Center of Lift Question

hambone

hambone

Well-Known Member
Hello, I haven't instructed in a while, and am in the midst of putting together a bunch of powerpoint grounds from Private through Commercial and have a quick center of lift question.

In straight-and-level flight, the center of lift is slight behind the center of gravity. Just to refresh something that seems to have escaped me, when you pitch the a/c up, the CL moves ahead of the CG, and when you pitch down, it moves behind the CG yes?

Thanks in advance.
 
hambone said:
Hello, I haven't instructed in a while, and am in the midst of putting together a bunch of powerpoint grounds from Private through Commercial and have a quick center of lift question.

In straight-and-level flight, the center of lift is slight behind the center of gravity. Just to refresh something that seems to have escaped me, when you pitch the a/c up, the CL moves ahead of the CG, and when you pitch down, it moves behind the CG yes?

Thanks in advance.
Click to expand...

If this is true it is new news to me. When you pitch up the CL moves forward and probably gets larger due to the greater AOA but I don't know about it moving forward of the CG. Would that not then require the application of nose down elevator to prevent a continuing nose up increase in pitch? I know of no aircraft that exhibit static instability to this degree. Curious to hear Tgrayson's take though.
 
Eh, reading a bit further my understanding of CL is a bit skewed. I always thought of it like the CL moved ahead of the CG when pitch up because I imagined the CL to be pivoting around the CG. If the CL was ahead of the CG, the a/c would have to be pitching up, and if the CL were behind the CG, the a/c would have to be pitching down.

Upon reading further, I realized that the CL does not move ahead of the CG, and it only moves as far as about 25% behind the leading edge of the wing. And in straight-and-level flight, the CL is located slightly behind the trailing edge of the wing, not the CG.

I think I thought the CL moved ahead of the CG, because I read it as the CL being slightly behind the CG, and therefore any increase in AOA would result in the CL being ahead of the CG.
 
Ok, let's move onto a different question now. I keep reading that the Horizontal Stabilizer is designed with a negative angle of attack which is what creates a tail down force. My initial CFI instructor taught to me that the Stabilizer was designed to have an upside down camber, which would create an opposite pressure differential and thereby create the down force? Which is which or are they both true, because the latter makes way more sense in explaining why the plane pitches down when you reduce the power.
 
Center of lift never moves ahead of CG unless you are talking about a weight shift control aircraft.

Aircraft stability comes from Lift (pitch down) and TDF (tail down force- pitch up). If the cg = center of lift, the aircraft is very unstable. The more aft you go from normal, the worse it gets.

This is the aircraft cg:

imageck7.jpg



This is the wing cg:


chapter_2_img_23.jpg


That's just the wing, and it's useful for drag and force equations. Not for determining where the wing cg moves. This does change the amount of TDF needed, but depending on aircraft design it may or may not be significant.
 
hambone said:
Ok, let's move onto a different question now. I keep reading that the Horizontal Stabilizer is designed with a negative angle of attack which is what creates a tail down force. My initial CFI instructor taught to me that the Stabilizer was designed to have an upside down camber, which would create an opposite pressure differential and thereby create the down force? Which is which or are they both true, because the latter makes way more sense in explaining why the plane pitches down when you reduce the power.
Click to expand...

I'm a simpleton, and have always understood the H. stab. to be an upside down wing, which would create low pressure on the bottom and suck the tail down. Seems to make sense to me.
 
I've always thought it easiest to explain it like an old fashioned balance scale. If you imagine the longitudinal axis of the plane as the scale arm, the CL would be the point from which it hangs. The CG then acts as the weight on one side of the balance point, and the horizontal stab acts as the other.

Trimming the horizontal stab just adds or removes weight to one side of the balance to keep the bar level. Then when you pitch with the elevator, you're again adding or removing weight to tilt the bar up or down.

Granted, this is a simplified explanation... and one that's easier to draw... but it helped me learn the concepts involved in the CG/CL relationship.
 
hambone said:
Ok, let's move onto a different question now. I keep reading that the Horizontal Stabilizer is designed with a negative angle of attack which is what creates a tail down force. My initial CFI instructor taught to me that the Stabilizer was designed to have an upside down camber, which would create an opposite pressure differential and thereby create the down force? Which is which or are they both true, because the latter makes way more sense in explaining why the plane pitches down when you reduce the power.
Click to expand...

I have seen aircraft designed both ways. It is up to whatever engineer worked on that part of the plane because they both end up essentially doing the same thing.
 
hambone said:
Eh, reading a bit further my understanding of CL is a bit skewed.
Click to expand...



The start to unconfusing oneself is to use the right terminology. The term "center of lift" doesn't have any real meaning in the aerodynamic literature...you mainly only see it in pilot training publications. There are two similar terms that have actual meaning:
  • center of pressure
  • aerodynamic center
The center of pressure moves with AoA. The CP moves backward with decreasing AoA and moves forward with increasing AoA. Its forward limit is approximiately the 1/4 chord point.

If the CP didn't move, then it would be true that the CG must be in front of the CP in order to have a stable aircraft. However, the CP does move, so you might have the CG in front of the CP at one AoA, but an increase in AoA will put the CG behind the CP, since the CP just moved forward.

That's no good.

That's why the discipline of stability and control uses the concept of the aerodynamic center. This is located at the 1/4 chord point and doesn't change with AoA. As a first approximation, the CG must lie ahead of the aerodynamic center in order to have a stable aircraft. However, even this isn't the whole truth, because we are only talking about the wing here, when in reality the wing is attached to a fuselage and tail section. Adding these other components to the wing creates a whole new aerodynamic center for the airplane which is called the neutral point. The CG must lie ahead of the neutral point for the airplane to be stable, but this point may well lie behind the aerodynamic center of the wing alone.

I always thought of it like the CL moved ahead of the CG when pitch up because I imagined the CL to be pivoting around the CG.
Click to expand...

Keep in mind that the CP and AC are aerodynamic concepts and they have no "knowledge" of where the CG lies. The best we can do is place the CG in relation to where we know the AC lies.

If the CL was ahead of the CG, the a/c would have to be pitching up
Click to expand...

If we replace the CL with AC, this may well be true, but consider too that lift acting ahead or behind the CG is not the only aerodynamic force producing the pitching motion.

And in straight-and-level flight, the CL is located slightly behind the trailing edge of the wing, not the CG.
Click to expand...

Assuming you mean CP when you say CL, this would likely be highly aircraft dependent.


If you want to read more accurate info on the subject, google "aerodynamic center" or "aircraft neutral point".
 
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