jetcareers - View Single Post

ananoman · October 6th, 2004, 21:39

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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.

October 6th, 2004, 21:39	#2
ananoman Senior Member Join Date: Dec 2002 Posts: 916	Re: Controllability & CG [ QUOTE ] 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. [/ QUOTE ] 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) [ QUOTE ] Why are the flight controls easier to manipulate with an aft CG? [/ QUOTE ] 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.) [ QUOTE ] Is the airplane more or less controllable with an aft CG? [/ QUOTE ] 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.