CG vs Center of Pressure?

[CC268]

Been reading Chapter 5 of the PHAK and making notes...but I have a question about stability in regards to CG vs. Center of Pressure.

So as CG moves forward, stability increases (although performance decreases across the board due to flying at a higher AOA). If CG moves rearward, stability decreases (but performance increases across the board due to flying a lower AOA - that said stall/spin recovery can be hindered due to a shorter moment arm).

The above all makes clear sense. However, what is it that makes a rearward CG inherently less stable? Is it because the CG is closer to the Center of Pressure, so the effective moment arm is smaller?

Thanks

 

[dmspilot]

Yes. The reason most aircraft are designed with the CG in front of the Center of Pressure is for stability, and it's balanced out by the horizontal stabilizer. Imagine if they were in the same place and you had no horizontal stabilizer, it would be like balancing an elephant on a pin.

 

[CC268]

Yes. The reason most aircraft are designed with the CG in front of the Center of Pressure is for stability, and it's balanced out by the horizontal stabilizer. Imagine if they were in the same place and you had no horizontal stabilizer, it would be like balancing an elephant on a pin.

Yea you would have moment with no counter force - that is what I thought, I just wanted to verify that was indeed what they were referring to.

Thanks for the help.

 

[Half Fast]

However, what is it that makes a rearward CG inherently less stable? Is it because the CG is closer to the Center of Pressure, so the effective moment arm is smaller?

Thanks

Yes. Having the CG forward of the CP means that the moment provides a restoring force, ensuring nose-first stable flight. If the CG were behind the CP, the moment would act to flip the plane over. (In the extreme case, imagine trying to throw a dart feathers forward.) The farther forward the CG, though, the less effect the control surfaces have. This is why there both fore and aft CG limits. They establish the optimal point between stability and controllability.

Some of the missiles my company manufactures have heavy guidance systems and warheads that are forward of the CG. Fuel burn during flight reduces aft weight, moving the CG even farther forward. This results in a missile that is too stable to make end-game aim point corrections without excessively large tailfins, so the missile has strakes ahead of the CG that move the CP forward to compensate.

 

[Stewartb]

Increased stability isn't necessarily a good thing in the context of our airplanes. It's a paper description that translates to resistance to control response. More isn't always better. My planes have all flown better with CG favoring aft in the approved envelope.

 

[MauleSkinner]

Yes. Having the CG forward of the CP means that the moment provides a restoring force, ensuring nose-first stable flight. If the CG were behind the CP, the moment would act to flip the plane over. (In the extreme case, imagine trying to throw a dart feathers forward.) The farther forward the CG, though, the less effect the control surfaces have. This is why there both fore and aft CG limits. They establish the optimal point between stability and controllability.

Some of the missiles my company manufactures have heavy guidance systems and warheads that are forward of the CG. Fuel burn during flight reduces aft weight, moving the CG even farther forward. This results in a missile that is too stable to make end-game aim point corrections without excessively large tailfins, so the missile has strakes ahead of the CG that move the CP forward to compensate.

This.

Think of it in terms of positive static stability, rather than just "more stable". Positive static stability is the tendency for the airplane to self-correct when a gust takes it out of equilibrium.

Stability does affect controllability, but as @Stewartb said, more isn't necessarily better. There are the certified CG limits that keep the level of stability and control within design limits, but within that is a "sweet spot" where controls are light and responsive. The definition of the best "sweet spot" will vary from pilot to pilot, and even from flight to flight...more stable might be preferable on a bumpy day, where less stable might be better on a smooth day.

 

[Let'sgoflying!]

This would be a good one for a diagram. Instant understanding.

 

[FastEddieB]

If anyone doubts the effect of CG on performance, try this...

Go up on a calm day and set cruise power. Note the airspeed. Now slide your seat back a bit and see what happens.

In short, less tail downforce needed, so less lift required from the wing, so less induced drag from both the wing and the tail.

Rumor has it Mooney salesmen would take advantage of this by sliding their seat back to show what the plane could do!

 

[Clip4]

Been reading Chapter 5 of the PHAK and making notes...but I have a question about stability in regards to CG vs. Center of Pressure.

So as CG moves forward, stability increases (although performance decreases across the board due to flying at a higher AOA). If CG moves rearward, stability decreases (but performance increases across the board due to flying a lower AOA - that said stall/spin recovery can be hindered due to a shorter moment arm).

The above all makes clear sense. However, what is it that makes a rearward CG inherently less stable? Is it because the CG is closer to the Center of Pressure, so the effective moment arm is smaller?

Thanks

Try this. http://media.aero.und.edu/interactive-trainers/weight-cg-effects/

 

[CC268]

Try this. http://media.aero.und.edu/interactive-trainers/weight-cg-effects/

That is a great interactive graphic. Only thing is that the graphic itself doesn't really comment on WHY (from a technical perspective) an aft CG means less stability. That said, the folks above have confirmed original thoughts which was, "having the CG forward of the CP means that the moment provides a restoring force" - the closer you get to the CP, the less stability due to a smaller moment arm.

 

[CC268]

If anyone doubts the effect of CG on performance, try this...

Go up on a calm day and set cruise power. Note the airspeed. Now slide your seat back a bit and see what happens.

In short, less tail downforce needed, so less lift required from the wing, so less induced drag from both the wing and the tail.

Rumor has it Mooney salesmen would take advantage of this by sliding their seat back to show what the plane could do!

I fully understand the effects of CG on performance - that said a lot of books/resources seem to comment on performance effects, but never really explain WHY an aircraft is less stable as the CG is moved aft. They just simply state that it is less stable. Half Fast, dms, and Maule provided a good explanation of it though.