CG changing stall characteristics

pilotod

pilotod

Pre-takeoff checklist
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What will change if the CG is moved aft?

My Gleim book says...."As the CG is moved aft, the airplane flies at a lower angle of attack at a given airspeed because of reduced tail-down force....thus the critical angle of attack will be exceeded (causing the airplane to stall) at a lower airspeed.

Someone told me that's wrong.

If the horizontal stabilizer is keeping the nose from dropping it seems like more weight back there will make the nose come up faster meaning the stall will come sooner...meaning at a lower speed.
 
More weight in the back will be offset by less downward force from the stabilizer
 
Whatever the case, I think the moment where an aft CG really matters is when you try to recover from said stall... :eek:
 
The wing stalls at the same angle of attack regardless of CG. The wing doesn't know or care where the weight inside the plane is. The point of what the Gleim book was that as the CG goes aft the tail is required to push down less. The total weight the wings must support is the actual weight plus that tail down force. So, if you move the CG back the wings have less weight to support so at a given speed the angle required to support that weight goes down.

That means you have further to go to reach your critical angle of attack and that means you lowered your stall speed.
 
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moving weight aft effectively makes the airplane lighter. between that and less drag from trim drag on the tail its the best way to pick up a few knots. With 2 up in a club 172 all the baggage goes as far aft as it can, preferably with something like a case of water as far back as possible.
 
And this person told me that the airlines do the same thing....except instead of beer cans in the back they pack the bodies in the back.


tonycondon said:
moving weight aft effectively makes the airplane lighter. between that and less drag from trim drag on the tail its the best way to pick up a few knots. With 2 up in a club 172 all the baggage goes as far aft as it can, preferably with something like a case of water as far back as possible.
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pilotod said:
And this person told me that the airlines do the same thing....except instead of beer cans in the back they pack the bodies in the back.
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The only time the airlines move passengers or bags is if they are OUT of CG. They never do it just to optimize. Maybe they should but I've never seen it.

With that said, I've heard B747s have a fuel tank in the horizontal stab that the computer uses to manage CG. I certainly don't know it's algorithms and don't know if it just maintains or tries to optimize the CG.
 
tonycondon said:
moving weight aft effectively makes the airplane lighter. between that and less drag from trim drag on the tail its the best way to pick up a few knots. With 2 up in a club 172 all the baggage goes as far aft as it can, preferably with something like a case of water as far back as possible.
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Sliding seats back in cruise makes a notable difference too.
 
The big issue with moving the cg aft is the reduction in static and dynamic stability. As a result, it takes less stick force to induce a stall, and it becomes easier to overcontrol in the recovery. In addition, the aircraft becomes more spin-prone because less yaw force is required to create a yaw divergence.
 
Ron Levy said:
The big issue with moving the cg aft is the reduction in static and dynamic stability. As a result, it takes less stick force to induce a stall, and it becomes easier to overcontrol in the recovery. In addition, the aircraft becomes more spin-prone because less yaw force is required to create a yaw divergence.
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Hence the cg range of the plane
 
Captain said:
The only time the airlines move passengers or bags is if they are OUT of CG. They never do it just to optimize. Maybe they should but I've never seen it.
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Hey, Mr. Airline Pilot... :) Something I've always wondered about airliners. How do you calculate W&B? I know you do it before leaving the gate... I've even been moved forward once, with a few other bulky individuals, for that reason. Are there load cells that tell you the weight on the gear? That would seem a pretty good approach, given the level of engineering and equipment in your average Airbus, Boeing or Embraer.
 
No, the FA's count passengers in zones. The plane is split into zones and it's a straight number count. Airlines have an 'average weight program'. They use a standard weight for passengers and split it into summer and winter weights. (winter weighs more because people tend to wear more clothes).

So you have the passenger count from the FA. The bag count from the ramp and you just plot it out. Bigger / newer planes use the FMS to calculate it. I've always used a CG calculator witch is just a sliding ruler that figures out the CG.

It's all done at the gate. We have to figure out a maximum Takeoff weight based on MATOW (structural), Landing weight (MLW plus the fuel burn), MZFW (max zero fuel weight plus fuel on board) and cruise weight (drift down if applicable).

As far as CG goes the goal is always to move as few people as possible. Seldom is it necessary...but it does happen. It happens more on smaller planes (ie CRJ and ERJs) but I've seen it on B737's with a light load.
 
I've hauled cattle. They do the same thing with the zones. They build pens in the plane and keep the cattle in the pens. So the bulls move around in the pen but that's okay as the average weight is limited to the zone. That keeps the plane balanced.

Weight shift of cargo planes can be a big problem. One went down a decade or so ago in Miami due to weight shift.
 
Captain said:
With that said, I've heard B747s have a fuel tank in the horizontal stab that the computer uses to manage CG. I certainly don't know it's algorithms and don't know if it just maintains or tries to optimize the CG.
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From my 747-400 sim ride and long chat with a pilot friend who flies them, he mentioned that the horizontal stab tank is fairly critical, in that if you experience just the right failures, you can end up with fuel stuck in that tank, and be out of rearward CG for landing.

I got the distinct impression that his opinion was not to leave fuel in there for any longer than necessary. If you can't pump it out, you're going to have a bad day. He wanted to get it empty and the fuel transferred to the centerline body tank.

Not a word about efficiency, just words related to how much fuel that beast needs on board and how to move it all around to feed hungry engines.

The system:
http://www.boeing.com/commercial/aeromagazine/aero_09/fuel_fig01_02.html

From an article about troubleshooting fuel system imbalances in Boeing aircraft:
http://www.boeing.com/commercial/aeromagazine/aero_09/fuel_story.html

The article interestingly keeps reassuring the reader that even imbalanced, the pilots have *lateral* control a'-plenty. But it never mentions the 747-400 and 777 horizontal stab tanks and control of the pitch axis.
 
Captain said:
...... One went down a decade or so ago in Miami due to weight shift.
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I lived in Miami when that happened.. Probably 4 decades ago.. Maybe early 70's ? It was a two block long bloody streak from 9L I think.

Edit... there was a cattle hauler I was speaking about..:redface: Maybe a 707 or DC-8 ?
 
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tonycondon said:
moving weight aft effectively makes the airplane lighter. between that and less drag from trim drag on the tail its the best way to pick up a few knots. With 2 up in a club 172 all the baggage goes as far aft as it can, preferably with something like a case of water as far back as possible.
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Exactly. The downforce on the horizontal stab is reduced and the wing flys at a lower AOA reducing induced drag. You loose some of the pitch stability designed in but should not be a problem in smooth air if you remain in CG range. The Kings have it on one of the "pilot tips" tapes as a way to pick up a bit of speed
 
it shouldn't be a problem in rough air either.
 
Did you guys forget that the tailplane is an upside down wing? In a too heavy situation, there may not be enough lifting force to bring the tail UP, thereby forcing the nose down during stall recovery and the airplane goes in nose high.
 
Jaybird180 said:
Did you guys forget that the tailplane is an upside down wing? In a too heavy situation, there may not be enough lifting force to bring the tail UP, thereby forcing the nose down during stall recovery and the airplane goes in nose high.
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If the cg is that far forward, you won't hardly be able to get enough tail downforce to stall it in the first place, and even you do, the nose will drop and speed will pick up until you have the extra downforce again.
 
But the OP was trying to understand what happens when you do that. I find it ofen helps to understand when you throw subtlety out the window.
 
In that case make a paper plane and tape a couple of quarters to the back. See what happens.
 
Skylane81E said:
In that case make a paper plane and tape a couple of quarters to the back. See what happens.
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Unless the paper is something like stiff board and big enough, the experiment will be poor. 8-1/2 x 11 airplanes are really only for 2-5 year olds.
 
Experiment is a perfect example of CG and it's affect on stability. We all make paper planes, put a paperclip on the nose and it gets more stable. Put weight on the back and the thing just flops around the air.

FWIW I still enjoy a good paper plane from time to time, am I the equivalent of a 5 year old for it?
 
Skylane81E said:
Experiment is a perfect example of CG and it's affect on stability. We all make paper planes, put a paperclip on the nose and it gets more stable. Put weight on the back and the thing just flops around the air.

FWIW I still enjoy a good paper plane from time to time, am I the equivalent of a 5 year old for it?
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Yes, but whoever said that was a bad thing?
 
Ron Levy said:
Differrent story entirely. If you're tail-heavy (aft cg), your problem will be over-controlling as described above, not lack of control authority.
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So how do you setup the groundschool CoG scare scenario of aft-CG? I was taught that in an extreme example a stall would be unrecoverable due to lack of elevator authority.
 
Jaybird180 said:
So how do you setup the groundschool CoG scare scenario of aft-CG? I was taught that in an extreme example a stall would be unrecoverable due to lack of elevator authority.
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Near aft limit of CG range vs wildly aft of CG range are different discussions.
 
Jaybird180 said:
So how do you setup the groundschool CoG scare scenario of aft-CG? I was taught that in an extreme example a stall would be unrecoverable due to lack of elevator authority.
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It can become unrecoverable because the plane wants to fall tail first, imagine trying to fly a dart backwards...
 
Jaybird180 said:
So how do you setup the groundschool CoG scare scenario of aft-CG? I was taught that in an extreme example a stall would be unrecoverable due to lack of elevator authority.
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Then i think you were taught wrong because the main problem would be instability, not lack of control authority.
 
I'm going to, for the first time, fill a 172 with 4 people and hopefully not full fuel (at least that is what I requested but you never know). Useful is only 847 so if we have 40 gal we will be at max gross.

This is Sunday, I'll let you know how it changes things for me. Don't worry, I'll have a CFI right seat. I've been told I should notice a difference if I usually only do 2 people in a 172. It is a new 172 so I don't know anything about it. N12161.
 
If you've never flown four before it will be noticeably different. Have fun.
 
Ron Levy said:
Then i think you were taught wrong because the main problem would be instability, not lack of control authority.
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I will see if I can find the Sporty's video. I got that reinforced from them.
 
Captain said:
If you've never flown four before it will be noticeably different. Have fun.
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I hope to be briefed on this. Fun it will be yes. I debated not bringing one of them but then thought "why not" - it is fun to share flying with others. Plus they get a free airplane ride.
 
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