What does it mean to get "behind the power curve?

Q:
I mean, every now and then I hear this term and specifically that you should not get behind the power curve. I figure this should mean keeping enough power for each aircraft configuration / altitude combination.

Anybody can explain me this concept ? Also, when I am flying, how do I know that am behind, at or ahead (possible ?) of the power curve ? It is OK to redirect me to a tutorial, link or manual I might have overlooked.

Thanks and regards

Sergio Almendra

Curitiba Brazil.


A:
Good question Sergio.

The power curve as we speak of it refers to the amount of power applied in an aircraft to maintain a specific airspeed or altitude, or combination of both.

What do I mean by that? Well lets suppose that you decide to cruise you garden variety Cessna 172 at 110 knots indicated at 5000 feet. I can tell you to do that you will pull about 2450 RPM in trim. Technically RPM has nothing to do with this manifold pressure is more closely related to power, but since I don't fly a 172 with a MP gauge we'll use rpm as a reference. I will tell you that at say 25 degrees C at 5000 feet and 2450 you are probably looking at about 23" MP if you cared to know.

Now to maintain altitude I have my power set to 2450 and a given angle of attack and in level unaccelerated flight, all forces are balanced.

If I want to fly slower, but maintain altitude I can pull power and start to trim nose up which effectively increases my angle of attack and generates greater lift, but since I have changed power settings, thrust has been reduced and something must go, that something is airspeed. reduced airspeed means less airflow over the wing, less lift, but in balance I am compensating by greater angle of attack so I can maintain altitude.

We can continue to do this, pull power, increase angle of attack (alpha) and to a certain point, which typically coincides closely with LDmax, we can get away with this, meaning I can hold 5000 feet with aft trim and slower airspeed by increasing alpha.

But if I pull power even further, I will no longer be able to hold altitude at that high angle of attack, because a high alpha generates a lot of drag. In the absence of something to counteract that, I will start to descend.

But let's say at this point I increase alpha even further, technically my ability to generate lift is greater because of the increased angle of attack, but in the absence of greater thrust, drag will slow my airspeed sufficiently that I will descend. The best example I can show you of this is a well coordinated (ball centered) stall. The airplane is at very high alpha and will fall like a stone. The airplane is configured to generate lift by nature of its angle of attack, or alpha, but without thrust it will not maintain altitude.

So I decide at this very slow, descending airspeed to add power. I do not change alpha. As I add thrust, airflow will increase over the wing and I will start to generate lift. Keeping in mind that my very high alpha causes a lot of drag. At a certain point, my power will be sufficient to create enough thrust to overcome that drag and I will stop descending.

We duplicate this in a maneuver called a departure stall. Essentially we configure the airplane at stall speed plus 10 or so, add full power and start pulling back on the yoke. At some point the alpha we create will pass the critical angle of attack and the wing will stall. We have full power but we are stalled. How can that be?

Back to the question. What is the backside of the power curve?

The backside is the area of power vs. airspeed where are airspeed is very low but our power setting is very high. In essence we are using thrust not wing lift, to maintain altitude. Our nose is pitched up at such an angle that we maintain our altitude by keeping very high thrust with a very high angle of attack and very high drag. The advantage is that we have very low airspeed.

Why is that? Well if I want to plant my Helio Courier into say a 500 foot airstrip with trees on either end, I want to use as much runway as I can to touch down and slow down. If I carry too much airspeed I will float and use up ground I could be landing on. But if I configure for maximum lift with minimal airspeed I can ride the airplane down very steeply, essentially by "hanging on the prop" and touch down at the close end of the runway so that I have the benefit of full landing area to slow down.

What's the drawback? The drawback is reverse control. When you fly the backside of the power curve, your airspeed is pitch dependent. That is, if I lower the nose, reduce alpha, I pick up speed, but lose lift and start to sink momentarily. If I raise the nose to a higher alpha, I am already very close to the critical angle of attack and could stall very quickly. In fact it is alpha that is controlling my rate of descent, not power, which is how we fly on the "front side" . In a normal approach, I don't change pitch to descend, I pull back five inches of MP or 500 RPM and leaving all things the same start a 500 foot per minute descent at my trimmed airspeed.

On the backside if I pull power or raise the nose even slightly I descend. So it is critical that I manage pitch carefully because as I am closing on the ground I have little room to suffer big altitude changes quickly. So I am at high thrust and keep it there and I carefully manage pitch to maintain a rate of descent.

What happens if the engine quits?

Loss of thrust, drop very rapidly, airspeed decays, stall...

What can I do? My only choice is to lower the nose to get airspeed up but this will result in an increase rate of descent because I have reduced alpha. Close to the ground this could mean a very rough landing. But it is my only option. If I do not lower the nose I will stall and slam into the ground or worse, spin and crash inverted.

For that reason such flying is best limited to carefully planned approaches which are conducted in phases. When I fly short fields, I am always thinking at which point am I committed, where is my energy, what am I going to do if the engine pukes, where is my touchdown point? My go around decision height is very high because I know if I reject, I have to lower the nose and I will lose precious altitude until my airspeed builds to generate significant lift. To start the climb.

In a normal landing I am close to idle, I have available power and an alpha nowhere near stall, so I pour on power and pretty rapidly start to climb without much if any pitch change. That means I can reject at a relatively low altitude and be pretty certain that I can go around without a problem.

Backside, I have no more available power and the only thing I can do is trade altitude for airspeed and I do this by LOWERING the nose.

In practice I do a lot of backside flying even at my large airport. I will pull power at say 2000 feet agl and very short final and ride the stall down. I know this sounds crazy, but when coordinated and when you have a feel for your airplane it is a very safe maneuver. It is not something for a beginning PPL student to mess with nor anyone who has not had some acro time. You need to understand the airplane and have spent a lot of time practicing stalls and understanding your airplane's stall behavior. You cannot have lazy feet, or you could get injured or killed in a spin.

But I have been able to put a Cessna 150 down in a 10 knot headwind in less than 200 feet, by doing just this. I have landed my 172 in 400 feet the same way.

What is the advantage to this? Well, when you get comfortable flying at very slow airspeeds you begin to understand that if you have to you can place an airplane into a very small place and walk away from that. The importance of that skill arises when, heaven forbid, you lose a crankshaft over rugged terrain and the only thing below you is a four lane highway or some cleared farm fields that are 800 feet long with trees at both ends.

Hope that helps.

Todd :-wave



A very good question, Sergio. At best glide speed, the aircraft is in its lowest drag configuration, when you consider both parasitic and induced drag. So it takes more power to maintain level flight when either accelerating or slowing down from that speed, because you have increased drag. When this is plotted on a graph, it makes a more-or-less bell shaped curve, with best glide (lowest drag) at the peak.

So, technically, anytime you are maintaining altitude at a speed less than best glide speed you are "behind" (to the left side of) the power curve.

However in general pilot jargon, it means that you are far enough to the left of the peak of the curve that either full power will not accelerate the aircraft at all (in level flight) or that it takes quite a while to do so. This, obviously, can cause serious problems if you don't have much altitude to trade for airspeed and you need to climb and/or speed up.

For example if you are in a Cessna 150 with full flaps and just above the stall, you may have to lose a little altitude to gain enough speed to ease off the flaps, or it may not even maintain altitude, depending on density altitude, load on board, etc. If you are in the flare and suddenly need to go around because of, say, an obstacle on the runway (truck, cow, deer, etc.), you may not have enough performance to do so, thus you may hit the obstacle.

Larry N.



Others have added far better explanations of the power curve. One of the reasons that pilots practice slow flight with airplanes is to get familiar with the feel of the plane when you push it too far. A pilot has to realize instinctively that he is getting behind the curve and correct the problem before things get to the point that only the hand of God will help lift him.

The solution to the predicament is to do something that seems alien to student pilots and terrifying to passengers. When you are stuck and the trees are coming up you have to LOWER the nose to go faster than you will start to climb. Trust me, the urge to pull up close to the ground is as strong as the urge to take a breath when your lungs run out of air underwater. Doing either is usually fatal.

Bob K.

Similar Pages

  1. Comments: 0
    Last Post: 12-20-2007, 03:22 PM
  2. Comments: 0
    Last Post: 11-09-2007, 08:29 PM
  3. Comments: 0
    Last Post: 11-09-2007, 08:29 PM

Posting Permissions

Posting Permissions
  • You may not create new articles
  • You may edit articles
  • You may not protect articles
  • You may not post comments
  • You may not post attachments
  • You may not edit your comments