Cg Effects

[christina3hunt]

Could anyone give me a simplified straight to the point discussion on the difference between a forward CG and an aft CG as they pertain to stability, stall speed, and performance. Not just what happens, but why??

I understand these concepts, but I am having a hard time explaining them...:crazy:

 

[exleardriver]

taylor..? lol. it's late..i'm tired..and just don't wanna risk being wrong..not on this forum..

 

[ScottG]

Aft CG - The tail of the aircraft needs to create a slight downward force in straight and level flight because of the weight up front (think see-saw). When you load further back you decrease the amount of "negative lift" the tail needs to create because the weight is doing it for you. This reduces the load on the wings and they don't have to do as much work to counteract this "negative lift". This enables you to fly at an angle of attack closer to 0, (flatter wings). When you start at a lower angle of attack you have further to go before you reach the critical angle of attack, say 15 degrees and stall the aircraft. 0-15 degrees is harder to stall than 5-15, so a lower angle of attack gives you a lower stall speed. However with all the weight at the back of the plane pushing down you will have a much harder time lowering to nose to recover if you did stall. Also when you are closer to a 0 degrees angle of attack you have less drag as a horizontal wing moves easier than a vertical one so then you have higher performance ie. faster and more fuel efficient.

Forward CG is exact opposite.....low performance due to high tail down pressure being compensated for by the wings creating more lift and thus drag. Higher stall speed because of a higher starting angle of attack due to more lift needed. BUT if you stall rest assured the nose will fall very easily as the weight is already up front. Making recovery easier\possible.

I missed stability but its late and my answer will hopefully be understandable.

 

[CoffeeIcePapers]

Think of the horizontal stab. as a lever. The longer the lever, the more force you get (leverage). An aft CG shortens the moment-arm (leverage) that the horizontal stab has, so it decreases the effectiveness of the horizontal in stall recovery. Also, the force of the aft cg is going to naturally increase the angle of attack, working against stall recovery.

Here is a picture that explains stability:

The forward CG will counteract a pitch up motion, as the picture shows. It will also counteract a pitch down motion, as more air will be flowing over the lifting surfaces, which will help correct the pitch down motion.

 

[Pilot Hopeful]

Think of the horizontal stab. as a lever. The longer the lever, the more force you get (leverage). An aft CG shortens the moment-arm (leverage) that the horizontal stab has, so it decreases the effectiveness of the horizontal in stall recovery. Also, the force of the aft cg is going to naturally increase the angle of attack, working against stall recovery.

Here is a picture that explains stability:

The forward CG will counteract a pitch up motion, as the picture shows. It will also counteract a pitch down motion, as more air will be flowing over the lifting surfaces, which will help correct the pitch down motion.

BUT . . .

When the CG moves to its aft limit, it does not necessarily have to move behind the center of lift. Aft CG movement promotes a LESS stable aircraft, not necessarily an UNstable aircraft.

ALSO . . .

An aircraft with a forward CG must produce more lift in steady, unaccelerated flight than an aircraft with an aft CG for reasons noted previously. A aircraft producing more lift looks very much like an aircraft that weighs more. Remember, stall speed and weight are directly related: heavier aircraft stall at higher speeds than lighter aircraft in the same conditions. Conversely, when CG moves aft, total lift can decrease as required tail downforce decreases. The aircraft appears to weigh less, and stall speed decreases.

Similar arguments may be made for aircraft stability. A loaded (heavy) aircraft tends to be more stable than the same aircraft when empty. Because a forward CG mimics heavier weights, such an aircraft is more stable than one with an aft CG.

 

[tgrayson]

pertain to stability, stall speed, and performance. Not just what happens, but why??

Scott G already described the effects on stall and performance.

For stability, the key relationship is the distiance between the CG and the aerodynamic center (often referred to in pilot literature as the "center of lift"), but we'll call the AC.

When an aircraft pitches, it pitches around the CG, so it really doesn't make sense to talk about the CG location forcing the nose down or the tail down...that implies the existence of some other pivot point, and there isn't one.

The nose down pitching (AOA change, really) is produced by the changes in lift; an increase in lift with a forward CG provides a strong nose down pitching via the lever arm principle. For a rear CG, an increase in lift provides a nose up tendency. This latter is bad, because the nose up produces even more lift, which produces even more nose up, which produces more lift, etc, until the aircraft stalls.

So as the CG moves from front to rear, the aircraft changes from fighting your AOA changes to helping you far to much.

 

[tgrayson]

A loaded (heavy) aircraft tends to be more stable than the same aircraft when empty.

Why do you think so?. The CG is the pivot point, so it doesn't matter how much the aircraft weighs. Once the airplane is built, the amount of stability is controlled only by the distance between the AC and the CG. Since the AC doesn't move, the CG location is the controlling factor.

Now, a heavier airplane isn't as succeptible to turbulence, but that isn't longitudinal stability.

 

[Pilot Hopeful]

Why do you think so?. The CG is the pivot point, so it doesn't matter how much the aircraft weighs. Once the airplane is built, the amount of stability is controlled only by the distance between the AC and the CG. Since the AC doesn't move, the CG location is the controlling factor.

Now, a heavier airplane isn't as succeptible to turbulence, but that isn't longitudinal stability.

Correct.

To clarify, just because an aircraft weighs more does not mean it will inherently be more stable. If CG is constant . . .
(1) a heavier aircraft will be more stable---less responsive to turbulence and other disturbances---than a lightly loaded aircraft; and
(2) a heavier aircraft will have the same measure of longitudinal stability (i.e., tendency to return to predisturbed conditions) as its lightly loaded counterpart.

No?

 

[tgrayson]

To clarify, just because an aircraft weighs... No?

Yes, using your definitions. But I wouldn't recommend using the word "stable" the way you did when talking about a heavier aircraft, particuarly when we're talking about a different sort of stability. "Stable" connotes returning to a pre-existing condition; resistance to change alone isn't stability.

 

[ScottG]

Yes, using your definitions. But I wouldn't recommend using the word "stable" the way you did when talking about a heavier aircraft, particuarly when we're talking about a different sort of stability. "Stable" connotes returning to a pre-existing condition; resistance to change alone isn't stability.

Bingo, an inherit difference between positive and negative "return to a pre-existing condition" stability and Newton's mass resisting change\inertia stability. Takes A LOT to move an airliner, that may be called stable, but has zero to do with CG and more to do with inertia. Now a forward CG will create said positive "return to pre-existing condition" stability, regardless or weight.....unless you are falling from the sky

 

[kevmor99]

The nose down pitching (AOA change, really) is produced by the changes in lift; an increase in lift with a forward CG provides a strong nose down pitching via the lever arm principle. For a rear CG, an increase in lift provides a nose up tendency. This latter is bad, because the nose up produces even more lift, which produces even more nose up, which produces more lift, etc, until the aircraft stalls.

So as the CG moves from front to rear, the aircraft changes from fighting your AOA changes to helping you far to much.

I'm having a hard time understanding this part. A book I have says the AoA is increased on the tail when the nose pitches down, causing it to nose up with a forward CG? Wouldn't this be the same for an aft CG but a lesser extent since it has less down force?

Does the nose pitch up with an aft CG when there is increased lift because the tail has less down force, and pitches down with fwd CG because it has a greater down force?

 

[tgrayson]

I'm having a hard time understanding this part. A book I have says the AoA is increased on the tail when the nose pitches down, causing it to nose up with a forward CG? Wouldn't this be the same for an aft CG but a lesser extent since it has less down force?

Does the nose pitch up with an aft CG when there is increased lift because the tail has less down force, and pitches down with fwd CG because it has a greater down force?

First understand that no horizontal stabilizer (H.S.) is needed for an aircraft to be stable. All that is required is that the CG be ahead of the A.C. However, the aircraft cannot be trimmed without a horizontal stabilizer (H.S) and so will not be flyable. The reason is that lift doesn't usually act at the CG and so will be generating a pitching moment around that point either nose down (negative) or nose up (positive). You need an H.S. to counteract that.

Stability is all about how the aircraft reacts to changes in lift. Picture an airfoil with the CG ahead of the leading edge; imagine an increase in AOA due to a gust. You can see that the airfoil's lift would try to pitch it down, around the CG, lowering the AOA. This is stability.

So the airplane is in a delicate balance. The H.S. keeps the aircraft from nosing over due to the fact the lift is acting behind the CG. However, when a gust strikes, lift increases and tries to pitch the nose down again. Also, the down force on the tail is reduced and it helps the inherent stability of the main wing.

Now, as you move the CG back, the wing's stability decreases; once you move it to coincide with the A.C., the wing makes no contribution to the stability of the aircraft. However, the horizontal tail still does. In fact, you can move the CG a bit behind the A.C, due to the extra stability the H.S. provides, but the instability of the main wing is now fighting the H.S.

At some point, as you move the CG back, the stability of the aircraft becomes neutral, and that point is called the "neutral point". If you move the CG back any further, the airplane is unstable. (But still flyable.)

So the H.S. contributes to the stability of the aircraft and can greatly increase the safe CG positions, particularly when you have a real long moment arm, such as on transport category aircraft.