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When it comes to airline businesses the world watches closely as airlines pass through rough waters. Operators have no control over the major costs incurred in running the business. For example, fuel accounts for nearly 30 to 40 percent of an airline's total expenses. Using the Theory of Inventive Problem Solving (TRIZ) there are several ways fuel efficiency can be improved.
Aircraft engineers could say that aerodynamics is a mature science. That was until NASA's Richard T. Whitcomb invented vertical winglets, which could reduce the drag by nearly 20 percent. The problem explored in this article considers the trouble of drag reduction at an aircraft's wingtips. This is one of the major contributors of problematic lift force, which adversely affects fuel efficiency.
The following includes what the problem is, where it occurs, what the contradictions are and how TRIZ principles could be used to create this concept. The proposed solution is tested and validated for improved fuel efficiency and includes several benefits.
It is a challenge to reduce fuel cost. The airline operator can try to do it by way of fuel hedging or by having a better business model, however, the aircraft manufacturer has to see profit growth from its customers.
This case study shows how the fuel cost reduction problem was tackled by the aircraft manufacturers. The broader problem, therefore, becomes "How to reduce fuel consumption." The first step is to define "ideality" (i.e., maximum benefits / (zero cost + zero harm). The solution should include more good things than bad. When talking about a technical system it means maximization of a useful function performed by the system and reduction or elimination of the harmful function at no cost.
First, look at the forces on an aircraft wing in cruise mode. Figure 1 illustrates how these forces are provided by the engine, the lift generated (due to the wings of the moving body), its own weight and the drag that is induced (by the stream of air as it passes through).
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The engine design can be examined for how to improve thrust per unit of fuel burnt. This involves mapping the engine system and maximizing useful functions with the elimination of harmful functions. Similarly, weight can be reduced by using knowledge from several fields where the similar problem of weight reduction is successfully solved. As a result, this is one of the directions that can be taken to tackle the fuel saving problem. The focus is on the reduction of harmful drag force, which will give better lift and will help get closer to an ideal solution.
An airfoil is the shape of a wing or blade (a propeller, rotor, turbine or sail) in the cross-section. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with an asymmetric camber, shown in Figure 2.
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Designing a wing would have been simple if it were a two dimensional airfoil. But the wing has a finite length. The difference in air pressure between the lower and upper surfaces of a wing causes the air to escape around the wingtip, which reduces lift.
Drag means the harmful forces acting on the wing that reduce available lift. There are two types of drag the first type is lift-induced drag. It accounts for 40 percent of the total drag and the other is parasitic drag.
The lift-induced drag occurs as the result of the creation of lift on a three dimensional lifting body. Induced drag consists of primarily two components: the vortex drag and the viscous drag.
The motion of the air rushing around the wingtip coupled with the velocity of the airflow through the wing causes a vortex to form near the wingtip as shown in Figure 4. The vortex tip causes upwash and downwash air currents that alter the currents of the free stream flow around the wing, shown in Figure 5. The tip induces a decrease in the angle of attack from the average relative wind flowing around the wing. This results in two undesirable by-products. First, the wing generates lift perpendicular to the average relative wind. This diverts the lift vector away from the desired direction, which is perpendicular to the free steam. Diverting the lift vector causes a drag component to be generated that is parallel to the free stream airflow. The drag component varies as the cosine of the angle and the total lift vectors as shown in Figure 4.4
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The upwash / downwash effect (shown in Figure 5) of the tip vortices (shown in Figure 6) has its greatest influence on the wing section closest to the tip. The tip vortex has little effect on the average relative wind of the wing sections far from the wingtip.4,6
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If the wingtips are pushed outboard, a smaller section of the wing will be affected by the tip vortex, which will reduce the upwash and downwash effect. In other words, if the span were infinite the induced drag would be zero because there would be no wingtip.
Parasitic drag is caused by moving a solid object through fluid. This is around 60 percent of the drag. Parasitic drag is made up of multiple components including form drag, drag as a result of form from the object and skin drag (which occurs because of interaction with the object skins). A large, long wing with infinte span- to chord-ratio (shown in Figure 7) would have enormous parasitic drag. But by optimizing the wingspan, chord and airfoil from the drag can be controlled. The vortex drag remains and affects the wingtip by reducing the overall lift.
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The vortex drag can be minimized by having an infinite span- to chord-ratio. If the designers optimize the aspect ratio for improvement in lift coefficient then the result shows that there is a marginal difference in the lift coefficient from a ratio of eight to infinity. For example, there is not a significant difference in the lift coefficient when the aspect ratio (wing length to width) is eight. The wing length is eight times the wing width or when it was increased to infinity where wing width is zero and wing length is infinite. This means the system has hit the limit and cannot be improved by further optimization, shown in Figure 8.4
Plane wingtips create a drag by the vortices it generates. This drag causes more fuel burn increasing the operational cost of airline companies. Drag can be reduced by increasing the wingspan. Optimization tools can optimize the wingspan- to tip-ratio to reduce the drag, however, it needs a lot of experimentation. The wingspan cannot be increased as it cannot fit the airport gates. This causes a contradiction.
A contradiction is recognized when two technical parameters are 180 degrees opposite. This means when an individual wants to improve one parameter the other side stops it from improving. In this case the parameter to improve is force (in order to reduce the drag, which is classified by TRIZ researchers under the parameter "force") and the parameter that stops it from improving is the length of the wingspan.
There are two parameters that can worsen the wingspan (or length of the wing). The first one is the weight of the wing. If the wing is infinitely long then the structural weight would increase. This would increase the stress, which would lead to worsening the strength (as per the definition of contradiction parameters such as resistance to breaking or fatigue) as the wing structure will be subjected to cyclic stresses more than it is designed to accommodate. So the first sets of contradictions include the following:
The inventive principles include:
| Table 1: Force and Strength | ||
| Parameter being improved | Parameter that stops object from improving | Inventive principles used by other inventors to solve similar problems |
Force (one of the equivalent meanings of force from several that are indicated in the Matrix 2003 is drag.) | Strength (equivalent meaning: fatigue, creep, bond, join, muscle, droop, distortion, stiffness, rigidity, etc.) Parameter is defined as: "The extent to which an object is able to resist changing in response to force, resistance to breaking. Can mean elastic limit, plastic limit or ultimate strength, tensile or compressive; linear or rotational. Also includes toughness and hardness. | 35 – Parameter changes 14 – Curvature 9 – Preliminary anti-action 3 – Local quality 17 – Another dimension 5 – Merging 27– Cheap, short-living objects 7 – Nested doll |
Increasing the length of the wing will affect the weight of the wing, however, the plane cannot be accommodated at airport gates.
As per Matrix 2003, "length" is represented by two separate parameters. The first of those parameters is "length of a stationary object" and the second is "length of a moving object." The task is to choose the right parameter for defining the contradiction.
For instance, consider the wingspan length under the parameter "length of the stationary object" as it seems that the length of the wingspan in the stationary condition (when the plane is grounded and about to stop in the parking lot) is the problem of its accommodation. What will be shown next is the effect of the cause that is making it longer.
The drag experienced by the wing, which affects the lift, is experienced in mid air because of the relative motion between the air and the wingspan; therefore, the parameter to select is "length of the moving object" and not the "length of the stationary object."
The contradiction table and the corresponding inventive principles are summarized as follows:
| Table 2: Contradiction Table and Inventive Principles | ||
| Parameter being improved | Parameter that stops object from improving | Inventive principles used by other inventors to solve similar problems |
Force (one of the equivalent meanings of force from several others is drag). | Length of the moving object. | 17 – Another dimension 35 – Parameter changes 9 – Preliminary anti-action 3 – Local quality 14 – Curvature 19 – Periodic action 28 – Mechanical substitution 36 – Phase transitions |
Before revealing how the inventive principles can be applied to the proposed solution, first an individual needs to see the developed solution.
By stretching wingspan or increasing aspect ratio it reduces induced drag. Designers, however, have to balance the benefits of less induced drag against the costs of structural weight increases, more parasitic drag or cost considerations. Winglets work because they efficiently produce aerodynamic side forces that divert the inflow of air from the tip vortex, therefore, the drag- to lift-ratio is not affected as it is with the normal wing and there is an improvement in lift-to drag-performance resulting in less fuel consumption.
A blended winglet (as shown in Figure 9) is intended to reduce interference drag at the wing / winglet junction. A sharp interior angle in this region can interact with the boundary layer flow causing a drag inducing vortex, negating some of the benefits of the winglet. The blended winglet is joined to the wingtip in a constant radius curve, rather than a relatively sharp angle junction. According to Aviation Partner's Inc. this smooth junction reduces shock interference between the winglet and the wing near the tip. This has achieved further improvement in the fuel consumption and has also moved the winglet forward eliminating problems with the roll up vortex. This reduces the drag. Figure 9 illustrates the design and the difference in the structure of conventional and blended winglets.
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Raked wingtips (shown in Figure 10) are a feature on some Boeing airliners, where the tip of the wing has a higher degree of sweep than the rest of the wing. The stated purpose of this additional feature is to improve fuel efficiency, climb performance and to shorten takeoff field length. It has been found, however, that wingspan is generally more effective than a winglet of the same length, but may present difficulties in ground handling. For this reason, the short-range Boeing 787-3 design currently calls for winglets instead of the raked wingtips featured on all other 787 variants. Figure 10 uses principle 17, another dimension and principle 3, local quality.
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Figure 11 demonstrates the higher vortex intensity without a winglet (lefthand portion of the wing) and reduction of vortex intensity with a winglet (righthand portion of the wing).8
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The Theory of Inventive Problem Solving is a wonderful innovation science. Several of its problem formulations and solution generation tools have been validated through extensive research conducted over the past 50 years.
The contradiction of drag reduction versus length and weight is faced by several inventors in various fields and they have conquered it with a solution based on the inventive principles. If problem solving, strategy teams use TRIZ as their thinking methodology then the time to arrive at the best solution will be reduced. Interested readers can refer to reference seven to further augment any innovation efforts in a systematic manner.
One of the ways the author found to master its application is through reverse ideation (by working backwards to find out what the "general problem" is then applying the general solution to the specific problem). If a TRIZ facilitator is involved in system analysis, functional modeling, problem formulation or solution generation processes then it could help experts in the field to have access to knowledge of other fields for creation of new knowledge. In practice, after the design is frozen the next phase is to use optimization methods to arrive at the optimal design parameters.
"If the 20th century was of experts creating SILOS then the 21st century is of innovation generalist using systematic innovation to connect those experts and create a value which is truly ideal." - Darrell Mann
Prashant Y. Joglekar is a TRIZ enthusiast. He has studied and applied this innovation science for the past five years. He holds a masters degree in engineering from the Indian Institute of Technology, Mumbai. He has completed several practical applications of TRIZ. He believes that more people will be interested in using TRIZ, if they are educated with simple day-to-day examples. Joglekar can be found on Twitter @ideabound. Contact Prashant Y. Joglekar at joglekarprashant (at) gmail.com or visit http://innovationnukkad.blogspot.com.
