Drag Coefficient ?

[Ashraf Ahmed Khan]

I'm using FCalc for calculating accurate friction values for locomotives and coaches. I have two doubts :-

1. What will be the Drag Coefficient for locomotives ? 0.65 or 1.00 ? Is there any way to calculate the drag coefficient itself ?

2. The friction value is calculated at what speed(s) exactly ? 0 or 100 ?

Please suggest

[R. Steele]

I'm using FCalc for calculating accurate friction values for locomotives and coaches. I have two doubts :-

1. What will be the Drag Coefficient for locomotives ? 0.65 or 1.00 ? Is there any way to calculate the drag coefficient itself ?

2. The friction value is calculated at what speed(s) exactly ? 0 or 100 ?

Please suggest

For detailed answers and information regarding your questions please refer to the documentation included with FCalc2 >> "Methodology and Theory of FCalc 2.doc"

It's my understanding that for North American freight the wind resistance starts to affect performance at around 65mph ( 105kph ) so that's what I use for freight. With high speed passenger trains you should probably choose a speed that reflects the average/normal running speed of the train and use that.

For drag coefficient I'll provide a quote from the above document ( suggested reading )

While the “big three” are very important, another parameter of some importance is the drag coefficient. You are generally only asked for the drag coefficient if the vehicle is in front of the train. Basically, this means locomotives and leading motor cars only. For a good portion of the rolling stock out there it is pretty safe to assume a value of 1. If it looks boxy, you probably can’t go wrong picking 1 for the drag coefficient. However, if you have a locomotive that will only be used as one of the following units in a multiple unit lash-up then it is probably more realistic to use a value of less than 1 for the drag coefficient. A value around 0.7 might be appropriate in that case. For cases when the leading unit is streamlined, appropriate values would be 0.65 to 0.75 if partially streamlined, and 0.5 or less if fully streamlined. I’ve found that a value of 0.35 for very streamlined trains like the TGV combined with proper frontal area will give results that are in near perfect agreement with empirically measured values. Because data regarding drag coefficients is very hard to come by, the best method is to take a look at the locomotive, estimate the degree of streamlining, and choose an appropriate value based on the guidelines I just gave for various types of locomotives. Slight streamlining might mean a drag coefficient of 0.9, something like an F7 might be around 0.6 to 0.7. An HHP-8 might be about 0.55. A TGV or bullet train will probably be under 0.4, and in some cases under 0.3. All of these examples only apply to units in front of the train. Trailing units will have less drag, and the drag coefficient should reflect this. A value of from 30% to 70% of the leading unit’s drag coefficient is appropriate, depending upon how closely coupled the units are. For instance, an F7 or PA B-unit might use a drag coefficient of 0.25, while an unstreamlined freight locomotive might use 0.6 to 0.7. In the end a good portion of the train resistance is made up of the cars being pulled, not the locomotive(s), so errors of 20% or more in the drag coefficient won’t have that large of an effect except for a very short train.

One other thing ( see screenshot ), if you are going to use the Davis numbers, make sure that you've chosen the metric units under "file" -- that way the values will be given in the default units OR uses.

bandicam 2021-03-02 13-34-21-950.jpg

[Ashraf Ahmed Khan]

Thanks a lot, Sir. Reading all of it carefully.