Dear Bob and Richard,

Does a change in IAS during level flight affect longitudinal stability?
For example, if I decrease IAS from cruise speed to speed for minimum power while maintaining level flight, does my pitch stability change?

Cheers, rubber

Interesting question, I had the same thought when studying aerodynamics aswell.

My thinking was along the lines that in normal cruise, the arm (horizontal distance) between the elevator and CG would be at a maximum, and when at lower speeds and higher AOA the effective horizontal distance between the elevator and CG would be lower. But I am not sure if it is to be thought of in this way.

One thing though, flying at a lower speed (especially near stall speed), their would be controllability issues, so I would imagine that stability would definitely be influenced in one way or another.

But I could be totally wrong so would be interesting to see other people's thoughts.

Things to remember: CofG position doesn't change with angle of attack (unless you have things rolling around in the cabin ). The position of the centre of pressure does change though and this will affect the pitching moment generated between the CofG and Centre of Pressure.

If flying level at changing IAS the following occurs:

1) Slower flight:
You will have a greater angle of attack so the centre of pressure moves forward which will increase the nose-up pitching moment if the CofG is aft of the Centre of Pressure. If the CofG is ahead of the Centre of Pressure, the increased angle of attack will actually reduce the nose down pitching moment. In both cases the pitching moment needs to be overcome by an elevator which is less effective because of the slower airflow over the control surface.

2) Faster flight:
You will have a smaller angle of attack so the centre of pressure moves aft. If the CofG is aft of the centre of pressure, the nose-up pitching moment will decrease. If the CofG is forward of the Centre of Pressure the nose down pitching moment will increase. Again, the pitching moment will be overcome with elevator which is now more effective due to the higher speed airflow over the control surface.

Cheers,

Rich

Thanks Richard, as usual, clarity is your strong point!

Just to be sure I understand...

Even if I don't know the location of the CoG in relation to the moving CoP, I can confidently say that my longitudinal stability decreases as I slow down due to the decreasing elevator authority.

Therefore ANY aircraft that slows down has less longitudinal stability.

Right?

rubber

I think that would be a fair enough assumption and I would go so far as to say that directional and lateral stability will suffer with the lower airspeed as well since the aircraft will take more significant control inputs to maintain control.

Cheers,

Rich