Does the CG envelope ever go AFT of the Center of Pressure?
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ananoman
New Member
If it does, you will be in for an interesting ride.
You can think of an aircraft as a teeter-totter with the center of pressure being the pivot point, it is what supports the aircraft. The CG is on one side of the center of pressure and the other side is controlled by how much tail down force the horizontal stabilizer is creating. So, in a normal situation, the CG is always trying to pull the nose down, and the tail is creating a down force to balance this and keep the pitch attitude you desire.
If you load the CG too far foreward, you may not be able to rotate to takeoff at the normal speed. If you have a short runway, you might go off the end before you get going fast enough to have enough elevator authority to get the nose up. Or, on final approach you might run out of elevator and have the nose pitch down, making you into a lawn dart.
Aft CG is easier to achieve in most airplanes and is even more dangerous. As the CG moves closer to the center of pressure, the airplane will get more unstable. Think of a throwing dart with the weight in the middle instead of the nose. If the CG gets behind the center of pressure, the nose of the aircraft will try to pitch up uncontrollably. If you are lucky you will have enough elevator authority to stop the pitch up and be able to maintain control. You may have heard stories of this happening, when someone overloads an aircraft, or improperly loads cargo. Sometimes the pilots are lucky. Many times they are not.
You can think of an aircraft as a teeter-totter with the center of pressure being the pivot point, it is what supports the aircraft. The CG is on one side of the center of pressure and the other side is controlled by how much tail down force the horizontal stabilizer is creating. So, in a normal situation, the CG is always trying to pull the nose down, and the tail is creating a down force to balance this and keep the pitch attitude you desire.
If you load the CG too far foreward, you may not be able to rotate to takeoff at the normal speed. If you have a short runway, you might go off the end before you get going fast enough to have enough elevator authority to get the nose up. Or, on final approach you might run out of elevator and have the nose pitch down, making you into a lawn dart.
Aft CG is easier to achieve in most airplanes and is even more dangerous. As the CG moves closer to the center of pressure, the airplane will get more unstable. Think of a throwing dart with the weight in the middle instead of the nose. If the CG gets behind the center of pressure, the nose of the aircraft will try to pitch up uncontrollably. If you are lucky you will have enough elevator authority to stop the pitch up and be able to maintain control. You may have heard stories of this happening, when someone overloads an aircraft, or improperly loads cargo. Sometimes the pilots are lucky. Many times they are not.
ananoman
New Member
This is a link to an accident that made big news a few years ago caused by an aft CG on takeoff: http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20010907X01905&ntsbno=MIA01RA225&akey=1
meritflyer
Well-Known Member
As mentioned it would make for a very interesting aircraft design and ride. The CG keeps the nose heavy as previously stated. Its then balanced by a specific amount of tail down force. Now, imagine the CP ahead of the CG. This would theoretically cause a nose up moment instead of down and eliminate the need for tail down force. You'd require tail up force to keep the nose down if the CP was ahead of the CG.
The CG position as relative to the CP position is mostly design factor. A manufacturer builds in CG/CP position to ensure a nose heavy - for stalls, turbulence, and load factors - along with stability. Remember longitudinal stability is primarily dependent of the CG/CP location and arm between the two. An airplane requiring greater tail down force as found with a long CG/CP arm tends to be more stable. At the same time, if there were an excessive amount of tail down force our stalling speed goes up as we are close to CLmax, our cruise is slower due to increase induced drag, our takeoff roll can be severly impacted, our maneuverability is decreased due to the amount of stabilator already being applied just to maintain level flight (reduced authority), and last but not least, our aircrafts climb performance is decreased.
So as mentioned, the position of the CP/CG is mostly a design attribute. Keep in mind that pilots have the ability with loading to alter or decrease the margin of safety though by improperly calculating W/B which can often be deadly.
The CG position as relative to the CP position is mostly design factor. A manufacturer builds in CG/CP position to ensure a nose heavy - for stalls, turbulence, and load factors - along with stability. Remember longitudinal stability is primarily dependent of the CG/CP location and arm between the two. An airplane requiring greater tail down force as found with a long CG/CP arm tends to be more stable. At the same time, if there were an excessive amount of tail down force our stalling speed goes up as we are close to CLmax, our cruise is slower due to increase induced drag, our takeoff roll can be severly impacted, our maneuverability is decreased due to the amount of stabilator already being applied just to maintain level flight (reduced authority), and last but not least, our aircrafts climb performance is decreased.
So as mentioned, the position of the CP/CG is mostly a design attribute. Keep in mind that pilots have the ability with loading to alter or decrease the margin of safety though by improperly calculating W/B which can often be deadly.
ananoman
New Member
Well I guess you can call them interesting. Canard aircraft have two lifting surfaces, the main wing and the canard, with the CG located somewhat in front of the wing's center of pressure.meritflyer said:
The Avanti is another unconventional design, with 3 lifting surfaces, although it is not a true canard aircraft. Looking at one, the CG is probably in front of the wing's center of pressure, although I could be wrong. Cruising at 390kts in a turboprop is pretty impressive though.
meritflyer
Well-Known Member
ananoman said:
It could be. It would make sense as when an aircraft such as the Piaggio. Conventional design is CG ---- CP though.
sharkey
kind of a big deal
ananoman said:
Correct me if I'm wrong but Martha King and my 8th grade physics tells me that everything, aircraft included, will rotate about its CG not the CP. The CP will make an upward force that needs to be balanced by the elevator's downward force. More like a baseball bat than a teeter-totter. Everything else I agree on 100%
I heard this exact explination from my CFI instructor and I took a little exception to it then and I still do since I never got a good enough explination from him to convince me.
meritflyer
Well-Known Member
Yep. The aircraft pivots around the CG not the CP. The CP is a balancing force or fulcrum for the airplane.
seagull
Well-Known Member
Canards still will have the CG forward of the CP. In fact, in those designs, it would be impossible to NOT have it that way!
I got to fly an aircraft with the CG moved (via computer) aft of the CP. The computer allows for programming the flight controls so it would handle as if the CG was moved to different locations (among other things). Pretty interesting, definitley requires anticipatory control inputs. See the PIO article in the Safety Section on the Home Page of this website for some issues that you might find with this stuff.
I got to fly an aircraft with the CG moved (via computer) aft of the CP. The computer allows for programming the flight controls so it would handle as if the CG was moved to different locations (among other things). Pretty interesting, definitley requires anticipatory control inputs. See the PIO article in the Safety Section on the Home Page of this website for some issues that you might find with this stuff.
fish314
Well-Known Member
meritflyer said:
This is exactly right. The reason you may have trouble picturing this is because you are thinking about a see-saw as the example of your lever. Think instead about a wheelbarrow. In a wheelbarrow the fulcrum (pivot point) is at one end and there are two forces in opposite directions on one side of the pivot point. In a wheelbarrow, the pivot point is the wheel, and the load (whatever you are hauling) provides a down force that is balanced by your hands providing the up force.
In an airplane, the same thing happens. The Cg is the pivot point, and aft of it is the Cp, providing an up force. Even further aft of that is the tail, providing a down force to balance the Cp. If you moved the Cg aft until it was aft of the Cp, the Cp's up force would tend to raise the nose and the tail's nose down force would tend to raise the nose also. The aircraft would be virtually uncontrollable.
In a canard aircraft, the see-saw analogy works better. Both forces are in the same direction (UP) with the Cg (pivot point) in between.
sharkey
kind of a big deal
I'm with you on the wheel barrow. Maybe more correctly: an upside down wheel barrow would have the fulcrum/pivot point, up force and down force in that order. Thats a lot better than the baseball bat I came up with. But the CP is not the fulcrum. The fulcrum is the CG the CP is the up force.
seagull
Well-Known Member
Not sure that the wheel barrow is that good of an analogy, because the design forces it to rotate about a point that is NOT the CG, which really confuses things. I follow what you are trying to do, but the pivot point is mucked up.
Trying to think of an example that would show it. A model airplane on a string, with it balanced out with the stab gets close, but the string, being a fixed length, forces rotation at, what would be, the CP. In reality, if you rotate a model on a string, the "lift" force is "adjusting" to force rotation where the string attaches, but you can't see it (the string can provide what we can consider, in this example, essentially infinite lift, and that force increases, or decreases, as the model rotates in pitch).
Perhaps a model suspended by a rubber band could make it work, depending on the size of that band! The rubber band would allow the CP to move the aircraft vertically to keep the rotation about the CG.
The problem with trying to use ground based analogies is they usually don't work. Same problem occurs when people use the idea of the pendulum effect to explain dihedral effect in high wing aircraft. Flat out wrong, but seems intuitive. Just doesn't work, in reality.
Trying to think of an example that would show it. A model airplane on a string, with it balanced out with the stab gets close, but the string, being a fixed length, forces rotation at, what would be, the CP. In reality, if you rotate a model on a string, the "lift" force is "adjusting" to force rotation where the string attaches, but you can't see it (the string can provide what we can consider, in this example, essentially infinite lift, and that force increases, or decreases, as the model rotates in pitch).
Perhaps a model suspended by a rubber band could make it work, depending on the size of that band! The rubber band would allow the CP to move the aircraft vertically to keep the rotation about the CG.
The problem with trying to use ground based analogies is they usually don't work. Same problem occurs when people use the idea of the pendulum effect to explain dihedral effect in high wing aircraft. Flat out wrong, but seems intuitive. Just doesn't work, in reality.