<h1>TLDR;</h1>
<p>Aft CG means stalls are unrecoverable!</p>
<h1>Why is CG relevant?</h1>
<p>The plane's elevator is the control surface for pitch angle. The elevator is its own proper wing, and as such only has a certain amount of force it can develop before it stalls. Once it stalls, then pitch control is lost. Not a happy day.</p>
<p>The force that the elevator must develop is the lever arm required to balance the torque stemming from the distance between the airframes center of gravity (CG) and the wing's <a href="https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra">aerodynamic center</a>.</p>
<p><a href="https://i.sstatic.net/RC0dT.gif" rel="noreferrer"><img src="https://i.sstatic.net/RC0dT.gif" alt="enter image description here" /></a></p>
<p>Think about it as a see-saw with an elephant sitting close to the center, being balanced by a mouse 100x farther out.</p>
<p>The upshot is that if the CG is aft of the aerodynamic center, then the plane's nose naturally wants to rotate up, and the elevator has to push the tail up to counterbalance.</p>
<p>And if the CG is fore of the aerodynamic center, then the plane's nose naturally wants to rotate down, and the elevator has to push the tail down to counterbalance.</p>
<h2>Aft CG</h2>
<p>Aft CG is a huge no-no. It all comes down to stall recovery. If the CG is behind the aerodynamic center, then if the plane stalls the nose will simply not come down. The upshot is that you can never regain airspeed and you will enter into what's called a <em>deep stall</em> or <em>leaf stall</em>.</p>
<p>One common reason for cargo aircraft crashes is improperly secured loads which shift backwards immediately after rotation on takeoff. This moves the CG aft, and the plane can no longer keep its nose down and very quickly stalls unrecoverably. There is a famous video a decade years back of a US military cargo jet which had its cargo load shift back on takeoff while departing a Middle East airbase. All aboard died, and I won't post the link out of sensitivity.</p>
<h2>Excessive forward CG</h2>
<p>If it's too far forward then your elevator will have to produce a ton of downward force to keep the nose up. This is draggy and inefficient. But it's also (within the limits of reason) survivable.</p>
<h1>So where to put the CG?</h1>
<p>As far back as you safely can! Competition glider pilots move the CG to the very limits of safety because they want to stop the elevator from producing downward thrust which costs precious meters of altitude.</p>
<p>Jetliners want an aft CG because it saves fuel. But don't panic, they have strict limits on how far back they can move it. Because in equal parts no pilot wants to die and no amount of saved jet fuel ever covers the cost of a crashed plane.</p>

CC BY-SA 4.0
2020-05-11T22:11:35.337Z
+1, but wording nitpick: the "upshot" of something is a positive - it's not clear to me how "that you can never regain airspeed and you will enter into what's called a *deep stall* or *leaf stall*" is an upshot - sounds more like a "downside". (Unless you really do mean that it is somehow a good thing, but in that case I think that warrants more explanation of why it's a good thing.)
mtraceur
CC BY-SA 4.0
2020-05-11T23:06:03.970Z
That is not the connotation of "upshot" I am familiar with. Merriam-Webster doesn't suggest any synonyms which have a positive-leaning connotation: https://www.merriam-webster.com/dictionary/upshot.
Kenn Sebesta
CC BY-SA 4.0
2020-05-12T04:20:28.460Z
Huh. I guess I just mislearned it from context then, because I don't think I've ever seen it used with a non-positive connotation before. This does suggest that it's perhaps more liable to misinterpretation than you might realize, but I take back my nitpick - your usage seems correct!
mtraceur
CC BY-SA 4.0
2020-05-12T09:51:03.293Z
Of course there is an [english.stackexchange question](https://english.stackexchange.com/questions/68948/does-upshot-denote-something-positive-negative-or-neutral) about this :)
ROIMaison
CC BY-SA 4.0
2020-05-14T09:32:43.117Z
This is generally incorrect. CP by itself has very little to do with stability; it is the dynamics of its movement that matters, leading to the concept of Aerodynamic Centre (or Neutral Point). Stall, even though dangerous, is not the determining factor either: you can make an airplane with enormous elevator which will provide enough authority even with a very rear CG (preventing _necessity_ of stall), yet it may still be unstable. By definition, instability is the tendency to depart from equilibrium (and nothing more), and it doesn't prohibit controlled flying.
Zeus
CC BY-SA 4.0
2020-05-14T14:51:26.333Z
Thank you for pointing out my error in using CP instead of AC. The wording has been changed. Nonetheless, I still stand by my answer. An unrecoverable stall is a very stable configuration, that's the problem. It is of course possible to design an airplane with any number of configurations, including canards and tailless airplanes. However, aside from certain exotic models, which are the exception proving the rule, these configs do not exist in practice. Standard config is large wing, small tail, which needs forward CG to recover from a stall.
Kenn Sebesta
CC BY-SA 4.0
2020-05-15T00:40:10.770Z
My point was that "unrecoverable stall" is one of the practical problems, but is not _the_ problem. It's not fair to say that "it all comes down to stall recovery". The actual problem is the very tendency to make things worse: any disturbance lead to even greater departure from equlibrium. If left unchecked, it will indeed lead to stall, but if corrected in time, the aircraft will fly, and in some sense will even fly better: there is a reason why most modern fighters are statically unstable. But it (normally) takes a computer to stabilise such aircraft; it's _difficult_ to fly with a rear CG.
Zeus
CC BY-SA 4.0
2020-05-15T05:18:34.170Z
You're not wrong, but IMHO your approach is too high level for this question. You're absolutely right that there are nuances to this, and with aerodynamic design you can do all kinds of crazy things. But for the bog standard airplane, which looks like a Cessna or an Airbus or a C-135, an aft CG is dangerous because of the possibility of good flight characteristics up until the moment of stall. If you have other information, link to it in the comments and I'll be sure to add to the answer.
Kenn Sebesta
CC BY-SA 4.0
2020-05-11T11:10:14.857Z
2020-05-14T12:20:48.563Z
2020-06-18T08:37:30.297Z
Kenn Sebesta
<p>It's a common saying that "A nose heavy plane flies poorly, a tail heavy plane flies once", but why does moving the centre of gravity backwards or forwards change the stability?</p>

CC BY-SA 4.0
2020-05-14T09:23:24.977Z
This is a proper aviation question for [aviation.SE](https://aviation.stackexchange.com/) (which has been discussed there many times). Things are not very intuitive, such that even pilots often have incorrect understanding of it.
Zeus
CC BY-SA 4.0
2020-05-14T15:01:43.243Z
Agreed. Somewhere we've taken the decision to allow a certain amount of redundancy here. I would have preferred we more aggressively close questions which are not uniquely related to drones, but by popular vote the community prefers otherwise. Feel free to jump into Drones.Meta.SE and make your voice heard.
Kenn Sebesta
<fixed-wing><center-of-gravity>
Why does moving the centre of gravity forward increase stability of a plane?
Robin Bennett
# Why does moving the centre of gravity forward increase stability of a plane?

(Asked by: username_0 on 2020-05-11)

It's a common saying that "A nose heavy plane flies poorly, a tail heavy plane flies once", but why does moving the centre of gravity backwards or forwards change the stability?

**Tags:** fixed-wing, center-of-gravity

> **Comments:**
> **username_1**: This is a proper aviation question for [aviation.SE](https://aviation.stackexchange.com/) (which has been discussed there many times). Things are not very intuitive, such that even pilots often have incorrect understanding of it.  
> **username_2**: Agreed. Somewhere we've taken the decision to allow a certain amount of redundancy here. I would have preferred we more aggressively close questions which are not uniquely related to drones, but by popular vote the community prefers otherwise. Feel free to jump into Drones.Meta.SE and make your voice heard.  

---

## 1 answers

### Answer by username_2 on 2020-05-11. Score: 12

# TLDR;

Aft CG means stalls are unrecoverable!

# Why is CG relevant?

The plane's elevator is the control surface for pitch angle. The elevator is its own proper wing, and as such only has a certain amount of force it can develop before it stalls. Once it stalls, then pitch control is lost. Not a happy day.

The force that the elevator must develop is the lever arm required to balance the torque stemming from the distance between the airframes center of gravity (CG) and the wing's [aerodynamic center](https://aviation.stackexchange.com/questions/19388/what-is-the-difference-between-centre-of-pressure-aerodynamic-centre-and-neutra).

[![enter image description here](https://i.sstatic.net/RC0dT.gif)](https://i.sstatic.net/RC0dT.gif)

Think about it as a see-saw with an elephant sitting close to the center, being balanced by a mouse 100x farther out.

The upshot is that if the CG is aft of the aerodynamic center, then the plane's nose naturally wants to rotate up, and the elevator has to push the tail up to counterbalance.

And if the CG is fore of the aerodynamic center, then the plane's nose naturally wants to rotate down, and the elevator has to push the tail down to counterbalance.

## Aft CG

Aft CG is a huge no-no. It all comes down to stall recovery. If the CG is behind the aerodynamic center, then if the plane stalls the nose will simply not come down. The upshot is that you can never regain airspeed and you will enter into what's called a *deep stall* or *leaf stall*.

One common reason for cargo aircraft crashes is improperly secured loads which shift backwards immediately after rotation on takeoff. This moves the CG aft, and the plane can no longer keep its nose down and very quickly stalls unrecoverably. There is a famous video a decade years back of a US military cargo jet which had its cargo load shift back on takeoff while departing a Middle East airbase. All aboard died, and I won't post the link out of sensitivity.

## Excessive forward CG

If it's too far forward then your elevator will have to produce a ton of downward force to keep the nose up. This is draggy and inefficient. But it's also (within the limits of reason) survivable.

# So where to put the CG?

As far back as you safely can! Competition glider pilots move the CG to the very limits of safety because they want to stop the elevator from producing downward thrust which costs precious meters of altitude.

Jetliners want an aft CG because it saves fuel. But don't panic, they have strict limits on how far back they can move it. Because in equal parts no pilot wants to die and no amount of saved jet fuel ever covers the cost of a crashed plane.

> **Comments:**
> **username_3**: +1, but wording nitpick: the "upshot" of something is a positive - it's not clear to me how "that you can never regain airspeed and you will enter into what's called a *deep stall* or *leaf stall*" is an upshot - sounds more like a "downside". (Unless you really do mean that it is somehow a good thing, but in that case I think that warrants more explanation of why it's a good thing.)  
> **username_2**: That is not the connotation of "upshot" I am familiar with. Merriam-Webster doesn't suggest any synonyms which have a positive-leaning connotation: https://www.merriam-webster.com/dictionary/upshot.  
> **username_3**: Huh. I guess I just mislearned it from context then, because I don't think I've ever seen it used with a non-positive connotation before. This does suggest that it's perhaps more liable to misinterpretation than you might realize, but I take back my nitpick - your usage seems correct!  
> **username_4**: Of course there is an [english.stackexchange question](https://english.stackexchange.com/questions/68948/does-upshot-denote-something-positive-negative-or-neutral) about this :)  
> **username_1**: This is generally incorrect. CP by itself has very little to do with stability; it is the dynamics of its movement that matters, leading to the concept of Aerodynamic Centre (or Neutral Point). Stall, even though dangerous, is not the determining factor either: you can make an airplane with enormous elevator which will provide enough authority even with a very rear CG (preventing _necessity_ of stall), yet it may still be unstable. By definition, instability is the tendency to depart from equilibrium (and nothing more), and it doesn't prohibit controlled flying.  
> **username_2**: Thank you for pointing out my error in using CP instead of AC. The wording has been changed. Nonetheless, I still stand by my answer. An unrecoverable stall is a very stable configuration, that's the problem. It is of course possible to design an airplane with any number of configurations, including canards and tailless airplanes. However, aside from certain exotic models, which are the exception proving the rule, these configs do not exist in practice. Standard config is large wing, small tail, which needs forward CG to recover from a stall.  
> **username_1**: My point was that "unrecoverable stall" is one of the practical problems, but is not _the_ problem. It's not fair to say that "it all comes down to stall recovery". The actual problem is the very tendency to make things worse: any disturbance lead to even greater departure from equlibrium. If left unchecked, it will indeed lead to stall, but if corrected in time, the aircraft will fly, and in some sense will even fly better: there is a reason why most modern fighters are statically unstable. But it (normally) takes a computer to stabilise such aircraft; it's _difficult_ to fly with a rear CG.  
> **username_2**: You're not wrong, but IMHO your approach is too high level for this question. You're absolutely right that there are nuances to this, and with aerodynamic design you can do all kinds of crazy things. But for the bog standard airplane, which looks like a Cessna or an Airbus or a C-135, an aft CG is dangerous because of the possibility of good flight characteristics up until the moment of stall. If you have other information, link to it in the comments and I'll be sure to add to the answer.  

---

