Sean,
Thank you very much for this, I will be saving it in my construction file. This will be a good exercise when I get a little better using the CAD. (Rhino, about three months now).
You my friend, are an engineer. I am still having trouble with the basics (surfaces) so a ready made prop model is less of a headache for me. I found my people models as you suggested, and one prop model but, Rhino apparently wont read .cob files.
There seems to be something about the propeller (or helix screw) that has fascinated engineering types going back to da Vinci. I suspect something about elegant simplicity...simple yet complex at the same time. The engine guys were the same way about their turbine blades when I was in the airline industry.
When the time comes, I will take the path of least resistance and go to Miami Propeller with my engine specs, displacement and application, and ask them something along the lines of "What do you recommend?" :)
In the meantime, your effort is not wasted because rest assured, I will give this a try as a modeling exercise.
Thanks again
Joe
From: "Sean T. Stevenson" <cast55@telus.net>
Reply-To: personal_submersibles@psubs.org
To: personal_submersibles@psubs.org
Subject: propellors, was Re: [PSUBS-MAILIST] concept sketch
Date: Fri, 24 Feb 2006 09:31:58 -0500
>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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