This extended essay focuses on analysing different solutions aimed at reducing the overall effects of a phenomenon in Aviation, known as wake turbulence. Wake turbulence has been one of the prime causes to accidents involving two planes flying in a close vicinity to each other[footnoteRef:0]. In this extended essay, the causations and effects of wake turbulence, as well as different approaches made to reduce such effects by plane manufacturers and safety procedures proposed by the FAA, will be investigated. These approaches will be identified, defined and analyzed theoretically, coupled with experiments to verify predictions made theoretically. In the end, these approaches will be compared together to see which one is the most effective, with possible improvements for future uses. The approaches proposed include different variations of wingtip devices and other spacing requirements by the Federal Aviation Administration (FAA) for pilots of different types of aircrafts, but this extended essay will focus primarily on wingtip devices.
The study of wake turbulence have been known and experimented on since the 1950s, but the subject was never taken seriously or became well known to governmental agencies and aircraft manufacturers, until the incident of American Airlines flight 587 on November 12, 2001. The aircraft of this flight, an Airbus A300-600, was en-route from New York’s JFK airport to Santo Domingo, the Dominican Republic, when it crashed into a neighborhood in Queens, New York, shortly after take-off. The Airbus A300’s vertical stabilizer snapped off the plane after a series of aggressive rudder control applied by the pilots of flight AA587, which unfortunately put too much stress on the vertical stabilizer. Such an action was made by the pilots to counter the induced turbulence that they encountered from a bigger Boeing B747-400 that took off a minute before, also flying in the same path.
Since then, a lot more have been studied about wake turbulence, followed by different solutions from aircraft manufacturers and the FAA in hope to make air travelling safer. One cost effective, reliable and flexible option is the utilization of wingtip devices, which was originally introduced by NASA Langley’s aeronautical engineer Richard Whitcomb during the 1973 oil crisis that prompted airline manufacturers to find a way to reduce fuel consumption, without a substantial decrease in output.
The inspiration for this extended essay comes from a TV show on National Geographic Channel, called Air Crash Investigation. I remember watching the incident involving flight AA587 and thought to myself that such incidents could have been prevented if we had a better understanding of the effects of wake turbulence. Growing up in a house that lies adjacent to my city’s biggest airport, I also had a chance to observe planes taking off and landing in intervals, and discovered that once a large plane had just taken off or landed, the interval tends to be longer, but could not figure out why. This question urges me to further investigate and finally, came across the topic of wake turbulence.
Turbulence is generally defined as the chaotic movement of particles in a fluid. Wake turbulence is particularly defined as the turbulence created by an aircraft as it passes through the air, from the movement it takes off from the ground to the point it touches down[footnoteRef:3]. There are two types of wake turbulence: wingtip vortex, which is going to be the main focus of this extended essay, and jet wash, which is simply the highly pressurized streams of air blasted out from plane engines. Jet washes are extremely turbulent, but only last for a few seconds. Wingtip vortices, on the other hand, are not as violent but can last up to three minutes after they are caused by the aircraft, which has long flown away.
Wingtip vortices occurs whenever lift is created. Aircraft wings are shaped in an airfoil-shape that allows high pressure wing to be at the bottom surface and low pressure wings to be at the upper surface. According to Bernoulli’s principles, lower air speed equates to to higher pressure and vice versa, making the unevenly shaped surfaces of airfoils perfect to create lift. As high pressured air from the lower surface escapes up to the upper surface at the upper surface through the tips of the wings, alongside streams of air coming at the wings, wingtip vortices are formed[footnoteRef:6]. As the plane continues to fly, more and more wingtip vortices are formed, which move slowly in the direction opposite to the movement of the plane, leaving behind a turbulent area that potentially could be harmful for other aircrafts, as exemplified by the case of American Airlines flight 587. These turbulent area can sometimes last up to 3 minutes, requiring aircrafts on the same path to be safely spaced apart, either vertically or horizontally.
The effects of wingtip vortices vary greatly, based on different circumstances. On the aircraft producing the wingtip vortices itself, more wingtip vortices creates more drag, as the vortices spiral backward, causing the wing to tip up. Since lift is perpendicular to the surface of the wing, tipping the wing up and backward will partially turn lift into induced drag. As a result, more induced lift and thrust are needed to even out the effect of induced drag created by wingtip vortices, making flights less efficient. Aside from the economical issue caused by wingtip vortices, a more serious effect of wingtip vortices is on aircrafts trailing the one that produces wingtip vortices. In most cases, trailing aircrafts, especially smaller ones, can be caught in the turbulent region of leftover wingtip vortices, causing them to lose lift and the overall controllability of the aircraft.
There are 5 major factors that have been experimentally proven to affect the severity and strength of wingtip vortices. The 5 factors are the speed of the aircraft, its weight, angle of attack, wing configuration and finally, its proximity to the ground. Particularly, the aircraft speed and wing configurations will be investigated further in the experimental section of this extended essay. The faster an aircraft flies, the less wingtip vortices can be generated as they will be smoothed out as straight, non-vortex-like streams of air trailing the aircraft. The heavier the aircraft is, the more lift is needed to be generated and thus, more wingtip vortices are generated. Larger angle of attack, often results from the deployment of flaps during take off and landing and/or the increase in vertical speed when an aircraft is climbing to a desirable altitude, also creates more wingtip vortices as more lift is generated and again, more induced drags in the form of wingtip vortices are created.
Different wing configurations in the form of different length to width ratio, curvature of edges and the implementations of different types of wingtip devices also have various effects on the wingtip vortices produced. Lastly, the closeness of an aircraft to the ground also has an effect on the wingtip vortices produced, as the spiraling effect of the vortices will be canceled out if come into contact with another surface, in which it would be the ground if a plane is still on the ground or is flying to low. Among the FAA’s regulations, there are 4 common rules that can be simplified below:
A wind tunnel is needed to simulate an environment suitable for a flying aircraft. Since the school does not have a wind tunnel and there is no access to nearby wind tunnels for testing purposes, I decide to build a wind tunnel made out of transparent, smooth-surfaced glass, with a high-speed, adjustable PWM fan attached at the end of the tunnel. To better visualize the effects, a smoke creator is also placed near the fan so that traces of air being blown at the wing and the result of that can be clearly seen. There will also be guiding pipes made out of plastic straws stacked together in a honeycomb shape in order to minimize turbulent air and make the visualization of smoke trails better.
As for wing models, I will be using highly accurate, custom-made 3D printed models of wing models and wingtip devices. All wing models will have identical main airfoil section, in every way possible except for the color codings. The only aspect that varies is the wingtip devices that each wing model will be equipped with. There will be 4 models that are going to be tested: A bare model (model 1) without any wingtip devices; model 2 with an outward and upward pointed sharklet (similar to those on new versions of the Airbus A320 family aircrafts); model 3 with a 2-edged winglet (similar to those AT winglets on Boeing B737 MAX aircrafts), where 1 edge points outward and upward while the other, smaller one points outward but downward. Finally, model 4 will feature blended sharklets (similar to those on the Airbus A350 XMB aircrafts), where the wingtip device will be curved outward and upward.
These wing models will then be put on a scale inside the wind tunnel so that the lift generated by different wingtip configurations could be measured effectively. On the second part of the experiment, a high speed camera is placed at the other open end of the tunnel to record the shape and size of the induced wingtip vortices, in order to determine which wingtip device induces the less wingtip vortices at various speed. The diagram below visually illustrates how the experiment is setup and what component is placed at what position. FanGuiding pipesWing ModelScaleHigh speed Camera.
The way the experiment is going to take place is explained below. The wing model is put at a fixed point (at a constant angle of attack) by attaching it to the inner side of the wind tunnel, using permanent glue. At one open end of the tubing lies the high speed fan with the smoke creator. At the other end is the high speed camera, which will be used to take snapshots of the wingtip vortices outlined by smoke trails. The relative size of the vortices will determine the strength of the induced wingtip vortices. The reading from the scale will also be taken, initially to determine the weight of a wing model without any wingtip devices, then other wings equipped with wingtip devices and finally, the weights of all wing models once air is being moved inside the tunnel, so that the lift generated from different wing configurations can be measured and analyzed. All measurements will be the averages of all values taken every 5 seconds for 1 minute and will be recorded in a value table below.
The other part of the experiment, involving the use of a high speed camera, is responsible for determining the relative diameter of the wingtip vortices (as outlined by smoke trails), in order to confirm whether or not there is a solid correlation between vortices size and induced-drag created from different wingtip configurations. Diameters of wingtip vortices are measured using a straight-edged ruler, using images taken by the high speed camera.
It can be reasonably hypothesized that wingtip devices with higher surface area will more likely mitigate the strength of induced drag, an effect of wingtip vortices, as hypothetically seen in the lift equation: , where L represents lift, CL is lift coefficient (fixed for airfoils of the same material and shape), r is air density, V is airspeed and A is wing area. That being said, later generations of wingtip devices, such as the 2-edged split-tips seen on Boeing B737-MAX aircrafts, are more effective than older generations, such as the 1-edged sharklets on A320 aircrafts. This trend will be tested by measuring the effective lift generated between model 2 and model 3. Another trend that will be tested is that curved winglets are better at minimizing induced drag compared to straight-edged winglets of similar size, material and weights. This trend will be tested by measuring the effective lift generated between model 2 and model 4. As for the vortices sizes, it can be reasonably inferred that the less induced-drag a model produces, the smaller the size of the wingtip vortices.
There are a couple of areas that this experiment could have improved upon, not because the overall results could have been altered greatly, but so that the accuracy could have been increased. As mentioned above, for every trial, the wing models are glued onto and then taken off the side of the windtunnel, where their relative angle of attack are approximated to be the same, using bare eyes. This should not be allowed because without a constant angle of attack, the amount of lift and induced drag will vary from time to time, leading to incorrect measurements. Instead, the windtunnel should have been designed to have a rotatable mounting rack that allows wing models to be attached and detached easily, as well as to be fixed at a desirable, constant angle of attack throughout the experiment. Another area of improvement could be the use of a highly accurate electronic scale (often used to measure chemical mass), instead of a regular kitchen scale that does not have as high of accuracy or as small of scale division.
The experiments would have yielded even more concrete trends if more trials and more fan speed selections can be chosen. Nevertheless, the above 2 experiments have further solidified the aviation industry’s ongoing trend of having curved, or “blended” wingtip devices on various types of aircrafts, mainly commercial aircrafts. The reasons behind such trends can be attributed to growing economical demands on fuel efficiency, in order to cut down on operation costs or to extend operational range, and the need for a safer airspace, where tragic accidents caused by wingtip vortices in the past can no longer be the threats to commercial airplanes. With more and more planes being manufactured and put into service every day, the sky is getting more and more densely populated than ever, challenging aircraft manufacturers to continue innovating. Because of this, more advanced wingtip technologies would have to be invented, in order to help existing FAA guidelines coordinate the sky in a safer, more efficient manner.
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