-Straight & Level -

LESSON OBJECTIVES

1.- Understand the physics that make S&L flight possible

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

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

-Introduction-

This is the first lesson in which the student (in other words, you) will have to demonstrate how to use the controls in adequate manner as they learnt in Effects of Controls

You will also learn the interrelation between a lot of variables that interfere in the flight such as airspeed, power, pitch ...etc

And lastly but not less important, the lesson in which you will be required to fly coordinated (Review Lesson Nº 3 - Adverse Yaw)

-Principles of Flight-

As we already know in an aircraft in flight there are 4 forces involved (Review Lesson Nº1):

- Lift

- Weight

- Thrust

- Drag

In order 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 

We can translate this definition into the next principle. For an aircraft to be in S&L flight:

- Lift = Weight

- Thrust = Drag

It is not difficult to understand, right? Well, that was the easy part, now comes the difficult one

If you remember Lesson Nº1 - The Four Forces of Flight, there was a diagram indicating these forces, all of them acting over the same point. At the bottom of that document, there was a note that said "These forces do not act in the same point (Simplification Purpose)"

Now, we will see the real point from where each one of the four forces acts:

- Weight -> Center of Gravity -> 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 -> 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)

As you see 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

- Thrust / Drag Couple -> Thrust Line is slightly below the Drag Line, as you saw in 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

-Air Exercise-

Now we will discuss, how the manoeuver is accomplished, but before that, we must introduce a new concept into the equation:

- Power + Attitude = Performance

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 2200 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 (2200 RPM) + Attitude (4 fingers below horizon) = Performance (S&L at 90 kts)

Easy, right? 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

Now, let's see the steps required for:

- Establishing S&L flight

- Maintaining S&L flight

- Regaining S&L flight

-Establishing S&L-

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

- Power -> Set the power

- 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)

In order to know how much power / pitch we must apply to get the performance required, we need to understand the effect that both of them have in the flight path.

The effect of pitch maintaining constant power:

- Pitch -> Pitch UP -> Altitude INCREASE / Airspeed DECREASE

             -> Pitch DOWN -> Altitude DECREASE / Airspeed INCREASE

 The effect of power without pitch input:

- Power -> Power UP -> No pitch input -> Altitude INCREASE / Airspeed CONSTANT

               -> Power DOWN -> No pitch input -> Altitude DECREASE / Airspeed CONSTANT

As you can see modifying airspeed/altitude is a game of relationship between power and pitch. And that takes us into the next topic - "Maintaining S&L flight"

-Maintaining S&L-

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

Well, now the objective is to maintain the good work that you have been doing. In order to do that there are 3 steps more that you must do:

Lookout -> You must scan your environment at regular intervals to check any traffic that could be flying around you. To do that scan the sky from left to right passing over the nose of the aircraft at intervals of 20º per 2 seconds

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

                 -> Ensure the attitude is correct (4 fingers below horizon, wings level and aircraft coordinated)

- Instruments -> We are humans and we can commit errors that's why 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)

Now imagine that you want to maintain S&L but you want a higher/lower airspeed but maintaining the altitude. How would you do it?

The answer is that you would have to follow the first 3 steps again. In the case of requiring a higher airspeed:

- Power -> Power UP -> Aircraft tends to INCREASE the altitude and yaw RIGHT

- Attitude -> Pitch DOWN -> We counteract the effect of the power by pitching down and trying to get a CONSTANT altitude, so the power now is used increase our airspeed

                -> Bank -> Maintain wings level 

                -> Yaw -> We counteract the effect of the power by pressing the right rudder and trying to get the aircraft coordinated

 - Trim -> As you get the correct attitude, and you need to apply forward pressure, use nose-down trim

In the case of requiring a lower airspeed:

- Power -> Power DOWN -> Aircraft tends to DECREASE the altitude and yaw LEFT

- Attitude -> Pitch UP -> We counteract the effect of the power by pitching up and trying to get a CONSTANT altitude, so decreasing our airspeed

                -> Bank -> Maintain wings level 

                -> Yaw -> We counteract the effect of the power by pressing the left rudder and trying to get the aircraft coordinated

 - Trim -> As you get the correct attitude, and you need to apply back pressure, use nose-up trim

Captura_edited.jpg

-Regaining S&L-

Everybody can commit an error and is very common to commit errors in every manoeuvre that you do. The important thing is to know how to solve them and continue. 

In this topic, we will discuss how to correct an error in the manoeuvre such as being at a higher / lower altitude/airspeed, or dont follow a straight line while flying S&L

Let's see each one of the possible errors

As an example, we will suppose our original situation is flying at 8000 ft at 100 kts, and we will go over different situations that can occur:

- Higher Altitude / Lower Airspeed -> 9000ft/90 kts -> Pitch DOWN only -> Altitude DECREASES / Airspeed INCREASES

- Lower Altitude / Higher Airspeed -> 7000ft/110kts -> Pitch UP only -> Altitude INCREASES / Airspeed DECREASES

- Higher Altitude / Higher Airspeed -> 9000ft/110kts -> 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 -> 7000ft/90 kts -> 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

As you can see 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). This error is much more simple to solve.

 

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 also simple:

- 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

-- In a similar way, if you are using the right aileron to level the wing, you will need to apply the right rudder to stay coordinated