small, square piece of paper twice diagonally.
More about how these values are calculated later.
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The lower part of the spreadsheet calculates the
shape of the line that you will need to draw on
your block of wood in order to carve the forming
block. I have to admit that for me the greatest
surprise in this business of propeller forming was
the idea that you can draw a single straight line
on a rectangular block of wood, do a bit of whittling,
and produce a helical shape. The shape of that straight
line (that is, the distance of the line from the
edge of the block) is simply 1/Tangent(Blade Angle)
at any given radius from the center of the propeller.
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To aid in visualizing what the forming block should
look like, I used Excel to create a graph showing
blade radius versus block width. This graph is
shown below as Figure 1. The
shaded area represents the part of the block that
would be cut away, as viewed from the top of the
block. The graph enables us to quickly see how
wide the forming block will need to be for a given
thickness of wood and pitch of the propeller.
By playing with the numbers a bit we can also
see that the thicker the forming block is the
wider it must be, and the more wood we have to
cut away and the more work we have to do.
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Fig. 1
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I also included on the graph a plot of blade
angle versus propeller radius (the red line).
I found this line to be interesting, as it shows
something that is difficult (for me, at least)
to see when looking at an actual propeller: the
fact that the angle, or twist, of the blade changes
at a decreasing rate as we move from the hub to
the tip.
If you want to build your own spreadsheet, here
is a guide to setting up the formulas:
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For the first part of the spreadsheet –
Pitch = Propeller Diameter * P/D Ratio
45° Radius = Pitch / (2 * Pi)
30° Radius = (45° Radius) * 1.73,
where 1.73=Tangent 45° / Tangent 30°
Tip Angle = Arctangent (Pitch / (Pi * Propeller
Diameter)).
For the forming block part of the spreadsheet
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Blade Angle = Arctangent (Pitch / (2 * Pi
* Blade Radius))
Forming Block Line = Block Thickness * (1
/ (Tangent (Blade Angle))
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