A380 PUSHING TECHNICAL BOUNDARIES

Airbus has always been at the forefront of pushing technical boundaries in aviation and excelling in aircraft design and technology. Airbus says efforts to lower the weight of the world's largest airliner lay behind recent A380 wing cracks and pledged to learn from mistakes that lay dormant for a decade, as repair costs looked set to climb towards 500 million euros ($A642 million).

DOWNWARD FACING WINGLETS

Aircraft designers have traditionally used winglets as a means to reduce induced drag and save on consequent fuel burn aka operating costs. Over the years several designs have emerged and the classic end plates and winglets are common in Boeing and airbus models. General aviation aircraft on the other hand have more innovative designs of winglets to enhance operational cost benefits.

While an equivalent increase in wingspan would be more effective than a winglet of the same length, the bending force becomes a greater factor.  Typically, a three-foot winglet has the same bending force as a one-foot increase in span, yet gives the same performance gain as a two-foot wing span increase.  For this reason, most designers have concentrated their efforts in winglets designs to reduce drag.

MITSUBISHI FLIGHT TEST

Mitsubishi Aircraft Corporation has completed the first flight test on Pratt & Whitney’s PurePower  PW1200G engine for the MRJ, just a week after it announced a delay to the programme

The PW1217G for the 90-seat MRJ90 flew on a specially designed stub wing aboard Pratt & Whitney’s Boeing 747SP flying test bed from Pratt &Whitney’s Mirabel Aerospace Centre in Canada.

Tests to validate the engine's fuel efficiency, durability and other performance metrics, will be conducted over roughly one year so they will be completed in time for the MRJ's maiden flight. In-flight testing will be done with the engine installed in an ultralarge aircraft.


The PW1217G engine uses geared turbofan technology, which the company says can reduce noise and fuel consumption by having the large fan rotate slower. This is expected to improve the MRJ's fuel efficiency by around 20% over rival jets in this class, which seat up to 100 passengers.

Mitsubishi Aircraft has announced a new schedule for the MRJ, delaying its launch by more than a year. Behind the pushed-back schedule were inadequate inspections of aircraft parts by parent Mitsubishi Heavy Industries Ltd. (7011).

The company will step up its sales campaign in such markets as the U.S. and Southeast Asia now that engine tests have begun.(adapted from Nikkei and Mitsubishi)

NEW GENERATION PURE POWER ENGINES: GEARED TURBOFAN

Today, the turbofan engine has found a home on practically all jet-propelled aircraft. However, the ambitious emission goals of ACARE 2020 cannot be fully met with the current turbofan concepts and industry majors need to look elsewhere to find a viable solution.

A high bypass engine is the key to reducing both fuel consumption and noise and developments are underfoot aimed to raise the bypass ratio above ten and optimize individual components for better aerodynamic efficiency.

BOEING WINGLET DESIGN FOR 737 MAX

B737 MAX

In continuation of our coverage on WINGLETS and FUEL SAVING A320 SHARKLETS, we bring the news about Boeing announcement and breakthrough on new winglet design concept for the 737 MAX. The new Advanced Technology winglet will provide MAX customers with up to an additional 1.5 percent fuel-burn improvement, depending on range, on top of the 10-12 percent improvement already offered on the new-engine variant.

FLIGHT AT EXTREMELY LOW SPEEDS

S tall speed is defined as the lowest airspeed at which 1 'G' level flight can be achieved. However it is also possible to fly the airplane at speeds below the defined stall speed. This regime is outside the certified flight envelope. There are several important factors that a pilot must know when the airplane is at extremely low speeds.

NEXT GENERATION ENGINE

PW1000G- Pure Power

PurePower PW1000G engine with Geared Turbofan™ technology is a state of the art gear system that separates the engine fan from the low pressure compressor and turbine, allowing each of the modules to operate at their optimum speeds. This enables the fan to rotate slower and while the low pressure compressor and turbine operate at a high speed, increasing engine efficiency and delivering significantly lower fuel consumption, emissions and noise. This increased efficiency also translates to fewer engine stages and parts for lower weight and reduced maintenance costs.

The PurePower PW1000G engine’s fan-drive gear system is just one component of this next-generation engine. The PurePower PW1000G engine also incorporates advances in aerodynamics, lightweight materials and other major technology improvements in the high-pressure spool, low-pressure turbine, combustor, controls, engine health monitoring and more.

KUCHEMANN'S CARROTS

A

s we do the pre-flight walk around , and inspect  the wing underside we rarely give a second glance at the canoe shaped flap track fairings under the wings. Some are slender but many appear somewhat oversized to accommodate just  the flap fairings. Or do they serve some other function?

The physics of airflow alteres violently as it expands from subsonic to supersonic speeds. As the aircraft passes through the transonic speed range, local airflow approaches sonic speeds over the wing and body of the aircraft and leads to the formation of shock waves and consequent large increase in drag.

At transonic speeds, it was found, that the time-honoured principle that the drag of the individual elements of an airplane could be added in a linear manner to give the approximate drag of the entire configuration could no longer be relied upon.

Researches by Dietrich Kuchemann in the UK and Richard Whitcomb of NASA , in early 1950s, established that this wave drag can be minimised by a fuselage wing configuration synthesis, where the cross sectional area changed smoothly along the length of the aircraft. Known as the Area Rule, its basic tenet postulates that the wave drag of a simple equivalent body of revolution would be the same as a more complex wing body arrangements.

Initial  application of area rule designs can be seen in the   “Coke Bottle” or “Marylin Monroe” indented fuselage body shapes to reduce the effect of the presence of wings as in F-102 and F-106 aircraft. This, however had practical limitations and alternate efforts to address the local discrepancies in cross sectional areas led to the concept of attaching conical and pod shaped bodies along the wing , nacelle and fuselage. First successful application of this principle to combat wave drag effects was in Convair- 990. Following applications of the local area rule, several pylon, nacelle, and wing fairings were embodied, to smooth out the area distribution and facilitated in raising the cruise speed from 0.8M for the basic aircraft to 0.89 M for the modified airframe.

 These anti shock bodies  christened as ‘Whitcomb After-bodies’ or ‘Kuchemann’s Carrots’ are widespread in today’s designs.

Anti shock bodies were also apparently developed by the Soviet Designers during the same time , as seen in their installations in TU-16 and through subsequent designs such as the TU 154.

On most modern designs, the mechanism for deploying the wing flaps are encased in canoe shaped pods , which serve as anti-shock bodies and can be seen in A300/310, A 380 and Boeing 757 to name a few. Known to most as flap track fairings, garnering little attention, these pods nevertheless have an important role in transonic drag reduction and fuel economy.

WINGLETS

It’s a nearly vertical airfoil at an airplanes wingtip that reduces drag by inhibiting turbulence.

( Merriam-Webster dictionary)

First known use of winglet dates back to as early as 1611.

EVOLUTION OF WINGLET

NASA’s pioneering research in the 1970’s as part of energy efficiency program to conserve energy in aviation resulted in Winglets finding acceptance with airplane manufacturers and airlines alike.

Richard Whitcomb was instrumental in conducting test to explore hypothesis that a precisely designed vertical wingtip device could weaken wing tip vortices and thus diminish induced drag which translates into less fuel burn and better cruise efficiency.(NASA website)

American, Southwest, Ryanair, and others took advantage of fuel efficiency that comes with winglets and partnered with Boeing –Aviation Partners group(ABP) to have winglets installed.

Wing Tip fence

Wingtip fence is the preferred device of Airbus to tackle and reduce induced drag on wingtip.Airbus also has ambitious project in introducing Sharklets, akin winglets on its A320 neo and also an active proposal for the same to be introduced on A330.

AERODYNAMICS OF WINGLET

Winglets

Lift is the force that makes the aircraft fly. Lift is a result of unequal pressure in a wing as air flows around it with positive pressure underneath the wing and negative pressure above.

Drag is the resistance encountered while moving through the airflow. Considerable amount of drag is also generated from the high pressure under the wing, which causes air to flow up over the wing tip and spin off in a vortex.. These vortices produce what is called induced drag which hampers aircraft fuel consumption, range, speed and so on.

Sharklets

 Hence, the primary aim of winglet is to break the wing tip vortices and reduce the induced drag and increase aircraft performance. Fuel savings are estimated between 4-6% by employing winglets.  Initial results as released by airbus for A330 program indicate fuel saving in excess of 3.4% and increased take off weight. Also, the noise footprint will be reduced along with better carbon footprint in light of emissions being the centre stage of aviation policies.
(Acknowledgements: Airbus, NASA and Merriam-Webster)

BUFFET BOUNDARIES- COFFIN'S CORNER

Stall speed is defined as the minimum airspeed required to maintain 1g level flight. Any further reduction in speed will result in the lift produced by the wings to be less than the weight of the aircraft and leads to a loss of altitude. The increase in angle of attack will in turn cause flow separation from the upper surface of the wing . In a swept back high speed aerofoil, this flow separation and associated pitch down will not be a marked phenomenon. Instead the aircraft enters into a descent. The descent rate further tilts the relative airflow downwards and leads to an increase in angle of attack further driving the aircraft into the stall regime. Any attempt by the pilot to raise the attitude by aft pressure on the elevator will cause a further increase in angle of attack and further loss of altitude.

The lift, however, depends on both air density (kg/m³) and on the plane’s velocity, and air density decreases with altitude. So, the higher you go, the faster you have to fly to stay above the stall speed. As you go higher, temperature also decreases, at least in the troposphere were commercial planes are flying. As the temperature decreases, so does the speed of sound.

Similarly, the critical Mach number is the maximum speed at which the airflow can sustain over the wings without losing lift due to flow separation and shock waves,. Any increase in speed in will cause the airplane to encounter stall effects. When the critical Mach number is exceeded, there is an abrupt rise in drag rise as well as a pitch down due Mach tuck. This can result in aircraft upset, altitude loss and loss of control. As the aircraft descends, the airspeed increases. Excessive pull forces during recovery may lead to further loss of control or structural damage to the airplane.

Modern commercial jet aircraft may suffer both high and low speed stall buffet. The associated boundaries are depicted in the FCOM of the aircraft.  The high speed buffet is caused by flow separation from the wings as occurs behind a shockwave at high altitudes and/or Mach numbers. The low speed buffet is caused by the same airflow separation as the aircraft approaches the stall angle of attack. With stall speed increasing with altitude and sound speed decreasing, the velocity window in which an aircraft can operate becomes narrower and narrower.

Turning manoeuvres at these altitudes increase the angle of attack and results in stability deterioration with  a decrease in control effectiveness. The relationship of stall speeds to critical     Mach number (Mcrit) narrows to a point where sudden increase in angle of attack , roll rates and disturbances cause the limits of the airspeed to be exceeded.

The Coffin corner or the Q corner is the altitude at or near which a high speed fixed wing aircraft’s stall speed is equal to the critical Mach number.  Coffin corner exists in the upper portion of the manoeuvring envelope of an aircraft, for a given gross weight and G – Force.

VMO is an aircraft’s indicated airspeed limit. Exceeding the Vmo may cause aerodynamic flutter and G load limitations to become critical during recovery. Structural design  integrity is also not predictable at airspeeds greater than Vmo.

A deeper understanding of the stall characteristics and recovery procedures are important proficiency issues.  When flying at high altitudes, the crew needs to be aware of the margins of safety available, especially when manoeuvring and while riding out turbulence.

To recover from a stall, the attitude needs to be decreased to reduce the angle of attack. The old maxim of Power for ROD or altitude control and Attitude for airspeed control holds good. A burst of power is not the solution for a stall recovery. In all cases, remember  “attitude before power” when you are in a stall.

WING LOADS

In the aftermath of wing root cracks appearing on A380, lets refresh ourselves on how the wing loading and wing root cracks surface.


Lift produced by a wing is not linear across the wing surface.As a matter of fact,lift is produced by the wing as a result of pressure differential between the top and bottom side of the wing.The pressure differential gives birth to wing shear force and a bending moment, which is the highest where the wing meets the fuselage.

However, aircraft like A330,whose engines are wing mounted, their weight is near the area where maximum lift is being produced.This reduces the total weight, thereby reducing the shear force and the bending moment at the wing root.

Wing loads are also subject to the fuel distribution in the wing.Aiming to achieve lesser moment at the wing root is the objective in effectively managing the wing loading.

OVERWEIGHT LANDING

Definition
All aircraft are certified to land at a particular maximum landing weight. Any landing carried out in excess of that weight is termed as an overweight landing.


Design
Landing at or below overweight landing ensures that normal performance margins as per certification are ensured.
Regulatory certification criteria require that landing gear design be based on 

• A sink rate of 10ft/sec at the maximum landing weight, and 
• A sink rate of 6ft/sec   at the maximum takeoff weight.

Commercial airliners normally make a sink rate of 2-3 ft/sec. A so called hard landing barely exceeds 6ft/sec.

When would it be required to carry out overweight landing?

• In case of any uncontrollable fire, damage, malfunction, etc
• In case of crew incapacitation, medical cases on board requiring immediate attention, etc.
• Any other situation where crew perceive an immediate landing is required.

airbus.com

Is it safe to carry out overweight landing?

Enough debates have been done on this subject. Airline crew are trained to handle overweight landing and the performance criteria and design aspects have been catered to allow for such an event should an emergency arise. Overweight landing provision is limited only to non-normal operation and crew shall not land overweight  on a normal flight  due to direct routings and strong tailwinds.

Aircraft are designed with adequate strength margins for overweight landings.Performance margins are generally well above maximum landing weight. Brakes are designed to withstand reject takeoff at the maximum takeoff weight. So oversight landing should not be a problem.

Is there a special procedure for maintenance after landing overweight?

An overweight landing entails a maintenance procedure even if the landing was smooth!!!!!

Inspections aim at checking for structural distress.

Is there a procedure for crew to follow?

Crew have a procedure to follow which aims at sensitizing crew on the sink rate for touchdown and the technique thereof.

Summary

Overweight landing provision is allowed for by the manufacturer in case of exigencies and the procedure is outlined.Design and performance margins allow for overweight landing.Overweight landing is prohibited in a normal operation. Procedures are outlined for the crew and the maintenance teams  to follow in the event of an overweight landing.