Straight And Level

Question 1

Explain the four forces acting and the conditions required for steady straight and level flight.

Answer 1

Four forces are lifting force which point up and must counterbalance the weight which points down. Thrust which is pointed forward which must be equal and opposite to drag which opposes thrust and points rearwards.

All must be in equilibrium for straight and level.

Question 2

For a conventional aeroplane configuration, describe:

(a) the lift/weight and thrust/drag couples;

Answer 2

Lift/weight produce a nose-down pitching moment and thrust/drag produce a nose-up pitching moment

Question 3

For a conventional aeroplane configuration, describe: (b) pitching moments and the tailplane stabilising moment

Answer 3

Weight acts through the CoG whereas lift acts through the CoP and the CoP changes with AoA.

Tailplane stabilising moment provide the necessary balancing force for straight and level.

Question 4

For a conventional aeroplane configuration, describe: (c) pitching moments caused by power changes

Answer 4

Increase in power and thrust increases the strength of the T/D couple resulting in a nose-up pitch.

Reduction in power and thrust weakens the T/D couple and results in a nose-down pitch.

Question 5

For a conventional aeroplane configuration, describe: (c) pitching moments caused by flap extension

Answer 5

lift increases and the CP moves rearward, resulting in an increased nose-down pitching moment.

drag increases and the line through which the total drag acts is usually lowered with a low-wing aircraft resulting in a nose-down pitching moment.

depending on design, sometimes the downwash is reduced, a stronger nose-down pitching moment generally results and if the downwash is increased, the nose will generally tend to pitch up.

Question 6

For a conventional aeroplane configuration, describe pitching moments caused by undercarriage extension

Answer 6

Raising undercarriage usually cause a nose-up pitch while lowering will result in a nose down.

Question 7

Explain the power and attitude relationships at various airspeeds in straight and level flight

Answer 7

Level flight, high speed is associated with a low nose attitude and a low speed with a high nose attitude.

To remain in level flight when the speed is changing, the use of elevator to control the nose attitude must be properly coordinated with changes in power and speed.

Question 8

For unaccelerated level flight:

(a) state the power required formula);

Answer 8

This is the power available from the propellor.

Power required = drag x TAS

Question 9

For unaccelerated level flight:

distinguish between the minimum drag speed and the minimum power speed.

Answer 9

Minimum drag speed is speed with least drag and minimum power is speed with least power. However, minimum power sped is lower than the minimum drag speed

Minimum drag = best L/d ratio speed.

Question 10

Given typical power available and power required curves, explain:

(b) the effects of increased weight.

Answer 10

Any given airspeed, If weight is increased, AoA will also increase if lift is to remain equal to weight so drag increases. Resulting in higher PR. (PR = TAS x drag)

PR curve therefore moves upward and to the right which means more PR to maintain level flight. Maximum flight speed is reduced and stall speed is increased.

Conversely the reverse for less weight