The power curve

The Power Curve

Counter courtesy

Beware the back side of the Power Curve

Wow, almost sounds like something Darth Vador would say from a Starwars episode, doesn't it? It isn't of course, but to you the pilot it could be every bit as dangerous as some ot the tools that Darth has available to him from the "Dark Side."

What is the "Power Curve?"

First off, you have to accept the fact that a powered object, be it the Space Shuttle or your trusty Speedwing 200 climbs for one reason and one reason only- an excess of power. "Now wait a minute", you're thinking, "I can just raise the nose at cruise and my aircraft will climb." Well, yes it will- until you reach a point where, without adding any power, the old gal just won't go any higher. It was able to climb because you had "excess power" set in.

OK, I admit it. An airplane can climb for two reasons- excess power and excess airspeed. And you can trade off excess airspeed for some altitude. And, you only have "excess airspeed" (any speed above the aircraft's stall speed) because you have excess power. But only "some", because you'll start slowing down as soon as you bring the nose up and will continue to slow down as long as you climb in this condition. So really excess power allows you to have a sustained climb. So, when I say "climb" I'm really saying a sustained climb.

"Excess power?" What's that?

Let's just say that you're cruising along in that Speedwing 200 and you have just enough power set in to maintain level flight and that if you remove any power the old Speedwing will stall. If you just raise the nose of the aircraft in order to climb the aircraft will stall because of the increased drag from your pitching the nose up. Well then, how come you were able to climb in the earlier example? Ah, excess power Grasshopper. In that case the excess power was what enabled you to go faster than the "just before stall speed." Now I appreciate that the word "excess" makes it sound like you're doing something wrong or abusing your engine/s. You're not, it's just one of those aviation phrases that is commonly used.

So, bottom line is that "excess power" is any more power than is required to maintain straight and level flight and be just above the stall speed at your given altitude.

The exceptions

There's always exceptions in aviation. Many military aircraft can climb straight up, as can the Space Shuttle, due to.... bingo- excess power. The military fighter can only go so high before he runs out of that stuff. And, similarly, the Space Shuttle can only go into so high of an orbit around the Earth for the same reason. So, no matter what equipment you're driving, at some point or another you will run out of excess power.

Airspeed

Please bear with me for a minute while I backtrack for a discussion of airspeed. If you have a complete understanding of how airspeed and an aircraft work together you can skip this section and move on to the next heading.

Airspeed, indicated airspeed, true airspeed, actual airspeed, calibrated airspeed...the list goes on. Let's take a look at how this "airspeed thing" works with airplanes.

The 90 degree angled piece of metal at the nose or wing of the aircraft is the Pitot tube. it is orientated to get the most clear and least turbulent air flow available. It is usually heated in most aircraft so that ice will not clog it.

Because the air gets thinner as you increase altitude there is less and less air entering the pitot tube. In addition, barometric pressure has an affect because of the change in air density. And (wouldn't you know there would be an "and") temperature also has its input on the reading what with warm air being less dense than cold. OK, that's a lot of variables- how can the thing even start displaying information? Well, knowing all of this, a standard was set for airspeed readings.

That standard is:
1. at sea level
2. standard day (temperature, humidity)
3. a barometric setting of 29.92 inches of pressure

If any of these criteria are off then the indicated airspeed will be different than the actual airspeed of the aircraft. It's pretty obvious that most pilots will go their entire careers without having the airspeed reflect the actual speed of the aircraft. Lousy deal, no? Well, not quite. The aircraft doesn't care! When an aircraft is flight tested it is loaded with instruments that feed in all of these corrections so that when the plane stalls the INDICATED airspeed is recorded. Now, that means given the air that was flowing over (and under) the wings on that day it stalled at, let's say 100 knots.

OK, let's dick with the standards and increase the altitude by 10,000 feet. We know that the air is thinner up there. And, because of this there are fewer molecules slamming into the pitot tube. And, because of that the airplane really has to go faster (if you were an observer on the ground you could tell that) to get an indicated airspeed of 100 knots. But, and here is the key thing, the wing doesn't care. It will still stall at an indicated airspeed of 100 knots because the airspeed indicator and the wing are both affected by the thinner air.

Let's increase the temperature. The air is thinner. The aircraft will have to go faster to obtain an indicated airspeed of 100 knots (relative to you standing on the ground watching all of this). But, it will still stall at 100 knots indicated.

The bottom line on this is its simplicity. The guy driving doesn't have to care about conversions for barometric pressure, humidity, temperature etc. All he cares about is that the airplane will stall at 100 knots, indicated airspeed. It will stall at this indicated speed at the height of Mt. Everest or down at Death Valley. So when the pilot glances down at his airspeed indicator he doesn't have to make any mental corrections- what you see is what you get.

So, what about those figures of "cruises at 494 knots @ 30,000 feet"? Well, that's where "true airspeed" comes in. Now, if indicated airspeed is only true & correct at those standards I mentioned before it holds that if you want to find out what your true airspeed is you will have to account for any deviations. And that is exactly what is done. First the indicated airspeed is noted, a correction is made for the outside air temperature, another correction is made for the barometric setting, and unless you really want to get to the nits they ignore humidity (that comes into play with density altitude- another subject). So, figuring in all of these deviations the pilot is able to come up with his true airspeed.

(There is one more factor on high-performance aircraft. Some heating occurs at the pitot tube because of the friction of the air molecules slamming into the tube. There is a correction table in the aircraft file that compensates for this warmer air.)

An aircraft goes faster up high because the air is thinner and causes less form drag on the aircraft. (the drag produced by lift is referred to as induced drag and pretty much remains the same in all flight regimes). So, higher means faster true airspeed and less indicated airspeed.

So, outside of the sales department why the heck would anyone care about true airspeed? Well, the pilot wants to know so that he can plan his trip. Using charts he will pick what altitude he is going to cruise at and find out what the true airspeed will be. Factoring in winds aloft will give him his expected ground speed and then the times can be calculated for each leg of the trip. Also, ARTC (Center) wants to know so they can plug the information into their computers for tracking purposes. (they can recalculate based on actual readouts, but it gives them a starting point)

So, the higher you go, the faster you go, the lower the indicated airspeed is.

The "Coffin Corner"

As you climb, you will finally reach a point called "the coffin corner". It's usually associated with what is known as "upsets". You're way up there in your Speedwing aircraft and have full power in...if you raise the nose the aircraft will stall, if you lower the nose you will descend- this is it, the max. you can get out of the old Speedwing. Oh, one other detail here- the airspeed indicator will be indicating near-stall, even though you are now going the fastest the Speedwing is capable of going. Any turbulence will cause a stall and the aircraft will fall a considerable distance (20,000 feet or more is not unheard of) before the air becomes thick enough to recover from the upset.

Welcome to the "back side of the power curve.

Back to the Power Curve

For the same power setting, every airplane has two positions on the power curve- the "front side" and the "back side." On the "front side" you are happy as a clam- climbing, descending or level flight. For the same power setting, and with increased drag- be it from raising the nose or adding drag by lowering the flaps or gear- you will reach a point where the aircraft will stall and no longer maintain level flight. In fact, it will drop like a stone.

So, why is this important?

Because you can flirt with the back side of the curve when you takeoff and climb. Get that nose up too high and even with full power the old Speedwing will stall.

You can flirt with the back side of the curve if you don't add power for a sustained climb enroute. The Speedwing will keep slowing down and stall if you don't level off before too long.

You flirt with it during the landing phase when the aircraft is down and dirty- flaps, gear and whatever hanging out. Lots of drag there slowing you down (which is what you want during landing) and causing you the pilot to add more power to overcome all of that drag. Get too much drag, and get slow enough and you won't have enough power to get you out of a stall.

What if you find yourself on the back side of the power curve?

There is only one way out of this situation- you have to lower the nose of the aircraft. If you are "at altitude" you can trade off airspace for airspeed. If you are close to the ground... well, you stand a good chance of making contact. This is why it is good practice for you as a pilot to go out and do stalls in your aircraft from time to time, and especially important for you to do them in a new aircraft. You have to know how the aircraft that you are flying performs in adverse conditions. When you do a power on stall you are on the back side of the power curve.

It doesn't sneak up on you.

Being in this regime usually doesn't happen rapidly. It is the end result of a situation going bad. Recognize that if you are low on airspeed and adding power that the end result could put you in this position. Early recognition, situational awareness, can save the day.

Summation

The Back Side Of The Power Curve is just there, you can't make it go away. What you can do though is be an informed pilot. Understand how this situation unfolds and be aware of the implications. An informed pilot is a safe pilot.

Fly safely.

Hal Stoen

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