Straight and Level

2. Objectives

To establish and maintain straight and level flight, at a constant airspeed, constant altitude, in a constant direction, and in balance.

To regain straight and level flight.

To maintain straight and level flight at selected airspeeds or power settings.

Principles of Flight

In VFR flight, flying straight and level should only be accomplished with reference to the horizon. Remember the location of the horizon and how it can be identified if it is not visible, for example with hills or weather in the way.

The Four Forces

The four forces acting on the aeroplane should be remembered.

Arrangement of Forces

Lift acts through its centre of pressure and is slightly behind the centre of gravity, where weight acts (small moment arm), creating a nose-down pitching couple. The comparative size of the lift and weight forces to thrust and drag forces should be discussed. 

For general aviation aeroplanes the lift/drag ratio is said to be about 10:1. Your diagram should reflect this ratio approximately – a picture is worth a thousand words.

The ideal arrangement is for the thrust line to be well below the drag line. This provides a large moment arm to compensate for the smaller forces of thrust and drag, and creates a nose-up couple that balances the nose-down couple of lift and weight.

In the previous lesson Effect of Controls, you saw the pitch change when power was increased and decreased. The arrangement of these couples is the reason for the pitch changes. A decrease in power will pitch the nose down into a descent, without pilot input, and an increase in power will pitch the nose up.

In practice, getting the thrust and drag lines separated far enough to balance the lift/weight couple is not possible. Therefore, the tailplane is set at an angle of attack that will provide a down force on the tailplane in level flight, which combined with the large moment arm, balances the forces.

Any further imbalance between the couples, as a result of weight or airspeed changes for example, are compensated for by the elevator.

Lift

Lift is generated by air flowing faster over the top surface of the wing, compared with air flowing under the wing. Air is made to flow faster by shaping the top surface – called camber. Due to this, air on top of the wing has a lower pressure than below the wing. 

This high pressure under the wing, wants to move to an area of lower pressure which is above the wing; think of high pressure air inside a bicycle tube moving to lower pressure ambient air when the valve is opened. 

The formula for lift is:

L = CL ½ ρ V² S

Where;

CL is the co-efficient of lift (angle of attack)

½ is a constant

ρ (rho) is the density of the air

V is the airspeed, and

S is the surface area of the wing.

The two elements the pilot can easily control are airspeed and angle of attack, so in essence;

L = angle of attack x airspeed

Angle of attack (α) is the angle between the relative airflow and the chordline of the aeroplane’s wing.

The most efficient angle of attack is approximately 4 degrees, but as no angle-of-attack indicator is fitted to light aeroplanes, the airspeed is used as a guide to the aeroplane’s angle of attack.

In order to keep lift constant, any change in the angle of attack must be matched by a change in the airspeed. For example if airspeed increases, less angle of attack is required to maintain a constant lift. A decrease in airspeed will require an increase in the angle of attack to maintain constant lift and consequently altitude.

Performance

Power + Attitude = Performance

Power is set by reference to rpm – (use the organisation’s recommended rpm setting for training flights), in the example below we have used 2200 rpm.

The attitude will depend on the aeroplane type, in this example we will use four fingers below the horizon.

In this case the performance we want is a constant altitude, direction and airspeed – straight and level.

3. Principles of Flight

In VFR flight, flying straight and level should only be accomplished with reference to the horizon. Remember the location of the horizon and how it can be identified if it is not visible, for example with hills or weather in the way.

4. The Four Forces

The four forces acting on the aeroplane should be remembered.

5. Arrangement of Forces

Lift acts through its centre of pressure and is slightly behind the centre of gravity, where weight acts (small moment arm), creating a nose-down pitching couple. The comparative size of the lift and weight forces to thrust and drag forces should be discussed. 

For general aviation aeroplanes the lift/drag ratio is said to be about 10:1. Your diagram should reflect this ratio approximately – a picture is worth a thousand words.

The ideal arrangement is for the thrust line to be well below the drag line. This provides a large moment arm to compensate for the smaller forces of thrust and drag, and creates a nose-up couple that balances the nose-down couple of lift and weight.

In the previous lesson Effect of Controls, you saw the pitch change when power was increased and decreased. The arrangement of these couples is the reason for the pitch changes. A decrease in power will pitch the nose down into a descent, without pilot input, and an increase in power will pitch the nose up.

In practice, getting the thrust and drag lines separated far enough to balance the lift/weight couple is not possible. Therefore, the tailplane is set at an angle of attack that will provide a down force on the tailplane in level flight, which combined with the large moment arm, balances the forces.

Any further imbalance between the couples, as a result of weight or airspeed changes for example, are compensated for by the elevator.

6. Lift

Lift is generated by air flowing faster over the top surface of the wing, compared with air flowing under the wing. Air is made to flow faster by shaping the top surface – called camber. Due to this, air on top of the wing has a lower pressure than below the wing. 

This high pressure under the wing, wants to move to an area of lower pressure which is above the wing; think of high pressure air inside a bicycle tube moving to lower pressure ambient air when the valve is opened. 

The formula for lift is:

L = CL ½ ρ V² S

Where;

CL is the co-efficient of lift (angle of attack)

½ is a constant

ρ (rho) is the density of the air

V is the airspeed, and

S is the surface area of the wing.

The two elements the pilot can easily control are airspeed and angle of attack, so in essence;

L = angle of attack x airspeed

Angle of attack (α) is the angle between the relative airflow and the chordline of the aeroplane’s wing.

The most efficient angle of attack is approximately 4 degrees, but as no angle-of-attack indicator is fitted to light aeroplanes, the airspeed is used as a guide to the aeroplane’s angle of attack.

In order to keep lift constant, any change in the angle of attack must be matched by a change in the airspeed. For example if airspeed increases, less angle of attack is required to maintain a constant lift. A decrease in airspeed will require an increase in the angle of attack to maintain constant lift and consequently altitude.

7. Performance

Power plus attitude is 95% of your flying for the future if not more.

What were talking about is a “chosen performance” which is this scenario straight and level at a speed.

Thus in most situations there is one power setting and one attitude that will deliver your chosen performance.

Power + Attitude = Performance

Power is set by reference to rpm – (use the organisation’s recommended rpm setting for training flights), in the example below we have used 2200 rpm.

The attitude will depend on the aeroplane type, in this example we will use four fingers below the horizon.

In this case the performance we want is a constant altitude, direction and airspeed – straight and level.

8. Objectives

Objectives 

• Understanding factors which affect during straight and level flight
• How flying controls and instruments are used to achieve and maintain a constant height, heading, and airspeed -> aircraft in balance 
• Use of co-ordinated controls and understanding aircraft performance 

9. Introduction

Introduction 

• Straight and level flight is about keeping aircraft laterally level -> with height and direction constant 
• Aircraft in equilibrium -> recognized by pilot with constant airspeed and height
  
  • Lift = Weight 
  • Thrust = Drag 

10. The Forces

The Forces

• Maintain condition of horizontal level flight -> lift = weight 
• Obtain aircraft’s forward movement in the air -> thrust = use of engine and propeller 
• Speed is constant -> thrust = drag produced from all sources (includes aircraft’s air resistance) 
• When Lift = Weight and Thrust = Drag; equilibrium is achieved
  
  • Only small movements to maintain this condition  

11. Weight

Weight

Acts straight down through the centre of gravity.

12. Lift

Lift

Is produced by the wings and acts upwards through the centre of pressure.

13. Drag

Drag

Is the resistance to motion felt by all bodies within the atmosphere.

{{acard,- Thrust}}

14. Thrust

Thrust

Is provided by the engine through the propeller.

Equilibrium requires a constant airspeed and constant direction (the combination of these is velocity). A constant direction is maintained by the wings being level and the aeroplane in balance. Equilibrium is achieved when lift = weight and thrust = drag.

15. Arrangement of Forces

Arrangement of Forces

Lift acts through its centre of pressure and is slightly behind the centre of gravity, where weight acts (small moment arm), creating a nose-down pitching couple. The comparative size of the lift and weight forces to thrust and drag forces should be discussed. 

For general aviation aeroplanes the lift/drag ratio is said to be about 10:1. Your diagram should reflect this ratio approximately – a picture is worth a thousand words.

The ideal arrangement is for the thrust line to be well below the drag line. This provides a large moment arm to compensate for the smaller forces of thrust and drag, and creates a nose-up couple that balances the nose-down couple of lift and weight.

In the previous lesson Effect of Controls, you saw the pitch change when power was increased and decreased. The arrangement of these couples is the reason for the pitch changes. A decrease in power will pitch the nose down into a descent, without pilot input, and an increase in power will pitch the nose up.

In practice, getting the thrust and drag lines separated far enough to balance the lift/weight couple is not possible. Therefore, the tailplane is set at an angle of attack that will provide a down force on the tailplane in level flight, which combined with the large moment arm, balances the forces.

Any further imbalance between the couples, as a result of weight or airspeed changes for example, are compensated for by the elevator.

16. Couple

Couple

couple, in mechanics, pair of equal parallel forces that are opposite in direction. The only effect of a couple is to produce or prevent the turning of a body. The turning effect, or moment, of a couple is measured by the product of the magnitude of either force and the perpendicular distance between the action lines of the forces.

17. Lift Weight Couple

Lift Weight Couple

Lift acts through the centre of pressure.  Weight acts through the centre of gravity.  The tailplane balances the Lift/Weight couple.

18. Thrust Drag Couple

Thrust Drag Couple

Thrust acts through the propeller.  Drag acts through the wing.  The drag position can change, eg in a low wing aircraft.  

19. Longitudinal Stability and Control in Pitch

Longitudinal Stability and Control in Pitch 

• Main factors that influence longitudinal stability -> relative positions of Centre of Pressure (lift) and Centre of Gravity (weight) + design of tailplane and elevators 
• Initially achieved by arranging forces so that the centre of weight acts ahead of the centre of lift
  
  • Creates nose down moment -> compensated for action of horizontal tailplane 
  • Angle at which tailplane is set causes it to carry download 
  • Arrangement of force + action of the tailplane will ensure a stable/nose down moment of the aircraft whenever engine is throttled back 
• High wing aircraft -> high drag line + low thrust line = pitch up moment
  
  • Counteracts nose down moment couple of light and weight 
• If centre of gravity behind centre of pressure -> unstable
  
  • Tendency to pitch up -> slower airspeed -> tailplane and elevators less effective 
• Exact positions of centre of gravity and centre of pressure will vary during flight
  
  • Due to variations in angle of attach, weight, speed and use of flaps 
  • Designer -> balance action of tailplane and effects of elevators is sufficient to control attitude in lowest speed of flight -> provided centre of gravity limits have not been exceeded 
• Different angles of incidence for wings and tailplane -> provide longitudinal balance wherever disturbing influences of air are encountered
  
  • Disturbed by gust = new attitude -> aircraft has inertia so it will continue temporarily on original flight path 
  • Both wings and tailplane have a change of angle of attack of the same amount 
  • Due to difference in angle of incidence, more lift on the tailplane = rise -> results in pitch down moment 

20. Relationship ofCentre of Gravity to Control in Pitch

Relationship of Centre of Gravity to Control in Pitch 

• Aircrafts designed to be longitudinally stable over limited centre of gravity range
  
  • Moves outside -> performance + controllability over attitude will be limited or uncontrollable 
  • Loaded at forward limit -> aircraft will be most stable
    
    • Disturbed in flight -> quickly return to original attitude 
    • Exceeds limit -> aircraft tiring to maneuver in pitch as it is too longitudinally stable 
    • Becomes nose heavy at lower airspeeds -> elevator not effective -> might not be able to raise nose for touchdown 
  • Loaded aft -> stability decreases
    
    • Takes longer to resume original attitude when disturbed 
    • Becomes tail heavy -> nose will rise -> aircraft will eventually stall 

21. Attitude and Balance Control

Attitude and Balance Control 

At Normal Cruising Power 

• This means to fly at a power setting specified by Flight Manual 
• Power setting adjusted -> straight and level achieved by
  
  • Pitch = Elevators 
  • Roll = Ailerons (Laterally level)
  • Yaw = Rudder 
• Aircraft must be laterally level to fly straight
  
  • Small angle of bank -> causes a yaw in direction of lower wing 
  • Can correct by applying opposite rudder -> cross controls -> inefficient way of flying 
• Aircraft flying without sideslip -> path of aircraft is in line with longitudinal axis 
• Most propeller driven aircraft -> yawing tendency due to slipstream -> counteracted with use of rudder 
• Aircraft held level laterally -> rudder is used prevent yaw -> aircraft in balance and maintaining a constant heading 

Effect of Inertia 

• Changing attitude to straight and level flight -> short time lapse before airspeed settles -> due to inertia of aircraft 
• Essential after adjusting -> wait for airspeed to become steady 
• Failure = tendency to chase airspeed to the detriment of maintaining a constant altitude 

22. Trimming

Trimming 

• Assumed correct attitude -> appropriate airspeed for power used -> final part is trimming 
• Because of stability -> aircraft if well trimmed -> remain steady on own accord
  
  • Unless disturbed by alterations in power, turbulence, or changes in disposition of load 
• Frequent use to trim -> necessary to adjust for changes in power load and airspeed -> maintain accurate straight and level flight Notes