FWD CG VS Controllability
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FWD CG VS Controllability
Just a simple question. How does FWD CG has an effect on the controllability of the airplane? I know that FWD CG would make the controllability worse. But how? Can anyone give me an answer based on the aerodynamic principle?
Thanks in advance!
Thanks in advance!
Moderator
Think of (static) stability and controllability as being two complementary considerations at either end of a teeter-totter .. as one goes UP the other goes DOWN. Both involve, amongst other things, consideration of stick forces in the pitching plane - ie pilot push/pull loads on the control yoke or column as the case may be - especially when considering the situation of maintaining the aeroplane in an away-from-trimmed state.
Stability involves higher stick loads and a greater resistance to pilot efforts to effect pitch change. It is concerned with ease of maintaining aircraft attitude. This is important to the civil airliner and military transport .. but not the military fighter.
Controllability involves lower stick loads and a lesser resistance to pilot efforts to effect pitch change. It is concerned with ease of changing aircraft attitude. This is of great importance to the fighter but adds a significant pilot workload to the crew of the transport or airliner. As an aside, the fighter pilot wants both controllability and, very importantly, the ability to track accurately (ie be able to point the aircraft where the pilot requires) so that weapons can be brought to bear effectively
As CG moves aft, stick forces to effect a change in pitch reduce. That is to say, it is "easier" for the pilot to make the aircraft change its pitch attitude by input to the control. We say that the stability is lower but the controllability is higher.
Conversely, as CG moves forward, stick forces to effect a change in pitch increase. That is to say, it is "harder" ("more difficult") for the pilot to make the aircraft change its pitch attitude by input to the control. We say that the stability is higher but the controllability is lower.
The aerodynamics behind the phenomenon involve consideration of the interrelationship of pitch attitude (angle of attack, if you like, or, more particularly, lift coefficient) and the resulting pitching moments (ie does the aircraft, of itself, want to pitch up or down) associated with a given attitude. For the aerodynamicist, it is necessary to investigate the contribution of the various lifting surfaces on an aeroplane to arrive at an overall picture of what is going on.
Considering CG variation and a simple overview, the main pitching moment contributions come from
(a) the wing, for which the relevant measure is the distance between the CG and aerodynamic centre - the latter being the CG position for which pitching moment is constant as the attitude varies. For an aeroplane to be nice to fly, the CG must be forward of the AC and the greater that distance, the greater the stability and less the controllability.
(b) the fuselage, for which the contribution generally is destabilising - ie not what we want to see but we have to put up with the effect and balance out the problem by clever wing and tailplane design and relative location of wing and tailplane.
(c) tailplane, for which the contribution is made to be quite stabilising - ie what we need for adequate levels of stability for satisfactory flight characteristics. The sums involve consideration of
(i) tailplane lift characteristics
(ii) wing/fuselage downwash characteristics which affect what airflow the tailplane sees in flight - flap configuration can have a significant effect on this.
(iii) distance of the tailplane from the wing (ie tailplane arm, if you prefer - a bit like weight and balance sums)
(iv) effects of engine thrust on the airflow which the tailplane sees
There are other considerations which can have a significant effect on stability/controllability. One which is important at high incidence (typically in the missed approach case) is the "normal" (ie at right angles to the airflow) force generated as the incoming airflow is deflected through the propeller disc or the jet nacelle lip plane.
If you need to get a more detailed feel than this simple explanation might provide, it will probably be necessary to get into the maths associated with stability and control analyses. These you can find in any of the standard engineering textbooks.
Main thing to keep in mind is that
(a) a more forward CG is good for stability, not so good for control
(b) a more aft CG is good for control, not so good for stability
Another matter to be aware of
(a) in general, an aeroplane which is both statically (ie considering stick forces and pitching moments) and dynamically (ie considering flight motion characteristics after a disturbance) stable (to a reasonable extent) is likely to be flyable
(b) if it is statically unstable but dynamically stable, it may be flyable but will require a high level knowledge of what is going on, a high level of specific training and skill to know how to fly it and what control inputs are required. This is the stuff of test pilots and is very high workload. For those of us in the normal real world, it doesn't bear thinking about too much.
(c) if both statically and dynamically unstable, a human pilot is not up to the task and such an aircraft will need a sophisticated AFCS to allow flight by direct computer control. This may be acceptable in the military world (where the crew has an alternative means of landing via the Martin Baker system) but is not the approach adopted in civil design.
Stability involves higher stick loads and a greater resistance to pilot efforts to effect pitch change. It is concerned with ease of maintaining aircraft attitude. This is important to the civil airliner and military transport .. but not the military fighter.
Controllability involves lower stick loads and a lesser resistance to pilot efforts to effect pitch change. It is concerned with ease of changing aircraft attitude. This is of great importance to the fighter but adds a significant pilot workload to the crew of the transport or airliner. As an aside, the fighter pilot wants both controllability and, very importantly, the ability to track accurately (ie be able to point the aircraft where the pilot requires) so that weapons can be brought to bear effectively
As CG moves aft, stick forces to effect a change in pitch reduce. That is to say, it is "easier" for the pilot to make the aircraft change its pitch attitude by input to the control. We say that the stability is lower but the controllability is higher.
Conversely, as CG moves forward, stick forces to effect a change in pitch increase. That is to say, it is "harder" ("more difficult") for the pilot to make the aircraft change its pitch attitude by input to the control. We say that the stability is higher but the controllability is lower.
The aerodynamics behind the phenomenon involve consideration of the interrelationship of pitch attitude (angle of attack, if you like, or, more particularly, lift coefficient) and the resulting pitching moments (ie does the aircraft, of itself, want to pitch up or down) associated with a given attitude. For the aerodynamicist, it is necessary to investigate the contribution of the various lifting surfaces on an aeroplane to arrive at an overall picture of what is going on.
Considering CG variation and a simple overview, the main pitching moment contributions come from
(a) the wing, for which the relevant measure is the distance between the CG and aerodynamic centre - the latter being the CG position for which pitching moment is constant as the attitude varies. For an aeroplane to be nice to fly, the CG must be forward of the AC and the greater that distance, the greater the stability and less the controllability.
(b) the fuselage, for which the contribution generally is destabilising - ie not what we want to see but we have to put up with the effect and balance out the problem by clever wing and tailplane design and relative location of wing and tailplane.
(c) tailplane, for which the contribution is made to be quite stabilising - ie what we need for adequate levels of stability for satisfactory flight characteristics. The sums involve consideration of
(i) tailplane lift characteristics
(ii) wing/fuselage downwash characteristics which affect what airflow the tailplane sees in flight - flap configuration can have a significant effect on this.
(iii) distance of the tailplane from the wing (ie tailplane arm, if you prefer - a bit like weight and balance sums)
(iv) effects of engine thrust on the airflow which the tailplane sees
There are other considerations which can have a significant effect on stability/controllability. One which is important at high incidence (typically in the missed approach case) is the "normal" (ie at right angles to the airflow) force generated as the incoming airflow is deflected through the propeller disc or the jet nacelle lip plane.
If you need to get a more detailed feel than this simple explanation might provide, it will probably be necessary to get into the maths associated with stability and control analyses. These you can find in any of the standard engineering textbooks.
Main thing to keep in mind is that
(a) a more forward CG is good for stability, not so good for control
(b) a more aft CG is good for control, not so good for stability
Another matter to be aware of
(a) in general, an aeroplane which is both statically (ie considering stick forces and pitching moments) and dynamically (ie considering flight motion characteristics after a disturbance) stable (to a reasonable extent) is likely to be flyable
(b) if it is statically unstable but dynamically stable, it may be flyable but will require a high level knowledge of what is going on, a high level of specific training and skill to know how to fly it and what control inputs are required. This is the stuff of test pilots and is very high workload. For those of us in the normal real world, it doesn't bear thinking about too much.
(c) if both statically and dynamically unstable, a human pilot is not up to the task and such an aircraft will need a sophisticated AFCS to allow flight by direct computer control. This may be acceptable in the military world (where the crew has an alternative means of landing via the Martin Baker system) but is not the approach adopted in civil design.
