Hi,

Can someone tell me what the general method for calculating CG is please? I remember calculating this in the past by using a drawing and adding the tip chord to the root chord, vice versa, diagonal lines, 25% MAC.....

Can someone just tell me please?!!

Thanks a lot,
Al

See:
http://www.scalesoaring.net/EMM/cg%20calc.htm
which has an on line program to calculate the MAC. You have to pick the percent of the MAC to place the CG. What that percent of the MAC should be for the CG location depends on the tail moment arm length in telation to the MAC, the horizontal tail area in relation to the wing area, the desired degree of stability and a whole bunch of additional minor considerations. The desired degree of stability depends on the purpose of the model and the flying style of the pilot.

As a practical matter it is best to begin flight testing with plenty of stability and adjust it by moving the CG in small increments.

Once you have the M.A.C. of your wing & stab calculated, go to http://www.aalmps.com/tv.htm and use the Tail Volume Coefficient formula listed there. Once you have that number, use Al's rule of thumb CG formula and you'll have a very educated point to start from.

I have used this method very successfully of late. In fact, I have gone back and checked some of my previous designs that have had their CG positions arrived at by the "lots of flight testing" method and found that I could have saved a lot of time by using Al's calculations from the start! The differences between the calculated positions and the as tested positions were very small.

Dickeybird,
I generally design to a tail volume coefficient of 0.5 as in aalmps' example but I end up with a CG at about 40% of the MAC instead of 34% recommended by aalmps. Some of that difference may be due to flying style and some due to the fact that my fuselages are skinny with no destabilizing prop or landing gear and my horizontal tails are higher aspect ratio. The fact that you like aalmps' recommendation just means that you have similar flying styles and/or taste in plane types.

A trainer which is setup with lots of stability will have a CG that is about 5 to 7% of the MAC farther forward than an aerobat that is set up with little or no static stability even if the TVC's are the same.

Is there a rule/theory about the relationship between the tail moment and the tolerance of moving the CG aft?

I have found, if the tail moment is short, it is less tolerant of a change in CG back. . . . Hard to explain.. . . In a model I have with a tail moment of ~1.3, flies great with a CG of 25%( 28% is unflyable ). A model with a tail moment of 2.5, flies great with a CG of 33%. I like them at neutral stability.

The longer the moment, the less critical a CG change will have?

If you gradually move the CG aft between flights, you will come to a point where the plane goes where it is pointed and has no tendency to correct itself to its original flight path when disturbed by a gust or brief control input. That is called the Neutral Point (NP). If you move the CG aft just a tiny bit the plane will become difficult to control because it will always try to diverge from its flight path and with a little more aft CG the plane will become uncontrollable because it diverges so violently. The neutral point is the aerodynamic center (AC) of the whole aircraft. As the CG is moved ahead of the NP the plane becomes more stable. The distance that the CG is ahead of the CG is called the static margin (SM) which is expressed as a percent of the mean aerodynamic chord of the wing. As the SM becomes larger the tendency of the plane to return to its original flight path gets stronger and the response to yaw and pitch controls get weaker. The tendency to correct can become so strong that the plane overshoots the corrections. The corrections can even become violent enough as to throw the plane into a stall. The damped oscillations from a correction have a period and if the period is similar to the reaction time of the pilot, then pilot induced oscillations can result. There is a range of safe CG locations. The best place for the CG within that range is a matter of the purpose of the aircraft (eg. trainer or aerobat) and the skill and flying style of the pilot.

The location of the NP is a matter of the horizontal tail area as a fraction of the mean aerodynamic chord (MAC) of the wing times the tail moment arm length expressed as multiples of the MAC. This product is called the tail volume coefficient (TVC). The larger the TVC the farther aft the NP will be. Also, the damping factor for oscillations increases as the square of the tail moment arm length and linearly with tail area. The actual location of the NP is affected a little by the location of props, landing gear and the size, the aspect ratio of the tail and the shape of the fuselage. In models, the AC of the wing airfoil can actuall shift a little with changes in angle of attack. the uncertainty which these small influences introduce make the calculation of precise NP location a little uncertain. It is because of this that the safe starting point for CG adjustment should be conservative for first flight and for the flight testing and adjustment process of the CG to begin.