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7 Answers

Moving weight around

Asked by: 5489 views , ,
Aerodynamics, General Aviation, Student Pilot

I am still new to this; thank you for taking time to consider a few more questions.

-Is it safe to assume that lighter planes (Cessna 172) are naturally heavier in the front because of where the engine/powerplant is located? 

If this is the case, in order to move the CG aft, does the weight added to back of the plane have to EXCEED the weight in the front?  I'm not talking about an aft CG that is out of limits; I'm just talking about having the CG move aft (even by a little bit).

For instance, If the plane has a full tank and only one pilot, the aircraft will probably have a more forward CG (still in limits...just more forward).  Now, if a tiny bit of weight is added to the extreme back of the plane, will the CG still move aft?  I'm guessing the front is still heavier...but SOME weight has been added to the back.  Therefore, the CG should move back just a bit.  Am I correct in my assumption?

I guess that's why it's called weight AND balance...HAH!  Additional aft weight = CG moves aft.  Additional forward weight = CG moves forward

-Can an aircraft be OVERWEIGHT but still IN CG LIMITS?

-In my practice problems, oil has a negative arm.  Is this because it's FORWARD of the datum line?

I've always wondered why the formula is weight multiplied by arm and not weight added to arm.  I guess if you have a weight of 25 pounds...those 25 pounds are exerted equally over the entire arm (thus multiplied).

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7 Answers

  1. Kent Shook on Nov 21, 2010

    1) If you add weight to the back (behind the CG), yes, your CG will move aft – And the further from the CG you add this weight, the more effect it has on the CG. That is why we use “moments” which have both a weight and an arm (distance from the datum plane). If you think of a seesaw, if you put a 50-pound kid on one end, 10 feet from the fulcrum, that’s an arm of 500 foot-pounds. If dad weighs 250 pounds, he’ll need to sit only two feet from the fulcrum to balance with the kid – 250 lbs * 2 feet = 500 foot-pounds. Airplanes are the same, except we measure from the datum rather than a fulcrum.

    2) Yes, an aircraft can be overweight but still within CG limits. Most airplanes’ weight and balance envelopes are shaped like a rectangle with the upper-left corner cut out – The top is maximum gross weight, the bottom is empty weight, the left is forward CG, and the right is aft CG. If you were to extend the slanted line at the upper left corner and the right-hand line upwards, the space in that triangle represents where the plane is overweight but within CG limits. Note this is a conceptual exercise only – The maximum gross weight is there for a reason – many reasons, actually. Do not exceed it.

    3) You’re exactly right – The oil has a negative arm because it’s forward of the datum. Let me guess, you’re flying a Cessna? Generally, Cessna chooses to use the firewall as the datum plane, while Piper and some other manufacturers use a spot that’s either at the tip of the spinner, or in front of the plane entirely. If you were flying one of those planes, the oil would have a positive arm.

    4) Moment is weight *times* arm because we’re effectively working with torques here. Using the seesaw example, if Dad moves from 2 feet from the fulcrum away to 4 feet from the fulcrum, the upward force on Junior will double. If they were added together, there would be very little effect (252 and 254 are very close to each other). If you think about it in terms of units, distances and weights are not equal and the units we want are inch-pounds (or foot-pounds, or…) Hope this helps clear it up. 🙂

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  2. The Red Baron on Nov 22, 2010

    Interesting! I always thought torque was tendency of a force to ROTATE AROUND an object…more of a TWISTING force.

    Not so in those case?

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  3. The Red Baron on Nov 22, 2010

    Typo…THIS case.

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  4. Brian on Nov 22, 2010

    …Qt: The Red Baron…”I guess if you have a weight of 25 pounds…those 25 pounds are exerted equally over the entire arm (thus multiplied)”…
    Not so, the arm is a measure of where the weight is applied in reference to the fulcrum (rotating point). The force from that weight is exhibited directly on the point it is applied, not distributed throughout. This is no different then placing a piece of wood across two cinderblocks. Push down in the middle and the wood bends, right? By your logic the board would not bend because all weight is evenly distributed, see the problem?
    Unless I’m misreading again?
    …Qt: The Red Baron…”I always thought torque was tendency of a force to ROTATE AROUND an object”…
    It is. For your purposes, unless you intend on becoming a PHD in the sciences, you can view moment and torque as one and the same. As you will see, even some scientific communities view them as such. There is an interesting piece on this in the first few paragraphs of wikipedia, they explain it better than I can. Here:

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  5. The Red Baron on Nov 22, 2010

    Lol, Brian…what’s “QT?”

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  6. Brian on Nov 28, 2010

    Qt = quote. Apologies for my non standard phraseology. 🙂

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  7. Antonio Barreto on Aug 18, 2015

    I am having a little problem solving weight shift using the Gleim formula for the FAA Advance ground instructor test. I know one has to do some algebra here , but I took algebra before the dinosaurs disappear from the face of the earth. Can somebody has a better way to explain the steps necessary to solve these problems , that is the only issue preventing me to go ahead and take the first test. Any help will be greatly appreciated.


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