Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Drag on rotating airfoil (tail wind?)

Drag on rotating airfoil (tail wind?)

Drag on rotating airfoil (tail wind?)


I have a question regarding drag on a moving airfoil, as in a wind turbine. In the simplest form, say an airfoil is moving forward at zero angle of attack at a speed V with a tail wind of U. Is the drag:

a.) D = Cd*1/2*rho*A*(U^2-V^2)

b.) D= Cd*1/2*rho*A*(U-V)^2

Difference of the squares or square of the difference? (there are plenty assumptions in the above - one being that Cd is equal at 0 and 180 AoA, if it wasn't there would be a Cd term before each U and V).

Thank you.

RE: Drag on rotating airfoil (tail wind?)

It probably doesn't matter what kind of machine or wing you're interested it.  This is about the basic concept of force on an aerofoil, regardless of its application.

In a simulation of an aerofoil or a wing, the wing is usually said to move with respect to the air.  In a wind tunnel, the air moves with respect to the wing.  A semantic difference to us now, but 100 years ago there was some discussion on the matter.

The wing doesn't really care if it is moving through the air, or if the air moves over it.  Forces will develop nonetheless.

In your case, find the net relative airspeed of the aerofoil through whatever situation the air is in, and that net velocity is your V vector.  This makes equation "b)" correct.

Steven Fahey, CET

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close