Weight and Balance Design Question

Discussion in 'Pilot Training' started by brcase, Apr 27, 2012.

  1. brcase

    brcase Cleared for Takeoff

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    So looking at a C-182C Weight and balance information I show the attached chart (Blue Line).

    Why does the forward CG limit change from 33.5 inchs to 40 inches from 2100lbs to 2650?
    My theory was that it was to limit the maximum down force on the tail, so I plotted this on the Green line, But it actually appears that as the weight increases it is designed to actually have less maximum down force on the tail?

    Anyone else have any idea why the change in forward CG limit?
    Or at least why so much change?

    Brian
    CFIIG/ASEL
     

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    Last edited: Apr 27, 2012
  2. skidoo

    skidoo Line Up and Wait

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    I have thought about that for a long time. I think you are on the right track as to why. But, the chart you show is for the moment envelope, not the CG limits that you reference for 33.5 to 40 inches...
     
  3. brcase

    brcase Cleared for Takeoff

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    True, I did some of the math from Moments to CG location. It is just taking the Moment and dividing by wieght. I also looked at the TCDS to get the CG locations.

    Brian
     
  4. denverpilot

    denverpilot Taxi to Parking

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    Could have had bad stall recovery behavior at higher weights in flight test? :dunno:
     
  5. skidoo

    skidoo Line Up and Wait

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    Here is a chart of mine. It is very similar, but a newer model. One thing about it is the two charts do not agree. The CG limits chart has a vertex at about 2700 lbs and I believe that is the correct one.

    It does make sense that with forward CG more elevator down force is required to maintain a certain attitude. So, at 36" CG, the max weight is about 2700 lbs. To go to 2900 lbs, you would need to move that CG rearward by at least 2". This probably amounts to the same down-force on the tail to maintain the same attitude. Just some reasonable speculation here...
     

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  6. Henning

    Henning Ejection Handle Pulled

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    Have you considered that the graph is about force available rather than force allowable?
     
  7. brcase

    brcase Cleared for Takeoff

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    I though I had. But at a higher weight the stall speed will be higher which would provide more available down force on the tail. Which doesn't tell me why they are limiting it to less down force on the tail at higher wieghts.

    Brian
     
  8. Henning

    Henning Ejection Handle Pulled

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    Are force available and force required linearly proportional with weight? Look at the low speed side of the LD chart, weight is induced drag, the one that peaks before stall.

    This is why you always use full flaps, throttle is more effective on the tail than speed.
     
    Last edited: Apr 28, 2012
  9. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Final Approach

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    Could it have something to do with how much weight ends up on the nosewheel? Does the 185 have the same kink in the envelope?
     
  10. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Waitaminute. Utility category is bigger than Normal? I don't buy that chart at all...
     
  11. brcase

    brcase Cleared for Takeoff

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    Yes, ignore that. I had forgot to relabel the chart. Actually I hadn't figured out how to do it in Numbers on my Ipad.
    C-182C isn't rated for Utility Category.
    and the green area is a fictional area I calculated using about a constant load on the tail.
     
    Last edited: Apr 28, 2012
  12. brcase

    brcase Cleared for Takeoff

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    Interesting Idea,
    C-180A shows
    Landplane (+40.0) to (+45.8) at 2650 lbs.
    (+34.5) to (+45.8) at 2100 lbs. or less

    So it doesn't allow one to go as far forward at lower weights (By 1") Perhaps this supports the nose wheel idea. I will have to look at this further?

    Brian
     
  13. John Collins

    John Collins Pattern Altitude

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    The forward CG is often limited at higher weights to limit the required stick force on landing. This is particularly true for airplanes that are nose heavy to begin with. In other words, it limits the forward CG to prevent running out of elevator on landing.
     
  14. alfadog

    alfadog En-Route

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    Remember that the moment envelope graph is in reference to an arbitrary datum plane well ahead of the actual CG. I would expect the forward CG limit reference to the datum to move rearward toward the actual CG as weight increases in order to limit maximum moment around the actual CG.
     
  15. brcase

    brcase Cleared for Takeoff

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    That is what I was kind of expecting to see, and if you hold the moment constant around what I guessed to be the center of lift you get the green line. But you do have a good point and this is what I was looking for, a few more things to look at.

    Perhaps if I adjust my location of the center of lift I will get a better match to the actual published limits.

    As mentioned in an earlier post I probably also need to look at the loading on the nose wheel.

    Thanks for the ideas.

    Brian
     
  16. denverpilot

    denverpilot Taxi to Parking

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    Don't discount that control force idea either. We all know one needs big ape-sized arms to land a 182 without trimming. Look at me! ;)

    Haha.
     
  17. Henning

    Henning Ejection Handle Pulled

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    The center of lift moves with CG though as the force on the tailplane changes.
     
  18. alfadog

    alfadog En-Route

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    Except that I phrased it wrong as the weight and balance moment around the actual CG will always be zero. That is why it is called the CG :goofy:.

    The maximum moment of interest is the moment between the center of lift and the CG. And yes, the COL moves rearward as you increase AOA with increased weight. So that would carry all allowable CG's rearward with it. CG must remain forward of COL to have downforce on the elevator, which provides pitch stability.

    The forward CG limit would be expected to move rearward faster than the COL with increasing weight to maintain a max downforce on the tail. It would be interesting to plot the COL position line as part of the weight/CG graph.

    I find the change in the rear CG with weight more puzzling. Why does it move as much as the forward limit??
     
    Last edited: Apr 29, 2012
  19. brcase

    brcase Cleared for Takeoff

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    I think a better way to say that is that as the Angle of Attack changes the Center of lift moves. Directly the CG obviously doesn't change the Center of lift. But a change in weight will change the Angle of attack. I had kind of assumed that the elevator would have had enough power to get you near the Maximum Angle of attack at any allowable weight and CG and thus at about the same Center of lift. I will have to think about this some more, If the limit is the maximum down force on the elevator perhaps you can't get to the maximum AOA at certain allowable Weight and CG location. This might mean I need to model an airfoil to figure out how much the CG moves.

    Understand this is just a mental excise, The Engineer part of me just likes to know how stuff works.


    Brian
     
  20. Henning

    Henning Ejection Handle Pulled

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    Think about what CG position does to to tail force required at any given weight. When that force is changed, wouldn't the center of lift be changed by the difference in the component of lift the tail is providing, be that positive or negative, since the center of lift is the culmination of all lift moments of the plane?

    The having sufficient force designed in is exactly why we have that envelope and do the calculation.
     
    Last edited: Apr 29, 2012
  21. John Collins

    John Collins Pattern Altitude

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    I think it is simpler than all that. The forward CG is limited so that you can flare the sucker when landing.
     
  22. Henning

    Henning Ejection Handle Pulled

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    Most definitely, all part and parcel of the same thing, one the outcome of the other.
     
  23. alfadog

    alfadog En-Route

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    In steady state straight flight (does not need to be level), the CG will always be directly below and equal to the vector sum of lift forces. In the envelope, the tail is providing downforce so wing lift > weight and CoL behind CG. CoL refers only to the wing.
     
  24. Henning

    Henning Ejection Handle Pulled

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    When the wing stalls, where is the CoL? :popcorn:
     
  25. skidoo

    skidoo Line Up and Wait

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    Not necessarily so... As fuel weight is added/removed, the CG location tends to move more vertical on the chart. For example, with two persons plus baggage and 87 gal of fuel, its almost at gross weight. If all that is done is remove fuel to 0, the CG location moves less than 1".

    The front seats on my 182 has a range of 32" to 50". So, if I have 25 gallons and zero baggage, the CG location is at 38". Then, if both seats are set at 38", then any weight added to the front passengers seats will move the CG location by zero. With a 150 lb pilot, no passengers, it CG is 38". Or, with a 350 lb pilot and a 350 lb passenger, CG is still 38". You would be under gross, but 1" forward or the limit.

    Now if you moved those seats all the way forward to 32", the CG location would move to 36.6" and you would be 2.6" forward of the limit. However, good luck being able to fit two 350 lb persons at full forward seat position and still being able to move the yoke.

    At the opposite end, move those seats to full rear position, and the CG moves to 40.8", and you are well within the limits.


    I tend to operate my seat 1" from full forward, and I can certainly tell the difference in trim required with just two persons, and no baggage, vs with baggage in the rear. It is worst, when I have an instructor aboard because he has to reach everything like I would. But, when I just have one passenger and little or no baggage, I always ask them to keep their seat back some.

    So, I believe, as others have suggested, that it is related to the tail force required on landing, or the amount of trim available. I have run out of trim to keep my 65kts with instructor only aboard. Next time out with the instructor, I added weight in the back, and the trim had sufficient range.
     
  26. skidoo

    skidoo Line Up and Wait

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    Are you suggesting that there is no lift in a stall?
     
  27. Henning

    Henning Ejection Handle Pulled

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    I suggest nothing whatsoever; the question stands on its own merit, you tell me. Consider the aircraft in a stalled condition, where would the sum total of remaining components of lift, if any, lie? If you have lift remaining in your answer please state the components providing that force and in what direction.
     
    Last edited: Apr 29, 2012
  28. skidoo

    skidoo Line Up and Wait

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    Well, as you know, a stall is not the same every time. It varies in degrees, typically beginning with just a little at the root, and progresses outward as conditions change for the worst. So, what condition of stall are you referring to? Complete, all the way out to the wing tips?
     
  29. Henning

    Henning Ejection Handle Pulled

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    When the wing is stalled, what is the force vector of the component of lift of the horizontal tail? Does it have an effect on the location of the center of lift of the aircraft as a whole?
     
  30. alfadog

    alfadog En-Route

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    You are misphrasing something. On the chart, CG can only move left/right on the X-axis. Weight moves up/down the Y-axis and thus moment can move XY.

    What you are telling me is that the CG of the fuel is relatively close to the CG of the airplane. That is a good thing.
     
  31. skidoo

    skidoo Line Up and Wait

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    Correct, my using the term vertical for CG movement was incorrect...
     
  32. denverpilot

    denverpilot Taxi to Parking

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    Yes. 182 tanks are almost centered at the real CG. And they're not shaped in such a way as to have odd CG shifts as you burn fuel.

    This (I hear) bites 182 drivers when they transition to a Bo, for one example. We aren't used to needing to calculate CG shift for fuel burn and we get lazy about it.
     
  33. Skylane81E

    Skylane81E Final Approach

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    it is nice to know that if you are in CG at take off you should still be there at landing. I just need to be sure that I will have burned suffecent fuel to be back down to 2950 or less before landing.
     
  34. Henning

    Henning Ejection Handle Pulled

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    For the most part user it's hardly an issue because he'll fly mostly himself, at that and a tool box in back with another 35-50 luggage (used to have a 35lb flight bag to get you around the country if you like your VFR charts, that was movable ballast as well) and you've got enough to put weight here and there that you work it out. Remember, this is not a big deal. Your typical airplane operator has only a few load profiles he'll run in his private course of use. You only figure each profile once and you know what you have to do to comply.
     
  35. denverpilot

    denverpilot Taxi to Parking

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    We're thinking about putting the STC on while the bird's logbooks are within striking distance of the IA.

    $750 = 150 lbs of useful load and your same 3100 MGTOW...

    Decisions, decisions.
     
  36. Henning

    Henning Ejection Handle Pulled

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    You'd pay $750 for a piece of paper that says it's ok to take your current as is piece of equipment and operate with another 150lbs? How often would you be taking advantage of this increase? Are you planning on selling? What is the resale improvement of the STC?
     
  37. brcase

    brcase Cleared for Takeoff

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    I went back and looked at my numbers for calculating the down force on the tail. By moving my estimate of the center of lift farther back I can pretty much duplicate the factory numbers for the CG envelope by limiting the amount of down force on the tail.

    Some how it hadn't occurred to me that the CofL location would affect this. So it looks like my initial hypothesis is likely correct, i.e. the shift in maximum forward CG is to limit the maximum down force required on the tail. I just made one assumption too many to make it work in my initial set of numbers.

    Brian
     
  38. Henning

    Henning Ejection Handle Pulled

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    It's the other way around but the same thing; the limitation isn't force required, the limitation is force available.
     
  39. Skylane81E

    Skylane81E Final Approach

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    We did on the schools Q model. But I also had a coupon for $250 off, if I had another...


    For Henning, I have used the full gross of my several times, we put it on the school plane after repeated need to offload fuel.

    Silly to spend so much on a scrap of paper, but in order to operate legally at the higher weight...
     
  40. Skylane81E

    Skylane81E Final Approach

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    In fact it occurs to me that the up gross stc is in line with the topic of this thread. Cessna achieved the higher take off weight of the R and up model 182s by narrowing the CG range from the prev Q model. The STC applies the narrower CG range to the aircraft when operating at weights above 2950lbs. A P or Q with the STC actually has a larger legal operating envelope than my R does.