January 2002Features

Behind The Power Curve

The Place You Don't Want To Be


Jet fighters are easier to fly than most general aviation aircraft. It's true! I came to this conclusion after 30 years of flying during which I logged more than 2,000 hours in the F-4 Phantom II, 500 hours in the F-16 Fighting Falcon, 500 hours in the F-15E Strike Eagle, and more than 1,000 hours in a wide variety of single-engine GA aircraft. And I say this for a number of reasons.

First, being considerably heavier than light airplanes, fighters are much more stable in the wind. You fly them to the runway and, hence, they are a lot easier to land in a crosswind. Second, redundant systems that are integral to jets make most emergencies a lot less critical in fighters. Besides, there's always an ejection system that allows you to walk home if circumstances so dictate. But, perhaps the greatest difference between the two that makes fighters so much easier to fly is excess power.

Each generation of fighters has had a progressive increase in the power-to-weight ratio. To give you an idea, consider that the latest model of the F-15 has 30 percent more power than the model that immediately preceded it. The latest fighter, the F-22, can even cruise supersonic. It is the excess power of jet fighters that can correct for a multitude of sins. But there is one sin it can't correct for - getting too far behind the power curve.

Every airplane has a power curve, and every power curve has a back side. Understanding the power curve is essential if you want to be in control of your airplane at all times. And, to understand the power curve you must first understand drag.

You may recall that during your ground-school days you learned drag in an airplane comes in two forms: parasite and induced. And after taking your FAA knowledge test, that is probably the last time you thought about drag at all. Parasite drag is primarily caused by the shape of the airplane and the friction of the air over the skin of the plane. The faster the airplane flies, the more parasite drag it creates. In fact, if you double your speed, you increase parasite drag by four times! Parasite drag is very noticeable. It is why those last few knots of increased airspeed increase fuel consumption so much. It is also why we normally fly at around 75-percent power.

Less noticeable, but even more important, is induced drag. Induced drag is the result of lift. Without going into great detail of the physics of the phenomenon, suffice it to say that induced drag is greatest at slow speeds and decreases at a geometric rate as speed increases. Of course, when you stall, you have no drag; you only have gravity.

The reason we need to know about these two types of drag is that they both act upon our airplanes any time we are flying, and together they dictate such factors as maximum endurance airspeed, minimum range airspeed, and power required.

In addition to displaying the total drag acting on your airplane, this "total drag" curve also dictates the required thrust to overcome it. But don't confuse the two. Thrust is the force generated by the propeller. Power is the actual work done by the engine turning the propeller.

Despite the similarity in the shape of their curves and the fact that they are both the result of drag, the power curve and the thrust curve do not have corresponding points. For example, the lowest point on the power curve represents the speed for maximum endurance and minimum sink rate, while the lowest point on the thrust curve represents optimum glide speed providing maximum range.

So let's take a closer look at that power curve. Remember that it shows the power required to maintain a given indicated airspeed while flying at a constant airspeed. Now let's put some numbers on the curve.

We are flying our theoretical airplane at a constant indicated airspeed and at a constant altitude. It is amazing to most pilots that it takes the same power to fly at 30 kt as it does to fly 100 kt. This is the effect of induced drag.

You will also notice that our power curve has a "front side" and a "back side." We spend most of our time flying around on the front side. To go faster while flying on the front side (maintaining constant altitude), you must add power. That makes sense. However, on the back side, to fly slower you must actually add power. This takes a little thought. So let's take a closer look at the back side of this power curve. If the numbers I've picked for our theoretical aircraft look a lot like those you see when on final approach, it's because that is when we spend most of our time flying on the back side.

The VASIs tell us that we are too low as we approach the airfield. So we maintain our airspeed of 65 kt and trim the aircraft to maintain our attitude. If you drop the nose momentarily, your airspeed will increase and move off the power curve. The laws of physics require you to reduce power if you are going to maintain this higher airspeed and not change your altitude. The opposite is also true.

If on your "dragged-in" final, you perceive that you are a bit low on altitude and you raise the nose, your airspeed will slip a little bit. And if you don't add power, you will actually increase your sink rate because induced drag has caused you to have a deficiency of power on the back side of the power curve.

Adding more back pressure to stop the sinking only exacerbates the situation. To get out of trouble, you must lower the nose and add power. If you wait too long or don't have enough power to recover, you are going to hit the ground short of the runway. This is why your instructor probably told you that, on final, you control altitude with power and airspeed with pitch.

So how does your airplane fly "behind the power curve"? I suspect that your pilot operating handbook doesn't have a set of power curves. The next time you have some free time, you might decide to collect data for drawing your own power curves. It's really very simple. Trim the aircraft for level flight at a given power setting. Write down both the airspeed and power setting. Then reduce your airspeed in 10-kt increments without changing your altitude and continue to record airspeed and power settings.

Eventually, you will reach a point at which more power is required to maintain a slower airspeed. You are now officially "behind the power curve." Once you know you're on the back side, push the nose over and observe what happens. Then raise the nose and make some similar observations. Continue to do this while maintaining altitude. When you have finished, you will know exactly how your airplane will act while flying under these circumstances.

Once you have mastered how to physically fly your airplane behind the power curve, you can better keep from entering this flight regime unintentionally.

Power Behind The Curve

Thrust couldn't keep this jet out of trouble.

It could happen to any pilot. You would never expect it to happen to a highly experienced fighter pilot in an F-16, but it did. Here's the true incident I witnessed:

According to the pilot, it had not been a good flight. He had been unable to accomplish much on this training mission as the head-up display in his F-16 had failed and the only thing he could do was serve as a target for the other fighters. This had been very frustrating, and now he was returning to his home base very low on fuel. Common sense dictated a straight-in approach, but from habit, he flew a standard overhead pattern.

On downwind, he noticed that the nose-gear light was not illuminated. By the time he realized that it was just a burned-out bulb, he had extended his pattern. This resulted in a dragged-in final approach. His airspeed was normal, well above stall. About a mile out on final, he realized he was too low, so he raised the nose. His airspeed slipped, and the vertical velocity indicator (vertical speed indicator to civilians) showed an increase in his descent rate.

He gingerly inched the throttle forward, but he was still descending, so he pulled the nose up farther. The airspeed declined, and the sink rate increased. Now he was in afterburner. But he was too late. The fighter jet fell out of the sky onto the runway approach and overrun, shearing off the right main gear and causing the aircraft to spin off the runway to the right.

The hapless pilot told the accident investigators that he was sure that he had been caught in the wake turbulence of the aircraft landing in front of him. But he was wrong. He had been caught on the back side of the power curve. And if it can happen to him, it can happen to you.


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