One of the first things that every pilot is taught is the importance of the aerodynamic characteristics of an aircraft, with a particular focus on the low-speed flight regime. Aircraft designers have many means at their disposal to improve the aircraft’s performance, and wing fences and strakes are perhaps among the most effective and often underappreciated aerodynamic aids that have a direct and substantial impact on low-speed handling and stall behaviour. Available on a range of aircraft, both old and new, these features are crucial for preventing undesirable airflow behaviour, especially in high-angle-of-attack situations.
The Problem: Spanwise Flow and Stall Behaviour
Before we can talk about wing fences and strakes, we need to understand why they are needed in the first place. The problem is spanwise flow. As a wing generates lift, the airflow doesn’t travel purely from front to back (chordwise). Airflow also tries to move outwards, towards the wingtips, spanwise. This is exaggerated at higher angles of attack.
Spanwise flow is the primary cause of premature flow separation near the wingtip, which leads to tip stall. Tip stalls are particularly undesirable as they result in decreased effectiveness of the ailerons, making the aircraft difficult or impossible to roll, at the most critical point in time – when a stall is beginning.
If left unchecked, this kind of airflow will lead to asymmetric stall behaviour, uncommanded roll, and deep stall conditions, especially in aircraft with swept wings or high wing loading.
Wing Fences: Vertical Barriers for Better Control
Wing fences are fixed vertical surfaces on the upper surfaces of a wing, normal to the leading edge. They obstruct spanwise flow, and channel more of the flow chordwise from the leading edge to the trailing edge.
Benefits of Wing Fences:
- Delay tip stall by maintaining airflow over critical control surfaces
- Improve aileron effectiveness during high angles of attack
- Enhance low-speed stability, particularly during takeoff and landing
- Provide predictable stall behavior, ideal for training aircraft
Wing fences are especially beneficial on aircraft with swept wings or pronounced taper, where spanwise flow is more aggressive. Aircraft such as the MiG-15 and early Learjets feature prominent wing fences to counteract these effects.
For those engaged in Flight Training in Australia, understanding the role of wing fences helps student pilots anticipate how their aircraft will behave during stalls and recoveries.
Strakes: Vortex Generators for Stability
Differing from wing fences that aim to halt spanwise flow, strakes are typically small aerodynamic surfaces added to the fuselage, engine nacelles, or wing roots. The purpose of a strake is to create controlled vortices that energize the boundary layer and promote better airflow attachment over critical surfaces.
A typical example of a strake is the leading-edge root extension (LERX) commonly seen on military jets. Strakes can also appear as nose strakes, canard strakes, or small additions to the wing-root.
Benefits of Strakes:
- Create vortices that delay airflow separation over control surfaces.
- Increase pitch and yaw stability, particularly at high angles of attack.
- Enhance stall margins by energizing critical sections of the wing.
- Maintain rudder and elevator effectiveness during aggressive maneuvers.
Even small general aviation aircraft may feature simplified strakes to enhance stability during slow flight. For instance, some light aircraft incorporate fuselage strakes to reduce adverse yaw during coordinated turns.
Applications in General and Commercial Aviation
Wing fences and strakes are found in military and high performance aircraft, but have use in general and commercial aircraft too. On light trainers, like the Cessna 152 or Piper Tomahawk, aerodynamic devices including stall strips and stall fences are used to give students benign stall characteristics. By ensuring that the stall occurs at the wing root first and not at the tip, the aircraft gives aileron control even during stall and is consequently a benign stall. On airliners from Boeing and Airbus, strakes and fences may be used on the nacelles or forward fuselage to direct airflow around complex fuselage shapes. This gives the aircraft stability during slow-speed flight, like a go-around or missed approach, or other high drag configurations.
For students at institutions like a Melbourne Flight school, learning to identify these features on pre-flight inspections and understanding their purpose reinforces the link between aerodynamic theory and real-world safety.
How They Influence Pilot Training
Flight instructors often stress the importance of coordinated flight, recognition of stalls early, and recovery to un-stall the aircraft. Knowledge of how wing fences and strakes can aid in aircraft handling can provide students and pilots with the foresight to make safer decisions when conducting slow-speed maneuvers.
For instance:
A student pilot might observe the difference in stall behaviour between an aircraft equipped with wing fences and one without during a power-off stall.
If a pilot is operating in high-density altitude conditions or turbulent weather, they may experience smoother control response with the assistance of strakes, which allows for more precise control during landings.
When undergoing advanced flight training, such as spin training or advanced stall/spin awareness modules, an understanding of these devices can give pilots a better understanding of what to expect and how to read the aircraft’s behaviour.
Summary
Although wing fences and strakes may be small and unassuming, their role in improving aircraft’s low-speed handling and safety should not be underestimated. By effectively managing airflow and boundary layer behaviour, these aerodynamic features allow the aircraft to maintain control authority during stalls, steep approaches, and high angle-of-attack (AoA) flight. Students, instructors, and aviation professionals alike will benefit from learning more about these aerodynamic devices to gain an appreciation of aircraft design and how form truly follows function in aviation.
If you’re an aspiring or current pilot, keep an eye out for these features during your flights, and you’ll gain a greater appreciation for the subtle language of wing design, as well as add an extra dimension to your decision-making and flying abilities.
