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leeminardi
en respuesta a: jimbir

 

 

Could you post a picture of the wing tip you are trying to draw?

 

A good place to start to learn about AutoCAD splines is here.

 

As noted, AutoCAD splines are technically known as NURBS Non Uniform Rational B-Splines. This is a fancy name for some fancy math.  If you would like to know more about NURBS you can start by learning about Bezier splines and then B-Splines (which can be considered as a series of Bezier splines head to tail). NURBS are essentially a B-spline divided by a B-spline.

 

You don’t need to know any of the math behind NURBS to use them.  The key concept is that in AutoCAD there are two methods for creating a spline, fit points (also know as interpolation points) or control vertices (CV).  Control vertices are the key data element used directly by the NURBS math. Newbies to spline creation will probably use the fit method for creating a spline.  The spline will pass through each point you specify in this case.  The tangent option lets you specify the slope of the curve at its ends.  See the first link above for more detail. Also in that link it states that tolerance “Specifies the distance by which the spline is allowed to deviate from the specified fit points. A tolerance value of 0 requires the resulting spline to pass directly through the fit points. The tolerance value applies to all fit points except the starting and ending fit points, which always have a tolerance of 0”.

 

If you create a spline using fit points you can drag them around to change the shape of the spline. Here you see  a spline with 7 fit points.

ss1.JPG

Here the 3rd fit point (from the left) has been moved by selecting and dragging it to a new location.

ss2.JPG

With the properties dialog open you can change the method to Control Vertices and the spline will look like this.

ss3.JPG

In this example, the spline is represented by 9 CVs. The exact number is a function of the number of fit points and the degree of the spline. You can click and drag the CVs like you did the fit points. In the image below the second CV was moved from above the start of the spline to a point directly to the left. The result is that the slope of the spline start out horizontally going to the left.  It is useful to know that the second and next to the last CV of a spline controls the slope and radius of curvature at the ends of the spline.

ss4.JPG

The splinedit command is a very powerful tool for editing splines.  For example, using the Edit vertex, and Add option an additional CV is added to the spline allowing for finer control in that area of the spline.

ss5.JPG

If a cv is dragged to be coincident with another CV the result is a sharp corner.  In the following image the newly added CV was moved to the top CV.

ss6.JPG

In general it is best to use as few fit points (or CVs) as possible.  This will help to ensure a smooth shape.  An excessive number of fit points (or CVs) increases the likelihood of getting ripples in the curve which are often undesirable.

 

If you want to create a shape like an airfoil it is a good tactic to use larger distances between fit points where the curvature is slight and closer together where the curvature is tight. You might consider a gradual decrease in spacing as the shape transitions from loose to tight curvature as you can see here.

image.png

 

 

I usually start spline creation method by using the fit point method and then use a combination of splinedit and the manipulation of the CVs to get the shape I want.  I will also add or delete CVs as needed.

 

I suggest experimenting with the creation of splines via fit points and CVs and then editing them with splinedit.

 

 

 

 

 

lee.minardi