Module 3

Aicraft Performance

Module 3

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

Weights / CG, engine performance, level flight

2

Weights / CG

 Introduction  Basic weights definitions  CG envelopes  Ground balance  Flight Balance  Trim drag  Aircraft loadability

Weights / CG, engine performance, level flight

3

Weights / CG - Introduction  Weights/CG considerations are very important in aeronautical engineering  Safety aspects • Aircraft controllability and stability is not guaranteed if CG is outside certified limits • Stall margin will be reduced if actual weight is greater than assumed weight

 Performance aspects • Significant change in performance when weight varies • Ex : Take-off distance is proportional to W2 • Aircraft empty weight has a great impact on payload / range capability and fuel burn for a specific mission • Drag increases as CG moves forward • Weight guarantees / remedies

Weights / CG, engine performance, level flight

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Weights / CG – Basic weight definitions  Many basic weight definitions must be introduced as they are directly related to aircraft performance  Manufacturer’s Weight Empty (MWE) • The MWE consists of the weight of the structure, power plant, systems and interior provisions as defined in the Type Specification. The MWE excludes the engine oil and unusable fuel

 Delivered Weight Empty (DWE) • The DWE consists of MWE, all fixed interior equipment (both standard and optional) and customer options.

 Operating Weight Empty (OWE) • The OWE consists of DWE plus operating items: -

Engine oil Unusable fuel Crew Crew equipment Passenger service items Other items required for normal operation

Weights / CG, engine performance, level flight

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Weights / CG – Basic weight definitions (Cont’d)  Zero Fuel Weight (ZFW) • OWE plus payload (passengers, baggage and cargo)

 Maximum Zero Fuel Weight (MZFW) • Maximum weight allowed before usable fuel is loaded on the aircraft (structural limit)

 Maximum Landing Weight (MLW) • Maximum aircraft weight for landing (structural limit)

 Maximum Take-Off Weight (MTOW) • The maximum weight allowed at the start of take-off roll (structural limit)

 Maximum Ramp Weight (MRW) • The maximum possible aircraft weight. Operation at MRW is permitted for ground handling and taxi only (structural limit)

Weights / CG, engine performance, level flight

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Weights / CG – CG envelopes  CG problems are not new !

Weights / CG, engine performance, level flight

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Weights / CG – CG envelopes (Cont’d)  But are more embarrassing in modern times!

Weights / CG, engine performance, level flight

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Weights / CG – CG envelopes (Cont’d) CG limits

Weights / CG, engine performance, level flight

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Weights / CG – CG envelopes (Cont’d)  Typical CG envelope 80000

75000

Aft limit

Fwd limit

Maximum Take-off Weight

70000 Aircraft Weight (lb)

Maximum Landing Weight 65000

Maximum Zero Fuel Weight 60000

55000

50000

Flight limit

Ground limit

Ground limit

45000

Flight limit

40000 -10

0

10

20

30

40

50

60

70

Xcg (% MAC)

Weights / CG, engine performance, level flight

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Weights / CG – CG envelopes (Cont’d)

 Why ground limits and flight limits • Optimize structure (gear loads) • Optimize take-off / landing performance • Allow in flight movement of passengers • Flight Attendant with cart

Weights / CG, engine performance, level flight

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Weights / CG – Ground Balance

W

NZ NLG

NZ MLG Weights / CG, engine performance, level flight

12

Weights / CG – Flight balance LW

LT

W

Weights / CG, engine performance, level flight

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Weights / CG – Trim drag Aft CG

LW

When flying at fwd CG :

LT

• Additional nose-down pitching moment due to CG being more forward

D

• LT must be increased to compensate nose-down pitching moment

W

• LW is increased so that total lift is equal to weight ( LW+LT = W ) • Greater LW results in greater α and greater induced drag

Fwd CG

LW

LT

• Drag increase due to shift in CG is referred to as trim drag • Flight at aft CG results in lower drag

D

W Weights / CG, engine performance, level flight

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Weights / CG – Loadability •Typical Wt build-up

Item Description

(lb)

(in)

40000

790.0

3000.0

790.0

43000

790.0

1400.0

580.0

44400

783.4

14000.0 1400.0

790.0 1000.0

59800

790.0

Addition of Fuel

15200.0

776.6

Ramp Weight

75000

787.3

M.W.E. Bare Interior

M.W.E. Delivered Addition of Operating Items

O.W.E.

% MAC 35.2 35.2 30.2

Addition of Payload Passengers Aft Baggage

Zero Fuel Weight

Weights / CG, engine performance, level flight

35.2 33.2

15

Weights / CG – Loadability (Cont’d) 80000

Fuel ctr

75000

70000

Aft Bag

Aircraft Weight (lb)

65000

Fuel mains

60000

Back to Front

55000 Aisle

50000 Front to Back

45000

Window PASSENGERS

40000 -10

0

10

20

30

40

50

60

Xcg (% MAC)

Weights / CG, engine performance, level flight

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70

Engine Performance

 Introduction  Turbofan engine  Basic thrust equations  Thrust ratings  Installation effects  Factors affecting thrust  Fuel consumption  Reverse thrust  Reduced thrust and derate

PW500

Weights / CG, engine performance, level flight

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Engine Performance - Introduction  Engine performance has a direct impact on aircraft performance • Take-off distance, rate of climb, climb ceiling and maximum cruise speed are function of the thrust available • Endurance and range are function of the fuel consumption

 Focus on turbofan engines – widespread use in commercial airplanes  Intent is to give background information for performance calculations • Many technical details are covered in Turbomachinery courses

Weights / CG, engine performance, level flight

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Engine Performance – Turbofan engine

 Dual rotor assembly – fan rotor (N1) and compressor rotor (N2)  Core air is compressed, ignited, exhausted through turbines and accelerated through exhaust nozzle  Bypass air passes through the fan and is ducted around the engine  Thrust is controlled via N1 or EPR (Engine Pressure Ratio)  BPR = Bypass Ratio = (dm/dt)bypass/(dm/dt)core

Weights / CG, engine performance, level flight

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Engine Performance – Basic thrust equations

Ve (dm/dt)e

Vo (dm/dt)o

 Net thrust = FN or T = (dm/dt)eVe + A (pe–po) – (dm/dt)oVo  Gross thrust = FG = (dm/dt)eVe + A (pe–po)  Ram drag = FR = (dm/dt)oVo  Pressure thrust = A (pe–po)

(normally negligible)

Weights / CG, engine performance, level flight

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Engine Performance – Thrust Ratings 

Thrust ratings can be achieved either by setting thrust levers is specific detents or by adjusting lever angle to obtain a specific N1 or EPR



Full Authority Digital Engine Control (FADEC) simplifies selection of thrust ratings for the pilot •



Thrust settings corresponding to the various ratings are programmed as a function of altitude, temperature and airspeed

Ratings are associated specific engine temperature limitations – Inter Turbine Temperature (ITT) or Exhaust Gas Temperature (EGT) limits are defined (red lines)

Weights / CG, engine performance, level flight

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Engine Performance Thrust

Flat rating

Constant ITT

Thrust Setting Parameter

Flat rating temp.

EPR N1 Outside air temperature

Weights / CG, engine performance, level flight

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Engine Performance – Thrust Ratings (cont’d) 

Take-off (TO) •

Maximum thrust available for take-off

•

May be different for All-Engines-Operating (AEO) or One-EngineInoperative (OEI) -



•

Time limited (5 minutes for AEO, 5 or 10 minutes for OEI)

•

Highest thrust level and highest ITT/EGT limit

Maximum Continuous (MCT) •



Normal Take-Off (NTO) for AEO Maximum Take-Off (MTO) or Automatic Power Reserve (APR) for OEI APR system OEI thrust ≤ 1.10 AEO thrust

Maximum thrust which may be used continuously and is intended only for emergency use (OEI) at the discretion of the pilot

Maximum Climb (MCL) •

Maximum rating approved for normal climb Weights / CG, engine performance, level flight

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Engine Performance – Thrust Ratings (Cont’d) 





Maximum Cruise (MCR) •

Maximum rating approved for cruising

•

MCR is normally slightly lower than MCL

Go-around (GA) •

Maximum rating approved for go-around

•

Normally equal to TO and may be different for AEO or OEI conditions

Maximum Reverse •



Not a rating but a thrust lever position associated with use of reverse thrust on the ground

Idle •

Not a rating but a thrust lever position suitable for minimum thrust operation on the ground or in flight

•

Ground idle versus approach idle

•

Impact on brake heating during taxi Weights / CG, engine performance, level flight

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Engine Performance – Installation effects 

Engine manufacturers provide engine performance models for uninstalled engines



Addition of an engine nacelle and attachment of the engine to the aircraft result in losses in engine performance (installation losses)



Engine performance models must be adjusted to take installation effects into account



Main installation effects include



•

Engine inlet efficiency (ram recovery)

•

Exhaust nozzle efficiency

•

Nacelle leakage

•

Scrubbing drag

•

Interference with the pylon

Installation effects may have a significant impact on engine performance

Weights / CG, engine performance, level flight

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Engine Performance – Factors affecting thrust 

Effect of altitude •



Effect of airspeed •





T reduces as altitude increases T reduces as airspeed increases

Effect of temperature •

Gas turbine engines are sensitive to to variation in air temperature

•

Thrust output can vary by +/- 20 % from the specified rating on a cold day or a hot day

Effect of humidity •

Thrust decreases slightly as humidity increases

•

FAR 25 defines humidity levels that must be assumed for thrust determination

Weights / CG, engine performance, level flight

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Engine Performance – Factors affecting thrust (Cont’d) 

Fan speed N1 • •



The pilot can set any thrust level between idle and rated thrust by varying N1 Relationship between FN and N1 is not linear

Engine bleed extraction •

Compressed air may be extracted from the engine core for use in other systems -

• • • •

Anti-icing, air conditioning, …

For a given fan speed N1, bleed extraction has essentially no effect on thrust but it causes an increase in ITT / EGT When operating at temperatures above the flat rating, N1 must be reduced in order not to exceed the ITT / EGT limit The reduction in N1 may have a significant impact on thrust Typical thrust reductions associated with bleed extraction : 0 – 2 % for air conditioning, 0 - 10 % for anti-icing

Weights / CG, engine performance, level flight

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Engine Performance (Cont’d) Thrust (lb)

Ze

En

ro

gi ne

en

gin

eb

bl ee

d

op

lee

d

en

Thrust Setting (N1-%)

Ze

ro

En g

ine

en

gin

ble

eb

ed

lee

d

op

en

Outside air temperature Weights / CG, engine performance, level flight

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Engine Performance – Fuel consumption 

Specific Fuel Consumption (SFC) •

SFC = FF / FN FF = Fuel flow (lb / hr) SFC units : (lb/hr)fuel / lbthrust

   

SFC levels of approximately 0.3 to 0.5 are obtained with modern turbofan engines at SL Turbojet engines have SFC levels of approximately 1.0 (up to 2.0 with after burner) SFC varies as a function of flight condition (altitude, Mach number, thrust level) (typically, SFC = 0.6 to 0.8 in cruise) SFC varies also as a function of time (SFC deterioration) • • • • •

Engine wear results in reduced efficiency SFC deterioration is partially recovered at engine overhaul Deterioration may reach important levels ( up to 6 % and more) Fuel loads must be adjusted accordingly SFC has a significant impact on aircraft operating costs Weights / CG, engine performance, level flight

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Engine Performance – Reverse thrust



Reverse thrust is used as a means to decelerate the aircraft during landing or during a rejected take-off



Reverse thrust can also sometimes be used to back the aircraft at the gate (reverse taxi)



Two types of thrust reversers •

Cascade-type : bypass air is reversed – relatively low reverse thrust levels (illustrated above)

•

Bucket-type : both bypass and core airflows are reversed – higher reverse thrust levels Weights / CG, engine performance, level flight

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Engine Performance – Reverse thrust (Cont’d) 

Utilization of reverse thrust may result in significant reductions of stopping distances when runway is wet or contaminated •





Reverse thrust is needed operationally (ref. to early EMB 145)

Use of high levels of reverse thrust at low speed may cause engine damage •

Hot air re-ingestion

•

Foreign Object Damage (FOD) due to debris on the runway

•

Limitations are normally applied to restrict the use of high reverse thrust levels at lower airspeeds (e.g. selection of reverse idle at speeds lower than 60 knots) – through FADEC or manually

Reverse thrust levels are difficult to predict •

Complex airflow patterns due to interaction with the ground and other aircraft parts

•

Theoretical reverse thrust levels are adjusted with a “reverse thrust efficiency factor” determined from flight tests in order to account for such effects Weights / CG, engine performance, level flight

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Engine Performance – Reverse thrust (Cont’d) 

Dynamic response of the thrust reverser system may have an important impact on aircraft stopping distance •



Time delays for reverse thrust selection, thrust reverser deployment and engine spool-up to maximum reverse

Controllability aspects •

Deterioration of directional control may happen when thrust reversers are used (rudder blanking effect with aft mounted engines) and particularly when crosswinds are present

•

Nose-up pitching moment when reverse thrust is selected on aircraft with thrust line above CG

•

Controllability with one engine inoperative and one engine at maximum reverse thrust on the ground at lower airspeeds -

Performance credit can only be taken for the amount of reverse thrust that is controllable

Weights / CG, engine performance, level flight

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Engine Performance – Reduced Thrust and Derates 

Use of take-off rated thrust levels has an impact on engine SFC deterioration with time – it reduces engine life



Two methods are used operationally to preserve engine life : reduced thrust take-off and derates



Reduced thrust take-off •

Sometimes referred to as “flex” take-off procedure or “assumed temperature method”

•

Can only be used when excess performance is available with rated take-off thrust

•

Pilot uses a lower thrust setting that allows to meet all take-off performance requirements (field length, obstacles, …)

•

Thrust reduction must not exceed 25 %

•

Pilot may, at his discretion, increase thrust at any time during takeoff (e.g. in case of engine failure)

•

Not allowed on contaminated runways

•

Used routinely by airlines Weights / CG, engine performance, level flight

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Engine Performance – Reduced Thrust and Derates 

Derates •

Set of distinct thrust ratings that can be selected by the pilot before take-off

•

A set of AFM performance charts is available for each derate level

•

The selected rating is a limitation and cannot be changed during take-off (minimum control speed considerations)

•

Pilot uses a derate that allows to meet all take-off performance requirements (field length, obstacles, …)

•

No limit on thrust reduction relative to rated take-off thrust

•

A reduced thrust procedure can be applied to each derate level

•

Not as much used as reduced thrust take-off procedure but it can provide somewhat greater savings in engine life

Weights / CG, engine performance, level flight

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Level Flight  Understand physics associated with level flight • Balance of forces • Maximum cruise speed • Minimum drag speed and speed stability

Weights / CG, engine performance, level flight

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Level flight – Balance of forces  In straight and level flight, the balance of horizontal and vertical forces can be defined as follows T=D L=W

(T is assumed to act along the speed vector)

Lift

Thrust

Drag

V

Weight

Weights / CG, engine performance, level flight

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Level flight – Drag Versus Speed

Total drag Parasite drag

Drag (lb)

Min. drag

Induced drag

Compr. Drag

Mach number

Weights / CG, engine performance, level flight

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Level flight – Thrust and Drag Versus Speed D, T and Excess thrust (T-D) versus Mach number Negative thrust response at constant TLA Drag

Force (1000 lb)

Thrust

Excess thrust

Mach number

Trim Mach number

Weights / CG, engine performance, level flight

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Level flight – Vmd and Speed Stability  Speed stability can be defined as the tendency of the aircraft, when stabilized in a given flight condition, to return to that speed if it is disturbed from that stable condition  Speed stability is not only a function of drag (back side of the drag curve), but it is also a function of thrust variation as a function of speed (unless thrust is constant as a function of speed)  Positive speed stability when excess thrust decreases with a speed increase  Negative speed stability when excess thrust increases with a speed increase  Aircraft are not generally operated in areas where speed stability is negative because of the increased pilot workload  Speed stability can be improved with an auto-throttle system • Can incorporate a logic that modifies the relationship between thrust and speed (relative to the relationship at fixed TLA)

Weights / CG, engine performance, level flight

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