# Thrust Coefficient vs. Drag Coefficient

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 April 21, 2018, 14:51 Thrust Coefficient vs. Drag Coefficient #1 Member   Matt Ridzon Join Date: Jun 2014 Posts: 91 Rep Power: 11 Admittedly, I'm ignorant of what the Thrust Coefficient of an airfoil is. I've only ever studied lift and drag coefficients of an airfoil, but never thrust coefficients in an academic setting. However, after a little digging through textbooks and some web searches, the thrust coefficient equation appears identical to that of the lift and drag coefficients. So intuition tells me the thrust acts opposite of drag, therefore the thrust coefficient would be identical to the drag coefficient but opposite in sign. Is my understanding correct? Just as background information, here's what I'm doing...I am modeling an airfoil in Fluent and will extract its drag, lift, and thrust coefficients from steady-state laminar flow with low Re. Thanks in advance, M Ridzon

April 23, 2018, 00:55
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Lucky
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During aerial manuevers (i.e. turns, banks, loop-the-loops), part of the airfoil reaction forces can be in the direction of acceleration so that the thrust coefficient is not always equal and opposite the drag because the drag in the direction of the velocity.

Quote:
 Originally Posted by m_ridzon ...the thrust coefficient would be identical to the drag coefficient but opposite in sign.
The coefficients are all positive.

April 23, 2018, 09:25
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Matt Ridzon
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Quote:
 Originally Posted by LuckyTran During aerial manuevers (i.e. turns, banks, loop-the-loops), part of the airfoil reaction forces can be in the direction of acceleration so that the thrust coefficient is not always equal and opposite the drag because the drag in the direction of the velocity. The coefficients are all positive.
Thanks for the insight. I would agree with you under transient conditions. But as mentioned in my initial post, my conditions are steady-state flow over the airfoil. So if my intuition is right, drag would equal thrust. Does that seem correct?

M Ridzon

 April 23, 2018, 14:53 #4 New Member   Join Date: Mar 2018 Posts: 11 Rep Power: 8 Never heard of thrust coefficient for an airfoil. I've always used thrust coefficient for jet nozzle thrust. There is the concept of leading-edge suction for airfoils.

April 23, 2018, 15:22
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Matt Ridzon
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Quote:
 Originally Posted by steve_podleski Never heard of thrust coefficient for an airfoil. I've always used thrust coefficient for jet nozzle thrust. There is the concept of leading-edge suction for airfoils.
I've never heard of it either until now. I found a research paper from another student at my college. I was able to speak to the author about his use of the term in reference to his airfoil studies. He said it is just equal and opposite of the drag coefficient. So unless I learn something more here on this forum, I suppose that's what I'll go with.

M Ridzon

April 23, 2018, 17:20
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Quote:
 Originally Posted by m_ridzon I've never heard of it either until now. I found a research paper from another student at my college. I was able to speak to the author about his use of the term in reference to his airfoil studies. He said it is just equal and opposite of the drag coefficient. So unless I learn something more here on this forum, I suppose that's what I'll go with. M Ridzon

Just had another thought: for flapping wings, .e.g. birds and insects or ornithopters, the concept of thrust coefficient may be appropriate. Also for rotors and propellers which use airfoil sections

April 24, 2018, 08:43
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Matt Ridzon
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Quote:
 Originally Posted by steve_podleski Just had another thought: for flapping wings, .e.g. birds and insects or ornithopters, the concept of thrust coefficient may be appropriate. Also for rotors and propellers which use airfoil sections
Yes, thank you. My college colleague affirmed this also, noting that experts in the area of flapping wing technology use the idea of "thrust coefficient" to help define their research findings. And that is precisely the area of my current work is in.

April 24, 2018, 12:01
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Quote:
 Originally Posted by m_ridzon Yes, thank you. My college colleague affirmed this also, noting that experts in the area of flapping wing technology use the idea of "thrust coefficient" to help define their research findings. And that is precisely the area of my current work is in.
Great! Many, many years ago, I was involved in trying to build a mechanism to flap wings for wind tunnel study but it was stopped because of mechanical difficulties but in the meanwhile I did a literature search in the wonderful world of bird and insect flight. My advisor, much later, had his students build and fly and ornithopter!

 Tags drag coefficient, lift coefficient, thrust