In today's cutthroat competition between airlines, only those airlines operating the entire gamut of air and ground operations at optimum efficiency will survive. The principal contributor to an airline's bottom line is the quality of aircraft it operates. The same competition exists between the manufacturers as well, with an identical base criterion, i.e., the quality of aircraft they produce. Essentially, the governing factor is the maximum number of passengers that can be flown the longest distance in one trip with the minimum consumption of expensive aviation fuel in each category of aircraft─ short, medium, long and ultra-long range. "Every Kg. of basic aircraft weight saved is a million dollars saved over a life span of twenty-five years," according to Dr. Kota Harinarayan, Project Director, LCA, India, who did his T-75 MCF with me (1977) (n.p.).
Designing an aircraft and sections thereof take millions of man-hours from the drawing board to first flight to fully operational status. Aircraft have to be sleek and light, its surfaces as smooth as a baby's skin, no unwanted drag creating protrusions, etc. Everything is centered about minimum weight, minimum drag and optimal use of CAD/CAM in the field of aerodynamics to extract maximum lift at minimum engine thrust in flight, with highest passenger density. Any aerodynamic innovation is most welcome.
Non-Aerodynamic Considerations
I do not propose to look at ground operations like ticketing, loading, salaries, etc. That leaves only the aircraft. Here, there are two factors to be considered, the engine and the airframe. Since this Paper is specifically aimed at an aerodynamic concept, I will discuss engines only in brief. NASA (2008) describes four propulsion systems: The Propeller, the Ramjet, the Rocket and Turbine Engines (www.grc.nasa.gov). The Ramjet and Rocket are outside the purview of this Paper and will be left out.
Propeller-driven airplanes use heavy internal combustion engines to turn propellers to generate thrust in a noisy and inefficient manner. Mechanical problems limit the velocity of the piston-engine aircraft and the thrust generated limits its gross take off weight. There are many other problems too, making the propeller-driven aircraft unsuitable for passenger flights (n.p.).
Air is sucked into a jet engine and compressed in a specifically designed compressor section before being pushed into a combustion section where fuel is added and the mixture burnt. The hot mixture passes through a turbine, rotating it, before exiting at high speed through the exhaust nozzle. The reaction to this huge mass flow exiting at high speed drives the aircraft forward, as Newton's third law decrees. In some engines, the turbine drives a propeller, and the combination is called a turbo-prop engine (n.p.). "For short distance flights, i.e., up to 150 miles or so, the turboprop airliner proves to be most cost-efficient. For longer hauls, a jet engine of a type called a turbofan is the most effective and cost efficient engine" (www.probertencyclopaedia.com). Aerodynamic Considerations
An aircraft in flight experiences four forces as shown in the figure below:
Figure 1. Forces on an Aircraft (www.myaeromodelling.com)
In this simplistic photograph, the forces are shown are Thrust, Wright, Lift and Drag. Only the last two are aerodynamic forces. When the aircraft is at equilibrium, i.e., at a steady velocity and altitude, thrust is equal to drag (T=D) and lift is equal to weight (L=W). For the moment, let us focus on the aerodynamic forces only. All this data is readily available in any text book on basic Aerodynamics. In this Paper, the IAF text book, the IAP 3106 will be used extensively.
Lift
As the term suggests, Lift (L) is the aerodynamic force that keeps an aircraft in the air. It is given by the formula: L=CL½ÏV2S, where CL is the coefficient of lift, Ï is the static pressure, V is the true air speed (TAS) and S is the surface area exposed to the airflow. As NASA explains, "coefficient CL contains all the complex dependencies and is usually determined experimentally" (www.grc.nasa.gov).
Figure 2. Angle of Attack (www.myaeromodelling.com)
The free airflow approaching at a relative wind velocity V (TAS) splits up as it meets the aerofoil (a vertical cut view of a wing, shown in blue). "The aerofoil has a distinct curved shape, called camber. The dotted line joining the midpoint of the leading edge to the trailing edge is called the chord line and the angle between the chord and the free airstream is called the angle of attack (α)" (IAP 3106). Assuming that two adjacent air particles, A and B, split at the leading edge. Let A travel above the aerofoil and B below. According to the Equation of Continuity, which states that 'Air mass flow is a constant' (Smith.H.C., IAP 3106, Part 1, Sect.1, Chap.2, Para 15), they have to meet up at the trailing edge, so A has to speed up as the distance involved is longer than for B. "Such an increase in speed causes a drop in pressure", as proved by Bernoulli (library.thinkquest.org) more than two hundred years ago. Thus the pressure below the aerofoil is higher than that above it. The net result is an upward force, Lift.
An increase in α makes 'A' travel faster and more lift is generated. In Figure 3 below, this is seen as an extreme where α=15°. Note that the airflow attaches itself to the wing. But at α=25°, the airflow can no longer negotiate the curve smoothly and the upper flow breaks up. Lift drops to almost zero and the aircraft will sink as L<W. "The wing has stalled! CL is close to zero. The airflow has detached itself and is in a state of turbulence" (IAP 3106).
Figure 3. Pressure Patterns at Various Angles of Attack (www.myaeromodelling.com)
Drag
Drag is the resistance offered to the wing as it travels forwards. It is given by the formula:
D=CD½ÏV2S, where CD is the coefficient of drag. Note that both lift and drag vary directly with the square of the TAS. There are essentially two types of drag (ibid):
Zero Lift Drag. When an aircraft is flying at zero lift α, as shown in Figure 3, the resultant of all aerodynamic forces acts parallel and opposite to the direction of flight and is called Zero Lift Drag. It is composed of surface friction drag, form drag and interference drag.
Lift Dependent Drag. In producing lift, the whole aircraft will produce additional drag which is composed of Induced drag (vortex drag) and increments of the components of zero lift drag. This additional drag is called 'Lift Dependent Drag' (ibid).
The Boundary Layer: Because air is viscous, the layer of air immediately next to the skin of the aircraft slows down due to adherence. The next layer is also affected, but not as much. This carries on till a layer reaches free-stream velocity. "The affected layers are known collectively as the boundary layer" (ibid). The nature of this boundary layer determines the maximum lift coefficient, the value of form drag and the stalling characteristics of a wing. "The boundary layer is defined as that region of flow in which the speed of the flow is less than 99% of the free stream flow" (ibid). It normally exists in two forms, laminar and turbulent. The flow at the front of the body is laminar and somewhere along the flow of the boundary layer, it turns turbulent. This point is called 'transition point' (ibid). The turbulence in the boundary layer causes it to mix with the free stream air flow and extract some kinetic energy out of it. Thus, "turbulent boundary layers tend to have more energy than purely laminar flows" (ibid).
When moving along the curved surface of the wing, the boundary layer is actually traveling into a region of an adverse pressure gradient. The "transition point is generally found where its energy is at its lowest, which is usually the thickest portion of the wing" (ibid). The effect of skin friction is depletion of the boundary layer's kinetic energy, exacerbated by the adverse pressure gradient along a curved wing. The layer comes to a stop somewhere close to the trailing edge and separates, creating a turbulent wake. The separation point comes so close to the trailing edge only because the boundary layer is turbulent (ibid).
Vortex Drag: The high pressure air under the wing tries to move up into the low pressure area, but cannot do so as there is a physical barrier-the wing. But at the wingtip, there is no barrier and the air spills over onto the upper surface. "Thus the airflow over the wing is deflected towards the fuselage, while below the wing, it drifts towards the wingtip. Also, when the two airflows meet at the trailing edge, a sheet of vortices is formed, tending to drift towards the wingtip. The other factors affecting vortex drag are aspect ratio, planform, lift & weight, and lastly, speed" (ibid). There is an interesting relationship between aspect ratio, planform and lift. When operating an aircraft with swept back wings, the airflow is parallel to the fore and aft axis of the aircraft and meets the wing at an angle. It has to be divided into two parts, V1 parallel to the leading edge which has no effect on the lift and V2, normal to the leading edge which does affect the lift. V2=Vcos ø. Hence, "the CL of a swept wing is reduced in the ratio of the cosine of the sweep angle" (ibid).
V2 ø v v1
Figure 4. Flow Velocities on a Swept Back Wing
This means that if there are three types of wings, e.g. unswept, moderate sweep like airliners and high sweep like the Concorde, all with an aspect ratio of 2.5, the stall on the latter pair would occur much later and at a higher angle of attack. Also, when a wing is swept back, the boundary layer tends to change direction and flow towards the tips. "This outward drift is caused by the boundary layer meeting an adverse pressure gradient and flowing obliquely to it over the rear of the wing. This drift tends to thicken the boundary layer over the outer parts of the wing, making it more prone to separation. Chances of tip stall increase. At the same time, at high α, the airflow is separating along the leading edge and re-attaching itself over the inboard section of the wing, behind a short 'bubble'. It rarely re-attaches itself to the outboard section. All vortices are coming together and in Figures 5 & 6, we see an embryonic Ram's horn vortex forming". (IAP 3106).
Figure 5. Swept Wing Vortices (www. pprune.org) Figure 6. The Ram's Horn Vortex (www. pprune.org)
Tip Stalling
As explained in IAP 3106, the wing of an aircraft is designed to stall progressively from the root to the tip. The reasons for this are threefold: Firstly, to induce early buffet symptoms over the tail surface; secondly, to retain aileron effectiveness up to the critical angle of attack and finally,
to avoid a large rolling movement which would arise if the tip of one wing stalled before the other (wing drop).
The most common features designed to prevent wing tip stalling (ibid) are: Washout, where a reduction in incidence at the tips will result in the wing root reaching its critical angle of attack before the wing tip; Root Spoilers, where, by making the leading edge of the root sharper, the airflow has more difficulty navigating the contour of the leading edge, inducing an early stall (a crude form of this device can be seen on the Chipmunk/HT2); Change of Section, when an aerofoil section with more gradual stalling characteristics may be employed towards the wing tips (increased camber) and the complex Slats/Slots. The use of slats and/or slots on the other portion of the wing increases the stalling angle of that part of the wing (ibid).
Alleviating the Tip Stall
As given in IAP 3106, most of the methods described demand re-energizing a weakened boundary layer. Some of these methods are the Boundary Layer Fence, which restricts boundary layer outflow and checks span-wise growth of the separation bubble along the leading edge; Boundary Layer Suction, where suitably placed holes draw off the bottom layers and allow the upper stronger boundary layers to take their place; Boundary Layer Blowing, where high velocity air taken from behind the compressor is injected into the boundary layer, increasing its energy level; Leading Edge Extensions, also known as saw-tooth leading edges, which cut down the growth of the main vortex. Instead, a small fresh vortex starts from there. Since it lies across the wing, it acts as a barrier to boundary layer outward flow; Leading Edge Notches, which have the same effect as the leading edge extensions and Leading Edge Slots which re-energize the boundary layer (ibid). It should thus be obvious that "the boundary layer has a major role to play in optimizing desirable aerodynamic forces on an aircraft" (n.p.).
Two simpler and far cheaper options are vortex generators and vortilons, with the latter being a modern-day innovation. Both stir the airflow above the wing, intermixing low energy air kissing the wing with the high energy free airstream, re-energizing it. Both will be examined in closer detail in the rest of this Paper.
Vortex Generators
Vortex generators are 'simple small rectangular plates that jut above the wing surface. They look like tiny little winglets poking out of the wing. As air moves past them, vortices are created off the tips of the generators. These vortices interact with the airflow moving over the wing to speed it up and help reduce the possibility of separation' (www.aerospaceweb.org) . Vortex generators are generally used as follows:
On swept wings at transonic speed: Many early swept wings suffered from separation at transonic speeds because shocks formed on the wing create an increasing pressure that slow the air and cause flow separation. The Buccaneer and Javelin fighter are good examples of such aircraft (ibid).
Three sets of vortex generators are used along the Javelin's outer wing as visible below. The generators on both planes serve to break up the shocks formed at transonic speeds thereby delaying
the effects of separation (ibid).
Figure 7. A Gloster Javelin Showing Three Sets of Vortex Generators (www.aerospaceweb.org)
For ineffective control surfaces: The separation problem becomes even more significant since control surfaces like flaps and ailerons are usually located along the trailing edge of a wing. "When the flow separates from the wing, these control surfaces have little or no air flowing over them and they become ineffective. Thus, not only will the aircraft lose lift when the wing stalls, but the pilot may not be able to control the orientation of the aircraft. To correct this problem, vortex generators are often placed just ahead of the control surfaces to create a faster flow of air over the surfaces and increase their effectiveness" (IAP 3106). The Kiran is a good example of this concept.
On short-takeoff and landing aircraft: These aircraft generally must operate at low speeds during takeoff and landing, so the flow speed over the wings tends to be low as well (www.aerospaceweb.org). Aircraft like the C-17 Globemaster III transport use vortex generators to create a higher-speed flow over the wings and control surfaces at these conditions to improve performance and controllability. In the C-17, "the vortex generators are located on the sides of the engine nacelles rather than on the wings", but they still produce the same beneficial effects (ibid).
Figure 8. Large Vortex Generator Plates Visible on the Engine Cowlings of a C-17 (www.aerospaceweb.org)
The one drawback of vortex generators is that they also create drag.
The Vortilon
Figure 9. Vortilons under the Left Main Wing of an Unknown Aircraft (www.berkut13.com)
Vortilons are very simple devices that perform an important function, somewhat like vortex generators, but without the penalty of drag. They are small fences fitted on the undersurface of an aircraft's wing, "but their main function is to generate a vortex of air over the top of the main wing only at high angles of attack. When the angle of attack on the main wing is raised, the lower surface airflow starts to move outboard at an increasing angle" (www.berkut13.com). They have no loss in performance in all other phases of flight (www.raisbeck.com). "The vortilons stick up and more forward as the wing angle increases and they start acting as little fences to the spanwise air flow. They don't stop it, they 'trip' it - causing a vortex. This vortex has the effect of keeping the air flow attached to the upper surface of the wing - reducing the wing's local stall angle and increasing aileron effectiveness at low speeds/high α" ( www.berkut13.com). Vortilons also work with different parts of the wing to enhance stall behaviour.
Vortilons on the DC-9: "The DC-9 wing featured vortilons on the lower wing surface that improved control at high angles of attack up to 30°" (Norris and Wagner, n.d.). In most attitudes, the vortilons were aft of the area where the airflow 'stagnated,' so they had little effect. However, when the aircraft was in a potentially dangerous, nose-up attitude, the vortilons "poked past the stagnation point and triggered vortices" (ibid). The vortices extended over the upper wing surface and limited the span-wise flow, thereby preserving lift on the outboard wing sections, so the inner wing would stall first. In a swept wing design, this makes the nose pitch sharply down, enabling the crew to recover control quickly. The vortilons also reduced the downwash from the wing on the tail, which helped crews recover from potential deep stalls (ibid).
Vortilons on the HS 125-800: These were also relatively small (www.pprune.org). The objective was to replace wing fences used on previous models, which minimized span-wise flow and tip stall, and predominantly maintained aileron effectiveness. The standard wing fence 'fix' on the -800 required more vortex generators just in front of the aileron hinge line; the combination added drag. "The vortilon solution had less drag than the wing fence and required fewer vortex generators. Also, there were advantages at low speed, and possibly with high speed cruise performance" (ibid).
Vortilons on the Lear Jet: As stated earlier, Vortilons work with different parts of the wing to enhance stall behaviour. According to a Learjet Newsletter (August 2006), the inboard pair of vortilons on the Lear Jet 35 and 35A are placed halfway between the stall strip and the stall fence. The latter forces the inboard portion of the wing to stall first, while the outboard section continues generating lift, giving the pilot better control of the ailerons for a longer period. The inboard vortilons act as a second stall fence, creating a high energy vortex, with concomitant benefits. The outboard pair of vortilons are placed directly in front of the ailerons, which we know is a desirable factor. Pilot workload during an impending stall is minimized, permitting simple recovery.
Vortilons on the Boeings: As the basic Boeing 737 evolved with time, extra performance became necessary. The 737-200 NG's have three vortilons on the underside of the leading edge slats to restrict the spanwise flow of air, as shown in Figure 10. (www.b737.org.uk)
Figure 10. The Three Vortilons on the Boeing 737-200NG (www.b737.org.uk)
The Boeing 767-400ER also features three vortilons under the leading edge of the outboard
slats. "Results of stall testing were not satisfactory, in that stick forces became light near the stall, and uncommanded and undesirable roll at the stall would tilt the aircraft up to a 20-degree bank. Installation of the vortilons eliminated the problem." (Terry L. Lutz, 2001).
Vortilons on the Embraer 145: "The shape and position of vortilons is not yet an exact science
Figure 11. Vortilons (Yellow) on the Wings of an Alitalia Embraer ERJ145 (www.airlines.net)
and requires considerable flight-testing and knowledge to locate them optimally"(ibid). Continuous experimenting is required with various shapes, sizes and positions to arrive at a decision. According to de Resende, of Embraer, Brazil, who worked on integrating vortilons on the ERJ145, the aircraft faced very much the same problem as the Boeing 767-400ER in terms of stall characteristics. Their test pilots found one wing dropping as α reached 20 degrees. Vortilons solved this problem. Furthermore, the ERJ145 uses state-of-the-art lifting devices, yet it fell short of 'the maximum lift coefficient values to meet the short take-off and landing field lengths required for regional airline operations'(www.scielo.br). Market surveys provided design margins to allow the leading edge to be modified with a fixed 'droop' and the four vortilons on the lower surface leading edge of the outboard wing panel contributed significantly towards achieving their aim. "Their interaction with the wing sidewash at high angles of attack produce strong vortices that are convected to the upper surface, where they modify the pressure distribution and boundary layer development, postponing flow separation and increasing maximum lift. Their shape and position were defined using advanced 3D programming. The combined effect of the leading edge droop and vortilons allowed an improved take-off and landing performance without resorting to more complex variable geometry leading edge devices (such as slats), for a small cruise performance penalty." Figure 12 shows a schematic representation of the ERJ145 droop and vortilon, while Figure 13 is a close up view of vortilons. Note that the aircraft is flying from your left to the right.
Figure 12. Schematic Representation of the Vortilon on the ERJ145 (www.airlines.net)
Figure 13: Close up View of Vortilons (www.airlines.net)
