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)

INITIATION OF EVACUATION

AIRCRAFT EVACUATION

TCAS TRAFFIC DISPLAY IS NOT RADAR DISPLAY

TCAS in aviation was mandated to facilitate greater situational awareness to the flight crew and serve as a last safety net to avoid mid-air collisions. Over the years, design Improvements in accuracy and response capability of TCAS systems are able to generate Traffic alerts and Resolution Advisories with manoeuvre guidance in case of RAs.

 Of late there have been several reported incidents of crew resorting to pre-emptive manoeuvres solely based on TCAS displays showing traffic proximity. Manoeuvres initiated to achieve self-separation or sequencing, solely on the information displayed on TCAS systems can often result in degraded safety margins and should never be attempted by the flight crew.

 "An A340 reduced its speed on its own, miles too early on approach, to increase the distance from the preceding aircraft [based on the TCAS traffic display]. It messed up the sequence and an A320 was then only 4 NM behind it was obliged to perform an "S" for delay".

A B737 is cleared to climb to 3000 ft. A VFR on an opposite track is level at 3500 ft, but offset horizontally. The controller provides traffic information to the B737. The pilot reports two targets on the TCAS traffic display and shortly after reports a left turn to avoid this traffic. Fortunately, the controller instructs the B737 to stop climb at 2500 ft, because the inappropriate turn reduced the separation.

Examples are many and continue to occur. Regulations on the use of TCAS traffic display at ICAO PANS-OPS Doc 8168, chapter 3, section 3.2, states that “Pilots shall not manoeuvre their aircraft in response to traffic advisories (TAs) only”. This point is also emphasised in ICAO ACAS II Training Guides for pilots. Confusing and nonstandard responses from pilots are also commonly encountered. Responses such as “TCAS Contact” or “We have the Traffic on TCAS”, on receiving traffic information from ATC, provide no added value. 

Flight crew need to appreciate that the TCAS traffic display is not designed to support self-separation manoeuvres, but to aid visual acquisition of an intruder. It gives only a snapshot of the relative horizontal and vertical position of other aircraft in the vicinity. The lack of speed vector and the possibility of rapidly changing relative bearings, it is extremely difficult to anticipate the evolution of the situation based solely on the TCAS traffic display.

Air traffic controllers, on the other hand, base their actions on the comprehensive information shown on the radar display, which enables them to provide a safe and expeditious air traffic flow. The radar display also provides velocity vectors and the controller is able to predictive information of crossing traffic. The TCAS traffic display does not provide the information necessary for the provision of self-separation and sequencing.

When operating IFR in a controlled airspace, ATC is responsible to provide safe separation between traffic. TCAS II is designed to trigger an RA command, should there be a potential or imminent risk of collision between two transponder equipped aircraft. Correct response to annunciated RAs will safely resolve such situations.

If an aircraft is close to its operational ceiling or where the indicated rate climb is excessive, it may simply be unable to follow the RA commands. In these cases, the pilot should continue to manoeuvre at the maximum rate possible consistent with safety and performance. If unable to climb, it may even maintain level flight( engine out conditions).The TCAS equipment in the target aircraft will be able to detect the reduced response and adjust the manoeuvre commands to achieve the desired separation.

Airline operators need to include an unambiguous policy in their Operations Manual procedures, prohibiting self-separation manoeuvres by the crew based solely on TCAS displays.