CG And Elevator Effectiveness

[uwochris]

Hey guys,

I realize that the elevators become more effective as the CG moves aft.

However... if the rudder is more effective with a more forward CG (i.e. with multi engine a/c, Vmc is lower with a forward CG because the arm between CG and the rudder is longer), then why doesn't this principle apply to the elevators?

It seems to me that all movements occur through the CG, as that is where the weight is centered. With a forward CG, the arm between the elevators and the point from which the aircraft pitches is longer, so more moment is produced here. But, this analogy is obviously wrong.

Am I omitting the Centre of Pressure, or should this not be a factor for pitch authority?

Thanks in advance.

 

[enigma]

Hey guys,

I realize that the elevators become more effective as the CG moves aft.

Do they? Define "more effective".

However... if the rudder is more effective with a more forward CG (i.e. with multi engine a/c, Vmc is lower with a forward CG because the arm between CG and the rudder is longer), then why doesn't this principle apply to the elevators?

Because the vertical stab/rudder doesn't produce any aerodynamic force in normal coordinated flight. It only makes force (yawing force) when called upon to counter torque, a dead engine, etc. On the other hand, the horizontal stab/elevator is almost always producing an aerodynamic force needed to trim the airplane in pitch. In general, for a normal configuration airplane, the elevator produces negative lift at forward c.g. in order to hold the nose up, and it produces positive lift at aft c.g. in order to hold the tail up. BTW, that answers your first question. Think about it.

To answer your question, "why doesn't this principle apply to the elevators?"..... It does, but the difference in elevator effectiveness between fwd and aft c.g. is very small in relation to the difference in elevator forces required to trim between fwd and aft c.g.'s. This is a ballpark estimate, but I'd guess that the differing arms between fore and aft c.g. would not change the elevators ability to produce a force by more than a few small percent; while the difference between force required to trim a full fwd c.g. vs a full aft c.g. could amount to 25 to 30% of the elevators available force.

It seems to me that all movements occur through the CG, as that is where the weight is centered. With a forward CG, the arm between the elevators and the point from which the aircraft pitches is longer, so more moment is produced here. But, this analogy is obviously wrong.

Am I omitting the Centre of Pressure, or should this not be a factor for pitch authority?

Thanks in advance.

See the above, you may be omitting the CofP, but the answer was more simple than you realized. It has almost everything to do with the amount of force necessary to trim the airplane.

regards,
enigma

 

[uwochris]

Enigma,

Thanks for the response.

I should have probably been more clear in my initial post... by "more effective," I meant to say that the controls are "lighter." ie) stick force required to get a response from the elevators.

In the Advanced Pilot's Flight Manual, Kershner explains that the controls are lighter for a more aft CG, but he never got into the "why" part.

Anyway, thanks again.

 

[A Squared]

Chris,

A conventional airplane, loaded within gc limits is stable, as the cg moves aft, it becomes less stable. As it becoems less stable it requires less force to initiate a pitch change. Think of a lamp with a wide base and another similar lamp with a narrow base. the one with the smaller base is less stable, and requires less of a push to topple it.

As far as the aerodynamics behind it, first you have to understand why an airplane is stable. I assume that you do, but bear with me. If a conventional aircraft trimmed in level flightexperiences a ptch disdurbance upward, the airspeed drops, the wing generates less lift, the stabilizer develops less force down so so the tial rises and the nose drops, and hte airplane tries to return to level flight. If the nose drops, the airspeed increases, the wing generates more lift, the stabilzer generates more downforce, both of which tend to pitch the airplane up to return to level flight.

That's kind of a simplified view of stability, but it's accurate enough to understand the concept.

Now, what happens in a an airplane on which the cg is well aft of the center of lift of the wing. On this airplane both the wing and the stabilizer must provide lift upward to balance cg. The tendancy of changes in lift and downforce to work together to correct pitch changes is gone. It this aircraft pitches up, the stabilizer loses lift as the aircraft slows and this may cause the aircraft to pitch up further, which further decreases airspeed, which furhter decreases stabilizer lift and results in even more pitch up. The amount of pitch will continue increasing until the pilot actively stops it with a control movement.

In the unstable second aircraft, a very small elevator movement will result in a very large pitch change because of the airplane's tendency to multiply pitch disturbances, rather than correct them like the stable first airplane.

Stability is not a yes/no situation. The stability varies continuously as cg moves aftl; from very stable with the cg at the forward limits, to uncontrollable with a cg location someplace well aft of the aft limit. As the cg moves further and further along the range, the airplane will experience larger and larger pitch changes with smaller and smaller elevator movements.

Does any of this help?

 

[uwochris]

A Squared,

Thanks for the response. That was very helpful.

I guess my confusion comes from the fact that I am trying to compare the rudder to the elevators. I figured that if one becomes more responsive at a more forward CG, then why shouldn't the other?

I guess it all comes down to issues of stability.

Thanks again, and I welcome any more comments.

 

[Illini Pilot]

i think what i gather from the previous explanations is that it is because the stabilizer is producing a down force, and as the CG moves aft, the tail down force is less, thus making any inputs more effective. On the other hand, a fwd cg makes rudder inputs more effective because it creates a longer arm, which leads to a greater force

 

[mar]

Just to underline what's been said

I'm not gonna review Enigma and Asquared.

I just want to underline this for you:

The rudder is more *effective* with a forward CG.

The elevator is more *sensitive* with a rear CG.

*Responsiveness* really has nothing to do with it. They both *respond* but with different qualities.

Effectiveness and sensitivity are two different things. Your confusion may be a result of trying to correlate them.

Your questions are excellent as always. You're gonna be an outstanding instructor.