Flight control - Rudder - vertical, transverse and longitudinal axis

Flight control – simply explained

Tim Takeoff
05.11.2019
6 pictures
6 minutes

In the early days of flying, flight control was one of the largest hurdles. Those breaking ground in aviation had to understand how lift is generated, have a fundamental grasp of aerodynamics, but also have their inventions under control in the air at all times.

Those pioneers such as the Montgolfier Brothers with their hot air balloons, the Wright Brothers with the first motorised flight or Otto Lilienthal with his glider had the central goal of getting an object into the skies (more on the history of flying in our article). The main objective was clear: to get airborne and enable manned flights to take off.

Re-directing air streams

As far back as the famous inventor Leonardo Da Vinci, it was clear that taking off in itself is insufficient. You also need to be able to control flying objects in the air stream. Right from his early drawings, you can find ideas intended to control his inventions.

In the modern world, it is absolutely evident that re-directing air streams leads to pressure differences both in water and air. Depending on the points of an object where these air streams are re-directed, the object’s direction can be influenced.

Countless shapes and genres of aircraft were born in the erstwhile pioneers’ trials. Some of them proved to be fatal misguided designs, others continued to be pursued. These successful designs have evolved to become ‘taken as read’ in today’s aviation.

From kites to airliners

If you initially take a non-motorised flying device in its most simple form, such as a rigid kite flown by a human, the principle is relatively simple. By shifting weight, this device can be controlled in the air around two axes. Weight at the front, the nose goes down. Weight to the back, the nose goes up. The device can be controlled left and right using the same principle.

If you now add various control surfaces in the form of a rigid foil, you quickly achieve what we today call gliders. Certain areas are hollowed out, fitted with hinges and connected via the central unit (the control joystick or control column). The pilot can then operate all control surfaces directly, mostly using push rods or hoists.

Controlling flight around three axes

Chaotic composition of metal

For a left-hand curve, the control surfaces on the inside of the curve protrude upwards, thus deflecting the air flow upwards in various manners. On the opposite side, the air particles are diverted downwards and thus generate an increased overpressure. The aircraft starts to enter a curve. You can also deploy associated flaps for landing to reduce the minimum speed. More lift is generated, saving material and reducing emissions.

And what role does the engine play here?

From control ropes to cables

In a modern airliner, the control impulse distribution is no longer primarily generated by hoists and push rods. This would be too laborious and is practically antique if you consider the opportunities provided by modern software. The pilot’s control column or joystick is connected to a primary computer using kilometres of cable. It processes the pilot’s (or auto-pilot’s) entries in fractions of a second, checks for plausibility and forwards signals to the control surfaces’ hydraulic transmitters. These transmitters are then directly connected to the rudder and ‘forces’ them to move.

A closed circuit

Further sensors then recognise the rudder protrusion that has been generated and feeds this back to the flight computer. This verifies whether the transmitted signal has also been implemented exactly. If there are deviations, the computer may carry out fine-tuning and create the desired flight position. If errors were to crop up again, the system reports this to the pilot immediately who can switch off the primary flight computer at any time. In this way, the pilot can access the hydraulic actuators on the rudders directly. Airbus and Boeing call this state ‘direct law’. The flight properties alter significantly in this state, as there is no further software-related adjustment and optimisation.

Sombre example – 737 Max

A prominent example of sub-optimal software is the Boeing 737 MAX. On multiple occasions, this software caused uncontrollable control entries and ultimately led to the pilot not being able to override the system, tragically leading to two crashes. Boeing is currently working with the authorities on a large-scale update for all Boeing 737 MAX aircraft, which currently are able to take to the skies with special permission only.

Flight control relies on software

Without modern software, it would not be possible to fly economically and maintain the exact flying position in the world’s ever more crowded airspace. The entire industry relies on the most stringent safety standards. It is not least for this reason that aviation is an extraordinarily specialised industry field. Nothing is checked and tested more than a new aircraft before it is allowed to leave the ground for the first time.

Are you still not absolutely in the picture about how flight control works? Then take a look at this video:

by Tim Takeoff

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