# Where do I find coefficient values for these 2 formulas? (drag coefficients)

• Tim Leijten
In summary, the conversation is about calculating the air and wheel drag of a train. The formulas for calculating drag are mentioned and the person is looking for the coefficient values needed for the calculations. They have tried searching for these values but want to know if there is a way to calculate them themselves. Suggestions are made to look at drag coefficient and coefficient of rolling resistance online, as well as in manuals and databases. The conversation ends with recommendations for books and sources to find the necessary coefficients.
Tim Leijten
Hi there,

So I asked a question here how I would calculate the air drag and the wheel drag of a train and I got an answer that made me very happy.
Unfortunately i don't know where i could find the coefficient values required for the formulas.
So i am wondering if someone knows where i can find these values for trains or how I can calucate them myself.
This are the formulas:
Fd = Cd 1/2 p v2 A
and: F = Crr N
So I need to know how i get the Cd and Crr values.(original formulas in the link in the first sentence)Thanks!

Have you tried Googling drag coefficient or coefficient of rolling resistance?

gleem said:
Have you tried Googling drag coefficient or coefficient of rolling resistance?
I have, but I want to calculate them myself.
But i did find something for drag coefficent.
The only values i could find for wheels were in % which i believe is not what i am looking for.

I think they are usually determined experimentally. The Wikipedia articles give some guidance and you could look up drag coefficients of locomotives too.

@gleem is right, these are usually not calculated theoretically, they are measured, for example in a wind tunnel.

gleem said:
I think they are usually determined experimentally. The Wikipedia articles give some guidance and you could look up drag coefficients of locomotives too.
Thanks, I see.
Do you maybe know if there is some database for that?
Or are these put in like the manuals of locomotives or something?

Tim Leijten said:
Do you maybe know if there is some database for that?

Maybe there are such databases but I don't expect them to be free for everybody. Such experiments are quite expensive and the results are intellectual property of the sponsors.

Tim Leijten said:
Or are these put in like the manuals of locomotives or something?

Yes, that should be a good place to look for the coefficients (or something equivalent) if you have access to such documents.

There was a lot of excellent experimental work done in Nazi Germany (Prandtl, et al) in the 1930's as that nation prepared for war. This work made its way into two large books:
Fluid Dynamic Drag
Fluid Dynamic Lift

both by S.F. Hoerner
These books are rather hard to find, but they are by far the best source I know about.

jrmichler
In addition to the books listed by Dr.D, which mostly apply to aircraft, there is also Aerodynamics of Road Vehicles by Wolf-Heinrich Hucho.

Okay, thanks!
I think I got an idea where to look.

## 1. What are drag coefficients and why are they important in scientific research?

Drag coefficients are values used to quantify the amount of drag, or resistance, an object experiences as it moves through a fluid. They are important in scientific research because they help scientists understand and predict the behavior of objects in various fluid environments, such as air or water.

## 2. Where can I find the coefficient values for specific objects or materials?

The coefficient values for specific objects or materials can typically be found in published research papers or in databases compiled by scientific organizations or institutions. You can also calculate the coefficients using experimental data and mathematical equations.

## 3. How do I determine the appropriate drag coefficient for my research or experiment?

The appropriate drag coefficient depends on the shape and properties of the object or material you are studying, as well as the fluid environment it is moving through. It is important to carefully research and consider these factors to determine the most suitable coefficient for your research or experiment.

## 4. Are there any standard drag coefficient values that can be used for general calculations?

Yes, there are standard drag coefficient values for basic shapes, such as spheres, cylinders, and plates, that can be used for general calculations. However, for more complex shapes or materials, it is recommended to use experimentally determined or calculated coefficients for more accurate results.

## 5. Can drag coefficient values change over time or in different fluid environments?

Yes, drag coefficient values can change over time or in different fluid environments due to factors such as changes in speed, temperature, and fluid viscosity. It is important to consider these variables and to update the coefficient values accordingly in order to accurately predict the behavior of objects in different conditions.

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