Around which point does an aircraft rotate during the landing flare?

[Stefan428]

TL;DR Summary

When an aircraft is coming in to land and starts the landing flare, around which point does it rotate?

Without giving possible options, as to not lead people in a wrong direction perhaps, I have a question I am struggling with. Mostly because I get different answers from different people 🤔

When an aircraft is coming into land and starts the landing flare, around which point does it rotate? It would be of great help if you can also explain why :-)

 

[Lnewqban]

Welcome, Stefan428!

The airplane normally rotates around is center of mass, through which three imaginary axis of rotation pass: pitch, roll and yaw.
During flare, the steady descending trajectory of the center of mass experiments a negative acceleration due to increased lift and drag, slowing down the velocity with which the landing gear touches down.

Please, see:
https://www.grc.nasa.gov/www/k-12/airplane/rotations.html

 

[Stefan428]

Thanks @Lnewqban
From that, would you conclude that the following sentence is false:
"During the flare, an aircraft will change its pitch attitude around the Center of Pressure (Center of Lift)."

 

[anorlunda]

Thanks @Lnewqban
From that, would you conclude that the following sentence is false:
"During the flare, an aircraft will change its pitch attitude around the Center of Pressure (Center of Lift)."

I don't see the two statements as conflicting. Any rotation is likely to change pitch relative to the center of gravity and the center of lift.

Changing pitch does not identify the center of rotation.

 

[Stefan428]

Fair point @anorlunda
This is how I understand it though, and I am looking for somebody to prove me that I am wrong (really, I want to be wrong, because only then something makes sense :-) )

The Center of Gravity (CG) is basically the center point through which the mass of the aircraft acts.
The Center of Lift (CoL) is the center point through which the lift of a wing acts or, if we consider the whole aircraft, all lift generating surfaces together, which seems to be most commonly called the Center of Pressure (CoP).

During unaccelerated level flight, the CoP will be at the same point as the CG, otherwise it would creating a pitching moment with the a/c pitching up or down and the speed at which that happens would depend on the arm. The lift generated by the horizontal stabilizer will be either positive or negative, depending on whether or not the CoP is forward of the CG (upward force required from the tail) or aft of the CG (downward force required from the tail).

As angle of attack changes like it does during the flare, the CoP also changes and lift will increase or decrease. However, the CG stays the same (assuming we don't consider the aircrafts fuel usage).
The aircraft will now rotate around the CG with the movement of the CoP being the reason it does so.

So please tell me, where do I go wrong?? Or is it correct and would that sentence be wrong (that I posted earlier) that says the aircraft rotates around the center of pressure?

 

[FactChecker]

A rotating airplane is rotating around all points -- the CG, the aerodynamic center, the location of its gyroscopes, etc. In studying the aerodynamic consequences of the rotation, it is convenient to consider how it is rotating around the aerodynamic center. In studying other consequences of the rotation, such as the inertial guidance system, it is convenient to consider how it is rotating around the gyroscope location.

 

[Lnewqban]

For pitch stability, the CG and CP must keep certain distance.
The moment on the vertical plane that that distance and the two forces create is compensated by the opposite moment created by the elevator.
For flaring, the that elevator moment is increased, inducing the rotation and the AOA increase.

Please, see:
https://en.m.wikipedia.org/wiki/Longitudinal_static_stability