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A graph showing coefficient of pitching moment with respect to angle of attack. The negative slope for positive α indicates stability in pitching.

In aerodynamics, the pitching moment on an airfoil is the moment (or torque) 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.[1] More generally, a pitching moment is any moment acting on the pitch axis of a moving body.

The lift on an airfoil is a distributed force that can be said to act 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.[2]

Measurement[edit]

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_\text{indicated} + \mathbf{x}\times (D_\text{indicated}, L_\text{indicated})

where the indicated values of D and L are the drag and lift on the model, as measured by the force sensor..

Coefficient[edit]

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[3]

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,[2] 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.

The 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.[4]

References[edit]

  • 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)
  • Low-Speed Stability Retrieved on 2008-07-18

Notes[edit]

  1. ^ Clancy, L.J., Aerodynamics, Section 5.3
  2. ^ a b Clancy, L.J., Aerodynamics, Section 5.10
  3. ^ Clancy, L.J., Aerodynamics, Section 5.4
  4. ^ Ira H. Abbott, and Albert E. Von Doenhoff (1959), Theory of Wing Sections, Dover Publications Inc., New York SBN 486-60586-8

See also[edit]


Original courtesy of Wikipedia: http://en.wikipedia.org/wiki/Pitching_moment — Please support Wikipedia.
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35 news items

Ottawa Citizen

Ottawa Citizen
Mon, 30 Mar 2015 09:14:52 -0700

The engines are angled upward by nine degrees to cancel out the nose-down pitching moment they would otherwise generate due to being mounted above the aircraft's aerodynamic center, avoiding the need to trim the control surfaces against the force.[57]

Aviation Week

Aviation Week
Fri, 13 Mar 2015 11:37:42 -0700

There is a noticeable pitching moment when the spoilers extend, but panel extension and retraction in flight is relatively slow, so it's easily controllable. Also, there is mild airframe buffet with the spoilers extended. The Garmin GWX68 weather radar ...
 
Scuttlebutt Sailing News
Tue, 07 Oct 2014 04:38:48 -0700

Luna Rossa skipper Max Sirena believes the Italian syndicate can finally win the America's Cup. “For the first time I feel in a position to win the America's Cup,” Sirena told a media conference at the just-completed Genoa Boat Show. “It would be the ...

That Balls Outta Here

That Balls Outta Here
Thu, 30 Oct 2014 11:52:28 -0700

“Greatness In Base Ball Yearly”, that's what MLB's GIBBY Awards celebrate. The awards were begun by Major League Baseball back in 2002. They are awarded for extraordinary performances by players, managers, executives, and others in the game.
 
The Aviation Herald
Thu, 31 Jul 2014 08:14:23 -0700

A Ryanair Boeing 737-800, registration EI-EFB performing flight FR-2369 from Ostrava (Czech Republic) to London Stansted,EN (UK) with 171 passengers and 6 crew, landed on Stansted's runway 04 at 22:42L (21:42Z) but struck its tail onto the runway.

Grantland

Grantland
Fri, 11 Jul 2014 10:57:48 -0700

Kershaw's streak was starting to overshadow this season's signature pitching moment: the no-hitter he threw against the Rockies on June 18. In that game at Dodger Stadium, Kershaw struck out 15 batters and walked none, giving him the most punchouts ...
 
SB Nation
Mon, 21 Apr 2014 10:29:00 -0700

The interesting part? It was a seven-run game, which in Las Vegas is like ... dunno, a four-run game? It was a bit closer than the 11-4 score made it seem, at least. So maybe this wasn't just a position-player-pitching moment. Maybe this is a new career!

Superyacht Times

Superyacht Times
Fri, 30 May 2014 01:01:23 -0700

Addressing the final criteria is a long slender hullform with plumb bow and semi-bulbous bow sections, creating a fast hull with low resistance and dampened pitching moment. This is all driven by a hybrid diesel-electric propulsion system given an 8 ...
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