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Why do people have to worry about CG? Is it primarily due to fuel penalty, or take-off and landing performance, or is it mostly just to keep the aircraft from tipping over during loading and unloading, or other?

As a bonus, does tipping over of aircraft actually occur these days due to improperly managed CG?

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    $\begingroup$ a badly managed CG can crash your plane $\endgroup$ – ratchet freak May 14 '14 at 10:31
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    $\begingroup$ It's all about stability in flight (and, more importantly, ability to recover from a spin should one be permitted to develop). None of the factors addressed in the question are a CG concern that I've ever heard. $\endgroup$ – mah May 14 '14 at 10:48
  • $\begingroup$ Why worry? Because you want to live. Does tipping (assuming you mean loss of control) happen because of unmanaged CG? Yes. $\endgroup$ – GdD May 14 '14 at 14:55
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Why do people have to worry about CG?

Because the manufacturer has designed the aircraft following certain criteria, and one of those is where the CG will be during flight. If the CG is outside that interval, the elevator will not have enough autority to keep level flight.

does tipping over of aircraft actually occur these days due to improperly managed CG?

Was not improperly "managed", but it went out of bounds (in particular, the aft limit), unfortunately, soon after take-off and a 747 with 7 people on board was lost.

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  • $\begingroup$ wow. good example. For some reason I was not thinking of the obvious when I raised the question. I had just read an article about CG as it relates to a bunch of secondary issues. Thanks. $\endgroup$ – esé May 14 '14 at 12:03
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    $\begingroup$ In particular, as mentioned before, an out-of-limits CG may make a stall recovery difficult or impossible (outside of the other handling problems the CG may cause). $\endgroup$ – dougk_ff7 May 14 '14 at 12:49
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    $\begingroup$ Video is of National Air Cargo, 747-400 freighter, N949CA, Bagram Apr 29th 2013 $\endgroup$ – RedGrittyBrick May 14 '14 at 14:45
  • $\begingroup$ @RedGrittyBrick, what is the meaning of "gear up" in that video? Is that mean to pull up the landing gears? $\endgroup$ – AirCraft Lover Dec 14 '18 at 7:39
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To balance the aircraft, the sum of the lift from all lifting surfaces has to be at the same position as the weight. If the c.g. is ahead of the center of lift, the aircraft will pitch down. The pilot can counteract this by pulling on the elevator, which in effect reduces lift at the tail and thus shifts the center of lift forward. As you can imagine, there is only so much lengthwise change which can be effected with the elevator. So the first limit for the c.g. position is given by controllability.

Another important consideration is stability. You want the airplane to return to the original flight attitude if it has been disturbed, say by jolt on the stick or yoke. This can be done by creating proportionally more lift with the forward surfaces (= the wing in a conventional configuration) than with the tail surfaces. Say the jolt pitches the aircraft up. Now the aerodynamic forces should change in a way which increases lift proportionally more on the tail surfaces, so the aircraft pitches down and returns to it's original attitude. By distributing lift the way I mentioned above, the same change in angle of attack on both wing and tail will create a proportionally higher lift increase on the tail, making the aircraft (statically) stable. Thus, the second important limit for c.g. location is stability.

As for tipping over …

Two pictures with tipped-over donkey cart and DC-10

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    $\begingroup$ good explanation and pictures $\endgroup$ – esé May 15 '14 at 23:36
  • $\begingroup$ ultraligero.net/Cursos/mecanica/… Chap.9 Effect of cg position on angle of attack stability could you please point me out where I am wrong? $\endgroup$ – Federico May 17 '14 at 6:44
  • $\begingroup$ @Federico: Yes: The neutral point is fixed and is the point where the air loads can be expressed as a sum of a variable force plus a constant moment. This is the quarter chord point of a single wing and is used to express the c.g. position (26% of MAC would mean the c.g. is 26% of the mean aerodynamic chord ahead of the neutral point). The aerodynamic center is the point where all air loads can be summed up without leaving a residual moment. This point is variable and moves forward with higher AoA and can be shifted with the elevator. $\endgroup$ – Peter Kämpf May 17 '14 at 9:59
  • $\begingroup$ @Federico: Wow - I just read the PDF you linked, and the author has them confused, too (chapter 1.6 versus 8.3). He uses the same term "aerodynamic center" for both concepts. He also uses the term neutral point, but only to express the stability consequences when the c.g. is at this point. I need to read more, but from a first glance I would say he has not really understood the difference. Maybe this is a good topic for a new question here! $\endgroup$ – Peter Kämpf May 17 '14 at 10:13
  • $\begingroup$ @PeterKämpf, what is the center of lift here? Is it the wing? $\endgroup$ – AirCraft Lover Dec 14 '18 at 7:44
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In flight the wing is generating upward lift, the tail plane a downward force. As you move the c.g. forward, more downward force is required from the tail plane, which means the tail plane is causing more drag, which means you will burn more fuel. Move the c.g. aft and you'll burn less fuel. However, the farther aft you move the c.g., the less stable the airplane becomes in response to gusts. As an example, 747 freighter operators typically aim for a zero fuel weight c.g. of 26% MAC (mean aerodynamic chord) as an optimal balance.

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  • $\begingroup$ good point on the fuel $\endgroup$ – esé May 16 '14 at 2:30

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