General Principles - Cruise — Page 241, Lesson 283

We're moving into the cruise phase of flight now, and I want to start with something that ties directly back to the weight and balance work you've been doing. Let's look at how the centre of gravity position affects the aeroplane's performance in cruise. You'll remember that in straight and level flight, the aeroplane is balanced around its centre of gravity. The lift acts upward, the weight acts downward, and together they form a couple — a pair of equal and opposite forces separated by an arm. Now, if the centre of gravity moves forward, that arm between the lift and the weight gets longer. That means the lift/weight couple becomes stronger, which increases the nose-down pitching moment. To balance that greater nose-down moment and maintain level flight, the tailplane has to produce more down force. And here's the performance penalty: that extra tailplane down force increases the effective weight of the aeroplane and increases drag. More drag and more effective weight means the aeroplane's performance suffers — both its range and its endurance are reduced. Now let's move the centre of gravity aft, toward the rear. The arm between lift and weight becomes shorter, so the lift/weight couple is weaker. That decreases the nose-down pitching moment. To balance it, you need less tailplane down force to maintain level flight. Reducing tailplane down force reduces drag and reduces effective weight, which increases both the aeroplane's range and its endurance capability. So you can see the trade: a more aft centre of gravity is generally better for cruise performance, but you must always ensure the centre of gravity stays within the limits published in the aircraft manual. And I want to add a practical caution here — be extra careful when handling data produced by countries whose units of measurement are different from those you're used to. A simple unit conversion error in a weight and balance calculation can put you outside the envelope. Now let's turn to aeroplane speeds. This section deals with maximum speed, minimum speed, and the relationships between the various expressions of speed — indicated airspeed, calibrated airspeed, true airspeed, true ground speed, and Mach number. Let's start with maximum speed. You've learned that an aeroplane remains at a constant speed when the forward and rearward forces are balanced — that is, when thrust equals drag. In straight and level flight, if thrust equals drag, the speed stays constant. To accelerate, thrust must exceed drag. The pilot achieves that by opening the throttle further. With thrust greater than drag, the aeroplane accelerates. But as it accelerates, drag increases. When drag rises to the point where it equals the thrust again, acceleration ceases and the aeroplane is back in balanced flight — but now at a higher speed. So here's the key idea: the highest level flight speed the aeroplane can achieve occurs at the speed where thrust is maximum and drag is maximum. Let me show you this on a graph. If we plot thrust and drag curves for a typical jet aeroplane, the intersections of those two curves represent the maximum and minimum straight and level flight speeds. The maximum speed is achieved at the point where thrust and drag are equal on the high-speed side. It's impossible in straight and level flight to accelerate any faster than that, because beyond that point the drag would exceed the thrust. That speed is the fastest the aeroplane can achieve in level flight. So to summarise what we've covered: the centre of gravity position directly influences the tailplane down force needed, which in turn affects drag, effective weight, range, and endurance. And the maximum level flight speed is set by the balance point where thrust and drag are equal — you can't push past it in level flight. Next we'll look at the minimum speed side of that same graph, and then we'll get into the different expressions of speed.