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propellors, was Re: [PSUBS-MAILIST] concept sketch
Joe - what CAD software are you working with? There may be public
domain solid models available - the one shown in my drawing is such a
download, but even if not, props are not that difficult to draw. I
didn't use my own in the drawing because I'm still tweaking it, but
here's the basic process (for SolidWorks - other packages may behave
differently) for a non-controllable constant pitch prop:
Set up equations or global variables to define propellor pitch, outer
diameter, hub base diameter, hub taper angle, number of blades, rake
angle, base thickness, tip thickness and area ratio (this one is a fudge
factor - it is the projected area ratio at the complete sector stage -
i.e. before profiling the blade).
1) Create a helical path defined by pitch and revolution. Pitch will
equal your pitch variable. Revolutions will equal the area ratio
divided by the number of blades. If your CAD package uses angle instead
of revolutions, just convert this to degrees accordingly..
2) Parallel to the axis of this helix, create a sketch of the cross
section of your blade. SolidWorks does not seem to like helical sweeps
which intersect the axis, so start at some minimum distance from it -
this will be consumed later by the hub anyway. Draw a line from this
point, out to a point at the outer radius of the blade, using your
diameter variable. This line is not necessarily horizontal, but angled
down according to your rake angle variable. This line represents the
working (high pressure) surface of your prop. Next, draw lines upwards
parallel to the axis from the endpoints of this first line, with lengths
according to the base thickness and tip thickness variables you
defined. Finally, connect these ends with another line. Now you have
your profile.
3) Sweep the profile you just created along the helical path. The
result will be a single blade which is a circular sector with no
profiling, as might be used within a Kort nozzle. Well, almost - since
the leading and trailing edges of this blade are flat surfaces. This is
obviously undesireable, so we will correct this next.
4) In a new sketch on either the leading or trailing face, convert the
boundary of the face to sketch geometry.
5) Use this sketch to create a rotated cut around the helix axis. While
you can rotate this cut 360 degrees, it is better to keep the angle
equivalent to the angle of rotation of the blade. The software seems to
do strange things sometimes when you create circular patterns (as we are
going to do) with self-intersecting features. Once this is done, you
should have eliminated the flat face, reducing the blade edge instead to
a sharp edge, with a new flat face at the edge of the blade which is
normal to the helix axis, instead of parallel to it as it was before.
6) Repeat the previous step for the flat face on the other end of the
blade.
7) Now, we are going to profile the blade. This is where we change the
blade's shape from a simple circular sector to the lobed or curved shape
we are all familiar with. There are two ways to do this - we'll start
with the simplest one. Create a sketch normal to the helix axis which
will be used to create an extruded cut. You can create a symmetrical
lobe, or a shape with some skew, or whatever. Just sketch what you want
to cut away, and extrude it through the blade. Be aware, though, that
when you do this, you will cut through the blade at locations where the
blade has some thickness, resulting in flat leading or trailing edges
again which will have to be dealt with.
8) The second method, which I prefer, is to create a sketch parallel to
the helix axis, and similarly define a shape which will cut away the
unneeded part, only this time the sketch is a rotated cut, rather than
an extruded one. The simplest illustration of this is a half ellipse
(assuming your rake angle is zero), rotated through the same angle of
revolution as the blade. The advantage of this method, given that we
already made the leading and trailing edges of the blade sharp, is that
the cut enters and exits the existing solid at sharp edges, so you don't
create new flat faces when you make the cut. Rake angle makes it
difficult to determine the correct geometry for the cut. Play with this
and you'll see what I mean.
9) Now that you have defined your blade's shape, the last thing you
need to do is round off the sharp edges with a minimum radius, and fair
out any flat leading or trailing edges that you created when defining
the blade shape. Small details can usually just be filleted, but for
any location where you are removing material significantly, you want to
do this from the low pressure (forward) side only, leaving the high
pressure side untouched. Lofts are useful here, as the geometry can get
complicated. Essentially, you want to profile the forward edges only so
that the profiling curve is parallel to the helix axis when it meets the
high pressure side of the blade. The resulting edge can have a minimum
radius, but you don't want to go shaving a bunch of material off the
high pressure side of a leading edge, since this destroys the
theoretically perfect helix profile.
10) Now that the blade is done, create a circular pattern using the
number of blades variable
11) Create a sketch normal to the helix axis which is a circle
according to the hub base diameter variable, and extrude it with the hub
taper angle you defined.
That's pretty much it. Note that this is only one way of doing it, and
there are many prop variables not addressed with this method (variable
pitch, cupping, etc.) If you really want to understand what's going on
with props, an excellent reference is "The Propellor Handbook", by Dave
Gerr. I think it costs about twenty bucks.
-Sean
Joseph Perkel wrote:
Sean.
Your model is very "Typhoon" like, were you inspired by it for the
exterior profile? Very nicely done.
Also, where did you get the prop model files? I could really use these.
Thanks
Joe
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