Straight & Level

Question 1

4 main forces acting on an a/c in flight

Answer 1

• Lift
• Weight
• Thrust
• Drag

Question 2

Forces acting on an a/c that is maintaining a constant altitude, IAS and heading

Answer 2

The forces acting on the a/c must be in equilibrium, meaning the lifting force must counterbalance weight and the thrust must be equal and opposite to the drag

Question 3

What causes pitching moments

Answer 3

In flight the 4 main forces don’t act through the same point, this results in 2 couples:
- L/W couple
- T/D couple

Question 4

Tailplane stabilising moment

Answer 4

a/c are designed so that the pitching moments of the L/W couple and T/D couples oppose each other, however they are rarely in balance and it is the function of the tailplane to provide the necessary stabilising force known as the tailplane stabilising moment

Question 5

L/W couple

Answer 5

Weight acts through the CG (which changes depending on the a/c loading + fuel burn), and lift acts through the CP (which changes with AoA). because of these changes CP and CG rarely act through the same line. The usual design is to have the CP behind the CG and as a result the lift / weight forces set up a couple that causes a nose down pitching moment

Question 6

T/D couple

Answer 6

The thrust and drag forces rarely act through the same line and form a couple that can cause a nose up or down pitching moment depending on the arrangement of these forces. Usually, the thrust line acts below the line of drag and a nose up pitching moment results

Question 7

Pitching moment caused by power changes

Answer 7

Power increase = nose up
Power decrease = nose down

Question 8

Pitching moment caused by flap changes

Answer 8

Flap lowered = nose down
Flap up = nose up

(exception being with high wing training a/c, as when the flaps go up it may cause the nose to go down and vice versa)

Question 9

Pitching moment caused by retractable undercarriage

Answer 9

Undercarriage up = nose up
Undercarriage down = nose down

Question 10

How do you decrease or increase speed whilst remaining in level flight

Answer 10

Controlling both power and nose attitude

Question 11

Flying at a faster airspeed whilst remaining level

Answer 11

Nose must be prevented from pitching up at the same time power is applied, whilst the a/c is accelerating the AoA must be progressively decreased (nose attitude lowered) to keep lift equal to weight. As airspeed is increased drag will also increase and eventually build up to be equal to thrust, the forces will be in a new state of equilibrium

Question 12

Reducing airspeed whilst remaining level

Answer 12

As power is decreased the nose attitude must be progressively raised

Question 13

Nose attitude at a higher airspeed for level flight

Answer 13

It will be lower

Question 14

Flying level at speeds well below the min drag speed

Answer 14

Drag begins to increase again as speed is reduced (a large reduction in power may result in the a/c decelerating into this region). So to fly level the power will have to be increased again to maintain speed, the a/c will otherwise continue to decelerate until the stalling AoA is reached

Question 15

For unaccelerated level flight PR =

Answer 15

Drag x TAS

Question 16

How do you determine the power required to move an a/c through the air at a constant speed

Answer 16

By multiplying the thrust required (which is the same as drag) by TAS

Question 17

How are PR curves obtained

Answer 17

By multiplying drag x TAS and plotting against TAS

Question 18

Points to note when comparing a PR curve with a drag curve

Answer 18

• although drag and therefore thrust is the same at 2 points on the drag curve, on the PR curve the power required at those different airspeeds is different (higher power is required at a higher TAS) this shows that power and thrust are not the same thing
• the bottom of the drag curve (min drag) doesnt coincide with the bottom of the PR curve (min power), hence the min power speed is lower than the min drag / best L/D ratio speed

Question 19

Where is the speed for min drag / best L/D ratio on a PR curve

Answer 19

Occurs where a line drawn from the origin of the graph is tangential to the curve

Question 20

Comparing a PR curve and a PA curve - highest intersection point

Answer 20

Is the max speed for level flight at any given power setting, as at this point PR = PA and thus the thrust developed will = drag

Question 21

Comparing a PR curve and a PA curve, reducing power

Answer 21

As power is reduced from maximum the PA curve moves down the graph, the intersection point of the curves moves to the left and the max speed at the lower power setting is reduced

Question 22

Comparing a PR curve and a PA curve - min speed for level flight

Answer 22

Occurs either where the curves intersect at the lower TAS or at the stalling speed (whichever is reached first), for most a/c the stalling AoA will be reached before max PA and thrust are required to counterbalance drag

Question 23

Comparing a PR curve and a PA curve - effect of increasing weight

Answer 23

If weight is increased the AoA will have to increase so that lift remains equal to weight, this increase in AoA results in an increase in drag and therefore the PR to maintain level flight at all speeds is increased. This moves the PR curve up and to the right, we can also see a slight reduction in max level flight speed and an increase in stalling speed

Question 24

Comparing a PR curve and a PA curve - effect of altitude

Answer 24

• Altitude effects both PA and PR curves
• As altitude increases the max TAS in level flight decreases
• However with a supercharged engine an increase in max TAS will be possible up to such altitude as the supercharger is able to maintain max boost, after which the effect of alt will be the same as a normal engine
• The min level flight speed is increased with alt
• The TAS at which the a/c stalls will be increased but the rate at which the low-speed intersection of the PR and PA curves moves to the right on the graph is faster which means in most a/c an alt will be reached where min level flight speed is above stalling speed

Question 25

If an a/c is flown at a constant IAS but altitude is increased what effect does this have on the PR curve

Answer 25

TAS at that IAS is steadily increasing, since the PR to fly at any IAS is a function of TAS x drag, the PR to maintain an IAS increases as altitude increases, this causes the PR curve to move up and to the right

Question 26

PA curve - the effect of altitude

Answer 26

As altitude increases, density decreases, this causes the PA curve to move down on the graph