I use AutoCad but any CAD program will have the different line types or if your on a board then draw out a centerline and start from there.Īs I learned from my early start in aviation, the datum line is a weight and balance reference. It's like plotting an airfoil, so many inches from the ( God I hate to confuse you here ) front end of the chord line and so many inches above and below the chord line. If your doing scale then just loft from the centerline to the points reference. From there it is just a matter of picking the points and drawing the lines or lofting as it is referred to. I start with my fuse length and divide it by 2 to get the center of the fuse length and plot a reference on the centerline. When I start the line is usually two to three inches longer than my planned subject. In all my years of designing I've never heard it revered as a horizontal datam line, however I guess someone could call it that. When designing, the first line I draw is a centerline and then all measurements for formers, height, width are referenced from there. Gord, I think what your referring to is the CENTERLINE. How about the FAAs reference to the horizontal datum line. Key words in FAA reference are from which all measurements OF ARM are taken. I have never seen the negative and positive weights reversed but it is possible. So, in reference to the datum line, all weight forward is indicated as a negative while anything aft is referenced as positive. The datum line is only a starting point from which to measure the moment arms (distance) from the line. It has nothing to do with aircraft design whatsoever. To answer the original question at the beginning of the thread, the datum line is not a calculation but an arbitrary reference line in the vertical plane. range must be taken with reference to that point." Once the datum has been selected, all moment arms and the location of permissible c.g. The datum is established by the manufacturer. "Datum (reference datum)-is an imaginary vertical plane or line from which all measurements of arm are taken. Here is the official FAA definition of the Datum Line.
The Datum line you guys are talking about isnt called a datum line its some other reference line that I would agree with what Mike explained it as though I personally didnt know what it was called but knew it wasnt a datum line. And it always seems to follow in a line along the widest part of the fuse formers which is very helpful when Lofting your fuse formers and has nothing to do with CG or its location. From what I always was told is the datum line was basically a Horizontal reference line that followed the thrustline on the plane. Ok U guys cornfusing me on the Datum Line. Again, this is not necessary for small models but when you have something cracking the 55# limit, it's not a bad idea. It give a complete description of the weight and balance process. Search for AC91-23A, Weight and Balance for Pilots. The FAA website (faa.gov) has the Advisory Circular available for free. Divide the moment by the total weight and you will then have the location of the c/g. When you have recorded each moment, total them up. Record the weight on each scale referencing the distance you measured. It dosen't matter whether the airplane has a nose or tail wheel. Measure from this line to the center line of each gear leg. Mark this with a line perpendicular to the longitudinal axis of the airplane. Level the aircraft and suspend the plumb bob at your datum point. You would need three decently calibrated scales, a plumb bob and tape measure. The basic calculation is W/A=M or Weight times Arm equals Moment. The most common locations can be anywhere from the nose to the wing leading edge. The underside of the movable jaw is 10mm from the datum surface but it is in the opposite direction and hence is labelled '-10'.As Starcad said, the datum is an arbitrary vertical line used for c/g calculations. The 15mm dimension to the centre line of the bolt and the 30mm dimension to the top of the movable jaw are dimensioned from the datum surface. In the example shown in Figure 4.2, the datum faces are the flat faces where the underside of the hardened insert contacts the step in the movable jaw.
The definition of a datum feature will be dependent upon the functional performance of a particular component. Thus, the various features on a component are dimensioned with respect to datum features. Reality datum features since it is with respect to features that other features relate. The datum point or points on a component are in A datum, is a point, line or surface of a component to which dimensions are referred and from which measurements are taken during inspection.
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