Behind the Power Curve

 

I've heard about this one since the time I started flying. "Don't get behind the power curve." Every airplane has a power curve. And every power curve has a backside. It's an area of the performance envelope in which induced drag rises dramatically, necessitating considerably more power to maintain a given airspeed and altitude.

So what?

I've been following a post by Rod Machado on Facebook in which he has taken issue with the FAA for changing the test requirements for flying at minimum controllable airspeed.

Isn't it ironic that the FAA wants all pilots to have better stick-and-rudder skills while, at the same time, it dumbs down the flying skills required to obtain a pilot certificate? In case you've been in the Himalayas practicing chants with the Maharaji for the past year, the private pilot ACS no longer requires a demonstration of flight at minimum controllable airspeed. Same with the commercial ACS, but the FAA goes a bit further in dumbing down pilot skills in this document. The link is here.

Which brings us back to the Power Curve.

My airplane doesn't have a stall warning claxon. The pilot must figure out (on his own) if she is getting into a situation where she might stall. It is pretty obvious with a PiperSport. As I reduce airspeed (by reducing power), the controls become mushy, the wind noise increases and she desperately wants to descend. Unless you forcefully increase the Angle of Attack she'll just mush along in a slow descent. If she does stall it is a very gentle nose over, seldom dropping one wing or the other. Recovery is to let the nose drop and add power.

But what if I held my altitude by adding power as I raised the nose? At high enough AoA I could fly at a slower speed by increasing power. See how the red line on the graph starts to bend up on the left as you get close to minimum airspeed? Flying slower by increasing power. That's the "back side of the curve".

So what?

You’re on final, and see you’re losing altitude. The VASI lights are all red. The trees are getting closer. It’s clear that on your present approach path you’re on target to land well short of the runway. In an attempt to arrest the descent and set things right, you add power and raise the nose.

Bad move. You’re still sinking, even though you’ve gone to full power in what is now a desperate effort to climb. Welcome to what’s commonly known as the back side of the power curve, or the “region of reversed command.” It’s not a happy place, and it’s worth a review of some basics in order to avoid it. These basics have to do with the relationship between drag and power. The link is here.

Finally a break in the weather pattern for central Florida. Sally really need some exercise and I wanted to experiment with the power curve. After takeoff we flew over to the training area near Kidney Lake. Smooth air and good visibility and all of the instruments were green. I always like to start with a few steep turns (45°) and use them for clearing turns.

This "back side power theory" seems to work. Try it for yourself, just not on final.

Video: The Backside of the Power Curve.