The very mention of the word “stall” in the context of flying conjures up images of aircraft falling out of the sky, for most people. While stalling is definitely an issue that can occur in an aircraft, all pilots are trained to recognise an imminent stall and take corrective action, or to quickly recover from a stall, should one actually occur.
On the second day of flight training, my instructor showed me what is called an “incipient stall”, i.e. a situation where any increase in the angle of attack, or a reduction in power would result in a stall.
On day two, we actually practiced stalls - both powered and “power off”. So, what’s the difference between the two stalls, and can a stall actually occur when the aircraft is powered?
What is a Stall?
A stall occurs when the “angle of attack” of the aerofoil exceeds what is called the “critical angle of attack”. So, what is this thing called the “angle of attack”?
Angle of Attack
The textbook definition of “angle of attack” is that it is the angle between the chord line of the aerofoil and the relative wind.
In order to understand what we mean by the chord line, let us take a look at the cross section of an aerofoil given in the figure, from Wikipedia, below.
In this figure, you can see the chord line in approximately the middle of the aerofoil (the dashed line). The angle that the chord line makes with the “relative wind” is called the angle of attack.
So, what is this relative wind, you may ask? Let me try to explain. The simplest explanation is that the relative wind is the direction of airflow opposite to the flight path vector of the aircraft.
How does a stall occur?
In order for the aircraft to stall, we know that the angle of attack must exceed the critical angle of attack.
One way to force that situation in flight is for the pilot to slow the aircraft down significantly while holding the aircraft in a substantial nose-up attitude. As the aircraft slows down, its flight path starts to move more and more towards the horizontal, due to the reduced thrust that can no longer overcome drag. In other words, the aircraft is no longer climbing, but is starting to follow a level flight path.
In such a situation, the angle of attack starts to increase until, at one point, it exceeds the critical angle of attack, and a stall occurs.
Indications of an Imminent Stall
As the aircraft starts to get closer to a stall, a number of things occur. First the controls of the aircraft get very “mushy” and unresponsive. The aircraft is in a significant nose-up pitch attitude. In addition, there is a clear reduction in the sound caused by wind blowing against the aircraft. A trained pilot can recognise these signs and take corrective action before the stall occurs.
If the pilot is relatively inexperienced, or is unable to recognise the symptoms, most aircraft are equipped with one or more warning devices. In the case of the PA–28–140, the device is in the form of a red lamp on the dashboard that starts flashing as stall becomes imminent. On other aircraft, there is a siren, or a combination of a light and a siren.
Power Off Stall
The power off stall does not require that the aircraft be powered off in mid-air. Rather, it intends to simulate conditions that may occur when the aircraft is landing.
This means that while you’re flying, you slow the aeroplane down into close to landing speed, lower the flaps and the undercarriage (if applicable), and then start to pitch the aircraft upwards until a stall occurs.
Power On Stall (Full Power Stall)
The power on (or full power stall) seeks to simulate a stall when the aircraft is in the takeoff mode. To simulate this stall, you slow the aircraft down to takeoff speed (close to Vx or Vy), then rapidly increase power to maximum (or 75% of maximum) while raising the pitch attitude until the stall occurs.
Recovery from a stall
Recovering from a stall essentially requires the pilot to reduce the pitch attitude of the aircraft while simultaneously increasing engine power (or, in the case of a full power stall, doing nothing to engine power). Also, the pilot should ensure that the wings are brought level with coordinated rudder control to prevent one wing stalling, thereby forcing the aircraft into a spin.
Once the aircraft starts gaining airspeed, the pilot needs to gradually pull back on the control column until the aircraft demonstrates a positive rate of climb. Pulling back too quickly on the control column could result in a secondary stall.