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The aerodynamics of a spin

By Scot Campbell and Donald Talleur

 

If pilots were to get together and rank the most dangerous situations they could encounter over the course of a flight, stall/spin incidents would be near the top. 

While stall/spin accidents are not as frequent as other types of accidents, they are in general more deadly. The statistics show that although stall/spin encounters make up only 8% of all general aviation accidents, they account for 25% of the accidents involving serious or fatal injuries. Therefore, general knowledge of spins is stressed throughout pilot training and reiterated in aviation publications.

However, a deeper understanding of spins is commonly lacking among the majority of the pilot population. Hopefully this article will shed light on some of the basic aerodynamic principles that govern the behaviour of aircraft before, during, and after a spin is encountered.

To understand the aerodynamics of a spin, it is important to first understand how lift and drag behave at high angles of attack. This includes not only an understanding of what happens to lift and drag near stall, but also where the stall occurs along the span of the wing.

It is convention to say that a stall occurs when the aircraft exceeds its critical angle of attack. Aerodynamically speaking, this means that at the critical angle of attack, separated flow dominates the airflow over the wing resulting in a decrease in lift and an increase in drag. The location along the wingspan where the stall begins depends on many factors including the wing planform and any stall control device installed on the wing.

Typically, a wing is designed to stall from root to tip, resulting in more effective aileron control during stall. This is important in understanding the effect of aileron input during spin recovery, and will be discussed a little later in the article.

The generally accepted definition of a spin is an aggravated stall that results in autorotation. This is an accurate and concise definition, but it does not explain or provide good understanding to the underlying cause and persistence of a spin. 

The aerodynamics of a spin are very complicated, and for ease of understanding the aerodynamics of each phase of a spin should be analyzed separately. The phases of a spin are: entry, incipient, and fully developed. 

The entry phase begins with the aggravated stall and ends when the aircraft departs controlled flight. The incipient phase occurs between the departure from controlled flight and the point when the forces acting on the aircraft equilibrate. The fully developed phase is characterized by equilibrium between the aerodynamic and inertial forces.

The entry phase of a spin is characterized by an aggravated stall, causing the aircraft to depart controlled flight. A stall can become aggravated in two ways: a prolonged slip (or more importantly a skid) or a sudden yawing motion at the time of stall.

To answer the question why a prolonged slip aggravates a stall you must first understand that as an airplane slows down, its natural roll damping decreases. This means that the inherent stability of the airplane to keep wings level decreases with airspeed, which makes it much harder to keep wings level in a slip or skid.

Eventually it will result in a rolling motion in the direction of the prolonged slip or skid. 

This rolling motion induces a higher angle of attack on the downward wing, resulting in an aggravated stall situation where the downward wing is more stalled than the upward wing. A sudden yawing motion at the time of stall causes the outside wing to travel faster than the inside wing.

This creates more lift on the outside wing compared to the inside wing, which results in rolling motion toward the inside wing and causes the inside wing to be at a higher angle of attack than the outside wing. In either case the airplane enters a state of aggravated stall where one wing is stalled more than the other. 

The wing that is more stalled creates more drag and less lift than the less stalled wing, and this imbalance of forces pulls the aircraft away from controlled flight in the direction of the more stalled wing

Figure 1.  Lift and drag during entry.

(Figure 1 shows the behaviour of lift and drag during the entry phase).

The incipient phase of a spin is characterized by a continued imbalance of lift and drag that continues to pull the aircraft into the spin. In general, the incipient phase lasts for approximately two rotations, during which the rotation rate of the spin increases. 

The increase in rotation rate causes the outside wing to increase its velocity, which corresponds to a lower angle of attack. This deepens the aggravated stall, causing a greater imbalance of forces that increases the rotation rate until the spin reaches equilibrium (Figure 2 shows the imbalance of forces on the aircraft during the incipient phase).

Figure 2.  Forces on aircraft during incipient phase.

The fully developed phase of a spin is characterized by an equilibrium between the aerodynamic and inertial forces on the aircraft. Once the aircraft reaches this state it will remain in a fully developed spin until action is taken to recover. 

There are four categories of fully developed spins: steep, moderately steep, moderately flat, and flat (Table 1 shows the distinctions between the categories).
Spin recovery procedures are necessary to remove the aircraft from its spin equilibrium and return it to normal flight. It is important to understand that different aircraft have different spin characteristics and therefore spin recovery procedures are unique to each aircraft.

This makes it critical to know the procedures in your aircraft’s operating handbook for the proper recovery. A general spin recovery procedure taught during primary flight training is known by the acronym “PARE”, which states Power to idle, Ailerons neutral, Rudder opposite rotation, and Elevator forward. 

Most spin recovery procedures, even though different, incorporate all of these elements, albeit sometimes in a different order. Understanding the reason behind each of these steps can give insight into the specific spin recovery procedure and complicated spin aerodynamics of your aircraft.

The effect of power on the dynamics of a spin depends on the orientation of the spin (left or right), and also the configuration of the aircraft. The first effect of power on a spin is the torque effect. Torque affects a spin because of the equal and opposite reaction of the airplane to the rotation of the propeller. 

For a propeller that rotates clockwise when viewed by the pilot, torque acts to tighten a left spin and flatten a right spin. The second effect of power is the gyroscopic effect. For a propeller that rotates clockwise as viewed by the pilot, a gyroscopic force acts to raise the nose in a left spin and lower the nose in a right spin. 

The last effect of power is a thrust line effect, where the thrust line is a line parallel to the longitudinal axis of the airplane along which the thrust acts. If this line is below the center of gravity of the aircraft, power results in a flatter spin. The opposite is the case if the thrust line is above the center of gravity.

The effect of ailerons on the dynamics of a spin are probably the most complicated to explain as well as understand. Going back to the second paragraph, it was said that the vast majority of aircraft are designed so that their wings stall at the root before the tip.  This is incredibly important to understand because it implies that the wing tip of the outside wing (less stalled wing) might not be completely stalled. This means that the outside aileron is still effective whereas the inside aileron is not. 

To show the full effect of ailerons on spin behaviour, cases of both pro-spin and anti-spin aileron, need to be analyzed for situations with both wing tips stalled and for only one wing tip stalled. 

If both wing tips are stalled, pro-spin aileron acts to level the wings and slow down the spin rate, whereas anti-spin aileron acts to steepen the wings and increase the spin rate. If only one wing tip is stalled, pro-spin aileron acts to steepen the wings and slow down the spin rate, and anti-spin aileron levels the wings and increases the spin rate (Figure 3 shows the effect of pro-spin aileron and Figure 4 shows the effect of anti-spin aileron).

Figure 3.  Effect of pro-spin aileron.

Figure 4.  Effect of anti-spin aileron.

From a practical standpoint it is hard to know if one or both of your wing tips are stalled during a spin. Therefore, the best course of action during recovery is to keep the ailerons neutral because you do not know if aileron application will help or hurt recovery.

The last two steps of the general spin recovery procedure are relatively straightforward to understand. The rudder opposite rotation is used to stop the rotation of the spin, and the elevator forward step is to break the stall (decrease the angle of attack) so that the aircraft can be returned to normal flight.

Understanding the aerodynamics behind spins helps the pilot better understand how his or her airplane flies. Of course the critical point in all this is that the aircraft must be stalled in order to spin. No stall, no spin!

As a result, it’s important to be proficient at stall recovery so that the spin condition is never reached. Especially at low altitude, successful spin recovery may be difficult if not impossible in many aircraft.

Many pilots also find it educational to seek out spin training so that if a spin is inadvertently encountered they will know what to expect and also how to make an effective recovery. If you haven’t had spin training, consider seeking some instruction in this area. The life you save by having spin recognition and recovery skills may be your own!

This month’s Pilot Primer is written by Scot Campbell and Donald Talleur. Scot is a doctoral candidate in Aerospace Engineering at the University of Illinois at Urbana-Champaign. He has been flying for 8 years and is on the Instructional Staff at the University of Illinois Institute of Aviation. Donald Talleur is an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He has been flying for 25 years. Donald is also a research staff member of the Institute’s Human Factors Division