CG Questions

FlyHigh423

Well-Known Member
Hello!

I just have a couple of center of gravity questions that my students asked me today and I honestly did not know the answer to. First one was, I was explaining the relationship between center of pressure and CG and they asked me where on the CG limit graph in the POH does the CG move behing the center of pressure? I thought that it would have to be beyond the most aft limit but I was not completely sure.

Second Question: I was showing him how to calculate where the CG is by using (moment/weight) and explained how the number you got was how many inches aft of the reference datum the airplane would balance if suspended. But he was wondering where that point was lateraly as well. I explained how it would be the point where the longitudinal axis and lateral axis intersected but I was wondering if there is actually a point in lateral measurement where this was. But then I know there is no reference datum for Lateral measurements so I would just assume there is no specific lateral point for this.

Thank you! Hopefully that all made sense!
 
1: who knows only the AC manufacture. But it would depend for the AC as well

2: Lateral for AC does not matter that much, but for Heilos yes it does. Everything is referenced from the datum.
 
Second Question: I was showing him how to calculate where the CG is by using (moment/weight) and explained how the number you got was how many inches aft of the reference datum the airplane would balance if suspended. But he was wondering where that point was lateraly as well. I explained how it would be the point where the longitudinal axis and lateral axis intersected but I was wondering if there is actually a point in lateral measurement where this was. But then I know there is no reference datum for Lateral measurements so I would just assume there is no specific lateral point for this.

The datum for the lateral CG would be the center-line of the aircraft. Since the ailerons can create a substantial amount of roll force, it's rarely ever a safety of flight issue.

The Caravan had a max demonstrated fuel imbalance of 400 pounds. If you were past that, then the heavy wing would be quite noticeable and the auto pilot would not stay engaged.

Helicopters can have limits to their lateral as well as fore/aft CG. If you watch the videos of the guys who work on powerlines from helicopters, you will see that there are counterweights on the opposite skid.
 
Can't answer the second question at this late hour, but to answer your first question, it doesn't matter, only the manufacturer knows. Also, remember that center of pressure changes relative to angle of attack and flap configuration.


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I just want to say I think it's great you took the time to ask on behalf of your student. Many instructors would simply put the question on the back burner and forget, or simply ignore it!
 
Oh thank you Mark! Yeah I am always trying to find the answers because then they learn more and so do I.
 
One thing that always helps them, and you, is to send them home with the study assignment and when they come back they should be able to not only give you the answer, but show you where they found it.
 
Hello!
First one was, I was explaining the relationship between center of pressure and CG and they asked me where on the CG limit graph in the POH does the CG move behing the center of pressure? I thought that it would have to be beyond the most aft limit but I was not completely sure.

The CG would have to be aft of the legal limit to get behind the center of pressure. The aircraft is designed so the horizontal stab has a negative angle of attack to balance the forward CG. Aft of the CL would be very unstable.

longitudinal_stability.gif
 
The CG would have to be aft of the legal limit to get behind the center of pressure. The aircraft is designed so the horizontal stab has a negative angle of attack to balance the forward CG. Aft of the CL would be very unstable.

longitudinal_stability.gif

Pretty much this. It would be trying to swap ends constantly. I'd venture to guess that the plane would just crash immediately if you got anywhere near the center of lift. I'd also imagine that most tricycle gear airplanes would fall on their tails as well, so you wouldn't be going anywhere anyways. :) As others have pointed out, the real relationship is unknown to us.
 
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