Controllability & CG

FlyboyZR1

New Member
I don't quite understand what happens to the controllability of an airplane as the C.G. moves fore/aft.

With an aft CG it's harder to recover from a stall/spin, so it seems like the airplane would be less controllable, BUT with an aft CG the flight controls are easier to manipulate and it's easier to overstress the airplane.

Why are the flight controls easier to manipulate with an aft CG?

Is the airplane more or less controllable with an aft CG?
 
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With an aft CG it's harder to recover from a stall/spin, so it seems like the airplane would be less controllable, BUT with an aft CG the flight controls are easier to manipulate and it's easier to overstress the airplane.

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Most aircraft that we fly are built so that the center of gravity is always in front of the 'center of lift'. This 'nose heavy' situation is balanced by tail down force generated by the horizontal stabilizer.

This is a stable situation. Think about a throwing dart: if you remove the weight at the front, the dart is very unstable and will likely swap ends if you try to throw it at anything.

This is why it can be impossible to recover from a stall spin in an extreme aft CG situation. If the center of lift on the wing is in front of of the CG, the aircraft will try to pitch up. Since the wing is obviously able to make more 'lift' than the tail, you the pilot will not be able to recover from the stall. (In normal flight the tail has a negative angle of attack and provides 'tail down force'. During a stall recovery when you push foreward on the yoke, you are causing the tail to have a positive angle of attack and create 'lift', hopefully forcing the tail up. If you are not loaded to an extreme aft CG, the nose to drop and allow you to recover from the stall)
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Why are the flight controls easier to manipulate with an aft CG?

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The flight controls are easy to move for the same reason. The less 'nose heavy' the aircraft is, the less force the pilot has to exert on the controls to cause a pitch change. If you put a heavy person on a teeter-totter (do play grounds still have these?) and try to lift them in the air by pushing down on the other end, it will be difficult. If you have them sit closer to the pivot, your task will be much easier since they have less leverage that you must overcome. (In this example think of the heavy person as the 'CG', the pivot as the 'center of lift' and you pushing down as the 'tail down force' generated by the horizontal stabilizer.)
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Is the airplane more or less controllable with an aft CG?

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Be careful what you ask. I am too lazy to look in the Pilots Handbook of Aeronautical Knowledge to see how the FAA defines controlability, so here goes. The airplane is more 'controllable' (maneuverable) with an aft CG as long as you are loaded within limits. The aircraft will be easier to control since the pitch forces will be lighter and the horizontal stabilizer will have more leverage. This is why modern fighter aircraft are 'fly by wire', they are neutrally stable with the CG very close to the center of lift. This makes it very easy for them to change direction. The only problem is if the computer quits, the airplane is uncontrollable because it is so unstable. If you load your aircraft beyond the aft limit, your aircraft will become uncontrollable as well.

The foreward CG is much more stable (think throwing dart again) and will be less work for the pilot.

Keep in mind that as long as you stay within limits, your aircraft will be fine.
 
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I don't quite understand what happens to the controllability of an airplane as the C.G. moves fore/aft.

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Is the airplane more or less controllable with an aft CG?

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Less. One of the problems with understanding is that the terms, stability, maneuverability, and controllability =sound= like English, but mean something different in aviation-speak.

haven't pulled out this personal FAQ in a long time, but FWIW, here goes.

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Stability, Maneuverability, Controlability

I just know I'm going to get in trouble with this one. I once got completely taken apart, chewed up and spit out for even suggesting some of the examples:

Start with definitions. Like any field that has it's own language ("terms of art"), there are common words that have very specific aviation meanings. In aviation terms,

stability: the tendency to return to a desired condition when disturbed
controllability: how easily the body responds to control inputs. (Not how easily it is to maintain control)
maneuverability: the ability of the body to withstand the stresses of being pushed around. (Not how easy it is to maneuver something around)

I mention these because these are not necessarily the way these words are used outside aviation. For example, something can be highly "controllable" in the aviation sense because it responds too easily to control movements. If it is also unstable, though, it will be very, very difficult to "control" in the lay sense.

I sometimes use a skiing analogy. Let's stick to the flat blue runs. There is an ideal part of the ski for your weight to be centered. Essentially, it's the CG that gives you the best balance between the ability to turn (controllability) and yet prevents you from losing control completely as the skis fly off on their own (stability).

Sit too far back on your skis and the skis become very "controllable" in the aviation sense. The smallest movement of your foot will make the ski fly of in another direction. Unfortunately, the ski's aft CG also makes it very, very unstable. Not only does the slightest foot movement send the ski in a new direction, but the instability magnifies the movement.

Move your weight forward, and the ski becomes very stable. Put it on track and it will tend to stay there. But have your weight too far forward, and it suddenly becomes more difficult to turn.

Not a very aerodynamic explanation and unlike an airplane, skiers can constantly shift their CG to handle changing conditions. But a useful analogy for some.
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Can you apply this to your questions?
 
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Be careful what you ask. I am too lazy to look in the Pilots Handbook of Aeronautical Knowledge to see how the FAA defines controlability, so here goes. The airplane is more 'controllable' (maneuverable) with an aft CG as long as you are loaded within limits.

[/ QUOTE ]Great explanation unitl here. It's less controlable precisely because of the way the FAA defines "comtrollability". Not ease of maintaining control, but

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CONTROLLABILITY — A measure of the response of an aircraft relative to the pilot’s flight control inputs. [Pilot's Handbook of Aeronautical Knowledge]
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And this wonderful explanation from Aerodynamics for Naval Aviators
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The term "controllability" refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. Adequate controllability must be available to perform takeoff and landing and accomplish the various maneuvers in flight. An important contradiction exists between stability and controllability since adequate controllability does not necessarily exist with adequate stability. In fact, a high degree of stability tends to reduce the controllability of the aircraft.
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controllability: how easily the body responds to control inputs. (Not how easily it is to maintain control)

CONTROLLABILITY - A measure of the response of an aircraft relative to the pilot?s flight control inputs. [Pilot's Handbook of Aeronautical Knowledge]

And this wonderful explanation from Aerodynamics for Naval Aviators
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The term "controllability" refers to the ability of the aircraft to respond to control surface displacement and achieve the desired condition of flight. Adequate controllability must be available to perform takeoff and landing and accomplish the various maneuvers in flight. An important contradiction exists between stability and controllability since adequate controllability does not necessarily exist with adequate stability. In fact, a high degree of stability tends to reduce the controllability of the aircraft.
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Based on these definitions, an aircraft with an aft CG is more controllable than one loaded to a foreward CG, as long as both are within certain limits (Do not think certification limits here, think about the 'absolute limit', or what the aircraft is capable of). An aircraft loaded to it's aft certification limit is more 'controllable' than one loaded to it's foreward certified limit because it takes less pilot input to generate pitch changes. (Read my above post regarding the teeter-totter example.) Even as the aft certified limit is exceeded, the aircraft's controllability will continue to increase as the CG approaches the center of lift. As the center of lift and the CG merge, the aircraft may become so unstable that flight is not possible. In extreme examples you can fly a conventional aircraft that is loaded so that the center of lift is infront of the CG, but for our purposes I think it is safe to say that the 'absolute aft limit' is reached when the center of lift and the CG merge.

If the absolute aft limit is exceeded, then the aircraft may pitch up and stall on takeoff. If takeoff is successful, the aircraft will be prone to an unrecoverable stall/spin.

We have all probably heard of people taking off and only being able to fly level by holding full foreward on the controls, this is an example of the 'absolute' limit being exceeded. It would be impossible to recover from a stall in this situation. If the CG were only a little farther aft, the pilot would probably have been killed on takoff due to an over rotation and unrecoverable stall.

If the foreward absolute limit is exceeded then there may not be enough elevator authority to rotate the aircraft for takeoff or to flare for landing. As the above definitions note, in this situation the aircraft is very 'stable', but unfortunately it is not 'controllable'.
 
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