Pitching moment

Pitching moment

In aerodynamics, the pitching moment on an airfoil is the moment produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the airfoil. The pitching moment on the wing of an airplane is part of the total moment that must be balanced using the lift on the horizontal stabilizer. [Clancy, L.J., "Aerodynamics", Section 5.3] .

The lift on an airfoil acts at a point called the center of pressure. However, as angle of attack changes on a cambered airfoil, there is movement of the center of pressure forward and aft. This makes analysis difficult when attempting to use the concept of the center of pressure. One of the remarkable properties of a cambered airfoil is that, even though the center of pressure moves forward and aft, if the lift is imagined to act at a point called the aerodynamic center the moment of the lift force changes in proportion to the square of the airspeed. If the moment is divided by the dynamic pressure, the area and chord of the airfoil, to compute a pitching moment coefficient, this coefficient changes only a little over the operating range of angle of attack of the airfoil. The combination of the two concepts of "aerodynamic center" and "pitching moment coefficient" make it relatively simple to analyse some of the flight characteristics of an aircraft. Clancy, L.J., "Aerodynamics", Section 5.10]

Measurement

The aerodynamic center of an airfoil is usually close to 25% of the chord behind the leading edge of the airfoil. When making tests on a model airfoil, such as in a wind-tunnel, if the force sensor is not aligned with the quarter-chord of the airfoil, but offset by a distance x, the pitching moment about the quarter-chord point, M_{c/4} is given by:M_{c/4} = M_ ext{indicated} + mathbf{x} imes (D_ ext{indicated}, L_ ext{indicated})where the indicated values of "D" and "L" are the drag and lift on the model, as measured by the force sensor.

Coefficient

The pitching moment coefficient is important in the study of the longitudinal static stability of aircraft and missiles.

The "pitching moment coefficient" C_m is defined as follows [ Clancy, L.J., "Aerodynamics", Section 5.4] :C_m=frac{M}{qSc}where "M" is the pitching moment, "q" is the dynamic pressure, "S" is the planform area, and "c" is the length of the chord of the airfoil.C_m is a dimensionless coefficient so consistent units must be used for "M", "q", "S" and "c".

Pitching moment coefficient is fundamental to the definition of aerodynamic center of an airfoil. The "aerodynamic center" is defined to be the point on the chord line of the airfoil at which the "pitching moment coefficient" does not vary with angle of attack, or at least does not vary significantly over the operating range of angle of attack of the airfoil.

In the case of a symmetric airfoil, the lift force acts through one point for all angles of attack, and the "center of pressure" does not move as it does in a cambered airfoil. Consequently the "pitching moment coefficient" for a symmetric airfoil is zero.

Pitching moment is, by convention, considered to be positive when it acts to pitch the airfoil in the nose-up direction. Conventional cambered airfoils supported at the aerodynamic center pitch nose-down so the "pitching moment coefficient" of these airfoils is negative. [ Ira H. Abbott, and Albert E. Von Doenhoff (1959), "Theory of Wing Sections", Dover Publications Inc., New York SBN 486-60586-8]

References

* Clancy, L.J. (1975), "Aerodynamics", Pitman Publishing Limited, London, ISBN 0 273 01120 0
* Piercy, N.A.V, "Aerodynamics", pp384-386, English Universities Press. London (1943)
* [http://ciurpita.tripod.com/rc/rcsd/lowSpeedStability/lowSpeedStability.html Low-Speed Stability] Retrieved on 2008-07-18

Notes

ee also

* Aircraft flight mechanics
* Flight dynamics
* Longitudinal static stability
* Neutral point
* Lift coefficient
* Drag coefficient


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