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ATA 27 Flight controls

ATA 27

Flight Controls.

This chapter is one of the most important of all airplane systems. Second to the engines. “Without flight controls it’s impossible to control the flight, period!”

Within this chapter you’ll find two main groups of flight controls being; primary and secondary flight controls. Oké I hear you, how can a flight control system be secondary. Well, with the primary ones you can control the direct fight path or direction, what ever you want to call it, the secondary ones are designed to assist the primary ones during take-off, flight, and landing. Secondary flight controls are only used on larger airplanes like airliners.

Oké, but what is a primary flight control and what is a secondary one?!

Ailerons, elevator, and rudder, are the three main controls EVERY airplane has!

But the large ones have the need of secondary flight controls. This is because the size of this airplane gives it such momentum you will need even more input to get the correct output! Secondary flight controls are the flight spoilers on top of the wing for example, they assist the aileron when turning left or right. Ground spoilers, also on top of the wing, assist the reverse thrust system of the engine and the main wheel brakes together with the flight spoilers during landing. Slats are located at the very front of the wings, and create an improved aerodynamic profile over the wing during take-off.

“Dual or triple slotted fowler flaps”, let’s just call them flaps, are used during take-off for the same purpose as the slats, and during landing same as the ground spoilers. During take-off they create in a small setting of about 5 to 10 degrees, an extension of the wing, creating in this way a larger surface and therefore a better aerodynamic profile, but during landing they are lowered all the way down to the 35° flap setting, creating so much drag they break the aerodynamic principle. More drag then thrust = de-acceleration of the airplane.

NOTE: flaps assist only, they are not THE brakes during landing, this is MAINLY done by means of the reverse thrust system installed on the engine. Depending upon the engine type, the propeller (ATA 61) will go to reverse, or a reverse thrust system on a jet-engine (ATA 71) will de-accelerate the specific airplane.


Primary flight controls:

Lets take a closer look at these three main control surface systems, how do they work, what’s in the system, etc.

How the elevator moves:

At the flight deck or cockpit, you have the “yoke”, you know, the thing that looks like the steering wheel of a Formula 1 car! In small airplanes this yoke is installed on “a pipe” which can be pulled or pushed, in airliners this yoke is installed upon a control column, which runs through the cockpit floor and can be pivoted back and front with the pivot point at the floor level. Inside this control column, the cable’s run up and down for the ailerons, but more about this later. The movement of the yoke back and front is translated via cable’s or push-pull rods directly to the elevator, or via a electrical / hydraulic system. Airbus uses the “fly-by-wire” system, being in some way the same as you do during a session of Flight sim. With the difference that your input to your pc will be translated to your screen, with Airbus this will be translated to the electric-hydraulic system of the airplane, which is connected to the control surfaces. Also fighter airplanes use this Fly-by-wire system. Just think of the famous F-16!

In elevator systems are a wide variety, to be able to explain the differences we’ll split them up in groups.

1. A fixed stabilizer with a movable elevator attached at the rear.
2. A movable stabilizer with a movable elevator attached at the rear.
3. A movable stabilizer that consist of one surface without a movable part attached at the rear.
4. A “V” shaped tail in which the rudder and the elevator is combined, consisting of a fixed stabilizer with a movable part at the rear. V-tail called.

These are the most common versions of elevator systems. Note that all these systems (except Nr 3) consist of a stabilizer with an elevator part attached at the back. But what is a stabilizer?

The stabilizer:

A stabilizer is the “wing” like part of the tail that normally is positioned horizontally. On large aircraft the front of the stabilizer, called the leading edge, is heated from inside. This to prevent the build-up of ice during flight. For more information about Anti-Ice systems you can go to ATA 30.
At the back of the stabilizer, there are brackets riveted in to the structure of the stabilizer, on which the movable elevator part is mounted. 

The elevator:

The elevator part is the movable part installed at the back of the stabilizer, this part will force the plane to climb or descent. There are two 
elevator surfaces on most airplanes, which are interconnected with each other via cable’s or “push-pull rods”. This depends pure upon the manufacturer and model of airplane. It’s possible you will see airplanes with only one movable elevator part, running across the tail. You’ll see this in small airplanes.

Great! Now you know of which main parts the elevator system consist, we can look at the differences summed in the list above.

Version Nr 1, is off course the most used and the most common, let’s call this the “Common Elevator”. This type is also used by airliners from small up to the biggest, like the A380!!

NOTE: the way the elevator actually moves depends upon the size of the airplane, the elevator of a small plane can be moved by a simple steel cable, but the elevator of a large airliner like the Airbus A380 or a Boeing 747-800 will need a powerful hydraulic actuator to be able to be moved.

Version Nr 2, this is the one with the movable stabilizer, the movement is accomplished by means of a jackscrew. The jackscrew system has two possible editions, one; the jackscrew is connected to the structure of the plane and runs the drive nut up and down which is connected to the movable stabilizer. Or the other way around. Both systems give the same result and are both safe. In the past there has been an airplane crash due to a drive nut running of the jackscrew, but because of this incident world wide all operators of planes with a jackscrew where informed to update the maintenance program in relation to this system.

Version Nr 3, used the F-14, this system gives the pilot the opportunity to fly in a more aggressive manner, because of the large control surface that can be moved , and this can be done very quickly and at high airspeeds. This is a typical fighter plane elevator system. 

Version Nr 4, used both by small airplanes as by fighter airplanes. Beechcraft Bonanza, Fouga Magister, Eclipse 400, are a few planes who use this type of elevator.

NOTE: some planes also have a “canard”. These are two small control surfaces at the front of the plane on both sides of the cockpit, giving the plane the possibility for a steeper angel to climb. This is mostly used by small jet airplane and is inter-connected to the elevator.

So far for the elevator, now we know how an airplane moves up or down in flight, the proper word is; “climb” or “descent”, let’s look at the rudder system.

The rudder:

The rudder also at the tail section, controls the “yaw” movement. Yaw is the motion of an aircraft about its vertical axis. Let’s explain this in human language. Just think of the plane being the seat part of an office chair, which can turn around his axis. The rudder system with the exception of the V-tail has on every airplane the same rudder system principal. A stabilizer with a moving control surface behind it. The operation of the rudder is the same as the elevator, but in contrast to the elevator, the rudder is not  moved by means of “hands-on-a-yoke”, but by moving two rudder pedals with both feet. These pedals move opposite of each other. Whenever one pedal is pushed the other one will move in the opposite direction.  To remember which pedal to push, to go a certain direction, you can remember this easy trick;
you push the left pedal = you go left,… you push the right pedal = you go right. Easy!

NOTE: if both pedals are pushed in a pivoting forward manner, you  will activate the gear brakes! ( ATA 32 )

Together with the rudder being moved, in certain plane models, mostly small planes, like Cessna for instance, the nose gear will also be moved above a certain “driving speed” called ground speed. This is done to aid the pilot during the take-off roll, this way the airplane can be steered by means of rudder pedal input giving the pilot the possibility to hold the yoke with both hands and so fly the plane solo. This is an improvement of safety.

In an airliner the steering of the nose wheels will be done by use of a separate steering column wheel at the forward left and right side of the flight deck. For more information about this see also ATA 32.

Now that we looked at the elevator and the rudder, there’s only one primary flight control left being:….the ailerons!

The ailerons:

Are located at the outer wings, they move opposite of each other, and are controlled by turning the yoke left or right. The ailerons push the wings up or down to which they are respectively connected to. When the aileron goes down, that wing will do the opposite and will go up. Therefore, when the left aileron goes down he will push that wing up, simultaneously the opposite aileron will go up, and pushes that wing down. As a result the plane will turn,…. right! Exactly! You’ve got the hang of it! Easy!

Low and high speed…:

On large airplanes, you will notice two pair of ailerons, at the rear side of the wing beginning from the airplane body (called: ”the fuselage”) going to the wingtip, you will notice half way a flight control surface not being an flap or spoiler, you’re looking at the HIGH SPEED aileron. Going in the same direction you’ll find at the wing tip the LOW SPEED aileron. This one is lock-out at a certain airspeed, to prevent use. In case the low speed aileron would be used at high speeds the airplane would react very fast, and would be very hard to control. Therefore, the high speed aileron closer to the fuselage has a smaller leverage at high speed then the low speed aileron. Giving the pilot more control over the airplane.

For the rest there isn’t a lot more to tell you about the aileron system as such. Taking basic’s that is.

Some nice things to know:

The Vortex Generator.

The control surfaces deflect the airfoil over a wing or stabilizer surface. But in airfoils you have two types! Linear air and turbulent air. The big difference in relation to our control surfaces is that the effect improves when ever the airfoil is turbulent. Therefore, the VORTEX GENERATOR is invented. This are small  metal pieces that are located on top of the wing or stabilizer just in front of the specific flight control surface. These metal L-shaped pieces create turbulent air by disturbing the linear layer of air. Giving the control surface more leverage.


Static discharger:

During flight air flows over the complete surface of the airplane, creating a load of static electricity. This load is dangerous for an airplane and everything inside it! Furthermore, during a lighting strike the lighting will enter at one side and exit at the other side. Most of the time the outer points of an airplane, being the wingtips, tail, and nose radome, are the enter or exit point of a lighting strike. At the aileron, elevator, and rudder, there are electrical conductive sticks called “static discharger” or “static wick”, these “sticks” will discharge the static load from both electric sources. This is based upon the “corona effect”. For more info check out ATA 23.


Slats, Spoilers, and Flaps:

Let’s start with the slats and flaps, these devices are also called “lift augmenting devices”, they will make the wing profile longer, giving it more lift. But these devices will also be used during landing to slow the airplane down. It just depends upon the setting of degrees flap and/or slats. At flaps 5° or 10° you will enlarge the profile in a positive way and therefore the wing will created more lift. At flaps 25° up to 35°, the wing will create drag and slow the plane down.

Spoilers you have in two types, ground spoilers and flight spoilers. Flight spoilers are used to assist the high speed ailerons, ground spoiler are used during landing to break the aero dynamical airfoil over the wing and so reducing the lift to zero!

FOR INFO: a flat surface has 100% drag, a cylinder 50%, and a wing profile or aero dynamical profile 25% drag. So when the ground spoilers go to full deployed position the wing drag percentage goes from 25% up to 100% !!! You need these baby’s together with the other “landing” devices to get the plane to stop.


Winglets:

Are mainly to improve the “lift-to-drag” ratio, and therefore reducing fuel cost.


Artificial Feel:

During the explanation of the primary flight controls we talked about the difference between small and large airplanes, and the systems the large airliner need to be able to move the large control surfaces. To give the pilot some realistic feeling of what he asks of his plane to do, the airliner is equipped with a special system called artificial feel system. This system will counter act against the force the pilot is inputting and the force measured at the specific flight control surface. The to prevent that the pilot would be able to over stress the airliner. This system also takes airspeed in consideration. Just like expensive cars do now days with the steering at high or low speeds.

Balance tab, spring tab, trim tab:

A balance tab will move opposite of the control surface, and will limit the force needed by the pilot to be able to move the control surface at a certain airspeed. The spring tab is an improved balance tab, improved by use of a spring inside the linkage. The trim tab is used to keep the plane in a certain attitude in relation with airspeed and altitude. All of these tab’s are located at the trailing edge of the flight controls, note that every tab can only be mounded one time at every control surface.


Redundancy:

In airliners the control surfaces are moved by hydraulic powered actuators, these are on all primary flight controls in threesome mounded.

For example: an Airbus A300 has three hydraulic systems, every system having a system color, green for the main or largest system, blue for the secondary system, and finally yellow for the backup system. On the left aileron you’ll find three actuator, one powered by green, one by blue, and one by yellow. The same thing for the right aileron, both elevators, and the rudder also. This way incase one system fails the pilot has two other system still left to control the flight controls.

 

 

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