Straight & Level Flashcards
4 main forces acting on an a/c in flight
- Lift
- Weight
- Thrust
- Drag
Forces acting on an a/c that is maintaining a constant altitude, IAS and heading
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
What causes pitching moments
In flight the 4 main forces don’t act through the same point, this results in 2 couples:
- L/W couple
- T/D couple
Tailplane stabilising moment
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
L/W couple
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
T/D couple
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
Pitching moment caused by power changes
Power increase = nose up
Power decrease = nose down
Pitching moment caused by flap changes
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)
Pitching moment caused by retractable undercarriage
Undercarriage up = nose up
Undercarriage down = nose down
How do you decrease or increase speed whilst remaining in level flight
Controlling both power and nose attitude
Flying at a faster airspeed whilst remaining level
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
Reducing airspeed whilst remaining level
As power is decreased the nose attitude must be progressively raised
Nose attitude at a higher airspeed for level flight
It will be lower
Flying level at speeds well below the min drag speed
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
For unaccelerated level flight PR =
Drag x TAS
How do you determine the power required to move an a/c through the air at a constant speed
By multiplying the thrust required (which is the same as drag) by TAS
How are PR curves obtained
By multiplying drag x TAS and plotting against TAS
Points to note when comparing a PR curve with a drag curve
- 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
Where is the speed for min drag / best L/D ratio on a PR curve
Occurs where a line drawn from the origin of the graph is tangential to the curve
Comparing a PR curve and a PA curve - highest intersection point
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
Comparing a PR curve and a PA curve, reducing power
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
Comparing a PR curve and a PA curve - min speed for level flight
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
Comparing a PR curve and a PA curve - effect of increasing weight
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
Comparing a PR curve and a PA curve - effect of altitude
- 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
If an a/c is flown at a constant IAS but altitude is increased what effect does this have on the PR curve
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
PA curve - the effect of altitude
As altitude increases, density decreases, this causes the PA curve to move down on the graph
