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  1. #1

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    CG located on fuselage?

    Hi gentlemen, I found a CG calc tool that used the dimensions of the wing, tail, and distance between the wing leading edge and the elev leading edge. I have used this before with some success. For grins and giggles I played with changing the distance between the two leading edges. The recommended CG kept moving rearward. I went as far as to extend the distance enough where the airplane would probably not fly but I was trying to see where the CG would end .Well I got one spec that put the CG in the middle of the fuselage between the wing and the tail. I am trying to understand the physic behind this. I assume that the center of lift would always be somewhere on the wing. Would this work with the cg that far off the wing. If anyone can explain why the tail distance from the wing affects cg. I understand that the longer the arm the smoother the reaction etc. but how does this affect cg? anyone help/And thanks.

  2. #2

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    Sounds like your calculator has an error. Balance can be behind the wing, but the planform would be for a tandem wing model, balance could be forward of the wing, but it would be a canard layout. Models with a conventional planform can have the balance around 100% chord, but these would be specific models, free flight power comes to mind, and very much 'single speed' trims with long moments.
    Evan.

  3. #3

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    Something lost on some people is that tail should carry some of the plane's weight. All else being equal, I can see where a longer tail moment might move the CG back. Taken to extreme, it would be impossible to keep "all else being equal" due to the weight of the structure required to get the tail back that far and remain structurally sound.

  4. #4
    Moderator BMatthews's Avatar
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    There's no error. As you make the fuselage longer the overall stability neutral point moves back. Make the fuselage excessively long enough and the NP ends up behind the wing. And the CG set for some degree of stability follows the neutral point.

    Back in the 1950's there was a movement among free flight power flyers to make the fuselage longer and longer and the stabilizers larger and larger. This led to models such as the Civvy Boy where the CG was right on the trailing edge of the wing. And one particular extra long .049 competition power model had the CG indicated at 110% back from the leading edge. That's right, it was 10% behind the trailing edge.

    What happens is that as the CG moves to 30% and further back is that the design starts to behave more like a tandem wing airplane. That is the stabilizer lifts upward same as the wing. The further back the more the stabilizer lifts.

    What the CG calculator does not tell you is that for a smaller size stabilizer you can over load the lifting area and it will have too high a "wing loading" and not fly well at all. The stab ends up "stalling" when loaded too strongly unless it's a larger size stabilizer like those old free flight models had with their 25 and 30% size stabs. Bad and confusing things occur at about that time.
    Witty saying to be plagarized shortly.....

  5. #5

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    Thanks I think I got it now

  6. #6

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    Quote Originally Posted by ahicks View Post
    Something lost on some people is that tail should carry some of the plane's weight. ....
    Does this apply with a conventional design with positive stability?

    Kurt

  7. #7

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    would depend on weather the stab had an airfoil or incidence both

  8. #8

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    Quote Originally Posted by Bozarth View Post
    Does this apply with a conventional design with positive stability?

    Kurt

    If the center of gravity is forward of the center of pressure, as it is on most conventional designs, the horizontal tail surfaces keep the tail from rising.
    Al Gunn
    Ultra Sport Brotherhood No. 9; Sig Kadet Brotherhood No. 22

  9. #9
    Moderator BMatthews's Avatar
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    Quote Originally Posted by Bozarth View Post
    Does this apply with a conventional design with positive stability?

    Kurt
    The lift from a wing through the normal range of flight is focused on the 25% chord point. Then if there is some camber then you also see some pitching moment. But if we look at symmetrical airfoils for simplicity the lift occurs at the 25% point with no pitching moment.

    So if the CG of the model is set to some point behind this 25% point as is very often the case then the horizontal tail will be lifting upward by some amount. It is purely related to the location of the CG with respect to the wing. The rest such as airfoils used on the stabilizer and angles between the wing and stabilizer only act to trim the model for a given stability margin and flight speed. This can alter slightly when we use camber in the wing. At some flight trims a CG at something like 30% with a strongly cambered airfoil might be lifting upwards. At some other speed it might be creating no lift and at yet a third speed might be lifting downwards. It depends on how strong the pitching moment is and the flight speed at any given time.

    But even with a strong pitching moment as we enlarge the stabilizer or lengthen the tail moment and use that to allow us to shift the CG rearwards the stabilizer will shift to lifting upwards all the time and more and more strongly the more the CG shifts to the rear.

    This is one reason why old time free flight models so often use lifting airfoils on the stabilizers. The use of an airfoil does not automatically make the tail a lifting tail. Instead it is related to the strongly rearward CG locations such models typically used. Anything from 35 to 60% for a location of the CG was not at all unusual depending on individual designs. One of my own models is an electric powered Henry Struck Record Hound. And at 50% CG it is actually still more nose heavy than is optimal for the glide. I'm waiting on a brushless motor to arrive and then I can install it along with the swap to light Lipo packs and finally get the CG back where it should be. I've only avoided ballasting it further back up to now because it was already quite heavy thanks to the old school geared brush motor it has lived with through the late 80's and all of the 90's.
    Witty saying to be plagarized shortly.....

  10. #10
    All Day Dan's Avatar
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    Mr Mathews, Am I correct to assume that when you say "chord" you mean the Mean Aerodynamic Chord? Dan.
    Dan

  11. #11
    Moderator BMatthews's Avatar
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    Yes, that would be the understanding on an overall wing. But when I was describing the lift at the 25% chord of any given airfoil it was more a 2D airfoil thing. But of course when that translates to a real wing it would be related to the MAC.
    Witty saying to be plagarized shortly.....


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