Straight and Level Flight Flashcards
22.18.2 Explain the four forces acting and the conditions required for steady straight and level flight.
In steady straight and level flight, forces must be in equilibrium. Lifting forces must counterbalance weight, and thrust must be equal and opposite to drag.
- 18.4 For conventional aeroplane configuration, describe
(a) the lift/weight and thrust/drag couples
Lift acts through the centre of pressure and weight acts through centre of gravity, centre of pressure changes with angle of attack and centre of gravity changes from flight to flight depending on loading.
Centre of pressure and centre of gravity rarely act through the same point. The usual design arrangement is to have centre of pressure behind centre or gravity. This creates the lift/weight couple which causes a nose-down pitching moment.
Thrust and drag also rarely act through the same point and create a thrust/drag couple which causes nose-up or nose-down pitching moment depending in the forces. Its usually that the thrust line acts below the drag line creating a nose-up pitching moment.
- 18.4 For conventional aeroplane configuration, describe
(b) pitching moments and the tailplane stabilising moment
Aircraft are designed so that the pitching moments of the lift/weight couple and the thrust/drag couple balance each other. In reality they are rarely in perfect balance so the tailplane functions to provide the extra balancing force.
- 18.4 For conventional aeroplane configuration, describe
(c) pitching moments caused by power changes, undercarriage and flap extension.
In level flight when there is a change in magnitude or line of action in any of the forces the strength of the respective couple change and cause a nose-up or nose-down pitching moment.
An increase in power, increases the thrust/drag couple and creates an nose-up pitch. So a decrease in power, the thrust/drag couple will weaken creating a nose-down pitch.
Flap extension changes lift and drag.. therefore changing the respective couples. Flap up will create a nose up pitch and flap down will create nose down pitch.
The same principles apply to undercarriage in the way it affect the couples. Undercarriage up will create a nose-up pitch and undercarriage down will create a nose-down pitch.
22.18.6 Explain the power and attitude relationships at various airspeeds in straight and level flight.
To accelerate in level flight thrust must be greater than drag, and to decelerate thrust must be less than drag. In level flight at constant speed, if the pilot increases power intending to increase speed without moving any other controls the nose will pitch up. And if power is reduced the nose will pitch down and the aircraft will descend. The pilot can control these pitching moments using elevator (i.e holds same attitude).
This can be summarised by the saying “Power plus attitude equals performance”
- 18.8 For unaccelerated level flight.
(a) state the power required formula.
Power required = drag X Tas
- 18.8 For unaccelerated level flight
(b) explain the difference between the drag curve and the power required curve.
Refer graph illustration fig 9-11.
- 18.8 For unaccelerated level flight
(c) distinguish between the minimum drag speed and minimum power speed
Refer graph illustration fig 9-11.
Note: The bottom of the drag curve does not coincide with the bottom of the power required curve.
Meaning that minimum power speed is lower than the minimum drag/ best lift-drag ratio speed.
- 18.10 Given typical power available and power required curves explain
(a) the maximum and minimum speeds for level flight
Refer graph illustration fig 9-14.
- 18.10 Given typical power available and power required curves explain
(b) the effects of increased weight and altitude
Refer graph illustration 9-15.
At any given airspeed if weight is increased, the angle of attack will have to be increased to maintain the same amount of lift to equal weight. Increased angle of attack results in an increase of drag and therefore power required to maintain level flight.
Increased weight moves the Pr curve upward and to the right which means power required to maintain straight and level flight is increased.
Refer graph illustration 9-16.
The effect of altitude is to bring the 2 curves closer together, meaning the maximum speed is reduced and the minimum speed is increased. Also less power available.
