Cocerning aircraft, what is meant by the back of the power curve?

Question

Cocerning aircraft, what is meant by the back of the power curve?

Answer 1

I think you’re referring to the backside of the power curve. It’s probably a little complex to go over here as it involves quite a bit  of theory on power vs. drag but basically it’s not a good place to be. It typically occurs in a swept wing jet when the airspeed has reduced to a point below the peak of the power/drag curve. Think of a speedboat on a lake. Going fast it just planes the water effortlessly. When it slows down though, at some point it is now pushing its bow wave and to get over that bow wave again requires a lot of power. The same thing happens when a swept wing jet gets to slow. It starts to “Push” air and that ain’t good.

We practice flying on the backside during recurrent training recovery from the stick shaker and wind shear. An airplane will produce its maximum lift at a point just before the wing stalls. But that does not mean that’s a good place to be if lift is not what you’re looking for.  We fly the stick shaker to escape wind shear because that IS what we’re looking for but that’s about the only time you’d care to give up everything else just for lift. But since the ground and the rate that you are sinking is what’s about to kill you there is nothing more important than lift at that moment. But, once you have escaped the windshear, now you find yourself alive and on the backside of the power curve. You’re pushin air. Now what. Your throttles hopefully are already against the hard stops so just maintain altitude and let the airplane slowly accelerate over that hump that’s the peak of the power curve and she starts to fly right again.

I doubt that you will find yourself on the backside in a light, straight wing airplane. But get too slow in a transport jet with any kind of sink rate near the ground, as in a downburst under a thunderstorm, and you’ll be one busy sucker.

Answer 2

In simple practical terms.

If you are coming in to land, if your best glide is 70 for example and the plane/wing stalls at 49.  If you have a problem and have to go around for another try,   at 70 you just add power and the aircraft will quit settling and you can go around. If you let the airspeed drop to a certain point probably about 60 or so in this case you can add full power and it will continue to sink for a bit before it starts to climb again. It is possible if  you are too low you may not be able to go around, you have to land.

This is very common if you are flying in conditions of high density altitude, high altitude runways at gross weight.

It is very common where I fly in Idaho, most runways are at least 4500 feet ASL, summertime temps of 100+ degrees. That can make the runway altitude (density) be 9500 feet. The engine doesn't work as well either.

Answer 3

Power is the force that opposes drag in airplanes.  There are two types of drag: parasite drag and induced drag.  Parasite drag is sometimes called form drag.  It is the drag that occurs as a result of the structure of the airplane moving through the air.   Induced drag is a byproduct of the production of lift.

Induced drag is pretty minimal at normal flight speeds and actually decreases with increasing airspeed.  The opposite is true about parasite (form) drag: the faster a plane moves, the more parasite drag there is.

Induced drag is a funny thing.  The slower you go, the more induced drag there is.  To get really technical (ignore this paragraph if this goes over your head), lift acts perpendicular to the wing's angle of attack.  At slow airspeeds, an airplane needs a high angle of attack to produce enough lift.  At a high angle of attack, the lift force is acting up and to the rear.  The rearward component of lift is drag, specifically induced drag.  So the higher the angle of attack, the more rearward lift acts (since it is perpendicular to the angle of attack), which means more induced drag (because induced drag is the rearward component of lift).

Still with me?  The technical mumbo-jumbo is over now.  I'll give you the simple summary which will actually answer your original question.  Normally, the faster you want to go in a plane, the more power you'll need.  Makes sense, right?  This is because power is needed to overcome that parasite (form) drag.  That's really no different than driving a car.

The back side of the power curve (also called the region of reversed command) starts happening at slower-than-normal airspeeds.  You can actually get to a point that MORE power is needed the SLOWER you go.  Strange, huh?  This is because if you get slow enough, induced drag starts to increase... so the slower you go, the more power you need, which is exactly opposite of normal operation.  In fact, many training airplanes can slow to the point that they require full power just to maintain altitude, but they are only moving 40 knots or so!

I could give you a much better explanation if you were sitting across from me and I had the luxury of a whiteboard... Hopefully you at least caught the gist of this.

EDIT:  Correction, lift acts perpendicular to the relative wind, not the angle of attack.  It's a pretty small difference, but I don't want to give bad information...