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6 Answers

Weight and balance…commerical

Asked by: 4257 views Commercial Pilot, General Aviation, Student Pilot

I understand how you calculate weight and balance...However, the calculations only tell you if the plane's CG is within normal limits and if the weight is acceptable.

As far as I know (and I've only calculated for C172's), you never calculate for how many people are on the right side of the plane vs. the left.

For instance, my plane COULD be in balance...but what if everyone was sitting to the left of the plane...or heavier people were all on the left side.  It could techincally be in balance calculations wise...but the calculations do not take into account how far right or left the weight is.

From what I understand, the arms are the VERTICAL (not horizontal) distance from the datum.

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6 Answers

  1. Jason Schappert on Oct 30, 2010

    You’re correct most basic weight and balance calculations don’t account for sitting left or right.
    Not to throw you for a loop but also think about this. What about seat position. The “ARM” They give you is for an average “middle” seat position. Now I have to fly with mine all the way back while others are all the way forward. crazy huh 🙂
    However to answer your question your concern needs to be am I within CG? What does an Aft CG give me? What does a forward CG give me?
    I did a pretty neat video on CG and stall characteristics you can check out below
    Video: The Effects of CG on an Aircraft

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  2. Best Answer

    Kent Shook on Oct 30, 2010

    I don’t know of any small GA airplanes that have any sort of limitation on lateral (left-right) weight and balance – The ailerons will easily compensate even if everyone is sitting on the left side of the airplane. However, some helicopters like the Robinson R22 *do* have a lateral weight and balance envelope as well as a longitudinal one.

    On a normal GA training airplane, what you are calculating is the longitudinal weight and balance – And yes, that means horizontal distance from the datum, NOT vertical. You’ll see why in a moment. Usually, the datum plane is in the front part of the airplane – The tip of the spinner, the firewall, or some other point.

    The reason we calculate the weight and balance is mainly for stability and control effectiveness. If your center of gravity is too far forward, your elevator may not have enough authority at slow speeds to keep you from smashing your nose gear when you land. Also, the stability of our small airplanes depends on a balance between the lift generated by the wings, and a downward force generated by the tail. If the center of gravity is too far aft, the tail will be unable to maintain that stability, and you may lose control of the airplane.

    So, it’s forward and aft that really matters in an airplane, not left and right, and definitely not vertical (putting everyone in the airplane in a booster seat will not change the stability and control effectiveness of the airplane or the weight and balance).

    Hope this helps!

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  3. Lindsay Atkins on Oct 30, 2010

    By vertical…I meant more front and back (length wise down the plane) and by horizontal I meant more left and right (width wise across the plane)…sorry for the confusion.
    Got it!  Thanks!

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  4. Aviatrix on Nov 02, 2010

    “… The ailerons will easily compensate even if everyone is sitting on the left side of the airplane.”
    Wouldn’t it be unwise to fly a long route turning the alierons to the right (banking) the whole entire time?

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  5. Kent Shook on Nov 03, 2010

    You wouldn’t be banking – You’d simply be compensating for the extra weight on the left side of the airplane by holding a tiny bit of right aileron <B>simply to keep the wings level</B>.
    Technically, this would add just a little bit of drag, but it would almost certainly be negligible in terms of aircraft performance.

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  6. Steve Pomroy on Dec 30, 2010

    That’s an interesting question.  As noted, some helicopters (maybe all, I’m not sure) have lateral CG limits as well as longitudinal.  In the fixed-wing world, the width of the fuselage usually precludes getting out of balance laterally by shifting passengers around — although the folks who do the design work will double check this sort of thing before giving us a blanket “don’t worry about it”.  It’s likely that if any Blended-Wing-Body designs ever make it to market that they will have lateral limits published.
    On the aircraft that I fly (the Grob G-120A), we have a fuel imbalance limit of 25 litres, which is especially important when conducting spins and aerobatics.  No calculations are required to see where our CG is laterally, just keep the fuel imbalance within limits (by switching tanks as needed) and you’re good to go.  That’s the only type of lateral restriction I’ve seen on fixed-wing aircraft.
    With respect to Jason’s comment about seat position:  Piper Aircraft’s Service Bulletin 753 requires seat position to be accounted for in the PA-28-140 Cherokee when calculating weight and balance for spinning.  This is the result of several fatal accidents during spin training that occurred due to out-of-limit conditions that were missed during calculations using average seat position.  In all of the C-172’s that I’ve flown (’70’s and ’80’s models), the flight manual included information on the average seat position as well as the extreme forward and aft positions.
    Interestingly, the Cherokee, which can only run on one fuel tank at a time, doesn’t have a lateral or fuel imbalance limitation.  I’ve flown them out of balance far enough to have to hold aileron in order to maintain an attitude.  It gets irritating, and is a good reminder to switch tanks!

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