Ideal c.g location for an aircraft

ROHAN · September 10, 2015, 07:30

hi,
I understand that stability can be achieved for any position of cg, by adjusting the tail sizing, and cg ahead of ac is more stable considering the case of engine failure but is there any advantage of having cg little aft of the wing a.c or somewhere in between the wing and tail?

Thanks.

fluid23 · September 10, 2015, 17:08

Having CG ahead of AC isn't done in case of engine failure. It has more to do with the pitch response to longitudinal disturbances. Having CG forward of AC causes the aircraft to have a nose down response to an increase in lift caused by a longitudinal gust. So consider an aircraft flying near stall (perhaps landing) and a gust comes along and causes the lift to increase on the wing. If the CG is aft of the AC then this will push the plane into a stall by causing a nose up moment. What we want to happen in that situation is for the nose to go down, reducing lift and corrective moment.

You can get away with CG aft of the AC, but this creates a negative static margin. This requires massive amounts of attention from the pilot or control laws/system to help compensate. This means that your tail is also going to have to produce a lift force (rather than a down force) to keep you pointed forward.

In either case, having the CG and AC far apart is a very bad idea. The further these are separated, the greater the control forces needed. This also means bigger tail, bigger deflections, higher weight and so on.... you can see why that wouldn't make sense just based on our desire to keep aircraft weight to a minimum.

https://en.wikipedia.org/wiki/Longit...atic_stability

fluid23 · September 10, 2015, 17:16

If I recall, you typically want to keep AC and CG separated by 5%-10% of your root chord. I could be mis-remembering that however...

September 10, 2015, 07:30	Ideal c.g location for an aircraft	#1
ROHAN New Member ROHAN Join Date: Oct 2013 Posts: 4 Rep Power: 13	hi, I understand that stability can be achieved for any position of cg, by adjusting the tail sizing, and cg ahead of ac is more stable considering the case of engine failure but is there any advantage of having cg little aft of the wing a.c or somewhere in between the wing and tail? Thanks.

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September 10, 2015, 17:08		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	Having CG ahead of AC isn't done in case of engine failure. It has more to do with the pitch response to longitudinal disturbances. Having CG forward of AC causes the aircraft to have a nose down response to an increase in lift caused by a longitudinal gust. So consider an aircraft flying near stall (perhaps landing) and a gust comes along and causes the lift to increase on the wing. If the CG is aft of the AC then this will push the plane into a stall by causing a nose up moment. What we want to happen in that situation is for the nose to go down, reducing lift and corrective moment. You can get away with CG aft of the AC, but this creates a negative static margin. This requires massive amounts of attention from the pilot or control laws/system to help compensate. This means that your tail is also going to have to produce a lift force (rather than a down force) to keep you pointed forward. In either case, having the CG and AC far apart is a very bad idea. The further these are separated, the greater the control forces needed. This also means bigger tail, bigger deflections, higher weight and so on.... you can see why that wouldn't make sense just based on our desire to keep aircraft weight to a minimum. https://en.wikipedia.org/wiki/Longit...atic_stability

September 10, 2015, 17:16		#3
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	If I recall, you typically want to keep AC and CG separated by 5%-10% of your root chord. I could be mis-remembering that however...