Definitions and Calculations — Page 63, Lesson 85

Let's pick up right where the station-number conversion chart left off, because the two worked examples you just saw are the key to everything that follows. First, let me make sure the logic of that chart is crystal clear. The chart exists because the aircraft's body stations — the physical measuring points along the fuselage — do not line up with the balance arms, which are the distances from the datum used for weight and balance calculations. The centre section of the airframe, stations 540 to 727, is the original aircraft, so there the station numbers and the balance arms are coincidental — they match exactly. But forward of that, and aft of that, the numbers diverge, which is why you need the conversion chart at all. Now, the two examples. To convert body station 500E into a balance arm, you take 348 and add 110, giving you balance arm 458 inches. So station 500E sits 458 inches from the datum. To go the other way, converting balance arm 809 inches into a station number, you subtract 82, giving you station number 727. And the chart shows that balance arm 809 is actually station number 727D — the letter suffix distinguishes it from the plain station 727. Then there are four practice conversions for you to try yourself. Question one: what is the station number at the nose of the aircraft? Question two: what is the station number 1365 inches from the datum? Question three: what is the distance of station 500 from the datum? And question four: what is the distance of station 727C from the datum? The answers are on page 92, but I want you to work through them using the same add-or-subtract logic from the examples before you check. Now let's move to a completely different effect on the centre of gravity — flap movement. On a large aircraft, extending or retracting the flaps can shift the CG considerably, because moving those large surfaces changes the moment. Table 4.3 gives you the moment change for each flap position. For example, retracting the flaps from 30° down to 0° causes a total moment change of minus 15,000 kilogram-inches. Conversely, extending the flaps from 0° up to 40° causes a total moment change of plus 16,000 kilogram-inches. Notice the sign convention — retracting gives a negative moment change, extending gives a positive one, and the magnitude depends on how far the flaps travel. Finally, the stabilizer setting for take-off. This is extracted from the graph at Figure 4.4. The purpose of that setting is to allow the stabilizer trim to be positioned so that the elevator has sufficient authority — enough control power — to rotate the aircraft during the take-off run and to control it during the first stages of flight. And here's the crucial relationship: the position of the CG determines the stabilizer setting for take-off. So you read the CG position, go to the graph, and that gives you the stabilizer trim setting you need. Let me show you the cargo bay control panel and the linear loading diagram, because they tie into how you actually apply these numbers in practice. So to summarise what we've covered: the conversion chart lets you move between body stations and balance arms using fixed add or subtract values; flap movement changes the CG moment by known amounts depending on the flap angle; and the take-off stabilizer setting is read from a graph based on the CG position, specifically to give the elevator enough authority for rotation and early flight control.