Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/118927
F li g h t Te s t in g Te c hn i q u e s at Point E, or 75 knots. Remember, the power setting is unchanged, and all the points in Figure 1 represent a single power setting. If you're not aware that you're flying on the back side at Point A, and you want to make your flight path angle shallower, you might pull the stick back and slow down a few knots. Initially, your flight path would become shallower because you'd be trading airspeed for altitude. This result is only a balloon effect. With the increased lift comes increased induced drag. As your airplane stabilizes at its new, slower airspeed with no change in power, the increased drag results in a steeper descent angle than you had at the Point A airspeed. This is the insidious nature of the back side. At this point you have two options for achieving a shallower flight path angle: You can lower the nose and accept a temporarily steeper descent angle (same balloon effect in reverse) until the speed increases above the Point A value. At your new, faster airspeed, the descent angle is shallower. One problem with this noselowering option is that it goes against pilot intuition, especially on final approach, where you don't have a lot of altitude to trade for airspeed. That leaves your only other option—add power. Either way, completing the approach after wandering the back side of the stabil- 1 ity curve would be a salvage effort, and a go-around is probably the best idea at this point. Returning to our constant power assumption, we made it so we could see how changing one variable— airspeed—affects the vertical flight path. Increasing power would shift the entire flight path stability curve in Figure 1 upward. Every speed slower than Point B would still be on the back side and every speed faster would still be on the front side, but the corresponding descent angles would all be shallower. Add enough power and you might even climb at the Point A airspeed. Reducing power shifts the curve downward. Pull enough power, and Point B would be a descent, just like it would be in an engine-out situation. In short, power changes move the curve up or down, but they don't significantly affect the curve's shape. The Shape Different airplanes have different flight path stability curves. Figure 2 shows the curves for two airplanes, both of which have a 75-knot final approach speed. If the pilots of these airplanes pushed their respective sticks forward to establish an 83-knot approach Airspeed (knots) 30 40 50 60 70 80 90 0 Flight Path Angle (degrees) -1 Same airspeed change -2 -3 -4 Diferent fight path angle change Airplane X -5 -6 Airplane Y Figure 2 42 Vol.2 No.4 / April 2013