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Thread: Determining Center of Gravity on a new design

  1. #1

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    Determining Center of Gravity on a new design

    I have calculated CG for my two homebuilts using CG limits the designer provided. I would like a broad-brush explanation of how a designer first calculates or otherwise determines the CG limits. Approximation followed by trial and error? Calculation only? What? Just wanted a general idea. Thanks.

  2. #2
    steveinindy's Avatar
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    Establishing the weights of everything that goes into the design and where it goes should give you some idea of where you're limits will be. Then you design/modify the control surfaces to meet the stability requirements. Normally you want to keep your CG in the 25%MAC range according to Raymer (p. 29, Simplified Aircraft Design for Homebuilders). Basically, for the initial design, you pretty much just take a guess. Later you can go back and see how far off the mark you were.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  3. #3
    Matt Gonitzke's Avatar
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    In general, the aft CG limit is slightly forward of the stick-free neutral point (calculated first, and then probably verified in flight test), and the forward limit is the forwardmost CG at which there is enough elevator control power to stall the airplane in the landing configuration with the power at idle.

  4. #4

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    Experienced designers know the general weight of each of the major subassemblies and can calculate their affect on the CG using something as simple as a spreadsheet, or graphically. If you are starting at the bottom of the larning curve, I suggest starting by making a list of all of the large components of your project airplane. You might start the list with the engine, propeller, main aircraft battery, wheels and brakes, tires, pilot, passenger, etc. You can look up weights for those pretty easily in the Aircraft Spruce catalog and the flight manuals of common airplanes. I will note that at places like Boeing, the engineers work from a book of standard parts that the company has built up over years from working with vendors and designing parts for previous projects. For the homebuilder, the Spruce and Wicks catalogs server the same function.

    For subassemblies that you will build from scratch like the wings, fuselage, and tail feathers, you can get comparable weights by looking at kits like the RV's. Wings for a monoplane will weight something like 80 lbs example. Tail feathers are each much less. If you want to get more precision, you can write up a simple bill of materials or component list for each subassembly estimate the amount of material in each part, and look up the weight per volume for the material you intend to use for each part. Then add it all up.

    The exercise is important but not complicated. The interesting part is moving stuff around to get the approxamate CG location you want. As mentioned earlier, you want the CG to be somewhere around the 25% wing chord location for most types of wings that are used in a homebuilt airplane. There are books that explain the trade-offs in moving your CG around. An aft CG is better for speed and aerobatics. You almost never want a forward CG.

    Best of luck,

    Wes

    N78PS

  5. #5
    bigdog's Avatar
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    The CG limits of a design are determined by calculation using the specific aerodynamics of the design. The aft limit is generally chosen as a margin ahead of the neutral point (where pitch stability goes divergent) to maintain acceptable stability derivatives (think pitch damping) and stick force gradients (pounds per G). Forward CG is normally based on elevator effectiveness and stick forces. We used to do a passable job with slide rules. However the limits are calculated they are verified with flight tests. Testing starts somewhere in the middle of the calculated range and gingerly moves the CG fore and aft verifying the expected stability derivatives and stick forces at each point.
    Regards,
    Greg Young
    1950 Navion N5221K
    RV-6 N6GY - 90% done, 90% to go
    3.5 L-2 projects

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