Definitions and Calculations — Page 33, Lesson 41

Let’s start with the big idea that drives everything in this chapter: the position of the centre of gravity — the CG — decides how stable or how manoeuvrable your aircraft is. These two qualities pull in opposite directions, and the CG position is the lever that trades one against the other. Imagine the fuselage and its mid position. If the CG moves forward, towards the nose, stability increases progressively, and manoeuvrability decreases progressively. If the CG moves aft, towards the tail, manoeuvrability increases and stability decreases. So you can’t have maximum of both at once — you’re always balancing them. Why does that matter to you as a pilot? Too much stability means the flying control stick forces go up, and the workload on you — trying to overcome those forces — goes up too. Too much manoeuvrability makes the aircraft unstable and genuinely difficult to control. So the manufacturer sets a CG range of movement limits. The purpose of those limits is to ensure that the average pilot can control the aircraft safely through all stages of flight, with normal piloting effort, free of fatigue. Now, an important point: with regard to aeroplanes, the CG is not fixed. It moves in flight as a result of fuel burn, flap positions, and crew and passenger movements. And it is the aircraft operator’s responsibility — not the pilot’s alone, but the operator’s — to ensure that the CG movement stays within the limits imposed by the manufacturer. The next part of the text walks through what happens if the CG exceeds the forward limit. These effects are frequently asked in the exams, so I want you to learn them well. First, drag increases. Consequently, fuel consumption increases, and range and endurance decrease. Here’s the mechanism: to keep the nose from pitching downwards, the tailplane must produce a balancing down load — think of a see-saw. The resulting elevator deflection increases drag, which in turn increases fuel consumption and reduces range and endurance. Second, longitudinal stability is increased. That means higher control column forces during manoeuvres, a corresponding increase in physical effort to overcome them, and increased pilot fatigue. Third, the increase in tail down force is equivalent to an increase in weight. Consequently, the stall speed increases. And an increased stall speed has a knock-on effect on other performance aspects: take-off and landing speeds increase, the available speed range reduces, and the safety margin between low-speed and high-speed buffet narrows. Fourth, the ability to pitch the nose up or down decreases, because of that increased stability. Fifth — and this is a detailed one — take-off speeds V1, VR, and VMU all increase. Let me unpack those. V1 is the take-off decision speed, VR is the rotation speed, and VMU is the minimum unstick speed. On the ground, the aeroplane rotates about the main wheels and uses the elevators to raise the nose for take-off. The CG, being ahead of the main wheels, produces a down force that the elevators — together with the speed of the airflow passing over them — must overcome. The more forward the CG, the greater that down force, and for a particular elevator deflection, the greater the speed of the airflow required. So the aircraft must accelerate for longer to produce the airspeed required. Then there’s a summary table of the effects of a forward CG. Take-off and landing run increase. Range and endurance increase. Rate of descent increases. Max horizontal speed increases. Rate of climb decreases. Max altitude decreases. Fuel consumption increases. Braking energy increases. Tyre wear increases. Structural fatigue increases. Notice the pattern: a forward CG pushes several performance numbers in the wrong direction — climb, altitude, and fuel consumption suffer, while braking energy, tyre wear, and structural fatigue all go up. That’s the price of the extra stability. So the takeaway for you: the CG position is a trade between stability and manoeuvrability, the manufacturer’s limits protect the average pilot from fatigue and loss of control, and a forward CG — while more stable — costs you in drag, fuel, stall speed, take-off speeds, and structural wear.