Wednesday, May 1

CONSERVATION OF ENERGY IN ANY AIRCRAFT

Energy Awareness and Energy Management in Aircraft

Note: You can buy a used airplane for about the same price as a new sports car.

Riddle: What's the main difference between the sports car and the airplane?

Answer: If you speed up the sports car to about 75 miles per hour and pull back on the steering wheel, nothing very interesting happens.

When piloting an airplane, two of your most fundamental duties are (1) controlling the airplane's speed and (2) controlling its altitude.

Performing these duties would be easy if the airplane were equipped with ideal controls , so that you could (1) move a lever that would immediately change the airspeed by a few knots, with no change in altitude, or (2) move another lever that would immediately change the altitude by a few dozen feet, with no change in airspeed.

Alas, it is physically impossible to build an airplane with such ideal controls. One purpose of this chapter is to explain how real controls affect the airspeed and altitude of a real airplane.

For example, consider the seemingly simple maneuver of changing speed while maintaining a constant altitude. We will see that this requires a complex sequence of adjustments of several controls. There are two ways to deal with this maneuver. One way would be to discover (by trial and error) the required sequence of adjustments, and perform that sequence by rote forever after. A far easier and better way is to understand the fundamental relationships, so that the proper sequence seems logical and obvious.

Understanding how the airplane really responds to the controls makes your flying not only easier, but safer as well.

Generally, a pilot who tries to control airspeed and altitude separately winds up controlling one or the other rather poorly. Usually it is the airspeed that suffers. All too often, the airspeed gets too low, whereupon the wing stalls and the pilot rather abruptly loses control. This is how the all-too-common stall/spin accident begins. You can stay out of this sort of trouble if you understand what the controls really do.

The key to understanding the relationship between airspeed and altitude — and several other things — is the concept of energy.

Energy is not a new¹ or complicated concept. Most pilots understand that being "high and fast" is very, very different from being "low and slow"; the concept of energy just makes this notion a little more precise and gives it an official name.

Good pilots think about energy all the time. The more critical the situation, the more carefully they evaluate the energy before reaching for the controls.

Once you grasp the basic concept of energy, you will be able to apply it in many ways, to many different situations. This is a big improvement over trying to figure out all possible situations one by one. Energy gives you the "big picture".

Energy Cannot Be Created or Destroyed

As illustrated in figure 1.1, there are four types of energy that are crucially important for airplanes, namely:

potential energy, which is proportional to the airplane's altitude;
kinetic energy, which is proportional to the square of the airspeed;
the chemical energy in the fuel; and finally
the energy left behind in the air as the plane passes through, stirring the air and leaving it slightly warmer.

There are of course other types of energy, but the four forms listed above are the ones pilots use all the time, so let's concentrate on them for now.²

Figure 1.1: Total Energy Cannot Be Created or Destroyed

Energy has the remarkable property that it cannot be created or destroyed. Energy can flow from one region to an adjoining region, and it can be converted from one form to another ... but the amount of energy remains the same. This rule (which physicists call the law of conservation of energy) is not one of Newton's laws; it was not even known in Newton's day.

Consider the analogy with freezing water: liquid water can be converted to ice and back again, yet the amount of H₂O doesn't change in the process. Similarly, if some water leaks away and we lose track of where it is, the number of H₂O molecules hasn't changed.

Similar³ notions apply to energy, as illustrated in figure 1.1. Fuel energy can be converted to altitude; altitude can be exchanged for airspeed; altitude can be cashed in to pay for drag; et cetera. The amount of energy doesn't change. The energy is just converted from one form to another.

Some of these energy conversions are irreversible. Fuel burn, for example, is a one-way street; we cannot (alas) operate the engine backwards and replenish the fuel supply. Similarly, when energy is dissipated by drag, that energy can never be recaptured in a useful form.

The airspeed and altitude together are called the mechanical energy . Engine power increases the mechanical energy, while dissipation decreases the mechanical energy.

Energy Conversion

Figure 1.2: Energy Conversion – Glide

Altitude is being cashed in to pay for drag. The airspeed is not changing, and no energy is being taken from the fuel tank.

Figure 1.3: Energy Conversion – Climb

Fuel is being consumed to pay for drag and purchase altitude.

Figure 1.4: Energy Conversion – Cruise

Fuel is being consumed to pay for drag. Altitude and airspeed are not changing much.

Figure 1.5: Energy Conversion – Zoom

If you pull back on the yoke, the airplane will slow down and ascend. If you do it quickly enough, drag will not have time to consume very much energy, nor will the engine have time to convert very much fuel.

Figure 1.6: Energy Conversion – Pushover

Conversely, if you push forward on the yoke, the airplane will speed up and descend. Once again, if you do it quickly enough, drag and engine power will not affect the energy budget very much.

Figure 1.7: Energy Conversion – Initial Roll

During the early part of the takeoff roll, drag is negligible. There is no change in altitude, so virtually all engine power goes toward building up airspeed.

Figure 1.8: Energy Conversion – Flare

An important conversion is the flare maneuver, which occurs at the end of every flight. It is possible to maintain altitude without using the engine, by gradually cashing in airspeed to pay for drag.

Figure 1.2 through figure 1.8 show several examples of how one form of energy can be converted to another. We now investigate energy-conversion processes in a little more detail.

Energy Management Strategy

The next step is to combine what we know about energy and develop general rules for energy management. Let's consider the four situations depicted in figure 1.17.

In the figure, as we go from left to right the kinetic energy increases; similarly as we go from bottom to top the potential energy of the situation increases.