General Principles - Cruise — Page 241, Lesson 283

Let’s pick this up right where the centre of gravity left off, because the whole point of that discussion was performance. I want you to remember the key result: if the centre of gravity moves forward, the lift/weight couple gets stronger — the arm between the lift and the weight is longer — so the nose-down pitching moment increases. To balance that and hold level flight, you need more tailplane down force. That extra down force effectively adds to the weight and increases drag, which hurts performance: it reduces both range and endurance. Now flip it. If the centre of gravity moves aft, the arm between lift and weight gets shorter, so the lift/weight couple is weaker and the nose-down pitching moment decreases. That means you need less tailplane down force to maintain level flight. Less tailplane down force means less drag and less effective weight, so both range and endurance capability increase. So the takeaway is a direct trade: a more aft centre of gravity, within limits, is generally better for cruise performance. And that brings me to a critical operational point. Before you fly, when you do your weight and balance check, you must ensure the centre of gravity is still within the limits published in the aircraft manual. And be extra careful when handling data produced by countries whose units of measurement are different from the ones you’re used to. That’s a real trap — a kilogram versus a pound, or a metre versus a foot, can put you outside the envelope without you realising it. Now let’s move into the next big topic: aeroplane speeds. This section deals with maximum speed, minimum speed, and the relationships between the different expressions of speed — indicated airspeed, calibrated airspeed, true airspeed, true ground speed, and Mach number. We’ll get to those definitions shortly, but first let’s establish what maximum speed actually means. You’ll recall from earlier chapters that an aeroplane stays at a constant speed when the forward and rearward forces are balanced. In straight and level flight, that means thrust equals drag. So for the aeroplane to accelerate, thrust must exceed drag. The pilot does that by opening the throttle further. With thrust greater than drag, the aeroplane accelerates. But here’s the key: as the aeroplane accelerates, drag increases. When drag rises to the point where it equals thrust again, acceleration stops and the aeroplane is back in balanced flight — but now at a higher speed. So the highest level flight speed the aeroplane can achieve is the speed where thrust is maximum and drag is maximum. Let me show you that on a graph. Look at Figure 5.5 — those are the thrust and drag curves for a typical jet aeroplane. The intersections of the thrust and drag curves represent the maximum and minimum straight and level flight speeds. The maximum speed is achieved exactly where thrust and drag are equal. It is impossible in straight and level flight to accelerate any faster, because beyond that point drag would exceed thrust. That speed is the fastest the aeroplane can fly in level flight. So the picture you should hold in your head: thrust pushes forward, drag pulls back, and the speed at which they balance is your level flight speed. Push the throttle open, thrust wins, you accelerate, drag builds, and you settle at a new, higher balanced speed — up to the point where thrust and drag meet at their maximum. That’s your maximum level flight speed.