Helix Design
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NY Signal drawn by 3rd PlanIt


NY Signal drawn 
3rd PlanIt by Walter Griffin

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When incorporating a Helix into a 3rd PlanIt design, it's important to remember that the Helix is only the means of getting from one track to another. The elevation, direction and location of the approach tracks at each end, together with the radius of the Helix and its number of turns, define the Helix in detail. The best way to get a feeling for designing with a Helix is to start with a simple example.

Before getting started, it's important to define "Curve", "Circle" and "Helix" as they are used in 3rd PlanIt. The definition of a Circle is fairly intuitive: it is a curving line of constant radius, always equidistant from a Center Point, which is 360 degrees in length so that its start and end points are located at the same X,Y position. It may or may not have a grade (a change in elevation). A Curve is a portion of a circle, less than 360 degrees, which may or may not have a grade. A Helix is a "Circle" of more than 360 degrees, which always has a grade.

If you are designing a loopback curve that's less than or equal to 360 degrees, you must use a Circle or Curve. A Helix always is expected to have more than 360 degrees of curving track. With these definitions in mind, let's start learning how to use a Helix in your track plan.

First, draw a Helix using the Draw / Helix / Draw from center tool. Place a helix of reasonable size in the center of a new track plan. The helix shown in this example has a radius of about 39 inches (1 meter) to ensure reliable operation with HO scale equipment.

For this example, the "Color by Object Type" mode was chosen by pressing Alt+T on the keyboard. The helix appears red, like a circle would appear. That's because the default helix is made from a complete 360 degree Circle, plus an additional 360 degrees called a Turn in 3rd PlanIt. The track included in one or more Turns is colored a light purple, but it's not visible at this point because the entry and exit points have not yet been established. If you look at the Object Data Window at this point, the start and end points will have the same elevation. Ignore that for the moment; the program will determine the proper elevations for you in a later step.

Now choose the Connect with Easements tool. You can use the Connect Direct tool, but I prefer to include easements in the approaches to a Helix. Point the cursor to the top of the Helix, making sure it highlights to confirm you are pointing at it, and click the Left Mouse Button (press and release it). After clicking the Left Mouse Button (LMB), move the cursor to the right. A red line will follow the cursor from the top of Helix. Should the red line happen to start at the bottom of the Helix, simply click the Right Mouse Button (RMB) and it will swap to the top of the Helix.

Although the Connect Tool is most frequently used to connect two existing objects, it can also be used to make a connection from an existing object and terminates anywhere in the drawing. We'll use that feature to design our Helix. Hold down the Ctrl key on the keyboard, and the Arrow of the Connect Tool will change to a Crosshair, indicating that you may now "Connect" to any point in the drawing. Holding the Ctrl key down, position the tangent approach track as you desire. Still holding the Ctrl key down, click the LMB to achieve results similar to those shown above. You can now see a portion of the Turn of the Helix colored purple. The tangent approach track might be the higher level or the lower level... it does not matter. The details of elevation will be automatically set later in the process.

Using the Connect Tool in exactly the same manner, establish the other approach track. As you can see in the image to the right, more of the Turn of the helix is visible. Since 3rd PlanIt is in "Color by Object Type" mode, you can see the Tangent Track colored black, the curved track as red, the easements as green, and the visible Turn of the Helix colored purple. The "Color by Object Type" mode lets you easily select the desired track segment, even when the joint between two objects is not obvious, such as where the easement joins the tangent track and curved track.

Again, the elevation of the two approach tracks is unimportant at this point. The only important feature is the placement of the two approach tracks. Simply envision one track as the "upper" approach track and the other as the "lower", even though all track objects have the same elevation when you draw them.

To help demonstrate this point, two Elevation Markers have been added to the drawing in the images below:

As you can see, all track objects were originally drawn at an elevation of 0 inches. In this example, we will choose to make the upper level at 54" and the lower level at 32". To do this, select one approach track and set its elevation to 54". This is most easily done in the Object Data Window, setting both Z values to 54" in one case and 32" in the other. Experienced users need only set the "most distant" elevation. When you click on a Z data value in the Object Data Window, a shrinking circle will appear at the end of the line corresponding to the Z value you selected. You need only set the elevation of the ends of the Lines farthest from the Helix.

Select the Path Tool, available in the ToolKit just below the Connect Direct button. Point to the upper tangent track near the end closest to the helix and click the LMB. Don't click while pointing to the green-colored easement, or the grade will not be properly designed. As shown in the upper left image, if you have properly clicked the upper tangent track, it will highlight, first in magenta, then in light blue when the cursor points at other track objects. Point to the other tangent track approaching the spiral, make sure it highlights, then click the LMB once again to establish a Path between the two tangent tracks. The whole Path will then appear in Magenta, as seen above.

Setting the grade through the turnout is easy. Simple press Ctrl+E to bring up the Elevation dialog box, or click the Elevation and Grade button in the Property Bar (shown at the right). The Set Path Properties dialog box will appear with the Elevation control page as shown to the left.

You can see the elevations you chose for the approach tracks have already been set up in the dialog box. The resulting grade is -4.42%, a negative number because the grade is computed from the higher elevation to the lower elevation.

This grade is too steep for use in a real model railroad, but it is very useful in this design example. There's no need to change any values in the Elevation page, so click OK to establish a grade of 4.42% through all the track included in the Path. Swap to 3D using the Tab key or the Render 3D button in the Property Bar. You may need to adjust the view a bit using the Arrow keys, but in the end you will see a smooth grade through all the track highlighted  in the Path, including the Helix. The results are shown in the image at the right.

It looks good, but obviously a grade of 4.42% is not usable in a Helix that may be inaccessible when your layout is built. It is too steep, which may cause locomotives to lose traction, or in the worst case, the whole train might fall off the track to the inside of the Helix. There are several ways to adjust the grade of the Helix. The best place to start is to add a Turn to the Helix.

Swap back to 2D using the Tab key or the Render 3D button in the Property Bar. Choose the Select Tool from the ToolKit. Point to a blank area of the drawing and click the LMB to deselect all objects. Point to the Helix and click the LMB to select the Helix. Bring up the Property Sheet by pressing Enter, or choosing Edit / Properties. If you don't see the Data page, click the Data tab at the top of the dialog box. The Data page shows all the critical values of the Helix, including the Railhead-to-railhead distance. You can see that it is 10 13/16" in this example, far more than required for an HO scale Helix. Change the number of Turns to two, and note that the Railhead-to-railhead distance is now 6 21/32", quite sufficient for HO scale operation.

It's important to note that the exact number of turns, including the entry and exit segments of the Helix, is 2.610928 in this example. You can only adjust the number of complete "helical" turns as an integer value. The fraction to the right of the decimal point is determined exclusively by the geometry that connects the helix to the approach tracks. If you were allowed to change the decimal portion of the number of turns, the endpoints of the Helix would no longer be aligned to the approach tracks. This will be discussed in more detail below.

After changing the number of Turns from one to two, you have only changed the grade of the Helix itself. The approach tracks still have their original 4.42% grade. Press OK to set the number of Turns in the Helix to two, then use the Path Tool exactly as you did before. Bring up the Elevation dialog box again, as before. The endpoint elevations will be the same as the first setting, in this case, 54 and 32 inches.

Note that the overall grade, including the approach track, is -2.96%, rather than -2.726% shown in the Helix data page. That's because the Helix endpoint elevations were originally established with a grade of 4.42 percent. This "compressed" the vertical space available to the Helix, compared to what is available when the Helix has an additional Turn.

Now that you have made a Path that includes both approach tracks, as you did originally, the overall grade through all track segments is now 2.96 percent. This will result in a Helix whose endpoints are slightly higher and lower than in the original attempt. Again, this is a matter of geometry, not a fault in the track design or the use of the Helix in the design. Simply press the OK button to establish a constant grade throughout all the track segments in the Path.

Swap to 3D to view the results. As shown at the left, the Helix now has an extra Turn, and remains smoothly connected to the approach tracks. All track segments are at the same grade. To see the details of the Helix, swap back to 2D and select only the Helix. The Object Data Window will then show all critical dimensions and locations, as shown below.

Because the approach tracks have less grade than the first attempt, the vertical distance available for the Helix is greater. Therefore, the endpoints of the Helix are higher and lower, the grade is a bit steeper, and the Railhead-to-railhead distance is a bit greater. The resulting 3% grade and 7.25" spacing is quite reasonable, and this design could be built and operated successfully.

But what would you do if you couldn't achieve workable results by simply adding or removing Turns from the Helix? In the real world, there are only three ways to make more subtle changes to the grade or spacing of the helix: 1) change the upper and/or lower elevations, 2) change the geometry of the approach tracks, or 3) change the radius of the Helix.

Approach number one is the easiest. If you haven't locked-in the elevations of the upper and lower levels of your layout, simply change the elevation of one or both approach track endpoints. Use the Path tool and Elevation dialog as before until you find elevations for both levels that are comfortable for operation and result in a usable Helix.

Approach number two is accomplished by changing the location that the approach segments enter the Helix. You won't be able to make a large change in grade this way, but it may be adequate if your dimensions are only slightly out of tolerance. Delete one or both of the approach tracks, including the tangent track and easement. Use the Connect with Easements tool exactly as you did before, but relocate one or both approach tracks to change the length of track in the Helix. Again, use the Path Tool as before and see if you can obtain usable results.

Finally, you can change the radius of the Helix itself. Note that, if you change the radius of the Helix, it by definition changes the geometry of the approach track. The easements were originally created to transition to the radius of the original Helix, and the locations of the easements and tangent tracks were determined based on the Helix. Changing the radius of Helix necessitates changes to the approach tracks as well. The best way to do this is to delete the original Helix and draw a new Helix with a different radius. Move the new Helix until it is near the original approach tracks. Then delete the approach tracks, both tangents and easements, and redraw them using the Connect Tool in the same manner shown above. From experience, you may want to set the Number of Turns in the new Helix before using the Path Tool to set the grades. Next, use the Path Tool in the same manner to set the grade through the approach tracks and the new Helix. If you've chosen the new radius well, you should end up with a grade and spacing that meets your design goals.

With these techniques, you will never need to manipulate the endpoints or grade of the Helix directly. The Helix dimensions are set automatically with the Path and Elevation tools, based on the geometry of the approach tracks. You could even "flip" the Helix top-to-bottom, simply by swapping the elevations of the approach track endpoints and using the Path and Elevation tools in the same manner. If you find yourself tempted to change the values of the Helix directly, watch out! You'll need to change the approach tracks to correspond to the changes in the Helix, which can be very difficult and time consuming. It's always best to start with approach tracks that make sense, a reasonable radius and Number of Turns for the Helix, and use the Connect, Path and Elevation tools.

Think of the Helix as the result of the elevation and orientation of the upper and lower approach tracks, the radius of the Helix, and the number of Turns within the Helix. Focus on these variables, and use the Path and Elevation tools to automatically compute the proper Helix for the elevation, geometry, radius and number of turns you select. In fact, it's best to not even view the Helix in 3D until you have applied the Path and Elevation tools, or you might be concerned that something isn't right. Be confident that the Path and Elevation tools will compute a Helix that corresponds to the elevation of the approach tracks, considering the radius and number of turns in the Helix.

You can also use these procedures between Layers in the 3rd PlanIt track plan. You need to make sure that the "Allow only the Active Layer to be edited" check-box in the Layer Enable Dialog is not checked, and the layers containing the track objects are not frozen. There is no difference in technique, whether the track objects exist in the same Layer, or more than one Layer.


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Last modified: June 14, 2003