![]() If we think about a car going over a hill, however, the process is not hard to understand. These, of course, are vehicles limited to the altitude of the road or water surface. Most of us are conditioned by experience with cars, boats and bicycles to think of speed increase as a consequence of adding power. The concept of adding power to increase altitude (climb) is usually not intuitive. This can be determined from the power performance information studied in the last chapter. The maximum rate of climb at a given speed will then depend on the difference between the power available from the engine at that speed and the power required for straight and level flight. ![]() To climb at that same speed then requires extra power and the amount of that extra power will determine the rate at which climb will occur. We are aware that a certain amount of power is required for straight and level flight at a given speed. The engine provides the needed energy for climb and the engine energy output per unit time is power (work per unit time). Rate of climb then involves the change of potential energy in a given time. In climbing, the aircraft is increasing its potential energy. One of the questions above involved the rate of climb. In a glide we are converting potential energy into velocity (kinetic energy) which will give us needed lift for flight. In climb we are turning kinetic and internal (engine) energy into an increase in potential energy. To look at altitude changes we need to think in terms of energy changes. How fast can I get from altitude A to altitude B? How far can I glide after my engine fails? If I take off 600 feet from the end of the runway, can I clear the trees ahead? The question to be answered now is how do we get the aircraft from one altitude to the other? This discussion must include the investigation of possible rates of climb and descent, the distance over the ground needed to climb a given altitude and the range of the aircraft in a glide. We know that from the straight and level data we can determine the theoretical maximum altitude, or ceiling, for a given aircraft. We must now add another dimension to our study of performance, that of changes in altitude. Through the basic power and thrust performance curves considered in the last chapter we have been able to investigate the straight and level flight performance of an aircraft. ![]() If the pilot changes the throttle setting, or increases the wing angle of attack, the forces become unbalanced. The aircraft will move in the direction of the greater force, and we can compute acceleration of the aircraft from Newton’s second law of motion.\) With a constant ground speed, it is relatively easy to determine the aircraft range, the distance the airplane can fly with a given load of fuel. The motion of the aircraft is a pure translation. The ground speed is equal to the airspeed plus the wind speed using vector addition. If we take into account the relative velocity of the wind, we can determine the ground speed of a cruising aircraft. The aircraft maintains a constant airspeed called the cruise velocity. While the weight decreases due to fuel burned, the change is very small relative to the total aircraft weight. In this situation the lift is equal to the weight, and the thrust is equal to the drag. The closest example of this condition is a cruising airliner. In an ideal situation, the forces acting on an aircraft in flight can produce no net external force. If there is no net external force, the object will maintain a constant velocity. From Newton’s first law of motion, we know that an object at rest will stay at rest, and an object in motion (constant velocity) will stay in motion unless acted on by an external force. If the size and direction of the forces acting on an object are exactly balanced, then there is no net force acting on the object and the object is said to be in equilibrium. A force is a vector quantity which means that it has both a magnitude (size) and a direction associated with it. There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. Home > Beginners Guide to Aeronautics Airplane Cruise – Balanced Forces
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