CG effect on maneuverability

[GustavoMedeiros]

Greetings everyone!

I have a question about the effects of CG position on stability and maneuverability.

I understand that when we have a fwd CG we'd get more stability/higher stick forces to change the acfts attitude. Otherwise, with an aft CG, we'd get better maneuverabiliity and by consequence less stability.

BUUUT, when it comes to engine inop. procedures and we consider controlling the acft around it's vertical axis (yaw), it would be EASIER to control with an fwd CG. The question is: why is it so? I understand the forward CG is more stable, but at the same time, isn't it true that a aft CG would give me lighter forces on the controls? Heard some fellows talk about the arm size from the control surface (rudder) to the CG, what would make things easier for me. But at the same time, why doesn't it stand this way when we consider the elevator surface? :ugh::ugh:

Sorry for writing the New Testament in a question! Thank you.

[Genghis the Engineer]

At forward CG you have increased longitudinal static stability gradients, and generally speaking an increased directional static stability gradient and margin also.

Further aft, the reduction in LSS gradient as the CG approaches the neutral point, so the elevator displacements to give a required airspeed or Nz change will also reduce. One impact on this, particularly with an old fashioned reversible controlled aeroplane is reduced stick force per g. That is termed "manoeuver stability", but to equate it directly to manoeuverability is overly simplistic

Directionally, things are a bit more complex. With a further aft CG, the directional static stability reduces. This will tend to have no advantage for manoeuverability, it will have impact on other things. The aeroplane has a reduced correcting yawing moment in the event of sideslip. The impact of that is greater need for active rudder input to ensure zero sideslip in turns, but it is easier to fly deliberate sideslip if you want it. A further and undesirable secondary effect of this is that the wing drop at the stall tends also to be greater (you can take my word for that, but I doubt you'll find it in any ATPL textbooks).

If you want to go into this in more depth than the acceptable, but admittedly superficial treatment of most ATPL courses, Anderson's "Introduction to Flight" is the best and most accessible book widely used at present in university undergraduate courses (and vastly more accessible than Houghton's "Aerodynamics for Engineering Students" that I learned from a quarter century ago.)

https://www.amazon.com/Introduction-Flight-John-Anderson/dp/0078027675/ref=pd_sbs_14_t_0?_encoding=UTF8&psc=1&refRID=2WS17THJZ0KG09FHMHN4

[eckhard]

Yes, what Genghis said!

Maybe you are confusing 'manoeuvrability' with 'controllability'.

In this context, I think of 'manoeuvrability' as 'rate of yaw' (or pitch or roll) and 'controllability' as 'control deflection required'.

With an engine failure, you are not trying to manoeuvre around the yaw axis, you are trying to control (prevent) yaw.

In other words, you are not concerned with the achievable rate of yaw, but with the rudder deflection required to prevent yaw.

The forward CG gives a longer arm on which the rudder can act, so less rudder deflection will be needed to provide the required force to oppose the yaw when compared to that required with an aft CG.

Less rudder deflection means lighter foot-forces, so this is described as being 'easier'.

Less rudder deflection also means a lower Vmc, which is normally a good thing.

As you can tell, I am only a line-pilot and not a flight-test engineer or test-pilot!

[GustavoMedeiros]

Thank you guys very much!

eckhard, first of all thank you. But, the way you described it was exactly the way I used to think about it...until I started reading Handling the Big Jets, one week ago. It describes the forces on the controls for Aft CG as: EASIER AND LIGHTER. Fwd CG: Srong control forces. Thats what bothers me. This book is a great reference, at least inside my country (Brazil). Maybe I'm overrating it's content, maybe not.

The way Genghis described made me understand a bit more about the stuff. With a fwd CG I may say: ok, it's more easier to fly with an engine failure because of it's stability. The engine still operating will find it difficult to move the aircraft because of its CG position. But, if my job is to put the aircraft into a steep turn, let''s go with an aft CG. That's what I got.