Airplane Slow Flight

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Slow flight may be defined as maneuvering the airplane at airspeeds close to the stall speed of the plane. For the private pilot flight test, the examiner will be looking for speeds 5 knots above stall speed during several maneuvers centered on slow flight.

During flight at cruise airspeeds, the controls of most general-aviation aircraft have a solid, authoritative feel to them. When you roll in aileron, or use the elevator, the plane reacts in a crisp, responsive manner. As you slow a plane down, though, the airflow over the controls is reduced. As a result, the controls become less effective and acquire a “mushy” feeling. You will find that they not only feel softer due to the lower airspeed, but also the plane is much less responsive to control inputs and it takes a greater amount of control input to get the plane to execute the maneuver you are attempting to fly. When you are in slow flight mode, make the control inputs relatively gently, keeping in mind how the feedback on the controls is and how the plane reacts to the inputs. Depending on your familiarity with the aircraft, you may need to build a “feel” for control inputs at different airspeeds, from cruise down to just above stall speed. This will allow you to be able to anticipate how the airplane is going to react when you are flying at different speeds. Go out and practice flying at slow airspeeds until you can make smooth, coordinated inputs on the controls. As you become more practiced at slow flight you will find you can hold the airplane at just the desired attitude with smooth, coordinated control inputs.

Slow flight entry
Any time you are practicing slow flight, be sure you have sufficient altitude to recover from any unplanned stalls or spins.

Essentially, if you start in cruise configuration, begin slowing the plane by reducing power slowly. In airplanes like a Cessna 152 or a Piper Cherokee, start by reducing the engine’s power to about 1700 RPM, then adjust as necessary to maintain altitude when the desired airspeed has been achieved. As the plane begins to slow after the initial power reduction, ease back on the control yoke to slow the plane and establish a pitch altitude appropriate for the new speed. How much and how quickly you will need to input backpressure on the control yoke will depend on the plane and how much power has been reduced. As the plane slows and the pitch of the nose is raised, you will need to input right rudder to keep the ball centered as the left-turning tendencies begin to come into play. Many students forget to make the rudder correction, and the nose of the plane begins to drift slowly to the left. It is also common for students to lose altitude while the plane slows. Be sure to increase the pitch of the plane enough to help slow it down and to maintain altitude as it slows.

Work pitch attitude and power setting simultaneously to maintain the selected slow flight parameters. With a little practice you will be able to anticipate the correct power and pitch inputs and slow the plane to the correct airspeed while maintaining altitude very consistently. If you want to use flaps as part of the slow flight mode, add them once the plane has slowed to the white arc of the airspeed, then apply flaps in increments. You can use any flap settings, from no flaps to full flaps during slow fight practice.

When you are adding flaps, be prepared for the pitch change that will accompany their deployment. Each airplane will react differently, so learn the plane you are flying and become familiar with its flight characteristics.

If the plane is descending and you have the proper airspeed dialed in, you will need to increase your power setting slowly. Just as with the power reduction, you will also need to make a corresponding pitch change to maintain a constant airspeed. With an increase in power setting, you will need to increase the plane’s pitch to maintain the desired airspeed. By making power changes slowly, you will avoid overcompensating with too much or too little power.

The one thing you should notice immediately is the softness of the controls. Ailerons are mushy, with relatively large inputs needed to hold the wings straight and level as compared to cruise airspeed. The elevators are also less authoritative, and you will find that you will need a fairly large amount of right rudder to overcome the leftturning tendencies we discussed in the last section. The noise of the plane has also changed. The engine is producing less power, and the sound of airflow over the surface of the plane is also quieter at slower airspeeds. Get the feel for all of these sensations because you can detect changes in airspeed from these sensory inputs without ever looking at the airspeed indicator.

Figure depicts the load factor placed on the plane in relation to the angle of bank of the plane. As you can see, as the bank of the plane increases, the load factor also increases. This increase in load as a result  of a bank results in an increase in the stall speed of the airplane. If we are flying just five knots above stall speed, and we attempt to make a turn at a 45-degree bank, we are quite likely to end up in a stall due to the increase in stall speed as a result of the bank. The slower we fly, the more likely we are to stall during a steep turn as a result of this relationship between load factor increase and a corresponding increase in stall speed.

Turns are flown just as we would when making a turn in cruise flight, but we must realize the controls will be less effective, and you will need to keep your banks shallow to avoid a stall. You may need to keep right rudder inputs applied, even in turns to the left, to keep the ball centered. In slow flight, you can feel a certain amount of control buffet. The buffet may increase during the turn as a result of airflow disruptions and the increased load as a result of the turn. Monitor your airspeed and altitude during turns. The plane has a tendency to lose altitude and airspeed during a turn, and it may be necessary to add power and adjust the pitch to keep your airspeed and altitude locked in. The amount of the power and pitch change will depend on the plane and how steep the bank is, but you should practice at different bank angles to become familiar with these changing settings. As you roll out of the turn you will need to reset power and pitch to avoid gaining altitude or airspeed as the load on the wings is reduced. A final note about turns in slow flight. In turns to the right you will find that a large amount of right rudder is necessary to keep the ball centered. Not only are you trying to overcome the left-turning tendencies of the plane, but you are also overcoming adverse yaw with a rudder greatly diminished in control effectiveness.

Once you become comfortable with turns in slow flight, you may practice banking the airplane to the point of causing it to stall, to help you learn how the plane feels and reacts at this limit. As with any maneuver, you should have a qualified flight instructor along until you become familiar with the maneuvers. The instructor can help you learn and critique your performance and help prevent you from getting into trouble.

Slow flight recovery
Recovery is transition back to cruise power settings without gaining or losing altitude and maintaining heading. First, add engine power slowly, back to cruise power settings. As you add power you must reduce backpressure on the control yoke to avoid gaining altitude. If flaps were extended during entry into slow flight, remove them in increments. The reason for the incremental approach is to avoid a sudden loss of lift, with the associated risk of stall or aggressive pitch movement from the plane. After each notch of flaps is retracted, give the plane a few seconds to gain additional airspeed before retracting to the next setting. If you have extended landing gear, you will also want to retract them during this time. Do not exceed either the maximum flap extension speed, Vf, or the maximum speed the landing gear can be down or operated. If these airspeeds are exceeded while the flaps or landing gear are extended, you may cause damage to the airplane or the components themselves. As configuration changes, the plane accelerates to cruise airspeeds fairly quickly and you must adjust pitch attitude accordingly, back to cruise flight. With increasing speeds, you will also need to retrim the airplane.

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