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Load factor

By Oscar Elliott,2014-05-06 12:38
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Load factor

Turning Flight

    Turning flight is an unsteady manoeuvre where the aircraft is accelerating in a circular flight path. A pilot must use the ailerons, rudder, and elevators to complete a coordinated turn.

10.1 Turn Forces

    LL

    22ffWWVV××ggrr

    WW Figure 10.1: Forces on an aircraft in turning flight

    Consider an aircraft in turning flight and banked towards the turn as shown in Fig. 10.1. The only forces on the aircraft are lift, weight and centripetal force towards the centre of the turn.

Resolving forces horizontally, then

    2WV (10.1) L×sinfgr

    where: r = turn radius

Resolving forces vertically, then

     (10.2) L×cosfW0

Hence, the lift required in the turn is given by,

    W (10.3) Ln×WLcos f

From eq. 10.3, load factor can be expressed as,

    L1n (10.4) LWcosf

    This shows that the load factor in a turn is greater than the lift in steady level flight. Fig. 10.2 gives load factor values for different angles of bank. Note that at a 60º angle of bank, the load ( L.Smrcek 2004 - 10-1 -

    factor is 2 and begins to increase rapidly after this. A pilot needs to be careful flying at such steep angles of bank since not only are the g-forces much higher, but the aircraft could also stall which might result in a spin. This is especially critical during slow airspeeds such as in a landing approach when the aircraft is close to the ground.

     35

     30

    Angle of Bank Load Factor 25)Φ L nL(º) 200 1.00

    10 1.02 15

    20 1.06 Load Factor (n1030 1.16

    60 2.00 580 5.76

     0 020406080100 oAngle of Bank ()

    Figure 10.2: Load factor and bank angle

Dividing eq. 10.1 by eq. 10.3 then

    2V (10.5) tan fg×r

    Hence, the radius of the turn is given by,

    2V (10.6) rg×tanf

The time taken for the aircraft to complete φ radians is given by,

    V (10.7) t×g×tanf

    Isolated forces acting on an aircraft in different flight regimes and different manoeuvres cannot be a simple measure of aircraft performance assessment. The forces required to change the speed in flight also depend on aircraft weight. Heavier aircraft need a larger force to achieve the same change in speed as a lighter aircraft, and vice versa. For this reason, load factor is a performance assessment parameter used as a measure of aircraft manoeuvrability.

    Its definition is the ratio of the resultant aerodynamic force with the weight of the aircraft.

     ;;

    RRTR;;;rmnnor;;; (10.8)

    WWW

    When analysing flight performance, the load factor can be split into components similar to forces. For example, in the aerodynamic axes, the components are:

     n = drag load factor (component in drag direction) D

    ( L.Smrcek 2004 - 10-2 -

= lift load factor (component in lift direction) nL

n = side load factor (component in side force direction) Y

( L.Smrcek 2004 - 10-3 -

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