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Thread: Simple method to find CG. (interesting method)

  1. #1

    Lightbulb Simple method to find CG. (interesting method)

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  2. #2
    Not very useful in this hobby. Unless the material/shape is regular.

  3. #3
    For Aircraft, the c.g. is not true c.g as describe by a simple cardboard experiment in post#1.
    The c.g of the aircraft stability will shift forward when the aircraft is in forward motion.
    You can try the method in post#1 but I don't think it will work for a model aircraft. Most likely you will end up tail heavy situation.

    Cheers

  4. #4
    So primary school....

  5. #5
    Don't get confused. Centre of gravity is just a reference for every object, where all the difference masses within the same object can be representated as a single vector, and it is the same in static and dynamic conditions.

    For an airplane, what we are interested is to find a practical range of positions to conincide with the C.G. through the distributions of the masses, as such it'll provide inflight stability. The practical range is pretty much determined by the actual C.G., and all the dynamic forces acting on the various parts of the airframe, like wings and stabilizers. Fuselage do affect the dynamic loads as well, especially for airframe with wide 'lifting body', but for ease of calculations, they are often being left out. This practical range is NOT the CG itself, but rather a reference position where you should try to adjust the actual CG to fall within.

    Having said all these, I think the main interest of most of us here, is to try to find out the practical range of position of each individual model airplane, where you want to C.G. to fall within, and this is by another form of calculations, and not the ones shown earlier.

  6. #6

    CG as a reference point

    Quote Originally Posted by Mathieu View Post
    Not very useful in this hobby. Unless the material/shape is regular.
    I tried the method and find it somewhat interesting, hang the tail up with a string attached to a weight, mark the path, than hang the tip of the wing up with the string and weight hanging down, coincide part is roughly where the CG is, try balancing with my finger, plane almost balance lol

  7. #7
    Quote Originally Posted by joe yap View Post
    Don't get confused. Centre of gravity is just a reference for every object, where all the difference masses within the same object can be representated as a single vector, and it is the same in static and dynamic conditions.

    For an airplane, what we are interested is to find a practical range of positions to conincide with the C.G. through the distributions of the masses, as such it'll provide inflight stability. The practical range is pretty much determined by the actual C.G., and all the dynamic forces acting on the various parts of the airframe, like wings and stabilizers. Fuselage do affect the dynamic loads as well, especially for airframe with wide 'lifting body', but for ease of calculations, they are often being left out. This practical range is NOT the CG itself, but rather a reference position where you should try to adjust the actual CG to fall within.

    Having said all these, I think the main interest of most of us here, is to try to find out the practical range of position of each individual model airplane, where you want to C.G. to fall within, and this is by another form of calculations, and not the ones shown earlier.
    cg is cg lah, why complicate the matter?
    it is just a point where all the mass appears to act, where you can balance the plane. Vectors, reference, dynamic forces are all nonsense that complicate matters. No need using such complicated word as vector lah. The direction portion is always pointing down towards earth due to gravity.

  8. #8
    in fact what sunny suggested is a feasible method to determine the cg of each setup of the plane in more detail.

    What we usually do is only assessing the longitudinal [along the fuselage axis] cg by supporting at 2 pts on the wing, with the assumption that the lateral cg is exactly at the center.

    his method actually allows one to detect the lateral cg ie how far off to the side from the fuselage axis. Though this is usually not significant for normal single engine plane, it would hv significant effect on flying characteristic if too far off.

  9. #9
    Quote Originally Posted by dloh View Post
    I tried the method and find it somewhat interesting, hang the tail up with a string attached to a weight, mark the path, than hang the tip of the wing up with the string and weight hanging down, coincide part is roughly where the CG is, try balancing with my finger, plane almost balance lol
    ya, you are right. It is that simple.

    Thanks to Sunny. In future i would try this method when balancing my plane.
    just in case the lateral cg is out too much, i can either add some ballast at wingtip to balance it or dial some appropriate trim to take care of the lateral symmetry during maiden.

  10. #10
    Quote Originally Posted by GIWM View Post
    cg is cg lah, why complicate the matter?
    it is just a point where all the mass appears to act, where you can balance the plane. Vectors, reference, dynamic forces are all nonsense that complicate matters. No need using such complicated word as vector lah. The direction portion is always pointing down towards earth due to gravity.
    Yes indeed,but can you tell me where should I place my CG for example, a conventional plane and a flying wing with respect to MAC, in order to make your plane flyable? How would you determine where to place the CG for a model airplane you are going design? I think these are more meaningful for modellers than calculating something that you can simply find out by hanging your model on strings

    I wouldn't be bothered to post a reply if thing are as simple as you think. I wouldn't want to go ahead and explain so much if people are not interested to learn either.

  11. #11
    Quote Originally Posted by joe yap View Post
    Yes indeed,but can you tell me where should I place my CG for example, a conventional plane and a flying wing with respect to MAC, in order to make your plane flyable? How would you determine where to place the CG for a model airplane you are going design? I think these are more meaningful for modellers than calculating something that you can simply find out by hanging your model on strings

    I wouldn't be bothered to post a reply if thing are as simple as you think. I wouldn't want to go ahead and explain so much if people are not interested to learn either.
    joe, our bro Sunny is merely suggesting way of determining the cg location only.

    As to how far this cg should be in front of the center of pressure [ for static stability] of each particular type of plane, even as a aerodynamic engineer myself, i cannot tell without going into complicated analysis involving factors such planform, airfoil shape, AOA etc etc. The exact determination of where the cg should be with respect to the center of pressure [ which is a function of planform, airfoil shape, aoa etc and many other factors]for static stability, we should just based on recommended value given by model manufacturer.
    The rule of thumb estimation for cg for normal wing planform is about 1/4 to 1/3 chord from leading edge. For flying wing it might be a bit difficult to estimate.

    What we need to do as hobbyist is find the best/easiest way of measuring where our actual cg is, (based on our actual config such the type of motor we choose, the batt [its mass and location] we used etc) and then ensure that this is as close as possible to the recommended value.

    Bro Sunny has offered quite a good suggestion, in the sense that his method actually allow checks on the lateral cg location [which we seldom check, assuming that it is at the center] as well.

  12. #12
    Thank you for your explaination. While I do not have the luxury of a manufacturer to tell me where to put the CG for all the planes I have designed, I must have been so lucky than they all flew well without crashing. And now since you have explained all the nonsense yourself, and so I can save my breathe.

    Since you reminded me that the thread starter is merely suggesting a simple method for locating an object's actual CG, and not determining the practical flyable CG, I would not want to go on derailing the thread any further.
    Last edited by joe yap; 28-03-2011 at 07:34 AM. Reason: grammar

  13. #13
    Quote Originally Posted by joe yap View Post
    Thank you for your explaination. While I do not have the luxury of a manufacturer to tell me where to put the CG for all the planes I have designed, I must have been so lucky than they all flew well without crashing. And now since you have explained all the nonsense yourself, and so I can save my breathe.

    Since you reminded me that the thread starter is merely suggesting a simple method for locating an object's actual CG, and not determining the practical flyable CG, I would not want to go on derailing the thread any further.

    joe, why don't you start a sticky thread to share with everyone your past success of making planes flyable? yah?
    sticky threads are meant for important and indispensable concepts and ideas that can benefit everyone, esp newbies.
    This is a more positive approach. IMO
    We should all encourage good ideas from everyone. Instead of trying to down ideas.

  14. #14
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    Quote Originally Posted by GIWM View Post
    joe, our bro Sunny is merely suggesting way of determining the cg location only.

    As to how far this cg should be in front of the center of pressure [ for static stability] of each particular type of plane, even as a aerodynamic engineer myself, i cannot tell without going into complicated analysis involving factors such planform, airfoil shape, AOA etc etc. The exact determination of where the cg should be with respect to the center of pressure [ which is a function of planform, airfoil shape, aoa etc and many other factors]for static stability, we should just based on recommended value given by model manufacturer.
    The rule of thumb estimation for cg for normal wing planform is about 1/4 to 1/3 chord from leading edge. For flying wing it might be a bit difficult to estimate.

    What we need to do as hobbyist is find the best/easiest way of measuring where our actual cg is, (based on our actual config such the type of motor we choose, the batt [its mass and location] we used etc) and then ensure that this is as close as possible to the recommended value.

    Bro Sunny has offered quite a good suggestion, in the sense that his method actually allow checks on the lateral cg location [which we seldom check, assuming that it is at the center] as well.

    This is what makes the hobby interesting....even with the manufacturer's CG recommendation (if it were to be given at all), I would estimate that 50% of the time, it may not work properly. This could be due to the slight assembly errors if the model is in kit form, or ARF.
    In some cases, like Bi-plane, Tri-plane, or forward swap wings, unorthodox-designed objects, etc....the simple rule of thumb 1/4 to 1/3 from the LE may not be feasible, also depending on who's thumb.

    The other interesting part of the hobby is that we meet all sorts of funny people here who thinks other people are funnier than them

    MS

  15. #15
    Quote Originally Posted by GIWM View Post
    joe, why don't you start a sticky thread to share with everyone your past success of making planes flyable? yah?
    sticky threads are meant for important and indispensable concepts and ideas that can benefit everyone, esp newbies.
    This is a more positive approach. IMO
    We should all encourage good ideas from everyone. Instead of trying to down ideas.
    I did many posts so far. You just have to go through the achives. But that is just another story........

    I wouldn't want to do it just to entertain a troll anyway.

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