The Spiral/Helix command does not allow you to create a variable pitch helix, but you can create one with the Parametric Curve function.
Change the Z expression to change the pitch of the helix. For example, the Z value for the blue helix is 0.5*t^2. The Z value for the red helix is 0.2*t^2. The green circle is shown for reference. The example instructions below will help you understand the values expected to create a variable pitch helix.
To create a variable pitch helix,
1. Click 3D Geometry and then, in the 3D Curve group, click More next to Spline 3D.
2. Click Parametric Curve Approximation in the Direct section. The Parametric Curve dialog box opens.
3. Click Part and select a part in the Structure Browser, or type a new part name.
4. Do not change the Variable. The default value is t, which is used in the expressions for this example.
5. Click X Function and type 10*cos(t).
6. Click Y Function and type 10*sin(t).
7. Click Z Function and type 0.5*t^2.
8. Click Start and type 0.
9. Click End and type 14.0.
10. Type a Tolerance, which determines how far the represented curve can be from the exact curve.
Spline 3D.
Parametric Curve Approximation in the Direct section. The Parametric Curve dialog box opens.
to complete the operation.