STRAIGHT & LEVEL FLIGHT

LESSON OBJECTIVES

  1. Understand the relationship between the flight forces in S&L flight

  2. Learn to maintain a constant altitude and a constant airspeed during the manoeuver

  3. Learn the effect of changing power/pitch values while in S&L

In an aircraft in flight there are 4 forces involved:

- Lift

- Weight

- Thrust

- Drag

To perform an S&L flight, these forces must be in equilibrium:

- Equilibrium -> A body is said to be in equilibrium when the sum of the forces (resultant force) that act on it equals 0 

So, if an aircraft is in S&L flight, the following must be accomplished:

- Lift = Weight    (Lift - Weight = 0)

- Thrust = Drag    (Thrust - Drag = 0)

In Lesson Nº1 - Familiarisation with the aircraft, there was a diagram representing these forces, all of them acting over the same point. How the forces are represented in that diagram was a simplification because lift, weight, thrust and drag do not act at the same point

- Weight -> Center of Gravity (CG) -> The centre of gravity is the imaginary point where is supposed that all the weight of the aircraft acts. (Green Dot)

- Lift -> Center of Pressure (CP) -> Same concept that centre of gravity but with the lift force instead of the weight force (Blue Dot)

- Thrust -> Thrust Line -> An imaginary line passing through the centre of the propeller, perpendicular to the plane of the propeller rotation (Red Line)

- Drag -> Drag Line (Yellow Line)

These forces act over different points/lines, these differences make the combination of forces create couples. We have:

- Lift / Weight Couple -> CP is slightly behind the CG creating a nose-down pitching moment. The downward force created by the horizontal stabilizer help to reduce this effect

- Thrust / Drag Couple -> Thrust Line is slightly below the Drag Line, as you saw in  Lesson Nº3 -Effects of Controls if you increase/decrease power, the aircraft pitch up/down respectively. The couple form by these two forces is the explanation for this phenomenon

Captura.JPG

-Introduction-

-Practical Knowledge-

Now we will discuss, how the S&L flight is accomplished. But before that, we must introduce a new concept into the equation:

- Power + Attitude = Performance

With the same power and attitude you always get the same performance. Interiorizing this concept from the beginning will be very useful for your future career. Because you will know by heart, the power setting required and the attitude that the aircraft must maintain to achieve different airspeeds, rate of climb/descents ...etc

The formula written above can be a bit confusing at first, but you will understand better what it means with the next example:

- An aircraft with a power setting of 2000 RPM and with an attitude of 4 fingers down between the horizon and the cowling will result in an S&L flight at 90 Kts

- Power (2000 RPM) + Attitude (4 fingers below horizon) = Performance (S&L at 90 kts)

Power + Attitude = Performance.png

Establishing S&L flight is achieved using the next 3 steps in order:

- Power -> Set the power. In our last example, it was 2000 RPM

- Attitude -> Composed by three elements (pitch, bank and yaw) controlled by elevator, ailerons and rudder respectively:

                 --- Pitch -> Set in reference with the horizon (fingers above/ below the horizon)

                 --- Bank -> Maintain wings level - 0º bank angle

                 --- Yaw -> Maintain aircraft coordinated (Ball in the centre - Step on the ball)

- Trim -> We already describe how the trim must be used (Refer to Lesson Nº3 -Trim)

To know how much power/pitch we must apply, we need to understand the effect that both of them have on the flight path.

The effect of pitch maintaining constant power:

- Pitch -> Pitch Up -> Altitude ↑ / Airspeed ↓

            -> Pitch Down -> Altitude ↓ / Airspeed ↑

 The effect of power without pitch input:

- Power -> Power Increase -> Altitude ↑ / Airspeed CONSTANT

              -> Power Decrease -> Altitude ↓ / Airspeed CONSTANT

As you can see modifying airspeed/altitude is a game of relationship between power and pitch. 

Let's assume you have followed the 3 steps correctly and now you are cruising at 5000 ft at 90 kts. What now?

Lookout -> Scan your environment at regular intervals to check any traffic that could be flying around you.

-> Scan the sky from left to right passing over the nose of the aircraft at intervals of 20º per 2 seconds

- Attitude -> Look outside 90 % of the time and fly in reference to the horizon (VFR flight).

-> Ensure the attitude is correct (four fingers below the horizon is a common reference to maintain the  altitude), check wings level (same space between the horizon and the left/right wingtips) and that the aircraft is coordinated (ball in the centre)

- Instruments -> Instruments exist to help us maintain an accurate flight (maintain it - not set it).

-> Do a quick scan from left to right (Airspeed, Attitude, Altitude) and then to the left again (VSI, Directional Gyro, Turn Coordinator). After a few scans also check the T&Ps (Temperatures & Pressures of the engine)

---Increase airspeed maintaining altitude---

- Power -> Power UP -> Aircraft tends to Pitch UP and yaw RIGHT

- Attitude -> Pitch -> We counteract the effect of the power by pitching down to maintain altitude, so the power now is used to increase our airspeed, instead of gaining altitude

                -> Bank -> Maintain wings level 

                -> Yaw -> We counteract the effect of the power by pressing the right rudder to fly coordinated

 - Trim -> To get the correct attitude, you are applying forward pressure, so nose-down trim is required

---Reduce airspeed maintaining altitude---

- Power -> Power DOWN -> Aircraft tends to Pitch DOWN and yaw LEFT

- Attitude -> Pitch -> We counteract the effect of the power by pitching up to maintain altitude, so decreasing our airspeed

                -> Bank -> Maintain wings level 

                -> Yaw -> We counteract the effect of the power by pressing the left rudder to fly coordinated

 - Trim -> To get the correct attitude, you are applying back-pressure, so nose-up trim is required

Possible deviations and appropriate corrections

- Higher Altitude / Lower Airspeed -> Pitch DOWN only -> Altitude ↓ / Airspeed ↑

- Lower Altitude / Higher Airspeed -> Pitch UP only -> Altitude ↑ / Airspeed ↓

- Higher Altitude / Higher Airspeed -> First, reduce power and lower the nose to regain altitude, when at the desired altitude follow the steps of the last topic to decrease airspeed

- Lower Altitude / Lower Airspeed -> First, increase power and put the nose up to regain altitude, when at the desired altitude follow the steps of the last topic to increase airspeed

When one parameter is high and the other is low a simple pitch change can solve it by interchanging airspeed for altitude and vice versa. This is called energy management and it will be a very important concept to understand future lessons:

- Speed -> Kinetic Energy

- Altitude -> Potential Energy

When both parameters are high or low, you need to use both pitch and power accordingly.

- Pitch controls airspeed -

- Power controls altitude -

We have discussed altitude and airspeed errors but we also need the aircraft to fly straight (in the same heading with no deviation to the left/right).

 

If the aircraft is not flying straight there are only 3 causes:

- Wings are not level (Bank angle > 0º)

- Aircraft is uncoordinated (Ball is left/right of the centre)

- Both of them

The solutions are:

- Regain wings level using the ailerons (Bank angle = 0º)

- Check turn coordinator and put the ball in the centre (Step on the ball - Ball to the left/right - Use left/right rudder)

Remember that ailerons and rudder are correlated (Refer to Lesson Nº3 - Effects of Controls):

---- If the ball is in the left and you apply the left rudder, the secondary effect will be an induced roll to the left that you will have to counteract applying the right aileron ----

---- Similarly, if you are using the right aileron to level the wing, you will need to apply the right rudder to stay coordinated due to the adverse yaw effect ----