Module 12

Aircraft Performance

Module 12

Where are we? 1 : Introduction to aircraft performance, atmosphere 2 : Aerodynamics, air data measurements 3 : Weights / CG, engine performance, level flight 4 : Turning flight, flight envelope 5 : Climb and descent performance 6 : Cruise and endurance 7 : Payload-range, cost index 8 : Take-off performance 9 : Take-off performance 10 : Enroute and landing performance 11 : Wet and contaminated runways 12 : Impact of performance requirements on aircraft design

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Agenda – Module 12  Introduction  Marketing requirements  High speed performance • Engine performance • Aerodynamics and wing loading • Weight and CG

 Low speed performance • Engine performance • Aerodynamics and wing loading • Braking • Ground lift dumpers

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Introduction  Aircraft design is strongly influenced by the performance objectives • High speed requirements • Low speed requirements

 High speed and low speed performance requirements are affected by many parameters and are not necessarily compatible • As part of the aircraft design process, trade-off studies must be carried out to find an acceptable compromise between high speed and low speed requirements

 Some important factors affecting high speed and low speed performance are reviewed in the following slides

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Marketing requirements  Marketing Requirements and Objectives (MR&O) form the basis for aircraft design  Typical MR&O that are performance-related: • Initial cruise altitude – T.O. at MTOW • Maximum cruise speed

High speed requirements

• Maximum range with design payload • Take-off field length at MTOW • Landing field length at MLW

Low speed requirements

• V REF at MLW

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High speed – Engine performance  MCL and MCR ratings must be selected so as to meet mission requirements • MCL rating must allow the aircraft to reach the target initial cruise altitude (ideally up to at least ISA+10) • MCR rating must allow the aircraft to reach the target maximum cruise speed (ideally up to at least ISA+10)

 SFC must be as low as possible • High SFC will lead to an increase of the fuel quantity required , and therefore MTOW, for the mission Highly undesirable

 Good engine performance is the basis for good aircraft performance Impact of performance requirements on aircraft design

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High speed – Aerodynamics and wing loading  High speed performance can be optimized with proper aerodynamic design  For high cruise speeds: • Swept wing • High aspect ratio • Good high speed airfoil section (high critical Mach number) • Wing must not be over-sized (need a relatively high wing loading W/S reduced skin friction)

 For best fuel mileage: • High value of ML/D at the design cruise condition

 For a suitable margin to buffet onset: • Good high speed airfoil section (high critical Mach number) • Wing loading W/S must not be too high (operation at higher CL reduces margin to buffet onset) Impact of performance requirements on aircraft design

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High speed – Weight and CG  A low OWE is necessary in order to minimize fuel burn and to maximize range • Ex. : For a regional jet aircraft, an OWE increase of 1000 lb will - Increase fuel burn by 1 % for a given mission - Reduce maximum range by about 60 nm

 Considerations for low OWE • Composite materials, fly-by-wire and careful selection of aircraft interior

 MTOW must be selected such that target range can be achieved with the design payload condition • The MTOW/OWE ratio will increase in proportion with the target range • An increase in MTOW may require an increase in OWE (snowball effect)

 Flight at aft CG will reduce trim drag and maximize range • Tail fuel tank may be used to obtain control on CG position during flight

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Low speed – Engine performance  Take-off rating must provide acceptable • take-off field lengths • take-off WAT limits

 Adverse effects of high idle thrusts • High taxi speeds (brake heating) • Longer accelerate-stop and landing distances • Lower descent angle

 Adverse effect of low idle thrusts • Higher spool-up time to maximum reverse or GA rating

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Low speed – Engine performance (Cont’d)  For optimum performance on wet and contaminated runways Reverse thrust levels must be high (but controllable) Time delays for actuation of the thrust reverser system must be small

 Impact of engine bleed on thrust must be acceptable • Significant thrust reduction may result when using anti-ice

 Derates and / or reduced thrust procedure are necessary in order to preserve engine life

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Low speed – Aerodynamics and wing loading  Aerodynamic requirements for optimum low speed performance • Low operating speeds • High L/D in the take-off configuration • Low L/D in the landing configuration

 Low operating speeds • The key to short take-off and landing distances TOFL is proportional to V2 2 LFL is proportional to VREF 2 Impact of performance requirements on aircraft design

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Low speed – Aerodynamics and wing loading (Cont’d)  How can we design the aircraft to obtain low operating speeds ? • Need high CLMAX and low wing loading W/S

- Efficient flaps and slats - Good stall characteristics - Stall speed not limited by elevator power

• V2 and VREF should be based on minimum stall speed ratio as defined in FAR / JAR 25 - Operating speeds are limited by VSR rather than by VMCA, VMCG , VMU or by aircraft handling considerations

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Low speed – Aerodynamics and wing loading (Cont’d)  How can we design the aircraft to obtain low operating speeds ? • VMC considerations must not limit take-off and landing speeds - Limitation by VMC considerations may be acceptable at light weights - Directional and lateral flight controls must have sufficient power

VMCA Frudder DWM

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Low speed – Aerodynamics and wing loading (Cont’d)  How can we design the aircraft to obtain low operating speeds ? • VMU considerations must not limit take-off speeds - Adequate tail clearance is required, e.g. tail contact at a pitch attitude of not less than 15 degrees with landing gear fully extended - Need sufficient elevator power to ensure that high pitch attitudes can be achieved at low speeds

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Low speed – Aerodynamics and wing loading (Cont’d)  Other considerations • Aircraft handling must be acceptable at minimum take-off and landing speeds - Speed stability during landing approach (i.e. need high CDO ) Aircraft must be controllable during take-off and landing in crosswinds of 20-30 knots

• High values of VREF may result in operational restrictions - Approach categories are defined based on VREF at MLW - For approach category C , VREF must be less than 141 KCAS at MLW - If VREF is not less than 141 KCAS at MLW, the aircraft will not be allowed to operate at some airports

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Low speed – Aerodynamics and wing loading (Cont’d)  Lift-to-drag ratio • L/D should be as high as possible in the take-off configuration - Results in higher climb gradients and higher WAT limits - Lower certificated noise level - For aircraft equipped with slats, Improved climb procedure may be used to improve L/D at the expense of longer take-off distances - Multiple take-off slat / flap settings provide operational flexibility

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Low speed – Aerodynamics and wing loading (Cont’d)  Lift-to-drag ratio • L/D should be as high as possible in the take-off configuration - Results in higher climb gradients and higher WAT limits - Lower certificated noise level - For aircraft equipped with slats, Improved climb procedure may be used to improve L/D at the expense of longer take-off distances - Multiple take-off slat / flap settings provide operational flexibility

• L/D in the landing configuration -

High CLMAX required for landing generally results in relatively low L/D Low L/D results in better speed stability High L/D results in lower certificated noise levels High L/D results in better go-around and landing climb performance Multiple landing configurations provide operational flexibility Impact of performance requirements on aircraft design

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Low speed – Braking  Braking performance plays a major role in field performance • Significant impact on landing distance • Somewhat smaller impact on BFL as a result of the balancing process

 Brakes are designed to meet specific requirements • Landing and RTO brake energy levels • Brake torque required for maximum braking force on a dry runway • Turnaround time - function of brake temperatures following a stop and brake cooling characteristics - Cooling fans are sometimes included in brake systems

• Carbon brakes versus steel brakes Impact of performance requirements on aircraft design

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Low speed – Braking (Cont’d)  Some important characteristics of the braking system • Time to reach optimum braking pressure • Anti-skid efficiency (dry or wet runways)

 Anti-skid parameters are tuned during flight tests in order to optimize braking performance

μ

Time - sec

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Low speed – Ground lift dumpers  Ground lift dumpers

• Ground lift dumpers provide an increase in weight on wheels - Significant increase in braking force - Significant increase in drag

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Conclusion  I hope that you have liked this course  Your comments / suggestions are welcome • Please send comments / suggestions in an email

 Thank you for your attention!

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