AOPA Pilot Magazine - March 2012 - 30

PROFICIENTPILOT

BY BARRY SCHIFF

Behind the curve

he expression “behind the curve” apparently has to do with being on the left side of the so-called bell curve, the type of curve that many teachers use to

It varies inversely with the square of the airspeed. When airspeed is halved, parasite drag decreases by 75 percent but induced drag quadruples; when airspeed is doubled, parasite drag increases fourfold but induced drag decreases by 75 percent. Another characteristic worth noting is that an airplane is speed stable on the front side of the power curve. This means that if the airplane is trimmed for, say, 120 knots and the pilot pulls the control wheel slightly aft, airspeed will decrease. issue grades. Being behind the curve also refers to As soon as he relaxes back-pressure on the wheel, something or someone unable to keep up with the however, the nose will eventually return to about Barry Schiff has been a general norm. It wouldn’t surprise me, though, if the expreswhere it originally was, and the airplane will resume aviation test pilot sion originated in aviation, where flying behind the its trimmed airspeed of 120 knots. and member of power curve can have serious and sometimes fatal If an airplane is trimmed to 60 knots and is on the Society of implications. Unfortunately, some of us do not the back side of the power curve, pulling back on Experimental Test understand what to expect when operating an airthe control wheel still causes airspeed to decrease. Pilots. plane on the back side of the power curve. When the pilot releases back-pressure, however, the My first eye-opening experience with the power airplane will not return to its trimmed airspeed of curve was provided by Hunter Blackwell, a factory 60 knots. Instead, airspeed will continue to erode, test pilot who checked me out in a new Helio Courier Mark II another reason why the area behind the power curve is called (N6310V) at Hanscom Field in Bedford, Massachusetts, in 1968. the region of reverse command. Airspeed behavior is contrary He wanted to ensure that I understood the potential conse- to what pilots have learned to expect. quences of improperly decelerating on short final approach to Although the Helio Courier is designed for STOL (short the airplane’s incredibly low touchdown speed. takeoff and landing) operations, a similar demonstration of During cruise flight, he had me gradually slow the airplane the power curve can be made in most other airplanes. Without while maintaining altitude. This resulted in reduced parasite actually experiencing such a demonstration, the concept of drag and corresponding power reductions. Eventually, though, operating on the back side of the power curve is largely acawe got to a speed where power was at a minimum and further demic, and a pilot cannot fully appreciate the grief he could speed reductions required adding power to hold altitude, the encounter when inadvertently and unexpectedly flying on the result of increasing induced drag. The lower the airspeed, the back side of the power curve. more power I needed to maintain altitude. Because the Helio Assume, for example, that a pilot is approaching to land and Courier does not stall in the conventional sense, the lowest air- notices that his airplane is drifting below the glideslope (elecspeed attainable was determined by the amount of available tronic or visual). He attempts to reduce sink rate only by raising power. In other words, both minimum and maximum airspeeds the nose. Airspeed obviously will decrease in the process. (This were power limited. He then had me perform the same exer- also can occur insidiously when descending through a decreascise with the flaps and leading-edge slats extended. With full ing-headwind type of wind gradient, or more dramatically in throttle, the Helio could be held at an incredible 24 knots. We a wind shear.) The pilot recognizes the airspeed reduction and presumably could have flown more slowly with a more powerful adds power but not sufficiently to arrest the speed bleed. Before engine. This demonstrated the effect of the power curve. he realizes it, the airplane has entered the back side of the power An airplane is on the front side of the curve when reduc- curve. Sink rate will increase and pulling farther back on the ing airspeed requires reducing power. This is referred to as the control wheel will have no effect other than to further reduce “region of normal command.” To reduce airspeed further, the airspeed and increase sink rate, a situation that could have been pilot must add power. He is on the back side of the power curve, avoided by a greater, more timely application of power. Once behind the curve, a pilot must aggressively add power a regime of flight known as the “region of reverse command.” The point where minimum power is needed to maintain altitude and simultaneously lower the nose to increase the airspeed is the dividing point between the front and back sides of the required to return to the front side of the power curve. Hopefully curve. This is when parasite and induced drag are approximately he has the height needed to do this safely, otherwise the result equal. Parasite drag predominates on the front side of the power can be an embarrassingly hard landing—more common than curve, and induced drag predominates on the back side. you may realize—or worse, and that would be a real drag. Remember that although parasite drag varies in proportion to the square of the airspeed, induced drag is just the opposite. Visit the author’s website (www.barryschiff.com). 30 • MARCH 2012

AOPA PILOT •

http://www.barryschiff.com

AOPA Pilot Magazine - March 2012

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