Usually your second flight lesson with teach you straight and level flight.
A pilot must be able to fly an aircraft in a straight line, with a constant heading and at a constant altitude. Maintaining a constant altitude requires a constant attitude and a constant heading requires the aeroplane to be wings level and in balance.
Principles of Flight
In VFR flight, straight and level flight should only be accomplished with reference to the horizon.
The Four Forces in flight
The four forces acting on an aircraft are:
Weight
Acts straight down through the centre of gravity.
Lift
Is produced by the wings and acts upwards through the centre of pressure (C of P)
Thrust
Is provided by the engine through the propellor.
Drag
Is the resistance to motion experienced by all bodies within the atmosphere.
Equilibrium requires a constant airspeed and constant direction (the combination of these two factors is velocity). A constant direction is maintained by the wings being level and the aircraft in balance. Equilibrium is achieved when lift equals weight and thrust equals drag.
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 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 Effect of Controls lesson, you will have seen 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) whilst 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. Thus, 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 change (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 – this is known as camber.
The lift formula 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 things that the pilot can easily control are airspeed and angle of attack, so for the pilot;
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 there is no angle-of-attack indicator fitted to light ircraft, the airspeed is used as a guide to the aircraft’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 – in the example below we have used 2100 rpm.
The attitude will depend on the aircraft type, in this example we will use three fingers below the horizon.
In this case the performance we want is a constant altitude, direction and airspeed – straight and level.
Power + Attitude = Performance
(2100) (three fingers) (straight and level)
The following are some of the exercises you will undertake in the Straight and Level lesson
Identify the horizon, and what attitude is required relative to the horizon, with the appropriate power setting, to establish and maintain straight and level flight.
Power + Attitude = Performance
(2100) (four fingers) (straight and level)
With the use of the ‘windscreen view’ show the attitude in the correct position as well as in the too low and too high positions.
Establish Straight and Level
Establishing straight and level flight is achieved by using the mnemonic PAT.
Power
Set the power for selected (normal) straight and level performance.
Attitude
The attitude for straight and level is made up of three elements.
Elevator Set the nose attitude – for level (eg, three fingers)
Aileron Wings are level relative to the horizon – for straight
Rudder In balance – for straight
If a constant direction is not being maintained on the reference point (and the DI should confirm this) either the wings are not level, or the aeroplane is out of balance, or both.
Balance is confirmed with the balance ball indicator. The method used to achieve balance is ‘stand on the ball’. If the ball is out to the left, increased pressure on the left rudder pedal is required. This is a pressure increase, more than a movement and ‘stand’ implies continued pressure. Once the ball has been centred, reducing pressure will allow it to move out again.
Trim
Take the time to teach this thoroughly, make sure the student relieves all of the control pressures so that their hands can come off the controls and the aeroplane remains level.
Maintaining Straight and Level
Maintaining straight and level is achieved by using the mnemonic LAI.
L – Lookout
In a scan look ahead, look out to the left and scan 20 degrees for 3 seconds from left to right, passing over the nose of the aeroplane.
A – Attitude
Ensure the attitude is correct relative to the horizon and is also constant.
I – Instruments
Used to confirm accurate flight. Scan from From right to left.
Power + Attitude = Performance
It should be emphasised that every time power or airspeed is changed, a change in rudder pressure is necessary to maintain balance. Therefore, during those phases of flight where power or airspeed are changing, rudder will need to be applied to maintain balance. In addition, when rudder is being used to centre the ball, the wings must be held laterally level with aileron.
List the various power, airspeed and attitude required to maintain straight and level flight. See example below.
Power 2200 1800 2500
Airspeed 80–90 knots 60 knots 110 knots
Attitude Normal High Low
You will note, a high power setting means a higher airspeed, requiring a lower nose attitude whilst a low power setting means a low airspeed, requiring a higher nose attitude.