So we'd like to see the CG lying on the x-axis somewhere between 1.590 and 1.770. Running some quick calculations, we find the allowable range on the x-axis is: YASim prefers solutions with the CG in the aft range, so let's shoot for something in the back half of that range, between 25% and 33% MAC. A handbook calls for a CG range of 19% to 33% MAC (mean aerodynamic chord). Let's assume we have an small aircraft with an empty weight of about 2700 lbs, a nose at x=4 and a mid-chord wing located at x=1.446. Don't worry about getting a solution, at this point all you care about is the "CG: x:" value. Run a test against the command line solver. It may be helpful in placing the CG.Īfter choosing a CG position within the range and identifying its position along the x-axis, the next step is to get YASim to place the CG at this desired location. This is a typical estimate used for many airliner designs to be certain enough weight rests on the nose gear to provide good traction for steering. If you don't know the CG range and your model uses tricycle landing gear, you can make a good guess at the CG position by assuming about 8% of the aircraft's weight rests on the nose gear. ![]() Find the location of this point along the x-axis and you have your target x-axis CG position. Once you know the MAC, you can locate your CG range as a percentage of the chord from its leading edge. Now, treat these two chords as the root and tip chord of a new "wing", and repeat the procedure to give a single MAC for both surfaces. Perform the above exercise for each wing section to find the MAC for each. Treat the inboard and outboard sections as two different wings. Easy-peasy.įor aircraft with multiple wing segments featuring different geometries like many airliners, you can use the same strategy repeated. Draw lines between the extremes at either end, and their intersection indicates the location of the MAC, the mean aerodynamic chord. Add the length of the root chord to the leading and trailing edges of the tip chord, and do the opposite at the root chord. The image at right illustrates the use of geometry to find the MAC for a swept and tapered wing. In these situations you'll need to do a little work to find where the MAC lies along the wing. It's trickier with wings that have a taper and a sweep, and worse with airliner wings that often have two fundamental geometries, one for the long-chord inboard section, and another for the slender outboard section. In these cases, 25% MAC is just 25% of the way back from any point on the leading edge. This is easy with a wing shaped like a Hershey bar, where the airfoil shape and location is constant for the entire span. The goal is to learn where the CG range exists along the station-line or x-axis. If you lack a datum reference for placing the CG range, you'll have to calculate MAC. It's easier to find YASim solutions for aft-placed CGs for reasons I'll explain later. I suggest choosing a location in the 25-30% range, favoring the aft end. If you cannot find a CG range, you can make a guestimate based on ranges of 20-30% of MAC. The datum will given on the certification sheet. ![]() The CG limits can be found on FAA certification sheets expressed as a percentage of MAC (mean aerodynamic chord, aka SMC, standard mean chord) or as offsets from some datum, usually the aircraft's nose or the wing's leading edge at the fuselage. If it isn't, the plane could become unstable in normal flight attitudes, a Bad Thing. For most general aviation aircraft, the CG needs to be located forward of the aerodynamic center for the entire plane. The first step is to identify where the real CG should be. If the aircraft CG is positioned right, it's possible to get a solution that flies well on your very first try with the solver. Don't even think about tuning other YASim numbers until you have the CG positioned correctly. After geometry, center of gravity (CG) placement is the most important aspect of creating your FDM. You can get in all sorts of bad situations with a CG that is too far forward or too far aft. In a real plane, it's also critical for safety. Good weight distribution and aircraft balance is necessary for good flight characteristics.
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