For that to happen, the forces have to be in perfect balance — Page 234, Lesson 276

This is the start of Chapter 5, "General Principles – Cruise." We're moving from the takeoff and climb phases into the heart of the flight — the cruise. This chapter is all about the balance of forces that keep the aeroplane in steady, level flight, and then it builds a complete picture of the speeds we use, and how fuel flow, endurance, and range all fit together. Let me start with the very first topic: the balance of forces in level flight. I want you to picture the aeroplane in straight and level flight, cruising along at a constant altitude and a constant speed. For that to happen, the forces have to be in perfect balance. We have lift acting vertically upwards, and weight acting vertically downwards. In steady level flight, lift equals weight. That's the first couple — a couple being two equal and opposite forces that are separated by a distance, which creates a turning effect, a moment. Now, here's the key point I want you to grasp. The lift from the wing doesn't act through the same point as the weight. The centre of lift and the centre of gravity are at different positions along the fuselage. This separation creates a pitching moment — a tendency for the nose to pitch up or down. To keep the aeroplane balanced, we need a third force. That's where the tailplane comes in. The tailplane produces a downward force, a download, to counteract the pitching moment from the lift and weight couple. So in complete balance, we have lift up, weight down, and the tailplane providing a downward force to trim the aeroplane. That's the complete balance of forces in straight and level flight. Now, let's look at what happens when we move the centre of gravity. If we move the C of G forward, the distance between the centre of lift and the centre of gravity increases. That means the lift/weight couple gets bigger — the turning moment is greater. To balance that greater nose-down moment, the tailplane has to produce a larger download. So a more forward C of G requires more tailplane download, which means more drag, and that costs us performance. Conversely, if the C of G moves aft, the couple gets smaller, the tailplane download reduces, and we get less drag. But we can't just push the C of G as far back as we like — there are stability limits, and that's a critical balance for the pilot to manage. Let me show you this balance with a diagram. Here's the first illustration showing the lift and weight couple in straight and level flight. And here's the complete balance with the tailplane force. And finally, this one shows how a more forward C of G increases the couple and demands a greater tail download. Now, from that balance of forces, the chapter moves into aeroplane speeds. This is a crucial section because we have several different speeds, and each one has a precise definition. Let me walk you through them in order. First, we have Indicated Airspeed, or IAS. This is the speed shown directly on the airspeed indicator in the cockpit. It's what the instrument reads, before any corrections are applied. It's the raw reading from the pitot-static system. Next is Calibrated Airspeed, or CAS. This is the IAS corrected for instrument and position errors. The position error comes from the way the airflow around the aeroplane disturbs the static pressure at the pitot-static ports. So CAS is the accurate airspeed after we've removed those errors. Then we have Equivalent Airspeed, or EAS. This is the CAS corrected for compressibility effects. At high speeds, the air compresses in front of the aeroplane, which affects the pressure readings. EAS accounts for that, and it's the speed that relates directly to the dynamic pressure and the aerodynamic loads on the structure. After EAS, we get to True Airspeed, or TAS. This is the EAS corrected for air density. At altitude, the air is less dense, so for the same dynamic pressure, the aeroplane is actually moving faster through the air. TAS is the actual speed of the aeroplane through the air mass. This is the speed we use for navigation and performance calculations. Then there's True Ground Speed, or TGS. This is the TAS corrected for wind. If we have a headwind, the ground speed is less than the TAS. If we have a tailwind, the ground speed is greater. TGS is the actual speed over the ground — the speed that determines how long the flight takes. Finally, we have Mach Number. This is the ratio of the aeroplane's true airspeed to the local speed of sound. It's a dimensionless number. At high altitudes and high speeds, Mach number becomes the critical parameter because the speed of sound decreases with temperature, and the aeroplane's behaviour changes dramatically as it approaches Mach 1. Now, after speeds, the chapter moves into fuel flow, endurance, and range. Let me define these clearly. Fuel flow is simply the rate at which the engines consume fuel, usually measured in kilograms per hour or pounds per hour. Endurance is the total time an aeroplane can remain in the air on a given amount of fuel. Range is the total distance it can travel on that fuel. For a jet aeroplane, endurance is maximised by flying at the speed that gives the minimum fuel flow — the lowest fuel burn per hour. For a propeller aeroplane, the considerations are different because the propeller's efficiency changes with speed, so the optimum endurance speed is different. There are many factors that affect endurance — altitude, weight, wind, and the specific fuel consumption of the engines. Similarly, range is affected by factors like the cruise speed, the altitude, the wind, and the aeroplane's weight. The chapter goes into detail on each of these. Finally, we have Optimum Altitude and Long Range Cruise, or LRC. The optimum altitude is the altitude at which the aeroplane achieves the best fuel efficiency for a given weight — the altitude where the range is maximised. Long Range Cruise is a specific technique where we fly at a slightly higher speed than the maximum range speed, sacrificing a tiny amount of range for a significant gain in speed. It's the practical compromise used in airline operations. So that's the full map of this chapter. We start with the balance of forces, then the speeds, then the fuel management concepts of endurance and range, and finally the operational techniques of optimum altitude and long range cruise. Each of these builds on the last, and we'll work through them in detail.