EFFECTS OF CONTROLS
Identify the primary flight controls, how to use them, how they affect the aircraft and secondary effects
Identify the secondary flight controls, how they affect the aircraft
Comprehend the effects of inertia, airspeed, and propeller slipstream on the aircraft controls
All the flight controls work by modifying the quantity of lift that the wings and the tail create. So, in order to understand the flight controls, we need to know how lift is created.
The physic principle that makes lift possible is called Bernoulli´s principle:
- Bernoulli's principle states that an increase in the speed (V ↑) of a fluid (in this case, air) occurs simultaneously with a decrease in pressure (P ↓). A practical application of Bernoulli’s Principle is the Venturi Tube, this tube has an inlet that narrows to a throat (constricted point) and an outlet section that increases in diameter toward the end
Now that we know what Bernoulli's Principle is, we will see how it is applied to an aircraft wing. To do that we need to define another essential concept, the airfoil:
- Airfoil -> A body shaped to produce an aerodynamic reaction (lift) for a small resistance force (drag).
An airfoil, using Bernoulli´s principle, accelerates the air that flows over the upper surface, so it flows faster than the air underneath the wing:
Lift produced varies depending on some factors that are arranged in one formula:
- L = 1/2dV² x CL x S
- d -> Air density
- V² -> Aircraft speed through the air squared
- CL -> Coefficient of Lift -> A value with many variables that for instructional purposes we will use as a substitute for the angle of attack (We will explain the angle of attack concept later in this lesson)
- S -> Area of the Wing
In a car at a moderate speed, take your hand out of the wind and parallel to the ground. If you increase the angle, your hand will move upwards (lift created). If you decrease the angle, your hand will go down (lift destroyed). The airfoil works similarly, the angle of your hand and the air is called the angle of attack
The Angle of Attack (AoA) is the angle between the relative wind and the chord line. Let's define these concepts
Airfoil Working Principle
- Upper Surface -> Airflow Speed ↑ -> Pressure ↓ -> Low Pressure (LP)
- Bottom Surface -> Airflow Speed ↓ -> Pressure ↑ -> High Pressure (HP)
--- HP air tends to mix with LP air creating an upward force that counteracts gravity, this force is called lift ---
- Relative wind -> Airflow produced by aircraft own movement through the air
- Chord Line -> It is a straight line that goes from airfoil leading edge (foremost edge) to trailing edge (rear edge) (Refer to Lesson Nº1 - Familiarisation with the Aircraft)
-- L = 1/2dV² x CL x S -> ↑ AoA -> ↑ CL -> ↑ Lift
-The Bernoulli´s Principle-
-The Primary Flight Controls-
As we already saw in section Aircraft Parts (Refer to Lesson Nº1), an aircraft has three main controls surfaces:
- Elevator -> Nose Up/Down (Movement Name - Pitch)
- Ailerons -> Wings Up/Down (Movement Name - Bank)
- Rudder -> Nose Left/Right (Movement Name - Yaw)
These are the primary flight controls, when they are moved, their shape change, modifying the chord line of the wing or the tail, which causes a variation in the value of the angle of attack (AoA), thus lift
- Elevator -
The tail function is to create a downward force to compensate for the upward force (lift) produced by the wings. The elevator is a primary control surface positioned in the tail that moves up or down.
- The elevator is controlled by the stick/yoke by pushing/pulling the control column, which moves the elevator down/up respectively producing more/less downward force
- The effect is a movement known as pitch, the pitch angle is the angle between the aircraft longitudinal axis (imaginary line joining the nose and the tail) and the horizon
- Push Input -> Elevator moves DOWN -> AoA ↑ -> More lift to counteract the downward force -> Total Downward force ↓-> Tail moves UP -> Nose moves DOWN
- Pull Input -> Elevator moves UP -> AoA ↓- > Less lift to counteract the downward force -> Total Downward force ↑ -> Tail moves DOWN -> Nose moves UP
It is a primary control surface that is located in both wings and rolls the aircraft left/right around the longitudinal axis of the aircraft. This movement is known as bank, the bank angle is the angle between the lateral axis (imaginary line joining the left and right wingtips) and the horizon
The ailerons work by creating a lift difference between the right and the left-wing. If the right-wing produces more lift, moves upwards, banking the aircraft to the right
- The ailerons are controlled by the stick/yoke by moving the control column left/right, banking the aircraft to the left and the right respectively
- The ailerons work in opposite directions, so when the right aileron moves up, the left aileron moves down and vice-versa
- Left Input -> Left Aileron UP -> Left Wing AoA ↓ -> Lift ↓ -> Left wing moves down -> Right Aileron DOWN -> Right Wing AoA ↑ -> Lift ↑ -> Right Wing moves up
- Right Input -> Right Aileron UP -> Right Wing AoA ↓ -> Lift ↓ -> RW moves down -> Left Aileron DOWN -> Left Wing AoA ↑ -> Lift ↑ -> Left wing moves up
It is a primary flight control located at the trailing edge of the vertical section of the tail. The rudder act as a vertical wing, it also creates lift, but instead of vertical lift, it creates a horizontal force that moves the tail around.
Its function is to move the aircraft to the left or to the right around its vertical axis. This movement is also known as yaw.
- The rudder is a movable surface that is mounted on the trailing edge of the horizontal stabilizer.
- The rudder is controlled through the rudder pedals located on the flight deck. The pedals are mechanically connected to the rudder
The rudder changes the aerodynamic shape of the tail and produces/destroys horizontal lift to yaw the aircraft:
- Right Pedal Input -> Rudder deflects to the RIGHT -> Horizontal lift ↑ -> Tail moves LEFT -> Nose moves RIGHT
- Left Pedal Input -> Rudder deflects to the LEFT -> Horizontal lift ↑ -> Tail moves RIGHT -> Nose moves LEFT
-The Secondary Effects of Primary Flight Controls-
- Adverse Yaw-
The ailerons work by increasing the lift in one wing and decreasing the lift to create an unequal motion that banks the aircraft. The aileron that comes down to produce extra lift also produces extra drag. This extra drag causes the wing to slow down (more aileron deflection - more drag produced) resulting in a yaw movement to the opposite direction of the roll, that we must correct
What can we do about it?
- The solution is easy, only apply the rudder in the direction of the turn to bring the nose to the correct position
How much rudder do I need to apply?
- You need to check one of the flight instruments on the cockpit, the turn and slip indicator (Refer to Lesson Nº1), just follow the "Step on the Ball" rule
Let's see this secondary effect and its solution in a practical example:
- Right Input -> Left aileron DOWN -> Left-Wing Lift ↑ -> Left-Wing Drag ↑ -> Aircraft Banks to the RIGHT and yaws to the LEFT
- Left Input -> Right aileron DOWN -> Right-Wing Lift ↑ -> Right-Wing Drag ↑ -> Aircraft Banks to the LEFT and yaws to the RIGHT
Left Bank -> Adverse Yaw -> Right Yaw -> Ball to the left -> Press Left Pedal to centre the ball
Right Bank -> Adverse Yaw -> Left Yaw -> Ball to the right -> Press Right Pedal to centre the ball
Similarly, as roll triggers a yaw motion, the opposite also occurs when a yaw motion is applied. Yaw induce a roll motion. Imagine an aircraft in S&L (straight and level) flight and the pilot suddenly applies full left or right rudder. What would happen?:
- Right Rudder -> As we apply full right rudder, the aircraft starts yawing to the right. This movement also causes the wing in the outside (left-wing) to travel through the air at a higher speed than the inner wing (right-wing), thus generating more lift and triggering a roll motion to the right
- Left Rudder -> As we apply full left rudder, the aircraft starts yawing to the left. This movement also causes the wing in the outside (right-wing) to travel through the air at a higher speed than the inner wing (left-wing), thus generating more lift and triggering a roll motion to the left
-Secondary Flight Controls-
The flaps are a control surface designed to increase the lift produced by the wing in exchange for a drag penalty. Flaps are mounted on the wing trailing edges, from the wing root to the inboard part of the ailerons (Refer to Lesson Nº1 - Familiarisation with the Aircraft)
- The flaps can be hydraulically or electrically powered and in opposition with the ailerons, they only extend downward. The flaps are usually controlled by a lever with a series of markings to indicate the number of degrees that they are extended
How do they work?
- When flaps are extended (only move downward), they ↑ the curvature of the wing, modifying the chord line (Straight line from leading edge to trailing edge), ↑ the AoA and ↑ the maximum amount of lift produced by the wing
- As the flaps move downward they protrude into the air flowing around the wing. The amount of drag generated depends on the degree of extension of the flaps (↑ number of degrees -> ↑ drag produced)
Flaps are not used in the cruise phase of the flight where we have sufficient speed to produce lift and where any drag penalty is not beneficial. So, when are the flaps used?
Flaps are used in take-off and landing
- For Take-Off-> They are extended a few degrees producing more lift for a small drag penalty. Allowing us to take off in shorter runways
- For Landing -> They are extended to a maximum producing maximum lift (to maintain flight at slow speed) but also maximum drag (to avoid increasing speed with the nose down). Allowing us to touch down at a minimum speed and in shorter runways
If the pilot needs to move the nose up (commonly refer as pitching up), back-pressure on the yoke/stick must be applied and maintained to colocate the elevator in the required position. If the pilot has to climb from sea level (SL) to 10000 ft at 500 ft/min, the pressure on the controls has to be maintained for 20 min!!!
In the end, his arm would be aching and he would be too tired to keep flying
That´s why the trim exists:
- The trim adjust the aerodynamic forces on the control surfaces so that the aircraft maintains the attitude required without any control input
- Although trim devices exist for the three control surfaces (aileron trim, elevator trim and rudder trim). Not all aircraft have all of them. Usually, most common training aircraft only have elevator trim.
The trim use is uncomplicated. In some aircraft, the elevator trim is controlled by buttons and in others, like the C172 it is controlled by a wheel. In both of them, the concept is the same:
- If you need to apply back-pressure -> Use the trim bottom for nose-up / Rotate the trim wheel from up to down
- If you need to apply forward pressure -> Use the trim button for nose-down / Rotate the trim wheel from down and take it up
- Apply Control Input -> Establish required Attitude -> Trim -> Release all pressure from control
--Aircraft maintains attitude -> Trimmed correctly
--Aircraft does not maintain attitude -> Re-establish Required Attitude -> Trim Again
-Factors affecting Aircraft Control-
The effect of airspeed is simple and easy to comprehend. It affects:
- Control feeling
- Aircraft response rate
- Control deflection required to change the aircraft flight path
Why the airspeed affects all these variables?
- L = 1/2dV² x CL x S
The control surfaces alter the lift distribution of the aircraft making it pitch/bank/yaw, by modifying the AoA of the wings and the tail. In the lift formula, there is another factor that as pilots we can influence and that factor is airspeed.
To change the attitude of the aircraft, the amount of deflection required (change in AoA) is smaller/higher at higher/lower airspeed respectively to get the same amount of lift
- Low Airspeed -> Controls are easier to move / Less effective / Larger control deflections required / Slower response
- High Airspeed -> Controls are harder to move / More effective / Smaller control deflections required / Faster response
Because an aircraft has mass it is subject to inertia, but what exactly is inertia?
- Newton's first law -> If a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by a force
Just imagine yourself in a car at 100 kph, and suddenly braking to a stop, the car has stopped but your body has a tendency to keep moving and you lean forward. That is the effect of inertia, the tendency to keep moving in the same direction
The inertia affects the change of direction of the relative airflow (airflow caused by aircraft own movement) which modifies the actual AoA.
If in an S&L flight, the pilot suddenly abruptly applies back-pressure, the aircraft will not climb immediately. There would be a delay between the control input and the aircraft response, the plane would keep moving straight for a relatively short period (relative airflow still coming from the same direction) with the nose up (chord line position changes) which creates a temporarily higher AoA
- Higher Mass -> More Inertia -> More delayed response
- Lower Mass -> Less Inertia -> More immediate response
Slipstream is the name of the spiral column of air being accelerated by the propeller. Slipstream is always present whenever the propeller is rotating regardless of the aircraft speed
How does it affect aircraft controls?
- The slipstream affects the elevator and rudder. Its effect is proportional to the power setting applied.
- Due to their position, the ailerons are outside the area of influence of the slipstream, so their effectiveness is not affected
The effect of slipstream is similar to the effect of airspeed with the exception that it does not affect ailerons but the concept is the same
- Faster / Slower Airflow -> More / Less Effective Controls
The variation of the power setting also has immediate effects (disregarding the slipstream effect) on the aircraft attitude. The explanation for the aircraft reactions to power changes is complex and beyond the objectives of this lesson and we will omit it for now.
Reactions to the power changes:
- Power Increase -> Nose pitch UP / Yaw RIGHT
- Power Decrease -> Nose pitch DOWN / Yaw LEFT
--- Yaw reactions correct for clockwise rotating propellers, for counterclockwise rotating propellers, opposite applies ---