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Home » John E. McLain

Flaps, Power and Airspeed: Understanding, rather than rote memorization should be every pilot’s goal

By John E. McLain (September 2001)

The inspiration for this article comes from you, the readers. My articles on Rudder, Yaw, Spins, Slips, and Skids elicited more reader response than anything else I had written in my “literary career.” I am happy to say the responses were, for the most part, complimentary. To all of you, I say “Thanks.”

I looked for a thread in the responses and found a very common one: understanding and correlation. Flight Instructors and, I hope, all teachers are taught the four levels of learning. They are Rote, Understanding, Application, and Correlation.

Rote is the ability to repeat something that has been taught without understanding or being able to apply or correlate it to new situations. What I learned from the comments to my articles was that all too many students had only learned about yaw, rudder, etc. to Rote. They did not really understand what was happening. The extent of their knowledge was that proper coordinated flight means “keeping the ball in the middle.” Even the method to do this was Rote. “Step on the ball” is a common instruction. Few pilots are able to relate rate of turn and angle of bank to proper coordination. Thus they lack understanding, and do not have the ability to apply and correlate the concept of coordination to other situations that may be encountered in flight.

I hope my articles on rudder, yaw, slips, and skids gave the readers a better understanding of them so that they can apply and correlate the concepts to become better and safer pilots. Now, I would like to address another area where understanding seems to be lacking by many pilots.

This is the correlation among power, airspeed, and flaps. Somehow pilots seem to have very little understanding about the relationship. I think it can be traced to the simple fact that they are taught each of these factors individually, to Rote, with little or no attempt at Correlation. So here we go.

All pilots, I hope, realize that it requires thrust and lift to keep an airplane aloft. It also requires energy to provide the thrust and lift. Three things, power, gravity, and airspeed provide this energy. Consider this. You are cruising along at a safe altitude when suddenly the engine decides to take a vacation and totally quits. You have now lost power, one of your energy sources. So how long can you maintain altitude? Gravity won’t do it. You have to lose altitude to use gravity as an energy source. The answer is that you can maintain altitude as long as you have airspeed in excess of the stall speed. In other words, without power you can trade the stored up energy of airspeed to maintain altitude. Once you reach the stall speed, the only energy source left is gravity, and you must lose altitude to use gravity.

To help you understand this to a greater extent, let’s look at a takeoff. Sitting on the end of the runway, getting ready to go, you only have one energy source: power. In goes the throttle and away you go, no problem with power management. However, one thing most pilots don’t consider is that they are beginning to store up the other two energy sources, gravity and airspeed. Once you lift off, you have altitude and excess airspeed, as well as power, to provide the energy you might need. Therefore, energy management enters your flight equation.

By now, you should have a reasonable understanding of the energy sources available to you, so now come application and correlation. How do you apply and correlate this knowledge to flight situations? Let’s look at the management of available energy to several flight situations.

First we will consider flight at minimum controllable airspeed. The entry to MCA is a good example of energy management. You first throttle back and get rid of power, then you use airspeed in excess of stall speed to maintain altitude. Finally, you reach a speed slightly above stall where you no longer have any speed energy available. You then bring the power back in, since it is the only energy source available to maintain altitude. There you have the extreme example of power management. You start by using only airspeed to maintain altitude and finish with only power to maintain it. This is one reason why it is called the area of reverse command.

Now, before I go any farther I must bring drag into our equation. The reason is that energy is used to overcome drag. Without drag, we would need no energy to sustain flight. If we could eliminate drag, we would need no energy to maintain speed and altitude, either, once sufficient speed was reached to create enough lift to overcome gravity. This may be confusing concept, but I will revert to Rote for the moment, and ask that you accept it.

While I know there are all sorts of sophisticated drag items on some airplanes, such as slats, spoilers, speed brakes, and the like, for the purposes of this article we will only consider two: lift and flaps. I think you all know that when you create lift, you create drag. To be more precise, when you increase lift or angle of attack you increase drag.

This is known as induced drag. Flaps create induced drag to some degree, because they create lift. They also create parasite drag.

This leads to the other reason why flight at MCA is called the area of reverse command. Without power, maintaining altitude means using airspeed as your energy source by increasing pitch, which means an ever-increasing angle of attack, which means ever-increasing drag. Ultimately we would reach a point where the available airspeed cannot overcome the increased drag, and any additional pitch results in loss of lift and the inability to maintain altitude. Thus, the area of reverse command. Pull the nose up and the airplane descends. Old-timers had a simple explanation for this. “If you want to go up, pull the nose up. If you want to go down, pull it up some more.”

Now let’s look at the most serious area where energy and drag management become important: the approach and landing. Unfortunately, this is an area of flying where Rote all too often rears its ugly head.

If I do ten private pilot flight tests next week, I can guaranty that no fewer than eight applicants will follow the same routine. Somewhere on downwind, usually opposite the touchdown point, they will reduce power and start application of flaps. On base leg there will be another application of flap, and then on final the remainder of the landing flaps are applied. If, at any point in this procedure, I ask the applicant why flaps were applied, the answer will be something like: “That is the way I was taught,” or “I always do it that way.” Does it sound familiar?

Let’s look at how we are really trying to manage flaps and energy to accomplish the normal approach and landing. To begin with, let’s define what we mean by the normal approach and landing. To me it is very simple. The normal approach and landing procedure is the safest one. With this in mind, let’s look at how to apply energy and drag management to achieve it.

Before we go any farther, I want to stress that I am not trying to tell any pilot how to fly a traffic pattern. I am trying to deal in understanding and correlation, so that each pilot can develop his own traffic pattern and landing procedure. More important, I hope that an understanding of energy and drag management can give each reader an understanding of how to adapt to unusual situations. For what it is worth, my personal philosophy is simple. I want to conserve energy as long as possible. In other words, minimize drag until the situation dictates otherwise.

As you approach the traffic pattern, your first concern is to slow down from cruise speed. You have two options: reduce power or increase drag. What is your choice? Mine is to reduce power. Once in the pattern, you must slow to a reasonable approach speed. All too often, this is accomplished by application of flap, which means sacrificing power. I want the reader to ask, why add flap to slow down when you are still carrying power? When driving your car, do you apply the brakes before you reduce power? If you have a good reason, fine, but “that’s what I was taught to do” is not a good reason.

I can come up with many situations where the use of flaps while sacrificing an energy source, such as power, is perfectly appropriate. That is not the point of this article. Rather, I hope that the reader will gain understanding of the energy-drag relationship, apply it more appropriately to normal flight situations, and be able to correlate it to unusual situations.

It’s all about energy management, and making the best use of the energy available to you as a pilot.