May 7, 2024
Understanding Forward vs. Aft CG
As a DPE, one of the most commonly confused concepts I see during practical exams pertains to center of gravity, or weight shift. While the math is easy enough to do, the theory behind the concept of aft vs forward CG is a bit harder to conceptualize.
For those of you following along in your reference documents, this pertains to both the Private Pilot ACS and Commercial Pilot ACS, Area of Operation I Task F, as well as the new CFI ACS, Area of Operation I Tasks D and F.
The easiest way I've found to visualize an otherwise purely invisible concept is with a simple pen.
Watch the video for a better, visual explanation and the demonstration I use when teaching CG. But the gist is as follows:
With your center of lift over the wing, it can hold your airplane up at the plane's center of gravity. But it's not a terribly stable system. To solve some of this instability, we must, at least in training aircraft, ensure that the center of gravity is forward of the center of lift.
But now this is even less stable. So we add a stabilizer.
A horizontal stabilizer, to be precise.
Knowing what we know about the aerodynamic design of standard configuration aircraft, the h-stab is designed to provide down-force at the tail. It does this, of course, by generating lift in the same manner that the wing does, but with the total lift vector in a downward direction.
With the CG near its forward limit, the CG has a very long arm to the center of lift, meaning the tail must do much more work (produce a lot more lift, downward), to keep the system balanced. If we're producing more lift, though, we're producing more drag. More drag means less efficiency, higher fuel burn, and lower top speed for a given power setting. Additionally, the increased tail down-force means the effective weight the wing my carry, or its load factor, is much higher.
The POH for any aircraft, as well as the generally accepted laws of physics tell us that a higher load factor means a higher stall speed.
Conversely, an aft CG results in a lower tail down-force requirement, a lower load factor, a general improvement to cruise speed and efficiency, and a lower stall speed. On the other hand, it does mean that our elevator and rudder have a much shorter arm to the CG, the axis of rotation, and so our controlability and stall recovery characteristics are worse.
Hopefully that provides a little clarity to an easily confused aerodynamic principal. Let me know in the comments on this website or the video on Youtube what you think, or more importantly, what other ACS or checkride related topics you'd like to hear about.