Monday, February 1, 2010

Practicing Turns and Power-Off Stalls

I had my second lesson at East Coast Aero Club last week. This one was in the early morning on a clear and fairly cold day. I arrived to the airport only a bit after sunrise to a ramp full of very cold and frosty planes. For those who don't know, small aircraft generally require preheating prior to starting the engine when the temperature dips below freezing. This is critical for bringing the aluminum engine up to near its operating temperature, getting the battery to more effectively produce power for the electrical systems, and reducing the viscosity of the engine oil. At Hanscom, a third party provider, Signature Flight Support, is called for the morning preheat. They show up with a van that has two tanks of propane hanging off the back. They fire up a burner and blow hot air through a flexible 3" diameter hose (think your drier vent) into the engine compartment while the front of the plane is covered in a nice, snug blanket. Depending on the temperature outside (in this case, about 20F) and how long the plane has been sitting there (since yesterday), this process can take about 10-15 minutes. During that time, I had the pleasure of helping my flight instructor to brush and scrape the wing surfaces until they were devoid of ice. Any buildup, even fine, on the surface of the wing can cause undesirable friction as the air sweeps over the wing during flight. This friction can lead to turbulent (as opposed to smooth, "laminar") air flow on the wing surface, reducing lift and increasing the likelihood of a stall. Needless to say, even though I (stupidly) forgot to wear gloves for my early morning ride, I was eagerly scraping away the entire time we waited for the engine to heat.

Once we were warmed up and ready to go, we completed our inspection of the aircraft and our pre-flight checklist operations and I had another chance to taxi out to the runway. This was a lot more comfortable (if not yet natural) than my first taxiing experience. I also had the chance to show off that I had been studying my text books. We had a slight breeze coming from the front, left side of the aircraft (a "quartering" wind). When asked by my CFI how to account for this during taxiing, I correctly responded that the procedure was to raise the left aileron (move the control wheel left) to reduce the lift of that wing, while keeping the stabilator in a neutral position. (Had the quartering wind been from the left-rear rather than the left-front, the correct response would have been to turn the control wheel right while also pushing it forward.)

After taxiing to the runway and performing our run-up checks, I again had the opportunity to bring the plane up to speed and roll into our takeoff. It was a bit of a bumpy ride as we lifted off and began gaining altitude, but things soon smoothed out. After following the path of runway 29 (290 degrees, or West-Northwest, until we climbed to 800', we turned to heading 33 (330 degrees) and headed out towards the practice area, continuing to climb to an altitude of 3000'. This happened to coincide with pointing the aircraft roughly towards Mt. Monadnock in the distance, an excellent visual landmark. The view was amazing - a perfectly clear day with unlimited visibility - and the mountain looked much closer than I expected it to.

After a while we turned more westardly towards the practice area and I made a few 90-degree "clearing turns" to check for traffic. We then practiced slow-flying, where I reduced the power of the engine. This requires a compensatory increase in the pitch of the aircraft (increasing nose-up attitude by pulling back on the control wheel) to maintain sufficient lift for horizontal flight. The effect is a bit strange at first - reducing power and then gradually easing pulling back, back, back on the control wheel to tip the aircraft upward, yet still fly level at the same level altitude. This reduce-power, pull-back-to-compensate-for-loss-of-lift, continue-flying-level set of operations can only be kept up for so long. After reducing the speed of the aircraft from it's regular cruising speed (~110 knots) to about 60 knots, the aircraft is pointed fairly nose-high to maintain lift. At this point, I practiced extending the flaps, which change the camber of the wing, making them produce more lift at a relatively low airspeed. After spending a bit of time in this slow-flight condition, where responses to the aileron control inputs are significantly reduced and sluggish in comparison to cruising flight, I practiced entering into power-off stalls. The stall is a condition where the angle of the flowing air striking the downward-angled wing surface (known as the "angle of attack") becomes so severe that the air can no longer flow smoothly over the top surface of the wing. Instead, the airflow becomes a collection of turbulent vortices and the wing "stalls," no longer producing lift. At this point, the airplane begins to buffet a bit and starts to sink, and a loud stall alarm buzzer begins to sound. When this happens, the pilot should respond by returning full power to the engine and to pressing forward on the control wheel. This causes the airplane to pitch downward, which has two important effects: (1) the relative angle with which the air strikes the forward moving aircraft (angle of attack) decreases, bringing the wings out of the stalled condition, and (2) the forward airspeed of the aircraft increases, providing more lift over the wing surface. Entering into this stall regime and practicing recovery is an important part of learning how to fly a plane as it prepares the pilot to recognize the condition (which could be caused by a loss of engine power) and respond quickly to recover without significant loss of control or altitude. I had read up a bit about this procedure and, while I was expecting to be a bit nervous about it, I actually ended up feeling comfortable and in control the entire time. It was actually reassuring to see how easy and intuitive the recovery maneuver was.

Interestingly, we practiced the power-off stalls for about 20 minutes in spitting distance from both the Fitchburg Airport and a well-frozen lake, in case of the actual loss of the engine and the need to glide to a quick emergency landing. The lake was none other than Lake Whalom, where Lauren's parent's live. We ended up practicing the stalls almost directly over their house. I wondered how they would feel to know that the flight school regularly took completely inexperienced pilots over their house to practice making their airplanes drop out of the sky...

After the slow-flight and power-off stall practicing, we headed back to Hanscom for a nice, uneventful return flight. My instructor handled all of the communications with the tower as we got into the landing pattern (something she will continue to do until I have a bit more experience with the flying part of flying, although I am beginning to understand the radio patter a bit more now), and I helped configure the airplane and set us up for the final approach. All in all, a great morning flight and another 1.2 hours in my log book!

1 comment:

  1. I could almost feel the seat drop under me as I read this.