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szastoupil  
#1 Posted : Thursday, October 6, 2011 5:37:25 PM(UTC)
szastoupil

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I don't know what to make of this. All of this is importing as a SPLINE.

When importing the coordinates below, the airfoil is very faceted.

0.99977 0.000140
0.99959 0.000150
0.99907 0.000210
.....

When importing the coordinates below, the airfoil is nice and smooth.
POLYLINES
0.99977 0.000140
0.99959 0.000150
0.99907 0.000210
.....

The problem when using the polyline version, the surface extrusion has many lines in the surface...minifacets. The first one is a severely modified foil. Is the second one the proper way to import? Is there a method to smooth out the surface?

I'm a new VCP user and trying to figure out how to loft a wing and guess I'm stuck on step 1. Any pointers lofting with multiple airfoils along the wing and defining a curve shape is welcomed.
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Tim Olson  
#2 Posted : Thursday, October 6, 2011 6:43:53 PM(UTC)
Tim Olson

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Try turning up the display resolution on the spline. In the image attached I turned it to super fine with even distribution.


Tim
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Tim Olson
IMSI Design/Encore
szastoupil  
#3 Posted : Friday, October 7, 2011 9:33:29 AM(UTC)
szastoupil

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Strange...to me. I noticed when extruding with guides that the resulting surface is nice and smooth, but doesn't match what looks like a faceted profile (interpolated splines). Anyway, with the resolution maxed and even distribution, all is well.

So is what I'm seeing only a display property and not the actual profile? What is the main reason for this, PC graphics performance?

I found this in the guide that seems to hint that it's only an artifact on the display or printouts.

"Wireframes are selected by picking a location along the length of the wire. Wires are selected when any portion of the wire passes through the rubberbanding select box or single pick box. Conics,
ellipses, splines, and circles have resolution attributes that impact the screen display and printer output. Use the Edit:Change Resolution command from the Menu Bar to change a wireframe display resolution. "
Tim Olson  
#4 Posted : Friday, October 7, 2011 10:48:16 AM(UTC)
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>>What is the main reason for this, PC graphics performance?

Mathematically our curves are defined as parametric equations. To visualize the equations on your screen, the curves are evaluated and displayed as line segments. Same thing happens for surfaces and solids. In those cases, the surfaces are evaluated and displayed as facets. So resolution is just a display thing as internally all calculations are done on precise equations.

Tim
Tim Olson
IMSI Design/Encore
ALBANO  
#5 Posted : Saturday, October 8, 2011 9:31:08 AM(UTC)
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Hi szastoupil,
something I ran into earlier. I also punched in profile coordinates. But discovered that you can not easily take them as points for any spline type in Shark/Viacad to build aircraft profiles (maybe Im wrong?) You can only take this as collision data to build a second spline with the same geometry but with continous curvature!!!!!
The wing built directly from the coordinate data will NOT behave like in the books. Discontinuos curvature in the leading edge section will definitvely have an impact on your design.... see attached image....

ALBAN
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Tim Olson  
#6 Posted : Sunday, October 9, 2011 11:24:27 AM(UTC)
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Alban gives some good advice from my understanding as well especially if curvature is an issue.


>>But discovered that you can not easily take them as points for any spline >>type in Shark/Viacad to build aircraft profiles (maybe Im wrong?)

This is true for any CAD system. Airfoils are typically derived from equations which may be different for each family of foils. I use multiple equations to define the NACA airfoils in AeroPack. The issue is that CAD systems typically use NURBs to define a spline. So although you may fit the same points between the two definitions, the derivatives between the points are not the same resulting in the curvature anomalies pointed out by Alban.

Another important area to keep an eye on is the slope off the leading edge. The actual slope from the airfoil equation is frequently reinterpreted in the point fitting process when the airfoil definition combines the upper and lower profile into one spline that starts and ends at the trailing edge.

Tim
Tim Olson
IMSI Design/Encore
szastoupil  
#7 Posted : Sunday, October 9, 2011 7:01:24 PM(UTC)
szastoupil

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Thanks Alban for the curvature information. I was able to get a similar plot to what you have posted. I'll pose this question to the aero designer and not choose to take on that challenge at this point. This airfoil is somewhat unique with built in reflex with the default airfoil.

So I've been trying to loft a surface between a two airfoils and have had a few frustrations. This is just a practice exercise as the rails are kind of weird shaped.

1. I cannot get the leading edge rail spline flat into the plane. In evaluating VCP I'm finding it very difficult to draw things into a fixed plane. I haven't found the trick in doing this as I've used other 3D modelers that naturally inference into a plane. Anyway, I can't get a spline flat even with all of the control points at 0. How do I do this?

2. I can't get the line and spline to snap to the airfoils. If you zoom in close enough, the line endpoints are floating in air. How do I get them to land on the airfoil?

3. Lastly in trying to loft the surface using Guide Skin Surface, I select the ends hold shift, then the two rails, but always end up with an error, "The net mapping curve is bad". I'm not what this is and what I'm doing wrong. I think I'm following the Help instructions.

Scott
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szastoupil  
#8 Posted : Sunday, October 9, 2011 7:05:19 PM(UTC)
szastoupil

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Originally Posted by: ALBANO Go to Quoted Post
But discovered that you can not easily take them as points for any spline type in Shark/Viacad to build aircraft profiles (maybe Im wrong?) You can only take this as collision data to build a second spline with the same geometry but with continous curvature!!!!!


I'm not sure what you're suggesting on how to create continuous curvature. Does this moving the individual spline points to ride of the sharp transitions.
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