Talk:Center of lift

Verifiability of center of lift
This article began in September 2006 when Ericg inserted the first text. He summarised his work by writing center of lift is absolutely not the same as center of pressure. He left no reference or citation to show that his text was verifiable.

The present text states An aircraft's center of lift is the location at which the net lift produced by all airfoils is located. I am not aware of any author who has used the term center of lift, either for this purpose or any other purpose. (However, there may be one or two authors who make use of the term. If so, they should be cited.)

Perhaps the reason mainstream authors don’t define a point called center of lift is because the net lift produced by all the airfoils on an aircraft must act through (or extremely close to) the point called center of gravity! It is the function of the horizontal stabilizer to generate just enough lift, usually in the downwards direction, to keep the aircraft balanced in pitch. In steady flight the vector sum of the lift on the horizontal stabiliser and the lift on the wing and fuselage passes through the center of gravity. If the center of lift and the center of gravity were not aligned vertically the aircraft would begin to tumble nose to tail.

In unsteady flight, such as pitching nose up or down in a maneuver, the net lift does not act through the center of gravity in order for the lift vector and the weight vector to generate the necessary pitching moment. However, any mismatch of lift and center of gravity for the purpose of conducting a maneuver is a very small distance and a brief period of time.

Similarly, if the thrust vector and drag vector are not aligned they will generate a small pitching moment. This pitching moment must be counteracted by an equal and opposite pitching moment generated by the net lift not passing exactly through the center of gravity.

The present text also states the center of lift is an important factor in the performance and stability of an aircraft. I disagree. The important parameters in assessing the stability of an aircraft are aerodynamic center and neutral point. The center of pressure on an airfoil is also useful for a preliminary understanding of aerodynamic center and longitudinal stability, but the usefulness of center of pressure is limited because it moves fore and aft, depending on angle of attack (or lift coefficient). See movement of center of pressure.

Many authors in the field of aircraft stability use the terms center of pressure and aerodynamic center, but I am not aware of any author who has used the term center of lift. The center of pressure and aerodynamic center are defined for an airfoil and are arbitrarily located on the chord line at the point where the lift vector intersects the chord line. A complete aircraft does not have a chord line so if there is a formal definition of center of lift I am puzzled as to where it is located vertically.

WP:Verifiability contains the policy statement that the threshold for inclusion in Wikipedia is verifiability, not truth. Unless someone can provide a citation to support the verifiablity of this article, I propose that it be deleted or again redirected to center of pressure. Dolphin51 (talk) 03:40, 26 June 2008 (UTC)

I kind of forgot about this project when I purged my watchlist. Just a quick google search shows that fiu.edu (an educational site connected to NASA) and How It Flies (which is an extensive print/online textbook about flying) both refer to the 'center of lift', and are both well-regarded. My college aerodynamics courses used the term CL, and I will hit my library the next time I'm at work to get citations from a couple of aerodynamics texts. ericg ✈ 00:58, 28 June 2008 (UTC)


 * Ok, quick update. Aerodynamics for Naval Aviators (NAVWEPS 00-80T-80, p 47-49) uses aerodynamic center to refer to the same concept. I'll save everybody the trouble and replace this page's contents with a redirect. ericg ✈ 01:03, 28 June 2008 (UTC)

The problem is that center of lift for the aircraft moves as angle of attack moves. Generally, airfoils are listed as having a lift vector at an arbitrary point, and torque around that point. This was decided perhaps a hundred years ago, simply to make wind tunnel testing easier, and to provide a consistent method of defining airfoils. Center of lift is a valid concept, but from an engineering standpoint, it is generally accepted practice to define the airfoil with variable lift and torque vectors rather than a moving center of lift. I've never seen the term used for the whole aircraft, since in steady flight it must be collated with the CG, or the aircraft will rotate one of its axis'.

It is an important concept, and is tied to stability. As the airspeed of a typical aircraft increases, the downward force on the tail increases, and the aircraft pitches up, and slows down. The CG must be ahead of the center of lift for the aircraft to have pitch stability, and for a pilot, the concept of center of lift is necessary to understand weight and balance and aircraft stability. I would argue that the concept is necessary for engineers, as well, but is more easily defined as lift and torque vectors. — Preceding unsigned comment added by 2600:1005:B11A:F695:4D3C:1E23:BEA8:79E8 (talk) 15:14, 30 December 2014 (UTC)
 * The expression center of lift is not in common usage. The correct expression is center of pressure. That is why Wikipedia does not have an article called Center of lift. If you search for that expression it merely re-directs you to Center of pressure (fluid mechanics).
 * The lift and drag on an airfoil, or on an aircraft, are not independent forces - they are the two components of the aerodynamic force. The aerodynamic force acts at the center of pressure; consequently both its components, lift and drag, must also act through the center of pressure. Center of lift, and center of drag, are not defined and I doubt you will find any published source that describes either of them.
 * You have written "The CG must be ahead of the center of lift for the aircraft to have pitch stability". That is a common misconception and it is incorrect. If an aircraft is to be longitudinally stable its CG must be ahead of the neutral point. The neutral point does not move during flight whereas, as you have written, the center of (pressure) does move during flight. (How could the CG be forward of a point that moves during flight? Are you suggesting the CG must also move during flight?)
 * Wikipedia has an article on Center of pressure. I recommend you have a look at it. Dolphin  ( t ) 05:28, 31 December 2014 (UTC)