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2nd Year Aircraft Design Takeoff performance

Second Year Aircraft Design 2007-2008Take off performance

Aim

To understand the influence of take off performance on aircraft design

A320 view from cockpit during takeoff

http://www.airliners.net/open.file?id=1298852&size=L&width=1024&height=695&sok=&photo_nr=27

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2nd Year Aircraft Design Takeoff performance

Takeoff is the most dangerous part of flying Take off of a civil transport aircraft constitutes the most common

and widespread transfer of lethal kinetic energy to human

beings as part of a public transport system

This kinetic energy is equivalent to the energy at impact fromdropping the passengers (and aircraft) vertically from around

300m

It should also be remembered that at takeoff over half the weight

of a typical transport aircraft is fuel

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2nd Year Aircraft Design Takeoff performance

Objectives Define reference runway lengths and aircraft takeoff distances

used for takeoff performance analysis

Define reference speeds during aircraft takeoff and comment on

how these speeds affect the design of the aircraft Describe the options for coping with engine failure during takeoff

and the concept of a balanced field length

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2nd Year Aircraft Design Takeoff performance

Runway reference lengths

Runwaypaved surface used for normal operation

Stopwayflat area at end of runway clear of obstructions

Clearwayarea of obstacles above 35ft (11m)

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2nd Year Aircraft Design Takeoff performance

Manchester airport. No 1 runway length 3048m

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2nd Year Aircraft Design Takeoff performance

Aircraft takeoff performance The takeoff and climb to 1500ft is the most safety critical part of

a civil transport flight for a number of reasons:

The aircraft is close to the groundthis means that height can be

exchanged for air speed

The aircraft has maximum fuel on board and is at maximum weight, and

The engines are typically operating at maximum thrust

A detailed definition of the airfield limits for the aircraft will be

held as part of the Aircraft Flight Manual used by pilots

In the project design phase the takeoff field requirement is

regarded as a key part of the aircraft specification and may

dictate the engine sizing (though more likely now days engines

sized for top of climb performance)

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7/142nd Year Aircraft Design Takeoff performance

Normal takeoff The aircraft, starting from rest, accelerates along the runway at

a low angle of attack (nose wheel on the ground) and passes a

speed Vs equal to the aircraft stall speed in the takeoff

configuration and weight At a speed VR the pilot rotates the aircraft nose up

The aircraft will continue to accelerate along the runway until the

lift off speed Vlof is reached

The aircraft now climbs and continues to accelerate such that at

the screen height of 35ft a speed of V2 is reached V2 is a critical design speed that is dictated by various

airworthiness criteria to ensure adequate safety margins during

takeoff

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8/142nd Year Aircraft Design Takeoff performance

Takeoff speed definitions

V2 >1.1 Vmc , V2 >1.2 Vs

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9/142nd Year Aircraft Design Takeoff performance

Influence of take off reference speeds onaircraft design (1) Stall speed during takeoff dictates required CLmax for take off

Note that climb angle depends on L/D and T/W and that the climb angle

immediately after takeoff with a failed engine is typically a major design

driver. This typically means that CLmax for takeoff is lower than for landing in

order to maximise L/D and hence minimise T/W requirements

The ailerons and rudder are typically sized based on the controlrequirements at minimum control speed

Lower speed means larger control deflections to achieve same control

moment. Control deflections typically limited to +/- 15o . If this maxes out

than control chord or span must be increased

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10/142nd Year Aircraft Design Takeoff performance

Influence of take off reference speeds onaircraft design (2) The decision speed V1 depends on the engine out performance

of the aircraft and the braking performance.

Increased achievable effective coefficient of friction through use of

advanced braking systems enables V1 to be increased towards V2 and

hence reduces the speed no mans land following an engine failure

The minimum unstick speed depends on how much of CLmaxtakeoff can be obtained with the wheels on the ground

This is depends on the angle of attack achievable with the wheels on the

ground, which depends critically on the length and geometry of the aft

fuselage

For some stretch aircraft, e.g. A321, an increase in CLmax was required

because the longer aft fuselage reduced the max achievable alpha duringrotation

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11/142nd Year Aircraft Design Takeoff performance

Engine failure during takeoff

A "balanced field" with respect to aircraft takeoff performance refers to the minimum length

of runway that will allow for an aircraft to accelerate to V-1 (decision speed), experience

failure of the critical engine, and then either stop in the remaining runway or continue to a

successful takeoff meeting all applicable takeoff performance criteria. (wiki answers)

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12/142nd Year Aircraft Design Takeoff performance

Takeoff options following engine failure

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13/142nd Year Aircraft Design Takeoff performance

Calculation of balanced field length

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14/142nd Year Aircraft Design Takeoff performance

Graphical determination of balanced field length