What does a negative moment tell us about the characterisics of the airfoil?

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Discussion Overview

The discussion revolves around the implications of negative pitching moments in airfoils, particularly focusing on the NACA-0012 airfoil. Participants explore the relationship between angle of attack, pitching moments, and the overall stability of aircraft, considering both theoretical and practical aspects of aerodynamics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that as the angle of attack increases, the moment becomes more negative, prompting questions about the significance of this observation.
  • Another participant suggests that negative pitching moments occur because lift is concentrated towards the front of the airfoil, specifically in the first quarter of the chord region.
  • Several participants emphasize the importance of negative pitching moments in analyzing the longitudinal stability of an aircraft, particularly with cambered wings, which generate lift even in level flight.
  • There is a discussion about the convention of positive and negative pitching angles, with some participants asserting that a negative pitching moment indicates a nose-down attitude, while others challenge this interpretation.
  • A participant raises a question about how to quantify negative pitching moments and their relationship to airfoil characteristics, seeking a formula or explanation.
  • Another participant discusses the implications of coordinate systems on the interpretation of moments, indicating a potential misunderstanding in the context of the discussion.
  • There are inquiries about the specific point about which the moment is taken, with one participant asserting that the pitching moment should be zero at the quarter chord point for the NACA 0012 until stall.
  • A later reply elaborates on the concept of moments, stating that they impart torque to the fuselage and discussing the role of horizontal stabilizers in maintaining stability.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of negative pitching moments and their implications for aircraft stability. There is no consensus on the exact nature of these moments or their significance, indicating ongoing debate and exploration of the topic.

Contextual Notes

Some discussions involve assumptions about the definitions of pitching moments and stability, as well as the specific conditions under which these moments are evaluated. The conversation reflects a range of interpretations and understandings of aerodynamic principles.

  • #31
Right... both of those are named specifically as subheadings because they aren't treated the same.

Another list of books...
In fluids:
"Principles of Ideal-Fluid Aerodynamics" by Karamcheti - Moment has its own entry in the sense I use it.

"Viscous Fluid Flow" by White - Moment is used as I have described it.

"Fundamentals of Aerodynamics" by Anderson - Moment is used in several contexts as I have described it.

"Fundamentals of Fluid Mechanics" by Munson, Young and Okiishi - Separate entries for moment of inertia and moment of momentum as I have described.

"Theory of Wing Sections" by Abbott and Von Doenhoff - Moment is used as I have described; has a lot of stuff about pitching moments in particular.

"Low-Speed Aerodynamics" by Katz and Plotkin - Index doesn't refer to "moment" specifically, but points to moment coefficient. That section uses it as I have described it in relation to the pitching moment.

Other areas of mechanics:
"Dynamic Systems: Modeling and Analysis" by Vu and Esfandiari - Moment is used as I have described it and has a separate entry for moment of inertia.

"Classical Mechanics" by Taylor - Only refers specifically to moment of inertia

Those are the only books in arm's reach for me that talk about moments. All of them support me. I don't know how much more plain I can make it for you. In common engineering practice, the term moment refers to the first force moment. All other moments that are important are referred to by name.
 
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  • #32
boneh3ad said:
Right... both of those are named specifically as subheadings because they aren't treated the same.

Another list of books...
In fluids:
"Principles of Ideal-Fluid Aerodynamics" by Karamcheti - Moment has its own entry in the sense I use it.

"Viscous Fluid Flow" by White - Moment is used as I have described it.

"Fundamentals of Aerodynamics" by Anderson - Moment is used in several contexts as I have described it.

"Fundamentals of Fluid Mechanics" by Munson, Young and Okiishi - Separate entries for moment of inertia and moment of momentum as I have described.

"Theory of Wing Sections" by Abbott and Von Doenhoff - Moment is used as I have described; has a lot of stuff about pitching moments in particular.

"Low-Speed Aerodynamics" by Katz and Plotkin - Index doesn't refer to "moment" specifically, but points to moment coefficient. That section uses it as I have described it in relation to the pitching moment.

Other areas of mechanics:
"Dynamic Systems: Modeling and Analysis" by Vu and Esfandiari - Moment is used as I have described it and has a separate entry for moment of inertia.

"Classical Mechanics" by Taylor - Only refers specifically to moment of inertia

Those are the only books in arm's reach for me that talk about moments. All of them support me. I don't know how much more plain I can make it for you. In common engineering practice, the term moment refers to the first force moment. All other moments that are important are referred to by name.

I've already said 'moment' commonly refers to the first moment of force by convention.
 
  • #33
Then what are you arguing about? That has been my point all along yet you keep telling me I am wrong.
 
  • #34
boneh3ad said:
Then what are you arguing about? That has been my point all along yet you keep telling me I am wrong.

No it hasn't been your point all along. I made it first, and seem to have missed it. So what are you arguing about?
 
  • #35
Phrak said:
No it hasn't been your point all along. I made it first, and seem to have missed it. So what are you arguing about?

Claim what you want, that was not your point. I am done with this as it is going nowhere and serves no purpose.
 
  • #36
boneh3ad said:
Claim what you want, that was not your point.
It was.
I am done with this as it is going nowhere and serves no purpose.
See you later.
 
  • #37
Phrak said:
No, and this is the point. There is also the angular inertia, center of mass and total mass, and the moments involved in Euler buckling and torsional rigidity to obtain the radius or gyration.

None of which has anything to do with the question asked in the OP.
 
  • #38
mugaliens said:
None of which has anything to do with the question asked in the OP.

No. Indirectly, it does.
 

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