Definitions and Calculations — Page 63, Lesson 85

Let’s start with the conversion chart idea, because that’s the heart of what we’re looking at. On this aircraft, the station numbers and the balance arms don’t always line up directly. The chart exists to convert between the two. The key point is that the centre section of the airframe — stations 540 to 727 — is the same as the original aircraft, so those station numbers are coincidental with the balance arms. That means for that section, the station number and the balance arm are the same value. But outside that section, they’re not, which is exactly why the conversion chart is needed. Let me walk you through the two examples. First, converting body station 500E into a balance arm. Body station 500E equals 348 plus 110, which gives a balance arm of 458 inches. So the conversion adds 110 to the station number to get the balance arm. Second, converting a balance arm of 809 inches into a station number. Balance arm 809 minus 82 gives station number 727. So here, the conversion subtracts 82 to go from balance arm back to station number. And further examination of the chart shows that balance arm 809 is actually station number 727D — the letter D is a suffix that identifies a specific location within that station. Now, there are four practice questions here. 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? Question four: what is the distance of station 727C from the datum? The answers are on page 92, but I want you to think through the logic we just used — adding or subtracting the conversion factor depending on which direction you’re going. Now let’s move to the flap position effect on the centre of gravity, which is a separate but related concept. On a large aircraft like the MRJT 1, moving the flaps can have a considerable effect on the CG position. Table 4.3 shows the moment change to the aircraft when the flaps are extended or retracted. For example, retracting the flaps from 30 degrees to 0 degrees causes a total moment change of minus 15,000 kilogram-inches. Conversely, extending the flaps from 0 degrees to 40 degrees causes a total moment change of plus 16,000 kilogram-inches. So retracting moves the CG one way, extending moves it the other way, and the magnitude depends on the flap movement. Finally, the stabilizer setting for take-off. This is extracted from the graph at Figure 4.4. The purpose of this is to allow the stabilizer trim to be set so that the elevator has sufficient authority to enable the aircraft to be rotated during the take-off run and controlled during the first stages of flight. The position of the CG will determine the stabilizer setting for take-off. So the CG position drives the stabilizer setting — that’s the cause-and-effect relationship you need to remember.